A off–on pH fluorescence probe derived from phenanthro[9,10-d]imidazol-fluorescein based on ESIPT and ICT

A novel pH fluorescence probe (PIF), derived from phenanthro[9,10-d]imidazole bound to the fluorescein, is designed and synthesized based on the intramolecular hydrogen bond and then its spectra properties are investigated. Probe PIF exhibits negligible fluorescence intensity at acidic pH condition due to ESIPT quenching caused by the formation of the intramolecular hydrogen bonds, and a significant fluorescence enhancement in neutral and weak alkaline environments because of the strengthened ICT process arose from the fracture of the intramolecular hydrogen bonds. The studies of UV–Vis and fluorescence spectra mean that probe PIF is able to respond linearly and reversibly over pH from 6.5 to 9.0 with pKa of 7.0. Moreover, the pH responses of probe PIF are stable and not disturbed by the common cations and anions. The possible response mechanism has been proposed according to the analysis of 1H NMR and DFT calculations.