Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator

In order to investigate the possibility of a fullerene acting as an electron and/or singlet energy relay between a donor chromophore and an acceptor, a triad consisting of a fullerene (C₆₀) covalently linked to both a porphyrin energy and electron donor (P) and a β-tetracyanoporphyrin energy and electron acceptor (CYP) was synthesized. Steady state and time-resolved spectroscopic investigations show that the porphyrin first excited singlet state donates singlet excitation and an electron to the fullerene and also donates singlet excitation to the CYP. All three processes differ in rate constant by factors of ≤1.3, and all are much faster than the decay of ¹P-C₆₀-CYP by unichromophoric processes. The fullerene excited state accepts an electron from P and donates singlet excitation energy to CYP. The P^•+-C₆₀^•--CYP charge-separated state transfers an electron to CYP to produce a final P^•+-C₆₀-CYP^•- state. The same state is formed from P-C₆₀-¹CYP. Overall, the final charge-separated state is formed with a quantum yield of 85% in benzonitrile, and has a lifetime of 350 ps. Rate constants for formation and quantum yields of all intermediate states were estimated from results for the triad and several model compounds. Interestingly, the intermediate P^•+-C₆₀^•--CYP charge-separated state has a lifetime of 660 ps. It is longer lived than the final state in spite of stronger coupling of the radical ions. This is ascribed to the fact that recombination lies far into the inverted region of the Marcus rate constant vs thermodynamic driving force relationship.