Excitonic Coupling between B and Q Transitions in a Porphyrin Aggregate

Aggregation distinctively changes the absorption and fluorescence anisotropy spectrum of β-tetraethyl-tetrapyridinyl-porphyrin 1. A large excitonic splitting of the B band occurs, and the Q band absorption is distinctively enhanced. The latter effect must be attributed to intensity transfer from the B- to the Q-band region that is mediated by an excitonic coupling between B and Q transition dipoles. In the fluorescence anisotropy spectrum of the aggregate of 1, all Q bands and the red-shifted B band are polarized parallel, reflecting the strong interband excitonic coupling. A qualitative explanation of the observed excitonic effects is given.