Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Four-Step Synthesis of B2N2‑Embedded Corannulene Soichiro Nakatsuka,† 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
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S Supporting Information *
such report for corannulene derivatives bearing BN units. Herein, we report the first scalable synthesis of the π-extended B2N2-embedded corannulene, 1, in which two CC units on the spoke of the corannulene skeleton were replaced by BN units. The key to success was the use of one-shot multiple borylation13 based on electrophilic arene borylation,12c−g,14 which enabled us to construct the corannulene skeleton in four steps from commercially available compounds. Moreover, an organic light-emitting diode (OLED)15 was fabricated employing 1 as a blue emitter for demonstrating its promising potential in material science. The four-step synthesis of 1 is summarized in Figure 2. First, in the presence of 1.0 mol % Pd(PPh3)4 and 3.0 equiv of
ABSTRACT: A corannulene possessing two BN units on the spoke, 10b1,18b1-diaza-10b,18b-diboratetrabenzo [a,g,j,m]corannulene, was synthesized on a multigram scale in four steps from commercially available compounds. Its shallow bowl-shaped structure was confirmed by X-ray crystallography. The B2N2-embedded corannulene showed strong blue fluorescence and was employed as an efficient emitter for an organic light-emitting diode.
C
orannulene1,2 derivatives are an important class of materials for liquid crystals,3 molecular tweezers,4 polymers,5 single-chirality carbon nanotubes (CNTs),6 organic field-effect transistors (OFETs),7 and solar cells.8 While doping of heteroatoms in polyaromatic hydrocarbons (PAHs) is an attractive technique to modulate their physical properties and molecular structures and thus realize new functions, this strategy has not been widely applied to the corannulene motif because of challenging syntheses.9 It is only recently that corannulene analogs having one nitrogen atom have been synthesized and their crystal structures, photophysical properties, and C60 binding behavior have been studied (Figure 1).10
Figure 2. Four-step and gram-scale synthesis of 1. Conditions: (a) phenylboronic acid (2.2 equiv), Pd(PPh3)4 (1.0 mol %), K2CO3 (3.0 equiv), THF/H2O (1/1), reflux, 3 days. (b) Zn (10 equiv), acetic acid, 40 °C, 22 h. (c) 2,2-diphenylacetic acid (2.0 equiv), neat, 180 °C, 62 h. (d) BBr3 (4.0 equiv), o-dichlorobenzene, reflux (bath temperature: 200 °C), 16 h. Figure 1. Azacorannulenes and B2N2-embedded corannulene 1.
K 2 CO 3 , the Suzuki−Miyaura coupling between 4,7dibromobenzo[c][1,2,5]thiadiazole 2 and phenylboronic acid took place smoothly to give the coupling product 3 in 96% yield.16 In the second step, the sulfur extrusion reaction of compound 3 with zinc produced the corresponding diamine 4 in 86% yield. The third step involved the condensation reaction of 4 with 2.0 equiv of diphenyl acetic acid under neat conditions at 180 °C for 62 h and afforded 5 in 96% yield.
The incorporation of BN units into arenes has generated a lot of interest owing to their potential applications in the fabrication of biomedical and (opto)electronic materials.11 The substitution of CC double bonds in PAHs by isoelectronic BN bonds leads to dramatically altered optical and electronic properties because the isostructural molecules bear strong local dipole moments or polarized frontier orbitals. Recently, considerable effort has been devoted to the introduction of more than one internalized BN unit in PAHs.12 However, to the best of our knowledge, there is no © XXXX American Chemical Society
Received: August 1, 2018 Published: September 25, 2018 A
DOI: 10.1021/jacs.8b08197 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
that of A perpendicular to the molecular plane was calculated to be 0.90 D. The γ-motif packing structure20 of 1 is shown in Figure 4a. In the γ-motif, 1 formed one-dimensional slipped-stacks along
Finally, the electrophilic CH borylation of 5 proceeded in the presence of 4.0 equiv of BBr3 under reflux conditions (bath temperature: 200 °C) to afford the target compound 1 in 32% yield. These processes were robust and scalable, allowing multigram quantities (3.6 g) of 1 to be prepared. Notably, 1 did not decompose in air even at 350 °C. The shallow bowl shape of 1 was determined by X-ray crystallography (Figure 3a,b). The bowl depth, defined as the
Figure 4. Three views of the packing structure (a, b, c) of 1. The green arrows represent the C−H···π interactions. Hydrogen atoms are omitted for clarity in panels b and c. The gradient arrows represent the dipole moment. Figure 3. ORTEP drawings (a) and side view (b) of 1 obtained by Xray crystallographic analysis. Thermal ellipsoids are shown at 50% probability and hydrogen atoms are omitted for clarity. NICS(0) values for 1 (c) and tetrabenzo[a,d,g,j]corannulene A (d) calculated at the B3LYP/6-311+G(d,p) level of theory. The gradient arrow represents the dipole moment.
the a axis and a form of herringbone (edge-to-face) shape with CH−π interactions (2.8−3.0 Å). Interestingly, in these infinite stacks, each molecule showed concave/concave (cc/cc) and convex/convex (cv/cv) interactions with its nearest neighbors (Figure 4b). In the cc/cc interaction, the π−π distances were 3.47 Å and in the cv/cv interaction with the neighboring molecules, the shortest π−π distances were 3.28 Å. In this arrangement, the dipoles were not perfectly aligned along the stack because of the sequential shifting of the bowls along the stack axis (Figure 4c). It is likely that the molecular arrangement in 1 is predominantly governed by the offset of dipoles and the π−π interactions between the adjacent molecules. The photophysical properties of 1 are summarized in Figure 5. In CH2Cl2, the absorption spectrum of 1 exhibited characteristic vibronic structure bands. The absorption maximum of 411 nm was attributed to the HOMO−LUMO transition by TD-DFT calculation at the optimized S0 structures at the B3LYP/6-31G(d) level (Figure 6). Furthermore, 1 exhibited a strong blue emission band at 424 nm with a photoluminescence (PL) quantum yield of 69%, which was much higher than that of corannulene (7%).21 This could be attributed to the significantly larger radiative rate constant of 1 as compared to that of corannulene (kr = 1.6 and 0.07 × 108 s−1, respectively),22 which was consistent with the larger oscillator strength (f) of 1 than that of corannulene (0.1762 and 0.0013, respectively) for their optimized S1 structures. Considering the medium oscillator strength (0.1275) of the carbon analog A and the parent B2N2embedded corannulene (0.0854), it was evident that both,
perpendicular distance from the center of the hub N2C3 ring to the parallel planes containing the ten terminal carbon atoms of the corannulene skeleton C1−C10, was 0.15 Å (Figure 3b). Therefore, the bowl in 1 was much shallower than that of corannulene (0.87 Å)1 because the rim bond lengths of the corannulene skeleton in the former (B1C1, B1C10, B2 C4, and B2C5 are 1.554(3), 1.578(3), 1.542(3), 1.579(4) Å, respectively) were longer than those of the latter (1.44−1.45 Å). The BN bond lengths (1.414(3) and 1.410(3) Å) were shorter than those in typical BN aromatics (1.45−1.47 Å),12a,17 thus confirming the double bond character.18 The spoke bond lengths of the corannulene skeleton in 1, C11C12, C13 C14, and C15C16, were highly olefinic (1.354(3), 1.365(3), and 1.363(3) Å, respectively). The N1C12, N1C16, N2C12, and N2C14 lengths in 1 were 1.381(3), 1.382(3), 1.379(3), 1.381(3) Å, respectively, and the C14 C16 bond length was 1.375(3) Å. Nucleus-independent chemical shift (NICS) calculations at the B3LYP/6-311+G(d,p) level of theory indicated the nonaromatic character of the N2C3 ring (NICS(0) = −2.3) (Figure 3c).19 In contrast, positive NICS(0) values (8.6) were observed for the corresponding C5 rings of the carbon analog, tetrabenzo[a,d,g,j]corannulene (A) (Figure 3d). The dipole moment vector of 1 parallel to the molecular plane was 1.01 D, whereas B
DOI: 10.1021/jacs.8b08197 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Table 1. Properties of the OLED Employing 1 as an Emitter λmax
Va
ηca
ηpa
ηexta
Lmaxb
467
4.81
5.36
3.50
2.61
10400
Driving voltage (V), current efficiency (cd A−1), power efficiency (lm W−1), and external quantum efficiency (%) at 1000 cd m−2. b Maximum luminance. a
be explained by the π−π interactions with the host material and optical interference in the device. To the best of our knowledge, this is the first reported trial of a corannulenebased OLED, and the device performances can be further improved by modifying the molecular and device structures. In summary, the first B2N2-embedded corannulene 1 was synthesized via electrophilic CH borylation. The total yield of 1 over four steps starting from commercially available 4,7dibromobenzo[c][1,2,5]thiadiazole was 25% (3.6 g). The shallow bowl shape and one-dimensional slipped-stacks in the solid state of 1 were studied. Furthermore, 1 showed strong blue fluorescence owing to the introduction of the BN units and enabled the fabrication of the first corannulene-based OLED. Investigations aimed at realizing further applications of B2N2-embedded corannulene analogs and the incorporation of other heteroatoms are ongoing in our laboratory.
Figure 5. Normalized absorption (blue), fluorescence (red) with absorption/emission maxima (nm), absolute fluorescence quantum yields (φF) of 1 in CH2Cl2 excited at 340 nm, along with the oscillator strengths (green) obtained by TD-DFT calculations at the (TD)B3LYP/6-31G(d) level of theory.
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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.8b08197. Syntheses, analytical data, NMR spectra, DFT studies, photophysical studies (PDF) X-ray crystallography data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Takuji Hatakeyama: 0000-0002-7483-9525 Notes
The authors declare no competing financial interest.
Figure 6. Kohn−Sham molecular orbitals (HOMO and LUMO) and the oscillator strength (f) of the S0−S1 transition of 1 (a), carbon analog A (b), parent B2N2-corannulene (c), and corannulene (d) at the S1 structure, calculated at the (TD)B3LYP/6-31G(d) level of theory.
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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 (JP18H02051), Challenging Research (Exploratory, JP17K19164), and JSPS Fellows (16J02975) from Japan Society for the Promotion of Science (JSPS), the Iketani Science and Technology Foundation, the Mitsubishi Foundation, and the Sumitomo Foundation. Synchrotron X-ray diffraction measurements were performed at the BL40XU beamline in SPring-8 with the approval of JASRI (2016A1052, 2016B1059, 2017A1132, 2017B1073, 2018A1114). We are grateful to Dr. Yasuyuki Sasada and Mr. Yasuhiro Kondo (JNC Petrochemical Corporation) for experimental supports.
asymmetrization by tetrabenzo-annelation and polarization parallel to the molecular plane induced by the two BN units, played an important role in increasing the oscillator strength. For the optimum use of the fluorescent B2N2-embedded corannulene, an OLED employing 1 as an emitter was fabricated with the following structure: indium tin oxide (ITO, 120 nm); dipyrazino[2,3-f:2′,3′-h]quinoxaline2,3,6,7,10,11-hexacarbonitrile (HAT-CN, 10 nm); tris(4carbazoyl-9-ylphenyl)amine (TCTA, 95 nm); 2-tert-butyl9,10-di-2-naphthylanthracene (TBADN, 15 nm); 2 wt % of 1 and 98 wt % of TBADN (20 nm); 2,2′,2″-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi, 40 nm); LiF (1 nm); Al (100 nm). As shown in Table 1, the device exhibited sky blue electroluminescence (EL) with an external quantum efficiency of 2.61% at 1000 cd m−2 and maximum luminance of 10400 cd m−2. The considerable red-shift of the EL spectrum (λmax = 467 nm)23 from the PL spectrum (λmax = 424 nm) can
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DOI: 10.1021/jacs.8b08197 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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D
DOI: 10.1021/jacs.8b08197 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
