Evaluation of solvent effects and phototautomerism and their consequences on the degradation of the methyl benzimidazole-2-ylcarbamate

The photolysis and protonation mechanisms of methyl benzimidazol-2-ylcarbamate (MBC) were evaluated by determining the dipole moments and pKa in the ground and excited states using solvatochromic correlations and the Förster cycle, respectively. The bathochromic shift observed in the absorption and fluorescence spectra of MBC with increasing solvent polarity indicates that the transitions involved are π-π*. Three methods depending on the theory of solvatochromism were applied to estimate the difference in the excited-state (µe) and the ground-state (µg) dipole moment. In all cases, the excited-state dipole moment value was higher than the ground-state value, which indicates a substantial redistribution of the π-electron densities in a more polar excited state than the ground-state. The excited state pKa* < pKa shows that the molecular form of MBC is more acidic in the excited state. As a result, in strengthened acidic conditions, we observe an excited-state intramolecular transfer (ESIPT) responsive to dual fluorescence emission. The theoretical calculations proved consistent with experimental data. Finally, the determination of bond orders highlighted the significant weakening of the amide bond in acidic media, promoting the bond cleavage. Thus, the collected data improve our understanding of the degradation mechanisms of carbendazim to amino-benzimidazole.