2-(4-Boronophenyl)quinoline-4-carboxylic acid derivatives: Design and synthesis,aggregation-induced emission characteristics,and binding activity studies for D-ribose with long-wavelength emission

Abstract

Long-wavelength fluorescent sensors with large Stokes shifts show useful applications in chemical biology and clinical laboratory diagnosis. We have recently reported [4-(4-{[3-(4-boronobenzamido)propyl]carbamoyl}quinolin-2-yl)phenyl]boronic acid that can selectively recognize d-ribose in a buffer solution of pH 7.4. However, the short emission wavelength (395 nm) and aggregation-caused quenching effect are not conducive to applications as a sensor. Novel diboronic acid compounds are synthesized using 2-(4-boronophenyl)quinoline-4-carboxylic acid as the building block and p-phenylenediamine as the linker. These compounds show aggregation-induced emission and fluorescence emission at about 500 nm. In addition, after binding to most carbohydrates, the aggregated state of the boronic acid–containing compounds is dissociated, resulting in fluorescence quenching. Using [4-(4-{[4-(3-borono-5-methoxybenzamido)phenyl]carbamoyl}quinoline-2-yl)phenyl]boronic acid as an example, addition of 55 mM of d-ribose resulted in the strongest quenching of 83% for all the tested carbohydrates, indicating selectively recognizing d-ribose. The reciprocal of the fluorescence intensity change showed a good linear relationship with the reciprocal of d-ribose concentration (R² ⩾ 0.99), indicating sensor binding to d-ribose in a ratio of 1:1 to form an inclusion complex. The fluorescence emission is red-shifted compared to 2-(4-boronophenyl)quinoline-4-carboxylic acid and its common derivatives, which provides a new method for the development of long-wavelength fluorescent sensors.

Introduction

As luminescent materials, organic compounds with good optical properties and biocompatibility^1–3 are widely used in various research fields such as in light-emitting diodes, fluorescent sensors,^4–6 and biosensors.^7–9 Due to the hydrophobic conjugated system, traditional fluorescent molecules generally have a large emission wavelength and a high luminous efficiency in a single-molecule dispersion state. But when the molecules are in a state of aggregation, intermolecular interactions are enhanced, and the non-radiative transition is increased. The luminescence ability is weakened or even disappears, and this phenomenon is named “aggregation-caused quenching” (ACQ).^10,11 ACQ tends to limit the applications of organic light-emitting materials. To avoid the ACQ phenomenon, the development of compounds with strong luminescence in the aggregated state as fluorescent sensors has become an interesting topic of research.^2,12

In 2001, Tang reported ¹³ that a silicon heterocyclic pentadiene compound had weak fluorescence in the solution state, and that the fluorescence intensity increased sharply along with the turbidity due to a change of the solvent’s polarity leading to compound aggregation, and while the quantum efficiency increased remarkably. Subsequently, this phenomenon was named aggregation-induced emission (AIE).^14,15 The restriction of intramolecular rotation (RIR) was identified as the main cause for enhancing the fluorescence. ¹⁶ When dissolved in a suitable solvent, the benzene ring undergoes intramolecular rotation with its single bond between the other rings as an axis, consuming excitation energy. However, when the internal rotation of the molecule is limited in the aggregate state, the benzene ring rotation amplitude is greatly reduced, and energy is released in the form of light. ¹⁷ Typical AIE compounds are mainly tetraphenylethylene, distyrylfluorene, hexaphenylsilole, and so on.^18–21 Fluorescent sensors are usually constructed using these structures as the parent nucleus in order to recognize analytes and to generate biological images.

In addition to chemical synthesis intermediates and enzyme inhibitors, ²² boronic acids can be used as a biochemical tool for various purposes, including interference signaling pathways and cell delivery systems. ²³ Moreover, due to the characteristics of the interaction of boronic acid with cis-diols, there are many reports on the design and synthesis of boronic acid sensors.^24,25 In recent years, boronic acid–based sensors have been widely developed as chemsensors to detect d-glucose, d-ribose, catecholamines, and so on.^26,27 The sensors bind selectively to sialyl Lewis A/X, that is, specific carbohydrates in tumor cells, and enable tumor cell fluorescein imaging and early diagnosis of tumors to be realized. ²⁸

2-(4-Boronophenyl)quinoline-4-carboxylic acid (PBAQA) (1) is a water-soluble fluorescent sensor reported by Wang and colleagues. ²⁹ The fluorescence intensity of PBAQA increased significantly after binding to the tested carbohydrates, including d-fructose, d-mannose, d-galactose, and d-glucose. Our group has performed a lot of work on the synthesis and derivatization of PBAQA. ³⁰ Recently, we reported that [4-(4-{[3-(4-boronobenzamido)propyl]carbamoyl}quinolin-2-yl)phenyl]boronic acid (BPCQB) recognizes d-ribose in a buffer solution of pH 7.4. The structures of PBAQA and BPCQB are shown in Figure 1. However, they have short emission wavelengths (395 nm) and small Stokes shifts, leading to many limitations in detection and fluorescence imaging.

Figure 1.

The structures of PBAQA and BPCQB.

In this paper, we describe the synthesis of novel diboronic acid compounds 7a–c using PBAQA as the building block and p-phenylenediamine as the linker (Scheme 1). These compounds showed AIE and fluorescence emission at about 500 nm. The AIE effects of 7a–c were very significant and the fluorescence emission wavelengths of the compounds were red-shifted to 500 nm. The fluorescence intensity of the compounds decreased after binding to d-ribose, d-maltose, d-mannose, and d-glucose. This was completely opposite to the fluorescence phenomenon of PBAQA and carbohydrates reported in the literature. In particular, after compound 7c combined with d-ribose, the fluorescence is quenched to a higher degree. There was a good linear relationship in the range of 0.00447–0.04646 M and the binding ratio of 7c and d-ribose was determined to be 1:1. This result indicated that compound 7c binds to d-ribose selectively. Hence, the reported problem ³¹ that the fluorescent sensor previously had a short emission wavelength was solved.

Scheme 1.

Synthetic route to the target compounds: (i) DMT-MM, rt, 20 h; (ii) (Boc)₂O; (iii) HCl, rt, 18 h; and (iv) DMT-MM, rt, 20 h.

Results and discussion

The synthetic procedure for 2-(4-boronophenyl)quinoline-4-carboxylic acid was based on the previously reported synthetic method.^30,31 In this work, a highly efficient condensing agent, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM), was used as a condensing agent for the amide formation. This condensing agent is cheap and efficient. In addition, the low toxicity and low boiling point of the alcohol used as the solvent and the simple post-treatment of the reaction are advantages. There was no irritating odor during the reaction and fewer by-products formed. After the reaction was complete, water was added to the reaction mixture to produce a precipitate. The crude product was filtered and washed with water. Furthermore, direct recrystallization generally resulted in a product of higher purity. The method avoided the use of cumbersome column chromatography and overcame a major problem in the purification process of the diboronic acid.

Fluorescent properties of the diboronic acid compounds

The maximum emission wavelength of 1 was around 395 nm. ²⁹ The absorption spectra of compounds 7a–c are shown in Figure S1 and the maximum absorption wavelength of the compounds occurred at around 350 nm. The maximum emission wavelength of compounds 7a–c occurred at around 500 nm when the excitation wavelength was set at 350 nm (Figure 2). Compared with 1, the fluorescence emission wavelengths of compounds 7a–c were red-shifted by about 90 nm. Compounds 7a–c have large rigid structures, which causes the electron energy levels to become small, and the radiation light energy decreases as the energy level difference decreases. The maximum emission wavelength shifted to a long wavelength. The Stokes shift of 1 was around 50 nm. The excitation wavelength of compounds 7a–c was 350 nm and the Stokes shifts were more than 100 nm. The larger Stokes shifts could be detected spectrally at low signal intensities and overcame some of the interference from the fluorescent background to clearly identify the detected signal. Among them, the maximum emission wavelength of 7c was slightly larger than the emission wavelengths of 7a and 7b. The maximum fluorescence emission wavelength was red-shifted due to the increase in the degree of conjugation of compound 7c with the electron-donating substituent. Compound 7c had an electron-donating substituent, which resulted in an increased degree of conjugation and a red shift in the maximum fluorescence emission wavelength.

Figure 2.

Fluorescence spectra of compounds 7a–c in MeOH–90% H₂O solution (c = 5 × 10⁻⁵ M, λ_ex = 350 nm, slit: 5/5).

Diboronic acid AIE effects

At present, the most common method for identifying whether an organic compound has an AIE effect is to add a poor solvent to a good solvent of a luminescent compound. The fluorescent compound is insoluble in the poor solvent. When the content of the poor solvent reaches a certain level, it will cause aggregation or even precipitation of the fluorescent compound. An organic luminescent compound with the AIE effect is weakly fluorescent in a good solvent. The fluorescence intensity was greatly enhanced when the compound aggregated. ³² PBAQA has a strong fluorescence in methanol and the fluorescence intensity did not change much with the change of water content. When the water content exceeded 50%, the fluorescence intensity decreased (Figure 3(a)). A solution of PBAQA with different water content could not be observed clearly aggregation or fluorescence with the naked eye under an ultra-violet (UV)-lamp (365 nm), which indicated that PBAQA did not have AIE phenomenon (Figure 3(b)).

Figure 3.

(a) Fluorescence spectra of 1 in MeOH–H₂O with different solvent ratios (c = 5 × 10⁻⁵ M, λ_ex = 337 nm, slit: 5/5). (b) Photographic images of 1 in different ratios of MeOH–H₂O under a UV-lamp (λ_ex = 365 nm).

Compounds 7a–c show significant fluorescence intensity changes in MeOH–H₂O with different ratios (Figures 4 and 5). Compound 7a had no fluorescence intensity in methanol solution. A significant fluorescence peak appeared at 500 nm when the water content in the solution reached 70%. As the water content in the solution increased by more than 70%, the fluorescence intensity decreased. When the volume fraction of water was 70%, the fluorescence intensity reached a maximum.

Figure 4.

Fluorescence spectra of 7a, 7b, and 7c (10^–4 M) in MeOH–H₂O with different solvent ratios (c = 5 × 10⁻⁵ M, λ_ex = 350 nm, slit: 5/5).

Figure 5.

Photographic images of 7a, 7b, and 7c in different ratios of MeOH–H₂O under a UV-lamp (c = 5 × 10⁻⁵ M, λ_ex = 365 nm).

The AIE of 7b could also be observed visually under a UV-lamp when the water content reached 70%. Moreover, when the water content of the solution of 7b was less than 50%, no fluorescence occurred. When the water content rose to 70%, the fluorescence was significantly enhanced and a new fluorescent peak appeared at 500 nm. As the water content in the solution was further increased, the intensity of the fluorescence peak of 7b continuously increased. When the volume fraction of water in the solution reached 90%, the fluorescence intensity reached a maximum. At 365 nm (UV-lamp), it was observed that 7b aggregated at a water content of 70%, resulting in green fluorescence. Compared 7c also exhibited different degrees of AIE effects. There was no obvious fluorescence when the water content was 70%. When the water content was 80%, the fluorescence was significantly enhanced, and the maximum emission peak was at 508 nm.

Subsequently, time-resolved fluorescence measurements were introduced to investigate the AIE effect. The luminescence lifetimes of 7a in MeOH–90% H₂O (Figure 6(a)) and pure MeOH solution (Figure 6(b)) were measured at an emission maximum of about 500 nm with average lifetime values of 1.2483 and 1.1028 ns, respectively (Table 1). It was found that the luminescence lifetime of 7a in MeOH–90% H₂O was longer than that in pure MeOH. The data for 7a in MeOH–90% H₂O is best fitted to a multiexponential decay curve with an χ ² value close to 1.00.

Figure 6.

Time-resolved fluorescence decay profile of 7a (10^–4 M) monitored at the emission maximum in MeOH–90% H₂O (a) and pure MeOH (b) solution.

Table 1.

Time-resolved fluorescence decay parameters of 90% H₂O–MeOH and pure MeOH solution. ^a

System	B 1	τ1 (ns)	B 2	τ2 (ns)	<τ> a (ns)	χ2
MeOH–90% H2O	3036.891	0.9266	335.203	2.382	1.2483	1.053
Pure MeOH	3120.943	1.1028	–	–	1.1028	0.784

a

$<\tau >=(B_1{\tau }_1^2+B_2{\tau }_2^2+B_3{\tau }_3^2)/(B_1{\tau }_1+B_2{\tau }_2+B_3{\tau }_3)$ ; the magnitude of χ² denotes the goodness of the fit.

The solvent effect

The solvent effect of the compounds was further investigated. Taking 7a as an example, the fluorescence emission change of compound 7a in different solvents under the same conditions was investigated (Figure 7(a)). Compound 7a had the strongest fluorescence intensity in the highly polar solvent acetonitrile. On the contrary, the fluorescence intensity was weaker in a weakly polar solvent. The effect of different solvents can be clearly seen as different fluorescence colors under the UV-lamp (Figure 7(b)). It can also be seen that the position of the fluorescent emission peak was red-shifted with the polarity of the solution. This might occur due to intramolecular charge transfer (ITC). Compound 7a had a significant solvent effect. This illustrated that the solvent had a significant effect on the molecular ground state of 7a. Compound 7a had a large Stokes shift and good fluorescence properties in aqueous solution (Table 2) and has the potential to be applied to certain substances detected in biological fluids.

Figure 7.

(a) Fluorescence spectra of compound 7a in various solvents (c = 5 × 10⁻⁵ M, λ_ex = 350 nm, slit: 5/5). (b) Photographic images of 7a in various solvents under a UV-lamp (c = 1 × 10⁻⁵ M; λ_ex = 365 nm). The order is H₂O, MeCN, CH₂Cl₂, propanone, EtOAc.

Table 2.

Stokes shifts of compound 7a in different solvents.

Solvent	Propanone	CH2Cl2	MeCN	EtOAc	H2O
Stokes shift (nm)	105	115	107	94	150

Selective fluorescence detection of carbohydrates

Boronic acids can combine with carbohydrates to produce different fluorescence changes. Therefore, boronic acids can be used for fluorescent sensors of carbohydrates. A stock solution (10^–3 M) was prepared using methanol as the solvent. Then 1 mL of the stock solution was diluted to 20 mL using 0.1 M phosphate buffer (pH = 7.4) to give a blank sensor solution (5 × 10⁻⁵ M). A carbohydrate-sensor solution was prepared by adding the carbohydrate to the blank sensor solution (0.15 M). The blank sensor solution (1 mL) was added to a cuvette, and then different volumes of carbohydrate-sensor solution were added to obtain different concentrations of the carbohydrate-sensor solution. The fluorescence spectra were subsequently recorded.

In order to study the ability of compounds 7a–c to recognize carbohydrates, concentrations of 0.03 M of d-ribose, d-glucose, d-mannose, d-glycosamine, d-maltose, and sialic acid were added to 7a–c solutions (5 × 10⁻⁵ M), respectively. According to the literature, the fluorescence intensity of 1 increased after binding to carbohydrates. ²⁹ Compared with 1, there was a different binding mechanism for the combination of compounds 7a–c and carbohydrates. This response mechanism was that after compounds 7a–c had been combined with carbohydrates, the equilibrium of the compound in solution is affected, breaking the aggregation of the compound and causing the fluorescence to decrease. This mechanism has been reported previously. ³³ Furthermore, the d-ribose-induced dissociation of the nanoassembly was also evidenced by a dynamic light scattering (DLS) study to measure particle dimensions, in which the incorporation of d-ribose formed precise ensembles with smaller dimensions (from 2461 to 1081 nm), as shown in Figure 8.

Figure 8.

Changes in the hydrodynamic diameters of 7a (5 × 10⁻⁵ M) upon the addition of d-ribose (0.03 M) in MeOH/H₂O (1:20, v/v, pH = 7.4, phosphate buffer, 0.1 M).

Interestingly, when the compounds were combined with d-ribose, d-maltose, d-mannose, and d-glucose, the fluorescence intensity decreased, as shown in Figure S2 and Figure 9. D-ribose caused the most significant fluorescence quenching. Among the studied compounds, 7c did not respond significantly to glucosamine and sialic acid. Moreover, 55 mM of d-ribose resulted in the strongest quenching of 83% for all the tested carbohydrates, indicating selective recognition of d-ribose.

Figure 9.

Variation of the fluorescence intensity of 7c (5 × 10⁻⁵ M) upon addition of various carbohydrates (0.03 M) in MeOH/H₂O (1:20, v/v, pH = 7.4, phosphate buffer, 0.1 M).

The binding properties of 7c to d-ribose were further investigated by fluorescence titrations in a phosphate buffer solution, as shown in Figure 10(a). With an increase in the d-ribose concentration, the fluorescence intensity of 7c gradually decreased.

Figure 10.

(a) Fluorescence response of 7c (5 × 10⁻⁵ M) with increasing d-ribose concentration (0–0.04646 M) in MeOH/H₂O (1:20, v/v, pH = 7.4, phosphate buffer, 0.1 M). (b) Benesi–Hildebrand plot of 7c 1/(I – I₀) versus 1/[d-ribose].

The reciprocal of fluorescence intensity quenching showed a good linear relationship with the reciprocal of d-ribose concentration in the range of 0.00447–0.04646 M (Figure 10(b)). The binding constant value is determined by processing the emission intensity data according to the Benesi–Hildebrand equation, as follows

$$\frac{1}{I-I_0}=\frac{1}{I_1-I_0}+\frac{1}{\left(I_1-I_0\right)K_a{\left[C\right]}_{D-ribose}}$$

where I₀ is the initial fluorescence intensity of the free guest, I₁ is the intensity of the guest–host complex, I is the observed fluorescence intensity of the guest and the guest–host mixture, and K_a is the ground-state association constant for 1:1 complex formation. [C] _D-ribose is the d-ribose concentration. Using the curve of (I₁ – I₀)/(I – I₀) versus 1/[C] _D-ribose , the K_a value was calculated from the intercept/slope.

The binding constant (K_a) is 51.8 M^–1, and the linear correlation coefficient (R) ² reaches 0.99, as shown in Figure 10(b). Due to the plot processed by the Benesi–Hildebrand equation and which exhibited a good linear relationship, the binding ratio of 7c to d-ribose is determined to be 1:1. The binding constants of d-maltose, d-mannose, and d-glucose are 30.0, 20.9, and 37.1 M^–1 respectively, as shown in Figure S3–S5. Sialic acid and D-glycosamine have no linear relationship with compound 7c. This suggests that 7c may serve as a D-ribose selective sensor.

Conclusion

Although their water solubility is good, PBAQA and its derivatives have a short emission wavelength of about 395 nm and demonstrate no AIE effects. Short emission wavelengths impose many limitations on future applications. In this work, novel diboronic acid compounds 7a–c having AIEs were designed and synthesized, and the fluorescent properties were systematically studied. The fluorescence emission wavelength of these compounds is red-shifted to about 500 nm. The position of the boronic acid on the phenyl ring and the different substituent groups result in different degrees of AIE properties. The AIE effect might be ascribed to the fact that the free internal rotation of their benzene rings is hindered, resulting in the production of fluorescence in poor solvents. After binding to the carbohydrates, the aggregated state of the boronic acid–containing compounds is dissociated, resulting in fluorescence quenching. These sensors have two boronic acid recognition sites that can be used to identify carbohydrates, and few fluorescent sensors have been reported to recognize d-ribose. The preliminary binding activity test results showed that 7c had a greater response to d-ribose. There is a good linear relationship in the range of 0.00447–0.04646 M, and the binding ratio of 7c to d-ribose is determined to be 1:1. The sensor 7c emits at a long wavelength of 508 nm and has the potential to recognize d-ribose selectively.

Experimental

Materials

p-Phenylenediamine, 4-bromoacetophenone, eosin, the Pd catalyst, thionyl chloride, bis(pinacol) diboron, potassium acetate, di-tert-butyl dicarbonate, 4-carboxy-phenylboronic acid, and 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM) were purchased from Macklin. All the reagents were obtained from commercial sources and were used without further purification.

Apparatus

Fluorescence spectra were measured using a fluorescence spectrometer (F-280, China). Time-resolved fluorescence decay profile spectra were obtained using a high-resolution spectrofluorometer (FLS980; Edinburgh Instruments, UK) equipped with a 500 W Xenon lamp as an excitation source. The NMR spectra were recorded on a Bruker AM-600 spectrometer (Billerica, MA, USA) with tetramethylsilane as the internal standard. Mass spectra were obtained with an Agilent Trap VL LC/MS spectrometer (Santa Clara, CA, USA). High-resolution mass spectra (HRMS) were recorded on an Agilent 1290LC-6540 Accurate-Mass Q-TOF using electrospray ionization (ESI).

Synthesis

(4-{4-[(4-Aminophenyl)carbamoyl]e-2-yl}phenyl) boronic acid ( 5 ): A mixture of 2-(4-boronophenyl)quinoline-4-carboxylic acid (1) (0.50 g, 1.71 mmol), N-Boc-p-phenylenediamine (0.39 g, 1.88 mmol), and DMT-MM (0.52 g, 1.88 mmol) in methanol (50 mL) was stirred at room temperature for 20 h. The reaction solution was concentrated and poured into 150 mL of ice water. A large amount of solid was produced. The mixture was filtered, and the filter cake was washed three times with water. The crude residue was recrystallized to give a pale yellow powder (4) (MeOH:H₂O = 1:1). To a solution of compound 4 in ethyl acetate (200 mL), hydrochloric acid (4 mL) was added at room temperature. The reaction mixture was stirred at room temperature for 18 h. After completion, the crude product was filtered and washed with ethyl acetate to give compound 5. Yellow solid, yield: 61%, ¹H NMR (600 MHz, DMSO-d₆): δ (ppm) (Figure S9): 11.19 (s, 1H), 8.46 (s, 1H), 8.36 (d, J = 8.3 Hz, 2H), 8.28 (d, J = 8.4 Hz, 1H), 8.21 (d, J = 8.4 Hz, 1H), 8.02 (d, J = 8.0 Hz, 2H), 7.96 (d, J = 8.8 Hz, 2H), 7.92 (t, J = 8.1 Hz, 1H), 7.73 (t, J = 8.1 Hz, 1H), 7.49 (d, J = 8.8 Hz, 2H). ¹³C NMR (151 MHz, DMSO-d₆): δ (ppm) (Figure S10): 165.63, 156.29, 147.39, 143.81, 139.18, 135.16, 131.31, 129.18, 128.30, 127.79, 127.07, 125.66, 124.39, 123.78, 121.53, 118.04. MS (Figure S11): [M + 1]⁺: 384.1.

Synthesis of compounds 7a , 7b , 7c : To a solution of compound 5 (0.17 g, 0.4 mmol) in methanol (15 mL), triethylamine (0.122 mL), 3-carboxyphenylboronic acid (0.075 g, 0.45 mmol), and DMT-MM (0.125 g, 0.45 mmol) were added sequentially at room temperature. The reaction mixture was stirred for 20 h. Subsequently, the reaction solution was concentrated and poured into ice water. The mixture was filtered, and the filter cake was washed three times with water. The crude product was recrystallized from methanol to give 7a. Compounds 7b and 7c were obtained using the same procedure.

[4-(4-{[4-(3-Boronobenzamido)phenyl]carbamoyl}quinol-ine-2-yl)phenyl]boronic acid ( 7a ): White solid, yield: 70%. ¹H NMR (600 MHz, DMSO-d₆): δ (ppm) (Figure S12): 10.90–10.80 (m, 1H), 10.32 (s, 1H), 8.43–8.31 (m, 4H), 8.30–8.17 (m, 6H), 8.07–7.97 (m, 4H), 7.91–7.79 (m, 5H), 7.69 (dt, J = 20.4, 6.6 Hz, 1H), 7.55–7.48 (m, 1H). ¹³C NMR (151 MHz, DMSO-d₆): δ (ppm) (Figure S13): 166.52, 165.55, 156.36, 148.45, 143.53, 139.94, 137.54, 136.04, 135.17, 135.03, 134.76, 133.94, 130.80, 130.15, 129.66, 127.89, 126.75, 125.65, 123.80, 121.16, 120.81, 117.41. HRMS (ESI) (Figure S14): Calculated for C₂₉H₂₄B₂N₃O₆⁺ [M + H]⁺: 532.1846; found: 532.1849.

[4-(4-{[4-(4-Boronobenzamido)phenyl]}carbamoyl)quino-line-2-yl)phenyl]boronic acid ( 7b ): Yellow solid, yield: 62%. ¹H NMR (600 MHz, DMSO-d₆): δ (ppm) (Figure S15): 10.90–10.80 (m, 1H), 10.32 (s, 1H), 8.43–8.31 (m, 4H), 8.30–8.17 (m, 6H), 8.07–7.97 (m, 4H), 7.91–7.79 (m, 5H), 7.69 (dt, J = 20.4, 6.6 Hz, 1H), 7.55–7.48 (m, 1H). ¹³C NMR (151 MHz, DMSO-d₆): δ (ppm) (Figure S16): 165.87, 165.55, 156.36, 148.45, 143.52, 139.93, 136.63, 135.89, 135.17, 134.50, 130.80, 130.16, 127.90, 127.00, 126.75, 125.65, 123.80, 121.25, 120.79, 117.41. HRMS (ESI) (Figure S17): Calculated for C₂₉H₂₄B₂N₃O₆⁺ [M + H]⁺: 532.1846; found: 532.1791.

[4-(4-{[4-(3-Borono-5-methoxybenzamido)phenyl]carba-moyl}quinoline-2-yl)phenyl]boronic acid ( 7c ): Yellow solid, yield: 54%. ¹H NMR (600 MHz, DMSO-d₆): δ (ppm) (Figure S18): 10.87 (s, 1H), 10.69 (s, 1H), 8.87–8.79 (m, 3H), 8.72 (s, 2H), 8.40 (s, 1H), 8.36 (d, J = 8.3 Hz, 2H), 8.21 (t, J = 8.4 Hz, 4H), 8.02 (d, J = 8.2 Hz, 2H), 7.89–7.84 (m, 5H), 7.72–7.68 (m, 1H). ¹³C NMR (151 MHz, DMSO-d₆): δ (ppm) (Figure S19): 165.52, 164.84, 156.35, 156.20, 148.45, 143.50, 139.93, 135.82, 135.16, 130.80, 130.15, 129.20, 127.90, 126.78, 126.75, 126.57, 125.66, 123.80, 120.89, 120.52, 117.53, 117.41, 56.26. HRMS (ESI) (Figure S20): Calculated for C₃₀H₂₆B₂N₃O₇⁺ [M + H]⁺: 562.1951; found: 562.1908.

Supplemental Material

Supplementary_Material – Supplemental material for 2-(4-Boronophenyl)quinoline-4-carboxylic acid derivatives: Design and synthesis, aggregation-induced emission characteristics, and binding activity studies for D-ribose with long-wavelength emission

Supplemental material, Supplementary_Material for 2-(4-Boronophenyl)quinoline-4-carboxylic acid derivatives: Design and synthesis, aggregation-induced emission characteristics, and binding activity studies for D-ribose with long-wavelength emission by Guiqian Fang, Hao Wang, Zhancun Bian, Guimin Zhang, Min Guo, Zhongyu Wu and Qingqiang Yao in Journal of Chemical Research