TIME-RESOLVED TRANSIENT RESONANCE COHERENT ANTI-STOKES-RAMAN SCATTERING SPECTRA OF DIPHENYLHEXATRIENE ION RADICALS IN POLAR-SOLVENTS AND EXCIPLEX-FORMING SYSTEMS

The transient resonance CARS (coherent anti-Stokes Raman scattering) spectra of all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) cation and anion radicals were measured in exciplex systems with a strong electron acceptor (1,4-dicyanobenzene, DCNB) or with a strong electron donor (N,N-dimethylaniline, DMA) in 1,4-dioxane, as well as in polar solvents with different polarity. Close correspondence between the signals of the DPH/DCNB system and those of DPH in polar solvents such as acetone and ethanol confirms that the species produced in the solvents is the cation radical. Time-resolved CARS measurements revealed that the effective lifetime of the cation radical is strongly dependent on solvent polarity giving values of 1.5-mu-s, 4.0-mu-s, and 72 ns in acetone, ethanol, and 2-methyltetrahdrofuran, respectively. The ion radicals in DPH/DCNB and DPH/DMA systems had respective lifetimes of 110 and 61 ns, which are much more larger than the lifetime of exciplex fluorescence (equal to or less than 10 ns), indicating that geminated ion pairs or solvated free ions were produced from the charge-transfer (CT) exciplexes. Furthermore, biphotonic production of the cation radical in polar solvents was found to set in at higher UV power, competing with solvent-assisted monophotonic ionization.