Proton transfer reaction of 4-methyl-2,6-diamidophenol in alcoholic solvents at room temperature and 77 K

Abstract

Ground and excited state proton transfer reaction of 4-methyl-2,6-diamidophenol (MDOH) has been studied in alcoholic solvents, using steady state and nanosecond spectroscopy, at room temperature (RT) and 77 K both in presence and absence of triethylamine (TEA). Solute-solvent interaction appears to play a major role in determining the nature of the absorbing and fluorescing species. The emission properties of MDOH have been examined in relation to those of 4-methyl-2,6-diformylphenol (MFOH). At 77 K the emission due to the open conformer is markedly suppressed and consists of phosphorescence both in presence and absence of TEA. The fluorescence decay rates are relatively slow in alcoholic solvents compared to those in non-polar solvents and non-radiative decay rates are always found to be dominant over the radiative rates. Generation of the trajectories for the intramolecular proton transfer reaction in the ground (S₀) and the lowest excited singlet (S₁) states using the semiempirical AM1-SCI method demonstrates that the intramolecular proton transfer process is favored in the S₁ state both thermodynamically and kinetically. The activation energy for the excited state intramolecular proton transfer (ESIPT) process has been calculated in various alcoholic solvents differing in polarity assuming Onsager's dielectric continuum model.