Add time:09/26/2019         Source:sciencedirect.com

2-(2′-Hydroxyphenyl)benzimidazole derivatives (X-HBIs), modified by various substituents X (X=H, CH3, OH, OCH3, NO2, NHCOCH3, NH2, N(CH3)2), were synthesized and their fluorescent behaviors and equilibriums in aqueous solution were studied. Strong fluorescence attributed to the tautomer emission was observed in aqueous solution at pH 7.4. The fluorescence intensities of the X-HBIs were enhanced selectively by addition of Zn2+ but not by addition of Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, and Cu2+. Additionally, the effective ratiometric fluorescence response to Zn2+ addition was observed in 5-NH2-HBI and 5-N(Me)2-HBI. The pH-titration and speciation studies proved that the X-HBIs have two or three protonation equilibriums and one complexation equilibrium corresponding to the formation of the [Zn(X-HBI)]+ complex. Further structural studies using extended X-ray absorption fine structure analyses and density functional theory calculations identified the dominant Zn2+ species as the [Zn(HBI)(H2O)3]+ complex in aqueous solution. Based on the substituent effect on the fluorescence properties of X-HBIs and their Zn2+ complexes in aqueous solution, the maximum fluorescence excitation and fluorescence wavelengths of both the tautomeric form and the Zn2+ complexes were dependent on the Hammett substituent constants of X, which was attributed to the change of the π–π* energy gap of HBI by introduction of the substituent.

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