Femtosecond-picosecond laser photolysis studies on the dynamics of excited charge-transfer complexes in solution. 2. Ion pair formation processes in the excited states of 1,2,4,5-tetracyanobenzene-aromatic hydrocarbon complexes in polar solvents

Abstract

Femtosecond and picosecond laser photolysis and time-resolved transient absorption spectral studies have been made-to elucidate the photoinduced charge separation (CS) process in the 1,2,4,5-tetracyanobenzene (TCNB)-aromatic hydrocarbon charge-transfer (CT) complexes in various polar solvents. We have confirmed that, in a polar solvent such as acetonitrile, solvent reorientation can induce CS with a time constant shorter than 1 ps to a considerable extent but not completely. Moreover, we have demonstrated that, for the complete CS leading to the ion-pair formation, further intracomplex structural change and solvation, which take place with a time constant (τ CS ) of several picoseconds to 30 ps depending on the natures of the electron-donating aromatic hydrocarbon and the polar solvent, are necessary