Dicyanomethylene-4 H -pyran-based NIR fluorescent ratiometric chemosensor for pH measurement

A D-π-A featured dicyanomethylene-4H-pyran-based dye (DCM-OH) is designed as a pH-sensitive fluorescent chemosensor. In neutral conditions, DCM-OH shows a long absorption wavelength at 431 nm attributed to the DCM chromophore via the intramolecular charge transfer (ICT), but very weak fluorescence around 574 nm. While pH increases from 7.15 to 11.00, DCM-OH shows a bathochromic absorption band at 574 nm due to the increase of the ICT process from the oxygen anion group, shifting the emission band to the NIR region. A DFT calculation has been performed to confirm the ICT enhancement. In this way, an obvious ratiometric fluorescent signal (I692/I574) allows the ratiometric detection of pH, induced by a large Stokes shift of about 118 nm. The fluorescent chemosensor exhibits high sensitivity to H+ with a pKa value of 7.21. The strong change in fluorescence intensity with pH allows the determination of pH over a much wider range (typically 7–11). Furthermore, the reversible absorption and fluorescence conversion was also performed with protonation and deprotonation, which makes DCM-OH a simple naked-eye sensitive NIR fluorescent chemosensor for pH measurement.