H-atom transfer and rotational processes in the ground and first singlet excited electronic states of 2-(2'-hydroxyphenyl)oxazole derivatives: Experimental and theoretical studies

The H-atom transfer and the rotational processes of 2-(2'-hydroxyphenyl)oxazole derivatives in both ground (S-0) and first singlet (S-1) excited electronic states have been respectively studied from experimental and theoretical points of view. Experiment and theory support the coexistence of two ground state rotamers, E and ER, with OH ... N and OH ... O hydrogen bonds, respectively, rotamer E being the most stable and the only one that experiences a photoinduced H-atom motion in the S-1 state. The fluorescence of 2-(2'hydroxyphenyl)-4-methyloxazole in a rigid polymeric medium suggests that in fluid media the phototautomer of the excited enol rotamer suffers a twisting motion around the C-C bond linking both moieties of the molecule. Ab initio calculations at the Hartree-Fock and CI-all-singles levels reveal (a) the existence of a high-energy barrier to the H-atom transfer in the So state, whereas in the S-1 state this transfer has a small or null energy barrier, (b) a coupling between a charge transfer and the nuclear rearrangement (OH and N ... O modes) that makes the system move from the enol to the keto form, and (c) the presence of excited state rotamers of the keto phototautomer in these oxazole derivatives.