Excited state acidity of bifunctional compounds Part 8. Competitive kinetics between solvent reorientation and proton transfer during ESIPT of 2-hydroxyphenyl-lapazole in protic solvents

Abstract

2-Hydroxyphenyl-lapazole (HPL) is shown to undergo excited state intramolecular proton transfer (ESIPT) in the protic solvents methanol, propan-2-ol and octan-1-ol at room temperature. Investigation of the kinetics of this process, using time-resolved single photon counting and transient absorption spectroscopy, indicates the presence of three different excited-state species. These results are very different from that already reported for HPL in non-protic solvents, where it was found that the ESIPT process attains equilibrium during the lifetime of the excited state. Factor analysis of the steady state spectra supports the conclusions drawn from the kinetic results. The difference in behavior as a function of solvent is attributed to two factors that depend on the stronger solute–solvent interactions in the case of the protic solvents. (1) The slight slowing down of the process of proton transfer, which prevents equilibrium from being established during the lifetime of the excited singlet state. (2) The weakening of the intramolecular hydrogen bond, which allows rotation of the hydroxyphenyl moiety.