Probing the Charge Transfer Reaction Coordinate of 4-(Dimethylamino)benzonitrile with Femtosecond Stimulated Raman Spectroscopy

Abstract

Femtosecond stimulated Raman spectroscopy (FSRS) and femtosecond transient absorption have been used to probe the photoinduced charge transfer (CT) dynamics of 4-(dimethylamino)benzonitrile in methanol and n-hexane. Through a combined analysis of temporal changes in the Raman modes and transient absorption kinetics, a more complete picture of the reaction coordinate of the intramolecular charge transfer process has been established. FSRS spectra of the phenyl C═C stretching mode (Wilson mode 8a) at 1607 cm(-1), which shifts to 1581 cm(-1) in the CT state, and transient absorption measurements ranging from 360 to 700 nm support internal conversion from the locally excited to the charge transfer state in 4-5 ps and then a subsequent vibrational relaxation within the CT state manifold on a 6-8 ps time scale. Dramatic shifting and narrowing of the 1581 cm(-1) quinoidal C═C stretch (ν(8a)) on the ∼7 ps time scale indicates that the quinoidal distortion is an important probe of the CT reaction dynamics. The cause of the spectral shifts is determined by comparing the observed shifts in the vibrational spectrum to anharmonic couplings computed for the benzonitrile radical anion by density functional theory (DFT) and with quantitative theoretical models of the solvent induced vibrational peak shifts. The DFT calculations indicate that the 10 cm(-1) downshift of the C═C stretch is most likely attributable to significant vibrational excitation in nontotally symmetric modes that are strongly anharmonically coupled to the C═C stretch.