A novel aggregation-induced enhanced emission aromatic molecule: 2-aminophenylboronic acid dimer

Abstract

Aggregation-induced enhanced emission (AIEE) molecules have significant applications in optoelectronics, biomedical probes and chemical sensors, and large amounts of AIEE molecules have been reported since the concept of AIEE was proposed. Most aromatic AIEE molecules have complex structures consisting of multiple aromatic rings and/or polycyclic skeletons. In this study, we find that 2-aminophenylboronic acid (2-APBA) with a simple structure is highly emissive in the solid state. Further studies reveal that 2-APBA exists in a dimeric form, and the 2-APBA dimer is a novel AIEE molecule. The underlying AIEE mechanism is that the 2-APBA dimeric units aggregate through intermolecular interactions to produce highly ordered molecular packing without the presence of π-π stacking interactions that would lead to aggregation-caused quenching. Furthermore, the 2-APBA dimer aggregates could reversibly transform into its non-fluorescent monomer form driven by new kinds of dynamic covalent B-N and B-O bonds, illustrating its good potential in molecular recognition, nanogating, chemo/bio-sensing and controlled drug release.

Fig. 1. (A) Luminescence behavior of HBQ under the influence of PBA, 2-APBA and Neu5Ac. (B and D) Fluorescence spectra and intensity changes (at 504 nm, inset) of HBQ (5.0 × 10⁻⁵ mol L⁻¹) on addition of various equivalents of PBA (B) or 2-APBA (D) to the water–DMSO (4 : 1, v/v) solution of HBQ. (C and E) Fluorescence spectra and intensity changes (at 504 nm, inset) of the HBQ–PBA mixture (C) or HBQ-2-APBA mixture (E) on addition of various equivalents of Neu5Ac to the water–DMSO (4 : 1, v/v) solution of HBQ–PBA or HBQ-2-APBA, λ_ex = 367 nm. The concentrations of HBQ, PBA and 2-APBA were 5.0 × 10⁻⁵, 3.5 × 10⁻⁴ and 3.5 × 10⁻⁴ mol L⁻¹, respectively.

Fig. 2. (A) Crystal structure of 2-APBA with thermal ellipsoids at 50% probability. The dashed green line represents an intramolecular N–H⋯O hydrogen bond. (B and C) ¹¹B MAS (B) and ¹H–¹⁵N CP-MAS (C) NMR spectra of the solid 2-APBA sample. (D) Explanation of the luminescence behavior of HBQ in response to 2-APBA and Neu5Ac. (E) Two dimers connected through intermolecular O–H⋯N hydrogen bonds (dashed orange lines). (F) Packing mode of 2-APBA viewed along the a axis (CCDC 2024024†). (G) Nearest distance between the phenyl rings (dashed plum lines) in the crystal structure.

Fig. 3. (A) Fluorescence spectra of the 2-APBA dimer (2.9 × 10⁻⁵ mol L⁻¹) in THF–water mixtures with different amounts of water (volume%), λ_ex = 300 nm. The amorphous 2-APBA dimer sample was used for preparing the solutions. The dashed purple line is the fluorescence spectrum of the solid 2-APBA dimer sample. (B and C) Optimized structures of the S₀ and S₁ states of one 2-APBA dimer (B) and four 2-APBA dimers (C).

Fig. 4. (A) Comparison of absolute Φ_F of the 2-APBA dimer, and 3- and 4-APBA in THF, water and the solid state. (B) XRD patterns of the 2-APBA dimer, and 3- and 4-APBA. (C, H and J) SEM images of the samples on the silicon wafer prepared from the aqueous solution of the 2-APBA dimer (C), and 3- (H) and 4-APBA (J). (D, I and K) SEM images of the solid samples of the 2-APBA dimer (D), and 3- (I) and 4-APBA (K). (E and F) XRD pattern (E) and SEM image (F) of the amorphous 2-APBA dimer sample. (G) Fluorescence spectra of crystalline and amorphous 2-APBA dimer samples recorded under identical measurement conditions, λ_ex = 300 nm.

Fig. 5. (A) Fluorescence spectra of the 2-APBA dimer in water with bubbling CO₂ for different times (2.9 × 10⁻⁵ mol L⁻¹). The gas flow rate is 60 mL min⁻¹. λ_ex = 300 nm. (B) Time dependence of the variation of the relative fluorescence intensities at 377.5 nm of the 2-APBA dimer (I/I₀) in water with bubbling the CO₂/N₂ mixture (2.9 × 10⁻⁵ mol L⁻¹). The proportions of CO₂ in CO₂/N₂ mixtures are 10.0% (black), 25.0% (red), 33.3% (green), 50% (blue), 66.7% (cyan) and 100% (magenta), respectively. The total gas flow rate is 60 mL min⁻¹. λ_ex = 300 nm. (C) Fluorescence spectra of the 2-APBA dimer (2.9 × 10⁻⁵ mol L⁻¹) and intensity changes (at 377.5 nm, inset in the upper-right corner) during bubbling N₂ through an aqueous solution of the 2-APBA dimer saturated with CO₂. The inset SEM image in the lower-right corner is of the 2-APBA dimer sample alternately treated with CO₂ and N₂. The gas flow rate is 60 mL min⁻¹. λ_ex = 300 nm. (D) Cycling experiment illustrating the variation of the fluorescence intensities of 2-APBA at 377.5 nm after alternately bubbling CO₂ and N₂ through an aqueous solution of the 2-APBA dimer. (E) Illustration of the reversible transformation of 2-APBA between dimer and monomer aggregates through alternate treatment with CO₂ and N₂.

Fig. 6. (A) Schematic of the design of CO₂-responsive nanochannels by using the 2-APBA dimer. (B) Current–voltage (I–V) response recorded before and after CO₂ and N₂ bubbling through PET nanochannels in KCl solution containing 2-APBA dimer aggregates. I–V curves corresponding to CO₂ and N₂ bubbling are the average of the curves in the eight cycles in (C). (C) Transmembrane ionic current (at −2 V) switching of the PET conical nanochannels in 0.1 M KCl solution containing 2-APBA dimer aggregates upon alternate treatment with CO₂ and N₂.