Dual fluorescence in aromatic nitriles: The role of the charge-transfer state

Dual fluorescence has been observed in the emission spectra of benzonitrile and tolunitrile seeded in a supersonic jet. This fluorescence has a sharp component that approximates the mirror image of the absorption spectrum and a broad, red-shifted component characteristic of emission from a charge-transfer state. In tolunitrile, the broad, red-shifted fluorescence is easily observed from all vibronic levels including the zero-point level. In benzonitrile, this component is weak in the lower vibronic levels. In both molecules, the intensity of the broad component increases with respect to the intensity of the sharp component as the energy of the initially excited level is increased. At the highest observed levels (approximately 1600 cm(-1) above the zero-point level for both molecules), only the broad component is observed. The broad component is assigned to emission from a charge-transfer state. Ab initio calculations show that the charge-transfer state is much higher in energy than the S-1 state at the equilibrium geometry of the ground electronic state but is substantially lowered in energy as rehybridization of the cyano carbon from sp to sp(2) causes the -C-C-N to bend and the C-N bond to lengthen.