Torsional and proton transfer dynamics in substituted 3-hydroxyflavones

The role of torsional motion in proton transfer dynamics of 4'-(N,N-dimethylamino)-alpha-naphthyl-3-hydroxychromone has been studied using time-resolved emission spectroscopy. Excited state proton transfer (ESPT) rates in hydrocarbon solvents of varying viscosities are obtained. The viscosity is varied so that ESPT rate prior to, and following, torsional relaxation may be determined. The results show that as the aryl 'torsional relaxation proceeds, the exothermicity of proton transfer decreases and the proton transfer rate increases; thus, the system exhibits ''Marcus inverted'' behavior. These results are interpreted in terms of torsional potentials. These potential energy surfaces are constructed from solvent-dependent static absorption and emission spectra and time-dependent Stokes shift measurements. Potential energy surfaces are constructed for the normal and tautomeric forms af the molecule in both torsionally relaxed and unrelaxed states. From these surfaces, a ratio of reaction rates is obtained, which is in good agreement with experimentally determined value.