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

The spectral properties and proton transfer reaction of 4-methyl-2,6-diamid ophenol (MDOH) in relation to 4-methyl-2,6-diformylphenol (MFOH) and 4-meth yl-2,6-diacetyl phenol (MAOH), have been investigated by steady-state and t ransient-fluorescence experiments in aqueous medium both in the presence an d absence of base and acid at room temperature and 77 K. The absorption, em ission and excitation spectra are recorded and fluorescence quantum yields are calculated. In the case of MDOH, similar to MFOH and MAOH, intramolecul arly hydrogen bonded closed conformer was detected predominantly in pure wa ter in the ground state. It is shown that the Stokes-shifted fluorescence i s likely to originate from the anion conformers for all the compounds. It i s proposed that owing to the structural difference between the three compou nds, the proton transfer to the solvent is relatively easier in the case of MDOH and MFOH, in comparison to MAOH. At 77 K the fluorescence due to anio n is markedly suppressed, showing that at this temperature anion conformer is less stable. At 77 K both MFOH and MDOH show phosphorescence in aqueous medium. The measured decay rates of MDOH and MFOH are relatively slower tha n those of MAOH. (C) 1999 Elsevier Science B.V. All rights reserved.