Letter Cite This: Org. Lett. 2018, 20, 4512−4515
pubs.acs.org/OrgLett
Soluble Twisted Diarenoperylenes: Synthesis, Characterization, and Device Performance Xinyue Liu,† Meng Chen,† Chengyi Xiao,† Ning Xue,† and Lei Zhang*,†,‡ †
College of Energy, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ State Key Lab of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
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S Supporting Information *
ABSTRACT: Two stable TIPS-ethynyl functionalized polycyclic aromatic hydrocarbons (PAHs), dibenzo[a,j]perylene (TIPS-DBP), and dinaphtho[a,j]perylene (TIPS-DNP), which contain two rows of linear acenes joined by benzene rings, have been synthesized and characterized. It is found that the two twisted PAHs easily form one-dimensional charge-transport systems with short C−C contacts. The crystal of TIPS-DBP shows a hole mobility up to 0.17 cm2 V−1s−1, while the crystal of TIPS-DNP shows a hole mobility up to 0.74 cm2 V−1 s−1.
D
uring the past decades, the linear acenes have been studied extensively for their interesting optoelectronic properties and potential device applications.1 However, it is now clear that large linear acenes that have relatively high carrier mobilities readily undergo photoinduced oxidation, dimerization, or polymerization due to the biradical character in the ground state.2 In recent years, different strategies have been developed to prepare stable linear acenes through the addition of either electron-withdrawing or bulky substituents on the reactive sites to decrease the lowest unoccupied molecular orbital (LUMO) energy levels.1−3 On the other hand, peri-fusing two rows of linear acenes together has recently emerged as another promising strategy to topologically tune their π electron structures, which can significantly affect stability and optoelectronic properties.4 In addition to providing the possibility of preparing two-dimensional nanographenes or double helicenes,5 this strategy indeed has allowed for the resulting polycyclic aromatic hydrocarbons (PAHs) with an additional Clar sextet, which typically are more stable than linear analogs with only one Clar sextet.6 Therefore, there has been a large amount of synthetic activity to prepare peri-fused PAH systems.7 For example, Briseno et al., Frigoli et al., Tykwinski et al., and Chen et al. independently reported a series of compounds so-called bistetracenes that contain two rows of tetracene moieties fused at the zigzag positions with different fusion modes (Figure 1).8 These molecules with two Clar sextets exhibit charge carrier mobilities in the range of 0.1 to 6.1 cm2 V−1 s−1 with remarkable stability in solution. However, Müllen and Feng synthesized another “bistetracene” molecule, tetrabenzo[a,f,j,o]perylene, in which two tetracenes are joined by the central benzene ring, which was found to possess a singlet biradical character (y0 = 0.61) in the ground state that easily undergoes Diels−Alder reaction with oxygen (Figure 1).9 Notably, a planar zigzag-edged nanographene, 4-peri-acene (peri-tetracene), reported by the same group very recently, also © 2018 American Chemical Society
Figure 1. Representative PAHs which contain two rows of tetracene moieties with different fusion modes.
displays a biradical character but with a relatively large biradical index (y0 = 0.72).10 In contrast, a Z-shaped acene which contains two tetracene moieties joined at the zigzag positions by an antiaromatic pentalene unit, reported by Chi and co-workers, is extremely stable with a half-life of 54 days (Figure 1).11 Interestingly, by replacement of six-membered rings with five-membered rings in tetrabenzo[a,f,j,o]perylene, a new nonplanar cyclopenta-fused PAH with two benzo[b]fluorene subunits, reported by Miao and co-workers in 2015, exhibits a hole mobility up to 1.0 cm2 V−1 s−1.12 These results have demonstrated that the fusion mode and molecular geometry have significant effects on the electronic properties, Received: June 11, 2018 Published: July 26, 2018 4512
DOI: 10.1021/acs.orglett.8b01810 Org. Lett. 2018, 20, 4512−4515
Letter
Organic Letters
anion and subsequently by nucleophilic addition reaction with diketones 4 and 10 gave the relative alcohol derivatives, followed by a reductive aromatization with SnCl2, providing the desired products TIPS-DBP and TIPS-DNP in 18% and 56% yields, respectively, which are soluble in common solvents and purified via silica gel column chromatography. The molecular structures were unambiguously characterized by NMR, mass spectrometry, and X-ray crystallography (see the Supporting Information). The UV−vis absorption spectra of TIPS-DBP and TIPSDNP were measured in chloroform solution (Figure 2A). The
stability, and solubility of the PAHs, which are critical for device applications.7−13 Despite the intriguing molecular properties, it is a particular challenge to prepare peri-fused PAHs, probably due to the extremely low solubility and unexpected reactivity of the intermediates.3c,14 Herein, we describe the synthesis of two triisopropylsilyl(TIPS)-ethynyl functionalized PAHs, dibenzo[a,j]perylene (TIPS-DBP) and dinaphtho[a,j]perylene (TIPSDNP), by fusion of benzenes and naphthalenes to perylenes, respectively. The resulting PAHs can be regarded as two rows of anthracene and tetracene moieties peri-joined by the benzene ring, which have twisted conformations due to the steric hindrance of their cove regions. It should be noted that recent studies have demonstrated that nonplanar PAH systems usually exhibit high solubility and stability, and self-assemble into special supermolecular structures demonstrating efficient charge transport.15 In particular, we envision that the fusion mode within dinaphtho[a,j]perylene and “bistetracene” molecules could lead to key differences in electronic structure, stability, solubility and transport properties, which provide deep understanding of the structure−property relationships in nonplanar peri-fused PAHs. In addition, TIPS-ethynyl substituents on the PAHs are expected to increase the solubility, crystallinity, and stability of acenes.1b,c Scheme 1 shows the synthesis of soluble twisted diarenoperylenes. To prepare TIPS-DBP and TIPS-DNP,
Figure 2. (A) UV−vis spectra of TIPS-DBP and TIPS-DNP in chloroform solution (10−5 M). (B) Electrochemical properties of TIPS-DBP and TIPS-DNP in dichloromethane with tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) as the supporting electrolyte.
Scheme 1. Synthetic Steps to TIPS-DBP and TIPS-DNP two spectra are generally similar in shape and exhibit two major absorption bands: a set of absorption bands at short wavelength, and a broad longest wavelength absorption band, which are characteristic of the acene-like derivatives.17 As expected, the longest wavelength absorption value for TIPSDNP (λmax = 719 nm) shows a red shift of approximately 100 nm relative to TIPS-DBP due to the extended conjugation length. The optical gaps are 1.84 and 1.59 eV for TIPS-DBP and TIPS-DNP, respectively. The cyclic voltammograms (CV) of TIPS-DBP and TIPSDNP exhibit two well-defined, reversible oxidation and reduction waves (Figure 2B). For TIPS-DBP, the half-wave reduction−oxidation potentials are at −1.86, −1.48, 0.38, 0.94 V and LUMO/HOMO are estimated to be −3.52/−5.11 eV (relative to Fc/Fc+, 4.8 eV). For TIPS-DNP, the half-wave reduction−oxidation potentials are at −1.61, −1.30, 0.21, 0.70 V, and the LUMO/HOMO are estimated to be −3.58/−4.94 eV. These values indicate that the fusion of two additional benzene rings onto the TIPS-DBP leads to a significant increase of HOMO levels, but a minor influence on LUMO levels. The density functional theory (DFT) calculations indicate that the two compounds possess very similar HOMO and LUMO distributions (see Figure S2, Supporting Information). Their orbitals are distributed over the aromatic cores and CC bonds. The calculated HOMO/LUMO by DFT are −4.48/−2.45 eV for TIPS-DBP and −4.29/−2.68 eV for TIPS-DNP, respectively, which are in good agreement with the trend of the experimental values. Single crystals of TIPS-DBP and TIPS-DNP that were suitable for X-ray structure determination, were readily grown from hexane and chloroform. Single-crystal XRD revealed that the two molecules have twisted conformations with significant bending of alkyne substituents (CAr−Csp−Csp ≈ 176−178°, and Csp−Csp−Si ≈ 171−176°) (Figure 3A,B). Like other twisted PAHs, these two PAHs are chiral, with P,P- and M,M-
dibenzo[a,j]perylene-dione 4 and dinaphtho[a,j]perylenedione 10 are the key intermediate compounds. Diketone 4 was first synthesized by rearrangement of dilactone in polyphosphoric acid.16 Herein, we developed a new synthetic route to prepare 4 by a simple two-step pathway. A Suzuki coupling reaction between commercially available compounds 1 and 2 allowed us to prepare diester 3 in a 30% yield. A subsequent acid-prompted (CF3SO3H/CH3SO3H) cyclization of 3 provided 4 in 63% yield. Initially, we attempted to prepare diester 9 via a Suzuki coupling reaction between 1 and 5, but only starting materials were recovered. When the Suzuki reaction between 6 and 7 was conducted, the expected 9 was isolated in 5% yield along with 30% yield of 8. Unfortunately, the change of catalyst, solvent, and base had a very minor positive effect on the yield of 9. However, we were pleased to discover that 8 could serve as a starting material to react with 7 to afford 9 in an acceptable yield of 41%, which was readily converted to diketone 10 with 46% yield. With diketones in hand, lithiation of TIPS-acetylene with n-BuLi to form its 4513
DOI: 10.1021/acs.orglett.8b01810 Org. Lett. 2018, 20, 4512−4515
Letter
Organic Letters
Figure 3. ORTEP drawing structures with 50% probability of (A) TIPS-DBP and (B) TIPS-DNP. Crystal packing of (C) TIPS-DBP and (D) TIPS-DNP. (M,M-Enantiomers are in red, P,P-enantiomers are in green; TIPS substituents and hydrogen atoms are omitted for clarity.) Figure 4. Transfer (left) and output (right) curves of OFET devices based on single-crystalline microfibers: (A) TIPS-DBP and (B) TIPS-DNP.
enantiomers in the crystals. For TIPS-DBP, it crystallizes in the Pca21 space group and has eight molecules per unit cell, which have slightly different cove region angles (see Figure S3, Supporting Information). In contrast to p-CF3Ph-substituted dibenzo[a,j]perylene, which adopts two-dimensional herringbone-like packing, in the crystal of TIPS-DBP, M,M- and P,Penantiomers form a one-dimensional (1-D) π-stacking motif with an alternate arrangement and the TIPS-ethynyl substituents in adjacent enantiomers are arranged in a rotation angle of 45° (Figure 3C). This type of intermolecular organization is reminiscent of the TIPS-ethynyl functionalized angular acenes with small aspect ratios, which give rise to a 1-D columnar arrangement with a rotation of TIPS-ethynyl groups in adjacent molecules to alleviate the unfavorable electrostatic interactions found in perfectly cofacial π−π stacking and steric repulsion between the bulky TIPS substituents as well.4d,e The short C−C contacts between adjacent enantiomers are in the range of 3.33−3.38 Å, which are shorter than double the van der Waals radius of carbon. For TIPS-DNP, it crystallizes in the P42/n space group and has four molecules per unit cell. The two tetracene moieties are slightly curved and lead to different cove region angles (42.6° and 43.8°, Figure S3, Supporting Information). Similar to TIPS-DBP, the TIPSDNP crystal contains a pair of enantiomers that form 1-D πstacking motif in an alternating fashion to each other with the TIPS-ethynyl groups arranged in a larger rotation angle of 85°, as shown in Figure 3D. There are multiple short C−C contacts between adjacent molecules in the range of 3.31−3.38 Å, which exist only between two tetracene moieties. The π−π distance between the benzene rings joining the two tetracene moieties, which are parallel with each other, is 3.87 Å within a given stack. Subsequently, we evaluated the intrinsic charge carrier mobilities of the semiconductors in the form of singlecrystalline microfibers with crystal dimensions of half μm wide and hundreds μm long, which were prepared by slow evaporation of toluene solution (see Figure S4, Supporting Information). The single crystalline field-effect transistors were fabricated by a “gold strips” technique on the octadecyltrichlorosilane (OTS) treated substrate (OTS-SiO2/Si) with Au as electrodes.18 Parts A and B of Figure 4 show the output and transfer characteristics of TIPS-DBP and TIPS-DNP microfibers tested under ambient conditions. Compared to p-CF3Phsubstituted dibenzo[a, j]perylene, which exhibits an ambipolar behavior with a maximum electron mobility of 0.002 cm2V−1s−1 and a maximum hole mobility of 0.04 cm2 V−1 s−1, TIPS-DBP shows exclusively p-type charge transport with
an average mobility of 0.09 cm2 V−1 s−1 and a maximum mobility up to 0.17 cm2 V−1 s−1. In contrast, TIPS-DNP shows an average hole mobility of 0.33 cm2 V−1 s−1, and the highest mobility is up to 0.74 cm2 V−1 s−1, which is one of the highest of the TIPS-ethynyl functionalized polyacenes, with 1-D packing that is unfavorable for efficient charge transport due to the insulating TIPS substituents between different stacks, resulting in inefficient electronic interactions between different stacks.8 Finally, we tested the stability of TIPS-DBP and TIPS-DNP in solution. TIPS-DBP and TIPS-DNP were dissolved in chloroform (10−5 M) solution and were then exposed to ambient light and air at room temperature (see Figure S5, Supporting Information). The measured half-lives of TIPSDBP and TIPS-DNP are 33 and 200 h, respectively. The improved stability is attributed to the additional Clar sextets and TIPS ethynyl substituents to kinetically stabilize PAHs, as well as the steric congestion at the central benzene ring which is known to slow the decomposition rate of linear acenes.19 Our results also confirm that the stability of the “bistetracene” molecules highly relies on its fusion modes. In conclusion, we have described the synthesis of two TIPSfunctionalized nonplanar PAHs with different conjugation lengths. A comparative study of the two compounds indicates that the molecular geometry plays an important role on the optical and electrochemical properties, stability, and device performance. Our results also demonstrate that the fusion mode has a significant effect on the stability of this class of molecules. In addition, the two twisted PAHs easily produce one-dimensional charge-transport systems with short C−C contacts. The crystal of TIPS-DBP shows a hole mobility up to 0.17 cm2 V−1 s−1, while the crystal of TIPS-DNP shows a hole mobility up to 0.74 cm2 V−1 s−1. We are currently exploring the relationship between the molecular geometry and the stability as well as for synthesis of even larger, expanded π-conjugated PAHs with highly twisted conformations for organic devices.
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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.8b01810. 4514
DOI: 10.1021/acs.orglett.8b01810 Org. Lett. 2018, 20, 4512−4515
Letter
Organic Letters
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Detailed synthesis and characterization, CV, DFT calcula-tions, X-ray crystallographic data, and NMR spectra (PDF) Accession Codes
CCDC 1840440−1840441 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
Lei Zhang: 0000-0002-0162-7222 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.Z. thanks the National Science Foundation of China (NSFC) (21672020) and Beijing Natural Science Foundation (2182049). X.L. thanks the Distinguished Scientist Program at BUCT (buctylkxj02). C.X. thanks China Postdoctoral Science Foundation (2017M610744).
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DOI: 10.1021/acs.orglett.8b01810 Org. Lett. 2018, 20, 4512−4515
