FEMTOSECOND PICOSECOND LASER PHOTOLYSIS STUDIES ON THE PHOTOINDUCED CHARGE SEPARATION AND CHARGE RECOMBINATION OF A PRODUCED ION-PAIR STATE OF SOME TYPICAL INTRAMOLECULAR EXCIPLEX COMPOUNDS IN ALKANENITRILE SOLVENTS

Femtosecond and picosecond laser photolysis and time-resolved absorption spectral studies have been made to observe directly the photoinduced charge separation (CS) and charge recombination (CR) of a produced CS state of aromatic amine (D) and aromatic hydrocarbon (A) combined systems: p-(CH3)2N-Ph-(CH2)n-(1-pyrenyl) (Pn, n = 1, 2, 3) and p-(CH3)2N-Ph-(CH2)n-(9-anthryl) (An, n = 0, 1, 2, 3), in alkanenitrile solvents. The time constants τCS of the photoinduced CS of these systems are considerably longer than the solvent dielectric relaxation time τL as well as the solvation time τS (determined from the time-dependent Stokes shift of fluorescent probe) except that τCS of A1 is close to τS. In the case of A0 where D and A groups are more strongly interacting than in A1, the photoinduced CS is much slower than that of A1, which cannot be interpreted simply by usual electron-transfer theories. The time constants τCR of the CR decay of the produced CS state have been confirmed to be more than 2-3 orders of magnitude longer than τCS and to become shorter with increase of chain number n contrary to the case of τCS. These results are discussed on the basis of the solvent dynamics and/or solvation, the magnitude of the D-A electronic interaction, structural rearrangements, and the energy gap dependence of electron transfer. © 1990 American Chemical Society.