Ultrafast photoswitched charge transmission through the bridge molecule in a Donor-Bridge-Acceptor system

A Donor-Bridge-Acceptor molecule, D-B-A, was synthesized to probe the effects of changing the electronic-state of the bridge molecule, B, on the rates of electron transfer within D-B-A. Selective photoexcitation of D in a tetrahydrofuran solution of D-B-A with 400 nm, 130 fs laser pulses at t = 0 ps results in photoinduced electron transfer to yield the ion pair D⁺-B^--A with τ = 60 ps, which undergoes a subsequent charge shift with τ = 140 ps to yield the long-lived ion pair D⁺-B-A^- (τ = 700 ns). Subsequent selective photoexcitation of B within D⁺-B-A^- with a 520 nm, 150 fs laser pulse at t = 500 ps results in about 20% of the D⁺-B-A^- population undergoing charge recombination with τ = 100 ps. This charge recombination rate is about 7000 times faster than the normal recombination rate of the ion pair. The results demonstrate that formation of the lowest excited singlet state of the bridge molecule B significantly alters the reaction pathways leading to charge recombination. Thus, D-B-A can be viewed as a molecular switch in which the D⁺-B-A^- state can be rapidly turned on and off using 400 and 520 nm laser pulses, respectively.