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 elect ron transfer in the triad molecule zinc methyl 13(1)-desoxopyrophespho rbide a-pyromellitimide-1,8:4,5-naphthalenediimide (ZC-PI-NI) in solut ion. The absorption spectra of the radical anions of PI and NI possess narrow and well-separated absorption bands which permit the direct ob servation of bath 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-dependen t changes in the band shape of its transient absorption spectrum. The rate of the initial charge separation reaction forming ZC(+)-PI--NI an d the subsequent charge shift reaction to form the final state, ZC(+)- PI-NI-, as well as the appearance of the vibrationally excited interme diate are all highly solvent dependent even for solvents with similar dielectric constants. Analogous dyad control molecules ZCPI and ZCNI w ere also studied and compared with the results for ZCPINI.