Structure and picosecond excited-states dynamics in isolated, supercooled 4-(N,N-dimethylamino)benzonitrile

The first experimental evidence is presented for a strong coupling between electronic states in isolated, supercooled 4-(N,N-dimethylamino)benzonitril e, DMABN. In the fluorescence excitation spectrum, measured under isolated supercooled conditions, two types of transitions were found (B-type and C-t ype). Rotational contour analysis of the B-type bands (pi pi* character) sh ows that there is hardly any difference between the geometries of the groun d and excited states; the molecule retains its planar conformation. In the out-of-plane transition (C-type band) the equilibrium position of the dimet hylamino group twists 90 degrees with respect to the benzonitrile part. The broadening of the rotational lines indicates that the fluorescence is prec eded by a fast radiationless intramolecular process (internal conversion) o n a picosecond time scale. For the B-type bands it was found that the broad ening increases with the vibrational energy.