Synthesis and spectroscopic characterization of multifunctional D-& pi;-A benzimidazole derivatives as potential pH sensors

We report multifunctional D-& pi;-A molecular systems having the N,N-diethylamino group as a pH sensitive donor unit connected to the electron accepting substituents such as the cyano moiety and the pH responsive benz-imidazole fragment. For that purpose we have synthesized three types of derivatives, namely acrylonitrile, Schiff base and iminocoumarin derived benzimidazoles, whose photophysical characterization has been carried out in several polar and non-polar organic solvents as well as aqueous solution buffers with different pH. Computa-tionally supported determination of species involved in prototrophic equilibria, including their respective pKa values, has been performed to aid in understanding structural and substituent effects on their UV-Vis and fluorescence spectral properties and pH sensing potentials. Both sets of results jointly indicate that all molecules are predominantly unionized under neutral conditions, while their transition towards monocationic forms, responsible for spectroscopic response changes, occurs at pH & AP; 1-4 for acrylonitriles 4-5, pH & AP; 4-5 for Schiff bases 11-12, and pH & AP; 4.5-6.5 for iminocoumarins 7-8, thus increasing their practical attractiveness for a broad range of applications in the same order.

Automated summary

We report multifunctional D-π-A molecular systems with a pH-sensitive donor unit and electron accepting substituents. Three types of derivatives, including acrylonitrile, Schiff base, and iminocoumarin derived benzimidazoles, were synthesized and their photophysical characterization was carried out. The results indicate that these molecules exhibit pH sensing potentials in a broad range of applications.