Intramolecular Energy Transfer in S₁- and S₂-States of Porphyrin Trimers

A set of four porphyrin trimers ( 1 − 4 ) consisting of an energy-accepting 5,15-diphenylethynyl-substituted Zn(II)-porphyrin core flanked by two energy-donating peripheral Zn(II)-porphyrins have been prepared as a new efficient energy-transfer functional unit. The peripheral porphyrin donor is either a TPP-type Zn(II)-porphyrin for 1 and 2 or a OEP-type Zn(II)-porphyrin for 3 and 4 and the diphenylethynyl substitution axis of the core porphyrin is aligned either orthogonal in 1 and 3 or parallel in 2 and 4 with respect to the long axis of the trimeric arrays. Femtosecond transient absorption spectroscopy and femtosecond up-conversion fluorescence measurement have revealed the very efficient S 1 −S 1 energy-transfer reactions in these porphyrin trimers. The S 1 −S 1 energy transfer is faster in the parallel trimers 2 and 4 than in the orthogonal trimers 1 and 3, reflecting larger electronic coupling in the former pair. The peripheral porphyrin S 2 -state lifetime is considerably shortened in 1 − 4, which has been ascribed to S 2 −S 2 energy transfer. Probably the strong Soret-transitions of both the donor and acceptor lead to large Coulombic interactions, thereby rendering S 2 −S 2 energy transfer effective enough to compete with rapid internal conversion to S 1 -state. These results encourage a new strategy for construction of porphyrin-based supramolecular artificial photosynthetic antenna.