Optically Controllable Secret Ink Based on Kinetics of Supramolecular Structure

Simultaneous manipulation of both fluorescence and transparency of organic fluorescent materials is one of the biggest challenges in various fields such as security inks, memory devices, organic light-emitting displays, and biomedical imaging. Conventionally, the development of materials with transparency and fluorescence has been made by mixing nano-sized fluorescent particles with transparent amorphous polymers. However, this production process has limitations such as the heterogeneous dispersion of fluorescent particles and the low fluorescence intensity from the low fluorescent substance loading. To solve this problem, we newly designed and synthesized a monopyrene-based dendron (abbreviated as MD) molecule that has both transparency and fluorescence. MD is a molecule in which three alkyl chain groups are linked by amide bonds to monopyrene. Because of the strict distinction between the solvophilic alkyl dendrons and the solvophobic pyrene groups, we predicted that MD molecules would assemble a hierarchical superstructure at the molecular level. To confirm our hypothesis, we conducted morphological analysis via both scanning calorimetry (DSC) and cross-polarized optical microscope (POM). Interestingly, it was confirmed that when there was insufficient time for molecular assembly a metastable crystalline phase (C_ms) appeared, and when there was sufficient time a stable crystalline phase (C_s) appeared. In addition, if the quenching process is performed with little time for assembling molecules, an optically transparent crystalline phase (C_t) is obtained. Surprisingly, the supramolecular structure of C_t was equal to C_ms. To confirm the molecular packing of the polymorphic MD superstructure in more detail, the molecular packing structures of C_s and C_ms were investigated with wide angle X-ray diffraction (WAXD) and transmission electron microscope (TEM). As a result, when the assembly time of molecules in a thermodynamic equilibrium state is sufficiently provided, stable C_sis a fully packed layered structure where a dimeric building block is assembled. In the opposite case, metastable C_ms is a layered structure where the head-to-head dimers are alternatively interdigitated. Furthermore, the optical properties of MD had a significant change due to the polymorphic superstructure induced by the balance between intermolecular interactions and nanophase separation. These optical properties were identified by transmittance and photoluminescence (PL) measurement. In addition, to control organic materials with transparency and fluorescence, the optical properties of MD were evaluated under various external stimuli such as solvent fuming, and thermal annealing. The transparent fluorescent superstructure of MD under UV and visible light was constructed by the quenching process. Because the supramolecular structure was changed by the fuming and annealing processes, the MD materials became opaque, and the fluorescence wavelength shifted. Based on the previous results, it was found that the transparency and fluorescence of MD were controlled by molecular packing kinetics as well as external stimuli. Our research highlights that the optically controllable smart materials were available as secret ink. The letter was written in an isotropic state using secret ink and quenched. Encrypted letters were not visible in daylight but, were easily observed under UV light. However, Secret letters could be observed with the naked eye through fuming and annealing processes. Also, the decrypted letters can be converted back into the initial state by manipulating the molecular packing structure of a fluorescent molecule via kinetic control. This work was supported by the BK21 FOUR, Mid-Career Researcher Program (2021R1A2C2009423), and Basic Research Laboratory Program (2020R1A4A1018259).