KETO-ENOL TAUTOMERIZATION OF 2-(2'-HYDROXYPHENYL)BENZOXAZOLE AND 2-(2'-HYDROXY-4'-METHYLPHENYL)BENZOXAZOLE IN THE TRIPLET-STATE - HYDROGEN TUNNELING AND ISOTOPE EFFECTS .2. DUAL PHOSPHORESCENCE KINETICS

In the metastable triplet state of 2-(2'-hydroxyphenyl)benzoxazole (HBO) and its deuteriooxy analogue (DBO), the keto tautomer (K) and the enol tautomer (E) have about the same energy, and a dual phosphorescence, 3K* --> 1K and 'E* --> 1E, can be observed. From the initial time dependence of the dual phosphorescence from DBO the effective rate constant theta-1 almost-equal-to k(KE) + k(EK) of the keto-enol equilibration 3K* half arrow right over half arrow left 3E* has been determined. 3E* of DBO is almost exclusively populated from 3K*. In the dual phosphorescence from DBO the relative intensity of the keto band is smaller than in the dual phosphorescence from HBO; this is attributed to a small isotope effect on the keto-enol equilibrium in the triplet state. In the metastable triplet state of 2-(2'-hydroxy-4'-methylphenyl)benzoxazole (m-MeHBO) and its deuteriooxy analogue (m-MeDBO) the keto-enol equilibrium is strongly shifted to the side of the enol. No keto phosphorescence from m-MeHBO is observed. Due to the slower keto-enol equilibration in m-MeDBO, an initial keto phosphorescence from m-MeDBO can be observed. The values of theta-1 obtained from the time dependence of the phosphorescence of DBO and m-MeDBO agree reasonably well with the values obtained from transient-absorption experiments. The rate constants for the nonradiative decays of 3K* and 3E* to the electronic ground state are strongly decreased by deuteration.