Strong Photophysical Diversity and the Role of Charge Transfer Excitons in Transition Metal Phthalocyanine β‑Phases

The photophysics of the three transition metal phthalocyanines NiPc, CuPc and ZnPc in their β-phase has been studied using complementary experimental techniques: photoluminescence, optical absorption and electron-energy-loss spectroscopy. We demonstrate that the strongly different photoluminescence signal can be understood based on the different exciton dispersions, and that tiny differences in the structure and thus the intermolecular orbital overlap is able to model these differences. Our results clearly indicate a significant admixture of charge-transfer excitations to the lowest exciton state. They additionally demonstrate the need of a detailed microscopic understanding of organic semiconductors for their application potential.