Ground- and excited-state proton transfer in some o-hydroxy aromatic compounds and solvent effect

Proton transfer reaction in the ground and excited state of 4-methyl-2,6-di formyl phenol (MFOH) and some of its derivatives in different nonpolar and polar solvents has been studied using steady-state and nanosecond transient emission spectroscopy at room temperature and 77 K. The stable molecular s tructure in the ground state is an intramolecularly hydrogen-bonded closed conformer from which the intramolecular proton transfer (ESIPT) takes place in the first excited singlet state in nonpolar solvents. An emission band originates from the proton-transferred enol tautomer of the closed conforme r. From a detailed study of the absorption, emission, and excitation spectr a in highly polar protic and aprotic solvents, two other conformers have be en identified. At 77 K in nonpolar solvents, emission spectra show phosphor escence only in the case of MFOH and 4-methyl-2,6-diamidophenol (MDOH). In the case of other derivatives, such as 4-methyl-2,6-diacetylphenol (MAOH) a nd 4-methyl-2,6-dicarbomethoxyphenol (MOOH), phosphorescence is observed on ly in the presence of a strong base, such as trimethylamine. This conversio n of fluorescence into phosphorescence is markedly slow in the case of MDOH , MAOH, and MOOH compared to that of MFOH. The relatively larger values of fluorescence lifetimes at 77 K are consistent with our observation. It is p roposed that the occurrence of phosphorescence is from an open conformer an d is caused by the rotation of the formyl group. From nanosecond measuremen ts and quantum yields of fluorescence, we have estimated the decay rate con stants. The fluorescence decay rates are slower in polar than in nonpolar s olvents; nonradiative decay rates are always found to be dominant over the radiative ones.