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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: [email protected]

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

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