Solute–solvent exciplex versus twisted intramolecular charge-transfer state in anomalous fluorescence of 4-NN-dimethylaminobenzonitrile
We report on the influence which solvent deuteration and solute–solvent interactions in glassy matrices have on the fluorescence of 3,5,NN-tetramethyl-4-aminobenzonitrile and of 4-NN-dimethyl-aminobenzonitrile. The widely accepted assignment of the anomalous fluorescence of the latter compound to emission from a twisted intramolecular charge-transfer state does not explain the observations. It is pointed out that the large deuterium effect on the anomalous fluorescence may be understood in terms of the theory of radiationless electronic relaxation when the emission comes from a solute—solvent exciplex. Spectral shifts, which arise from coupling of solute electric dipole moments with the polarization of the surroundings, have been calculated for the case of a solute in a homogenous polar glassy matrix, relative to a solution in a non-polar glass. The shift found for the fluorescence of 3,5,NN-tetramethyl-4-aminobenzonitrile is not in agreement with acceptable values for molecular excited-state dipole moments. Emission from an exciplex is more in accordance with the shift. Because the amino group of 4-NN-dimethylaminobenzonitrile cannot rotate in glassy solutions, there is no exciplex emission from this compound in a polar glass. This does not exclude the formation of planar-solute exciplexes, which decay predominantly by a radiationless process. Our earlier suggestion concerning the formation of exciplexes with polar solvent molecules provides an adequate explanation of the facts for the decay of the first excited singlet states of 4-NN-dimethylaminobenzonitrile and related compounds. It may explain our observation of ion formation from these states in polar solvents, which occurs even when the amino group is restricted to be in plane with the phenyl ring and there is no anomalous fluorescence. We suggest that a planar-solute exciplex (E1) is the precursor of an emitting twisted-solute exciplex (E2). Then competition between ionic dissociation and rotational isomerization of E1 becomes an important factor, determining the quantum yield of anomalous fluorescence. A quantum chemical analysis of the consequences of overlap of atomic orbitals on the N atom of the amino group with lone-pair orbitals of the solvent molecule shows that both planar and twisted solute–solvent exciplex states may be bound states.