Ultrafast Measurements of Excited State Intramolecular Proton Transfer (ESIPT) in Room Temperature Solutions of 3-Hydroxyflavone and Derivatives

Ultrafast pump−probe studies of room-temperature solutions of 3-hydroxyflavone (3-HF) and some 4‘-substituted derivatives have been undertaken. Transient absorption attributable to the two zwitterionic forms of the excited tautomer arising from excited-state intramolecular proton transfer (ESIPT) was observed across most of the visible spectral region. For 3-HF in methylcyclohexane and acetonitrile, the ESIPT was found to be so rapid that it was only possible to assign a time constant of 35 fs to the process. In ethanol, however, a time constant of 60 fs was determined. The slower ESIPT in this solvent was attributed to the greater strength of the solute−solvent interactions. For the derivatives of 3-HF in all three solvents, the ESIPT step was also found to be instrument-limited. In addition to the femtosecond kinetics, there was also a picosecond component of the kinetics that is attributed to ESPT in molecules that are intermolecularly hydrogen bonded to the solvent.