Excited State Proton Transfer Reaction of 4-Methyl-2,6-dicarbomethoxyphenol in Some Protic Solvents at Room Temperature and 77K: Interaction with Base

Abstract

The ground and excited state proton transfer reactions of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) have been studied in water and some alcoholic solvents both in the presence and in the absence of base by means of absorption, emission, and nanosecond spectroscopy at room temperature and 77 K. Solute–solvent interaction appears to play a major role in determining the nature of absorbing and fluorescing species in these solvent media. The emission properties of CMOH have been examined in relation to those of 4-methyl-2,6-diformylphenol (MFOH). The spectral characteristics obtained indicates that CMOH in methanol, ethanol, and water exists in more than one structural form in both the ground and excited states. At 77 K the emission spectra show phosphorescence only in the presence of a base. The fluorescence decay rates of CMOH are relatively slower than those of MFOH and nonradiative decays are dominant in the decay process of CMOH. It is proposed that intramolecular hydrogen bond in CMOH is stronger compared to that in MFOH. From our theoretical calculation at AM1 level of approximation suggested that the process is thermodynamically endothermic in the ground state and also encounter high barriers. However, the process becomes exothermic in the excited state.