Excitonic Coupling in Gradually Fluorinated β-Phase Zinc Phthalocyanine

Understanding how the crystallographic structure influences the excitonic properties of novel organic semiconductors plays a crucial role in fundamental research on this material class as well as for its implementation in devices with new functionalities. The class of phthalocyanines provides a perfect model system to study this relationship, as it offers a wide range of different crystallographic phases and the possibility to reliably steer the structural and electronic parameters by gradual fluorination of the molecular ligands [1]. The shift of the respective frontier orbital energies caused by fluorination while, simultaneously, leaving the optical properties almost unchanged has already led to phthalocyanine based devices with advanced functionalities, such as dual emitting organic LEDs or color-switchable subwavelength plasmonic organic light emitting antennas [2,3].<br/><br/>In this contribution, we report on a comprehensive study on the interplay between molecular spacing and excitonic coupling in β-phase zinc phthalocyanine (ZnPc) single crystals. Temperature dependent photoluminescence measurements were performed on non-fluorinated ZnPc, partially fluorinated F₄-ZnPc crystals as well as their co-crystals, all of which grown by horizontal vapor deposition. Our measurements yield clear evidence for a change in the emission characteristics as function of temperature and polarization. For neat ZnPc crystals, a narrow emission of only 20 meV linewidth occurs below about 100 K, which we attribute to a superradiant enhancement caused by the coherent excitation of next neighboring molecules within the crystal lattice perpendicular to the crystallographic b-direction. In contrast, this phenomenon is absent in pure F₄-ZnPc as well as mixed ZnPc:F₄-ZnPc crystals, where already at small admixtures of the fluorinated counterparts, the intermolecular arrangement and coupling is changed in such a way that a coherently coupled state between adjacent molecules is strongly suppressed or energetically less favorable than a coexisting charge-transfer state as in case of the co-crystals. This hypothesis is corroborated by temperature dependent x-ray structural analyses on the investigated crystals in combination with extended theoretical simulations by means of time-dependent DFT, which highlight the role of the anisotropic coupling of the excitonic states and its spatial variation with temperature.<br/><br/>Our results demonstrate the potential inherent to this material class and the possible strategy of tuning the excitonic properties of molecular semiconductors by controlling their crystalline packing via gradual chemical substitution.<br/><br/>LSM and JP thank the Bavarian Ministry of Science and the Arts for the generous support by the research program Solar Technologies Go Hybrid.<br/><br/>[1] Rödel M. et al., The Role of Molecular Arrangement on the Strongly Coupled Exciton-Plasmon Polariton Dispersion in Metal–Organic Hybrid Structures, <i>J. Phys. Chem. C</i> 126 (2022) 4163<br/>[2] Hammer S. et al., Phase transition induced spectral tuning of dual luminescent crystalline zinc-phthalocyanine thin films and OLEDs, <i>Appl. Phys. </i><i>Lett.</i> 115 (2019) 263303<br/>[3] Grimm P. et al., Color-Switchable Subwavelength Organic Light-Emitting Antennas,<b> </b><i>Nano Lett.</i> 22 (2022) 1032