Rational design of Novel Benzimidazole-Based Sensor Molecules That Display Positive and Negative Fluorescence Responses to Anions

Author(s): Jie Shaoa, Yan-Hong Qiaoa, Hai Linb, Hua-Kuan Lina*

Synthesis of 1,10-phenanthroline-2,9-dicarboxylic acid

1,10-phenanthroline-2,9-dialdehyde (3) 1

Selenium dioxide (36.0g, 0.324mol) in 700ml dioxane (4% H2O) is added to a solution of 2,9-dimethyl- 1,10-phenanthroline (16.0g, 0.077mmol) in 250 ml dioxane (4% H2O) under strong stirring at room temperature. The reaction mixture is then heated to reflux under strong stirring for 4.5 hours, then, filtered when hot and the filtrate was allowed to cool to room temperature. The formed precipitated is collected and dried by several Et2O washings. After removal of the solvent under reduced pressure, the crude material (1,10-phenanthroline-2,9-dicarbaldehyde) is obtained as a yellow powder and a pure product is obtained after recrystallization from acetone or THF. M.P. 234～236℃ (lit: 231～232℃)

1,10-phenanthroline-2,9-dicarboxylic acid (4) 1

1,10-phenanthroline-2,9-dicarbaldehyde (1.5, 6.3mmol) is then stirred in a solution of HNO3 (60%, 30mL) under reflux for 3 hours. After cooling to 0°C, slow addition of ice-water leads to the formation of a precipitate, which is collected and dried by several Et2O washings. After removal of the solvent under reduced pressure, the crude material obtained was crystallised from methanol to give the 1,10-phenanthroline-2,9-dicarboxylic acid as a pale yellow powder. M.P. 234～237℃ (lit: 238℃)

Synthesis of benzimidazole derivatives

2-aminomethylbenzimidazole (5) 2, 3

O-phenylenediamine (10.8g, 0.1mol) and glycin (11.2g, 0.15mol) were dissolved in 100ml HCl (6 mol L-1) and then the mixture solution was refluxed for 30 hours. The reaction solution was cooled to 4℃, placed overnight and filtered to obtain a pink solid. The crude product was crystallized from ethanol to give a pure colorless solid. M.P.263℃

2-α-aminoethylbenzimidazole (6) 2, 3

O-phenylenediamine (5.4g, 0.05mol) and alanine (6.6g, 0.075mol) were dissolved in 50ml HCl (6 mol L-1) and then the mixture solution was refluxed for 72 hours. After of removal of the solvent under reduced pressure, the residue was dissolved in 4 mol L-1 HCl solution and the solution was placed overnight and filtered to obtain a pink solid. The crude product was crystallized from ethanol to give a pure colorless solid. M.P.: 135-138℃

Figure 1 Fluorescence spectra (excitation at 368 nm) of the receptor 1 (2×10-5 mol L-1) in the presence and absence of acetate ion.

Figure 2 Fluorescence spectra (excitation at 368 nm) of the receptor 1 (2×10-5 mol L-1) in the presence and absence of bromide ion.

Figure 3 Fluorescence spectra (excitation at 368 nm) of the receptor 1 (2×10-5 mol L-1) in the presence and absence of iodide ion.

Figure 4 Fluorescence spectra (excitation at 368 nm) of the receptor 2 (2×10-5 mol L-1) in the presence and absence of acetate ion.

Figure 5 Plot of 1H NMR spectra of receptor 2 (5×10-3 mol L-1) on addition of F- in DMSO-d6 (from bottom to top: 0, 0.5,1, 2, 5, 10, 20 equiv., respectively).

Figure 6 Plot of 1H NMR spectra of receptor 2 (5×10-3 mol L-1) on addition of AcO- in DMSO-d6 (from bottom to top: 0, 0.5,1, 2, 5, 10, 20 equiv., respectively).

Figure 7 Plot of 1H NMR spectra of receptor 1 (5×10-3 mol L-1) on addition of AcO- in DMSO-d6 (from bottom to top: 0, 0.5,1, 2, 5, 10, 20 equiv., respectively).

Figure 8 1H NMR spectrum of the receptor 1

Figure 9 1H NMR spectrum of the receptor 2

References:

[1] J. C. Christopher, W. D. Leslie, A. R. James (1981) Synthesis of some 2,9-disubstituted-1,10-phenanthroline. J. Heterocyclic Chem18:599-601.

[2] T. M. Aminabhavi, N. S. Birad, S. B. Patil (1986) Structural and Biological Studies on Benzimidazolyl Acid Complexes of Dimethyldichlorosilane. Inorganica Chimica Acta 125:125-128.

[3] L. A. Cescon, A .R. Day (1962) Preparation of some Benzimidazolyl Acids. Reactions of Animo Acids with o-Phenylenediamines. J. Org Chem 27:581-586.

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