FEMTOSECOND PICOSECOND LASER PHOTOLYSIS STUDIES ON THE DYNAMICS OF EXCITED CHARGE-TRANSFER COMPLEXES - AROMATIC HYDROCARBON-ACID ANHYDRIDE, HYDROCARBON-TETRACYANOETHYLENE, AND HYDROCARBON-TETRACYANOQUINODIMETHANE SYSTEMS IN ACETONITRILE SOLUTIONS

Formation processes of contact ion pairs (CIP) from the excited Franck-Condon (FC) state of charge-transfer (CT) complexes of aromatic hydrocarbons with acid anhydride as well as cyano compound acceptors in acetonitrile solution and charge recombination (CR) rates (k(CR)CIP) of produced CIP states have been investigated by femtosecond and picosecond laser photolysis and time-resolved absorption spectral measurements covering a wide range of free energy gap-DELTA-G(o)ip between the ion pair and the ground state. It has been confirmed that the CIP formation becomes faster and k(CR)CIP of the produced CIP increases with increase of the strengths of the electron donor (D) and acceptor (A) in the complex, i.e., with decrease of the -DELTA-G(o)ip value. This peculiar energy gap dependence of k(CR)CIP, quite different from the bell-shaped one observed in the case of the solvent-separated ion pairs (SSIP) or loose ion pairs (LIP) formed by encounter between fluorescer and quencher in the fluorescence quenching reaction, has been interpreted by assuming the change of electronic and geometrical structures of CIP depending on the strengths of D and A.