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Ground‐ and Excited‐State Proton Transfer in Some o ‐Hydroxy Aromatic Compounds and Solvent Effect
Author(s) -
Guha Dipanwita,
Mandal Abhijit,
Das Ranjan,
Mitra Sivaprasad,
Mukherjee Samaresh
Publication year - 1999
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.199900043
Subject(s) - chemistry , phosphorescence , photochemistry , conformational isomerism , excited state , tautomer , emission spectrum , singlet state , fluorescence , ground state , proton , isopentane , solvent , spectral line , molecule , stereochemistry , organic chemistry , atomic physics , catalysis , physics , quantum mechanics , astronomy
Proton transfer reaction in the ground and excited state of 4‐methyl‐2,6‐diformyl 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 structure 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 conformer. From a detailed study of the absorption, emission, and excitation spectra in highly polar protic and aprotic solvents, two other conformers have been identified. At 77 K in nonpolar solvents, emission spectra show phosphorescence 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) and 4‐methyl‐2,6‐dicarbomethoxyphenol (MOOH), phosphorescence is observed only in the presence of a strong base, such as trimethylamine. This conversion 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 proposed that the occurrence of phosphorescence is from an open conformer and is caused by the rotation of the formyl group. From nanosecond measurements and quantum yields of fluorescence, we have estimated the decay rate constants. The fluorescence decay rates are slower in polar than in nonpolar solvents; nonradiative decay rates are always found to be dominant over the radiative ones.