Contorted Naphtho- and Fluorenocoronenes: Syntheses and

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

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Contorted Naphtho- and Fluorenocoronenes: Syntheses and Properties of Polycyclic Aromatics beyond Benzo- and Thiophenocoronenes Sushil Kumar and Yu-Tai Tao* Institute of Chemistry, Academia Sinica, Taipei 11521, Taiwan S Supporting Information *

ABSTRACT: Selective oxidative cyclodehydrogenation reactions on olefins carrying larger aromatic units are described to offer modular access to contorted polyaromatics of naphtho- and fluorenocoronenes, which are more extended frameworks than the known benzocoronenes. The annulation proceeded regioselectively when more than one mode was possible, based either on electronic or steric effect. The single-crystal field-effect transistor based on one of the derivatives, dinaphtho[a,d]coronene, gave a p-carrier mobility of 0.04 cm2/(V s) and on/off current ratio of 102−103.

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developed by Müllen and others,10 a comprehensive approach to larger yet contorted polyaromatics is less well-developed. In particular, we are interested in systems that fuse coronene with other aromatic moieties such as naphthalene, fluorene, carbazole, and so on, into more extended yet contorted molecules. In achieving this, one key intermediate is the diaryl olefin, through which oxidative cyclodehydrogenation with the anthracenyl moiety on the other side of the double bond would give the coronene framework fused with the prepositioned aryl groups having a fjord region to cause twisting of the molecule. Depending on the connecting position of the aryl group and the direction of fusion, various frameworks are possible (Scheme 1).

olyfused aromatics have been vigorously pursued over the years because of their rich photophysical and electronic properties associated with π-conjugate frameworks.1 Particularly in the new era of organic electronics, where organic semiconducting molecules play central roles, the availability of various polyaromatics is ever important. A unique virtue of using organic compounds is that the molecule can be tailored in size, shape, geometry, bandgap, and so on, through rational design and synthesis, to possess the properties required in various applications, such as organic light-emitting diodes, field-effect transistors, solar cells, and sensors.2 Charge transport between semiconducting molecules is one property highly sought after in most of these applications. It is generally accepted that the charge transfer rate depends on the electronic coupling between neighboring molecules and the reorganization energy of a molecule upon charged/neutral state change, both of which are expected to benefit from a more extended π-system because it increases the intermolecular contacts and thus chances of greater π−π overlap. In addition, the structural changes and thus reorganization energy between the charged and neutral states may be reduced through charge delocalization over a larger framework.3 The electronic coupling is much dominated by the molecular packing, which relates to the size, geometry, substituent, and electron distribution in the molecular framework.4 Contorted polyaromatics have a tendency to pack cofacially due to self-complementarity in shape,5 so that a larger intermolecular electronic coupling and thus a higher charge transport rate along the π-stacking direction can be expected. Contorted tetrabenzocoronene, hexa-cata-hexabenzocoronene, and thiophenocoronenes have been explored by a number of groups for their charge mobility study in solution or solid/ crystalline state.6−8 The p-carrier mobility of these derivatives are higher compared to some of the known planar benzocoronenes.9 While a number of synthetic platforms for planar hexa-perihexabenzocoronenes and other larger polyaromatics have been © XXXX American Chemical Society

Scheme 1. Selective Formation of Naphtho- and Fluorenocoronenes from Their Precursor Olefins

Received: February 25, 2018

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DOI: 10.1021/acs.orglett.8b00666 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Here, we report the design and synthesis of naphthalene- and fluorene-fused coronenes through oxidative cyclodehydrogenations of respective key olefins. As it turns out, the cyclodehydrogenation reaction was selective in that only one of the two possible annulation modes was obtained, either for steric reason or electronic reason.11 The overall preparations of the targeted molecules are sketched in Schemes 2−4, while the preparations of the prefinal

Scheme 4. Formation of Tetranaphthobenzo-, Tetranaphthodibenzo-, and Tetrafluorenodibenzocoronenes (TNBC, TNDBCs, and Et-TFDBC)

Scheme 2. Synthesis of Dinaphtho- and Dinaphthodibenzocoronenes (DNCs and DNDBCs)

Scheme 3. Formation of Tetranaphthocoronenes (TNCs)

Table 1. Conversion of Diaryl Olefins to Annulated Derivatives

di- and tetraaryl olefins (4a−4c, 6a−6b, 8a−8d, 10a, and 12a− 12e) are given in the Schemes S2−S6 in the Supporting Information. The di- and tetraaryl olefins, the main precursors of oxidative annulation reactions, were synthesized in excellent yields by employing Suzuki coupling reactions on their dichloro or tetrabromo olefin intermediates. Some of the olefins (4b and 12a−12c), when treated with iodine12 (with 1:2.1−4.1 mol ratio of olefin and iodine), yielded directly the doubly annulated products, whereas others (4a, 6a− b, 8a−c, and 12e) gave a mixture of singly and doubly annulated products. Further treatment of these mixtures with iron chloride led to the doubly annulated products. However, the oxidative annulations of olefins 4c, 8d, and 12d having 1-naphthyl moieties were not successful with iodine, iron chloride, or with other more reactive reagents (Table 1, entries 3, 9 ,and 14). Thus, the abovementioned olefins did not furnish projected DNC1, TNC1, and TNDBC1. Rather, starting materials were recovered. The 2naphthyl-based olefins gave selective annulation reactions at the 1-position to give DNCs, DNDBCs, TNCs, TNBC, and TNDBCs, while annulation at the 3-position (TNC2 and

a

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Product did not form. bLess stable coronenes.

DOI: 10.1021/acs.orglett.8b00666 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

The naphtho- and fluorenocoronenes exhibited yellow to orange colors in their solid states. The electronic absorption spectra of those of soluble naphthocoronenes are sketched in Figures 2 and S59a,b, while the related data is enclosed in Table

TNDBC2) was not obtained (Schemes 2−4). An alkoxy substituent at the 6-position of naphthyl group increased the reactivity, as lower reaction time was needed for the cyclodehydrogenation reaction, with the same point of annulation at the 1-position (4b, 6b, 8b−8c, and 12b−12c and in Table 1, entries 2, 5, 7, 8, and 12−13). It was noted that for TNDBC, Hex-TNDBC fully “annulated” products with four additional cyclized six-membered rings were not obtained, even with further treatment with FeCl3. The selective annulation forming TNCs and TNDBCs at the 1-position rather than TNC2s and TNDBC2s was rationalized based on the suggested mechanism in which a more stable radical cation intermediate was formed at the 1-position, where a benzylic cation was involved (Scheme S7). In contrast, annulation at the 3-position would involve a nonaromatic cation.13 However, the recovering of 1-naphthyl-based olefins suggests even the first half annulation was disfavored. Molecular mechanics indicates that annulation at the 2-position requires a very unfavorable conformation due to steric repulsion between the two naphthalene rings. In the 2-fluorenyl-based olefin, oxidative cyclodehydrogenation at the 3-position is favored over the 1-position, again due to steric reason (Scheme 1). Thus, after the annulation reaction, olefin 12e afforded the less crowded, fused framework of Et-TFDBC, rather than the Et-TFDBC1. The same positional selectivity was observed for all the annulation sites for the di- or tetra-substituted derivatives. The chemical structures for naphthocoronenes and alkoxysubstituted ones were mainly established from NMR and highresolution mass spectra, with the crystal structures of DNC previously reported,14 and the newly prepared Oct-DNC determined in this study. The Oct-DNC molecule shows distortion due to steric congestion at the fjord area of the twonaphthyl units (Figure 1).

Figure 2. Normalized absorption spectra of alkoxy-substituted naphthocoronenes.

S2. The parent coronene hydrocarbon shows three absorption bands at the 220, 340, and 350 nm, while naphtho- and fluorenocoronenes exhibit a similar absorption pattern of coronene and benzocoronenes.15 The absorption maxima of naphtho- and fluorenocoronenes depend on the geometry and πconjugation of their fused frameworks. For example, tetranaphtho-fused Hex-TNC had a greater bathochromic shift in absorption maximum than dinaphthofused Oct-DNC, and expanded π-conjugation in fluorene-fused Et-TFDBC resulted in a bathochromic shift in its absorption maxima, compared to naphthalene-fused Hex-TNDBC. Although the naphtho- and fluorenocoronenes displayed more distortion of their molecular frameworks with respect to hexacata-hexabenzocoronene and thiophenocoronene analogues, they exhibited longer wavelength absorption with respect to later derivatives, possibly because of extended π-conjugation by naphthalene and fluorene units. The coronene derivatives showed green to orange emission in their dilute solutions (Figure S59c−e). The emission spectra of these derivatives exhibited pronounced longer wavelength emission bands with respect to reported benzocoronenes such as DBCs,12 TBCs,6,7 or HBCs.8 Oxidation potentials of the selected derivatives were measured by cyclic voltammetry, with dichloromethane as the solvent and 0.1 M tetrabutylammoniumhexafluorophosphate as the supporting electrolyte (Figure S60 and Table S3). Limited by the solubility, only Hex-TNC and Et-TFDBC derivatives could be measured, and both produced a single quasi-reversible oxidation wave, which was suggested to be due to oxidation of the central coronene. Their HOMO values were estimated to be 5.10 and 5.35 eV, respectively. To demonstrate the utility of these polyaromatic derivatives as a semiconductor, a single crystal field-effect transistor (SCFET) device (Figure S61a−c) was attempted. Among the derivatives, only DNC crystals were successfully grown by a vapor phase transfer method.16 A top-contact, top-gate FET device was fabricated by laminating the needle-like crystal on a glass substrate, with painted colloidal graphite as the source and drain. A thin film of perylene was grown on the crystals as the dielectric, whereas colloidal graphite on its top was the gate electrode.14 With the channel length, width, and parylene thickness of 1.0− 0.5 mm, 0.25−0.20 mm, and 1.8−2.5 μm, respectively, the

Figure 1. (a) π−π stacks of Oct-DNC molecule showing interplanar distance between two closer molecules. (b) Packing arrangement of Oct-DNC molecules.

Twisting at the fjord area forces Oct-DNC molecules to adopt slipped cofacial π−π stacks in which two adjacent naphthocoronene units were separated by 3.587 Å. This stacking pattern is somewhat different from parent DNC in which a better cofacial overlap is observed. Presumably, the two-octyl chains at the periphery much perturb the packing, so that the two molecules slide off from more cofacial packing found in DNC. As observed from their optimized molecular structures, the naphtho- and fluorenocoronenes have slightly more severe distortion in their fjord regions than their benzocoronene counterparts. Additionally, the HOMOs and LUMOs have maximum electron density centered on the coronene portion of the molecules (Figures S57−S58). Both of the above features may influence their optoelectronic properties. C

DOI: 10.1021/acs.orglett.8b00666 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters highest mobility of 0.04 cm2/(V s), an average mobility of 0.02 cm2/(V s), and an on−off current ratio of 102−103 (Figures S62−S63 and Table S1) were obtained. In conclusion, we have successfully synthesized new classes of polyaromatics through extending coronene frameworks by naphthalene and fluorene units. The key reaction of oxidative cyclodehydrogenation of diaryl olefins exhibited regioselectivity to give only specific frameworks when there was more than one mode of annulation. These derivatives were characterized by spectroscopic techniques, whereas the fused framework of derivative Oct-DNC was structurally evidenced by its singlecrystal X−-ray analysis. A single crystal based field-effect transistor with one of the derivatives DNC gave the highest ptype field-effect mobility of 0.04 cm2/(V s) and an on/off ratio of 102−103.

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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.8b00666. Detailed experimental procedures and complete spectroscopic analysis (PDF) Accession Codes

CCDC 1825674 contains 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 data_ [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]. Fax: +886-2-27831237. Tel: +886-2-27898580. ORCID

Yu-Tai Tao: 0000-0003-4507-0419 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Science and Technology, Taiwan (Grant Number: 106-2113-M-001-003), is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b00666 Org. Lett. XXXX, XXX, XXX−XXX