Soluble and Stable <i>N</i>‐Heteropentacenes with High Field‐Effect Mobility

Abstract

An exploratory study on a group of silylethynylated N-heteropentacenes as soluble and stable organic semiconductors is presented. An interesting finding is that a silylethynylated N-heteropentacene can function as a p-type, n-type, or ambipolar organic semiconductor depending on the structure of its π-backbone. The tetraazapentacene derivative is one of the best performing n-type organic semiconductors with an electron mobility of up to 3.3 cm2 V−1 s−1. Organic thin-film transistors (OTFTs) are essential building blocks of large-area, flexible, and low-cost organic electronics and offer promising applications in technology areas, such as light-weight and flexible displays; radio-frequency identification (RFID) circuitry;1 and physical, chemical, and biological sensors.2 The ultimate success of OTFTs requires developing organic semiconductors that allow 1) efficient charge transport, which is characterized by high field-effect mobility; 2) solution-based processing for low-cost fabrication; and 3) good operational stability of devices under ambient air, moisture, and light exposure.3 Although the field-effect mobilities of leading organic semiconductors in OTFTs have reached or even exceeded the value of amorphous silicon (≈1 cm2 V−1 s−1), organic semiconductors that can meet all the above requirements are still rare.4 Such challenge is particularly true for n-type (electron-transporting) organic semiconductors.5 In fact the development of n-type organic semiconductors for OTFTs has lagged behind that of p-type ones, primarily due to the inherent instability of organic anions in the presence of air and water and problems with oxygen trapping within these materials.4 Pentacene has led small-molecule organic semiconductors as a benchmark for applications in thin-film transistors, and functionalized pentacenes have been extensively studied for organic semiconductors with improved properties and better understanding of structure–property relationships.6 N-Heteropentacenes, which are members of a larger family of N-heteroacenes with five rings in the backbone,7 are interesting analogues of pentacene due to the opportunities for tuning the electronic structure, stability, solubility, and molecular packing by introducing nitrogen atoms to pentacene.8, 9 Although some N-heteropentacenes have been known for over a century,10 only a few N-heteropentacenes have been fully characterized structurally11 and even fewer have been characterized in electronic devices.8, 9, 12, 13 Some N-rich heteropentacene molecules were proposed as n-type organic semiconductors based on theoretical calculation14 and electrochemical measurements.15 However, most of these N-rich compounds were not synthesized and none of them were used in electronic devices. As suggested by the success of silylethynylated pentacenes, especially 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN as shown in Figure 1a), as a stable and solution-processable p-type organic semiconductor with high charge carrier mobility,16 substituting N-heteropentacenes with silylethynyl groups is a promising strategy to develop superior organic semiconductors based on N-heteropentacenes.13 Here, we report silylethynylated N-heteropentacenes as soluble and stable semiconductors with high field-effect mobility, and in particular high electron mobility. Figure 1 shows silylethynylated N-heteropentacenes 1–4 with the corresponding pentacene derivative TIPS-PEN. Among them, 1 and 3 are the reduced forms with two saturated nitrogen atoms, while 2 and 4 are the oxidized forms with all the nitrogen atoms unsaturated. Detailed below are their synthesis, electronic structure, stability, molecular packing, and use in thin-film transistors. a) Structures of silylethynylated N-heteropentacenes (1–4) and pentacene (TIPS-PEN). b) Synthesis of 1 and 2. As shown in Figure 1b, compounds 1 and 2 were synthesized from 5,14-diaza-6,13-pentacenequinone (5), which was prepared from 5,14-dihydro-5,14-diazapentacene8 by oxidation with K2Cr2O7. The low yield of 5 was due to formation of its isomer, 5,14-diaza-7,12-pentacenequinone,17 during the oxidation and the loss during separation of the two isomers. Addition of silylethynylide to 5 yielded diol 6, which was reduced with SnCl2 under acidic condition yielding 1. In a similar manner, 3 and 4 were synthesized following the reported methods15 with modification from 5,7,12,14,-tetraaza-6,13-pentacenequinone, which was easily prepared from very cheap starting materials.18 The reduced forms 1 and 3 are easily converted to the oxidized forms 2 and 4 by oxidization with MnO2. 1 and 3 are golden crystals, while 2 and 4 are black crystals with metallic luster. 1–4 are all soluble in common organic solvents, such as CH2Cl2, CHCl3, toluene, and chlorobenzene, with solubility larger than 15 mg mL−1. The electronic structures of 1–4 were investigated with UV-vis absorption spectroscopy and cyclic voltammetry and compared with that of TIPS-PEN. Figure 2a shows the UV-vis absorption spectra of TIPS-PEN and 1–4 as measured from their solutions in CH2Cl2. The longest-wavelength absorption of 2 and 4 shows a small red shift relative to that of TIPS-PEN, while that of 1 and 3 shows a large blue shift relative to TIPS-PEN, in agreement with the lower degree of conjugation in 1 and 3. The cyclic voltammograms of 1–4 recorded from solutions in CH2Cl2 are shown in the Supporting Information. The absorption edge and reduction and oxidation potentials of 1–4 are summarized in Table S-1 (Supporting Information). Based on these data, the energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for 1–4 were estimated19 and are shown in Figure 2b. An observation from Figure 2b is that replacing a benzene ring with a dihydropyrazine ring in a pentacene backbone (from TIPS-PEN to 1 and from 2 to 3) raises both the HOMO and LUMO energy levels, and the rise of LUMO is apparently greater than that of HOMO. In contrast, replacing a benzene ring with a pyrazine ring in a pentacene backbone (from TIPS-PEN to 2 and from 2 to 4) lowers both the HOMO and LUMO energy levels. These are in agreement with the electron-donating nature of saturated N atoms and the electron-withdrawing nature of unsaturated N atoms. Moreover, comparing 2 and 4 with the fluorinated and chlorinated derivatives of TIPS-PEN20 indicates that a pyrazine ring is more effective than a tetrafluorobenzene or tetrachlorobenzene ring in lowering the LUMO of TIPS-PEN. This can be attributed to the fact that N atoms withdraw π-electrons via a resonance effect while halogen atoms withdraw π-electrons via an inducing effect. With a high-lying HOMO, 1 and 3 are expected to function as p-type semiconductors, while 4 is expected to function as an n-type semiconductor with a low-lying LUMO. The relatively high HOMO energy level and relatively low LUMO energy level suggest that 2 may even function as an ambipolar organic semiconductor. In light of the easy conversion of the reduced forms to the oxidized forms (1 to 2 and 3 to 4), the frontier molecular orbital energy levels of N-heteropentacenes can be readily tuned. In contrast, it involves inconvenient multistep synthesis to adjust the LUMO energy level of pentacene by replacing hydrogen atoms with halogen atoms.20, 21 a) UV-vis absorption of 0.05 mM solutions of TIPS-PEN and 1–4 in CH2Cl2. b) HOMO and LUMO energy levels of TIPS-PEN and 1–4. c) The relative absorbance as a function of time as measured from the solutions of TIPS-PEN and 1–4 (0.05 mM in CH2Cl2) exposed to ambient light and air. The absorbance was measured at the longest wavelength absorption: 498 nm for 1, 697 nm for 2, 534 nm for 3, 679 nm for 4 and 642 nm for TIPS-PEN. The environmental stability of 1–4 in solution was monitored with UV-vis absorption spectroscopy because the characteristic absorptions decrease with the progress of photochemical degradation.22 To compare the stabilities of 1–4 and TIPS-PEN, the UV-vis absorption spectra of these compounds were recorded from their solutions in CH2Cl2 at the same concentration as the solutions were exposed to ambient light and air for a certain period of time. Shown in Figure 2c is the relative absorbance A/A0 (A is the absorbance and A0 is the initial absorbance) as a function of time for the longest-wavelength absorption of 1–4 and TIPS-PEN, indicating that 1–3 are slightly more stable than TIPS-PEN, and 4 is more stable than TIPS-PEN. Single crystals of 1–4 were grown from solutions in acetone, ethyl acetate, or chloroform, and X-ray crystallographic analysis revealed their crystal structures as depicted in Figure 3 and summarized in Table S-2 (Supporting Information).23 It was found that the π-backbones of 1–4 are all essentially flat. 1, 2, and 4 have very similar π-stacking of a 2D brickwork arrangement.7, 24 Such a brickwork packing motif is also shared by TIPS-PEN25 and fluorinated derivatives of TIPS-PEN26 in their crystal structures. The distances between π planes are 3.35 and 3.38 Å for 1, 3.34 and 3.42 Å for 2, and 3.28 and 3.38 Å for 4. The fact that the four molecules (1, 2, 4, and TIPS-PEN) have different π-backbones but share a common packing motif indicates that the molecular packing of these molecules is dominated by the triisopropylsilyl substituents. In contrast, molecules of 3 are arranged in 1D stacks with a π-to-π distance of 3.41 Å, as shown in Figure 3c. Such arrangement is possibly related to the charge distribution along the N-heteropentacene backbone due to the two types of nitrogen atoms. Another finding from the crystal structures is that molecules of 1, 2, and 3 have their unsymmetrical backbones stacking in a head-to-tail arrangement, which avoids an overlap between pyrazine rings or between dihydropyrazine rings. Among the four N-heteropentacene molecules, the unit cells of 1, 2, and 4 are similar, while that of 3 is apparently different as shown in Table S-2 (Supporting Information). 4 has the smallest unit cell volume (944.346 Å3) and the shortest π-to-π distance, indicating that 4 has the closest packing. In contrast, 3 has the largest cell volume (1015.99 Å3) and the longest π-to-π distance although the molecular dimensions of 3 and 4 are essentially the same (the N-heteropentacene backbone length is 13.6 Å for 3 and 13.5 Å for 4). The relatively loose packing of 3 is assumed to be related to the 1D π-stacking. Crystal structures of 1 (a), 2 (b), 3 (c), and 4 (d) showing π-stacks. The N-heteropentacene cores are shown with ball-stick models and the silylethynyl substituents are shown with stick models. Hydrogen atoms are removed for clarification. Carbon atoms are shown in grey, nitrogen atoms are shown in blue, and silicon atoms are shown in yellow. Semiconductor properties of 1–4 were first tested in vacuum-deposited thin films. To fabricate OTFTs, a thin film of silylethynylated N-heteropentacene was deposited onto a silicon wafer, whose SiO2 surface was pretreated with a self-assembled monolayer of octadecyltrimethoxysilane (OTMS),27 by thermal evaporation under a high vacuum. The fabrication was completed by depositing a layer of gold on the organic films through a shadow mask to form top-contact source and drain electrodes. The resulting devices had highly doped silicon as the gate electrode and a 300-nm-thick layer of SiO2 as the dielectric. The X-ray diffraction patterns (see Supporting Information) from the vacuum deposited films of 1–4 showed similar patterns indicating these films were polycrystalline. The three diffraction peaks from the films of 1, 2, and 4 are in accordance with the (0,0,1), (0,0,2), and (0,0,3) diffractions derived from the single crystal structures and indicate layer spacings of 16.4 to 16.6 Å, which are slightly smaller than the layer spacing in the thin film of TIPS-PEN (16.86 Å).22 In contrast, the diffraction peaks from the film of 3 can not be attributed to any diffractions derived from the single crystal structure indicating that 3 forms a different polymorph in the vacuum-deposited film. The atomic force microscopy (AFM) images displayed in Figure S-7 (Supporting Information) show that the vacuum-deposited films of 2–4 have a terraced morphology, which is typical of organic semiconductors and also observed from the thin films of halogenated derivatives of TIPS-PEN.20 In comparison, the film of 1 is composed of flatter grains. It was found that 1 functioned as a p-type semiconductor and 2 functioned as an ambipolar semiconductor. The field-effect mobilities of 1 and 2 varied with the temperature of substrate during deposition, and their mobilities measured from the films deposited at a substrate temperature of 100 °C are summarized in Table 1. 3 did not show any field effect in these studies, possibly related to its 1D π-stacking with relatively loose packing28 and its polymorph in the vacuum-deposited film. The best performing N-heteropentacene is 4, whose performance appeared almost independent on the substrate temperature during deposition in the range of room temperature to 100 °C. 4 functioned as an n-type semiconductor with field-effect mobility in the range of 1.0–3.3 cm2 V−1 s−1 as measured from 160 channels of 40 devices under vacuum (shown in Figure 4a). The average electron mobility measured from these channels was 1.6 cm2 V−1 s−1, which is ten times higher than the electron mobility of the recently reported ambipolar silylethynylated diazapentacene.13 When the transistors of 4 were tested in ambient air, the measured electron mobility decreased to 0.3–0.5 cm2 V−1 s−1. Shown in Figure 4b,c are the typical output and transfer current–voltage (I–V) curves for the thin-film transistors of 4. From the transfer I–V curve, a field-effect mobility of 1.6 cm2 V−1 s−1 was measured in the saturation regime using the equation: IDS = (μWCi/2L)(VGS – VT)2, where IDSis drain-source current, VGS is the gate voltage, VT is the threshold voltage, μ is the field effect mobility, W is the width, L is the length, and Ci is the capacitance for 300 nm SiO2(11 nF cm−2). The on/off ratio of the drain current obtained between 0 and 60 V gate bias was greater than 5 × 105. a) The statistics of measured field-effect mobilities of 4. b) Drain current (IDS) versus drain voltage (VDS) with varying gate voltage (VGS) for a thin-film transistor of 4 with the active channel of L = 1 mm and W = 100 μm. c) Drain current (IDS) versus gate voltage (VGS) with drain voltage (VDS) of 60 V for the same transistor of 4. To examine the solution processability of silylethynylated N-heteropentacenes, thin films of 1–4 were deposited by simply casting a solution in chlorobenzene onto a silicon wafer and tested with both top-contact and bottom-contact gold drain/source electrodes. In such OTFT devices, the SiO2 surface of the silicon wafer was not pretreated with OTMS because the hydrophobic OTMS surface was not wettable with the chlorobenzene solutions. The solution processed films of 1 and 4 performed as p-channel and n-channel transistors, respectively, while those of 2 and 3 did not show any field effect. The highest field-effect mobility measured from the solution processed films was 3 × 10−4 cm2 V−1 s−1 for 1 and 3 × 10−3 cm2 V−1 s−1 for 4. The low mobility of 4 in the solution-cast films can be attributed to the electron trapping by hydroxyl groups of SiO2 surface.29 In summary, the above study has shown that a silylethynylated N-heteropentacene can function as a p-type, n-type, or ambipolar organic semiconductor depending on the structure of its π-backbone. Particularly, tetraazapentacene 4 exhibited electron mobility of up to 3.3 cm2 V−1 s−1, which makes this molecule one of the best performing n-type organic semiconductors.30, 31 The high electron mobility of 4 may be attributed to its low LUMO energy level and close packing of molecules in a 2D brickwork arrangement. The solution processibility of these N-heteropentacenes was clearly demonstrated, although the field-effect mobilities measured from solution-processed films were much lower. This study together with the previous studies8, 9, 11, 13 on N-heteropentacene-based organic semiconductors indicates that N-heteropentacenes and their derivatives can be a general design for high-performance organic semiconductors. Because of its high electron mobility, good solubility in common organic solvents and strong absorption in visible light range, tetraazapentacene 4 may also find applications as an electron acceptor in organic photovoltaic solar cells. A preliminary study on photovoltaic cells with poly-(3-hexylthiophene) (P3HT) as the electron donor and 4 as the electron acceptor is in progress. Supporting Information is available from the Wiley Online Library or from the author. The authors thank Ms. Hoi Shan Chan (the Chinese University of Hong Kong) and Dr. Ken S. M. Yiu (City University of Hong Kong) for single crystal crystallography. This work was supported by grants from the Research Grants Council of Hong Kong (project number: GRF402508 and CUHK2/CRF/08) and the University Grants Committee of Hong Kong (project number: AoE/P-03/08). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.