Synthesis of Rigid Porphyrin-Quinone Compounds for Studying Mutual Orientation Effects on Electron Transfer and Their Photophysical Properties

A series of covalently linked porphyrin-quinone molecules in which a porphyrin ring is connected at two fixed distances with two different kinds of rigid spacers to benzoquinone were synthesized. The spacer is either spiro[4.4]nonane or trans-decalin. These compounds were designed to have a fixed orientation of different kinds with the same redox pair, the same separation distance, and the same number of intervening bonds. The forward electron-transfer rates were studied by fluorescence–lifetime measurements. Large differences in the rates were found in each pair, and the rates for those compounds having a spiro spacer were larger than those with a trans-decalin spacer. These differences were attributed to a difference in the molecular geometry of the spacer groups in each pair.