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Design and Development of Novel pH-Sensors Based on Iminocoumarin Derived Imidazo[4,5-b]pyridines

Design and development of molecules for (chemo)sensing and optoelectronic applications is of great interest in organic chemistry and sensing technology. Benzazoles are among the most privileged heterocyclic subunits in the medicinal chemistry due to a wide range of bioactivities, while iminocoumarins are known for interesting spectroscopic features. It is no surprise that their conjugates are being explored for pH sensing applications based on their excellent spectroscopic response in correlation with pH value. Here we present the synthesis and spectroscopic characterization of novel iminocoumarin derived imidazo[4, 5-b]pyridines in several polar and nonpolar solvents. The absorption spectra reveal a strong dependence on iminocoumarin substituents, solvent polarity and pH, which allows a potential use as sensitive and selective optical sensors in biological, environmental, and chemical processes. To determine their precise protonation state under different pH conditions, we calculated the matching pKa values with the (SMD)/B3LYP/6-31+G(d) model. The results are in very good agreement with experiments, and reveal that under neutral pH = 7, these systems are present as cations monoprotonated at the exo-imine, while in the pH range 1–13, they exchange between neutral and diprotonated forms, which is responsible for alterations in UV-Vis responses.

pH-sensors

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