Twisted intramolecular charge transfer states: Rotationally resolved fluorescence excitation spectra of 4,4 '-dimethylaminobenzonitrile in a molecular beam
SourceJournal of Chemical Physics, 122, 8, (2005), pp. 084309-084309-10
Article / Letter to editor
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Spectroscopy of Solids and Interfaces
Molecular and Biophysics
Journal of Chemical Physics
SubjectMolecular and Biophysics
We report the observation at high resolution of seven vibronic bands that appear within similar to200 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)less than or equal to30 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 (similar to1degrees) 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 (similar to3degrees) and twisted (similar to25degrees) 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. (C) 2005 American Institute of Physics.
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