A multicolor and ratiometric fluorescent sensing platform for metal ions based on arene–metal-ion contact

Despite continuous and active development of fluorescent metal-ion probes, their molecular design for ratiometric detection is restricted by the limited choice of available sensing mechanisms. Here we present a multicolor and ratiometric fluorescent sensing platform for metal ions based on the interaction between the metal ion and the aromatic ring of a fluorophore (arene–metal-ion, AM, coordination). Our molecular design provided the probes possessing a 1,9-bis(2′-pyridyl)-2,5,8-triazanonane as a flexible metal ion binding unit attached to a tricyclic fluorophore. This architecture allows to sense various metal ions, such as Zn(II), Cu(II), Cd(II), Ag(I), and Hg(II) with emission red-shifts. We showed that this probe design is applicable to a series of tricyclic fluorophores, which allow ratiometric detection of the metal ions from the blue to the near-infrared wavelengths. X-ray crystallography and theoretical calculations indicate that the coordinated metal ion has van der Waals contact with the fluorophore, perturbing the dye’s electronic structure and ring conformation to induce the emission red-shift. A set of the probes was useful for the differential sensing of eight metal ions in a one-pot single titration via principal component analysis. We also demonstrate that a xanthene fluorophore is applicable to the ratiometric imaging of metal ions under live-cell conditions.


Synthesis of 5-3
A solution of 5-1 S3 (2.20 g, 3,81 mmol) and K2CO3 (1.80 g, 13.0 mmol) in MeOH (140 mL)-H2O (7 mL) was heated at 80 °C for 2.5 h with stirring. After removal of the solvent in vacuo, the residue was filtered and washed with EtOH. The filtrate was concentrated by evaporation to give crude 5-2, which was dissolved in EtOH and concentrated by evaporation twice to azeotropically remove H2O. To a solution of crude 5-2 in dry MeOH (30 mL) added 2-formylpyridine (898 mg, 8.38 mmol) dissolved in dry MeOH (10 mL), and the mixture was stirred overnight at rt. Sodium borohydride (361 mg, 9.53 mmol) was added portionwise at 0 °C and the mixture was further stirred for 3 h at rt. After removal of the solvent by evaporation, the residue was diluted with sat.
NaHCO3 aq. and extracted with CHCl3 (x2). The combined organic layers were dried over Na2SO4 and concentrated in vacuo. The residue was purified by column chromatography on SiO2 (CHCl3:

Synthesis of 5-4
To a solution of 5-3 (40.4 mg, 0.10 mmol), 2-1 (38 mg, 0.10 mmol) and triethylamine (29 µL, 21.1 mmol) in dry CHCl3 (15 mL) was stirred at 40 °C for 24 h. After dilution with CHCl3, the organic layer was washed with sat. NaHCO3 aq. and brine followed by drying over Na2SO4. After removal of the solvent in vacuo, the residue was purified by column chromatography on SiO2

Synthesis of 6-2
To a solution of 6-1 (1.57 g, 12 mmol) in dry MeOH (35 mL) was added 2-formyl pyridine (2.57 g, 24 mmol) and the mixture was stirred overnight at room temperature. Sodium borohydride (1.30 g, 34.3 mmol) was added and the mixture was further stirred overnight at rt. After removal of the solvent by evaporation, the residue was diluted with water and extracted with CHCl3 followed by drying over Na2SO4. The organic layer was concentrated in vacuo and the residue was purified by column chromatography on SiO2 (CHCl3 : MeOH : NH3 aq. = 10 : 1 : 0.1) to give 6-2 (1.01 g, 27%) as a colorless oil. 1

Synthesis of 8-5
To a solution of 8-4 (80.5 mg, 0.21 mmol) in dry DMF (4 mL) was added dropwise thionyl chloride (80 µL, 1.10 mmol) at rt. The mixture was stirred for 15 min at rt. After dilution with AcOEt, the organic layer with sat. NaHCO3 aq. and brine followed by drying over Na2SO4. After removal of the solvent in vacuo, the residue was purified by column chromatography on SiO2

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Synthetic procedure of probe 9

Synthesis of 9-3
To a solution of 9-2 (51 mg, 46 µmol) in dry THF (2 mL) was added tetrabutylammonium fluoride in THF (1 M; 200 µL, 200 µmol), and the mixture was stirred for 1 h at rt. After removal of the solvent in vacuo, the residue was dissolved in dry EtOH (3 mL). 2,3-Dichloro-5,6-dicyano-pbenzoquinone (11 mg, 48 µmol) was added and the mixture was stirred for 15 min at rt. After removal of the solvent in vacuo, the residue was purified by column chromatography on SiO2

Synthesis of 10-7
To an ice-cooled solution of crude 10-6 (135 mg) in dry THF (12 mL) was added dropwise borane-THF complex (0.89 M in THF solution, 2.88 mL, 2.56 mmol), and the mixture was stirred for 30 min at 60°C. After quenching the reaction with H2O at 0 °C, the resultant mixture was extracted with AcOEt (x2). The combined organic layers were washed with sat. NaHCO3 aq. and brine followed by drying over Na2SO4. The solvent was removed in vacuo to give 10-7 (

Synthesis of 11-3
To a cooled (-78 °C) solution of 11-2 (891 mg, 3.0 mmol) in dry THF (9 mL) was added 1.6 M n-butyllithium solution in hexane (2.2 ml, 3.4 mmol). After stirring for 20 min at -78°C, dichlorodimethylsilane (291 µL, 3.0 mmol) was added dropwise and the mixture was stirred for 10 min at -78°C. The mixture was slowly warmed to rt and further stirred for 5 min at rt. After quenching the reaction with H2O, the resultant mixture was diluted with AcOEt. The organic layer was washed with sat. NaHCO3 aq. and brine followed by drying over N2SO4. After removal of the solvent in vacuo, the residue was purified by column chromatography on SiO2 (hexane : AcOEt
The organic layer was washed with NaHCO3 aq. and brine followed by drying over N2SO4.

Synthesis of 11-7
To an ice-cooled solution of lithium borohydride (33.9 mg, 1.60 mmol) in dry THF (6 mL) was slowly added the crude 11-6 (401 mg, 0.79 mmol) dissolved in dry THF (12 mL). The mixture was stirred for 30 min at rt. 1N NaOH aq. (4 mL) and MeOH (4 mL) was added at 0 °C, and the mixture was further stirred for 20 min at rt. After dilution with sat NaHCO3 aq., the resultant mixture was extracted with ethyl acetate (x2). The combined organic layers were washed with sat.