Proton transfer reaction of 4-methyl-2,6-diacetylphenol and an analysis with AM1 potential-energy surfaces

Abstract

The ground and excited state proton transfer processes of 4-methyl-2,6-diacetylphenol (MAOH) have been studied by means of steady-state absorption, emission and time resolved spectroscopy in different protic and aprotic solvents at room temperature and 77 K. The relative fluorescence quantum yield measurements are made at different excitation wavelength and both in presence and absence of added base. The emission properties of MAOH at 77 K have been examined in relation to those of 4-methyl-2,6-diformylphenol (MFOH). At this temperature, unlike MFOH, MAOH show phosphorescence only in presence of base like triethylamine in all the solvents studied here. From nanosecond measurements and fluorescence quantum yield we have estimated the decay rate constants. The nonradiative decays are always dominant in the decay processes of the excited states. The energetics of the ground- and excited-state proton transfer in MAOH molecule has been studied by the configuration interaction method at AM1 level of approximation. The ground singlet and the excited triplet are predicted to have considerable barriers on the respective proton transfer paths, while the barrier height is much lower on the corresponding singlet surface. The process is predicted to be endothermic in the ground state and exothermic in the excited singlet and triplet state.