Ratiometric pH responsive fluorescent probes operative on ESIPT

A series of C-8′-oxime-appended spirobenzopyrans 4a-4d were synthesized and developed as fluorescent pH ratiometric probes operative in HEPES/ACN (8:2, v/v) buffer solutions. Acidochromic conversion of spirobenzopyran to merocyanine open form can facilitate the subsequent enol-keto tautomerization giving dual emissive peaks via the excited state intramolecular proton transfer (ESIPT) mechanism. The pH titrations of 4a show a 68-fold increase in ratiometric intensities ratio (I645 nm/I522 nm) when the pH was switched from 8.0 to 4.0 with a pKa value of 5.90.