9,10-Azaboraphenanthrene-containing small molecules and conjugated polymers: synthesis and their application in chemodosimeters for the ratiometric detection of fluoride ions

Abstract

The introduction of main group elements into conjugated scaffolds is emerging as a key route to novel optoelectronic materials. Herein, an efficient and versatile way to synthesize polymerizable 9,10-azaboraphenanthrene (BNP)-containing monomers by aromaticity-driven ring expansion reactions between highly antiaromatic borafluorene and azides is reported, and the corresponding conjugated small molecules and polymers are developed as well. The BNP-containing small molecules and conjugated polymers showed good air/moisture stability and notable fluorescence properties. Addition of fluoride ions to the BNP-based small molecules and polymers induced a rapid change in the emission color from blue to green/yellow, respectively, accompanied by strong intensity changes. The conjugated polymers showed better ratiometric sensing performance than small molecules due to the exciton migration along the conjugated chains. Further experiments showed that the sensing process is fully reversible. The films prepared by solution-deposition of BNP-based compounds in the presence of polycaprolactone also showed good ratiometric sensing for fluoride ions.

Scheme 1. Aromaticity-driven ring expansion route to 9,10-azaboraphenanthrene-containing conjugated small molecules and polymers.

Scheme 2. Synthesis of BNP derivatives 5 and 6.

Fig. 1. X-ray crystal structure of 6. Selected bond lengths (Å): N(1)–C(12), 1.412(4); N(1)–B(1), 1.414(5); N(1)–C(19), 1.452(5); B(1)–C(1), 1.551(6); and B(1)–C(14), 1.576(5). Bond angles (deg): C(12)–N(1)–B(1), 123.2(3); C(12)–N(1)–C(19), 116.1(3); B(1)–N(1)–C(19), 120.7(3); N(1)–B(1)–C(1), 116.0(3); N(1)–B(1)–C(14), 121.8(3); and C(1)–B(1)–C(14), 122.2(3).

Scheme 3. Synthesis of the BNP-containing molecule 8 and polymers P1 and P2 by the Suzuki–Miyaura or Stille cross-coupling.

Fig. 2. Experimental orbital energy levels and HOMO/LUMO orbital plots of 5, 8, P1 and P2.

Fig. 3. Spectral changes in the UV/vis absorption (left) and fluorescence (right) after the addition of TBAF. (a and b) 5, λ_ex = 300 nm, [5] = 10 μM; (c and d) 8, λ_ex = 300 nm, [8] = 10 μM; (e and f) P1, λ_ex = 350 nm, [P1] = 10 μM; and (g and h) P2, λ_ex = 365 nm, [P2] = 10 μM. [F^–] = 1 mM.

Fig. 4. Stern–Volmer plots of fluorescence intensity (I₀/I) and lifetime change (τ₀/τ) as a function of [F^–] of 5 (λ_em = 355 nm) (a) and P2 (λ_em = 458 nm) (b) in THF upon addition of n-Bu₄NF.

Fig. 5. (a–d) Complexation of 5, 8, P1 and P2 (1 × 10^–5 M in THF) with different anions (THF, 5 equiv. of Bu₄N⁺ salts, 0.01 M).

Fig. 6. (a) Proposed interaction of 5 with fluoride anions; (b) illustration of the predicted shape changes for 5 upon fluoride ion binding; and the absorption (c) and emission (d) spectra of 5 in THF (solid line) upon addition of fluoride ions (dashed-dotted line) and BF₃·OEt₂ (dashed line).

Fig. 7. The 5 and P2-based film sensors for fluoride anions under UV irradiation (λ_ex = 365 nm).