Supersonic jet and solution studies of intramolecular complexes with TICT formation mimicking solute-solvent interaction

Three ester derivatives of N,N-dimethylaniline have been studied, which possess a flexibly linked intramolecular hydroxyl group amenable to mimic 1:1 complexation of donor-acceptor molecules with solvent molecules. The supersonic jet fluorescence excitation spectra indicate a small redshift for these intramolecularly complexed compounds with respect to the noncomplexed model compounds giving evidence for the intramolecular complexation by hydrogen bond formation at the ester group. The dispersed fluorescence spectra broaden upon an increase in the excitation energy. Semiempirical simulated annealing calculations using AMPAC/AM1 show that there exist many possible conformations of similar energy for the molecule, with H bonding at the carbonyl or alkoxy oxygen in the most stable conformer. A shift in the carbonyl vibrational frequencies is consistent with a carbonyl self-complexed species. The dual fluorescence observed in acetonitrile as solvent is very similar for all three compounds, with a slight redshift of the long wavelength charge transfer (CT) band for the complexed species with respect to the uncomplexed one consistent with the twisted intramolecular charge transfer model. The absence of a clear CT band in spite of complete intramolecular hydrogen bond complexation in alkanes is interpreted as evidence that formation of a specific solute-solvent complex alone is not sufficient to create the large anomalous CT band observed for N,N-dimethylaminobenzonitrile in polar and mixed polar/nonpolar solvents. (C) 2000 Elsevier Science B.V. All rights reserved.