Photoinitiated Electron Transfer in Carotenoporphyrin-Quinone Triads: Enhanced Quantum Yields via Control of Reaction Exergonicity

Carotenoporphyrin-quinone triad molecules have previously been found to successfully mimic many aspects of photosynthetic electron and energy transfer. These molecules generate long-lived, energetic charge-separated states via a biomimetic multistep electron transfer scheme. In many of these molecules, the overall quantum yield for charge separation has been limited by an unfavorable partitioning of an intermediate charge-separated state between charge recombination and further reaction to yield the desired species. One strategy for overcoming this limitation is based on the fact that, in general, electron transfer rate constants do not depend linearly on reaction free energy change. In the triads, raising the energy of the intermediate charge-separated state would be expected to speed up the desired electron transfer (“normal” Marcus behavior), but slow down charge recombination (which formally lies in the “inverted” region). Laser flash photolysis and transient fluorescence measurements on two new triad molecules reveal enhanced quantum yields of long-lived charge-separated states which are consistent with this expectation.