Peculiarity of triplet–triplet energy transfer from 2-(2′-hydroxy-phenyl)benzoxazole to diacetyl. Evidence for radiative keto–enol transitions 3K∗→1E and 1E→1K∗ Original Research Article

In the triplet–triplet energy transfer (TTET) from a donor D to an acceptor A, 3D∗+1A→1D+3A∗, the maximum transferable energy is equal to the maximum energy that can be emitted as phosphorescence 3D∗→1D. A peculiarity was expected for the TTET from 2-(2′-hydroxyphenyl)benzoxazole (HBO), which has two isoenergetic lowest triplet states, an enol triplet state 3E∗ and a keto triplet state 3K∗, with completely different phosphorescence spectra: 3E∗→1E (origin at 22400 cm−1) and 3K∗→1K (origin at ∼17500 cm−1). If the triplet energy of an acceptor A is between these two origin values, like in the case of diacetyl (19700 cm−1), then the TTET from HBO to A should be predominantly due to the process 3E∗+1A→1E+3A∗. The TTET from HBO, meta-methyl-HBO (m-MeHBO, 3K∗≳300 cm−1 above 3E∗) and ortho-methyl-HBO (o-MeHBO, 3K∗≳600 cm−1 below 3E∗) to diacetyl was studied in 3-methylpentane from 110 to 280 K, which corresponds to a viscosity range of more than three orders of magnitude. The postulated peculiarity of TTET was observed at low viscosity. At low temperature and high viscosity, however, the kinetic differences in the TTET from the three donors disappear. The TTET from o-MeHBO to diacetyl at low temperature is assigned to the process 3K∗+1A→1E+3A∗, which corresponds to a keto–enol phosphorescence 3K→1E. The temperature dependence of the second-order rate constant for TTET can be satisfactorily described with a kinetic model, whose distinctive feature is a finite rate of the triplet-state tautomerization 3K∗⇌3E∗. The only essentially donor-specific parameter is the energy difference between 3E∗ and 3K∗. The principal possibility of radiative keto–enol or enol–keto transitions has been verified: The phosphorescence excitation spectrum of m-MeHBO in a xenon matrix exhibits in the near UV a weak band that can be assigned to the transition 1E→1K∗.