Symposium F.EL05—Putting Photons to Work—Progress in Photomechanical Materials and Applications

Photochromic compounds undergo a photochemically reversible transformation reaction between two isomers. Such molecules in organic crystals, which are regularly oriented and fixed in the crystal lattice, may be potentially useful for optoelectronic devices. To apply photochromic diarylethene crystals to functional materials in photonics, electronics, mechanics, and medical fields, the materials are required to change large physical property by photoirradiation. We have reported photoinduced crystal shape change of diarylethene crystals.¹ In the photoinduced crystal shape changes, there are contraction/expansion, bending, twisting, and so on,² which depend on the crystal structure and the size of the crystal. In this work, we have focused on the photoinduced crystal shape changes of diarylethene crystals depending on the crystal size, irradiation power, irradiation wavelength, irradiation direction with polarized UV light, and the direction of the incident light.

The rod-like crystals composed of the diarylethene derivatives showed the crystal bending away from the incident UV light or toward the incident UV light, depending on the molecular structure or the crystal packing. The bending velocity depended on the crystal thickness, and the curvature change against the crystal thickness was well fitted to Timoshenko’s bimetal model.³ When the irradiation power was changed, the bending velocity was proportional to the power of the UV irradiation, indicating that local strain acts cumulatively due to the structural changes in the individual diarylethene molecule at the initial stage of the photochromic reaction.⁴ Moreover, the bending behavior depended on the wavelength of the incident light.⁵ When the crystal was irradiated under polarized UV light, the bending speed depended on the polarization angles.⁶ A unique mechanical behavior of a molecular crystal was induced by a combination of a photochromic reaction and a reversible single-crystal-to-single-crystal phase transition.⁷ The photoinduced reversible crystal twisting of a diarylethene crystal was observed upon alternating irradiation with UV and visible light. The crystal twisting takes place in both a left-handed helix and a right-handed helix. The direction of the twisting depends on the face irradiated with UV light. The control of the photomechanical twisting of a diarylethene crystal was studied from the viewpoint of illumination direction.⁸ Changing the UV illumination direction for the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. The control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.
References
1. Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Nature 2007, 446, 778−781.
2. Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Chem. Rev. 2014, 114, 12174−12277.
3. Kitagawa, D.; Kobatake, S. J. Phys. Chem. C 2013, 117, 20887−20892.
4. Hirano, A.; Hashimoto, T.; Kitagawa, D.; Kono, K.; Kobatake, S. Cryst. Growth Des. 2017, 17, 4819−4825.
5. Kitagawa, D.; Tanaka, R.; Kobatake, S. Phys. Chem. Chem. Phys. 2015, 17, 27300−27305.
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7. Kitagawa, D.; Kawasaki, K.; Tanaka, R.; Kobatake, S. Chem. Mater. 2017, 29, 7524−7532.
8. Kitagawa, D.; Tsujioka, H.; Tong, F.; Dong, X.; Bardeen, C. J.; Kobatake, S. J. Am. Chem. Soc. 2018, 140, 4208−4212.

In recent times various intriguing dynamic single crystals that respond to external stimuli such as mechanical stress, heat, light, solvent, humidity, have been reported, but their systematic design remained a challenge. In my talk, I will demonstrate a variety of mechanically reconfigurable and certain other stimuli responsive organic single crystals, where a range of supramolecular weak interactions such as van der Waals (vdW) and π-stacking, play a crucial role in absorbing the internal and external stresses generated.^1-3 I shall present a detailed case study of photomechanical work (i.e. mechanical bending) done by a crystal using visible light mediated photopolymerization chemical reaction, and recovery of the original shape in a reversible manner by a thermal depolymerization reaction.⁴ A few other examples covering waveguides and mechanochromic luminescence will be discussed. Our study demonstrated the potential of soft interactions for designing mechanical behaviour of ordered molecular materials, including those from π-conjugated semiconducting systems, which are of paramount importance for achieving flexible smart materials.

(1) Reddy, C. M.; Krishna, G. R.; Ghosh, S. CrystEngComm 2010, 12, 2296.
(2) Panda, M. K.; Ghosh, S.; Yasuda, N.; Moriwaki, T.; Mukherjee, G. D.; Reddy, C. M.; Naumov, P. Nature Chem. 2015, 7, 65-72.
(3) Krishna, G. R.; Devarapalli, R.; Lal, G. Reddy, C. M.; J. Am. Chem. Soc. 2016. 138, 13561−13567.
(4) R. Samanta, S. Ghosh, R. Devarapalli, C. M. Reddy, Chem. Mater. 2018, 30, 577−581.

CMR thanks DST for Swarnajayanti Fellowship and CSIR for research funding. IISER Kolkata is thanked for central facilities.

Phase-change materials have demonstrated to be capable of the largest energy densities in micrometer-sized actuators. Vanadium dioxide-based devices, for example, have shown energy densities near 1 MJ per cubic meter only within a temperature change of only 10 degrees. Comparable work/volume characteristics can only be obtained using difference thermal expansion mechanisms through temperature changes of hundreds of degrees, or by shape memory alloys. Another intrinsic advantages of using vanadium dioxide in micro-actuators include reversibility and ability to use the material’s hysteretic behavior to program mechanical states. However, even though vanadium dioxide is the solid-solid phase change material with the transition temperature closest to room temperature, micrometer-sized actuators based on this material still suffer from low efficiencies, especially when photothermal actuation is used. Designing vanadium dioxide-based photothermal actuators has typically involved unavoidable trade-offs that hinder the demonstration of ubiquitous devices with high energy density, speed, flexibility, efficiency, sensitivity, and multifunctionality. This talk will show techniques that use carbon nanotube thin films of different chiralities to overcome this low-efficiency issue; which also produced faster devices with increased sensitivity, and the demonstration of wavelength-selective photothermal actuators.

Crystals organize photoreactive molecules and coordinate their motions, leading to photoinduced deformations like expansion, bending, twisting, peeling and coiling. This class of materials provides a path to photomechanical actuators that can be scaled down to micro- or nanoscale dimensions, but along the way several challenges must be addressed: 1) optimization of the molecular structure to control the rate and mechanism of reversibility (photon versus thermal activation); 2) control of crystal morphology to generate actuating structures with a variety of shapes and sizes; and 3) the hierarchical organization of nano-scale and micro-scale crystals into a macroscopic actuator structure, usually by templated growth and organization. Recent progress in all three areas have led to the development of composite organic-inorganic structures with unprecedented power-to-weight ratios: under light illumination, less than 1 mg of a diarylethene photochrome can move a load weighing more than 100 g. These materials consist of organic photochromic crystals aligned in the pores of various types of inorganic host materials, including Al₂O₃ and SiO₂. The effects of the inorganic matrix material, organic photochromic molecule, and crystal alignment and orientation are all analyzed. The potential of these hybrid materials as practical photoactuators will be assessed.

In molecular crystals, reaction at one site can influence the reaction probability at other sites. This cooperative photochemistry can lead to interesting and possibly useful phenomena. The fluoro-9-anthracenecarboxylic acids comprise a family of thermally reversible (T-type) photomechanical molecular crystals. Their photomechanical response is driven by a [4+4] photodimerization reaction, while the dimer dissociation determines the reset time. A microscopic fluorescence-recovery-after-photobleaching experiment is used to monitor the kinetics of dimer dissociation and mechanical bending in single crystals. The recovery kinetics depend strongly on the initial concentration of photodimer, slowing down and becoming nonexponential at high dimer concentrations. A new feature in the photomechanical behavior is identified: the ability of a very weak control beam to suppress dimer dissociation after large initial dimer conversions. This phenomenon provides a way to optically control the mechanical response of this photomechanical crystal. To gain physical insight into the origin of the nonexponential recovery curves, the experimental results were analyzed in terms of a standard first-order kinetic model and a nonlinear Finke-Watzky (FW) model, but neither model can reproduce the suppression of the recovery in the presence of a weak holding beam. More sophisticated theoretical treatments that explicitly take spatial heterogeneity into account are probably required to analyze the role of cooperative molecular phenomena in solid-state crystalline reactions and their photomechanical response.

Phase change materials are of great interest for low-power high-throughput storage devices and next-generation neuromorphic computing technologies due to their contrasting properties between amorphous and crystalline phases and fast switching rate. Notably, Sb₂Te₃, an archetypical Ge-Sb-Te alloy, displays a fast and energy-efficient crystallization-amorphization cycle, due to its growth-dominated crystallization and low melting point. Here, we demonstrate a new nonthermal amorphization pathway of Sb₂Te₃ which can further reduce time required for amorphization from nanoseconds to picoseconds. We employ non-adiabatic quantum molecular dynamics simulations to investigate the time evolution of Sb₂Te₃ at a constant temperature of 500 K with 2.6%, 5.2%, 7.5%, 10.3%, and 15% photoexcited valence electron-hole carriers. Results reveal picosecond nonthermal amorphization at and above a critical excitation of 7.5% valence electrons. The rapid amorphization originates from an instantaneous charge transfer from Te-p orbitals to Sb-p orbitals upon photoexcitation. Subsequent evolution of the excited state, within picosecond timescale, indicates a Sb-Te bonding to antibonding transition. Concurrently, Sb-Sb and Te-Te antibonding decreases, leading to the formation of wrong bonds. For 7.5% valence electron photoexcitation or larger, the electronic changes destabilize the crystal structure, leading to large atomic diffusion and loss of long-range order. These results highlight an ultrafast energy-efficient amorphization pathway that could be used to enhance the performance of phase change materials based opto-electronic devices.

This work was supported as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0014607. The simulations were performed at the Argonne Leadership Computing Facility under the DOE INCITE and Aurora Early Science programs and at the Center for High Performance Computing of the University of Southern California.

The coupling between mechanical and optical properties can be surprisingly large, and has a long history of study. Optical absorption resonances are related to breaking, reforming, and rearranging bonds, on an energy scale of 1 eV/molecule, and it stands to reason that the effects of optical absorption on mechanical properties may be substantial; still these effects continue to surprise, and are often overlooked. Opto-mechanical effects may be especially strong when mediated by optically-active defects that store charge, induce local lattice distortions, and interact with modes of plastic deformation. The connection between defects and opto-mechanics provides a handle for controlling opto-mechanical effects through materials processing, and suggests new applications.
Here we show that the wide-band gap compound semiconductors ZnO, ZnS, and CdS all feature large opto-mechanical coupling that is mediated by point defects. We measure the mechanical properties of sintered and single crystal samples using a nanoindenter equipped with a purpose-built variable wavelength and intensity illumination system. We find that both elasticity and plasticity vary strongly with moderate illumination. For instance, the hardness and stiffness of ZnO can increase by up to 40-80% due to blue illumination of intensity 3 mW/cm². The wavelength-dependence shows that band-edge absorption (e.g. UV light) has the strongest effect, and the relative size of the effect of sub-band gap illumination varies between samples – a clear sign of defect-mediated processes. We further show that these giant opto-elastic and opto-plastic effects can be tuned by materials processing, and that the processing-dependence can be understood within a framework of point defect equilibrium. We tune point defect concentrations by annealing treatments, and measure composition by direct (inductively-coupled plasma optical emission spectroscopy, ICP-OES) and indirect (X-ray diffraction, XRD) methods. Our composition measurements for ZnO, ZnS and CdS all show that the magnitude of the optomechanical response varies with anion vacancy concentration. This is consistent with the hypothesis that photoconductivity and photoplasticity are positively correlated, and provides a rational guideline for engineering optomechanical response. We use density functional theory (DFT) calculations to interpret our experimental results. DFT suggests that photo-induced charge carriers migrate along strain gradients, and gather around anion vacancies. The high local net charge concentrations (10²¹ ~ 10²² cm^-3) create pseudo-tensile-strain (~5%), which opposes the compressive strain from the indenter, thereby enhancing the elastic modulus.
We will conclude with an outlook on opto-mechanical effects in wide-band gap semiconductors, and with several proposed, new applications for these still-surprising effects.

In 1948, Casimir described a bizarre force that acts between neutral objects based on quantum fluctuations of electromagnetic fields. This effect is of both fundamental interest in quantum field theory and practical importance in nanotechnology. Recently, it has been predicted that Casimir force can induce phonon coupling between nearby objects and thus transfer heat across a vacuum gap. Revealing this unique quantum effect experimentally would bring fundamental insights to quantum thermodynamics and practical implications to nanoscale thermal management. Here, we present the first experimental demonstration of the Casimir heat transfer. We use nanomechanical systems to realize strong Casimir phonon coupling and observe the exchange of thermal energy between individual phonon modes. The experimental observation agrees well with our theoretical calculations and is unambiguously distinguished from other effects such as near-field radiation and electrostatic interaction. This experiment opens up new opportunities for studying nanoscale energy transport and quantum thermodynamics.

Photomechanical organic crystals are fast becoming the next generation smart-materials that can convert light energy into mechanical work. While the majority of these photomechanical crystals are made from diarylethene or azobenzene derivatives our group has focused on the synthesis of anthracene-based compounds that can be grown into photomechanical crystals. These crystals can expand, bend, twist, curl, peel, or jump when exposed to UV or visible light. We will survey the synthetic routes used to make these anthracene derivatives. We will also describe how we made the corresponding photomechanical crystals using different techniques such as solvent annealing inside Anodic Aluminum Oxide (AAO) templates and aqueous surfactant-mediated precipitation technique.
Esterification of 9-Anthracenecarboxylic acid with aliphatic alcohols gives a family of photomechanically active anthracene derivatives. When these derivatives are grown into crystalline nanowires via solvent annealing inside AAO templates, the resulting nanowires expand anisotropically along their long axis when exposed to UV light. The intermolecular [4+4] photocycloaddition between parallel anthracene rings is the driving force behind the crystal expansion. A thoroughly studied molecule is the photomechanical expansion of tert-butyl anthracene-9-carboxylate (9TBAE) nanowires by 15% upon irradiation with 365 nm light. Crystalline microribbons grown from the commercially available 9-Anthracenecarboxylic acid (9AC) can twist reversibly (T-type) when exposed UV light. The microribbons require several minutes to reset by reversing the thermally unstable [4+4] photocycloaddition. The introduction of substituents on the 9AC anthracene ring helps expand our toolbox of T-type photomechanical crystals. Multi-step synthesis was used to introduce substituents on different positions of the 9AC anthracene ring. These substituents (F, Cl, or Br, NO2, CH3, etc.) either suppress or enhance the T-type character of the substituted 9-anthracene carboxylic acid crystals. While the 4-fluoro-9-anthracene carboxylic acid (4F9AC) shortened the reset time of the crystal by an order of magnitude, the 10-fluoro-9-anthracene carboxylic acid (10F9AC) derivative did the exact opposite. Extending the conjugation of the anthracene ring at the 9-position by attaching propenal using Mizoroki–Heck or Wittig reaction yields the very useful 3-(9-Anthryl)acrolein which was used as a building block for the synthesis of second-generation photomechanical anthracene derivatives. Adding different electron-withdrawing groups to the 3-(9-Anthryl)acrolein aldehyde via Knoevenagel condensation leads to a novel type of photomechanical crystals that are driven by trans/cis photochemistry. These crystals are dominantly P-type. For example, 2-(3-anthracene-9-yl-allylidene)-malononitrile (9DVAM) crystals and nanowires exhibit P-type photomechanical response under visible and UV light. When these crystals are grown into microwires from an aqueous surfactant they exhibit continuous photo-induced wiggling-like motion resembling flagella. Block-like microcrystals composed of cis-dimethyl-2(3-(anthracene-9-yl)allylidene)malonate (DMAAM) can peel off amorphous layers upon UV-light exposure while similar analog derivatives can super expand by transforming from crystalline-to-gel like material under intense green light exposure. The introduction of fluorine on the 9DVAM anthracene ring yields derivatives with P-type photosalient crystals and nanowires that may find future application in light-driven microactuators.
Synthesis of Anthracene-based photomechanical crystals coupled with non-classical crystal growth techniques has shown to be essential for both understanding the basic principles and advancing the field of photomechanical smart-crystals.

Photostrictive compounds are promising device materials because of their fundamental photophysical properties and light-induced cooperative behaviors. Although molecular charge-transfer crystals exhibiting light–matter interactions have been successfully deployed in optoelectronics, an air-stable molecular material that couples photons and electrons, achieving photostriction through the coupling of light and mechanical degrees of freedom, has not yet been discovered. Herein, we report a substantial light-induced dilation in a molecular material at room temperature, which is accompanied by simultaneous photocurrent generation. This finding opens avenues for coupling optical, electronic, and mechanical functionalities for possible use in remote wireless photoswitchable devices.

For the past few years we have been developing structurally simple, easy to make, modular, and tunable hydrazone-based functional materials (e.g., switches, sensors and fluorophores).¹ This presentation will deal with our recent advances with these systems, with an emphasis on newly developed photochromic compounds² that exhibit many interesting properties, including emission ON/OFF toggling in solution (see below) and the solid-state.³ The integration of these photochromic compounds into liquid crystals⁴ and liquid crystalline elastomers⁵ will also be discussed.

Representative references:
1. I. Aprahamian ChemCommun 2017, 53, 6674–6684
2. (a) H. Qian, S. Pramanik, I. Aprahamian J. Am. Chem. Soc. 2017, 139, 9140−9143
(b) Q. Li, H. Qian, B. Shao, R. P. Hughes, I. Aprahamian J. Am. Chem. Soc. 2018, 140, 11829–11835
3. B. Shao, M. Baroncini, H. Qian, L. Bussotti, M. Di Donato, A. Credi, I. Aprahamian J. Am. Chem. Soc. 2018, 140, 12323–12327
4. M. J. Moran, M. Magrini, D. Walba, I. Aprahamian J. Am. Chem. Soc. 2018, 140, 13623–13627
5. A. Ryabchun, Q. Li, F. Lancia, I. Aprahamian, N. Katsonis J. Am. Chem. Soc. 2019, 141, 1196–1200

We will describe our latest efforts to create new photochromic compounds based on sulfoxides and sulfoxide mimics. We will also detail our advances in the creation of first row transition metals, where we propose that isomerization is capable of stabilizing of high spin states available for first row transition metal complexes. In addition, we will detail our spectroscopic studies of these new materials, both in solution and in solid state.

Light is an attractive source of remote control and power: it is non-invasive, wavelength-selective, and provides high spatial and temporal resolution. However, the development of new materials is required to fully exploit these advantages by optimizing light energy transduction pathways and/or the light-mediated modulation of materials properties. In this talk, I will present my group’s collaborative efforts to advance the incorporation of donor–acceptor Stenhouse adducts (DASAs) into materials designed for light-driven generation and control of material motion. DASAs present unique advantages for actuation. These highly-absorbing, negatively photochromic molecules have highly tunable reaction kinetics and respond to visible light. We have shown them to be effective photothermal and photochemical agents that can drive the motion of amorphous polymeric materials as well as liquid flows. In particular, through control of DASA reaction kinetics and the applied light field, it is possible create self-regulating flow fields that hold promise for valveless optofluidic devices and soft robotics.

Making molecular machines that can be useful in the macroscopic world is a challenging long-term goal of nanoscience. Inspired by the protein machinery found in biological systems, and based on the theoretical understanding of the physics of motion at the nanoscale, organic chemists have developed a number of molecules that can produce work when triggered by various external chemical or physical stimuli. In particular, basic molecular switches that commute between at least two thermodynamic minima and more advanced molecular motors that behave as dissipative units working out-of-equilibrium when fueled with external energy have been reported. However, the ultimate challenge of coordinating individual molecular motors in a continuous mechanical process that can have a measurable effect at the macroscale has remained elusive until very recently. We will discuss advances developed by our group on light-activated molecular machines and involving their mechanical coupling within dynamic polymer systems. We will show that it is now possible to amplify their individual motions to achieve macroscopic functions in materials. In particular, we will present a dual-light controlled system operating fully out-of-equilibrium, and in which the integrated motions of two types of mechanically active units can be tuned by modulation of frequencies. Finally, we will discuss the possibility to use similar ideas to get control over mechanotransduction in living cells.
(1) E. Moulin, L. Faour, C. Carmona-Vargas, N. Giuseppone, Adv. Mat. 2020, 32, 201906036.
(2) D. Dattler, G. Fuks, J. Heiser, E. Moulin, A. Perrot, Y. Xuyang, N. Giuseppone, Chem. Rev. 2020, 120, 310.

We demonstrate the potential of spiropyran-functionalized, light-responsive hydrogels to be a promising new platform for nonlinear optical materials. Nonlinear optical behavior in these materials arises from the swelling/deswelling and resulting refractive index changes that occur due to photoisomerization between the hydrophilic ring-open merocyanine isomer and the hydrophobic ring-closed spiropyran isomer. These materials show an unprecedented combination of reversibility, soft easily processed material, low laser powers, and long-range interactions. These long-range interactions are especially promising for potential applications in all-optical computing or sensing. We show that the long-range interactions can be explained by thermal swelling or deswelling of the gel. The presence of a secondary temperature gradient that influences beam behavior leads to novel repulsive long-range interactions, which are not possible in conventional nonlinear optical materials in the absence of external symmetry breaking. Furthermore, the direction and magnitude of a hydrogel’s thermal response can be controlled by changing the gel composition, which allows the behavior of single and multiple beams in these gel materials to be rationally designed over a potentially vast design space. We demonstrate this potential by exploring two beam interactions in poly(N-isopropylacrylamide-co-acrylic acid) gels which contract rather than swelling as function of increasing temperature. We observe a lack of long-range interactions in these gels along with behaviors indicative of short-range attractive interactions, such as spiraling and beam fusion. This tunable diversity of behaviors suggests the enormous potential for photoresponsive hydrogels as extremely versatile, rationally designable, soft, nonlinear optical materials.

Molecular photoswitches – molecules that can switch between multiple states of differing properties – are useful to impart actuation and changes in optical properties of soft materials. The chemistry of these photoswitches can be conveniently tuned to control the resulting material properties and have found application in soft actuators, non-linear optics, and optical memories. Yet, there is often a mutual interaction between photoswitch and embedding matrix, in which the photoswitch not only determines material functionality, but in turn the embedding matrix influences photoswitching behavior, enabling self-regulated behaviors through feedback loops. Here, we show how such self-regulated behavior arises in spiropyran-functionalized hydrogels and what effects govern this behavior. We are able to disentangle influences of hydrogel chemistry, type of photoswitch attachment, ionic strength, changes in hydrogel volume, and aggregation on the isomer-distribution of the photoswitch. In particular, we find that by changing the spiropyran from appendage to crosslinker, we can not only control feedback in hydrogel swelling behavior, but also control the non-linear optical properties of the hydrogel. These studies directly inform how such feedback loops can be leveraged for the fabrication of evermore sophisticated self-regulated optical materials.

Actuation by light is attractive for a number of reasons: Objects can be actuated from a distance, distinct frequencies can be used to actuate and control distinct modes with minimal interference, and significant power can be transmitted over long distances through corrosion-free, lightweight fiber optic cables. Azobenzene and other molecules that are known to change conformation when exposed to light are the building blocks of photomechanical actuation. An important challenge is how the change of conformation of individual molecules can be harnessed into a robust change of deformation of the material. Approaches range from mean-field entropic coupling as in photoactive nematic elastomers to direct chemical bond coupling as in molecular crystal to hybrid composite approaches where molecular crystals are embedded an elastic matrix. The mean-field entropic coupling can lead to weak coupling while the stiffness of molecular crystal can inhibit actuation or cause them to shatter on actuation. This talk will present a series of theoretical models to understand various aspects of this molecular to material coupling, illustrate them with experimental observations and discuss how ideas from these studies can be harnessed in real materials and devices.

Liquid crystal polymer networks undergo reversible shape change in response to any stimulus that affects the strength of nematic order. These soft actuators can be fabricated as thin films, surface coatings, or 4D printed solids and have potential applications in soft robotics, biomedical devices, microfluidics, and sensors. Trajectories for shape change are “programmed” by patterning the nematic director when the polymer is cross-linked. Actuation is induced when the strength of nematic order is modulated by stimuli such as a change of temperature or chemical environment; for example, heating induces contraction along the nematic director and expansion in orthogonal directions. The addition of azobenzene derivatives that undergo photo-isomerization produces a soft actuator that flexes like an artificial muscle in response to illumination; if the cis-to-trans thermal relaxation is fast enough, the material can also flex rapidly back toward its original shape when illumination is removed. Such reversible photo-mechanical actuation is a highly attractive option to control a soft robot as it combines both control and power for induced motion without need for tethering or onboard batteries.

Modeling the dynamics of photo-mechanical actuation in these materials is important both to explain the mechanisms behind experimental observations and for device design. To model photo-mechanical actuation at the device scale, we use nonlinear finite element method (FEM) elastodynamics simulation based on a Hamiltonian approach. This method can simulate underdamped motion where momentum plays an important role. The code was developed in-house and implemented in CUDA for execution on a GPU-enabled computer.

We used this approach to model oscillatory mechanical wave motion in a soft robot whose motion was both controlled and powered by light [1]. We use a simplified ray tracing algorithm to determine which parts of the device were illuminated and which in shadow, and used a kinetic model of photo-isomerization/relaxation to describe the time evolution of nematic order. We found that the directionality of light-induced mechanical wave propagation is controlled by a self-shadowing mechanism and periodic pop-through transitions.

We also model a variety of other prototype devices including surface-adhered nematic polymer coatings that transform from flat to complex surface profiles with microchannels, spikes, or dimples recently observed in experiments by Babakhanova et al [2]. Next we model polymer coatings patterned with biomimetic “octopus-like" suckers, designed to grasp and release nonporous objects on command. Such a device would have the potential to achieve strong, reversible adhesion. Our results demonstrate that actuation geometry produced by a disclination in a nematic polymer is controlled not only by the defect’s topological charge and orientation but also by the details of its defect core structure. We compare our modeling results with relevant experiments by collaborators and discuss potential applications.

[1] Gelebart, A.H., Mulder, D.J.,Varga, M., Konya, A., Vantomme, G., Meijer, E.W., Selinger, R.L.B., Broer, D.J., (2017) Making waves in a photoactive polymer film, Nature 546, 632 (2017) https://doi.org/10.1038/nature22987
[2] Babakhanova, G., Turiv, T., Guo, Y. et al. Liquid crystal elastomer coatings with programmed response of surface profile. Nat Commun 9, 456 (2018). https://doi.org/10.1038/s41467-018-02895-9

Work supported by NSF CMMI-1663041, NSF DMR-1409658, and NSF-CMMI 1436565; computer resources provided by the Ohio Supercomputer Center.

The ability to harness light for photochromic switching that directs mechanical work generation within a macromolecular network is a platform for envisioning photomechanical machines, which are neither encumbered by onboard power sources, nor require microfabricated components and mechanisms. Light offers multiple control knobs for photomechanical regulation, including spatiotemporal modulation of intensity, direction, polarization and chirality. Unfortunately for actuators driven with light, the length over which the photons are attenuated (typically, few micrometers) limits the thickness of the photoactuators. The photomechanical strain generation is via bending, resulting from the gradation in the strains that are generated. Slender structures that respond by bending make for weak actuators. Work content that can be accessed is intrinsically bounded in the characteristically micrometer-scale thickness actuators.

Unusual opportunities emerge when we move beyond flat photoactive geometries, where the patterning of the molecular switches and the graded photomechanical strains conspire to result in non-linear mechanical responses. Snap-through in rod-like structures are one example. Creasing instabilities are another, where the bending of liquid crystalline polymers that are photoresponsive is used to drive non-linear transformations of curved shells, which are confined at their periphery. Torque densities >100 Nm/kg and flexural actuation rates >100 radians/s can be produced. Profiles of these photomechanical actuators places them in a property space that competes favorably with electromechanical systems. A third example is that of photoactuated tape-spring mimicking structures. Here, ms-scale actuation is triggered in shells that switch their principal curvatures, via a snap-through mechanism. Beyond these illustrations, opportunities also exist for utilizing a broader class of latch and torque-inversion mechanisms to magnify actuation by integrating active and passive (structural) materials.

Overall, accessing 3-dimensionally patterned freeform actuators is useful for magnifying photomechanical actuation. Realizing these geometries is simplified by the availability of an additive manufacturing platform, wherein the molecular order can be patterned voxel-by-voxel during the composition of multimaterial architectures. The interplay of magnetically assisted alignment and site-specific photopolymerization is a vehicle for achieving this in an inverted stereolithographic setup. In comparison to say, extrusion-based methods, decoupling the material accretion from the endowment of molecular order is key to expanding the design-space, which can be fabricated.

Magnifying photoactuation in monolithic polymeric structures, which can be scaled from the micrometer to mm-scale form factors holds implications for enabling photomechanical machines. It is possible to envision optically powered and reconfigurable devices in applications ranging from microrobotics, biomedicine and manipulation in environments hostile to electromechanical systems.

When illuminated, photomechanical materials move and demonstrate forces which cause motion. This response indicates that they have acquired momentum as consequence of photoexcitation. We characterize their response by identifying and quantifying the effectiveness of the physical mechanisms connecting photoexcitation and the mechanical response. We are particularly interested in the details of how photostress results from the exciting flux of photons, whose momentum is negligible compared to what is observed in the response, and how photostresses produced in the bulk are transported to the sample surfaces and do mechanical work. We introduce a simple relation linking a stress in light to the photostress, and propose that elements of the tensor relating these serve as measures of the effectiveness of the different aspects of the photoresponse, both in transforming photon energy to stress and in storing photon energy and making it available for mechanical work. Using this metric, we compare the performance of different classes of photomechanical materials using data from the published literature.

Light-driven strains can be large, controlled by colour and polarisation of light, and quickly activated in liquid crystalline solids, both rubbery and glassy.
The challenge for applications is to make the response strong, typically involving stretch rather than bend energies if the distortions are loaded. To achieve stretch in both pull and in push without instabilities, the director needs to be programmed in the solid to change the Gaussian curvature of sheet and shell-like objects.
We make contact with current experiments and suggest new Gaussian-curved evolving landscapes, including those arising from curved fold non-isometric origami objects, for effective applications.

Donor-acceptor Stenhouse adducts (DASA) are a promising new chromophore for photomechanical applications. We detail studies of the photochemistry of this chromophore, including both the initial light-induced isomerization and the thermal ring-closing reaction. We further provide details on a framework for modeling excited state dynamics under force, combining the ab initio multiple spawning formalism for nonadiabatic dynamics with the force-modified potential energy surface framework developed for ground state mechanochemistry. We discuss the role of external force (either compressive or extensional) on the photodynamics of the DASA chromophore and its implications for the use of DASA chromophores in light-triggered mechanical actuators.

Materials capable of directly converting photon energy into mechanical deformation offer promise in a wide variety of contexts including adaptive optics, remotely operated swimmers, and actuators controlled via lightweight optical cables that resist corrosion and electromagnetic interference. Organic photoswitches offer significant potential in this regard, thanks to their ability to undergo large changes in molecular conformation upon excitation, and their highly tailorable absorption spectra. To date, efforts have focused largely on either crystalline lattices of small photomechanical molecules, which offer the possibility for greater work output due to their high density and well-organized nature, or on polymer matrices containing a modest concentration of photochromes, which generally have superior processability and mechanical properties. We have recently considered two classes of hybrid materials that combine many of the advantages of these disparate approaches. In one case, we have studied the growth of micrometer-scale photomechanical diarylethene crystals within the pores of polymer membranes. For appropriately controlled orientations of the crystals, such materials can exhibit photomechanical work outputs rivalling the best previously reported systems, while enabling facile processing into mechanically robust samples with cm-scale lateral dimensions. In a second case, we have developed a new class of structurally regular addition polymers bearing photoisomerizable azobenzene groups alternating with chain extenders. These materials form semicrystalline solids that exhibit fundamentally interesting temperature- and light-dependent melting and recrystallization behaviors, and that can be processed into aligned films and fibers exhibiting two-way optical shape memory properties.

Photo-responsive liquid crystalline elastomers (LCEs) constitute ideal actuators for soft robots because their light-induced macroscopic shape changes can be harnessed to perform mechanical work. Conventional LCEs, however, do not typically exhibit complex modes of bending and twisting necessary to perform sophisticated maneuvers. Here, we model LCE microposts encompassing side-chain mesogens oriented along a specific axis characterized by the nematic director, and azo-benzene crosslinkers, which determine the deformations of illuminated posts. On altering the nematic director from 90o to 0o relative to the underlying substrate, the post’s bending respectively changes from light-seeking to light-avoiding. Moreover, with the director tilted at 45o, the initially achiral post reversibly twists into a right- or left-handed chiral structure, controlled by the angle of incident light. We exploit this photo-induced chirality to design “chimera” posts (encompassing two regions with distinct director orientations) that exhibit simultaneous bending and twisting, mimicking motions exhibited by the human musculoskeletal system.

The type of deformations and range of motion, which microstructures can undergo, determines their function, be it for soft robotics, for sensing or for changing properties in optical or metamaterials. While composite, multimaterial architectures allow imprinting of pre-determined deformation behaviors, their fabrication at the microscale is difficult and the deformation modes are fixed once fabricated. Herein, we demonstrate the use of an alternative strategy – the generation of a transient bilayer architecture within microstructures by an external light stimulus, – and a variety of deformation behaviors that the same structure can display as a result. Changing the direction of UV-irradiation of high-aspect-ratio compositionally uniform microstructures fabricated from photoresponsive liquid crystalline elastomers gives access to a wide range of sophisticated deformations such as clockwise and counter-clockwise twisting, light seeking and avoiding, particularly if rotational symmetry is broken. Light intensity is demonstrated to be a further control parameter that allows one to tap into non-monotonic bending motions and combinations of twisting, bending and expansions/contractions. Finally, because the light-generated layered architectures are transient, they adapt and evolve as the deformation proceeds, leading to highly complex, yet controllable, self-regulated deformation behaviors. The described LCE microstructures are rapidly and easily fabricated and they offer exciting opportunities to further explore essential features of their finely controllable deformation abilities for applications in next generation smart materials.

Photons have been “put to work” in polymeric materials for more than 50 years. Photogeneration of work has been observed in a range of polymeric materials spanning the continuum from soft elastomers to rigid, high performance materials. Of particular importance is the organization of the polymer chains within the network which can affect magnitude, rate, and directionality of photoinduced deformation. Photomechanical effects in liquid crystalline polymer networks and elastomers are particularly appealing in enabling distinctive conversion of light into work. This presentation will summarize the salient benefits of using light as an actinic stimulus as well as highlight recent developments in this topical area of materials research from our laboratory.

Inspired by the natural activated systems, realization of programmed deformations and locomotion in artificial materials has recently attracted great interest due to their promising applications in biomedical devices, soft robotics, and flexible electronics. Photo-responsive hydrogels are recognized as one ideal material to construct soft actuators and robots owing to their drastic volume change induced by contactless light irradiation with high spatial and temporal resolutions. The challenge is how to construct composite hydrogel with gradient structure and dynamically activate the specific regions of the gel toward programmed deformation and locomotion. We present here a photolithographic method to fabricate patterned composite hydrogel sheets with heterogeneous structures by embedding the photo-responsive hydrogel in a preformed nonresponsive hydrogel. Under photo irradiation, the swelling/contraction mismatch results in the built-up of internal stress and thus programmed deformations of the composite hydrogel. Furthermore, a moving light beam is imposed on the composite hydrogel to spatiotemporally actuate the gel that shows sophisticated motions, including crawling, walking, and turning. Experimental and simulation results reveal that multigait locomotion is realized by the mutual coordination of shape-morphing and dynamic friction of the gel against a substrate under the spatiotemporal light stimulation. The programmed deformations of motions of photo-responsive hydrogels should be instructive for the development of soft robotics with advanced technologies and versatile applications.

Light-fueled soft robots have recently demonstrated untethered manipulation of locomotion without the complex mechanical and electrical design principles of conventional robotic systems[1-2]. To achieve photomechanical motility, we photopolymerized light active molecular machines along with reactive liquid crystalline monomers. For energy effective and rapid photomotility, rolling is employed as an in-plane locomotive mode to reduce friction with the substrates in comparison with crawling. In this presentation, we will also discuss design strategy to implement and program jumping motion of liquid crystalline polymers by controlling the molecular alignment for miniaturized yet high-performance untethered soft robots.

Acknowledgements: This work was funded by NRF-2016R1D1A1B03931678 and NRF-2019R1A2C1004559.

References:
[1] J.J. Wie, M.R. Shankar, T.J. White, Nat .Commun., 7:13260 (2016)
[2] J.G. Kim, J.E. Park et al., Materials, 12, 19, 3065-3086, (2019)

The demonstration of tunable actuation performance at a single light intensity by the simple remodeling of material properties advances the field of light responsive materials one step closer to the development of “life-like” actuators. We introduce a novel approach for programmable actuation using negatively photochromic molecular photoswitches, termed donor-acceptor Stenhouse adducts (DASAs), capable of tuning the photothermal actuation amplitude under constant light intensity based on photochemically programed logic. We present a synthetic pathway for chemically attaching DASA conjugates to polymers through Diels-Alder click chemistry. We report a visible light-responsive bilayer actuator that can lift weight against gravity, as well as a simple light-powered crawler, that exploit photothermal energy conversion. Programmable control of the magnitude of actuation is achieved by slowly converting a highly-absorbing DASA photochrome into a non-absorbing form upon irradiation. Our results highlight the promising benefits of high molar absorptivity, negative photochromism, and broadband visible light absorption of DASAs for programmable actuation.

Liquid crystal elastomers (LCEs) can be functionalized with photochromic moieties to sensitize these materials to light exposure. Prior work has demonstrated that phototropic disruption of order via photochemical adjustment of the isomeric state of azobenzene (for example) disrupts the anisotropic chain configurations of the polymer network. The phototropic disruption of order in azobenzene-functionalized LCEs (azo-LCEs) with UV light via trans-cis isomerization generates comparatively large strain, exceeding 20%.
Fundamentally, the strength of intermolecular interactions between polymer chains defines the thermotropic response of both low molar mass and polymeric liquid crystal compositions. Here we show that by adjusting the intermolecular interaction by introducing a nontraditional liquid crystalline monomer, light irradiation can induce photomechanical responses in azo-LCE at lower energy inputs to achieve equivalent mechanical output.

The photothermal effect has been found in some porphyrin compounds including chlorophyll, chlorophyllin, hemoglobin, and phthalocyanine, all of which share similar structural characteristics. The porphyrin compounds are typically composed of four modified subunits bounded with carbon atoms forming a “ring-like” structure, therefore also known as “porphyrin rings.” Upon white light irradiation, these compounds can selectively observe light in a wide range of frequency from UV to NIR, and convert it to thermal energy. One of the major applications of these porphyrin compounds is in the energy-efficient windows via a newly developed concept of optical thermal insulation (OTI). Most of the thermally insulated windows in cold climate regions are double-paned, sandwiching transparent insulating materials (such as gases), creating additional and unnecessary volume and weight. Thermal transmittance (U-factor) is proportional to the temperature difference, ΔT, between the center of room and window interior surface. OTI does not rely on any conventional thermal insulating materials or gases, such as air or argon, as often used in conventional double-pane windows, but achieves its effectiveness via photothermally-heated building skins. If a spectral-selective thin film is applied on a window inner surface that exhibits strong UV/NIR absorptions and high visible transmittance, the pronounced photothermal (PT) effect will be photonically-activated to increase the surface temperature, resulting in much reduced heat transfer through the window. In this study, we report recent experimental results on deposition of transparent thin films of porphyrin compounds and show much lowered U-factors via OTI under simulated solar irradiation. Also identified is the operating mechanism of the photothermal effects for different porphyrin compounds.

Photothermal experiments of incident light angle dependence were carried out by using solar-simulated light on iron oxides (Fe₃O₄ and Fe₃O₄@Cu_2-xS) and porphyrin compounds (chlorophyll and chlorophyllin) thin films. All thin films were deposited on glass substrates by spin-coating method with the photothermal materials evenly dispersed in polymethyl methacrylate (PMMA) solutions. The Fe₃O₄ film was found to exhibit a strong absorption peak near the UV region but only with background absorptions in the visible and NIR regions. A broad NIR absorption was observed in the Fe₃O₄@Cu_2-xS thin film. Both chlorophyll and chlorophyllin showed pronounced peaks near UV and NIR characterized with the U-shaped spectra, ideal for energy-efficient single-pane window applications that require efficient solar harvest and high transparency. Thermal transmittance through the window can be significantly reduced by decreasing the temperature difference, ΔT, between the window inner surface and the room interior based on a so-called “optical thermal insulation” (OTI) without any intervention medium, such as air/argon. Upon solar irradiation, ΔT is reduced by increasing the inner surface temperature of the window through the photothermal heating of the iron oxide and porphyrin coatings. It is therefore possible to replace double- or triple- panes with single panes based on OTI. As the seasonal changes of sunlight inevitably affect OTI, an incident light angle dependence of the photothermal effect was carried on the oxide (Fe₃O₄ and Fe₃O₄@Cu_2-xS) and porphyrin (chlorophyll and chlorophyllin) thin films. It was found that their heating curves all reached maximum temperatures at small angles of incidence while that of which considerably reduced at large angles. The angle dependence data provides important references for OTI that the window exposure to the sun is higher at the winter solstice (small angle) and lower in summer (large angle). The enhanced solar harvesting at winter solstice can induce a stronger photothermal effect for effective optical thermal insulation of single-pane windows.

Azo-benzene functionalized polymers have tremendously been studied owing to the advantages of battery-free light trigger shape reconfigurable properties. However, the trade-off relationship between mechanical strength and deformability and insulating properties are great limitations of azo-functionalized polymer for shape reconfigurable devices. In this presentation, we introduce bilayered structure of azobenzene functionalized liquid crystalline polymer networks (Azo-LCN) with reduced graphene oxide (rGO) toward drastic enhancement in mechanical and electrical properties without sacrifice of strain responsivity. The Young’s modulus of the Azo-LCN/rGO bilayer increases by 5 times (6.4 GPa) in comparison with the neat Azo-LCN, and the electrical conductivity is recorded to be 340 S/cm. Remarkably, the Azo-LCN/rGO bilayer overcomes the tradeoff relationship between mechanical stiffness and elongational responsivity by mismatch of structural character of each layers. Upon exposure to broadband UV, the bending angle of azo-LCN/rGO bilayer is enhanced in forward and backward bending, respectively 2.4 times (97.5 to 229.5 degree) and 12 times (5.4 to 65.2 degree) compared to the neat Azo-LCN. In addition to in-plane bending, twisting actuation mode is achieved by simply adjusting the cutting angle of the linearly patterned azo-LCN/rGO bilayers. The Azo-LCN/rGO bilayer can also be shape-reconfigurable by thermal stimuli including near infrared (NIR), sunlight and lighter. The photothermal effect is selectively induced on the rGO layer by irradiation of NIR stimuli which drive bending actuation of azo-LCN/rGO bilayer. Then, spatially localized actuation of azo-LCN/rGO bilayer is demonstrated by focused irradiation of solar ray. Finally, a rapid and large actuation is also demonstrated by direct heat energy transfer by operation of a lighter.

The phenomena related to droplet migration at the micro-scale is expected to be applied to micro medical robots, new printing technologies, and basic research for the locomotion mechanisms of active matter such as cells or microbial populations. One of the main driving forces of droplet motion is Marangoni flow, which takes place from lower to higher surface tension regions. There are many studies on droplet manipulation based on Marangoni flow by generating surface tension difference under temperature gradient. Since, generally, surface tension decreases with temperature, a droplet moves from higher to lower temperature regions. In this study, we demonstrated light-driven microdroplet formation and transportation in a two-dimensional slit on a Ge₂Sb₂Te₅ (GST) substrate. GST as a substrate material is suitable for local laser heating due to its large light absorption and low thermal conductivity. We found an inverse Marangoni effect, which refers to the droplet motion from lower to higher temperature regions. The inverse Marangoni mechanism is more advantageous for droplet manipulation by laser beam positioning.

As a droplet material, polyethylene glycol (PEG) was mixed with toluene at a volume ratio of 7:1 to control the temperature dependence of surface tension of the droplet. The toluene/PEG solution was sandwiched with two coverslips, one of which was coated with GST. Droplet formation and transportation was observed under an inverse optical microscope. A local temperature gradient was induced by focusing subnanosecond pulsed laser (λ=532 nm) onto the GST layer through the microscope objective.

When the interface area between bulk-like toluene/PEG and its vapor was irradiated with the laser spot, many small droplets of toluene/PEG were generated in the vapor side and they merged together at the center of the laser spot to form a single larger droplet. When slightly shifting the laser spot, we found that the droplet immediately moved to follow the spot. The result implies that the droplet moved towards higher temperature region, which direction is opposite to that expected from the conventional Marangoni effect. When the laser was turned off, the droplet spread to its equilibrium diameter. We can control the diameter of droplet simply by changing the laser intensity.
We show possible explanation for the mechanism of inverse Marangoni effect. In the toluene/PEG mixed droplet under laser irradiation, the volume ratio of PEG becomes larger in the higher temperature region because the vapor pressure of PEG is much lower than that of toluene and evaporates more slowly. Also importantly, the surface tension of PEG is larger than that of toluene. Considering these facts, we can expect that the surface tension of droplet in the higher temperature side is larger and thereby Marangoni flow is directed towards the center of laser spot. In order to support this explanation, the same experiment was conducted using pure toluene.
The droplet moved towards the opposite direction with respect to the case of mixed solution. The result can be explained by conventional Marangoni effect. The efficient inverse Marangoni effect we obtained may be also attributed to the significantly low surface energy and its temperature dependence of GST.

Recently, magnetically reconfigurable micropillar arrays have been documented by aligning magnetic particles within the microstructure. When the orientation axis of magnetic particles is deviated from the external magnetic field axis, micropillars undergo directional twisting and bending deformation according to the alignment of magnetic particles. Previous studies have mostly been focused on cylindrical micropillars, resulting in identical actuation regardless of magnetic field direction. In this presentation, we discuss a strategy of anisotropic stress distribution by employing anisotropic pillar geometry in order to introduce a preferential direction for strain responsivity. Under the linear external magnetic field, the effects of geometric anisotropy on magnetic actuation will be investigated by comparing experimental results with magneto-mechanical simulation for rectangular and triangular micropillars. Furthermore, we will discuss the anisotropic liquid spreading and wetting on magnetically actuated micropillar arrays where the pillars twist and bend.

Construction of 3-dimentional (3D) architectures from 2-dimentional (2D) sheets has recently been drawn a great attention. In particular, printing of black ink hinges on pre-strained polymers provides facile and contactless shape-morphing by selective light absorption via photothermal effects. However, remaining ink hinges obstruct transparency and can cause undesirable over-actuation by thermal conduction to unpatterned regions. In this study, we suggest a facile strategy for formation of transparent non-Euclidean structures from pre-strained polystyrene (PS) sheets by employing adhesive tape. Upon exposure to near infrared (NIR) light, pre-stretched PS below the adhesive tape pattern undergoes relaxation of polymer segment into a random conformation above the glass transition temperature on account of local light absorption. Contraction of patterned areas induces 3D shape morphing and remaining tape patterns can simply be removed on-demand after the construction of the targeted 3D structures. Herein, we employed radial and chiral patterns to generate complex 3D architectures and conjugated the multi-material to endow heterogeneous strain-responsivity for each material in a single curvilinear structure. We will also discuss sequential folding by control of light absorption time with sequential removal of adhesive tapes.

Metal containing carbon nanocomposite coatings (Me doped DLC coatings) which include metal or metal carbide phase embedded in amorphous carbon matrix have attracted a significant attention for the past few decades. The association between the carbon matrix and the encapsulated metallic or metal carbide phase not only improve the overall mechanical properties of the nanocomposite coatings, but also attribute the optical properties by accompanying phase and microstructure changes in the coatings. Recently, it has been shown that DLC based coatings are very promising anti-reflection (AR) and protective coatings for photovoltaic applications due to their high chemical stability, radiation stability and high hardness with the possibility of changing their optical properties. Among the various DLC coating classification, tetrahedral amorphous carbon (ta-C) coatings, which showed high sp3/sp2 fraction over 60%, with extremely high hardness, excellent wear resistance, and better radiation stability have been paid much attention to an alternative anti-reflection protective coating materials. So in this study, various contents of Si metal were doped in the ta-C coating to improve optical properties of the ta-C coatings. Filtered cathodic vacuum arc (FCVA) and sputter hybrid PVD system was conducted to synthesize the metal doped ta-C coating. As increasing of Si content in ta-C coating, microstructure of the coatings changed from a dense featureless amorphous to a columnar “moth-eye” like structure by a formation of rhomb silicon carbide (SiC) phase, and that is especially useful for reducing reflections and increasing transmission between materials by the roll of light absorption. Raman spectroscopy analysis showed that all the coatings had high sp3/sp2 fraction about 63%, and the hardness showed high values of 47.5 GPa. The ta-C coating with 4at.% Si showed improved transmittance (over 93.4%) in the field of ultraviolet, and this value was higher than that of other carbon based coatings. These results indicate that the Si doped ta-C coating could be applied for protective & AR coating for photovoltaic applications.

Organic semiconductors (OSCs) are a rapidly emerging field in molecular electronics. Specifically, single crystals of OSCs have received considerable attention due to unique properties associated with long-range order, absence of grain boundaries, and extremely low defect density [1] leading to a wide range of multi-functional materials (e.g., field-effect transistors, light-emitting diodes, photovoltaics). However, the lack of strategies to control and promote π-stacking in single crystals of OSCs has limited their tunability and application. While covalent modification of OSCs has arguably received the most attention [2], the noncovalent approach (e.g., supramolecular modulation) has been largely unexplored [3].

In this contribution, we introduce of efforts in the use of small molecules (e.g., templates) to promote π-stacking of a pyridyl-containing anthracene through weak non-covalent interactions. Modulation of the geometry of π-stacking in the crystalline solid state is accompanied by changes in the photophysical properties. We envisage the supramolecular control of π-stacking in single crystals of OSCs could lead to rapid developments and diversification of multifunctional crystalline electronics.

References:
[1] Zhang, X., Dong, H., Hu, W., Adv. Mater. 2018, 30, 1801048.
[2] Anthony, J., Angew. Chem. Int. Ed., 2008, 47, 452.
[3] Ray, K. K., Campillo-Alvarado, G., Morales-Rojas, H., Höpfl, H., MacGillivray, L. R., Tivanski, A. V., Cryst. Growth Des. 2020, 20, 3.

Light-driven strains can be large, controlled by colour and polarisation of light, and quickly activated in liquid crystalline solids, both rubbery and glassy.
The challenge for applications is to make the response strong, typically involving stretch rather than bend energies if the distortions are loaded. To achieve stretch in both pull and in push without instabilities, the director needs to be programmed in the solid to change the Gaussian curvature of sheet and shell-like objects.
We make contact with current experiments and suggest new Gaussian-curved evolving landscapes, including those arising from curved fold non-isometric origami objects, for effective applications.

Actuation by light is attractive for a number of reasons: Objects can be actuated from a distance, distinct frequencies can be used to actuate and control distinct modes with minimal interference, and significant power can be transmitted over long distances through corrosion-free, lightweight fiber optic cables. Azobenzene and other molecules that are known to change conformation when exposed to light are the building blocks of photomechanical actuation. An important challenge is how the change of conformation of individual molecules can be harnessed into a robust change of deformation of the material. Approaches range from mean-field entropic coupling as in photoactive nematic elastomers to direct chemical bond coupling as in molecular crystal to hybrid composite approaches where molecular crystals are embedded an elastic matrix. The mean-field entropic coupling can lead to weak coupling while the stiffness of molecular crystal can inhibit actuation or cause them to shatter on actuation. This talk will present a series of theoretical models to understand various aspects of this molecular to material coupling, illustrate them with experimental observations and discuss how ideas from these studies can be harnessed in real materials and devices.

Light is an attractive source of remote control and power: it is non-invasive, wavelength-selective, and provides high spatial and temporal resolution. However, the development of new materials is required to fully exploit these advantages by optimizing light energy transduction pathways and/or the light-mediated modulation of materials properties. In this talk, I will present my group’s collaborative efforts to advance the incorporation of donor–acceptor Stenhouse adducts (DASAs) into materials designed for light-driven generation and control of material motion. DASAs present unique advantages for actuation. These highly-absorbing, negatively photochromic molecules have highly tunable reaction kinetics and respond to visible light. We have shown them to be effective photothermal and photochemical agents that can drive the motion of amorphous polymeric materials as well as liquid flows. In particular, through control of DASA reaction kinetics and the applied light field, it is possible create self-regulating flow fields that hold promise for valveless optofluidic devices and soft robotics.

In recent times various intriguing dynamic single crystals that respond to external stimuli such as mechanical stress, heat, light, solvent, humidity, have been reported, but their systematic design remained a challenge. In my talk, I will demonstrate a variety of mechanically reconfigurable and certain other stimuli responsive organic single crystals, where a range of supramolecular weak interactions such as van der Waals (vdW) and π-stacking, play a crucial role in absorbing the internal and external stresses generated.^1-3 I shall present a detailed case study of photomechanical work (i.e. mechanical bending) done by a crystal using visible light mediated photopolymerization chemical reaction, and recovery of the original shape in a reversible manner by a thermal depolymerization reaction.⁴ A few other examples covering waveguides and mechanochromic luminescence will be discussed. Our study demonstrated the potential of soft interactions for designing mechanical behaviour of ordered molecular materials, including those from π-conjugated semiconducting systems, which are of paramount importance for achieving flexible smart materials.

(1) Reddy, C. M.; Krishna, G. R.; Ghosh, S. CrystEngComm 2010, 12, 2296.
(2) Panda, M. K.; Ghosh, S.; Yasuda, N.; Moriwaki, T.; Mukherjee, G. D.; Reddy, C. M.; Naumov, P. Nature Chem. 2015, 7, 65-72.
(3) Krishna, G. R.; Devarapalli, R.; Lal, G. Reddy, C. M.; J. Am. Chem. Soc. 2016. 138, 13561−13567.
(4) R. Samanta, S. Ghosh, R. Devarapalli, C. M. Reddy, Chem. Mater. 2018, 30, 577−581.

CMR thanks DST for Swarnajayanti Fellowship and CSIR for research funding. IISER Kolkata is thanked for central facilities.

Inspired by the natural activated systems, realization of programmed deformations and locomotion in artificial materials has recently attracted great interest due to their promising applications in biomedical devices, soft robotics, and flexible electronics. Photo-responsive hydrogels are recognized as one ideal material to construct soft actuators and robots owing to their drastic volume change induced by contactless light irradiation with high spatial and temporal resolutions. The challenge is how to construct composite hydrogel with gradient structure and dynamically activate the specific regions of the gel toward programmed deformation and locomotion. We present here a photolithographic method to fabricate patterned composite hydrogel sheets with heterogeneous structures by embedding the photo-responsive hydrogel in a preformed nonresponsive hydrogel. Under photo irradiation, the swelling/contraction mismatch results in the built-up of internal stress and thus programmed deformations of the composite hydrogel. Furthermore, a moving light beam is imposed on the composite hydrogel to spatiotemporally actuate the gel that shows sophisticated motions, including crawling, walking, and turning. Experimental and simulation results reveal that multigait locomotion is realized by the mutual coordination of shape-morphing and dynamic friction of the gel against a substrate under the spatiotemporal light stimulation. The programmed deformations of motions of photo-responsive hydrogels should be instructive for the development of soft robotics with advanced technologies and versatile applications.

Photostrictive compounds are promising device materials because of their fundamental photophysical properties and light-induced cooperative behaviors. Although molecular charge-transfer crystals exhibiting light–matter interactions have been successfully deployed in optoelectronics, an air-stable molecular material that couples photons and electrons, achieving photostriction through the coupling of light and mechanical degrees of freedom, has not yet been discovered. Herein, we report a substantial light-induced dilation in a molecular material at room temperature, which is accompanied by simultaneous photocurrent generation. This finding opens avenues for coupling optical, electronic, and mechanical functionalities for possible use in remote wireless photoswitchable devices.

The ability to harness light for photochromic switching that directs mechanical work generation within a macromolecular network is a platform for envisioning photomechanical machines, which are neither encumbered by onboard power sources, nor require microfabricated components and mechanisms. Light offers multiple control knobs for photomechanical regulation, including spatiotemporal modulation of intensity, direction, polarization and chirality. Unfortunately for actuators driven with light, the length over which the photons are attenuated (typically, few micrometers) limits the thickness of the photoactuators. The photomechanical strain generation is via bending, resulting from the gradation in the strains that are generated. Slender structures that respond by bending make for weak actuators. Work content that can be accessed is intrinsically bounded in the characteristically micrometer-scale thickness actuators.

Unusual opportunities emerge when we move beyond flat photoactive geometries, where the patterning of the molecular switches and the graded photomechanical strains conspire to result in non-linear mechanical responses. Snap-through in rod-like structures are one example. Creasing instabilities are another, where the bending of liquid crystalline polymers that are photoresponsive is used to drive non-linear transformations of curved shells, which are confined at their periphery. Torque densities >100 Nm/kg and flexural actuation rates >100 radians/s can be produced. Profiles of these photomechanical actuators places them in a property space that competes favorably with electromechanical systems. A third example is that of photoactuated tape-spring mimicking structures. Here, ms-scale actuation is triggered in shells that switch their principal curvatures, via a snap-through mechanism. Beyond these illustrations, opportunities also exist for utilizing a broader class of latch and torque-inversion mechanisms to magnify actuation by integrating active and passive (structural) materials.

Overall, accessing 3-dimensionally patterned freeform actuators is useful for magnifying photomechanical actuation. Realizing these geometries is simplified by the availability of an additive manufacturing platform, wherein the molecular order can be patterned voxel-by-voxel during the composition of multimaterial architectures. The interplay of magnetically assisted alignment and site-specific photopolymerization is a vehicle for achieving this in an inverted stereolithographic setup. In comparison to say, extrusion-based methods, decoupling the material accretion from the endowment of molecular order is key to expanding the design-space, which can be fabricated.

Magnifying photoactuation in monolithic polymeric structures, which can be scaled from the micrometer to mm-scale form factors holds implications for enabling photomechanical machines. It is possible to envision optically powered and reconfigurable devices in applications ranging from microrobotics, biomedicine and manipulation in environments hostile to electromechanical systems.

When illuminated, photomechanical materials move and demonstrate forces which cause motion. This response indicates that they have acquired momentum as consequence of photoexcitation. We characterize their response by identifying and quantifying the effectiveness of the physical mechanisms connecting photoexcitation and the mechanical response. We are particularly interested in the details of how photostress results from the exciting flux of photons, whose momentum is negligible compared to what is observed in the response, and how photostresses produced in the bulk are transported to the sample surfaces and do mechanical work. We introduce a simple relation linking a stress in light to the photostress, and propose that elements of the tensor relating these serve as measures of the effectiveness of the different aspects of the photoresponse, both in transforming photon energy to stress and in storing photon energy and making it available for mechanical work. Using this metric, we compare the performance of different classes of photomechanical materials using data from the published literature.

Crystals organize photoreactive molecules and coordinate their motions, leading to photoinduced deformations like expansion, bending, twisting, peeling and coiling. This class of materials provides a path to photomechanical actuators that can be scaled down to micro- or nanoscale dimensions, but along the way several challenges must be addressed: 1) optimization of the molecular structure to control the rate and mechanism of reversibility (photon versus thermal activation); 2) control of crystal morphology to generate actuating structures with a variety of shapes and sizes; and 3) the hierarchical organization of nano-scale and micro-scale crystals into a macroscopic actuator structure, usually by templated growth and organization. Recent progress in all three areas have led to the development of composite organic-inorganic structures with unprecedented power-to-weight ratios: under light illumination, less than 1 mg of a diarylethene photochrome can move a load weighing more than 100 g. These materials consist of organic photochromic crystals aligned in the pores of various types of inorganic host materials, including Al₂O₃ and SiO₂. The effects of the inorganic matrix material, organic photochromic molecule, and crystal alignment and orientation are all analyzed. The potential of these hybrid materials as practical photoactuators will be assessed.

Phase change materials are of great interest for low-power high-throughput storage devices and next-generation neuromorphic computing technologies due to their contrasting properties between amorphous and crystalline phases and fast switching rate. Notably, Sb₂Te₃, an archetypical Ge-Sb-Te alloy, displays a fast and energy-efficient crystallization-amorphization cycle, due to its growth-dominated crystallization and low melting point. Here, we demonstrate a new nonthermal amorphization pathway of Sb₂Te₃ which can further reduce time required for amorphization from nanoseconds to picoseconds. We employ non-adiabatic quantum molecular dynamics simulations to investigate the time evolution of Sb₂Te₃ at a constant temperature of 500 K with 2.6%, 5.2%, 7.5%, 10.3%, and 15% photoexcited valence electron-hole carriers. Results reveal picosecond nonthermal amorphization at and above a critical excitation of 7.5% valence electrons. The rapid amorphization originates from an instantaneous charge transfer from Te-p orbitals to Sb-p orbitals upon photoexcitation. Subsequent evolution of the excited state, within picosecond timescale, indicates a Sb-Te bonding to antibonding transition. Concurrently, Sb-Sb and Te-Te antibonding decreases, leading to the formation of wrong bonds. For 7.5% valence electron photoexcitation or larger, the electronic changes destabilize the crystal structure, leading to large atomic diffusion and loss of long-range order. These results highlight an ultrafast energy-efficient amorphization pathway that could be used to enhance the performance of phase change materials based opto-electronic devices.

This work was supported as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0014607. The simulations were performed at the Argonne Leadership Computing Facility under the DOE INCITE and Aurora Early Science programs and at the Center for High Performance Computing of the University of Southern California.

The coupling between mechanical and optical properties can be surprisingly large, and has a long history of study. Optical absorption resonances are related to breaking, reforming, and rearranging bonds, on an energy scale of 1 eV/molecule, and it stands to reason that the effects of optical absorption on mechanical properties may be substantial; still these effects continue to surprise, and are often overlooked. Opto-mechanical effects may be especially strong when mediated by optically-active defects that store charge, induce local lattice distortions, and interact with modes of plastic deformation. The connection between defects and opto-mechanics provides a handle for controlling opto-mechanical effects through materials processing, and suggests new applications.
Here we show that the wide-band gap compound semiconductors ZnO, ZnS, and CdS all feature large opto-mechanical coupling that is mediated by point defects. We measure the mechanical properties of sintered and single crystal samples using a nanoindenter equipped with a purpose-built variable wavelength and intensity illumination system. We find that both elasticity and plasticity vary strongly with moderate illumination. For instance, the hardness and stiffness of ZnO can increase by up to 40-80% due to blue illumination of intensity 3 mW/cm². The wavelength-dependence shows that band-edge absorption (e.g. UV light) has the strongest effect, and the relative size of the effect of sub-band gap illumination varies between samples – a clear sign of defect-mediated processes. We further show that these giant opto-elastic and opto-plastic effects can be tuned by materials processing, and that the processing-dependence can be understood within a framework of point defect equilibrium. We tune point defect concentrations by annealing treatments, and measure composition by direct (inductively-coupled plasma optical emission spectroscopy, ICP-OES) and indirect (X-ray diffraction, XRD) methods. Our composition measurements for ZnO, ZnS and CdS all show that the magnitude of the optomechanical response varies with anion vacancy concentration. This is consistent with the hypothesis that photoconductivity and photoplasticity are positively correlated, and provides a rational guideline for engineering optomechanical response. We use density functional theory (DFT) calculations to interpret our experimental results. DFT suggests that photo-induced charge carriers migrate along strain gradients, and gather around anion vacancies. The high local net charge concentrations (10²¹ ~ 10²² cm^-3) create pseudo-tensile-strain (~5%), which opposes the compressive strain from the indenter, thereby enhancing the elastic modulus.
We will conclude with an outlook on opto-mechanical effects in wide-band gap semiconductors, and with several proposed, new applications for these still-surprising effects.

In 1948, Casimir described a bizarre force that acts between neutral objects based on quantum fluctuations of electromagnetic fields. This effect is of both fundamental interest in quantum field theory and practical importance in nanotechnology. Recently, it has been predicted that Casimir force can induce phonon coupling between nearby objects and thus transfer heat across a vacuum gap. Revealing this unique quantum effect experimentally would bring fundamental insights to quantum thermodynamics and practical implications to nanoscale thermal management. Here, we present the first experimental demonstration of the Casimir heat transfer. We use nanomechanical systems to realize strong Casimir phonon coupling and observe the exchange of thermal energy between individual phonon modes. The experimental observation agrees well with our theoretical calculations and is unambiguously distinguished from other effects such as near-field radiation and electrostatic interaction. This experiment opens up new opportunities for studying nanoscale energy transport and quantum thermodynamics.

Photomechanical organic crystals are fast becoming the next generation smart-materials that can convert light energy into mechanical work. While the majority of these photomechanical crystals are made from diarylethene or azobenzene derivatives our group has focused on the synthesis of anthracene-based compounds that can be grown into photomechanical crystals. These crystals can expand, bend, twist, curl, peel, or jump when exposed to UV or visible light. We will survey the synthetic routes used to make these anthracene derivatives. We will also describe how we made the corresponding photomechanical crystals using different techniques such as solvent annealing inside Anodic Aluminum Oxide (AAO) templates and aqueous surfactant-mediated precipitation technique.
Esterification of 9-Anthracenecarboxylic acid with aliphatic alcohols gives a family of photomechanically active anthracene derivatives. When these derivatives are grown into crystalline nanowires via solvent annealing inside AAO templates, the resulting nanowires expand anisotropically along their long axis when exposed to UV light. The intermolecular [4+4] photocycloaddition between parallel anthracene rings is the driving force behind the crystal expansion. A thoroughly studied molecule is the photomechanical expansion of tert-butyl anthracene-9-carboxylate (9TBAE) nanowires by 15% upon irradiation with 365 nm light. Crystalline microribbons grown from the commercially available 9-Anthracenecarboxylic acid (9AC) can twist reversibly (T-type) when exposed UV light. The microribbons require several minutes to reset by reversing the thermally unstable [4+4] photocycloaddition. The introduction of substituents on the 9AC anthracene ring helps expand our toolbox of T-type photomechanical crystals. Multi-step synthesis was used to introduce substituents on different positions of the 9AC anthracene ring. These substituents (F, Cl, or Br, NO2, CH3, etc.) either suppress or enhance the T-type character of the substituted 9-anthracene carboxylic acid crystals. While the 4-fluoro-9-anthracene carboxylic acid (4F9AC) shortened the reset time of the crystal by an order of magnitude, the 10-fluoro-9-anthracene carboxylic acid (10F9AC) derivative did the exact opposite. Extending the conjugation of the anthracene ring at the 9-position by attaching propenal using Mizoroki–Heck or Wittig reaction yields the very useful 3-(9-Anthryl)acrolein which was used as a building block for the synthesis of second-generation photomechanical anthracene derivatives. Adding different electron-withdrawing groups to the 3-(9-Anthryl)acrolein aldehyde via Knoevenagel condensation leads to a novel type of photomechanical crystals that are driven by trans/cis photochemistry. These crystals are dominantly P-type. For example, 2-(3-anthracene-9-yl-allylidene)-malononitrile (9DVAM) crystals and nanowires exhibit P-type photomechanical response under visible and UV light. When these crystals are grown into microwires from an aqueous surfactant they exhibit continuous photo-induced wiggling-like motion resembling flagella. Block-like microcrystals composed of cis-dimethyl-2(3-(anthracene-9-yl)allylidene)malonate (DMAAM) can peel off amorphous layers upon UV-light exposure while similar analog derivatives can super expand by transforming from crystalline-to-gel like material under intense green light exposure. The introduction of fluorine on the 9DVAM anthracene ring yields derivatives with P-type photosalient crystals and nanowires that may find future application in light-driven microactuators.
Synthesis of Anthracene-based photomechanical crystals coupled with non-classical crystal growth techniques has shown to be essential for both understanding the basic principles and advancing the field of photomechanical smart-crystals.

The development of soft synthetic materials that can emulate living organisms in their capacity to translate in space, change shape, and perform useful tasks with small energy inputs is a great challenge for materials science. Such “robotic soft matter” could be used to assist in productive functions that may range from robotics to smart management of chemical reactions and even communication with cells. This lecture will describe the development of molecularly designed hydrogels that can exhibit mechanical actuation in response to visible light or the combined action of photons and magnetic fields. The materials are created using components such as hybrid bonding polymers, which consist of supramolecular polymers integrated with light responsive covalent polymer networks. Relative to polymer networks, exposure of HBPs to photons results in faster bending and flattening actuation of objects, as well as longer steps during their light-driven crawling motion. In systems containing polymer networks and ferromagnetic nanostructures we have observed fast walking motion of macroscopic objects on surfaces with programmable trajectories and external activation of cargo delivery.

Materials capable of directly converting photon energy into mechanical deformation offer promise in a wide variety of contexts including adaptive optics, remotely operated swimmers, and actuators controlled via lightweight optical cables that resist corrosion and electromagnetic interference. Organic photoswitches offer significant potential in this regard, thanks to their ability to undergo large changes in molecular conformation upon excitation, and their highly tailorable absorption spectra. To date, efforts have focused largely on either crystalline lattices of small photomechanical molecules, which offer the possibility for greater work output due to their high density and well-organized nature, or on polymer matrices containing a modest concentration of photochromes, which generally have superior processability and mechanical properties. We have recently considered two classes of hybrid materials that combine many of the advantages of these disparate approaches. In one case, we have studied the growth of micrometer-scale photomechanical diarylethene crystals within the pores of polymer membranes. For appropriately controlled orientations of the crystals, such materials can exhibit photomechanical work outputs rivalling the best previously reported systems, while enabling facile processing into mechanically robust samples with cm-scale lateral dimensions. In a second case, we have developed a new class of structurally regular addition polymers bearing photoisomerizable azobenzene groups alternating with chain extenders. These materials form semicrystalline solids that exhibit fundamentally interesting temperature- and light-dependent melting and recrystallization behaviors, and that can be processed into aligned films and fibers exhibiting two-way optical shape memory properties.

For the past few years we have been developing structurally simple, easy to make, modular, and tunable hydrazone-based functional materials (e.g., switches, sensors and fluorophores).¹ This presentation will deal with our recent advances with these systems, with an emphasis on newly developed photochromic compounds² that exhibit many interesting properties, including emission ON/OFF toggling in solution (see below) and the solid-state.³ The integration of these photochromic compounds into liquid crystals⁴ and liquid crystalline elastomers⁵ will also be discussed.

Representative references:
1. I. Aprahamian ChemCommun 2017, 53, 6674–6684
2. (a) H. Qian, S. Pramanik, I. Aprahamian J. Am. Chem. Soc. 2017, 139, 9140−9143
(b) Q. Li, H. Qian, B. Shao, R. P. Hughes, I. Aprahamian J. Am. Chem. Soc. 2018, 140, 11829–11835
3. B. Shao, M. Baroncini, H. Qian, L. Bussotti, M. Di Donato, A. Credi, I. Aprahamian J. Am. Chem. Soc. 2018, 140, 12323–12327
4. M. J. Moran, M. Magrini, D. Walba, I. Aprahamian J. Am. Chem. Soc. 2018, 140, 13623–13627
5. A. Ryabchun, Q. Li, F. Lancia, I. Aprahamian, N. Katsonis J. Am. Chem. Soc. 2019, 141, 1196–1200

The type of deformations and range of motion, which microstructures can undergo, determines their function, be it for soft robotics, for sensing or for changing properties in optical or metamaterials. While composite, multimaterial architectures allow imprinting of pre-determined deformation behaviors, their fabrication at the microscale is difficult and the deformation modes are fixed once fabricated. Herein, we demonstrate the use of an alternative strategy – the generation of a transient bilayer architecture within microstructures by an external light stimulus, – and a variety of deformation behaviors that the same structure can display as a result. Changing the direction of UV-irradiation of high-aspect-ratio compositionally uniform microstructures fabricated from photoresponsive liquid crystalline elastomers gives access to a wide range of sophisticated deformations such as clockwise and counter-clockwise twisting, light seeking and avoiding, particularly if rotational symmetry is broken. Light intensity is demonstrated to be a further control parameter that allows one to tap into non-monotonic bending motions and combinations of twisting, bending and expansions/contractions. Finally, because the light-generated layered architectures are transient, they adapt and evolve as the deformation proceeds, leading to highly complex, yet controllable, self-regulated deformation behaviors. The described LCE microstructures are rapidly and easily fabricated and they offer exciting opportunities to further explore essential features of their finely controllable deformation abilities for applications in next generation smart materials.

We will describe our latest efforts to create new photochromic compounds based on sulfoxides and sulfoxide mimics. We will also detail our advances in the creation of first row transition metals, where we propose that isomerization is capable of stabilizing of high spin states available for first row transition metal complexes. In addition, we will detail our spectroscopic studies of these new materials, both in solution and in solid state.

Liquid crystal polymer networks undergo reversible shape change in response to any stimulus that affects the strength of nematic order. These soft actuators can be fabricated as thin films, surface coatings, or 4D printed solids and have potential applications in soft robotics, biomedical devices, microfluidics, and sensors. Trajectories for shape change are “programmed” by patterning the nematic director when the polymer is cross-linked. Actuation is induced when the strength of nematic order is modulated by stimuli such as a change of temperature or chemical environment; for example, heating induces contraction along the nematic director and expansion in orthogonal directions. The addition of azobenzene derivatives that undergo photo-isomerization produces a soft actuator that flexes like an artificial muscle in response to illumination; if the cis-to-trans thermal relaxation is fast enough, the material can also flex rapidly back toward its original shape when illumination is removed. Such reversible photo-mechanical actuation is a highly attractive option to control a soft robot as it combines both control and power for induced motion without need for tethering or onboard batteries.

Modeling the dynamics of photo-mechanical actuation in these materials is important both to explain the mechanisms behind experimental observations and for device design. To model photo-mechanical actuation at the device scale, we use nonlinear finite element method (FEM) elastodynamics simulation based on a Hamiltonian approach. This method can simulate underdamped motion where momentum plays an important role. The code was developed in-house and implemented in CUDA for execution on a GPU-enabled computer.

We used this approach to model oscillatory mechanical wave motion in a soft robot whose motion was both controlled and powered by light [1]. We use a simplified ray tracing algorithm to determine which parts of the device were illuminated and which in shadow, and used a kinetic model of photo-isomerization/relaxation to describe the time evolution of nematic order. We found that the directionality of light-induced mechanical wave propagation is controlled by a self-shadowing mechanism and periodic pop-through transitions.

We also model a variety of other prototype devices including surface-adhered nematic polymer coatings that transform from flat to complex surface profiles with microchannels, spikes, or dimples recently observed in experiments by Babakhanova et al [2]. Next we model polymer coatings patterned with biomimetic “octopus-like" suckers, designed to grasp and release nonporous objects on command. Such a device would have the potential to achieve strong, reversible adhesion. Our results demonstrate that actuation geometry produced by a disclination in a nematic polymer is controlled not only by the defect’s topological charge and orientation but also by the details of its defect core structure. We compare our modeling results with relevant experiments by collaborators and discuss potential applications.

[1] Gelebart, A.H., Mulder, D.J.,Varga, M., Konya, A., Vantomme, G., Meijer, E.W., Selinger, R.L.B., Broer, D.J., (2017) Making waves in a photoactive polymer film, Nature 546, 632 (2017) https://doi.org/10.1038/nature22987
[2] Babakhanova, G., Turiv, T., Guo, Y. et al. Liquid crystal elastomer coatings with programmed response of surface profile. Nat Commun 9, 456 (2018). https://doi.org/10.1038/s41467-018-02895-9

Work supported by NSF CMMI-1663041, NSF DMR-1409658, and NSF-CMMI 1436565; computer resources provided by the Ohio Supercomputer Center.

Photons have been “put to work” in polymeric materials for more than 50 years. Photogeneration of work has been observed in a range of polymeric materials spanning the continuum from soft elastomers to rigid, high performance materials. Of particular importance is the organization of the polymer chains within the network which can affect magnitude, rate, and directionality of photoinduced deformation. Photomechanical effects in liquid crystalline polymer networks and elastomers are particularly appealing in enabling distinctive conversion of light into work. This presentation will summarize the salient benefits of using light as an actinic stimulus as well as highlight recent developments in this topical area of materials research from our laboratory.

Donor-acceptor Stenhouse adducts (DASA) are a promising new chromophore for photomechanical applications. We detail studies of the photochemistry of this chromophore, including both the initial light-induced isomerization and the thermal ring-closing reaction. We further provide details on a framework for modeling excited state dynamics under force, combining the ab initio multiple spawning formalism for nonadiabatic dynamics with the force-modified potential energy surface framework developed for ground state mechanochemistry. We discuss the role of external force (either compressive or extensional) on the photodynamics of the DASA chromophore and its implications for the use of DASA chromophores in light-triggered mechanical actuators.

Photo-responsive liquid crystalline elastomers (LCEs) constitute ideal actuators for soft robots because their light-induced macroscopic shape changes can be harnessed to perform mechanical work. Conventional LCEs, however, do not typically exhibit complex modes of bending and twisting necessary to perform sophisticated maneuvers. Here, we model LCE microposts encompassing side-chain mesogens oriented along a specific axis characterized by the nematic director, and azo-benzene crosslinkers, which determine the deformations of illuminated posts. On altering the nematic director from 90o to 0o relative to the underlying substrate, the post’s bending respectively changes from light-seeking to light-avoiding. Moreover, with the director tilted at 45o, the initially achiral post reversibly twists into a right- or left-handed chiral structure, controlled by the angle of incident light. We exploit this photo-induced chirality to design “chimera” posts (encompassing two regions with distinct director orientations) that exhibit simultaneous bending and twisting, mimicking motions exhibited by the human musculoskeletal system.

We demonstrate the potential of spiropyran-functionalized, light-responsive hydrogels to be a promising new platform for nonlinear optical materials. Nonlinear optical behavior in these materials arises from the swelling/deswelling and resulting refractive index changes that occur due to photoisomerization between the hydrophilic ring-open merocyanine isomer and the hydrophobic ring-closed spiropyran isomer. These materials show an unprecedented combination of reversibility, soft easily processed material, low laser powers, and long-range interactions. These long-range interactions are especially promising for potential applications in all-optical computing or sensing. We show that the long-range interactions can be explained by thermal swelling or deswelling of the gel. The presence of a secondary temperature gradient that influences beam behavior leads to novel repulsive long-range interactions, which are not possible in conventional nonlinear optical materials in the absence of external symmetry breaking. Furthermore, the direction and magnitude of a hydrogel’s thermal response can be controlled by changing the gel composition, which allows the behavior of single and multiple beams in these gel materials to be rationally designed over a potentially vast design space. We demonstrate this potential by exploring two beam interactions in poly(N-isopropylacrylamide-co-acrylic acid) gels which contract rather than swelling as function of increasing temperature. We observe a lack of long-range interactions in these gels along with behaviors indicative of short-range attractive interactions, such as spiraling and beam fusion. This tunable diversity of behaviors suggests the enormous potential for photoresponsive hydrogels as extremely versatile, rationally designable, soft, nonlinear optical materials.

Light-fueled soft robots have recently demonstrated untethered manipulation of locomotion without the complex mechanical and electrical design principles of conventional robotic systems[1-2]. To achieve photomechanical motility, we photopolymerized light active molecular machines along with reactive liquid crystalline monomers. For energy effective and rapid photomotility, rolling is employed as an in-plane locomotive mode to reduce friction with the substrates in comparison with crawling. In this presentation, we will also discuss design strategy to implement and program jumping motion of liquid crystalline polymers by controlling the molecular alignment for miniaturized yet high-performance untethered soft robots.

Acknowledgements: This work was funded by NRF-2016R1D1A1B03931678 and NRF-2019R1A2C1004559.

References:
[1] J.J. Wie, M.R. Shankar, T.J. White, Nat .Commun., 7:13260 (2016)
[2] J.G. Kim, J.E. Park et al., Materials, 12, 19, 3065-3086, (2019)

The demonstration of tunable actuation performance at a single light intensity by the simple remodeling of material properties advances the field of light responsive materials one step closer to the development of “life-like” actuators. We introduce a novel approach for programmable actuation using negatively photochromic molecular photoswitches, termed donor-acceptor Stenhouse adducts (DASAs), capable of tuning the photothermal actuation amplitude under constant light intensity based on photochemically programed logic. We present a synthetic pathway for chemically attaching DASA conjugates to polymers through Diels-Alder click chemistry. We report a visible light-responsive bilayer actuator that can lift weight against gravity, as well as a simple light-powered crawler, that exploit photothermal energy conversion. Programmable control of the magnitude of actuation is achieved by slowly converting a highly-absorbing DASA photochrome into a non-absorbing form upon irradiation. Our results highlight the promising benefits of high molar absorptivity, negative photochromism, and broadband visible light absorption of DASAs for programmable actuation.

Molecular photoswitches – molecules that can switch between multiple states of differing properties – are useful to impart actuation and changes in optical properties of soft materials. The chemistry of these photoswitches can be conveniently tuned to control the resulting material properties and have found application in soft actuators, non-linear optics, and optical memories. Yet, there is often a mutual interaction between photoswitch and embedding matrix, in which the photoswitch not only determines material functionality, but in turn the embedding matrix influences photoswitching behavior, enabling self-regulated behaviors through feedback loops. Here, we show how such self-regulated behavior arises in spiropyran-functionalized hydrogels and what effects govern this behavior. We are able to disentangle influences of hydrogel chemistry, type of photoswitch attachment, ionic strength, changes in hydrogel volume, and aggregation on the isomer-distribution of the photoswitch. In particular, we find that by changing the spiropyran from appendage to crosslinker, we can not only control feedback in hydrogel swelling behavior, but also control the non-linear optical properties of the hydrogel. These studies directly inform how such feedback loops can be leveraged for the fabrication of evermore sophisticated self-regulated optical materials.

Liquid crystal elastomers (LCEs) can be functionalized with photochromic moieties to sensitize these materials to light exposure. Prior work has demonstrated that phototropic disruption of order via photochemical adjustment of the isomeric state of azobenzene (for example) disrupts the anisotropic chain configurations of the polymer network. The phototropic disruption of order in azobenzene-functionalized LCEs (azo-LCEs) with UV light via trans-cis isomerization generates comparatively large strain, exceeding 20%.
Fundamentally, the strength of intermolecular interactions between polymer chains defines the thermotropic response of both low molar mass and polymeric liquid crystal compositions. Here we show that by adjusting the intermolecular interaction by introducing a nontraditional liquid crystalline monomer, light irradiation can induce photomechanical responses in azo-LCE at lower energy inputs to achieve equivalent mechanical output.