Picosecond time-resolved study of 4-dimethylaminobenzonitrile in polar andnonpolar solvents

By applying a Kerr gate to reject fluorescence, the picosecond time-resolve d resonance Raman (TR3) spectrum of the intramolecular charge-transfer (ICT ) state of 4-dimethylaminobenzonitrile (DMABN) in a polar solvent has been obtained for the first time. To elucidate the geometric and electronic stru ctural changes occurring in DMABN in different solvent environments followi ng electronic excitation, the same method (without the Ken: gate) was also used to study the delocalized excited (DE) state of DMABN. The TR3 spectrum of the ICT state is dominated by a phenyl ring band, while the band corres ponding to the C=N stretching mode is absent. The TR3 spectrum of the DE st ate, observed in nonpolar solvents, implies a planar structure with double bond character in the C-ring-N(CH3)(2) bond. Conjugation therefore extends mainly between the dimethylamino group and the ring in the DE state. The Ke rr gate has also been used to measure the temporal spectral profile of the DMABN fluorescence in both nonpolar and polar solutions with 3 ps time reso lution covering the spectral window from 320 to 560 nm. Subpicosecond trans ient absorption spectra of DMABN in solvents of different polarity have als o been measured. The presence of an isosbestic point in the time-resolved f luorescence spectra at early time delays demonstrates the interchange of a two-state system during the initial relaxation process following the photoe xcitation. The results are discussed and placed in context with the wealth of work performed to date on this molecular system.