Spiropyran (SP) is an attractive molecular platform for the design of fluorescent OFF-ON probes for the detection of cations and anions operative on a ring-opening/closing mechanism. By incorporating a benzimidazole moiety at the ortho-position of the oxygen atom of the pyran unit, a ratiometric fluorescent probe in the NIR range for the measurement of pH change in aqueous ACN solution was designed and investigated. The ring-opening mechanism and cis-trans interconversion of the probe are also substantiated by 1H NMR titration. The probe showed a ratiometric change in the pH range from 7.0 to 5.5 and the pKa of the probe was calculated to be 5.9. The relative intensity ratio (I690 nm/I517 nm) changed from 0.08 to 2.11 with a more than 26-fold enhancement in response to the pH variation.
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