Structure of Cage Amines as Models for Twisted Intramolecular Charge-Transfer States

Abstract

6-Cyanobenzquinuclidine (CBQ) is widely assumed to be a good model to study twisted internal charge transfer (TICT), a hypothesis proposed to explain the appearance of solvent-induced dual fluorescence in 4-(N,N-dimethylamino)benzonitrile (DMABN) derivatives. This is due to the orthogonal arrangement of the nitrogen lone pair and the aromatic π orbitals in this cage amine and the exclusive appearance of long wavelength TICT fluorescence. Spectral and photophysical properties of CBQ are investigated by stationary and time-resolved fluorescence spectroscopy as a function of solvent polarity. The results are discussed in the context of conformational analysis studies performed for CBQ and benzquinuclidine by X-ray crystallography as well as by semiempirical calculations. 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 simulated and measured data is found. Rehybridization of the amino group, which leads to a less pyramidal geometry, and torsional vibration about the bond linking the donor group and the benzonitrile moiety contribute essentially to intramolecular charge separation and recombination processes.