Source of anomalous fluorescence from solutions of 4-N,N-dimethylaminobenzonitrile in polar solvents

Abstract

The interactions of the first excited singlet state of 4-N,N-dimethylaminobenzonitrile (DMABN) and of 3,5-N,N-tetramethyl-4-aminobenzonitrile (3,5-TMABN) with polar molecules have been studied. Mixtures of cyclohexane and a small amount of polar liquid have been used as solvents. The anomalous fluorescence of DMABN originates from a 1:1 complex between excited solute and a polar solvent molecule. The fluorescence of 3,5-TMABN in polar solvents arises only from exciplexes. A model is presented which describes the binding in the exciplexes in terms of a localized interaction between lone pair electrons of the polar solvent molecule with lone pair electrons of the dimethylamino group of the excited solute. The solvent-induced wavelength shift of any particular exciplex fluorescence of the two compounds can be correlated with the dielectric properties of the medium. If the conformation of the amino group is fixed as in 1-methyl-5-cyano-indoline (1-M-5-CI) and in 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (1-M-6-CTHQ) no formation of exciplexes with polar solvents is observed. The formation of exciplexes does not only lead to a new fluorescence, but also enhances the intersystem crossing in the two compounds, DMABN and 3,5-TMABN. There is no enhancement of the triplet quantum yield in polar solvents in the case of 1-M-6-CTHQ. The quantum yield of the normal fluorescence of both DMABN and 3,5-TMABN in polar solvents depends on the wavelength of excitation. The quantum yield of exciplex emission does not show this behaviour.

These two observations are explained by taking the existence of weakly bound complexes of ground state solutes as well as the high rate of exciplex formation into account.