Turning a fluorescent probe for Al3+ into a pH sensor by introducing Cl-substitution

Abstract

Here we report a new Schiff-base molecule, 4,4′-sulfonylbis(2-(5-chloro-2- hydroxybenzylideneamino)phenol) (H4LCl) as a colorimetric and fluorescent probe for pH. On increase in pH from 2.0 to 12.0, the fluorescence intensity of H4LCl increases by 45-fold at 509 nm in Britton-Robinson buffer (λex = 380 nm). Quantum yield of the probe also increases on increase in pH of the media. This enhancement of fluorescence intensity is mainly due to the deprotonation of the phenolic OH groups in the sensor molecule. The pKa of the probe has been determined to be 7.18. Theoritical studies show that the increment in intensity occurs via ICT-AIE controlled pathway. Naked eye detection is possible by noting the color change of the solutions observed under visible and UV light. Several cations and anions do not interfere in the pH sensing properties of H4LCl. It is to mention that the compound without Cl-substituion, namely, 4,4′-sulfonylbis(2-(-2-hydroxybenzylideneamino)phenol), was reported earlier as a chemosensor for Al3+ ion (New J. Chem. 41 (2017) 10677). Introduction of Cl changes sensing preference of the Schiff-base from Al3+ to pH. H4LCl has been used for pH detection of river water.