Mapping of upper electronic reaction surfaces by tuned laser photolysis and by absorption and emission spectroscopies.

摘要

Potential energy surfaces for photorotamerization of two intramolecularly hydrogen-bonded molecules, o-hydroxybenzaldehyde (OHBA) and methyl salicylate (MS), isolated in cryogenic matrices have been spectroscopically mapped. In addition, the external heavy atom effect of krypton and xenon matrices on the coupling between the S{dollar}sb1{dollar} and T{dollar}sb1{dollar} surfaces of 4-(dimethylamino)benzonitrile has been examined.; Based on steady-state and time-resolved emission measurements, the S{dollar}sb1{dollar} state of OHBA is postulated to be an n,{dollar}pi{dollar}* hydrogen atom transfer state with a 10.6 kcal barrier to formation of the non-intramolecularly hydrogen-bonded photorotamer, while S{dollar}sb2{dollar} is proposed to be a {dollar}pi ,pi{dollar}* proton transfer state, with a 17.7 kcal barrier. A higher quantum yield is detected upon photolysis of OHBA a few kcal above the S{dollar}sb1{dollar} barrier. The five-fold greater quantum yield for the forward over the reverse reaction observed within 15 kcal of the S{dollar}sb1{dollar} 0-0 energy is interpreted to be a manifestation of reaction on either the triplet surface or in the vibrationally excited ground state manifold.; In contrast, photolysis of MS in xenon or SF{dollar}sb6{dollar} matrices yields a rotamer having an intramolecular hydrogen-bond between the phenolic hydrogen and the ether oxygen of the ester moiety. The results from time-resolved and steady-state emission spectroscopies are consistent with MS having an S{dollar}sb1 pi ,pi{dollar}* proton transfer state with a 12.7 kcal S{dollar}sb1{dollar} barrier to formation of the ether-bonded conformer. The forward reaction proceeds at the S{dollar}sb1{dollar} 0-0 energy by either a triplet or vibrationally excited ground state mechanism. This reaction was found to not be photochemically reversible.; Heavy atom matrices are known to increase rates of spin-forbidden processes. The phosphorescence intensity of DMABN increases in krypton and xenon matrices, while the fluorescence intensity, and phosphorescence and fluorescence lifetime, decrease. These effects are interpreted in terms of a model in which the phosphorescence rate constant increases 300-fold in xenon compared to argon, while the rate constants for intersystem crossing and nonradiative relaxation from the triplet state increase by factors of less than 5. Lifetime measurements in argon matrices doped with heavy atoms indicate that even one heavy atom neighbor has a significant effect on both singlet and triplet lifetimes.