Molecular variation and fluorescent turn-on detection of chromium(III) by three ESIPT-reactive 2,2'-(1,4-phenylenebis(5-phenyl-1H-imidazole-4,2-diyl))diphenols

Three novel bis-imidazole-phenol dyes 1–3 have been designed with deliberate incorporation of a pair of O–H…N donor-acceptor units on each dye in order to enable Excited State Intramolecular Proton / electron Transfer (ESIPT) capability, its desirable red-shifted emissions and consequent reduction of fluorescence characteristics, which in turn became applicable for fluorescence turn-on response in metal ion sensing. Molecule 1 was confirmed by single crystal X-ray analysis. Their quantum yields in acetonitrile solutions are generally suitable (Φ = 0.30 for 1, Φ = 0.06 for 2 and Φ = 0.08 for 3) and they display fluorescence turn-on sensitivity towards chromium(III). The selectivity features of fluorophores 2 and 3 for Cr(III) are remarkable. The blue-shift of emission bands coupled with the emission turn-on after introduction of Cr(III) confirms that the ESIPT capability of the fluorophores contributes to the low emission properties of the molecules and that emission turn-on during fluorescence detection succeeds by elimination of the ESIPT possibility. Therefore, it could be concluded that the ESIPT feature is a vital tool to utilize for fluorescent sensor designs. Job plot analysis indicates a 2 : 1 stoichiometry of metal : sensor. Results of interference studies also favours potential application of the ESIPT dye 3 for detection of chromium(III). Molecular variation effects were discussed.