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

The ground and excited state proton transfer processes of 4-methyl-2,6-diac etylphenol (MAOH) have been studied by means of steady-state absorption, em ission and time resolved spectroscopy in different protic and aprotic solve nts at room temperature and 77 K. The relative fluorescence quantum yield m easurements are made at different excitation wavelength and both in presenc e and absence of added base. The emission properties of MAOH at 77 K have b een 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 nanosecon d measurements and fluorescence quantum yield we have estimated the decay r ate constants. The nonradiative decays are always dominant in the decay pro cesses of the excited states. The energetics of the ground- and excited-sta te proton transfer in MAOH molecule has been studied by the configuration i nteraction method at AM1 level of approximation. The ground singlet and the excited triplet are predicted to have considerable barriers on the respect ive proton transfer paths, while the barrier height is much lower on the co rresponding singlet surface. The process is predicted to be endothermic in the ground state and exothermic in the excited singlet and triplet state. ( C) 2001 American Institute of Physics.