Synthesis, Full Characterization, and Field Effect Transistor Behavior

Communication pubs.acs.org/cm

Synthesis, Full Characterization, and Field Effect Transistor Behavior of a Stable Pyrene-Fused N‑Heteroacene with Twelve Linearly Annulated Six-Membered Rings Pei-Yang Gu,†,∥ Zongrui Wang,†,∥ Guangfeng Liu,† Huiying Yao,‡ Zilong Wang,† Yang Li,† Jia Zhu,*,‡ Shuzhou Li,† and Qichun Zhang*,†,§ †

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore College of Chemistry, Beijing Normal University, 100875, Beijing, China § Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡

S Supporting Information *

A

In order to further extend the length of N-heteroacene, understand the stacking modes of N-heteroacene molecules in solid state, and enrich the family of large N-heteroacenes (n > 10), we are continuing our research on the synthesis of large pyrene-fused N-heteroacenes through attaching triisopropylsilyl units and 5-(octan-3-yl)thiophene moieties onto the backbone of pyrene-fused N-heteracenes because (1) the triisopropylsilyl unit has been widely demonstrated as a well-protecting and solubility-enhancing group since J. E. Anthony first introduced it into the acene system1 and (2) 5-(octan-3-yl)thiophene moieties might act as electron-donating groups to increase the intramolecular and intermolecular interactions through donor− acceptor interactions, leading to short C−C distances between two nearest molecules, suggesting enhanced π−π interactions between two nearest molecules. Note that such strong intermolecular interactions would benefit to exclude solvent molecules into the crystal lattice, and more importantly, substituted groups with long alkane chains could enhance the solubility of the as-prepared N-heteroacene. Scheme 1 shows the synthetic procedure for the preparation of compound 1. Compounds 3 and 4 were prepared according to the reported procedures.35−38 The intermediate diamine (compound 2) was obtained as a red powder in 93% yield through the reduction reaction between 3 and lithium aluminum hydride (LiAlH4) at 60 °C. The target compound 1 was harvested as a dark-brown powder in 52% yield through a one-step condensation reaction between 3 and 4. The asprepared compound 1 was characterized by 1H and 13C NMR spectroscopies (Figures S1 and S2), high resolution mass spectroscopy (HRMS, Figure S3), elemental analysis, meltingpoint, Fourier transform infrared spectroscopy (FTIR, Figure S4), and single-crystal structure analysis. The thermal stability of compound 1 was evaluated by thermogravimetric analysis (TGA, Figure S5) under nitrogen atmosphere. As shown in Figure S5, compound 1 shows a good thermal stability with an onset decomposition temperature of ∼412 °C (considering that there is a 5% weight loss at this temperature).

lthough it is very tedious and extremely challenging to experimentally obtain large acenes or N-heteroacenes (n ≥ 6) with precise structure analysis due to their poor solubility and high reactivity, their unusual optical and electronic properties as well as their interesting spatial arrangement strongly encourage scientists to devote their efforts to this field.1−23 The successful strategy to avoid the above-mentioned issues in higher acenes or N-heteroacenes is to selectively attach protecting groups (e.g., fluoride, silylethyne, arylthio, phenyl) onto the periphery of their frameworks.1−29 In fact, this strategy works well and a coronene-containing N-heteroacene with crystal structure has been reported by Bunz and co-workers.13 Very recently, many groups including ours found that employing a moderately conjugated pyrene unit as terminal groups or bridges would make the as-prepared acenes or Nheteroacenes more stable because of its cross-conjugated nature.15−32 Thus, preparing large and stable N-heteroacenes (n > 10) with pyrene units as building blocks would be feasible because (1) the moderately conjugated pyrene units should be able to enhance the stability of large N-heteroacenes and (2) the pyrene unit can be easily modified to enhance the solubility of the target compounds, which is very important for full characterization of the as-prepared molecules including the growth of high quality crystals for structure analysis. In fact, pyrene-fused pyrazaacene (PFP) units have been used to construct polymer-like structures with high thermal stability although their structures are not well-defined.33,34 Following this strategy, Wang and co-workers have attached tert-butyl groups and para-alkoxyphenyl substituents (alkyl groups refer to long chains, Chart S1, PFPA) onto molecular backbones to enhance the solubility of the as-prepared PFPAs.30 Unfortunately, single-crystal structures of PFPA in their paper were not reported. Recently, Mastalerz and co-workers employed more soluble triptycenylene moieties as terminal groups to be grafted onto the conjugated framework to produce a soluble PFPB (Chart S1),24 which can be further grown into suitable crystals for single-crystal X-ray diffraction analysis. To the best of our knowledge, until now, the number of large N-heteroacenes (n > 10) with crystal structures is only two.13,24 Thus, it is highly desirable to prepare more new large N-heteroacenes (n > 10) with crystal structures to deeply understand the structure− property relationship. © 2017 American Chemical Society

Received: March 31, 2017 Revised: April 29, 2017 Published: April 30, 2017 4172

DOI: 10.1021/acs.chemmater.7b01318 Chem. Mater. 2017, 29, 4172−4175

Communication

Chemistry of Materials Scheme 1. Synthetic Route of Compound 1a

a

(LUMO) energy levels of 1 were estimated to be −5.0 and −3.5 eV from the onset of the first oxidation/reduction potential with reference to Fc+/Fc (−4.8 eV) using the equation of EHOMO/LUMO = −[4.8 − EFc + Eox/reonset] eV. The band gap for 1 is calculated to be ∼1.5 eV. The LUMO energy level of 1 in film (Figure S6b) was estimated to be −3.6 eV using the equation of ELUMO = −[4.8 − EFc + Ereonset] eV (Fc+/ Fc (−4.8 eV) as a reference). Its HOMO energy level is estimated to be −4.9 eV according to the equation of EHOMO = ELUMO − Egopt. These results are similar to the calculated energies (HOMO: −4.96 eV; LUMO: −2.99 eV; and band gap: 1.97 eV) investigated by B3LYP/6-31G(d,p).39 In addition, calculations within B3PW91/6-31G(d,p) were also performed to verify the calculated results and obtain the energies of HOMO and LUMO. All results have been summarized and are provided in Supporting Information (Figure S7, Figure S8, and Table S1). The black prismatic crystals of 1 were obtained by slow evaporation of mixed solvents under ambient conditions (chloroform/toluene/isopropanol = 1:1:2; see details in Supporting Information). Note that although attaching 3ethylheptane chains onto thiophene groups can significantly improve the solubility of the pyrene-fused N-heteroacene, the diffraction quality of the as-prepared single crystals is reduced attributed to the disorder of the terminal alkyl groups.13,18 However, the atomic thermal ellipsoids of carbon and nitrogen atoms in the skeleton are acceptable when the single-crystal Xray diffraction (SCXRD) was performed at 80 K (Figure S9). The SCXRD analysis reveals that the as-prepared crystals of 1 belong to a monoclinic system with the following unit cell parameters: a = 16.281(3), b = 18.231(4), c = 20.595(4) Å, and β = 92.011(3), where the asymmetric unit is half of the molecular unit and the Z value is 2 (Table S2). Compared with PFPB,24 1 is more coplanar and no solvent molecules were incorporated in the crystal structures probably because the introduction of thiophene groups onto the backbone can increase the intermolecular interaction and make molecular stacking denser. As shown in Figure 2a, the main skeleton of 1 shows a double-dumbbell style and can be described in five parts, namely, three pyrene rings (labeled A, C, and E) and two pyrazino[2,3-g]quinoxaline rings (labeled B and D). Two endcapping pyrene rings (labeled A and E) are parallel to each other, and the dihedral angle between the end-capping pyrene rings and center pyrene ring (labeled C) is 2.84°. Two embedded pyrazino[2,3-g]quinoxaline rings (labeled B and D) are also parallel to each other, and the dihedral angles between pyrene rings and pyrazino[2,3-g]quinoxaline rings are 1.89° (∠A,B) and 3.65° (∠B,C), respectively, showing the reasonable planarity of 1. Two pairs of thiophene rings are placed on the side chains of 1. As shown in Figure S10, the dihedral angle of the F/G thiophene units with the pyrazino[2,3-g]quinoxaline rings are 24.73° and 33.87°, respectively. Given that the crystal does not have any solvent molecules in its lattice, the stacking properties of 1 can be investigated directly. Figure 2b shows that one molecule is strongly affected by its two above-andbelow adjacent molecules. The neighboring molecules selfassemble to form a 1D stacking feature. The mean distance and the shortest C−C distances between the two nearest molecules are 3.32, 3.313, and 3.334 Å (Figure S11), respectively, which means there are strong π−π interactions among the π-planes of neighboring molecules. Except for the 1D stacking of the nearest molecules, the molecules of 1 interact with each other through a large number of C−H intermolecular interactions

(i) LiAlH4/THF, 93%; (ii) acetic acid/chloroform, 52%.

The normalized optical absorption spectra of 1 in dichloromethane (DCM) solution and in the film state are shown in Figure 1. Compound 1 in DCM solution displays four

Figure 1. Normalized optical absorption spectra of 1 in DCM solution and film.

absorption bands (λmax) at 374, 415, 558, and 725 nm. Compared with compounds PFPA and PFPB, a new absorption band at 725 nm might be the result of charge transfer between thiophene moieties and molecular backbone, suggesting that 1 shows the stronger intramolecular charge transfer than PFPA and PFPB, while the absorption band at 558 nm red-shifts by 30 nm, which might be due to the higher degree of coplanarity, confirmed by single crystal structure analysis. Compared with DCM solution, the longest absorption peak of 1 in the film state shifts to 780 nm, and the absorption edge extends to 950 nm with an optical band gap (Egopt) of 1.3 eV due to the stronger intermolecular interaction at film state. It is noteworthy that 1 is stable for more than one year in the solid state and several months in solution. Cyclic voltammetry measurement of 1 has been conducted in DCM solution and at film state (0.1 M TBAPF6 as the electrolyte) to investigate its electrochemical behavior. As shown in Figure S6a, compound 1 in DCM solution exhibits a quasi-reversible redox process in the negative potential region and a reversible redox process in the positive potential region. Compared with PFPA and PFPB, a new reversible redox process in the positive potential region could originate from thiophene moieties. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital 4173

DOI: 10.1021/acs.chemmater.7b01318 Chem. Mater. 2017, 29, 4172−4175

Communication

Chemistry of Materials

Figure 3. (a) Representative transfer and (b) output characteristics of single crystal field effect transistors based on 1 with a BCTG geometry; inset is the microscopy image of a real compound 1 single crystal organic field effect transistor fabricated from the “organic ribbon mask technique”. (W = 1.47 μm, L = 5.08 μm, μh = 8.1 × 10−3 cm2 V−1 s−1, Ion/Ioff = 104, VT = −19 V.)

knowledge, it is the first organic field effect transistor device based on single crystals of large N-heteroacene (n > 10). In conclusion, a novel pyrene-containing N-heteroacene 1 with 12 linearly fused aromatic six-membered rings has been successfully synthesized and fully characterized. Due to the introduction of 5-(octan-3-yl)thiophene species and triisopropylsilyl units onto the molecular backbone, 1 can be dissolved in common solvents (e.g., DCM, chloroform, chlorobenzene) with a great tendency to form single crystals. To the best of our knowledge, 1 is the longest linearly fused pyrene-containing aromatic system with single crystal structure. Furthermore, no solvent molecules are part of the crystal lattice. Thus, the stacking properties of 1 can be investigated directly and accurately. Our success would provide more opportunities to prepare longer pyrene-fused N-heteroacenes.

Figure 2. Single-crystal X-ray structure of 1: (a) Molecular structure (color scheme: C, gray; N, blue; S, yellow; Si, pale yellow; H atoms are omitted for clarity). The different parts of the aromatic ring are marked with different colors and numbers (three pyrene rings (labeled A, C, and E) and two pyrazino[2,3-g]quinoxaline rings (labeled B and D)). (b) The arrangement of the neighboring molecules shows a 1D stacking motif. The mean distance of two adjacent molecules is 3.32 Å. The π−π stacking parts of the aromatic rings are highlighted by red and blue colors. (c) Crystal packing along a nonspecial crystallographic direction. To highlight the zigzag packing motif, every third neighboring molecules are depicted in a single color.

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ASSOCIATED CONTENT

S Supporting Information *

(not listed here), which leads to a zigzag packing motif in the structure (Figure 2c). The strong π−π interactions among the π-planes of neighboring molecules suggest that 1 might be a promising material for the potential applications in organic electronic devices. The charge transport property of 1 was investigated through the bottom-gate top-contact (BGTC) organic field effect transistors (OFETs) with n-octadecyltrimethoxylsilane (OTS)-modified SiO2 (300 nm, Ci = 11 nF cm−2) as the dielectric layer, highly n-doped silicon wafer as the gate electrode, and thermally evaporated gold as source/drain electrodes. We first investigated the charge transport in the organic thin-film field effect transistors. However, due to the poor polycrystalline nature of the spin-coated film with a nearly amorphous morphology (Figure S12), the transistors based on the spin-coated film exhibited a low hole transport property with an average mobility of μh = 8.2 × 10−5 cm2 V−1 s−1 (Figure S13), which is similar to other large N-heteroacenes.13 Furthermore, the single crystals are the best candidates for investigation of the intrinsic charge transport because of the superiorities of no grain boundary, no defects, and long-range molecular order.40 So the single crystal organic field effect transistors were also fabricated (Figure S14 and Figure 3) through an organic ribbon mask technique,41 and they exhibited a better charge transport with the mobility up to μh = 8.1 × 10−3 cm2 V−1 s−1 (Figure 3), which was about 100 times higher than that of spin-coated films. To the best of our

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01318. Crystallographic information file (CIF) Full experimental procedures and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.Z.) E-mail: [email protected]. *(Q.Z.) E-mail: [email protected]. ORCID

Shuzhou Li: 0000-0002-2159-2602 Qichun Zhang: 0000-0003-1854-8659 Author Contributions ∥

(P.-Y.G. and Z.W.) These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Fred Wudl and Prof. Dmitrii F. Perepichka for their valuable discussion. Q.Z. acknowledges financial support from AcRF Tier 1 (RG8/16, RG133/14, and RG 13/15), Singapore. J.Z. acknowledges financial support from the National 973 Programs (2011CB935702), China. 4174

DOI: 10.1021/acs.chemmater.7b01318 Chem. Mater. 2017, 29, 4172−4175

Communication

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DOI: 10.1021/acs.chemmater.7b01318 Chem. Mater. 2017, 29, 4172−4175