Transient IR spectroscopy and ab initio calculations on ESIPT in 3-hydroxyflavone solvated in acetonitrile

Abstract

Femtosecond polarization resolved UV/Vis and mid-infrared spectroscopy was used to thoroughly identify and characterize the relevant elementary chemical and physical processes in the photocycle of 3-hydroxyflavone (3-HF) in solution. In one set of experiments with the polar aprotic solvent acetonitrile-d₃, for the first time excited state intramolecular proton transfer (ESIPT), vibrational cooling/relaxation and rotational diffusion could be separated, and furthermore mid IR vibrational spectra of 3-HF excited states in solution phase were obtained. UV/Vis transient absorption data yield the time constant τ_Rot = 22 ps for rotational diffusion and the time constant τ_VR = 8.5 ps for vibrational cooling/relaxation in the tautomer excited state (S₁′). Biphasic ESIPT with τ < 120 fs and τ = 2.4 ps as well as slow ground state recovery with τ > 500 ps was found. The time resolved mid IR data yield a time constant of ≈3.4 ps for the slow ESIPT step as well as the vibrational frequencies of S_0, S₁′ and, in particular those of the short lived excited state S₁. Via quantum chemical calculations, structural parameters of these states are obtained. Various models were used, namely for the isolated molecule, aggregates with solvent as well as a polarizable continuum, that allow us to correlate the two ESIPT components with two mechanisms. Results are compared to those from previously published gas-phase experiments and indicate that the observed slow ESIPT is mediated by solute–solvent interaction via a hydrogen bond with the hydroxyl group of 3-HF.