Peculiarity of triplet–triplet energy transfer from 2-(2′-hydroxy-phenyl)benzoxazole to diacetyl. Evidence for radiative keto–enol transitions [formula omitted] and [formula omitted

Abstract

In the triplet–triplet energy transfer (TTET) from a donor D to an acceptor A, 3 D ∗+ 1 A→ 1 D+ 3 A ∗ , the maximum transferable energy is equal to the maximum energy that can be emitted as phosphorescence 3 D ∗→ 1 D . A peculiarity was expected for the TTET from 2-(2′-hydroxyphenyl)benzoxazole (HBO), which has two isoenergetic lowest triplet states, an enol triplet state 3 E ∗ and a keto triplet state 3 K ∗ , with completely different phosphorescence spectra: 3 E ∗→ 1 E (origin at 22400 cm −1) and 3 K ∗→ 1 K (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 3 E ∗+ 1 A→ 1 E+ 3 A ∗ . The TTET from HBO, meta-methyl-HBO ( m-MeHBO, 3 K ∗ ≳300 cm −1 above 3 E ∗ ) and ortho-methyl-HBO ( o-MeHBO, 3 K ∗ ≳600 cm −1 below 3 E ∗ ) 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 3 K ∗+ 1 A→ 1 E+ 3 A ∗ , which corresponds to a keto–enol phosphorescence 3 K→ 1 E . 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 3 K ∗⇌ 3 E ∗ . The only essentially donor-specific parameter is the energy difference between 3 E ∗ and 3 K ∗ . 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 1 E→ 1 K ∗ .