A new fluorescent pH probe for acidic conditions

A new imidazo[1,5-a]pyridine-based pH fluorescent probe has been synthesized and characterized. It is the first time that imidazo[1,5-a]pyridine is used as a fluorescent probe. This probe responds to acidic pH with fast response (within 3 min), high selectivity and sensitivity. It has good reversibility and nearly no interference from common metal ions. The probe is suitable for acidic conditions and can quantitatively detect pH value based on the equilibrium equation, pH = pKa − log[(Ia − Ix)/(Ix − Ib)], with a good linear relationship. Moreover, we verified that the protonation of the 2-nitrogen in the imidazole decreases the electron density on the fused ring by 1H NMR analysis and DFT calculation. We proposed that intramolecular charge transfer and solvation of the probe in aqueous solutions might be the mechanism of the fluorescence enhancement under strongly acidic conditions.