Symmetry-breaking charge transfer and intersystem crossing in copper phthalocyanine thin films

Intermolecular interactions in pi-stacked chromophores strongly influence their photophysical properties, and thereby also their function in photonic applications. Mixed electronic and vibrational coupling interactions lead to complex potential energy landscapes with competitive photophysical pathways. Here, we characterize the photoexcited dynamics of the small molecule semiconductor copper pthalocyanine (CuPc) in solution and in thin film, the latter comprising two different pi-stacked architectures, alpha-CuPc and beta-CuPc. In solution, CuPc undergoes ultrafast intersytem crossing (ISC) to the triplet excited state. In the solid state, both alpha-CuPc and beta-CuPc morphologies exhibit a mixing between Frenkel and charge-transfer excitons (Frenkel-CT mixing). We find that this mixing influences the photophysical properties differently, based on morphology. In addition to ISC, alpha-CuPc demonstrates symmetry-breaking charge transfer, which furthermore depends on excitation wavelength. This mechanism is not observed in beta-CuPc. These results elucidate how molecular organization mediates the balance of competitive photexcited decay mechanisms in organic semiconductors.

Automated summary

Intermolecular interactions in pi-stacked chromophores strongly influence their photophysical properties, and thereby also their function in photonic applications. The photophysical properties differ based on the molecular organization of alpha-CuPc and beta-CuPc, both in solution and in thin film.