C. Excited state proton transfer: Keto‐enol tautomerism in the triplet state of hydroxyphenylbenzoxazoles in an alkane glass: Hydrogen tunneling and isotope effects down to 2 K

Abstract

Abstract2‐(2î‐Hydroxyphenyl)benzoxazole (HBO) and its meta‐methyl derivative (m‐MeHBO) can exist in two tautomeric forms, an enol tautomer (E) and a keto tautomer (K). The stable ground state is an enol state, 1E, and the lowest excited singlet state is a keto state, 1K*. Intersystem crossing 1K*→3K* is followed by triplet‐state tautomerization (TST) 3K*→3E* if the energy relation E(3K* — E(3E* = hex > 0 holds. In an alkane glass, the wavenumber x is > 0 for ≈︁ 90% of all HBO molecules and for virtually all m‐MeHBO molecules. The kinetics of TST of HBO, m‐MeHBO and of the deuteriooxy analogues DBO and m‐MeDBO in an alkane glass was studied between ≈︁ 2 and ≈︁ 80 K. TST can be observed by measuring the transient absorption after pulsed excitation, due to the different absorption spectra of 3K* and 3E*. The basic results are: (1) TST is a tunnel reaction and its kinetics is independent of temperature between 2 and 25 K. (2) The rate constant kKE of TST depends on the energy difference hex. Due to the wide distribution of x, the kinetics of TST is multiexponential; an empirical biexponential description of TST yields the ratios k1k2 ≈︁ 5 for the rate constants and A1/A2 ≈︁ 2 for the preexponential factors. (3) The wide distribution of the rate constant kKE can be rationalized by a kinetic model, in which the generation of phonons is an important rate‐limiting factor. (4) The TST of DBO is ≈︁ 1700 times slower than that of HBO, and the TST of m‐MeDBO is ≈︁ 700 times slower than that of m‐MeHBO. ‐ Clear glassy solutions without cracks were obtained by using absorption cells made of polymethylmethacrylate (PMMA), whose thermal expansion coefficient is similar to that of hydrocarbon glasses.