Synthesis of Benzimidazoles Containing Pyrazole Group and Quantum Chemistry Calculation of Their Spectroscopic Properties and Electronic Structure

Five benzimidazole compounds containing pyrazole group were synthesized via one-step reaction of o-phenylenediamine and 1-arylpyrazole-4-carbaldehyde in ethanol under mild conditions. The composition and structure of resultant benzimidazole compounds were analyzed by means of elemental analysis, mass spectrometry, ¹H-nuclear magnetic resonance spectroscopy and X-ray single crystal diffraction. The ultraviolet–visible light spectra and fluorescent spectra of the products were measured. Their ground-state (S ₀) equilibrium geometries and vibrational frequencies were determined based on B3LYP method, and their first excited-state (S ₁) geometries were fully optimized based on 6–31G (d, p) basis set of TD-B3LYP method. Besides, the spectroscopic properties of the products were computed based on cc-pVTZ basis set of TD-B3LYP method and compared with corresponding experimental data. It has been found that benzimidazole compounds containing pyrazole group can be readily synthesized in a high yield via one-step reaction of o-phenylenediamine and 1-arylpyrazole-4-carbaldehyde in ethanol solvent. The fluorescence properties of the five synthesized compounds are closely related to their molecular structure; and their computed fluorescence spectra well correspond to their experimental values. Moreover, they have stable structure and strong fluorescence, showing potential application in time-resolved fluoroimmunoassay and DNA probe.