Air Stable Chalcogen-Doped Rubicenes with Diradical Character

Open AccessCCS ChemistryCOMMUNICATION7 Dec 2022Air Stable Chalcogen-Doped Rubicenes with Diradical Character Liangzhuo Ma†, Song Wang†, Yuan Li†, Qinqin Shi, Wenbin Xie, Hao Chen, Xin Wang, Weiya Zhu, Lang Jiang, Runfeng Chen, Qian Peng and Hui Huang Liangzhuo Ma† College of Materials Science and Opto-Electronic Technology, Center of Materials Science and Optoelectronics Engineering, CAS Center for Excellence in Topological Quantum Computation, CAS Key Laboratory of Vacuum Physic, University of Chinese Academy of Sciences, Beijing 100049 , Song Wang† College of Materials Science and Opto-Electronic Technology, Center of Materials Science and Optoelectronics Engineering, CAS Center for Excellence in Topological Quantum Computation, CAS Key Laboratory of Vacuum Physic, University of Chinese Academy of Sciences, Beijing 100049 , Yuan Li† Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 , Qinqin Shi *Corresponding authors: E-mail Address: huemail protected E-mail Address: email protected College of Materials Science and Opto-Electronic Technology, Center of Materials Science and Optoelectronics Engineering, CAS Center for Excellence in Topological Quantum Computation, CAS Key Laboratory of Vacuum Physic, University of Chinese Academy of Sciences, Beijing 100049 , Wenbin Xie College of Materials Science and Opto-Electronic Technology, Center of Materials Science and Optoelectronics Engineering, CAS Center for Excellence in Topological Quantum Computation, CAS Key Laboratory of Vacuum Physic, University of Chinese Academy of Sciences, Beijing 100049 , Hao Chen College of Materials Science and Opto-Electronic Technology, Center of Materials Science and Optoelectronics Engineering, CAS Center for Excellence in Topological Quantum Computation, CAS Key Laboratory of Vacuum Physic, University of Chinese Academy of Sciences, Beijing 100049 , Xin Wang Key Laboratory for Organic Electronics and Information Displays short-distance radical species tend to be reactive. To meet the above requirements, PAHs with large fused π moieties or introduced bulky groups to protect radicals must be designed.13 For example, although the classic 7,7,8,8-tetracyanoquinodimethane is a closed-shell structure,9,14 the p-quinodimethane possesses diradical character with a relatively long air stability when the π-conjugation length was extended.15 However, elongating the conjugation length between two radicals may need extra synthetic steps, which is detrimental for their future applications. Therefore, various strategies have been developed to achieve stable diradicals with short π-conjugation lengths (Figure 1a, red frameworks). For example, Zimmermann et al.16 reported the air-persistent quinoidal bisimidazole benzene by a heteroatom-doping strategy. A heptazethrene derivative diradical was stabilized by introducing the electron-withdrawing dicarboximide group to achieve a half-life time (t1/2) as long as 6 days.17 The antiaromatic indenofluorenes demonstrated m-quinodimethane diradical character with stabilities for weeks due to the protection of radicals by bulky mesityl substitution.18 Thus, developing novel strategies to achieve PAHs with small diradical distance and good air stabilities is under high demand. Figure 1 | (a) Diradicals with p- or m-quinodimethane diradical skeletons. (b) Chalcogen containing rubicene with a p-benzoquinoidal diradical skeleton from this work. Download figure Download PowerPoint Rubicene-based PAHs are molecular fragments of fullerenes,19 which have attracted much interest in exploring novel synthetic methods20,21 and broadening their applications, including organic field effect transistors (OFETs), organic solar cells, and sensors.22–24 Recently, we reported a simple synthesis of chalcogen-containing rubicenes (S- and Se-rubicenes) for OFET applications.24 We hypothesize that the rubicene rings could present diradical properties due to the aromaticity enhancement of breaking two double bonds (Figure 1b).9 Herein, a new furan fused rubicene (O-rubicene) was synthesized and characterized to investigate the chalcogen effects on the diradical characters of rubicenes. Their diradical character and air stability were probed by crystallography, UV–vis spectroscopy, cyclic voltammetry (CV), variable-temperature nuclear magnetic resonance (VT-NMR), variable-temperature electron spin resonance (VT-EPR), and density functional theory (DFT) calculations. Meanwhile, the chalcogen effects on the diradical properties of rubicenes were addressed, which may provide a guideline for designing air stable PAHs with small diradical distances. Results and Discussion The O-rubicene was synthesized according to the reported method (Scheme 1).24 The cross-coupling between 2,2′-(2,5-dibromo-1,4-phenylene)difuran 1 and 4-methyl-N′-(1,2-bis(4-tertbutylphenyl)-ethylidene)benzenesulfonohydrazone 2 produced trisubstituted olefin 3, which was then cyclized to afford O-rubicene by the Scholl reaction in a modest yield. The final compound was fully characterized by 1H and 13C NMR, and high-resolution electrospray ionization mass spectroscopy. The O-rubicene exhibits good solubilities in most common solvents, such as chloroform and toluene. The thermal gravimetric analysis showed the decomposition temperature (Td) of O-rubicene is 217 °C ( Supporting Information Figure S1), lower than those of S- and Se-rubicenes.24 Scheme 1 | The synthetic route of O-rubicene. Download figure Download PowerPoint Slow diffusion of methanol into toluene solution afforded orange crystals of O-rubicene suitable for single-crystal X-ray diffraction. As shown in Figure 2, O-rubicene adopts an achiral antifolded backbone. Notably, O-rubicene possesses a relatively planar structure with a small torsion angle of 0.3° (Figure 2a). Due to its planar structure, O-rubicene presents strong π–π interactions with an interlayer distance of 3.58 Å (Figure 2b), which is smaller than those of S- (3.85 Å) and Se-rubicenes (4.22 Å).24 As usual, the analysis of bond lengths of O-rubicene, as well as S- and Se-rubicenes, suggests they are centrosymmetric molecules. Especially, the bond length alternation pattern of four Clar's π-sextets of rubicenes crystals (Figure 2c and Supporting Information Figures S2a and S2b) resemble the open-shell diradical form of O-, S-, and Se-rubicene in Figure 1b. In general, the lengths of the bonds in the open-shell structures are proportional to the diradical character.25 It is of note that the bond lengths between the benzene core and apical carbon of the five-membered ring (Figure 2d and Supporting Information Figures S2c and S2d) are elongated (O-rubicene: 1.494(5), 1.397(8), and 1.418(5) Å; S-rubicene: 1.493(2), 1.420(2), and 1.411(2) Å; Se-rubicene: 1.487(3), 1.423(4), and 1.411(3) Å.) in comparison with the corresponding bond lengths of rubicene (1.486(5), 1.404(4), and 1.394(4) Å).26 The delocalized partial double-bond character indicates contributions from both pure open-shell and closed-shell structures.25 All these observations suggest the open-shell diradical nature of these chalcogen doped rubicenes. Since the radical centers are located at the center of the fjord region, the torsion of the rubicene molecules may impart their good air stabilities due to the natural steric hindrance. Figure 2 | (a) The torsion angles, (b) packing modes, and (c and d) multicentered lengths (in Å) of O-rubicene crystal. Download figure Download PowerPoint Next, the optical properties of O-, S-, and Se-rubicene in hexane solutions were investigated with UV–vis–near-infrared (NIR) spectroscopy. Similar to S- and Se-rubicene, O-rubicene exhibits featured characteristic clusters of absorption peaks with a fine vibronically structured band (max ca. 502 nm, ε = 1.0 × 104 M−1cm−1), together with a weak and long tail extending to 800 nm in solvents and films (Figure 3a and Supporting Information Figure S3). The existence of a weak long tail accords with the previous diradical species.5,27–31 In addition, the absorption peaks of O-rubicene are slightly bathochromically shifted to those of S-rubicene, while hypsochromically shifted to those of Se-rubicene, which agree well with the B3LYP/6-31G(d,p)-calculated electronic transitions for O-, S-, and Se-rubicenes ( Supporting Information Figure S10 and Tables S1–S3). The photoluminescence quantum yields for O-, S-, and Se-rubicenes in chloroform solution are 6.9%, 2.4%, and 0.5% ( Supporting Information Figure S4), respectively. This phenomenon indicates that the materials with strong radical properties have more channels for nonradiative transition, resulting in weak luminescence.29,32,33 Since diradical compounds usually present redox amphoterism characteristics,25 CV was employed to investigate their electrochemical properties (Figure 3b and Supporting Information Figure S5). Interestingly, the CV curves show two reversible reductions and two quasi-reversible oxidation peaks for O-rubicene. The two half-wave reduction and two half-wave oxidation potentials versus ferrocene are at −2.23 and−1.75 V, and 0.94 and 0.68 V, respectively, for O-rubicene similar to those of S- and Se-rubicenes.24 Whereas, we noticed one irreversible reduction and one irreversible oxidation at −1.0 to −1.3 V and 0 to 0.2 V versus ferrocene for O-, S-, and Se-rubicene (red circles in Figure 3b), which may lead to narrow bandgaps corresponding to the diradical species.28,34 To investigate the stability of these diradical compounds, the time-resolution UV–vis spectra of O-, S-, and Se-rubicenes in air-saturated chloroform solution under ambient condition were measured (Figure 3c). By monitoring their maximum absorption peaks ( Supporting Information Figure S6), the t1/2 were determined to be 62, 52, and 66 days (fitted from first-order kinetics35) for O-, S-, and Se-rubicenes, respectively. This excellent air stability may be ascribed to the twisted structures of rubicenes,36 which is rare and astonishing considering the diradical distances are very small in these rubicenes.6,37–39 Figure 3 | (a) UV–vis–NIR spectra of rubicenes in hexane. (b) CV of rubicenes in CH2Cl2/0.1 M n-Bu4N+PF6− with ferrocenium/ferrocene as an internal standard. (c) Absorption spectral loss for rubicenes under ambient condition over 20 days. Download figure Download PowerPoint Surprisingly, the 1H NMR spectra of the O-, S-, and Se-rubicenes at room temperature displayed well-resolved signals. This observation is different from those of other PAH diradicals, which mostly show broadened signals at room temperature.6,40 The VT-NMR data are used to evaluate whether the ground state of a diradical specie is a singlet or triplet.41 To probe the mechanism, 1H NMR spectra of O-, S-, and Se-rubicenes in 1,2-dichloro-3,4,5,6-tetradeuteriobenzene solution were recorded every 25 K when they were slowly heated to 423 K (Figures 4a–4c). It was observed that the proton peaks are obviously broadened and featureless due to the thermal population of paramagnetic triplet species. These observations indicate the ground states of all three rubicene diradicals are singlet, whereas the Se-rubicene may have the smallest singlet–triplet energy gap (ΔEST).10 Figure 4 | The aromatic region of VT-NMR spectra of O-rubicene (a, 9.60–7.29 ppm), S-rubicene (b, 9.50–7.35 ppm), and Se-rubicene (c, 9.40–7.40 ppm) in o-C6D4Cl2. Download figure Download PowerPoint Furthermore, magnetic properties of O-, S-, and Se-rubicenes solids were measured via EPR and superconducting quantum interference device analysis. These rubicenes possess broad and distinct EPR signals with g values of approximately 2.0026 at room temperature ( Supporting Information Figure S7). With every 20 K increase in temperature, the EPR signal strength gradually improved. By fitting the VT-EPR data (Figures 5a–5c) with the Bleaney–Bowers equation, the experimental ΔEST for O-, S-, and Se-rubicenes are −3.06, −4.11, and −3.78 kcal/mol, respectively. Such large energy gaps have been rarely reported, consistent with the observation in VT-NMR.5,28 The largest ΔEST of the S-rubicene indicate that portion of the open-shell diradicals is probably shorter than those of O- and Se-rubicenes, since it has to conquer the highest thermal energy barrier. This trend is similar to the results in VT-NMR. To rule out the effect of impurities, the single crystals and sublimated high purity S-rubicene were employed for EPR measurements, which show the identical EPR signals and similar ΔEST ( Supporting Information Figures S8 and S9), revealing the intrinsic diradical characteristics of these rubicenes.28 All these results indicate these rubicenes are mainly presenting singlet diradical ground states with large ΔEST, which explains why these rubicenes still show well-resolved peaks in NMR spectra even at high temperature. Figure 5 | The VT-EPR spectra of O-rubicene (300–480 K), S-rubicene (300–600 K), and Se-rubicene (300–540 K) in solids. Insets shows fitted IT-T curves according to the Bleaney–Bowers equation; g factor was taken to be 2.0026. Download figure Download PowerPoint To gain insight about the diradical properties of chalcogen doped rubicenes, nucleus independent chemical shifts (NICS)42 and anisotropy of the induced current density (ACID)43 calculations were performed based on the B3LYP/6-31G(d,p) level. As shown in Figures 6a–6c, the ground states of closed-shell O-, S-, and Se-rubicenes displayed NICS values of −19.76, −21.84, and −34.10, respectively, in the chalcogen containing rings, suggesting chalcogen doping enhances the aromaticity of rubicenes. In contrast, the aromaticity of the central benzene cores decreases to −21.44, −20.11, and −19.77 with the doping of O, S, and Se, respectively, indicating the propensity of the formation of the benzoquinoidal structures and tendency of diradical nature in comparison with rubicene's central core (−22.21) (Figure 6d). Therefore, such enhancement of the aromaticity from heterocycles makes the diradical forms of O-, S- and Se-rubicene energetically favorable. These results were further corroborated by ACID calculation (Figures 6e–6h),28 which clearly shows four clock-wise diatropic currents for O-, S-, Se-rubicenes and rubicene itself. The largest diatropic currents for O-, S-, and Se-rubicene should account for the enhancement in the aromaticity of diradicals. Both NICS and ACID results accord with the crystal structure and the open-shell diradical form, thus further confirming the formation of the open-shell rubicene diradical. To present the diradical character of rubicenes, a closely related quantity called fractional occupation number weighted electron density (NFOD; FOD = Fractional Occupation Number Weighted Electron Density) using finite-temperature density functional theory (FT-DFT) were performed, which quantifies the strong electron correlation, and a larger NFOD signifies multireference character.44 Four illustrative FOD plots (Figures 6i–6l) clearly identified the radical position at the central benzene core, which demonstrated two large and delocalized FODs reflecting their well-established diradicaloid characters. The NFOD values of O-, S-, and Se-rubicene are 1.651, 1.754, and 1.830, respectively, much larger than that of closed-shell rubicene (1.195) and consistent with that of reported tetracyclopentadef,jkl,pqr,vwx-tetraphenylene diradical (1.744).44,45 Additionally, the theoretical calculations at the spin-projected unrestricted Hartree–Fock level of theory and 6-31G(d,p) basis set were carried out to further confirm the ground state electronic structure of these small molecules. The radical character is defined by the indexes, y0 (diradical character index), where y0 (i = 0, 1) can have values as y0 = 0 (closed-shell), 0 < y0 < 1 (intermediate open-shell), and y0 = 1 (pure open-shell). The y0 values of O-, S-, and Se-rubicene are 0.20, 0.21, and 0.17, respectively, suggesting modest diradical character.34,35,46 The modest diradical character across this series is attributed to the proquinoidal nature of the benzene cores and the enhanced aromaticity from chalcogen doping.28 Figure 6 | NICS values (a, b, c, and d) and ACID plots (e, f, g, and h) of O-, S-, Se-rubicenes, and rubicene calculated at the B3LYP/6-31G (d,p) level. The red arrows indicate clockwise ring current. FOD plots (i, j, k, and l) at σ = 0.003 eBohr−3 (B3LYP/def2-SVP (T = 9000 K) level) for O-, S-, Se-rubicenes and rubicene (FOD in yellow). Download figure Download PowerPoint Conclusion A new O-rubicene-based PAH was synthesized together with S- and Se-rubicenes to investigate the chalcogen effects on the diradical characteristics. The detailed analysis of single crystal structures, UV–vis, CV, VT-NMR, and VT-EPR reveal that these chalcogens containing rubicenes possess diradical character with singlet ground states due to a large ΔEST. The increase of Clar's π-sextets is the driving force of the diradicals' formation, which is ascribed to the aromaticity enhanced by chalcogen atoms, supported by NICS and ACID studies. Impressively, these rubicenes possess the shortest π-conjugation lengths between two radicals and excellent air stabilities, which may be ascribed to aromaticity enhancement from the chalcogen effects and steric protection from twist structure in the fjord-region of rubicenes. Therefore, this contribution provides a simple strategy of constructing air stable PAHs with diradical character. Supporting Information Supporting Information is available and includes synthesis procedures, selected crystal parameters, EPR measurements, and theoretical calculations (PDF). Conflict of Interest The authors declare no competing financial interest. Funding Information The work was supported by the NSFC (grant nos. 51925306, 21774130, and 21905277), National Key R Suzuki S.; Sato K.; Takui T.Synthetic Organic Spin Chemistry for Structurally Well-Defined Open-Shell Graphene Fragments.Nat. Chem.2011, 3, 197–204. Google Scholar 2. Hu X.; Wang W.; Wang D.; Zheng Y.The Electronic Applications of Stable Diradicaloids: Present and Future.J. Mater. Chem. C2018, 6, 11232–11242. Google Scholar 3. Lombardi F.; Lodi A.; Ma A.; K.; K.; X.; from the Topological of Molecular Google Scholar and with a Chem. Google Scholar S.; and Diradicals from Google Scholar Chen Hu Y.; A.; with a Chem. Google Scholar S.; Y.; Wang State Diradical Chem. 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