title: Investigation of substitution effect on fluorescence properties of Zn²⁺-selective ratiometric fluorescent compounds: 2-(2'-Hydroxyphenyl)benzimidazole derivatives.
abstract: 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 Zn(2+) but not by addition of Na(+), K(+), Mg(2+), Ca(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), and Cu(2+). Additionally, the effective ratiometric fluorescence response to Zn(2+) 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 Zn(2+) 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 Zn(2+) complexes in aqueous solution, the maximum fluorescence excitation and fluorescence wavelengths of both the tautomeric form and the Zn(2+) 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.
authors.affiliation: Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan; Comprehensive Research Organization for Science and Society (CROSS), 162-1 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan. Electronic address: k_akutsu@cross.or.jp.
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Akutsu
authors.affiliation: Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan.
authors.forename: Seiji
authors.initials: S
authors.lastname: Mori
authors.affiliation: Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan.
authors.forename: Kenichi
authors.initials: K
authors.lastname: Shinmei
authors.affiliation: Comprehensive Research Organization for Science and Society (CROSS), 162-1 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Iwase
authors.affiliation: Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan.
authors.forename: Yoshiharu
authors.initials: Y
authors.lastname: Nakano
authors.affiliation: Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Fujii
keywords: 2-(2′-Hydroxyphenyl)benzimidazole; EXAFS; Ratiometric fluorescent probe; Substituent effect; Zinc-fluorescent probe