Solvation Dynamics and Proton Transfer in Diethylaminohydroxyflavone.

4'-N,N-Diethylamino-3-hydroxyflavone (DEAHF) exhibits dual fluorescence in most solvents as a result of a rapid excited-state intramolecular proton transfer reaction. The high sensitivity of its dual emission to solvent polarity and hydrogen bonding make DEAHF of interest as a ratiometric fluorescence sensor. In addition, prior work has suggested that the rate of this proton transfer should depend on solvent relaxation in an unusual manner. It has been proposed that rapid solvation of the initially excited reactant should retard reaction. The present work tests this idea by using femtosecond Kerr-gated emission spectroscopy to measure the reaction kinetics of DEAHF in mixtures of propylene carbonate (PC) + acetonitrile (ACN). This mixture was chosen to maintain constant solvent polarity and thereby constant reaction energies while varying solvation times ∼10-fold with composition. The reaction kinetics observed in these mixtures are multiexponential, consisting of resolvable components of ∼2 and ∼30 ps and a small fraction of reaction faster than detectable by the 400 fs resolution of the experiment. Average reaction times increase by approximately a factor of 2 as a function of ACN mole fraction, primarily as a result of changes to the slower kinetic component. This trend is opposite to the composition dependence of solvation times, thereby supporting the unusual role of polar solvation dynamics in this proton transfer. In n-alkane solvents, where electrostatic coupling is minimized, frictional properties of the solvent do not influence reaction rates.