Seeing Is Believing: A Wavy N-Heteroarene with 20 Six-Membered Rings Linearly Annulated in a Row

Open AccessCCS ChemistryCOMMUNICATION7 Nov 2022Seeing Is Believing: A Wavy N-Heteroarene with 20 Six-Membered Rings Linearly Annulated in a Row Zhongbo Zhang, Zilong Wang, Naoki Aratani, Xiaozhang Zhu and Qichun Zhang Zhongbo Zhang School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 , Zilong Wang School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 , Naoki Aratani *Corresponding authors: E-mail Address: email protected E-mail Address: email protected Division of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192 , Xiaozhang Zhu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 and Qichun Zhang *Corresponding authors: E-mail Address: email protected E-mail Address: email protected Department of Materials Science and Engineering, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR 999077 https://doi.org/10.31635/ccschem.022.202202013 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Structurally well-defined arenes with higher numbers of linearly-fused six-member rings (n ≥ 10) are important and highly desirable not only because of their experimental challenges but also due to their fundamental interests and the curiosity of their structure–property relationships. In this research, we successfully synthesized a novel large N-heteroarene ( N20) with 20 linearly-fused six-membered rings, which is the longest N-heteroarene (4.7 nm) confirmed by single-crystal X-ray diffraction (SCXRD) analysis so far. The as-prepared arene displays a wave-like framework rather than a rigid plane. Moreover, N20 shows high molar absorptivity of 364,000 M−1 cm−1 with its fluorescence extended to the near-infrared region. Download figure Download PowerPoint Introduction Although N-heteroarenes (including N-heteroacenes) have been widely demonstrated as promising active elements in optoelectronic devices,1–7 the effort to develop higher N-heteroarenes (n ≥ 10) never ends. For example, Mateo-Alonso et al. reported several larger N-heteroarenes ( NR10,8 NR13,9 NR20,8 NR30,8 NR33,9 and NR54,9 where N or NR represents N-heteroarene and the number stands for the amount of linearly annulated six-membered rings). Although these molecules have been confirmed through 1H NMR spectra and high-resolution mass spectra (HRMS), information regarding their exact crystal structures as well as their arrangement in the solid state is missing. Such gaps are frustrating, especially with respect to the structure-property relationships. Thus, pursuing the single-crystal structures of larger N-heteroarenes (n ≥ 10) and a deep understanding of their structure-property relationship has become a new focal area. Until now, a total of six larger N-heteroarenes ( N10,10 N11s,11,12 N12,13 N13,14 and N1515) with single-crystal structures have been reported, where N11s with different functional groups has been reported independently by the Mastalerz group11 and Baumgarten group,12 N13 has been developed by the Bunz group,14 and the rest ( N10, N12, and N15) are contributions from our group.10,13,15 These results have already provided hints that larger N-heteroarenes might possess twisted or wavy structures rather than rigid planes. However, a clear conclusion still requires more evidence. Continuing in this research direction, we successfully synthesized a novel N-heteroarene ( N20) containing 20 linearly-fused six-membered rings16 through the classic condensation reaction between diamines and diketones by adopting Anthony's " TIPS" (TIPS = triisopropylsilyl) strategy17,18 to increase solubility and stability as well as the cross-conjugated characteristics of pyrene units to quickly extend the conjugation.19,20 Although N20 was first synthesized by our group in 2017, it took us almost 4 years to grow the suitable single crystals of N20 for structural analysis.16 Eventually, the single-crystal structure of N20 was solved and reported here together with the theoretical study and physical properties. It is worth mentioning that N20 is the longest N-heteroarene (>4.7 nm) with its structure confirmed by single-crystal X-ray diffraction (SCXRD) analysis. Results and Discussion The synthetic detail of N20 is shown in Scheme 1. Diketone 115 and tetraamine 212 were prepared according to previously reported procedures. N20 was obtained by an acid-catalyzed condensation between diketone 1 and tetraamine 2 in a mixed solvent of chlorobenzene/acetic acid under 140 °C for 3 days in 48% yield. The medium solubility and excellent stability allowed for its purification in column chromatography. The as-prepared N20 was confirmed by 1H NMR spectroscopy ( Supporting Information Figure S1), HRMS ( Supporting Information Figure S2), and SCXRD analysis (CCDC number: 2130117). However, the limited solubility impedes the acquisition of 13C NMR spectra even at elevated temperature. Further solubility tests indicated that N20 is soluble in several solvents including carbon disulfide, chloroform, chlorobenzene, and 1,2-dichlorobenzene with limited solubility (<1 mg/mL), while it was insoluble in other solvents such as dioxane, ethanol, methanol, and acetonitrile. Scheme 1 | Synthetic route to N20 (TIPS = triisopropylsilyl). Download figure Download PowerPoint High-quality black single crystals of N20 suitable for SCXRD analysis were obtained by slow evaporation of a mixed solution (chloroform/carbon disulfide = 3:1) containing N20 under ambient conditions. SCXRD analysis shows that a single crystal of N20 belongs to a monoclinic system (P21/c space group). The unit cell volume is as large as 12517(2) Å3 with the following dimensions: a = 34.752(3) Å, b = 19.7170(18) Å, c = 18.2977(17) Å and β = 93.2741(19)° (more details in Supporting Information Table S3). As shown in Figure 1a, 20 linearly-fused six-membered rings in a row can be clearly observed. The end-to-end length of its backbone is more than 4.7 nm, which is the longest linearly-fused N-heteroarene proven by SCXRD so far. From the side view (Figure 1b), two end benzene rings are twisted in opposite directions from the central pyrene unit, leading to a wavy backbone of N20. The dihedral angle between the terminal benzene and central pyrene is 22.7°, where the two terminal benzene rings are parallel to one another ( Supporting Information Figure S16). This is different from the theoretically optimized planar structure ( Supporting Information Figure S7), which will be discussed later. Figures 1c–1e show the molecular packing details. Two adjacent molecules form a dimer with a close π–π distance of ∼3.26 Å, suggesting the existence of non-negligible π–π interactions in the intradimer although π-plane overlapping of two adjacent molecules is limited ( Supporting Information Figure S15). Then, the as-formed dimers adopt a one-dimensional face-to-edge herringbone stacking mode with very weak interactions (the shortest C–H…π distance between two adjacent dimers is ∼2.5 Å, Supporting Information Figure S14). This structure and packing details offer some guidelines for the exploration of large N-heteroarenes in device fabrication and theoretical investigation. Figure 1 | X-ray crystallographic structure of N20: (a) top view, (b) side view in which peripheral functional groups are omitted for clarity. Crystal packing of N20: (c) side view along the long axis of conjugated backbone, (d) top view, (e) side views perpendicular to the long axis of conjugated backbone, in which top molecules are blue and bottom molecules are green, and H atoms are omitted for clarity. Download figure Download PowerPoint We examined the UV–vis spectra of N20 in dilute chloroform at a concentration of 10−5 mol L−1 (Figure 2) and film state ( Supporting Information Figure S3), respectively. Seven peaks were observed in the range of 400–700 nm and the longest wavelength peak was located at 648 nm, where the strongest peak at 536 nm displayed a high ε value of 364,000 M−1 cm−1. The UV–vis spectrum of N20 in film state showed an obvious bathochromic shift, and the peaks in the range of 400–700 nm are more distinguishable compared with its spectrum in solution. This is attributed to the existence of strong intermolecular interactions of N20 in the solid state. The optical bandgap (Eopt) is 1.77 eV, which was calculated from the absorption onset of the film-based UV–vis spectrum. We also conducted time-dependent density functional theory (TD-DFT) calculations to discern the origin of the absorption features of N20. The simulated total absorption spectra and the contributions from main electronic transitions were conducted in Multiwfn21 ( Supporting Information Figure S11), which were in good agreement with experimental results. The excitation energies at 700 and 699 nm correspond to the S0 → S1 and S0 → S2 transition, which can be attributed to the contribution of HOMO → LUMO+1 (90%) and HOMO–1 → LUMO (92%), respectively ( Supporting Information Table S1). The excitation energies at 652, 649, and 530 nm are assigned to the S0 → S7, S0 → S8, and S0 → S21, respectively. Figure 2 | The UV–vis absorption (gray) and PL (red, λex = 536 nm) spectra of N20 in chloroform. Download figure Download PowerPoint It is worth noting that 700 nm might be the saturation of the UV–vis–near-infrared (NIR) absorption maximum for N-heteroarene without donor-acceptor structures because N1010 (667 nm (abs onset), 1.86 eV (bandgap)), N1111 (550 nm, 2.25 eV), N1112 (629 nm, 1.97 eV), N1515 (670 nm, 1.85 eV), NR139 (625 nm, 1.98 eV), NR208 (632 nm, 1.96 eV), NR308 (630 nm, 1.97 eV), NR339 (630 nm, 1.97 eV), and NR549 (640 nm, 1.94 eV)) have smaller absorption wavelengths. However, the D–A design would lead to a longer UV–vis–NIR absorption maximum ( N1213: 950 nm (abs onset), 1.30 eV; and N1314: 1300 nm (abs onset), 0.95 eV or 1199 nm (abs onset), 1.03 eV). According to these reported results, the saturation of the UV–vis–NIR absorption maximum of N20 may be a function of its length. The photoluminescence (PL) of N20 was excited at 393, 500, and 536 nm, respectively. We found that its fluorescence extends to the NIR region and covers a wide range from 600 to 850 nm with two peaks located at 673 and 728 nm, which are longer than those of the family members of N-heteroarenes.8–12,15 The emission characteristics do not depend on the excitation wavelength ( Supporting Information Figure S5a), suggesting that the emission arises from only one excited species. Excitation spectra with emission wavelengths at 673, 728, and 800 nm were also examined, respectively ( Supporting Information Figure S5b). The excitation spectra matched well with the absorption spectrum, indicating that the excitation stems from one single species in the ground state. The fluorescence quantum yield (Φ) of N20 is 12.9% ( Supporting Information Figure S6). Cyclic voltammetry (CV) was studied in film state in anhydrous acetonitrile (0.1 M n-Bu4NPF6 as electrolyte) with Fc+/Fc as a reference ( Supporting Information Figure S4). N20 shows one quasi-reversible reduction peak in the negative potential region, from which the lowest unoccupied molecular orbital (LUMO) energy level is calculated to be −3.58 eV from the onset of the first reduction potential. The highest occupied molecular orbital (HOMO) energy level is −5.35 eV, which is derived from the difference between LUMO energy level and Eopt. To further understand its optoelectronic properties theoretically, DFT calculations at the B3LYP/6-31G(d,p) level in the Gaussian 16 package were performed.22 The optimized structures and isosurface graphs of frontier molecular orbitals of N20 are shown in Figures 3a–3d. The optimized structures show good planarity of the N20 backbone, which is different from the experimentally obtained structure. This might be attributed to ignoring the molecular interactions during calculations. The calculated HOMO and LUMO energy levels are −5.43 and −3.42 eV, respectively, which are in good agreement with experimental results (±0.16 eV). The electronic distribution of HOMO orbital is mainly localized over one end of the molecule. The HOMO and HOMO–1, HOMO–2, and HOMO–3 are quasi-degenerated, respectively. The electron distribution of the LUMO orbital is mainly localized over the other end of molecule with longer spread. Four LUMO orbitals are quasi-degenerated ( Supporting Information Figure S9). The Anisotropy of the Induced Current Density (ACID) plot showed a clockwise (diatropic) ring current circuit around the N20 periphery, suggesting that the electrons are globally delocalized along the periphery (Figure 3e). Nevertheless, by comparing the ACID plots with different isovalues ( Supporting Information Figure S10), the ring current at the K-regions of pyrene is weaker than other parts, implying that the pyrene units restrict electron global delocalization to some extent. Because of this unique characteristic of pyrene, the optical bandgap of N20 does not show an obvious bathochromic shift compared with N15.15 This phenomenon was also observed in Mateo-Alonso's molecules ( N13, N33, and N54).9 Figure 3 | Optimal molecular structure and frontier molecular orbitals of N20 calculated by DFT at the B3LYP/6-31G(d,p) level: (a) top view, (b) side view, (c) isosurface graph of HOMO, and (d) isosurface graph of LUMO. Isopropyl substituents are replaced by methyl groups to simplify the calculation. (e) Calculated AICD plot of N20 with an isovalue of 0.01. Download figure Download PowerPoint Conclusions We have reported the synthesis of a novel large N-heteroarene with 20 six-membered rings linearly annulated in a row. The structure of N20 was confirmed by 1H NMR spectra, HRMS, and SCXRD analysis. The precise structure and wavy backbone are clearly presented. N20 sets a record of the longest linearly-fused N-heteroarene (4.7 nm) confirmed by SCXRD analysis so far. The as-prepared N20 possesses a narrow band gap of 1.77 eV, high molar absorptivity of 364,000 M−1 cm−1, and fluorescence in the NIR region, which endows it significant potential in organic optoelectronic devices. Supporting Information Supporting Information is available and includes experimental procedures, NMR spectra, Mass spectra, CV, emission and excitation spectra, theoretical calculations, and single-crystal X-ray analysis. Conflict of Interest There are no conflicts of interest to report. Funding Information Q.Z. thanks the support from starting funds from City University of Hongkong. The work at NAIST was supported by Grant-in-Aid for Scientific Research No. JP20H02711 (NA). References 1. Bunz U. H.; Engelhart J. U.; Lindner B. D.; Schaffroth M.Large N-Heteroacenes: New Tricks for Very Old Dogs?Angew. Chem. Int. Ed.2013, 52, 3810–3821. Google Scholar 2. Mateo-Alonso A.Pyrene-Fused Pyrazaacenes: From Small Moleculs to Nanoribbons.Chem. Soc. Rev.2014, 43, 6311–6324. Google Scholar 3. Miao Q.Ten Years of N-Heteropentacenes as Semiconductors for Organic Thin-Film Transistors.Adv. Mater.2014, 26, 5541–5549. Google Scholar 4. Bunz U. H.The Larger Linear N-Heteroacenes.Acc. Chem. Res.2015, 48, 1676–1686. Google Scholar 5. Li J.; Zhang Q.Linearly Fused Azaacenes: Novel Approaches and New Applications Beyond Field-Effect Transistors (FETs).ACS Appl. Mat. Interfaces2015, 7, 28049–28062. Google Scholar 6. Cai Z.; Awais M. A.; Zhang N.; Yu L.Exploration of Syntheses and Functions of Higher Ladder-Type π-Conjugated Heteroacenes.Chem2018, 4, 2538–2570. Google Scholar 7. Zhang Z.; Zhang Q.Recent Progress in Well-Defined Higher Azaacenes (n ≥ 6): Synthesis, Molecular Packing, and Applications.Mater. Chem. Front.2020, 4, 3419–3432. Google Scholar 8. Cortizo-Lacalle D.; Mora-Fuentes J. P.; Strutyński K.; Saeki A.; Melle-Franco M.; Mateo-Alonso A.Monodisperse N-Doped Graphene Nanoribbons Reaching 7.7 Nanometers in Length.Angew. Chem. Int. Ed.2018, 57, 703–708. Google Scholar 9. Dubey R. K.; Melle-Franco M.; Mateo-Alonso A.Twisted Molecular Nanoribbons with up to 53 Linearly-Fused Rings.J. Am. Chem. Soc.2021, 143, 6593–6600. Google Scholar 10. Wang Z.; Miao J.; Long G.; Gu P.; Li J.; Aratani N.; Yamada H.; Liu B.; Zhang Q.Full Characterization and Photoelectrochemical Behavior of Pyrene-fused Octaazadecacene and Tetraazaoctacene.Chem. Asian J.2016, 11, 482–485. Google Scholar 11. Kohl B.; Rominger F.; Mastalerz M.A Pyrene-Fused N-Heteroacene with Eleven Rectilinearly Annulated Aromatic Rings.Angew. Chem. Int. Ed.2015, 54, 6051–6056. Google Scholar 12. Hu B.-L.; Zhang K.; An C.; Schollmeyer D.; Pisula W.; Baumgarten M.Layered Thiadiazoloquinoxaline-Containing Long Pyrene-Fused N-Heteroacenes.Angew. Chem. Int. Ed.2018, 57, 12375–12379. Google Scholar 13. Gu P.-Y.; Wang Z.; Liu G.; Yao H.; Wang Z.; Li Y.; Zhu J.; Li S.; Zhang Q.Synthesis, Full Characterization, and Field Effect Transistor Behavior of a Stable Pyrene-Fused N-Heteroacene with Twelve Linearly Annulated Six-Membered Rings.Chem. Mater.2017, 29, 4172–4175. Google Scholar 14. Endres A. H.; Schaffroch M.; Paulus F.; Reiss H.; Wadepohl H.; Rominger F.; Krämer R.; Bunz U. H. F.Coronene-Containing N-Heteroarenes: 13 Rings in a Row.J. Am. Chem. Soc.2016, 138, 1792–1795. Google Scholar 15. Wang Z.; Gu P.; Liu G.; Yao H.; Wu Y.; Li X.; Rakesh G.; Zhu J.; Fy H.; Zhang Q.A Large Pyrene-Fused N-Heteroacene: Fifteen Aromatic Six-Membered Rings Annulated in One Row.Chem Commun.2017, 53, 7772–7775. Google Scholar 16. Wang Z.Large N-Heteroacene Derivaties: Synthesis, Characterization and Application as n-Type and Ambipolar Semiconductos. Ph.D Thesis, Nanyang Technological University, Singapore, 2018. Google Scholar 17. Anthony J. E.The Larger Acenes: Versatile Organic Semiconductors.Angew. Chem. Int. Ed.2008, 47, 452–483. Google Scholar 18. Payne M. M.; Parkin S. R.; Anthony J. E.Functionalized Higher Acenes: Hexacene and Heptacene.J. Am. Chem. Soc.2005, 127, 8028–8029. Google Scholar 19. Cortizo-Lacalle D.; Gozalvez C.; Melle-Franco M.; Mateo-Alonso A.A Thiadiazole-Capped Nanoribbon with 18 Linearly Fused Rings.Nanoscale2018, 10, 11297–11301. Google Scholar 20. Zhang Q.; Divayana Y.; Xiao J.; Wang Z.; Tiekink E. R. T.; Doung H. M.; Zhang H.; Boey F.; Sun X. W.; Wudl F.Synthesis, Characterization, and Bipolar Transporting Behavior of a New Twisted Polycyclic Aromatic Hydrocarbon: 1',4'-Diphenyl-naphtho-(2'.3':1.2)-pyrene-6'-nitro-7'-methyl Carboxylate.Chem Eur. J.2010, 16, 7422–7426. Google Scholar 21. Lu T.; Chen F.Multiwfn: A Multifunctional Wavefunction Analyzer.J. Comput. Chem.2012, 33, 580–592. Google Scholar 22. Frisch M. J.; Trucks G. W.; Schlegel H. B.; Scuseria G. E.; Robb M. A.; Cheeseman J. R.; Scalmani G.; Barone V.; Petersson G. A.; Nakatsuji H.; Li X.; Caricato M.; Marenich A. V.; Bloino J.; Janesko B. G.; Gomperts R.; Mennucci B.; Hratchian H. P.; Ortiz J. V.; Izmaylov A. F.; Sonnenberg J. L.; Williams D.; Ding F.; Lipparini F.; Egidi F.; Goings J.; Peng B.; Petrone A.; Henderson T.; Ranasinghe D.; Zakrzewski V. G.; Gao J.; Rega N.; Zheng G.; Liang W.; Hada M.; Ehara M.; Toyota K.; Fukuda R.; Hasegawa J.; Ishida M.; Nakajima T.; Honda Y.; Kitao O.; Nakai H.; Vreven T.; Throssell K.; Montgomery J. A.; Peralta J. E.; Ogliaro F.; Bearpark M. J.; Heyd J. J.; Brothers E. N.; Kudin K. N.; Staroverov V. N.; Keith T. A.; Kobayashi R.; Normand J.; Raghavachari K.; Rendell A. P.; Burant J. C.; Iyengar S. S.; Tomasi J.; Cossi M.; Millam J. M.; Klene M.; Adamo C.; Cammi R.; Ochterski J. W.; Martin R. L.; Morokuma K.; Farkas O.; Foresman J. B.; Fox D. J.Gaussian 16Rev. C.01; Gaussian, Inc.: Wallingford, CT, 2016. Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentVolume 4Issue 11Page: 3491-3496Supporting Information Copyright & Permissions© 2022 Chinese Chemical SocietyKeywordsN-heteroarenessingle-crystal structurelinear annulationoptical properties Downloaded 1,105 times PDF downloadLoading ...