CHARACTERIZATION OF GROUND AND ELECTRONICALLY EXCITED-STATES OF ORTHO-HYDROXYBENZALDEHYDE AND ITS NON-HYDROGEN-BONDED PHOTOROTAMER IN 12-K RARE-GAS MATRICES

Intramolecularly hydrogen bonded o-hydroxybenzaldehyde (OHBA-C) isolated in 12 K rare gas matrices photolyzes to a non-hydrogen-bonded rotamer (OHBA-F). IR spectra of OHBA-C, OHBA-F, and several model and isotopically substituted compounds are consistent with identification of the OHBA-F conformer as that formed by 180-deg rotation of both the hydroxy and aldehyde groups. For the two rotamers, electronic absorption, excitation, and emission spectra are presented together with time-resolved emission measurements and estimates of a ground-state reaction enthalpy. From these data, it is proposed that the S1 state of OHBA-C is an n,π* hydrogen atom transfer state, and S2 is a π,π* proton-transfer state with a large (∼18 kcal) barrier to reaction. Rotamerization is reversed by S1 or S2 excitation of OHBA-F. The conversion of OHBA-C to OHBA-F is ∼5 times as efficient as the reverse process upon excitation at the respective S1 0-0 energies. An increase in photolysis quantum yield of OHBA-C is measured at energies well above the 0-0 energy and may correspond to reaction over the proposed ∼8 kcal S1 barrier. © 1990 American Chemical Society.