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The spectroscopic properties of 2-(2‘-tosylaminophenyl)benzimidazole (TPBI) have been studied in a series of different solvents. As revealed by absorbance, steady-state, and time-resolved emission spectroscopy, the molecule undergoes fast excited-state intramolecular proton transfer (ESIPT) to yield emission of the corresponding tautomeric species with a large quantum yield (0.5). The fluorescence emission shows monoexponential decay kinetics (τ = 4.5 ns) regardless of the nature of the solvent. The ground-state equilibrium is dominated by a single rotamer and small amounts of the deprotonated anion. The X-ray structure of TPBI shows a substantial out-of-plane twist along the aryl−benzimidazole bond axis, which is presumably due to π-stacking interactions between the tosylamide and benzimidazole rings. Ab initio calculations suggest a different structure in the gas phase without π-stacking interactions and a substantially reduced twist angle. A large energy barrier for interconversion of the cis- and trans-rotamers in the ground and excited state has been predicted on the basis of DFT calculations, which is in agreement with all experimental data. The ground-state equilibrium and ESIPT process of TPBI are essentially unaffected by the nature of the solvent, which is of particular interest to sensing applications in cell biology.
Fahrni et al. (Wed,) studied this question.