Picosecond Time-Resolved Study of 4-Dimethylaminobenzonitrile in Polar and Nonpolar Solvents

Abstract

By applying a Kerr gate to reject fluorescence, the picosecond time-resolved resonance Raman (TR 3) 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 structural changes occurring in DMABN in different solvent environments following electronic excitation, the same method (without the Kerr gate) was also used to study the delocalized excited (DE) state of DMABN. The TR 3 spectrum of the ICT state is dominated by a phenyl ring band, while the band corresponding to the C≡N stretching mode is absent. The TR 3 spectrum of the DE state, observed in nonpolar solvents, implies a planar structure with double bond character in the C ring-N(CH 3) 2 bond. Conjugation therefore extends mainly between the dimethylamino group and the ring in the DE state. The Kerr 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 resolution covering the spectral window from 320 to 560 nm. Subpicosecond transient absorption spectra of DMABN in solvents of different polarity have also been measured. The presence of an isosbestic point in the time-resolved fluorescence spectra at early time delays demonstrates the interchange of a two-state system during the initial relaxation process following the photoexcitation. The results are discussed and placed in context with the wealth of work performed to date on this molecular system.