Mechanistic considerations for the dual fluorescence of dimethylaminobenzonitrile: a fluorescence anisotropy study

Fluorescence polarization experiments on N,N-dimethylaminobenzonitrile (DMABN) and derivatives bearing stronger acceptor substituents, and of amino-bridged model compounds have been conducted at low temperature. They allow the classification into two groups differing in the nature of the locally excited (LE) state. From the comparison with these and further observations, the suitability of different mechanistic models can be assessed. The twisted intramolecular charge transfer (TICT) model (twisted amino group) presents no conflict with the experiments. The RICT model (cyano bending) fails partially. The PICT model (planar charge transfer (CT) structure) fails to account for all four key observations considered.