Intramolecular Electron Transfer in Fullerene/Ferrocene Based Donor−Bridge−Acceptor Dyads

Abstract

A systematic steady-state fluorescence and time-resolved flash photolytic investigation of a series of covalently linked fullerene/ferrocene based donor−bridge−acceptor dyads is reported as a function of the nature of the spacer between the donor site (ferrocene) and acceptor site (fullerene) and the dielectric constant of the medium. The fluorescence of the investigated dyads 2 (Φrel = 0.17 × 10-4), 3 (Φrel = 0.78 × 10-4), 4 (Φrel = 1.5 × 10-4), 5 (Φrel = 0.7 × 10-4), and 6 (Φrel = 2.9 × 10-4) in methylcyclohexane at 77 K were substantially quenched, relative to N-methylfulleropyrrolidine 1 (Φrel = 6.0 × 10-4), indicating intramolecular quenching of the fullerene excited singlet state. Excitation of N-methylfulleropyrrolidine revealed the immediate formation of the excited singlet state, with λmax around 886 nm. A rapid intersystem crossing (τ1/2 = 1.2 ps) to the excited triplet state was observed with characteristic absorption around 705 nm. Picosecond resolved photolysis of dyads 2−6 in toluene showed light-induced formation of the excited singlet state which undergoes rapid intramolecular quenching, with rate constants of 28 × 109 s-1, 6.9 × 109 s-1, 3.4 × 109 s-1, 14 × 109 s-1, and 2.3 × 109 s-1, respectively. Nanosecond-resolved photolysis of dyads 3 and 4 in degassed benzonitrile revealed long-lived charge separated states (τ1/2 = 1.8 μs (3) and τ1/2 = 2.5 μs (4)) with characteristic fullerene radical-anion bands at λmax = 1055 nm. The nature of the spacer between C60 and ferrocene, weak electronic ground-state interactions, steady-state fluorescence, and picosecond-resolved photolysis suggest two different quenching mechanism: through-bond electron transfer for dyads 2, 5, and 6 and formation of a transient intramolecular exciplex for dyads 3 and 4.