Ultrafast photoinduced electron transfer in a chlorophyll-based triad: Vibrationally hot ion pair intermediates and dynamic solvent effects

We report ultrafast transient absorption studies of photoinduced electron transfer in the triad molecule zinc methyl 13¹-desoxopyropheophorbide a - pyromellitimide - 1,8:4,5-naphthalenediimide (ZC-PI-NI) in solution. The absorption spectra of the radical anions of PI and NI possess narrow and well-separated absorption bands which permit the direct observation of both the intermediate and final charge-separated states. Selective optical excitation of the ZC donor results in the formation of the intermediate charge-separated state, ZC⁺-PI^--NI, in less than 2 ps in nonpolar solvents. The PI radical ion within the ZC⁺-PI^--NI intermediate is vibrationally excited as illustrated by time-dependent changes in the band shape of its transient absorption spectrum. The rate of the initial charge separation reaction forming ZC⁺-PI^--NI and the subsequent charge shift reaction to form the final state, ZC⁺-PI-NI^-, as well as the appearance of the vibrationally excited intermediate are all highly solvent dependent even for solvents with similar dielectric constants. Analogous dyad control molecules ZCPI and ZCNI were also studied and compared with the results for ZCPINI.