FEMTOSECOND PICOSECOND LASER PHOTOLYSIS STUDIES ON THE DYNAMICS OF EXCITED CHARGE-TRANSFER COMPLEXES IN SOLUTION .1. CHARGE SEPARATION PROCESSES IN THE COURSE OF THE RELAXATION FROM THE EXCITED FRANCK-CONDON STATE OF 1,2,4,5-TETRACYANOBENZENE IN BENZENE AND METHYL-SUBSTITUTED BENZENE SOLUTIONS

Femtosecond and picosecond laser photolysis and time-resolved transient absorption spectral studies have been made to directly observe the charge separation (CS) process in the excited state of charge-transfer complexes in the case of 1,2,4,5-tetracyanobenzene (TCNB) in benzene and methyl-substituted benzene solutions. It has been demonstrated that, immediately after excitation, a slight change of absorption intensity accompanied by a slight sharpening of the band shape takes place with time constants of 2 ps, 1.5 ps, and 550 fs for benzene, toluene, and mesitylene solutions, respectively. This spectral change has been ascribed to the configurational rearrangement within the 1:1 donor (D)-acceptor (A) complex from the Franck-Condon excited state with asymmetrical configuration toward a more symmetrical overlapped one, which slightly increases the extent of CS. It has been shown that the slight sharpening of the band shape is not due to the mere vibrational relaxation (cooling), since the observed result in the TCNB-toluene system has not been affected by the change of the wavelength of excitation pulse from 355 to 295 nm. This structural change within the 1:1 complex, however, does not lead to the complete CS, but further interaction with donor and structural rearrangement including the formation of the 1:2 complex (A-·D2+) are of crucial importance for it. © 1990 American Chemical Society.