Dual phosphorescence from 2-(2′-hydroxyphenyl) benzoxazole in amorphous solid solution. Temperature dependence of dispersive kinetics fo nonexponential triplet decay

2-(2′-hydroxyphenyl) benzoxazole (HBO) can exist in two tautomeric forms, in an enol form (E) and a keto form (K). In the metastable triplet state of HBO in liquid alkanes, both tautomers have roughly the same energy, and a dual phosphorescence, ³K^*→¹K and ³E^*→¹E, is observed; both components of the dual phosphorescence exhibit the same monoexponential decay. In solid solution, the keto phosphorescence decays faster than the enol phosphorescence, and both decays are nonexponential. The dispersive kinetics of the dual phosphorescence from a solid solution can be described by a simple model with a minimum of parameters: The rate constants k_K and k_E for the decay of ³K^* and ³E^* to the respective electronic ground state and a distribution function for the energy difference between ³K^* and ³E^* in solid solution. The true distribution function seems to be better approximated by a Lorentzian than by a Gaussian. k_K and k_E are virtually independent of temperature.