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 ³E* → ¹E, can be observed. From the initial time dependence of the dual phosphorescence from DBO the effective rate constant θ₁ ≈ k_KE + k_EK of the keto-enol equilibration ³K* ⇌ 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 θ₁ 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.