Abstract
The nature of optical excitations and the spatial extent of excitons in organic semiconductors, both of which determine exciton diffusion and carrier mobilities, are key factors for the proper understanding and tuning of material performances. Using a combined experimental and theoretical approach, we investigate the excitonic properties of meso-tetraphenyl porphyrin-Zn(II) crystals. We find that several bands contribute to the optical absorption spectra, beyond the four main ones considered here as the analogue to the four frontier molecular orbitals of the Gouterman model commonly adopted for the isolated molecule. By using many-body perturbation theory in the GW and Bethe–Salpeter equation approach, we interpret the experimental large optical anisotropy as being due to the interplay between long- and short-range intermolecular interactions. In addition, both localized and delocalized excitons in the π-stacking direction are demonstrated to determine the optical response, in agreement with recent experimental observations reported for organic crystals with similar molecular packing.
Highlights
The nature of optical excitations and the spatial extent of excitons in organic semiconductors, both of which determine exciton diffusion and carrier mobilities, are key factors for the proper understanding and tuning of material performances
Porphyrin-based materials have attracted a great deal of interest due to their importance in several fields ranging from biology to optoelectronics; for example, light harvesting, energy capture and transfer, gas sensing, and photocatalysis are some of the most intriguing and useful applications.[3−6] The optical properties of porphyrin aggregates and crystals are generally treated in terms of the four frontier molecular orbitals of the Gouterman model[7,8] and adding the weak intermolecular interactions by means of the Kasha model.[9]
This study aims to go beyond this framework by unveiling the excitonic properties of crystalline π-stacked mesotetraphenyl porphyrin-Zn(II) (ZnTPP) by means of a joint theoretical−experimental approach that combines state-of-theart theoretical modeling with experimental measurements of highly crystalline and oriented ZnTPP nanowires
Summary
The nature of optical excitations and the spatial extent of excitons in organic semiconductors, both of which determine exciton diffusion and carrier mobilities, are key factors for the proper understanding and tuning of material performances. Given the epitaxial relation illustrated above, the spectra represent a very good approximation of the normal incidence response of a (100)-oriented ZnTPP triclinic single crystal, collected under light-polarized parallel and perpendicular to [001]ZnTPP, i.e., the π-stacking direction (for further details, see the Supporting Information).
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