Carbazole-Based Colorimetric Anion Sensors

Abstract

Owing to their strong carbazole chromophore and fluorophore, as well as to their powerful and convergent hydrogen bond donors, 1,8-diaminocarbazoles are amongst the most attractive and synthetically versatile building blocks for the construction of anion receptors, sensors, and transporters. Aiming to develop carbazole-based colorimetric anion sensors, herein we describe the synthesis of 1,8-diaminocarbazoles substituted with strongly electron-withdrawing substituents, i.e., 3,6-dicyano and 3,6-dinitro. Both of these precursors were subsequently converted into model diamide receptors. Anion binding studies revealed that the new receptors exhibited significantly enhanced anion affinities, but also significantly increased acidities. We also found that rear substitution of 1,8-diamidocarbazole with two nitro groups shifted its absorption spectrum into the visible region and converted the receptor into a colorimetric anion sensor. The new sensor displayed vivid color and fluorescence changes upon addition of basic anions in wet dimethyl sulfoxide, but it was poorly selective; because of its enhanced acidity, the dominant receptor-anion interaction for most anions was proton transfer and, accordingly, similar changes in color were observed for all basic anions. The highly acidic and strongly binding receptors developed in this study may be applicable in organocatalysis or in pH-switchable anion transport through lipophilic membranes.

Keywords: anion receptors; anion recognition; anion sensors; colorimetric sensors; supramolecular chemistry.

Figure 1

Model receptors investigated in this study.

Scheme 1

Synthesis of receptors 1 and 2 [19,22]. Reagents and conditions: (a) SO₂Cl₂, CH₂Cl₂, RT, 60%. (b) HNO₃ (100%), Ac₂O/AcOH, 1–110 °C, 73%. (c) H₂ (balloon), 5%Pt(S)/C (cat.), CH₃CN, RT, 90%. (d) NH₂NH₂, 10%Pd/C (cat.), EtOH, reflux, 6 h, 75%. (e) (CH₃)₃CH₂COCl, Et₃N, CH₃CN, RT, 40–88%.

Scheme 2

The synthesis of receptor 3. Reagents and conditions: (a) HNO₃ (100%), Ac₂O/AcOH, 1 °C to 110 °C, 12 h, 66%; (b) NH₂NH₂·H₂O, FeSO₄·7H₂O, EtOH, reflux, 48 h, 20%; (c) (CH₃)₃CH₂COCl, DMA, RT, 24 h, 36%.

Scheme 3

Nitration of 3,6-dicyanocarbazole 10. Reagents and conditions: (a) HNO₃ (100%)/Ac₂O/ AcOH, various temperatures.

Scheme 4

Synthetic pathway leading to the receptor 4. Isolated yields are given in parentheses. Reagents and conditions: (a) NH₂NH₂·H₂O, FeSO₄·7H₂O, EtOH, reflux, 20h; (b) 3,3-dimethylbutyryl chloride, DMA, RT, 20h.

Figure 2

¹H NMR titration curves of the (a) NH_carb. and (b) NH_amide signals of receptors 1–4 (10 mM) for TBACl in DMSO-d₆ + 0.5% H₂O at 298 K. The 1:1 modeled fits are shown as solid lines.

Figure 3

Crystals structures of (a) 2 × Cl⁻, (b) 3 × Cl⁻ and (c) 4 × Cl⁻.

Figure 4

UV-vis titration of: (a) 3 (10⁻⁴ M) with TBAPhCOO (top) and TBAH₂PO₄ (bottom); (b) 4 (10⁻⁴ M) with TBAPhCOO (top) and TBAH₂PO₄ (bottom) in DMSO + 0.5% H₂O at 298 K.

Scheme 5

Self-ionization of receptors 3 and 4.

Figure 5

Self-ionization studies. (a) Self-ionization of diluted solutions of 3 in DMSO + 0.5% H₂O at 298 K; (b) reversal of self-ionization of 4 (10⁻⁴ M) upon titration with TfOH in DMSO + 0.5% H₂O at 298 K.

Figure 6

Fluorescence color changes of 3 (left) and 4 (right) at 10⁻⁴ M in the presence of 2 equiv. of TfOH or 2 equiv. of TBAOH in DMSO + 0.5% H₂O observed under a UV lamp (365 nm).

Figure 7

UV-vis spectra of receptors 1–4 (10⁻⁴ M) in the presence of 2 equiv. of TfOH in DMSO + 0.5% H₂O at 298 K. Inset: the same solutions under visible light and under UV irradiation (365 nm).

Figure 8

Color changes of 10⁻⁴ M solutions of 3 and 4 in the presence of 10 equiv. of various anions (added as TBA salts) in DMSO + 0.5% H₂O: (a) under ambient light and (b) under a UV lamp (365 nm).