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.