High time resolution Q-band EPR study of sequential electron transfer in a triad oriented in a liquid crystal

Using high time resolution Q-band EPR we have been able to identify three different radical pairs generated by pulsed laser excitation of the fixed distance triad consisting of a zinc-9-desoxo-meso-methylpyrochloro-phyllide donor (ZC), a pyromellitimide primary acceptor (PI), and a naphthalene-1,8:4,5-diimide secondary acceptor (NI), i.e., ZC-PI-NI, oriented in a liquid crystal. Analysis of the transient EPR spectra provides direct evidence for sequential electron transfer from the primary to the secondary radical pair of the triplet channel. At room temperature, this process occurs with an exponential time constant of τ_T,2 = 50 ± 1 ns. In the singlet-initiated channel, the intramolecular electron-transfer rates are too fast for direct EPR detection. Thus, even the secondary singlet radical pair [ZC^.+PINI^.-]_s is formed instantaneously on the time scale of the EPR experiment. The species decays with a time constant of τ_s,3 = 36 ± 1 ns by charge recombination to the singlet ground state. The time evolution of the transverse magnetization exhibits fast initial oscillations, which disappear 50 ns after the laser pulse. Model calculations indicate that these oscillations can be assigned to zero quantum electron precessions in [ZC^.+PINI^.-]s. Thus, for the first time, quantum beats have been observed from a spin-correlated radical pair, oriented in a liquid crystal.