A 1-hydroxy-2,3,1-benzodiazaborine-containing π-conjugated system: synthesis, optical properties and solvent-dependent response toward anions

Yusuke Satta,† Ryuhei Nishiyabu,† Tony D. James,‡ and Yuji Kubo†*

†Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1, Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan

‡Department of Chemistry, University of Bath, Bath BA2 7AY UK

E-mail: yujik@tmu.ac.jp

ABSTRACT:

Our interest in the functionalization of –OH-substituted azaborines prompted us to synthesize a 1-hydroxy-2,3,1-benzodiazaborine conjugated with 1,8-naphthalimide 1. Its fluorescence was dramatically affected by the nature of the solvent. In particular, the use of DMSO, which has a relatively high donor number (DN = 29.8), led to a remarkable decrease in the fluorescence intensity (ΦF = 0.0014), possibly due to intermolecular hydrogen-bonding interactions (Me2S=O --- HO-B). The presence of the hydroxyl group on boron led to a solvent-driven colorimetric response towards anions; high selectivity for F− over other anions in DMSO, and responded to AcO− and F− in THF, as shown by UV/vis titrations, NMR, and mass spectroscopic analysis. The nucleus-independent chemical shift (NICS) indices suggested that hydrogen bonding interactions between Me2S=O and HO−B reduced the aromaticity of the benzodiazaborine macrocycle, causing an increase in the negative character of the boron. The increase in the polarity of the B−N bond may prevent acetate-binding of 1 in DMSO.

Keywords: Azaborine, 2,3,1-Benzodiazaborine, 1,8-Naphthalimide, Photophysical studies, Anion sensing

Introduction

Azaborines1 produced by replacing the C=C unit with a B−N unit serve as isoelectric aromatic scaffolds where the nitrogen can donate π electrons to the empty p-orbital of boron2 to form a π bond. Theoretical studies indicate that the aromatic nature of borazine is reduced,3 due to the difference in electronegativity between boron and nitrogen. Nevertheless, inspired by intriguing chemical features such as emission and bioavailability, considerable efforts have been devoted towards the synthesis of new aromatic, B−N-containing heterocycles, which may facilitate the creation of pseudoaromatic building blocks with applications in lighting and display technology.4-9 The ramifications of such studies are also attractive. For instance, boratriazoles are 5-membered aromatic boron-containing heterocycles,10-13 being B−N isosteres of imidazoles and pyrazoles. 1,3,2-Benzodiazaboroles14,15 and fused B−N indoles are, on the other hand, isosteres of indoles.16 In the former case, we took advantage of the fluorescence properties to develop cavitand-based sensor materials for the detection of tetraalkylammonium cations.17,18 Alternatively, BN-embedded polycyclic aromatics have been proposed as isosteres of polycyclic aromatic hydrocarbons.19 Their rich optoelectronic properties may have potential applications in electric devices such as organic light-emitting diodes (OLEDs),20-24 organic field effect transistors (OFETs),25,26 and organic solar cells.27 Moreover, the Lewis acidity of three-coordinate boryl heterocycles has prompted chemists to investigate the optical sensing of anions such as fluoride14,28,29 and cyanide ions.14,30-32 Notably, fluoride anion is one of the most important anions due to human health and environmental protection.33 Hydroxylated azaborines34 where –OH directly binds to boron in the ring, are among BN heterocycles. However, their potential as materials remains largely unexplored. To the best of our knowledge, no report has described the anion-responsive function of hydroxylated azaborines. Since the seminal report by Dewar and Dougherty,35 1-hydroxy-2,3,1-benzodiazaborines, B−N isosteres of isoquinoline, have been investigated as potential platforms for the construction of biologically active compounds.36-40 The structural chemistry has been clarified by X-ray analysis.41-43 In this context, Gillingham et al. showed that a Schiff base produced by the condensation of aldehydes with arylhydrazines is formed with an appropriately positioned boron atom, leading to the irreversible formation of aromatic B–N heterocycles.44 Notably, the optical properties of 1-hydroxy-2,3,1-benzodiazaborines have not been elucidated because the B-OH group was considered a potential fluorescence quencher.

In this work, the π conjugated system 1, composed of 1,8-naphthalimide and 1-hydroxy-2,3,1-benzodiazaborine, was synthesized for the first time. Electron deficient 1,8-naphthalimide units have been used not only as π-conjugation acceptors with excellent electron affinities and high charge carrier mobilities,45-49 but also as versatile building blocks for colorimetric and fluorescence chemosensors for cations and anions due to their absorption and emission at long wavelengths.50-52 It was therefore envisaged that such an acceptor group would induce an enhancement of electric perturbation by anions on the boron through π-conjugated system.53 As described below in detail, the π-conjugation character was rationalized by its solvatochromic features, and an increase in the donor number (DN) of the solvent led to a decrease in the fluorescence quantum yield. In particular, DMSO leads to significant quenching of the fluorescence, possibly due to intermolecular hydrogen-bonding interactions between Me2S=O and HO-B. Such solvation impacts the aromaticity and stability of the diazaborine heterocycle and alters the anion-responsive capabilities. Such intriguing feature of 1 as –OH substituted azaborine derivative has been demonstrated.

Results and Discussion

Synthesis and characterization

Scheme 1. Synthesis of 1.

The condensation of hydrazine-appended 1,8-naphthalimide 254 with 2-formylphenylboronic acid in dry EtOH gave 1 as a yellow solid in 44% yield after work-up. The chemical structure of 1 was determined using NMR, mass spectroscopy, and elemental analysis. A broad signal was observed at 27.3 ppm in the 11B NMR spectrum, which was assigned to an sp2 boron. The hydroxyl group bound to boron could be detected by 1H NMR in THF-d8, where a singlet was clearly observed at 7.68 ppm. These data indicate that the initial formation of the hydrazone intermediate was followed by cyclization via unimolecular dehydration44 to produce 1. A crystal of 1 suitable for X-ray diffraction analysis was successfully obtained;55 the analysis was consistent with the proposed structure of 1 shown in Scheme 1. As shown in Fig. 1a, the B‒N bond length was estimated to be 1.45 Å, which was almost equal to the 1.44 Å bond length of borazine.56 Considering the expected bond lengths of B‒N single bonds (1.58 Å) and B=N double bonds (1.40 Å),57 π-electron delocalization occurred in the heterocyclic ring. Indeed, the average root-mean-square deviation value of the benzodiazaborine was 0.0217 Å, indicating that the ring adopted a planar π-conjugation. The benzodiazaborine was tilted by 47.3º with respect to the naphthalimide unit (Fig. 1b), which may have caused the relatively large Stokes shifts in the fluorescence spectra (Table 1; vide infra). As inferred from Fig. 1c, 1 formed a supramolecular dimeric structure in which two molecules of 1 were linked through hydrogen bonding between B-OH and O=C of the naphthalimide unit, with a separation of 2.73 Å. The packing structure revealed that two neighboring molecules formed a π-stacked dimer with an intermolecular distance of 3.54 Å (Fig. 1d).

Fig. 1. (a) X-ray crystal structure of 1, where thermal ellipsoids are drawn at the 50% probability level, showing (b) the side view, (c) the front view and (d) packing structure.

Optical properties

Fig. 2 and Table 1 summarize optical properties of 1 in various solvents. The absorption intensity of 1 varied depending on the identity of the solvent; the molar extinction coefficient (εmax) in THF was 1.65 × 104 M−1 cm−1, which was 1.4 times larger than that in DMSO. However, the λmax value was nearly unaffected upon moving from toluene to DMSO. In regard to the fluorescence spectra, the emission shifted from 428 nm to 473 nm as the polarity of the solvent increased. This trend was rationalized by employing a Lippert-Mataga plot58,59 (Fig. S1), and the change in the static dipole moment of 1 (Δμ) was calculated to be 15.5 D. This value is larger than that of trans-ethyl-p-(dimethylamino)cinnamate, a typical intramolecular charge transfer dye,60 implying that 1 has an efficient D-π-A character when the benzodiazaborine acts as an electron donor. However, the fluorescent quantum yields (ΦF) in solution were found to be low in solvents other than toluene and CH2Cl2, indicating that the optical properties are affected by solvent polarity and DN. In particular, the ΦF value in DMSO was 0.14%, which was 290 times lower than that in CH2Cl2. Such a facile non-radiative pathway strongly suggests that DMSO interacts with 1. This speculation was supported by 1H NMR measurements; when THF-d8 was replaced with DMSO-d6, the proton resonance arising from the hydroxyl group bound to boron was shifted downfield by 1.26 ppm (Fig. S2). We postulated that the B-OH group would act as a hydrogen bonding donor, which may impact the reactivity of boron in 1 (vide infra).

Table 1. Absorption and fluorescence data of 1 at 25 °C.

solvent

λmax (nm)

εa

λemb (nm)

∆sc (cm−1)

ΦFd

toluene

354

1.55

428

4884

0.43

THF

355

1.65

451

5996

0.12

AcOEt

353

1.42

447

5957

0.15

CH2Cl2

354

1.54

444

5726

0.41

acetone

354

1.49

471

7017

0.095

DMSO

359

1.16

473

6714

0.0014

aMolar extinction coefficient, in 104 M−1 cm‒1. bλex = 360 nm. c∆s = (1/λmax) − (1/λem). dValues were determined against a 9,10-diphenylanthracene in cyclohexane (ΦR = 1.0).61

Fig. 2. (a) UV/vis spectra and (b) fluorescence spectra of 1 (20 μM) in various solvents at 25 °C; λex = 360 nm.

The optical properties were rationalized using time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G(d,p) level of theory (Gaussian 09).62 The calculated λmax of the longest absorption band (387 nm) was fairly consistent with the experimental data shown in Table 1. Although the first absorption band was characterized as a mixture of several configurations, it was mainly ascribed to the HOMO−LUMO transition. The corresponding surface plots (Fig. 3) supported the intramolecular charge transfer character from the benzodiazaborine heterocycle to naphthalimide unit. Accordingly, it occurred to us that electric perturbation on the heterocycle core upon interaction with anions may induce a change in the absorption spectra.

Fig. 3. Surface plots of HOMO and LUMO of 1.

Solvent-dependent colorimetric response to anions

Next, we investigated the response of 1 towards anions at 25 ºC in THF, where the ε value was the largest (Table 1). Upon adding various anions such as F−, H2PO4−, AcO−, Cl−, Br−, NO3−, I−, ClO4− and HSO4− as tetrabutylammonium (TBA) salts (2 × 10−5 M) to the THF solution of 1 (1 × 10−5 M), the color of the solution changed from colorless to purplish red with F− and AcO−, and new absorption bands appeared in the visible region (Fig. 4a). However, the use of DMSO instead of THF under similar conditions led to an improvement in anion selectivity, where only F− led to the appearance of an absorption band at 510 nm, although the intensity was relatively lower that in THF (Fig. 4b). The anion-sensing behavior of 1 was dramatically altered upon varying the solvent from THF to DMSO.

Fig. 4. Change in absorption spectra of 1 (dashed trace) (10 μM) upon adding 2.0 equivalents of various anions (F−, H2PO4−, AcO−, Cl−, Br−, NO3−, I−, ClO4− and HSO4−) as tetrabutylammonium (TBA) salts in THF (a) and DMSO (b) at 25 ºC.

In order to obtain further insight into the solvent dependency, we assessed the fluoride sensing of 1 in DMSO. Fig. 5a shows the change in absorption of 1 upon addition of various concentrations of TBAF at 25 ºC. A new absorption band appeared at 510 nm upon addition of F−, while the absorption intensity at 359 nm decreased and an isosbestic point was observed at 412 nm (Fig. S3). Plotting the change in absorption intensity at 510 nm as a function of F− concentration revealed a sigmoidal curvature, as expected for a cooperative binding process (Fig. 5a). Taking into account the formation of complex with a 1:2 stoichiometric ratio for 1:F−, as inferred from a Job’s plot (Fig. 5b), a Hill plot63 was employed to reproduce the sigmoidal curve. The binding and Hill constants were evaluated to be (1.45 ± 0.05) × 105 M−1 and 1.8 (R2 = 0.997), respectively. This suggested that stepwise F− binding occurred, where the mono(fluoride) adduct produced initially prompted the subsequent formation of the bis(fluoride) adduct. The association structures were subjected to 1H NMR measurements (Fig. 6a); in DMSO-d6, the signal intensities of the aryl resonances of 1 decreased as the fluoride ion concentration was increased up to 1 equiv. with respect to 1. Notably, the singlet at 8.94 ppm, which corresponded to the hydroxyl proton, disappeared completely upon addition of 1 equiv. of F−. Meanwhile, further addition of F− led to the appearance of new signals, which were assigned to the bis(fluoride) adduct. Also, under similar conditions, 11B NMR revealed a broad signal at 30.3 ppm, which was shifted up-field by 6.42 ppm, indicating that increasing amounts of F− led to a change in the hybridization of boron from sp2 to sp3 (Fig. 6b). To acquire deeper insight into the binding event, 19F NMR measurements were carried out, and the results are presented in Fig. 6c. The addition of F− (0.5 equiv.) to a DMSO-d6 solution of 1 revealed a peak at −127.7 ppm in addition to a weak peak at −135.4 ppm. Upon addition of ≥ 1 equiv. of F−, the intensity of the peak at −127.7 ppm increased gradually, suggesting that F− bound to the boron of 1 in a step-wise fashion.64 Upon further addition of F− (≥ 2 equiv.), a doublet appeared at −145.0 ppm (J = 150 Hz) due to the formation of bifluoride HF2−.65 Considering the small peaks observed at −101.6 ppm (free F−)64 and −131.5 ppm, additional TBAF may react with water to produce TBA+HF2− and TBA+OH−.65 The OH− would lead to the hydration of fluoride-bound 1, although only slightly.

Fig. 5. (a) Non-linear curve fitting plot obtained from titrations of 1 with F− in DMSO. Absorption intensities of 1 at 510 nm as a function of concentration of F−. (b) Job’s plot depicting binding interaction between 1 (10 μM) and F− in DMSO at 25 °C.

Fig. 6. (a) Partial 1H NMR titration (500 MHz) of 1 (5 mM), (b) 11B NMR (128 MHz) spectra of 1 (25 mM), and (c) 19F NMR (470 MHz) spectra of 1 (5 mM) upon the addition of F− in DMSO-d6 at 25 °C.

A plausible binding process between 1 and F− in DMSO is illustrated in Scheme 2; F− preferably binds to trigonal coordinate boron in 1 through Lewis acid-base interaction to give the tetracoordinate B−F adduct F@1, where exchange reactions between the hydroxyl group and F− provide F2@1. The B−F adduct may strengthen the donor character of benzodiazaborin unit to produce a new absorption band at around 510 nm (vide supra; Fig. 4b). A similar binding behavior between 1 and F− in THF was elucidated using UV/vis titrations, Job’s plot, 1H NMR, and 11B NMR (Fig. S4), although fluoride-dependent 1H NMR data exhibited a somewhat complicated signal pattern owing to the formation of side products (Fig. S4d). The binding constant (K) and Hill coefficient of 1 with F− in THF were estimated to be (1.35 ± 0.06) × 105 M−1 and 1.8 (R2 = 0.995), respectively. The production of F2@1was confirmed by ESI-MS spectroscopy carried out in the negative mode (m/z = 404.1, [F2@1 − TBA]−; Fig. S5); oxygenated product 3 (m/z = 372.1; [3 − H]− ) and related THF adducts (m/z = 454.1; [THF@1 – H] − ) were also detected. 1H NMR titration spectra of 1 with AcO− in THF-d8 revealed the formation of 3 and facilitated the structural assignment (Fig. 8; vide infra).

Scheme 2. Possible reactions of 1 with F− in DMSO.

Next, we focused our attention on why the acetate-triggered color change was observed in THF but not DMSO.66 This result was unexpected because previously reported azaborine-based receptors only responded to F− over other anions.67-69 UV/vis titrations of 1 with AcO− in THF (Fig. 7a and Fig. S7) were carried out, and the corresponding Job’s plot suggested a different binding mode from that in DMSO. As shown in Figure 7b, the Job’s plot revealed a plateau area ranging from 0.5 to 0.7 of [AcO−]/([1] +[AcO−]). However, this curvature would be expected for non-cooperative binding (Fig. 7a). Consequently, the reaction between 1 with AcO− may involve two independent binding sites for 1:1 and 1:2 binding modes between 1 and AcO−. We thus assessed those binding constants using a curve fitting procedure based on the following assumption: H + G ↔ HG; K1 = [HG]/[H][G], HG + G ↔ HG2; K2 = [HG2]/[HG][H], where H = 1, G = AcO−.70 As a result, the experimental data were perfectly reproduced with K1 ≥ 107 M−1 and K2 ≥ 105 M−1 (R2 = 0.999) (Fig. 7b), indicating that strong binding between AcO− and 1 occurred through a two-step association mode in THF. Structural information was obtained from 1H NMR titrations of 1 and AcO− in THF-d8 (Fig. 8). The resonance corresponding to the aromatic proton Hm, which was close to the diazaborine macrocycle, was quite sensitive to the concentration of AcO−; the doublet signal (J = 7.55 Hz) shifted from 8.23 ppm to 8.47 ppm upon addition of 1 equiv. of AcO−. The addition of more than 1 equiv. of AcO− caused a broadening of Hm and a downfield shift of Δδ = 0.37 ppm remarkably. On the contrary, other proton resonances of the benzodiazaborine ring (Hi, Hj, Hk, and Hl) were shifted up field upon addition of ≥ 1.0 equiv. of AcO−. In conjunction with the UV/vis titration results, the association of 1 with AcO− involves at least two binding steps, where upon adding more than 1 equiv. of AcO−, the second step may cause a significant change in the chemical structure of 1. Nevertheless, no change in the chemical shift at 27.3 ppm in the 11B NMR spectrum was observed upon adding AcO− into the THF-d8 solution of 1 (Fig. S8). This suggested that the acetate-binding process may contain two-step equilibrations during which the sp2-hybridization of boron was maintained in 1.

Fig. 7. (a) Binding curve obtained from the spectroscopic titrations of 1 with AcO− at 550 nm. (b) Job’s plot depicting binding interaction between 1 (10 μM) and AcO− in THF at 25 °C.

Fig. 8. Change in 1H NMR spectra (500 MHz) of 1 (5 mM) upon the addition of AcO− in THF-d8 at 25 °C.

In order to obtain further structural information, the sample obtained from the THF solution of 1 with 2.5 equiv. of AcO− was subject to APCI-MS measurements carried out in the positive mode (Fig. S9); peaks due to [(AcO)2@1 – TBA + H] and [(AcO)2@1 – TBA + 2H]+ were observed at 485. 3 (calcd. 485.2) and 486.3 (calcd. 486.2), respectively, and were assigned to the di(acetate) adduct, (AcO)2@1 (Figure S10). Our careful assessment in the low field ranging from 11 to 12 ppm enabled us to detect a small signal due to NH (Ho) proton (Scheme 3) upon addition of ≥ 2.0 equiv. of AcO−, indicating that some of (AcO)2@1 reacted with H2O to occur N−H protonation. Scheme 3 shows a possible reaction pathway between 1 and AcO− in THF. Considering that the trigonal structure of boron in 1 is maintained, the hydroxyl group that binds to the boron could be substituted with AcO− to give AcO@1 in the initial stage, followed by nucleophilic reaction by AcO−, which is accompanied by B−N bond cleavage to give (AcO)2@1 as the major acetate adduct. Figure 8 shows the existence of a side product, which was detected upon adding 0.25 equiv. of AcO− to 1 in THF-d8. Careful signal assignments coupled with 1H-1H COSY measurements (Fig. S10) allowed us to propose oxidation product 3 (Scheme 3),71 which was supported by ESI-MS (negative mode) measurement in Figure S11. We reasoned that 1 was hydrolyzed by water in THF, followed by aerobic oxidative hydroxylation of the dihydroxy boryl segment.72 Notably, titration in DMSO-d6 did not give oxidative product 3, suggesting that the use of DMSO as a solvent may contribute to the improved stability of 1 through intermolecular hydrogen bonding between Me2S=O and HO-B. Indeed, 1H NMR titrations of 1 with TBAF in THF-d8 indicated a somewhat complicated spectral pattern, where signals arising from byproducts were detected as the amount of F− increased, although F2@1 formed as the major species (Fig. S4d).

Scheme 3. Possible reactions of 1 with AcO− in THF, with oxygenated product 3 as a minor product.

In this way, the solvent-dependent anion response on 1 may be related to its aromaticity, as well as its stability in solution. Therefore, we calculated the nucleus-independent chemical shift (NICS) indices73 to assess the relative aromatic character of each ring in the presence or absence of intermolecular hydrogen bonding between Me2S=O and HO−B. Geometry optimization was carried out using in the gas phase at the B3LYP/6-311++G(d,p) level of theory. Subsequently, the NICS values were calculated using the GIAO B3LYP/6-311++G(d,p) level of theory at 0.0 Å [NICS(0)] and 1.0 Å [NICS(1)] from the center of the A and B rings in the bicyclic systems (Table 2). NICS(0) and NICS(1) of the A ring were more positive than those of the B ring, indicating the incomplete π-delocalization of the B−N bond.74 This calculation also supports F− and AcO− reactivities on the boron of 1. Furthermore, NICS(0) and NICS(1) of the A ring in 1−DMSO with intermolecular hydrogen bonding (Me2S=O --- HO−B) were calculated to be −1.81 and −4.42, respectively, which were more positive than the values of 1. This means that DMSO-induced hydrogen bonding interactions may lead to the reduced aromaticity of 1. Accordingly, an increase in the polarity of the B−N bond could stabilize the hydroxylated diazaborine ring.34 This may be the main reason why adding AcO− to the DMSO solution of 1 led to no reaction. As a result, a highly selective response towards F− was observed in a DMSO solution of 1 without any side reactions. Therefore, –OH substitution on the diazaborine core contributes to the stabilization of the heterocycle and endows it with chemosensing functions.

Table 2. NICS values of 1.

NICS(0)

A ring

NICS(1)

A ring

NICS(0)

B ring

NICS(1)

B ring

1

−2.63

−4.89

−7.57

−10.38

1-DMSO

−1.81

−4.42

−7.73

−10.23

Conclusions

In summary, the optical features involving not only solvatochromic but also anion-responsive behaviors of a π-extended and –OH-substituted azaborine were demonstrated for the first time using a naphthalimide-appended 1-hydroxy-2,3,1-benzodiazaborine, 1. It was found that an increase in the DN of the solvent led to a decrease in the fluorescence quantum yield. In particular, DMSO leads to significant quenching of the fluorescence, possibly due to intermolecular hydrogen-bonding interactions between Me2S=O and HO-B. Such solvation impacts the aromaticity and stability of the diazaborine heterocycle and alters the anion-sensing capabilities. Notably, HO-B part plays a significant role for AcO−-induced optical response, providing new perspective for functionalization of boron-containing macrocycles. In this way, application of –OH-substituted azaborines to optical materials deserves further study.

Acknowledgement

This research was partly supported by JSPS KAKENHI Grant Number 15H03799.

Experimental Section

General remarks

NMR spectra were measured on a 500 MHz spectrometer (1H: 500 MHz, 13C: 125 MHz, 19F: 470 MHz) and 400 MHz spectrometer (11B: 128 MHz). In 1H and 13C NMR measurements, chemical shifts (δ) are reported downfield from the internal standard Me4Si. 11B NMR and 19F NMR chemical shifts were referenced to external BF3·Et2O and internal hexafluorobenzene, respectively. Mass spectrometry data of 1 and fluoride and acetate adducts were taken by using an electrospray ionization (ESI) or an atmospheric pressure chemical ionization (APCI) method. The absorption and fluorescence spectra were measured using UV/vis/NIR spectroscopy and a spectrofluorometer, respectively. Elemental analyses were performed on an Elemental Analyzer. Melting point was measured using SHIMADZU DSC-60. IR spectrum was recorded with JASCO FT/IR-4100.

Materials

Reagents used for the synthesis were commercially available and used as supplied. Dry dichloromethane, dry acetone, dry ethyl acetate and dry toluene were prepared according to a standard procedure. Hydrazine-appended 1,8-naphthalimide 2 was synthesized according to literature procedure.54 DMSO and THF were used for analysis grade (purity: ≥99.9 %, impurity: ≤ 0.02% water) and HPLC grade (no stabilizer), respectively.

Synthesis and characterization of 2-(N-propyl-1,8-naphthalimid-4-yl)-1,2-dihydro-2,3,1-benzodiazaborin-1-ol (1)

2-Formylphenylboronic acid (0.19 g, 1.23 mmol) and 2 (0.30 g, 1.12 mmol) were dissolved in dry ethanol (30 mL) under a nitrogen atmosphere and the mixture was stirred under reflux conditions for 5 hours. After cooling to room temperature, the resulting solid was removed by filtration and the filtrate was concentrated under vacuum. The orange residue was washed with ethanol to obtain 1 as a yellow solid (0.19 g, 44%). Mp 183.9 ºC; IR (ATR cm−1) 3340, 2960, 1643, 1585, 1386, 1349, 1075, 897, 785; 1H NMR (500 MHz, THF-d8, 25 °C) δ (ppm): 8.61 (d, 1H, J = 7.80 Hz), 8.54 (dd, 1H, J = 7.15 and 1.20 Hz), 8.23 (d, 1H, J = 7.55 Hz), 8.20-8.18 (m, 2H), 7.80 (d, 1H, J = 7.40 Hz), 7.78-7.75 (m, 2H), 7.69 (dd, 1H, J = 8.50 and 7.25 Hz), 7.68 (s, 1H), 7.65 (td, 1H, J = 7.30 and 1.37 Hz), 4.14 (t, 2H, J = 7.42 Hz), 1.79-1.75 (m, 2H), 0.99 (t, 3H, J = 7.48 Hz). 13C NMR (125 MHz, THF-d8, 25 °C) δ (ppm): 164.5, 164.1, 149.2, 140.7, 136.7, 132.3, 131.9, 131.7, 131.6, 131.4, 130.2, 130.0, 129.9, 128.0, 127.2, 126.7, 124.1, 122.4, 42.2, 22.2, 11.8.75 11B NMR (128 MHz, THF-d8, 25 °C) δ (ppm): 27.3. APCI-MS: calcd. C22H18BN3O3, m/z = 383.1, found m/z = 384.2 [M + H]+. Elemental Anal. Calcd for C22H18BN3O3: C, 68.95; H, 4.73; N, 10.97. Found: C, 68.96; H, 4.75; N, 10.93.

X-ray crystallography for 1

A yellow prism crystal of 1 having approximate dimensions 0.09 × 0.05 × 0.03 mm was mounted on a glass fiber. All measurements were made on an X-ray diffractometer using multilayer mirror monochromated Mo Kα radiation (λ = 0.71075 Å). The structure was solved by direct methods (SHELXL Version 2014/7)76 and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. The refinement was made by using a full-matrix least-squares technique (SHELXL Version 2014/7). Crystallographic data for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1526123. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-336033 or e-mail:deposit@ccdc.cam.ac.Uk).

The measurement of fluorescence quantum yield

The fluorescence quantum yields (Φexp) of 1 was calculated from eq. (1).77

where F(λ) and FR(λ) describe the corrected fluorescence intensities of the compound and the reference, respectively, and A and AR describe the corresponding absorbance at the excitation wavelength (λex = 360 nm). The reference used was 9,10-diphenylanthracene in cyclohexane (ΦR = 1.0).61 The refractive indices are n = 1.50 for toluene, n = 1.41 for THF, n = 1.37 for AcOEt, n = 1.42 for CH2Cl2, n = 1.36 for acetone, n = 1.48 for DMSO, respectively.

Theoretical calculation

Geometry of 1 at the ground state was fully optimized by means of the B3LYP/6-31G (d,p) level method. Density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level were performed in the Gaussian 09 package.62 These molecular orbitals in Figure 3 were visualized using the Gauss view 5.0.8 program. NICS values were calculated using GIAO B3LYP/6-311++G(d,p) level of theory at 0.0 Å [NICS(0)] and 1.0 Å [NICS(1)] from the center of in A and B rings in the bicyclic systems.

References

1.Campbell, P. G.; Marwitz, A. J.; Liu, S. Y. Angew. Chem. Int. Ed. 2012, 51, 6074-6092.

2.The feature has been applied in organic synthesis, see: Yao, M.-L.; Kabalka, G. In Boron Science; CRC Press, 2011, 579-622.

3.Kiran, B.; Phukan, A. K.; Jemmis, E. D. Inorg. Chem. 2001, 40, 3615-3618.

4.Bosdet, M. J. D.; Jaska, C. A.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Org. Lett. 2007, 9, 1395-1398.

5.Jaska, C. A.; Piers, W. E.; McDonald, R.; Parvez, M. J. Org. Chem. 2007, 72, 5234-5243.

6.Ruman, T.; Jarmula, A.; Rode, W. Bioorg. Chem. 2010, 38, 242-245.

7.Baggett, A. W.; Guo, F.; Li, B.; Liu, S. Y.; Jakle, F. Angew. Chem. Int. Ed. 2015, 54, 11191-11195.

8.Huang, H.; Pan, Z.; Cui, C. Chem. Commun. 2016, 52, 4227-4230.

9.Liu, X.; Zhang, Y.; Li, B.; Zakharov, L. N.; Vasiliu, M.; Dixon, D. A.; Liu, S. Y. Angew. Chem. Int. Ed. 2016, 55, 8333-8337.

10.Loh, Y. K.; Chong, C. C.; Ganguly, R.; Li, Y.; Vidovic, D.; Kinjo, R. Chem. Commun. 2014, 50, 8561-8564.

11.Zurwerra, D.; Quetglas, V.; Kloer, D. P.; Renold, P.; Pitterna, T. Org. Lett. 2015, 17, 74-77.

12.Lu, W.; Hu, H.; Li, Y.; Ganguly, R.; Kinjo, R. J. Am. Chem. Soc. 2016, 138, 6650-6661.

13.Liew, S. K.; Holownia, A.; Tilley, A. J.; Carrera, E. I.; Seferos, D. S.; Yudin, A. K. J. Org. Chem. 2016, 81, 10444-10453.

14.Weber, L.; Böhling, L. Coord. Chem. Rev. 2015, 284, 236-275.

15.Weber, L.; Halama, J.; Hanke, K.; Böhling, L.; Brockhinke, A.; Stammler, H. G.; Neumann, B.; Fox, M. A. Dalton Trans. 2014, 43, 3347-3363.

16.Abbey, E. R.; Zakharov, L. N.; Liu, S. Y. J. Am. Chem. Soc. 2011, 133, 11508-11511.

17.Kubo, Y.; Tsuruzoe, K.; Okuyama, S.; Nishiyabu, R.; Fujihara, T. Chem. Commun. 2010, 46, 3604-3606.

18.Otsuka, K.; Kondo, T.; Nishiyabu, R.; Kubo, Y. J. Org. Chem. 2013, 78, 5782-5787.

19.Ishibashi, J. S. A.; Marshall, J. L.; Mazière, A.; Lovinger, G. J.; Li, B.; Zakharov, L. N.; Dargelos, A.; Graciaa, A.; Chrostowska, A.; Liu, S.-Y. J. Am. Chem. Soc. 2014, 136, 15414-15421.

20.Hatakeyama, T.; Hashimoto, S.; Seki, S.; Nakamura, M. J. Am. Chem. Soc. 2011, 133, 18614-18617.

21.Wang, X.; Zhang, F.; Liu, J.; Tang, R.; Fu, Y.; Wu, D.; Xu, Q.; Zhuang, X.; He, G.; Feng, X. Org. Lett. 2013, 15, 5714-5717.

22.van de Wouw, H. L.; Lee, J. Y.; Siegler, M. A.; Klausen, R. S. Org. Biomol. Chem. 2016, 14, 3256-3263.

23.Zhang, W.; Zhang, F.; Tang, R.; Fu, Y.; Wang, X.; Zhuang, X.; He, G.; Feng, X. Orga. Lett. 2016, 18, 3618-3621.

24.Hashimoto, S.; Ikuta, T.; Shiren, K.; Nakatsuka, S.; Ni, J.; Nakamura, M.; Hatakeyama, T. Chem. Mater. 2014, 26, 6265-6271.

25.Wang, X.-Y.; Zhuang, F.-D.; Zhou, X.; Yang, D.-C.; Wang, J.-Y.; Pei, J. J. Mater. Chem. C 2014, 2, 8152-8161.

26.Wang, X.-Y.; Lin, H.-R.; Lei, T.; Yang, D.-C.; Zhuang, F.-D.; Wang, J.-Y.; Yuan, S.-C.; Pei, J. Angew. Chem. Int. Ed. 2013, 52, 3117-3120.

27.Zhong, Z.; Wang, X.-Y.; Zhuang, F.-D.; Ai, N.; Wang, J.; Wang, J.-Y.; Pei, J.; Peng, J.; Cao, Y. J. Mater. Chem. A 2016, 4, 15420-15425.

28.Prakash Reddy, V.; Sinn, E.; Hosmane, N. J. Organomet. Chem. 2015, 798, 5-12.

29.Zhou, Y.; Zhang, J. F.; Yoon, J. Chem. Rev. 2014, 114, 5511-5571.

30.Kumar, G. R.; Sarkar, S. K.; Thilagar, P. Phys. Chem. Chem. Phys. 2015, 17, 30424-30432.

31.Misra, R.; Jadhav, T.; Dhokale, B.; Mobin, S. M. Dalton Trans. 2015, 44, 16052-16060.

32.Qian, X.; Xiao, Y.; Xu, Y.; Guo, X.; Qian, J.; Zhu, W. Chem. Commun. 2010, 46, 6418-6436.

33.Yahyavi, H.; Kaykhaii, M.; Mirmoghaddam, M. Crit. Rev. Anal. Chem. 2016, 46, 106-121.

34.Srivastava, A. K.; Pandey, S. K.; Misra, N. Mol. Phys. 2016, 114, 1763-1770.

35.Dewar, M. J. S.; Dougherty, R. C. J. Am. Chem. Soc. 1964, 86, 433-436.

36.Groziak, M. P.; Ganguly, A. D.; Robinson, P. D. J. Am. Chem. Soc. 1994, 116, 7597-7605.

37.Baldock, C.; Rafferty, J. B.; Sedelnikova, S. E.; Baker, P. J.; Stuitje, A. R.; Slabas, A. R.; Hawkes, T. R.; Rice, D. W. Science 1996, 274, 2107-2110.

38.Kanichar, D.; Roppiyakuda, L.; Kosmowska, E.; Faust, M. A.; Tran, K. P.; Chow, F.; Buglo, E.; Groziak, M. P.; Sarina, E. A.; Olmstead, M. M.; Silva, I.; Xu, H. H. Chem. Biodivers. 2014, 11, 1381-1397.

39.Dilek, O.; Lei, Z.; Mukherjee, K.; Bane, S. Chem. Commun. 2015, 51, 16992-16995.

40.Gillingham, D. Org. Biomol. Chem. 2016, 14, 7606-7609.

41.Groziak, M. P.; Robinson, P. D. Collect. Czech. Chem. Commun. 2002, 67, 1084-1094.

42.Sarina, E. A.; Olmstead, M. M.; Kanichar, D.; Groziak, M. P. Acta Cryst. C 2015, 71, 1085-1088.

43.Robinson, P. D.; Groziak, M. P.; Chen, L. Acta Cryst. C 1998, 54, 71-73.

44.Stress, C. J.; Schmidt, P. J.; Gillingham, D. G. Org. Biomol. Chem. 2016, 14, 5529-5533.

45.Gudeika, D.; Michaleviciute, A.; Grazulevicius, J. V.; Lygaitis, R.; Grigalevicius, S.; Jankauskas, V.; Miasojedovas, A.; Jursenas, S.; Sini, G. J. Phys. Chem. C 2012, 116, 14811-14819.

46.Mallia, A. R.; Salini, P. S.; Hariharan, M. J. Am. Chem. Soc. 2015, 137, 15604-15607.

47.Peebles, C.; Wight, C. D.; Iverson, B. L. J. Mater. Chem. C 2015, 3, 12156-12163.

48.Saini, A.; Justin Thomas, K. R. RSC Adv. 2016, 6, 71638-71651.

49.Zhang, J.; Xiao, H.; Zhang, X.; Wu, Y.; Li, G.; Li, C.; Chen, X.; Ma, W.; Bo, Z. J. Mater. Chem. C 2016, 4, 5656-5663.

50.Duke, R. M.; Veale, E. B.; Pfeffer, F. M.; Kruger, P. E.; Gunnlaugsson, T. Chem. Soc. Rev. 2010, 39, 3936-3953.

51.Li, J.; Yin, C.; Huo, F. Dyes Pig. 2016, 131, 100-133.

52.Wang, Y.; Zhang, Y.; Yi, X.; Qin, W. Asian J. Chem. 2012, 24, 323-326.

53.Sheshashena Reddy, T.; Maragani, R.; Misra, R. Dalton Trans. 2016, 45, 2549-2553.

54.Gan, J.; Tian, H.; Wang, Z.; Chen, K.; Hill, J.; Lane, P. A.; Rahn, M. D.; Fox, A. M.; Bradley, D. D. C. J. Org. Chem. 2002, 645, 68–175.

55.The crystallographic data of the structure are summarized in Electronic Supporting Information.

56.Islas, R.; Chamorro, E.; Robles, J.; Heine, T.; Santos, J. C.; Merino, G. Struct. Chem. 2007, 18, 833-839.

57.Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2009, 87, 8-29.

58.Lippert, V. E. Z. Elektrochem. 1957, 61, 962-975.

59.Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465-470.

60.Sanjoy Singh, T.; Mitra, S. J. Lumin. 2007, 127, 508-514.

61.Berlman, I. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York and London, 1965.

62.Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M. L., X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A. S., P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian, Inc: Wallingford, CT, 2010.

63.Connors, K. A. Binding constants; Wiley: New York, 1987.

64.Xu, Z.; Kim, S. K.; Han, S. J.; Lee, C.; Kociok-Kohn, G.; James, T. D.; Yoon, J. Eur. J. Org. Chem. 2009, 2009, 3058-3065.

65.Sun, H.; DiMagno, S. G. J. Am. Chem. Soc. 2005, 127, 2050-2051.

66.There is almost no perturbation in the 1H NMR titrations of 1 with AcO− in DMSO-d6; see Figure S6.

67.Agou, T.; Sekine, M.; Kobayashi, J.; Kawashima, T. Chem. Commun. 2009, 1894-1896.

68.Lepeltier, M.; Lukoyanova, O.; Jacobson, A.; Jeeva, S.; Perepichka, D. F. Chem. Commun. 2010, 46, 7007-7009.

69.Zhou, J.; Tang, R.; Wang, X.; Zhang, W.; Zhuang, X.; Zhang, F. J. Mater. Chem. C 2016, 4, 1159-1164.

70.Spectral data analysis SPANA (http://www.eonet.ne.jp/~spana-lsq/about-e.html) was employed to determine the binding constants.

71.1H NMR (500 MHz, THF-d8, 25 °C) δ (ppm): 9.08 (dd, 1H, J = 8.40 and 1.05 Hz), 8.35 (m, 1H), 8.17 (d, 1H, J = 8.80 Hz), 7.45 (d, 2H, J = 8.80 Hz), 7.39 (dd, 1H, J = 8.23 and 7.33 Hz), 7.20 (s, 1H), 7.00-6.97 (m, 2H), 6.61 (dd, 1H, J = 7.90 and 0.65 Hz), 6.32 (ddd, 1H, J = 7.68, 6.85 and 1.03 Hz).

72.Cheng, G.; Zeng, X.; Cui, X. Synthesis 2014, 46, 295-300.

73.Krygowski, T. M.; Cyrański, M. K. Chem. Rev. 2001, 101, 1385-1419.

74.Davies, G. H. M.; Molander, G. A. J. Org. Chem. 2016, 81, 3771-3779.

75.The peak of the carbon atom directly bound to the boron atom of 1 was not observed due to a quadrupolar relaxation. See, Wrackmeyer, B. Prog. Nuc.l Magn. Reson. Spectrosc., 1979, 12, 227−259.

76.Sheldrick, G. Acta Cryst. A 2008, 64, 112-122.

77.Crosby, G. A.; Demas, J. N. J. Phys. Chem. 1971, 75, 991-1024.

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