One-Shot Multiple Borylation toward BN-Doped Nanographenes

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX

One-Shot Multiple Borylation toward BN-Doped Nanographenes Kohei Matsui,† Susumu Oda,† Kazuki Yoshiura,† Kiichi Nakajima,† Nobuhiro Yasuda,‡ and Takuji Hatakeyama*,† †

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ‡ Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan S Supporting Information *

report the two-step synthesis of a B4N3-doped nanographene B4 (Scheme 1). The key to success is the quadruple borylation

ABSTRACT: One-shot double, triple, and quadruple borylation reactions of triarylamines were developed through a judicious choice of boron source and Brønsted base. With the aid of borylation reactions, a variety of BNdoped nanographenes were synthesized in two steps from commercially available starting materials. An organic lightemitting diode device employing BN-doped nanographene as an emitter exhibited deep pure-blue emission at 460 nm, with CIE coordinates of (0.13, 0.11), and an external quantum efficiency of 18.3%.

Scheme 1. Two-Step Synthesis of B4

H

eteroatom doping of graphene is one of the most promising strategies of engendering fascinating properties into this material. In particular, boron-, nitrogen-, phosphorus-, and sulfur-doped graphenes have been intensively studied as efficient materials for supercapacitors, anodes in Liion batteries, cathode catalysts in fuel cells, and sensors, among others.1 Syntheses of these heteroatom-doped graphenes has mostly been accomplished by chemical vapor deposition of carbon and heteroatom sources, or by the thermal annealing of graphene oxides with dopants. However, these approaches produce structurally nonuniform materials, which may prevent the detailed elucidation of the effect of the dopant, and the finetuning of physical properties. For these reasons, it is desirable to develop a bottom-up synthesis, based on the surface-assisted coupling2 or amplified growth3 of heteroaromatic compounds, which is potentially advantageous for the production of welldefined heteroatom-doped graphenes. Recently, heteroatom-doped nanographenes4 have attracted significant attention not only as starting units for bottom-up syntheses, but also as well-defined substructures for heteroatom-doped graphenes. A powerful synthetic method is dehydrogenative C−C coupling (Scholl reaction); N-5 and Bdoped6 nanographenes have been successfully synthesized by Müllen et al. and Yamaguchi et al., respectively. Another powerful method is the Friedel−Crafts-type carbon−heteroatom bond-forming reaction, which has provided a variety of polycyclic heteroaromatic compounds.7−11 However, this reaction has been mostly limited to intramolecular processes, and requires precursors possessing hydroxyl,7 amino,8 imino,9 halogeno,10 or trimethylsilyl groups,11 which can facilitate initial intermolecular reaction.12 As a consequence, conventional approaches to doped nanographenes13 often require multistep syntheses from commercially available sources. Herein, we © XXXX American Chemical Society

of triarylamines via intra- and intermolecular bora-Friedel− Crafts-type reactions, which converts 11 C−H bonds into C−B bonds in one shot. Moreover, we have succeeded in the development of the selective double and triple borylation of triarylamines, demonstrating the potential of the present strategy. The two-step synthesis of B4 is summarized in Scheme 1. The first step involves the palladium-catalyzed C−N coupling between the commercially available starting materials, 1,3,5tribromobenzene and di-p-tolylamine, which proceeded smoothly at 60 °C to afford N1,N1,N3,N3,N5,N5-hexakis(4methylphenyl)-1,3,5-benzenetriamine 1 in 98% yield. The second step is the one-shot quadruple borylation of 1; in the presence of 12 equiv of BI3, the borylation proceeded under reflux conditions (bath temperature, 200 °C) to afford the target compound B4 in 41% yield (35% isolated yield) and triple borylation compound B3 in 3%.14 While mono- and Received: October 4, 2017

A

DOI: 10.1021/jacs.7b10578 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society diborylated compounds were not observed under these conditions, a considerable amount of unidentified side products were formed. Judging by the formation of di-p-tolylamine, we assume that C−N bond cleavage completed with the borylation reaction. Notably, other boron sources, BCl3 and BBr3, did not give any B4 at all. After screening of the reaction conditions,14 we succeeded in the double and triple borylation of triarylamine 1 as shown Scheme 2. In the presence of 5.0 equiv of BI3 and 2.0 equiv of Scheme 2. Double and Triple Borylation of 1

Figure 1. ORTEP drawing (a) of B3 obtained by X-ray crystallographic analysis. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity. NICS(0) values for B3 (b) and the carbon analogue (c) calculated at the B3LYP/6-311+G(d,p) level of theory.

formation due to the large difference between the C−B and C− N bond lengths and the methyl substituents, which is in contrast to the isoelectronic carbon analogue, hexabenzo[a,d,g,j,m,p]coronene that adopts an almost D3d conformation.15 In particular, the dihedral angles between the planes of the peripheral benzene rings are very different to each other (A−A′, 30.3°; A−B, 46.4°; B−C, 33.1°; C−C′, 48.2°), while those of the carbon analogue are in the range of 43−45°. As shown in Figure 1b, the nonaromatic character of the BNC4 rings and the aromatic character of the central C6 ring are demonstrated by NICS calculations (NICS(0) = 0.9 to 1.1 and −4.8) performed at the B3LYP/6-311+G(d,p) level of theory, which is in contrast to negative (−3.6 to −4.0) and positive (5.0) NICS(0) values for the corresponding C6 rings of the carbon analogue (Figure 1c). Notably, the analogous helical structures of B2 and B4 were also determined by X-ray crystallography.14 The photophysical properties of B2−B4 in poly(methyl methacrylate) (PMMA) film (1 wt%, dispersed) are summarized in Figure 2. Interestingly, the absorption maximum of B3 (λab = 396 nm) is significantly blue-shifted compared to that of B2 (λab = 438 nm) and B4 (λab = 440 nm). Time-dependent density functional theory (TD-DFT) calculations suggest that the S0−S1 and S0−S2 transitions in B3 are forbidden and that the absorption band is assignable to S0−S3 and S0−S4 transitions (degenerate, f = 0.5200)14 The weak band around 430 nm is assignable to S0−S1 transition ( f = 0.0006).14 Three compounds show deep blue emission bands at 455, 441, and 450 nm, respectively (Φf = 0.53, 0.33, 0.57). Notably, the full width at half maxima (fwhm) for B2 and B3 are 32 and 34 nm, respectively, and are among the smallest values observed for organic emitting materials. The Commission Internationale de l’Eclairage (CIE) coordinates of B2 and B3 are (0.14, 0.08) and (0.15, 0.06), respectively, which perfectly satisfy the requirements for pure blue as defined by the National Television System Committee (0.14, 0.08) and Adobe RGB (0.15, 0.06). Since the frontier orbitals of these compounds are efficiently

Ph3B, the selective double borylation took place under reflux conditions (bath temperature: 190 °C) to give B2 in 80% yield (76% isolated yield). On the other hand, at elevated temperature (bath temperature: 200 °C) in a high boiling point solvent, 1,2,4-trichlorobenzene, the triple borylation took place to give B3 in 48% yield (45% isolated yield) with 6% yield of B2. Notably, other Brønsted bases, EtNiPr2 and N,Ndimethyltoluidine, did not give any B3 at all. In the absence of Ph3B, lower yields of the target compounds were obtained probably because of competition by the nonselective borylation or the C−N bond cleavage.14 We propose that Ph3B reduces the concentration of HI via the retro-Friedel−Crafts reaction to suppress the side reactions. Notably, the use of Mes3B instead of Ph3B gave 28% yield of B3, 23% yield of B4, and more than 2 equiv of mesitylene.14 The concentration of HI can be further reduced under reflux conditions, resulting in the selective formation of B2 at 190 °C. Compounds B2−B4 showed substantial stability; no decomposition was observed after treatment with 1 N HCl or NaOH, or heating at 300 °C in air, indicating that it is suitable for further transformation or practical application. While the double borylation of the electron-deficient fluorine-substituted substrate took place at 200 °C (58% isolated yield),14 the triple and quadruple borylation did not take place at all, even at 240 °C. The triple helical structure of triazatriborahexabenzo[a,d,g,j,m,p]coronene B3 was determined by X-ray crystallography (Figure 1a). B3 has a C2 axis of symmetry that passes through B1−C3−C6−N2, but exhibits an almost D3 conB

DOI: 10.1021/jacs.7b10578 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Table 1. Properties of OLED Employing B2 as an Emittera Von (V)

λmax (nm)

CIE (x,y) fwhm (nm)

ηc (cd A−1)

ηp (lm W−1)

ηext (%)

3.8b 5.1c

460

0.13, 0.11 37 (0.21 eV)

16.7b 11.5c

13.8b 7.1c

18.3b 12.6c

a Abbreviations: Von voltage required for 1 or 100 cd m−2; λmax, emission maximum; ηc, current efficiency; ηp, maximum power efficiency; ηext maximum external quantum efficiency. bData at 1 cd m−2. cData at 100 cd m−2.

In summary, we developed one-shot multiple borylation reactions for the successful two-step synthesis of BN-doped nanographenes. The key to success is the judicious choice of boron source and Brønsted base. The BN-doped nanographenes show deep pure-blue fluorescence and small ΔEST values. The OLED device employing B2 as a TADF emitter exhibited an excellent performance for blue OLEDs, demonstrating high potential in the field of materials chemistry. This simple and scalable protocol is useful not only for providing new functional materials but also for the further extension of the π-system, which can spur the further development of bottom-up approaches toward BN-doped nanocarbons.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b10578. Syntheses, analytical data, NMR spectra, DFT studies, and photophysical studies (PDF) X-ray crystallographic data for B2 (CIF) X-ray crystallographic data for B3 (CIF) X-ray crystallographic data for B4 (CIF)

Figure 2. Normalized absorption (blue), fluorescence (red), and phosphorescence (green, 77 K, 25 ms delay) spectra, with absorption/ emission maxima, absolute fluorescence quantum yields (ΦF), and fwhm of PMMA films of (a) B2, (b) B3, and (c) B4 (1 wt%, excited at 360 nm).

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separated by the multiple resonance effects of the boron and nitrogen atoms,14,16 the phosphorescence maxima at 77 K (λem = 488, 466, and 475 nm for B2−B4, respectively) are very close to the fluorescence maxima. Based on these emission maxima, the energy difference between the excited singlet and triplet states (ΔEST) are estimated to be 0.15−0.18 eV. These are promising characteristics for thermally activated delayedfluorescence (TADF) materials17 for organic light-emitting diodes (OLEDs). To demonstrate the potential of BN-embedded nanographenes, an OLED employing B2 as the emitter was fabricated with the following structure:14 indium tin oxide (ITO, 50 nm); N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD, 40 nm); tris(4-carbazolyl-9ylphenyl)amine (TCTA, 15 nm); 1,3-bis(N-carbazolyl)benzene (mCP, 15 nm); 1 wt% B2 and 99 wt% of mCBP (20 nm); diphenyl-4-triphenylsilylphenylphosphine oxide (TSPO1, 40 nm); LiF (1 nm); Al (100 nm). The results are summarized in Table 1.14 The device exhibited excellent performance with the external quantum efficiencies of 18.3% at 1 cd m−2 and 12.6% at 100 cd m−2, which are much higher than the theoretical limit (5%) for fluorescent OLEDs and comparable to those reported in the latest blue TADF OLEDs.17 Although a slight red-shift (455−460 nm) and broadening (32−37 nm) were observed in the electroluminescence spectrum due to π−π interactions with mCBP, the CIE coordinates are (0.13, 0.11), which represent record-setting values for blue TADF OLEDs.17

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Takuji Hatakeyama: 0000-0002-7483-9525 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “π-System Figuration” (JP17H05164), a Grant-in-Aid for Scientific Research (JP26288095), a Challenging Research (Exploratory, JP17K19164) from Japan Society for the Promotion of Science (JSPS), the Accelerated Innovation Research Initiative Turning Top Science and Ideas into High-Impact Values (ACCEL) program from the Japan Science and Technology Agency (JST), the Iketani Science and Technology Foundation, the Mitsubishi Foundation, the Sumitomo Foundation, and FY2017 Individual Special Research Fund from KG University. The synchrotron X-ray diffraction measurements were performed at the BL40XU beamline in SPring-8 with the approval of JASRI (2014B1815, 2015A1320, 2015B0123, 2016A1052, 2016B1059, and 2017A1132). We are grateful to Dr. Yasuyuki Sasada and Mr. Yasuhiro Kondo (JNC Petrochemical Corporation) for experimental support. C

DOI: 10.1021/jacs.7b10578 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

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DOI: 10.1021/jacs.7b10578 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX