NBN-embedded Polycyclic Aromatic Hydrocarbons Containing

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NBN-embedded Polycyclic Aromatic Hydrocarbons Containing Pentagonal and Heptagonal Rings Yubin Fu,†,∇ Ke Zhang,‡,∇ Evgenia Dmitrieva,⊥ Fupin Liu,⊥ Ji Ma,† Jan J. Weigand,§ Alexey A. Popov,⊥ Reinhard Berger,† Wojciech Pisula,‡,Δ Junzhi Liu,*,† and Xinliang Feng*,†

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Center for Advancing Electronics Dresden (cfaed) & Faculty of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ⊥ Center of Spectroelectrochemistry, Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Helmhol-tzstrasse 20, 01069 Dresden, Germany § Chair of Inorganic Molecular Chemistry, Technische Universität Dresden, 01062 Dresden, Germany Δ Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland S Supporting Information *

ABSTRACT: Two novel nonhexagonal ring fused NBNdibenzophenalenes (NBN-penta and NBN-hepta) were designed and synthesized. X-ray analysis reveals the planar structure of NBN-penta, while NBN-hepta displays a doublehelical structure. Both compounds possess higher oxidation potential compared to the NBN-type zigzag-edged polycyclic aromatic hydrocarbons. The resultant NBN-penta and NBNhepta exhibit global aromatic character based on X-ray analysis and DFT calculations. Furthermore, single crystal transistors of NBN-penta and NBN-hepta were fabricated, manifesting their promising potential in organic electronics.

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olycyclic aromatic hydrocarbons (PAHs) have attracted intense interest because of their unique electronic properties and supramolecular behavior, which render them as promising candidates for organic semiconducting devices such as organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and organic light-emitting diodes (OLEDs).1 Substituting the CC unit in PAHs using isoelectronic B−N moiety has emerged as an important strategy to develop novel aromatic compounds because it can significantly change the optical and electronic properties while keeping the same conjugated skeleton.2 Instead of the zero dipole CC bond, the positive dipole B−N bond is considered to be a zwitterionic π bonding in the neutral state, which leads to stable radical cations under oxidation process.3 Recently, we reported a class of dibenzo-fused 1,9-diaza-9a-boraphenalenes (NBN-DBP 1 and 2, Figure 1a), in which the NBN unit is decorated at the zigzag edge.3a However, NBN-DBP 1 can be easily oxidized into its σ-dimer (3) (Figure 1b). Very recently, Kumagai and Zeng reported the synthesis of BNB embedded PAHs (DATB 4 and DBAP-Ph 5, Figure 1c).3b,c Nevertheless, DATB 4 possesses weak aromaticity due to the existence of B−O bonds in its skeleton, while DBAP-Ph 5−2 is highly sensitive to moisture.3b,c Compared to the edge functionalization of PAHs with NBN units, the synthesis of PAHs containing internal NBN moieties remains challenging mostly due to the limited synthetic methodology.4 Typically, PAHs containing a pentagonal ring can introduce a positive curvature as well as stabilize injected electrons because © XXXX American Chemical Society

Figure 1. (a) NBN-type PAHs: dibenzo-fused 1,9-diaza-9a-boraphenalenes (NBN-DBP 1 and 2). (b) NBN σ-dimer 3 formation from NBN-DBP 1. (c) BNB-type DATB 4, BNB-type DBAP-Ph 5 with fivemembered ring, and NBN-6. (d) Our work: pentagonal (NBN-penta) and heptagonal (NBN-hepta) ring-embedded NBN-DBPs.

of the formation of aromatic cyclopentadienyl anion.5 More importantly, fusing a pentagonal ring onto the linear acenes can not only alter their electronic structures and physical properties but also preserve the same rigid planar skeleton as well.5b Instead of the positive curvature, saddle-shaped geometry with negative curvature can be achieved by fusing a heptagonal ring.6 In preparation of this Letter, a heptagonal ring embedded NBN PAH (NBN-6, Figure 1c) via oxidative coupling was reported by Received: January 6, 2019

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DOI: 10.1021/acs.orglett.9b00057 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Wang’s group, and its optoelectronic properties were examined.4 Nonetheless, pentagonal ring fused NBN PAHs have remained elusive. Herein, we demonstrate the efficient synthesis of novel pentagonal (NBN-penta, Figure 1d) and heptagonal (NBNhepta, Figure 1d) rings embedded NBN-DBPs using transition metal catalyzed intramolecular ring closure method. Single crystal structures reveal that NBN-penta adopts a planner geometry, while NBN-hepta displays a double-helical structure. Moreover, the resultant NBN-penta and NBN-hepta possess higher oxidation stability under electrochemical conditions than previous BN embedded PAHs7 and show blue fluorescence in solution. Besides, these two nonhexagonal ring embedded molecules exhibit unique global aromatic character8 with central antiaromatic NBN rings based on DFT calculations and X-ray analysis. The single crystal OFETs of both NBN-penta and NBN-hepta show hole charge carrier mobility above 0.01 cm2 V−1 s−1, indicating a potential in device applications of NBNembedded π-conjugated materials. The synthetic routes toward compounds NBN-penta and NBN-hepta are illustrated in Scheme 1. First, one side or two

Single crystals of NBN-penta and NBN-hepta were obtained by slow diffusion of a chloroform/methanol mixed solution. The NBN-penta crystallizes in the monoclinic space group C2 and shows a planar geometry (Figure 2a). The bond lengths of B1−

Figure 2. Single-crystal structures (hydrogen atoms are omitted for clarity) of (a) NBN-penta and (b) NBN-hepta with 50% probability of thermal ellipsoids. Colors: black, carbon; blue, nitrogen; pink, boron. R: benzene ring.

Scheme 1. Synthetic Routes toward NBN-Penta and NBNHepta

N1 (1.405 Å) and B1−N2 (1.423 Å) in NBN-penta are much shorter than a typical B−N single bond (1.58 Å) but slightly longer than a localized B−N double bond (1.403 Å), indicating that the B1−N1 bond is like a localized double bond.10 Compared to the reported NBN-DBP 1 and DBAP-Ph 5 (Figure 1),3a,c both B−N bond lengths in NBN-penta are slightly shorter, implying increased contribution of the delocalized πconjugation in NBN-penta. The packing diagram of NBNpenta shows a herringbone stacking mode with a π−π distance of 3.44−3.52 Å, which is below the sum of the van der Waals radius (3.60 Å).3a The NBN-hepta crystallizes in the orthorhombic space group Pbca. NBN-hepta possesses a twisted geometry due to the steric repulsion between two benzene rings (Figure 2b, R1/R2 and R3/R4), and the dihedral angles of C30−N1−C1−C2 and C13−N2−C12−C11 are 42.6° and 36.4°, respectively. The bond lengths of B1−N1 (1.451 Å) and B1−N2 (1.430 Å) in NBN-hepta are shorter than a typical B−N single bond (1.58 Å), meaning that the B−N bond is close to a double bond (1.403 Å).10 Compared to NBN-penta, the B−N bond lengths in NBN-hepta are much longer due to the nonplanar geometry of the seven-membered ring. Furthermore, the twisted conjugated skeleton contains double hetero[4]helicene substructures, which makes NBN-hepta a helical molecule with enantiomers MP and PM configuration in the crystal packing (Figure S2). NBN-hepta adopts slipped stacks by two enantiomers with a π−π distance of 3.43−3.56 Å (Figure 2b). The UV−vis absorption and fluorescence spectra of NBNpenta and NBN-hepta in anhydrous dichloromethane solution are presented in Figure 3a. There are two major electronic absorption bands in the regions of 225−302 and 302−369 nm for NBN-penta, and 225−333 and 333−397 nm for NBNhepta. The maximum absorption peaks of NBN-penta and NBN-hepta are 346 and 367 nm, respectively, which can be assigned to HOMO → LUMO transition based on the timedependent density functional theory (TD-DFT) calculations (Figured S76 and S77). NBN-penta and NBN-hepta exhibit blue fluorescence with a broad emission band between 350−450 and 375−500 nm, respectively (Figure 3a). The maximum emission peaks at 378 and 414 nm of NBN-penta and NBN-

side Buchwald−Hartwig reaction was performed based on 2′(trimethylsilyl)-[1,1′:3′,1″-terphenyl]-2,2″-diamine (6). Treatment of compound 6 with 1 or 2 equiv of 1-bromo-2iodobenzene provided N2-(2-bromophenyl)-2′-(trimethylsilyl)-[1,1′:3′,1″-terphenyl]-2,2″-diamine (7) and N2,N2″-bis(2bromophenyl)-2′-(trimethylsilyl)-[1,1′:3′,1″-terphenyl]-2,2″diamine (9) in 58% and 73% yield, respectively. Then heating of a solution of compound 7 or 9 in o-dichlorobenzene (o-DCB) at 180 °C in the presence of boron trichloride (BCl3) or boron tribromide (BBr3) with excess of trimethylamine (NEt3) gave 9(2-bromophenyl)-8H,9H-8,9-diaza-8a-borabenzo[fg]tetracene (8) or 8,9-bis(2-bromophenyl)-8H,9H-8,9-diaza-8a-borabenzo[fg]tetrac-ene (10) in 89% or 79% yield, respectively. Afterward, Buchwald−Hartwig reaction of 8 was performed to afford 4b,15b-diaza-4b1-borabenzo[fg]indeno[1,2,3-op]tetracene (NBN-penta) in 70% yield. Finally, 4b,15b-diaza-4b1borabenzo[fg]dibenzo-[4,5:6,7]cycl-ohepta[1,2,3-op]tetrac-ene (NBN-hepta) was obtained in 78% yield through intramolecular Yamamoto reaction (a useful method for the synthesis of boron-doped PAHs)9 of compound 10 under dilute concentration in toluene/dimethylformamide (DMF). NBN-penta and NBN-hepta were purified by chromatography using silica columns, then recrystallized from chloroform/ methanol (CHCl3/MeOH). The target compounds were fully characterized by 1H-, 13C-, 11B-NMR, and 2D NMR, high resolution mass spectroscopy and elemental analysis (see Supporting Information (SI)). B

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Figure 3. (a) UV−vis absorption and fluorescence spectra of NBN-penta and NBN-hepta in CH2Cl2 (concentration: 1 × 10−5 M). (b) Cyclic voltammograms of NBN-penta and NBN-hepta measured in CH2Cl2 (0.1 M n-Bu4NPF6) at the scan rate of 0.1 V/s. Insert: cyclic voltammogram recorded at the scan rate of 0.5 V/s.

hepta can be observed after excitation at 346 and 367 nm, respectively. The quantum yield of NBN-penta and NBN-hepta are estimated to be 0.22 and 0.23, respectively (quinine sulfate as the reference, Table S4). To understand the electrochemical behavior of NBN-penta and NBN-hepta, cyclic voltammetry (CV) was conducted in anhydrous dichloromethane (Figure 3b). NBN-penta displays an obvious irreversible oxidation wave with a peak maximum at 0.89 V vs Fc+/Fc. NBN-hepta shows a quasi-reversible oxidation process at the half-wave potential of 0.67 V vs Fc+/Fc if the scan rate increases (Figure 3b insert). Both compounds do not show any reduction processes in the cathodic potential range. Compared to our reported compound NBN-DBP 1 (a quasireversible wave at 0.51 V vs Fc+/Fc),3a NBN-penta and NBNhepta exhibit a stability advantage over oxidation. This is consistent with the titration results with SbCl5; NBN-penta and NBN-hepta were treated with excess of oxidant, but there was no obvious radical or radical cation signal arisen in the NIR region (Figures S68 and S69). To confirm this hypothesis, in situ ESR/UV-vis-NIR spectroelectrochemistry of NBN-penta was performed (Figure S70). No ESR signal was observed during the oxidation of the compound. In the backward potential scan, an ESR signal with hyperfine splitting originating from the interacting of the electron with a spin 1/2 nucleus was detected. A weak NIR band at 1180 nm appeared together with the ESR signal. The CV showed an additional reduction wave at low potential. All these observations pointed that a diamagnetic product was formed as a result of an electrochemical oxidation and follow-up chemical reaction. During the rereduction, this product underwent an electron transfer reaction with the formation of a radical structure. To gain deeper insight into the effect of benzocyclopenta- and dibenzocycloheptyl-fused rings on NBN motif, density functional theory (DFT) calculation at the B3LYP/6-31G(d) level was performed (Figure 4a). As expected, the lowest unoccupied molecular orbitals (LUMOs) of NBN-penta and NBN-hepta primarily reside on the NBN backbone instead of the nitrogen− boron−nitrogen atoms. The highest occupied molecular orbitals (HOMOs) of NBN-penta and NBN-hepta are fully delocalized over the entire molecule. The calculated HOMO levels are consistent with the CV experiments (Table S4). Compared to NBN-DBP 1,3a the LUMO and HOMO levels of NBN-penta and NBN-hepta become negative (∼0.4 eV), which may be caused by the introduction of pentagonal and heptagonal rings. To better understand the electronic structure, anisotropy of the induced current density (ACID) calculation (Figure 4b) and nucleus-independent chemical shift (NICS) calculation (Figure 4c) were conducted. The continuous current flow of the whole

Figure 4. (a) Calculated molecular orbitals and energy diagrams of NBN-penta and NBN-hepta. (b) Calculated ACID plots (isovalue = 0.05) of NBN-penta (top) and NBN-hepta (down). For ACID calculations, the direction of magnetic field is orthogonal to the XY plane and points upward. The clockwise (diamagnetic) and counterclockwise (paramagnetic) current flow are indicated by the red and black arrows, respectively. (c) NICS(0)zz values (in ppm) of compounds NBN-penta (top) and NBN-hepta (down) calculated at the GIAO-B3LYP/6-311+G(2d,p) level.

molecular backbone suggests that NBN-penta and NBN-hepta are globally aromatic, indicating that the π-electrons are delocalized over the molecules.8 Besides, the paramagnetic current flow on the center rings (the black arrow counterclockwise circle in Figure 4b) reveals the antiaromatic character. The NICS(0)zz values of the central NBN pentagonal ring in NBN-penta and NBN heptagonal ring in NBN-hepta are positive (Figure 4c), which reveal the strong antiaromatic character. On the contrary, the NICS(0)zz values of the outer benzene rings in NBN-penta and NBN-hepta are negative, indicating a global aromatic character in each compounds. Compared to the result of NBN-DBP 13a (Figure S75), NBNpenta and NBN-hepta exhibit extended π-conjugated structure as well as segregated aromaticity in different units. Single crystal OFETs of NBN-penta and NBN-hepta were fabricated to demonstrate their charge transport properties. The bottom gate and top contact geometry of single crystal OFETs are shown in Figure 5a. Heavily doped silicon substrates with 300 nm SiO2 were used for the device fabrication. Single crystal of NBN-penta was obtained by dip-coating from CHCl3 at a concentration of 1 mg/mL (Figure 5b). To achieve an efficient injection, 15 nm MoO3 and 100 nm Al were thermally evaporated as source and drain electrodes. The OFET measurement was performed in glovebox under nitrogen conditions. The average hole mobility of NBN-penta was around 0.05 cm2 V−1 s−1 with an on/off ratio of about 4 × 104. Single crystal of NBN-hepta was obtained by drop-casting from C

DOI: 10.1021/acs.orglett.9b00057 Org. Lett. XXXX, XXX, XXX−XXX

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emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yubin Fu: 0000-0002-2613-394X Evgenia Dmitrieva: 0000-0001-7490-617X Fupin Liu: 0000-0002-8454-726X Jan J. Weigand: 0000-0001-7323-7816 Alexey A. Popov: 0000-0002-7596-0378 Reinhard Berger: 0000-0002-8959-7821 Wojciech Pisula: 0000-0002-5853-1889 Junzhi Liu: 0000-0001-7146-0942 Xinliang Feng: 0000-0003-3885-2703

Figure 5. (a) Schematic illustration of single crystal FET geometry. Polarized optical microscopy images of the single crystals of (b) NBNpenta and (c) NBN-hepta. Transfer characteristics of FET devices under a Vds at −80 V based on the single crystals of (d) NBN-penta and (e) NBN-hepta.

Author Contributions ∇

CHCl3/methanol (1:1) mixed-solvent at a concentration of 1 mg/mL (Figure 5c). NBN-hepta showed a hole mobility of around 0.01 cm2 V−1 s−1 with an on/off ratio of about 5 × 105. The threshold voltages derived from the transfer curves in Figure 5d,e are around −5 V for both compounds, indicating a low injection barrier from the electrodes into the single crystals. In comparison to NBN-hepta, the higher mobility of NBNpenta is attributed to its planar molecular structure and herringbone crystal packing. The output curves of NBN-penta and NBN-hepta are shown in Figure S78. This charge transport performance is moderate in comparison to high performance organic semiconductors but impressive for NBN-embedded aromatic compounds. In summary, we have developed a facile synthetic approach toward pentagonal and heptagonal ring fused NBN-type PAHs. Employing pentagonal and heptagonal rings can internalize the NBN moieties as well as tune the geometry of the molecules. NBN-hepta displays a double hetero[4]helicene structure, while NBN-penta possesses a planar structure. Both compounds exhibit global aromaticity based on the DFT calculations and single crystal analysis, revealing the delocalized π-electrons in their entire molecules. Furthermore, single crystal transistors based on NBN-penta and NBN-hepta showed hole mobility of around 0.05 and 0.01 cm2 V−1 s−1, respectively. This work provides not only new possibilities for developing nonhexagonal, tailor-made NBN PAHs but also a broad class of NBNcontained semiconductors for organic electronics.

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Y.F. and K.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the European Union’s Horizon 2020 research and innovation program under grant agreement No. 696656 (Graphene Flagship Core2), the German Research Foundation (DFG) within the Cluster of Excellence “Center for Advancing Electronics Dresden (cfaed)” and EnhanceNano (No. 391979941) as well as the European Social Fund and the Federal State of Saxony (ESF-Project “GRAPHD”, TU Dresden) for financial support. K.Z. thanks the China Scholarship Council (CSC) for financial support. Diffraction data have been collected on BL14.3 at the BESSY II electron storage ring operated by the Helmholtz-Zentrum Berlin. We would particularly like to acknowledge the help and support of Manfred Weiss (BESSY II) and his group members during the experiment. We thank the Center for Information Services and High Performance Computing (ZIH) at TU Dresden for generous allocations of compute resources. We thank Dr. Tilo Lübken (Organic Chemistry, Technische Universität Dresden) for NMR analysis.

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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/acs.orglett.9b00057. Experimental details, synthetic procedures, DFT calculations, OFET characterizations, spectroelectrochemical data, and analytical data for new compounds (PDF) Accession Codes

CCDC 1885030 and 1885205 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by D

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DOI: 10.1021/acs.orglett.9b00057 Org. Lett. XXXX, XXX, XXX−XXX