Novel benzimidazole-based ratiometric fluorescent probes for acidic pH

2,6-Bis(1-alkyl-1H-benzo[d]imidazol-2-yl)naphthalene (2a-2c) as three new benzimidazole molecules have been designed, synthesized and applied as ratiometric fluorescent pH probes in the acidic pH region with high-efficiency and fast response. For probe 2a, upon decreasing the pH from 7.5 to 2.5, the emission spectra exhibit a large red shift from 402 to 472 nm, and the emission ratio (I472/I402) changes dramatically from 0.10 to 3.42 with a pKa value of 2.92. No significant interference on emission behavior is observed in the presence of various metal ions. Similar results are obtained for probes 2b and 2c. 1H NMR and DFT calculations studies demonstrate that the ratiometric response of the probes to acidic pH is due to H+ binding with one N atom in C=N of benzimidazole ring and the subsequent enhancement of the intramolecular charger transfer (ICT) process. In addition, these probes can be used in real-time and reversible pH sensing. Finally, the probes have been successfully applied to the visual detection of pH not only in solution, but also in test paper.