Inter- and intramolecular hydrogen bond in methyl 2-hydroxy-9H-1-carbazole carboxylate: effect of solvents and acid concentration

The photo-physics of methyl 2-hydroxy-9H-1-carbazazole carboxylate (MPCC) in different solvents and cyclohexane-trifluoroethanol (TFE) mixtures has been studied by means of absorption, fluorescence, fluorescence excitation spectra, time dependence spectrofluoremetery and AM1 semi-empirical quantum mechanical calculations. Only one small Stoke’s shifted fluorescence band is observed under all the environments, indicating that the geometry of the molecule is not changed much on excitation to the first singlet state (S1) and excited state intramolecular proton transfer (ESIPT) is not viable both in the ground (S0) and S1 states at the room temperature. AM1 calculation shows that the ESIPT is still endothermic in S1 state. Single exponential decay is observed in the fluorescence from MPCC in all the solvents except acetonitrile and methanol. This suggests that in these two solvents, at least two different conformers are present in the S0 state, whose absorption spectra are not different from each other. Spectral characteristics of MPCC in cyclohexane as a function of TFE have shown a slight blue shift in the λmaxab, decrease in the εmax, red shift in the λmaxfl and decrease in the φfl. This suggests that intermolecular hydrogen bonding is playing a major role in the deactivation of the fluorescence intensity than the intramoleuclar hydrogen bonding (IHB). Spectral properties of MPCC were also studied as a function of acid–base concentrations. pKa values for different prototropic equilibriums were determined in S0 and S1 states and discussed.