Electronic and vibrational spectra of the low-lying pisigma* state of 4-dimethylaminobenzonitrile: comparison of theoretical predictions with experiment.

Comparison of the TD-BP86cc-pVDZ electronic excitation energies and the CIScc-pVDZ vibrational frequencies of 4-dimethylaminobenzonitrile with the available experimental data indicates that the picosecond transient absorption at about 700 nm, and the excited-state vibration of frequency 1467 cm(-1), belong to the lowest-energy pisigma(CN) (*) state of bent geometry (CCN bond angle of about 120 degrees and a large CN bond distance). Consistent with these assignments, the 1467 cm(-1) Raman band, attributed to the CN stretch, exhibits a large resonance enhancement of intensity when the probe (Raman excitation) wavelength is set to the spectral region of the pisigma(*)<--pisigma(*) absorption. The result corroborates the occurrence of an ultrafast state switch from the initially excited (1)pipi(*) (L(b)) state to the (1)pisigma(*) state of lower energy.