Twisted intramolecular charge transfer states: rotationally resolved fluorescence excitation spectra of 4,4'-dimethylaminobenzonitrile in a molecular beam.

We report the observation at high resolution of seven vibronic bands that appear within approximately 200 cm(-1) of the electronic origin in the S(1)-S(0) fluorescence excitation spectrum of 4,4'-dimethylaminobenzonitrile (DMABN) in a molecular beam. Surprisingly, each band is found to be split into two or more components by a (coordinated) methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality [(Observed-Calculated)< or =30 MHz] fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized (approximately 1 degree) but otherwise (heavy-atom) planar molecule in its ground S(0) state, and that its electronically excited S(1) state has both a more pyramidalized (approximately 3 degrees) and twisted (approximately 25 degrees) dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S(1)<--S(0) electronic transition is accompanied by significant charge transfer from the nitrogen atom to the pi* orbitals of the aromatic ring. Thereby established is the participation of all three vibrational coordinates in the dynamics leading to the "anomalous" emissive behavior of DMABN in the condensed phase.