Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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From Phenanthrylene Butadiynylene Macrocycles to S‑Heterocycloarenes Yong Yang, Ming Chu, and Qian Miao* Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
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S Supporting Information *
ABSTRACT: The first member of S-heterocycloarenes was synthesized from an easily prepared phenanthrylene ethynylene macrocycle through diyne cyclocondensation and a Scholl reaction. The solution-processed thin film of this S-heterocycloarene behaved as a p-type semiconductor.
C
rings, as predicted by McWeeny8 and described by Clar with his aromatic sextet rule.9 The 1H NMR spectra of successfully synthesized cycloarenes all exhibit deshielded inner protons with chemical shifts larger than 7 ppm.4−6 This deshielding effect is indicative of π-electrons that are delocalized not in a large annulene, but only in individual benzenoid rings according to Clar’s bonding model. Cycloarenes are also macrocyclic nanographenes, which can serve as models for defects in graphene2 and may be envisaged as the repeat unit of periodic “graphene meshes”.6 In addition, the intrinsic cavity in cycloarenes can accommodate analyte molecules or ions6 for sensing applications. Replacing one or more benzenoid rings in cycloarenes with heterocycles leads to heterocycloarenes, which have similar macrocyclic π-systems as their hydrocarbon cousins. Despite their interesting structures and potential applications, only a handful of cycloarenes,4−6,10 N-heterocycloarenes,11,12 and π-extended cycloarenes13 have been synthesized and characterized, presumably because of the difficulties in synthesis. Herein, we report the first member of S-heterocycloarenes (1 in Figure 1), which is a hexathieno-fused derivative of Sheterocycloarene (highlighted in green). The parent hydrocarbon of 1 is cycloarene 2, which, unlike the known cycloarenes, encloses triphenylene in its cavity. The design of 1 and 2 involves phenanthrene as a building block, which was used in combination with different linkers to construct a variety of macrocycles by taking advantage of the angular geometry and regioselective functionalization of phenanthrene.6,14−16 Detailed below are the synthesis and characterization of 1 as well as a brief investigation of its semiconductor properties in thin film transistors. As shown in Scheme 1, we initially conceived the synthesis of S-heterocycloarene 3 through PtCl2-catalyzed cyclization17,18 of hexaethynylated macrocycle 4, which could, in principle, be
ycloarenes are polycyclic aromatics consisting of fully annelated benzene rings in a macrocyclic system with inward-pointing C−H bonds.1−3 As a result, cyclcoarenes enclose a cavity, which can be filled with a benzene ring in kekulene (Figure 1), the first member of cycloarenes synthesized
Figure 1. Structures of cycloarenes (highlighted in blue) enclosing the corresponding carbocycles and structure of S-heterocycloarene (1).
by Staab and Diederich in 1978,4 to yield a completely fused structure. The cavities in larger cycloarenes, septulene (Figure 1)5 and octulene,6 can be filled with seven- and eight-membered rings, respectively. Cycloarenes have served as ideal test cases for answering a long-standing and fundamental question about the nature of aromaticity: whether π-electrons in polycyclic arenes are globally delocalized throughout the entire system, as hypothesized by Pauling,7 or only delocalized in benzenoid © XXXX American Chemical Society
Received: May 28, 2018
A
DOI: 10.1021/acs.orglett.8b01668 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Attempted Synthesis of 3
Scheme 2. Synthesis of 1
synthesized from an easily prepared phenanthrylene thienylene macrocycle (5)15 by bromonation and subsequent Sonogashira coupling. However, this plan was frustrated by the first step. Attempted bromination of the thiophene moieties in 5 under various conditions failed to afford 6 in an acceptable yield. Only treatment with bromine in chloroform at room temperature for 15 min afforded a trace amount of 6, which was identified with 1 H NMR. Increasing the reaction time led to decomposition of 6 instead of a higher yield. To avoid the intractable β-bromination of thiophene moieties in 5, we modified the design of 4 by replacing the ethynyl groups with thiophene or benzene rings and attaching them to the phenanthrene moieties. As a result, the macrocyclic framework of 3 would be extended with six fused thiophene or benzene rings. Scheme 2 shows the synthesis of S-heterocycloarene 1 from 3,6-dibromo-2,7-diiodo-9,10-phenanthrenequinone (7)19 in five steps. The first step involved three subsequent reactions without purification of the intermediates: the Suzuki−Miyaura coupling of 7 with the corresponding arylboronic acid, followed by reduction with sodium hydrosulfite and treatment with 1bromohexane under basic conditions. As a result, diarylated phenanthrene 8a and 8b were isolated in yields of 46% and 39%, respectively. The Sonogashira coupling of 8a/b, followed by desilylation under basic conditions, resulted in the ethynylsubstituted phenanthrene monomer, which was subjected to the Eglinton reaction in diluted solutions for macrocyclization. Trimeric phenanthrylene macrocycles 10a and 10b were obtained in yields of 37% and 39%, respectively, while the corresponding dimeric or tetrameric macrocycles were not isolated. Treatment of 10a/b with Na2S·9H2O20 under conventional conditions15 failed to convert the diyne moieties into thiophene rings in an acceptable yield. This problem was solved by using a copper(I) catalyst,21 which increased the yields of 11a and 11b to 47% and 49%, respectively. Macrocycles 11a/ b were finally subjected to the Scholl reaction, which was rarely employed for macrocyclic precursors in the literature.13 After a few conditions including the commonly used DDQ/acid system22 were screened for the Scholl reaction, it was found that treatment of 11a with FeCl3 in CH2Cl2 at 0 °C for 10 min afforded 1 in a yield of 53%, which corresponded to a yield of
90% per C−C bond. Prolonging the reaction time led to gradual decomposition of 1. In contrast, the reaction of 11b under a variety of conditions did not afford macrocycle 12 as a detectable product. The distinct results of 11a and 11b toward the Scholl reaction can be rationalized by considering the most reactive site of the substrate in the cation radical (electron transfer) mechanism23 as discussed in Section 4 of the Supporting Information (SI). Single crystals of 11a and 11b suitable for X-ray crystallography were grown by slow diffusion of acetonitrile into their respective solutions in CH2Cl2. Unfortunately, our attempts to grow qualified single crystals of 1 did not succeed. The phenanthrylene thienylene macrocycles of 11a and 11b in the crystals exhibit essentially the same geometry, which indicates poor conjugation between the thiophene rings and phenanthrene rings in the macrocycle. Figure 2a shows the crystal structure of 11a, where the thiophene rings and the phenanthrene rings are connected in the macrocycle forming large dihedral angles (42.5°−73.0°) and long C−C single bonds (shown in green). Their bond lengths are in the range 1.46 to 1.48 Å, which is typical for nonconjugated single bonds between two sp2-hybridized C atoms.24 One interesting finding from the crystal structures of 11a and 11b is that they form supramolecular dimers of the same packing motif. Figure 2b shows the B
DOI: 10.1021/acs.orglett.8b01668 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 2. (a) Structure of 11a in the crystals (hexyl groups and hydrogen atoms are removed for clarity, and C, O, and S atoms are shown as ellipsoids at the 50% probability level); (b) supramolecular dimer of 11b in the crystals.
Figure 3. Partial 1H NMR spectra of 11a (top) and 1 (bottom).
effect indicates that π-electrons that are delocalized only in individual benzenoid and thiophene rings according to Clar’s bonding model. The three-dimensional structure of 1 was studied with density functional theory (DFT) calculations using a simplified model molecule (1′), which has methyl groups replacing the hexyl groups in 1. Geometry optimization at the B3LYP/6-31G* level of DFT revealed two diastereomeric conformers, (M, M, P) and (M, M, M)-1′, which are global and local energy minima, respectively, and are both shaped like shallow saddles as shown in Figure 4. (M, M, P)-1′ has two left-handed and one right-
dimeric structure of 11b, where the cavity in the macrocycle of one molecule accommodates one hexyl chain of the other molecule. A phenanthrene ring in one molecule of 11b overlaps with that in the other molecule with a π-to-π distance as large as 4.0 Å. On the basis of the unambiguously characterized structure of 11a, the molecular structure of 1 was determined with HRMS and NMR spectra. The MALDI-TOF HRMS exhibited a molecular ion peak with m/z of 1855.5188, indicative of a molecular formula of C114H102O6S9. The 1H NMR spectrum of 1 showed four sets of peaks in the aromatic region in a ratio of 1:1:1:1, in agreement with a 3-fold symmetry. The two singlet peaks at 8.22 and 8.67 ppm are attributable to the para protons (Ha and Hb in Figure 3) on the benzene ring, while the two doublets with a coupling constant of 4.8 Hz are attributable to the two thiophene protons at the 4- and 5-positions (Hc and Hd in Figure 3), which have a typical 3JH−H coupling constant of 4.8 Hz.25 In comparison to 1, molecule 11a exhibits six sets of peaks in the aromatic region in its 1H NMR spectrum. The three double−doublets are attributed to the side 3-thienyl group, and particularly, the double−doublet at 7.27 ppm with coupling constants of 1.2 and 2.8 Hz should be attributed to the 2-proton (He in Figure 3), which has typical 4JH−H coupling constants of 1.0 and 2.8 Hz with the protons at the 4- and 5-positions.25 The singlet at 7.02 ppm is attributed to the thiophene β-proton (Hf in Figure 3) in the macrocycle. Disappearance of the signals for He and Hf in the 1H NMR spectra of 1 indicates that the Scholl reaction of 11a occurred at the 2-position of the side 3-thienyl group to form a new C−C bond. This regioselectivity is in agreement with the fact that the α-sites in thiophene are more reactive than the β-sites toward electronphiles. In addition, a chemical shift larger than 8 ppm (8.22 or 8.67 ppm) indicates that the inner protons (Ha) in 1 are deshielded. This deshielding
Figure 4. Energy-minimized models of (M, M, P)-1′ and (M, M, M)-1′ as calculated at the B3LYP/6-31G* level of DFT.
handed thieno[5]helicene moieties, while (M, M, M)-1′ is D3 symmetric having three left-handed thieno[5]helicene moieties and is less stable than (M, M, P)-1′ by 2.6 kcal/mol. As a result, the observed broadening of the 1H NMR peaks for the aromatic protons of 1 (Figure 3) can be attributed to exchange of (M, M, P) and (M, M, M)-1 as well as their enantiomers in solution. In agreement with this, when a solution of 1 was heated to 100 °C, the 1H NMR peaks became sharp as shown in Figure S26 in the SI. S-Heteroarene 1 is a bright yellow solid with green fluorescence in solution when excited with UV light. As shown C
DOI: 10.1021/acs.orglett.8b01668 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
supported by the Research Grants Council of Hong Kong (GRF 14303614).
in Figure S3 in the SI, the UV/vis absorption spectrum of 1 exhibits the longest-wavelength absorption maximum at 455 nm. The fluorescence spectrum of 1 exhibits a relatively small Stokes shift of 0.20 eV, which suggests a fairly rigid π-backbone. The cyclic voltammogram of 1 in CH2Cl2 (Figure S4 in the SI) shows one irreversible oxidation wave with the peak potential at 0.35 V versus ferrocenium/ferrocene. From this oxidation potential, the HOMO energy level of 1 is estimated as −5.45 eV,26 which suggests that 1 can in principle function as a p-type semiconductor in the solid state. Thin films of 1 were deposited by drop-casting a solution of 1 in CH2Cl2 onto a silicon substrate, which had successive layers of silica, alumina, and 12cyclohexyldodecylphosphonic acid27 as a composite dielectric material.28 The device fabrication was completed by depositing a layer of gold onto the film of 1 through a shadow mask to form top-contact source and drain electrodes. As measured in ambient air, 1 functioned as a p-type semiconductor with a field effect mobility of up to 2.7 × 10−4 cm2/(V s) (Figure S5 in the SI). This low mobility is attributable to the amorphous nature of the film, which was found from the absence of diffraction peaks when the film was investigated with X-ray diffraction. In conclusion, we established a facile route to synthesize the first member of S-heterocycloarenes (1) from an easily prepared phenanthrylene ethynylene macrocycle through the diyne cyclocondensation and Scholl reaction. It was found that 1 functioned as a p-type semiconductor in solution-processed thin film transistors. Combination of semiconductor property and an intrinsic cavity suggests that cycloarenes and heterocycloarenes may have interesting applications. The cavity may accommodate counteranions for the purpose of doping or bind analyte molecules for the purpose of chemical sensing based on organic thin film transistors.
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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.8b01668. Details of synthesis and characterization, DFT calculations, fabrication and characterization of thin film transistors, NMR spectra (PDF) Accession Codes
CCDC 1840961−1840962 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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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qian Miao: 0000-0001-9933-6548 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Ms. Hoi Shan Chan (the Chinese University of Hong Kong) for the single crystal crystallography. This work was D
DOI: 10.1021/acs.orglett.8b01668 Org. Lett. XXXX, XXX, XXX−XXX
