A bowl-shaped sumanene derivative with dense convex–concave columnar packing for high-performance organic field-effect transistors

Beibei Fu a, Xueqing Hou b, Cong Wang a, Yu Wang a, Xiaotao Zhang a, Rongjin Li *a, Xiangfeng Shao *b and Wenping Hu a
aTianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, China. E-mail: lirj@tju.edu.cn
bState Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu Province, China. E-mail: shaoxf@lzu.edu.cn

Received 28th July 2017 , Accepted 28th September 2017

First published on 28th September 2017

The charge transport properties of a bowl-shaped sumanene derivative with dense convex–concave columnar packing were studied. Single-crystal microribbons were grown by solution casting. Organic field-effect transistors based on individual single-crystal ribbons displayed a high hole mobility up to 0.37 cm2 V−1 s−1, which was one of the highest mobilities for buckybowl aromatics.

Organic semiconductors with a dense columnar π–π stacking structure have long been desirable due to their expected high mobility.1–3 Planar polyaromatic hydrocarbons (PAHs) such as acenes and heteroacenes tend to exhibit herringbone (CH-π dominant) or slipped π-stacking structures with reduced π–π interaction.3 Theoretically, both tilt angles in the herringbone structure and the translation of neighboring molecule along the long or short axis in the slipped π-stacking will decrease the efficiency of π–π overlap and thus hamper the further increment of charge carrier mobility.1,2 Great efforts have been made to modify the chemical structures of the planar PAHs to enhance π–π stacking, such as the introduction of bulky groups at the peri-positions of acenes and hetero-acenes.4,5 However, due to the strong electrostatic repulsion between the adjacent molecules in the planar PAHs,6 it is still a great challenge to harness the solid state packing of the semiconductors for the desired columnar π–π stacking structure and enhanced charge carrier mobility.

In contrast to the exhaustively studied planar PAHs, aromatics with curved, especially bowl-shaped, conjugated backbones have been less studied.7–9 Among the bowl-shaped PAHs (also π bowls), the C5V symmetric corannulene (C20H10) and the C3V symmetric sumanene (C21H12, 1, Fig. 1a) were two representatives.10–13 Corannulene has attracted much attention since its first synthesis in 1966.10,14–21 In contrast, sumanene has been less studied, which was attributed to the synthetic challenges of sumanene with large tension in the π-bowl. The first synthesis of sumanene was achieved in 2003 and the solid state packing was elucidated two years later by Sakurai, Hirao and co-workers.12,13 Driven by the concave–convex π–π interactions,22,23 sumanene adopted the desired columnar π–π stacking structure with all the columns oriented in the same direction.24 However, due to the large steric hindrance at the benzylic positions, the interplanar distances reached 3.86 Å, which was larger than that of the ordinary π–π stacking24 and was unfavourable for charge transport.

image file: c7cc05889f-f1.tif
Fig. 1 (a and b) Chemical structure of sumanene (1) and 2,3,5,6,8,9-hexabutoxy triselenasumanene (2). (c and d) Side and top views of the packing structure of 2 in the single crystal. The bowl-to-bowl distances are shown in (c). All butyl groups were omitted for clarity.

To take full advantage of the columnar π–π stacking structure of sumanene, we introduced selenium atoms and butoxy groups into the backbone of sumanene to produce 2,3,5,6,8,9-hexabutoxy triselenasumanene (2, Fig. 1b).26 The substitution of the three methylene groups at the benzylic positions by Se atoms alleviated the steric hindrance and reduced the π–π stacking distance (Fig. 1c and d). Meanwhile, the butoxy groups guaranteed sufficient solubility for solution process.25 Taking advantage of the non-pyrolytic two-step synthetic routes, 2 could be obtained in multigram quantities.26 Judging from the dense columnar π–π stacking structure, molecule 2 might be an organic semiconductor with highly efficient charge transport properties. Herein, microribbons of 2 were grown from the solution and organic field-effect transistors (OFETs) based on individual single-crystal ribbons were constructed to study the charge-transport properties. Structural analysis revealed that 2 adopted a convex–concave packing motif along the long axes of the single-crystal microribbons, which provided an ideal tool to study the charge transport properties along the π–π stacking direction. A field-effect mobility as high as 0.37 cm2 V−1 s−1 was obtained under ambient conditions at room temperature. As far as we know, this is the first example of applying a sumanene derivative in OFETs and the mobility obtained here is one of the highest among π-bowls reported so far.

2 exhibited a buckybowl structure with a bowl depth of 0.47 Å in the crystal structure,26 which was shallower than that of sumanene and was ascribed to the significantly longer C–Se bond length (1.92 Å) compared to that of the C–C bond length in sumanene (1.55 Å). A highly ordered columnar structure with all the 1D columns oriented in the same direction was formed by concave–convex π–π interactions (Fig. 1d), which resembles the packing of sumanene. Due to the reduced steric hindrance at the benzylic positions, the π–π stacking distance was shortened from 3.83 Å in sumanene to 3.55–3.57 Å in 2.13,24,26 The ideal columnar π–π stacking structure and the reduced π–π stacking distance indicated that 2 might be an organic semiconductor with highly efficient charge transport properties.

By the elimination of structural defects, the organic single crystal offered the highest degree of order and was an ideal tool for the exploration of charge transport properties in organic materials.27–30 Owing to the good solubility of 2, single-crystal microribbons were easily prepared by drop casting of toluene solution (1 mg ml−1) onto octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) modified SiO2/Si substrates. The OTS modification of the SiO2 dielectric could promote molecular self-organization during the growth of the microribbons. After evaporation of the solvent, needle-like microribbons appeared on the subatrate.31 The crystals are shiny in color and regular in shape. As shown in Fig. 2a and b, the lengths of the ribbons ranged from several tens of micrometers to over one hundred micrometers, and the widths ranged from several hundreds of nanometers to near two micrometers. In the images recorded using a polarized optical microscope, the color of the entire microribbon changed simultaneously from bright to dark when rotating the substrate along an axis perpendicular to the substrate (Fig. 2c and d), indicating the crystalline nature of the microribbons.

image file: c7cc05889f-f2.tif
Fig. 2 (a) Optical microscopy image of the microribbons. (b) Scanning electron microscopy (SEM) image of one individual ribbon. (c) Polarized optical microscopy (POM) image of the crystals and (d) the image of the crystals rotated by 45°.

Fig. 3a displays an atomic force microscopy (AFM) image of one typical microribbon. Cross-sectional analysis showed a thickness of around 40 nm and a width of around 1 μm. A random scan on the surface of the microribbon revealed (Fig. 3b) a low root mean squared (RMS) roughness of 0.19 nm over an area of 15 by 15 nm2. A typical transmission electron microscopy (TEM) image of the microribbon is shown in Fig. 3c and the corresponding selected-area electron diffraction (SAED) pattern is shown in Fig. 3d. No changes of the SAED patterns were seen from different parts of the same microribbon, indicating that the microribbon is a single crystal. The SAED pattern has been indexed with the lattice parameters of 2 (Fig. 3d).26 It can be concluded that the ribbon grew along the [001] direction, which coincided with the concave–convex stacking direction of the π-bowls in the single-crystal structure (as shown in Fig. 1c and d).

image file: c7cc05889f-f3.tif
Fig. 3 (a) AFM image of a microribbon. The inset shows the section analysis of the ribbon. (b) AFM image of an arbitrary area on the surface of the ribbon shown in (a). (c and d) TEM image and the corresponding SAED pattern.

Encouraged by the dense columnar π–π stacking structure in the solids, OFETs based on individual single-crystal microribbons of 2 were fabricated to probe the charge transport properties. Devices were fabricated in the top-contact, bottom-gate configuration, with Au source and drain electrodes (60 nm) and OTS-modified SiO2 dielectric layers (Fig. 4a). Typical transfer and output curves are displayed in Fig. 4b and c. A hole mobility as high as 0.37 cm2 V−1 s−1 in the saturation range was obtained. The mobility distribution of 30 devices is shown in Fig. 4d. Of all the devices studied, 80% (24 devices) showed a mobility over 0.20 cm2 V−1 s−1, and the average value was 0.29 cm2 V−1 s−1. As far as we know, the field-effect mobility obtained here is the highest among corannulene and sumanene derivatives.21,32,33

image file: c7cc05889f-f4.tif
Fig. 4 (a) Optical microscopy image of one device. (b) Transfer characteristics of the device. (c) Output characteristics of the device. (d) Distributions of the mobilities based on thirty single-crystal OFET devices.

In conclusion, the charge transport properties of a bowl-shaped sumanene derivative were studied by the construction of single-crystal OFETs. The substitution of the methylene groups by Se atoms at the benzylic positions of sumanene alleviated the steric hindrance and thus reduced the cofacial π–π stacking distance, which was favorable for charge transport. Driven by the strong π–π interactions, single-crystal microribbons were easily obtained by solution casting. A highly ordered columnar π–π stacking structure along the long axes of the microribbon was found. A high hole mobility up to 0.37 cm2 V−1 s−1 (0.29 cm2 V−1 s−1 in average) was obtained at room temperature under ambient conditions, which was one of the highest mobilities for π-bowls. As far as we know, this study is the first example of a sumanene derivative for OFETs. Our work shed light on the further development of sumanene derivatives and buckybowl aromatics for organic electronics.

The authors acknowledge financial support from the National Natural Science Foundation of China (61674116 and 21522203) and the National Key Basic Research Program of China (2016YFA0202300 and 2015CB856505).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cc05889f

This journal is © The Royal Society of Chemistry 2017