STRUCTURE OF CAGE AMINES AS MODELS FOR TWISTED INTRAMOLECULAR CHARGE-TRANSFER STATES

6-Cyanobenzquinuclidine (CBQ) is widely assumed to be a good model to study twisted internal charge transfer (TICT), a hypothesis proposed t o explain the appearance of solvent-induced dual fluorescence in 4-(N, N-dimethylamino)benzonitrile (DMABN) derivatives. This is due to the o rthogonal arrangement of the nitrogen lone pair and the aromatic pi or bitals in this cage amine and the exclusive appearance of long wavelen gth TICT fluorescence. Spectral and photophysical properties of CBQ ar e investigated by stationary and time-resolved fluorescence spectrosco py as a function of solvent polarity. The results are discussed in the context of conformational analysis studies performed for CBQ and benz quinuclidine by X-ray crystallography as well as by semiempirical calc ulations. It is shown that only one singlet excited state is emissive. Fluorescence spectra and intersystem crossing rates can be modeled as charge recombination processes, and excellent agreement between simul ated and measured data is found. Rehybridization of the amino group, w hich leads to a less pyramidal geometry, and torsional vibration about the bond linking the donor group and the benzonitrile moiety contribu te essentially to intramolecular charge separation and recombination p rocesses.