Sulfur-Containing, Quinodimethane-Embedded Acene Analogue with

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Sulfur-Containing, Quinodimethane-Embedded Acene Analogue with Nine Consecutively Fused Six-Membered Rings Yang Chen, Huilin Kueh, Tullimilli Y. Gopalakrishna, Shaoqiang Dong, Yi Han, and Chunyan Chi* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543

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S Supporting Information *

ABSTRACT: Linear quinodimethane-embedded acene analogue 9L was synthesized, and its quinoidal structure was confirmed by X-ray crystallographic analysis. The multiple oxidation states of 9L could be achieved. Its dication is a triplet diradical, and its tetracation can be regarded as the isoelectronic structure of the nonacene, which was validated by experiments and theoretical calculations.

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cene-based materials have shown promising applications,1 but acenes longer than pentacene are highly reactive.2 Various strategies have been developed to stabilize long acenes, such as by kinetic blocking,2,3 substitution by electron-deficient groups,4 and incorporation of heteroatoms into the backbone.5 Our group demonstrated that incorporating two electron-rich sulfur atoms and embedding a para-quinodimethane (p-QDM) unit into the acene framework would disturb the diene-like conjugation and result in stable quinoidal acene analogues.6 Subsequent two-electron oxidation led to isoelectronic structures of acenes. However, the longest isoelectronic acene prepared by this method is limited to heptacene due to openshell diradical character of the precursors.6b We also successfully synthesized a dipolar quinoidal nonacene analogue, which has the same number of π-electrons to nonacene but exhibits different electronic properties.7 Therefore, synthesis of stable isoelectronic structures of even longer acenes is of great interest. Herein, our new design is to incorporate two p-QDM units and four electron-rich sulfur atoms into the π-conjugated acene framework, and four-electron oxidation of the resulting linear quinoidal nonacene analogue 9L would result in the isoelectronic structure of nonacene (Figure 1). Bulky mesityl groups are attached onto the terminal methylene sites of all pQDM units to guarantee good stability and solubility. The synthesis is shown in Scheme 1. Nucleophilic substitution of excess 1 with 1,4-benzenedithiol gave the intermediate dibromo compound 2 in 40% yield, and then the key intermediate tetraketone compound 3 was synthesized in 78% yield from compound 2 by direct nucleophilic substitution with 4-tert-butylthiophenol. Reduction of 3 by LiAlH4 gave the tetraol 4, and subsequent BF3·Et2O-mediated Friedel−Crafts alkylation generated the dihydro compound 5. It should be pointed out that the meta-dithia isomer, instead of the paradithia compound, was obtained, and it is believed that the 1,2sulfur migration via a spirocyclic cationic intermediate occurred.7 Finally, compound 9L was obtained by oxidative dehydrogenation with p-chloranil. It is very stable in both solution and solid state, which is similar to that of our previously reported heterocyclic quinodimethanes.6−8 No obvious decom© XXXX American Chemical Society

Figure 1. (a) Structures of 9L. (b) Representative canonical structures of the neutral form, dication, and tetracation of the backbones 9L′. Mes: mesityl.

position was observed when the solution of 9L was stored for 2 weeks or its solid was stored for 1 year in ambient air and light conditions. A single crystal of 9L suitable for X-ray crystallographic analysis was successfully grown (Figure 2a). The backbone of 9L is slightly disordered. The mesityl groups are almost perpendicular to the backbone. Large bond-length alternation (BLA) was observed for the p-QDM units in the molecule, indicating that the closed-shell quinoidal form contributes most to its ground-state geometry. DFT ((U)CAM-B3LYP/631G(d,p)) calculations indicate that 9L has a closed-shell ground state. The two-terminal benzenoid rings in the molecule display relatively small BLA, and nucleus-independent chemical shift (NICS)9 calculations show that they have a large negative NICS(1)zz value (−18 ppm; see Figure 2a), indicating their large aromatic character. The central benzenoid ring of each pReceived: March 5, 2019

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DOI: 10.1021/acs.orglett.9b00805 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 1. Synthesis of 9L

Figure 3. Absorption spectra of the neutral (N), radical cation (R), dication (D), radical trication (TR), and tetracation (TE) states of 9L in DCM (a) and cyclic voltammograms of 9L in DCM (b). Insets are photos of the solutions.

Stepwise titration of 9L by NO·SbF6 gave its corresponding radical cation 9L•+, dication 9L2+, radical trication 9L•3+, and tetracation 9L4+. 9L•+ displayed a long wavelength absorption with λmax at 2163 nm (Figure 3a). 9L2+, 9L•3+, and 9L4+ displayed a blue-shifted absorption band, with λmax at 1652, 1672, and 750 nm, respectively. The trend is consistent with TD-DFT calculations (see SI). Both 9L•+ and 9L•3+ showed an intense one-line ESR spectrum with ge = 2.0042 (Figure 4a), indicating that the spin was partially distributed to the sulfur atom (Figure 5a,c). For 9L2+, an intense ESR spectrum (ge = 2.0036) with hyperfine structure that matched well with

Figure 2. X-ray crystallographic structures, selected bond lengths (Å), and calculated NICS(1)zz values (red and blue numbers in hexagons) of (a) 9L and (b) 9L•3+. Hydrogen atoms are omitted for clarity.

QDM unit in the closed-shell form shows a nearly zero NICS(1) zz value, indicating nonaromatic character. Compound 9L in DCM displayed an intense absorption band with well-resolved peaks at 652/605/556 nm (Figure 3a). Due to the nearly C2v symmetry, 9L has partially segregated HOMO/ LUMO (Figure S11 in Supporting Information (SI)), with a notable dipole moment of 0.6107 D (Figure S16 in SI). 9L is not fluorescent. It exhibited four oxidation waves with a half-wave potential, E1/2ox, at −0.25, 0.07, 0.50, and 1.03 V and two reduction waves with E1/2red at −2.11 and −2.29 V (vs Fc+/Fc). (Figure 3b and Figure S1 in SI). The HOMO and LUMO energy levels were estimated to be −4.49 and −2.77 eV, respectively. As a result, its HOMO−LUMO gap is 1.72 eV.

Figure 4. (a) ESR spectra of (1) 9L, (2) 9L•+, (3) 9L••2+, (4) 9L•3+, and (5) 9L4+ (0.5 mmol/L) recorded in DCM at room temperature; (b) VT ESR spectra of 9L••2+ in DCM solution from 160 to 105 K in the frozen state. The ESR spectrum of 9L••2+ becomes broad when the temperature goes below 160 K. B

DOI: 10.1021/acs.orglett.9b00805 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

Figure 5. Calculated (UCAM-B3LYP/6-31G(d,p)) spin density distribution of (a) 9L•+, (b) 9L••2+, and (c) 9L•3+.

Figure 6. 1H NMR spectra (aromatic region) of 9L in benzene-d6 (a) and 9L4+ in CD2Cl2 (b). Insets are their corresponding ACID plots. (c) Change of 9L into 9L4+ with labels of the aromatic protons.

simulation was observed (Figure 4a and Figure S3 in SI), and the ESR intensity increased with decreasing temperature (Figure 4b). No NMR signal could be observed for 9L2+ from room temperature to −80 °C. Combination of these experimental observations and its spin density distribution map (Figure 5b) reveals that it is a triplet diradical (i.e., 9L••2+) (Figure 1b and Figure S10 in SI). The single crystal of 9L•3+ was obtained (Figure 2b). The lengths of C11−S3, C12−S3, C13−S4, and C14−S4 bonds decreased by about 0.5−0.6 Å compared with that of 9L, and the lengths of C7−S1 and C9−S2 bonds also show obvious C−S double bond character. The lengths of C6−S1 and C10−S2 bonds are between the C−S single bond and double bond. The calculated charge distribution map (Figure S18 in SI) and spin density distribution map (Figure 5c) of 9L•3+ indicate that the positive charges are mainly delocalized along the central backbone, whereas the radical is mainly delocalized along the C1−C5 edge (red color in Figure 2b). Clear 1H NMR spectrum of the tetracation 9L4+ was obtained in DCM-d2 via in situ oxidation by four-fold NO·SbF6. The resonances of most aromatic protons of the 9L appeared between 5.5 and 7.0 ppm due to its quinoidal character (Figure 6a). However, the resonances of 9L4+ were largely downfield shifted (Figure 6b), implying its aromatic character. Anisotropy of the induced current density (ACID) plots10 of 9L4+ clearly displays clockwise diatropic ring currents delocalized along the periphery, similar to nonacene (Figure 6 and Figure S14 in SI), whereas the ring currents are mainly localized at the terminal benzene rings for 9L (Figure 6). In addition, NICS calculations suggest that all of the benzenoid rings in the 9L4+ show negative NICS(1)zz values (Figure S15 in SI). The absorption spectrum of 9L4+ (Figure 3a and Figure S2 in SI) shows a band structure similar to that of Anthony’s nonacene derivative.2d This suggests that 9L4+ is a genuine isoelectronic structure of nonacene. It is relatively stable. No obvious decomposition was observed when the dry CD2Cl2 solution of 9L4+ was stored for 24 h in an inert atmosphere under ambient light conditions. In summary, linear sulfur-containing, p-QDM-embedded acene analogue 9L with nine consecutively fused six-membered rings was synthesized. It showed multiple oxidation states, and

its corresponding radical cation, dication, radical trication, and tetracation could be prepared by stepwise titration with NO· SbF6. The dication is a triplet diradical. The tetracation could be regarded as a genuine isoelectronic structure of nonacene. Our study provides a reasonable approach to access isoelectronic structures of the highly reactive polycyclic aromatic hydrocarbons such as long acenes, periacenes, and zigzag-edged graphene nanoribbons.

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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.9b00805. Synthetic procedures and characterization data of all new compounds, details for all physical characterization and theoretical calculations, and additional spectroscopic data (PDF) Accession Codes

CCDC 1833080 and 1833087 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 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 Author

*E-mail: [email protected]. ORCID

Chunyan Chi: 0000-0003-4677-3546 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MOE Tier 1 grant (R-143-000-692114), Tier 2 grant (MOE2018-T2-1-152), and Tier 3 program C

DOI: 10.1021/acs.orglett.9b00805 Org. Lett. XXXX, XXX, XXX−XXX

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(MOE2014-T3-1-004). We thank Dr. Tan Geok Kheng from National University of Singapore for the crystallographic analysis.

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DOI: 10.1021/acs.orglett.9b00805 Org. Lett. XXXX, XXX, XXX−XXX