Photoinduced charge separation in a carotenoid-porphyrin-diquinone tetrad: enhancement of quantum yields via control of electronic coupling

A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q_A-Q_B) has been synthesized, and its photochemistry has been investigated using time-resolved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P^{{radical dot}+}-Q^{{radical dot}-}_A-Q_B state, which is formed with arate constant of 2.3×10⁹ s^-1 and a quantum yield of 0.87. In chloroform, the rate is 4.1×10⁹ s^-1 and the quantum yield is 0.94. Transient absorption studies show that this state evolves by subsequent electron transfer pathways to a final C^{{radical dot}+}-P-Q_A-Q^{{radical dot}-}_B charge-separated state whose lifetime is 7.4 μs in dichloromethane and 740 ns in chloroform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P^{{radical dot}+}-Q^{{radical dot}-}_A-Q_B species is lower. This fact is interpreted in terms of the rates of charge-separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules may be altered in a predictable fashion using the basic tenets of electron transfer theory.