Synthesis of indolo[3,2-b]carbazole-based new colorimetric receptor for anions: A unique color change for fluoride ions

Abstract

A novel indolocarbazole-based chemosensor 1 containing hydrogen bond donor moieties has been established as a selective colorimetric and fluorometric sensor for F⁻ in CH₃CN/H₂O (4:1 v/v). Upon the addition of a series of tetrabutylammonium salts to receptor 1 in aqueous CH₃CN, only when the counter ion was F⁻ was a significant color change (from light violet to dark orange) observed.

Keywords: anion binding; colorimetry; fluorescence quenching; fluoride binding; indolocarbazole.

Figure 1

The structure of the indolocarbazole-based chemosensor 1.

Scheme 1

Synthesis of receptor 1.

Figure 2

The AM1 optimized structure of receptor 1 (heat of formation = −8.29 kcal/mol).

Figure 3

Color changes of receptor 1 (A) (c = 1.1 × 10⁻⁴ M) in CH₃CN/H₂O (4:1 v/v) on addition of tetrabutylammonium salt (TBAX), X = F⁻ (B), Cl⁻ (C), Br⁻ (D), I⁻ (E), AcO⁻ (F), HSO₄⁻ (G), and H₂PO₄⁻ (H) (left side); green fluorescence observed on addition of F⁻ ion to receptor 1 (right side).

Figure 4

UV spectral change of receptor 1 (c = 1.1 × 10⁻⁴ M) upon gradual addition of [Bu₄N]⁺F⁻ (left side) and [Bu₄N]⁺AcO⁻ (right side) in CH₃CN/H₂O (4:1 v/v) (c = 1.1 × 10⁻⁴ M).

Scheme 2

Schematic representation (the circles represent the indolocarbazole moiety) of the two-step process leading to receptor deprotonation with basic fluoride anions.

Figure 5

The Job plot of 1 with fluoride ion from UV method in CH₃CN/H₂O (4:1 v/v).

Figure 6

Fluorescence change of receptor 1 (c = 4.475 × 10⁻⁵ M) upon gradual addition of [Bu₄N]⁺F⁻ (left side) and [Bu₄N]⁺AcO⁻ (right side) in CH₃CN/H₂O (4:1 v/v) (c = 4.475 × 10⁻⁵ M) (λ_max = 443 nm).

Figure 7

Binding constant calculation curves for receptor 1 vs F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, HSO₄⁻, and H₂PO₄⁻ (left side); fluorescence spectra of 1 after addition of 1:1 equivalent of receptor and anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, HSO₄⁻, and H₂PO₄⁻) (right side).

Figure 8

¹H NMR spectra of receptor 1 (bottom), 1 with [Bu₄N]⁺F⁻ 1:2 [receptor 1:(Bu₄N)⁺F⁻] (middle) and excess [Bu₄N]⁺F⁻ (top).