Intramolecular Energy Transfer in S<INF>1</INF>- and S<INF>2</INF>-States of Porphyrin Trimers

A set of four porphyrin trimers (<BO>1</BO><BO>−</BO><BO>4</BO>) 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 <BO>1</BO> and <BO>2</BO> or a OEP-type Zn(II)-porphyrin for <BO>3</BO> and<BO> 4</BO> and the diphenylethynyl substitution axis of the core porphyrin is aligned either orthogonal in <BO>1</BO> and <BO>3</BO> or parallel in <BO>2</BO> and <BO>4</BO> 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<INF>1</INF>−S<INF>1</INF> energy-transfer reactions in these porphyrin trimers. The S<INF>1</INF>−S<INF>1</INF> energy transfer is faster in the parallel trimers <BO>2</BO> and <BO>4</BO> than in the orthogonal trimers <BO>1</BO> and <BO>3</BO>, reflecting larger electronic coupling in the former pair. The peripheral porphyrin S<INF>2</INF>-state lifetime is considerably shortened in <BO>1</BO><BO>−</BO><BO>4</BO>, which has been ascribed to S<INF>2</INF>−S<INF>2</INF> energy transfer. Probably the strong Soret-transitions of both the donor and acceptor lead to large Coulombic interactions, thereby rendering S<INF>2</INF>−S<INF>2</INF> energy transfer effective enough to compete with rapid internal conversion to S<INF>1</INF>-state. These results encourage a new strategy for construction of porphyrin-based supramolecular artificial photosynthetic antenna.