Structure property relationships of benzo[b]thiophen/benzo[b]furan end-capped naphthalene oligomers and their application for organic field effect transistors

Abstract

A series of new naphthalene oligomers were designed and synthesized through linking at the α- and β-position of the terminal substituents. Optical and electrochemical measurements and DFT simulation revealed the distinctly different electronic structure of the α-position and β-position linked oligomers. The naphthalene oligomer linked at the α-position of benzothiophene exhibits excellent field-effect performances, with mobility as high as 0.13 cm² V⁻¹ s⁻¹ and on/off ratio over 10⁶. The significant difference of the OFET performances and thin film microstructures between the naphthalene oligomers also confirmed that the substituted position has an important effect on the properties of semiconductor materials and the α-position linked oligomer is better than β-position linked isomers.

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Introduction

During the past two decades, organic field-effect transistors (OFETs) have attracted much attention for potential applications in low-cost, large-area electronic devices, such as flexible displays, RF-ID, and e-papers.^1–7 One of the key components in OFETs is the organic semiconductor layer, which has recently been developed significantly and dozens of organic semiconductor materials have exhibited high mobility over 1.0 cm² V⁻¹ s⁻¹.^8–10 However, searching for new organic semiconductor materials with excellent comprehensive performance is still highly desirable. One important way to develop organic semiconductor materials is the so called “building blocks approach”.¹¹ In this approach, conjugated segments of different electron accepting/donating properties are put together by appropriate chemical coupling to produce new molecules. Semiconductor molecules, prepared by using this approach frequently consist of a central unit, surrounded by one or two types of so called “intervening” units which modulate the electronic properties and aggregation of materials. The building blocks approach facilitates the design and synthesis of molecules (or polymer) with controlled electrical properties. The convenient in chemical modification of their structures could be beneficial to fine-tune their optical and electronic properties. Presently, most research is focused on the modification of the known central units through a variety of ways and developing the new building blocks. For example, once a new building block was synthesized or introduced, various intervening groups would be connected to produce novel semiconductor materials. Moreover, the connection mode of building blocks is also attracted considerable attention which includes σ-bond, double and triple bonds etc. Furthermore, the connection site, i.e., substituted position plays an important role for the chemical structures and physical properties of compounds,¹² however, the effect of connection site on properties of semiconductor materials was not received enough attention so far.

To explore the importance of substituted position in structure–property relationship so as to obtain desired organic semiconductor materials with high performance, in this study, a series of new oligomers, namely 2,6-bis(benzo[b]thiophen-2-yl)naphthalene (BBT2N), 2,6-bis(benzo[b]furan-2-yl)naphthalene (BBF2N), 2,6-bis(benzo[b]thiophen-3-yl)naphthalene (BBT3N) and 2,6-bis(benzo[b]furan-3-yl)naphthalene (BBF3N), in which comprising naphthalene as the central unit were synthesized through Suzuki coupling reaction. The benzothiophenes and/or benzofurans were connected to the both sides of naphthalene as the “intervening” units by σ-bond through linking at the α- and β-position of terminal substituents, respectively. By comparison of the optical, electrochemical property and simulated electronic structure, we found that the α-position linked oligomer have better π-conjugated degree than their β-position linked isomers although with identical building blocks. The further distinct difference of the OFETs performance and thin films microstructure among these compounds also demonstrates the crucial effect of substituted position on the properties of materials and it is deserved special attention in designing new semiconductor materials. The naphthalene oligomer linking at the α-position of benzothiophene exhibits high carrier mobility of 0.13 cm² V⁻¹ s⁻¹ when deposited at T_sub = 50 °C. These results suggest that the α-position linked oligomer is better than the β-position linked isomer.

Results and discussion

Synthesis

Naphthalene is the smallest acene and it has often been used as the building block for high mobility semiconductor materials.^13–17 Moreover, it has an advantage in batch synthesis of semiconductor materials over the other acenes due to the good solubility and easy commercial availability. The boronic acid intermediate of benzothiophene and benzofuran were synthesized according to the literatures^18,19 and/or purchased partly from commercial sources. The naphthalene oligomers prepared by the Suzuki coupling reaction between 2,6-dibromonaphthalene and corresponding boronic acid intermediate compounds which linked at the α- and β-position, respectively. The synthetic route is presented in Scheme 1, and all the oligomers could be easily purified through recrystallization. Their chemical structures were fully characterized by MS, ¹H NMR and elementary analysis.

Scheme 1 Synthetic route of the naphthalene oligomers.

Thermal, optical and electrochemical properties

The thermal stability of naphthalene oligomers was investigated by thermal gravimetric analysis (TGA, Fig. 1). All oligomers exhibit high thermal stability determined by the decomposition temperature (weight loss of 5%) which was observed under nitrogen atmosphere. The α-position linked oligomers exhibit higher decomposition temperature than their β-position linked isomers which indicates that the α-position linking is better than β-position. Moreover, the BBT2N shows highest thermal stability, while the BBF3N shows lowest decomposition temperature only of 321 °C. This also reveals that the benzothiophene is more stable end-group than benzofuran.

Fig. 1 TGA results of the naphthalene oligomers.

The optical properties of the naphthalene oligomers were investigated by using UV-vis and fluorescence spectroscopies in dilute dichloromethane solution (Fig. 2). The absorption maxima of β-position linked oligomers exhibit large blue-shift relative to their α-position linked isomers. Accordingly, the difference of optical band gaps levels which estimated by the onset of the low-energy side of the absorption spectra also reached over 0.4 eV between the different oligomers which linked at the α- and β-position. On the other side, no matter linking at the α-position or β-position, the thiophene-including oligomers exhibit little difference with the corresponding furan-including oligomers. These results demonstrate that the substituted position play more important role for the effectiveness of π-electron delocalization over the oligomer than the substituent. In fluorescence spectra, all compounds exhibit blue fluorescence in solution. Similar to the absorption spectra, the β-position linked oligomers exhibit large blue-shift than their α-position linked isomers, which also indicates that the α-position linking can produce better π-conjugated degree. Moreover, the thiophene-including oligomers exhibit slightly better conjugated degree than their furan-including analogues.

Fig. 2 Normalized UV-vis absorption (top) and fluorescence (down) spectra of the naphthalene oligomers in CH₂Cl₂ dilute solution.

The electrochemical properties of the naphthalene oligomers were explored by cyclic voltammetry (CV) (Fig. 3). The HOMO energy level of oligomers are in the range of 5.75–5.55 eV (see Table 1), which imply good anti-oxidation ability and environmental stability. The LUMO energy levels which estimated from the CV data and optical band gaps exhibit large difference between the α- and β-position linked oligomers over 0.4 eV. This reveals that the difference of conjugated degree of naphthalene oligomers is caused mainly by the LUMO energy level. To gain a better understanding of the relationships between the electronic structure and substituents, the molecular geometries and electron density distribution were simulated using density functional theory (DFT). As shown in Fig. 4, the optimized geometries of BBT2N and BBF2N have a linear and planar structure. In contrast, due to the large torsion angles between the thiophene or furan rings and naphthalene, BBT3N and BBF3N possess a twisted structure, which restricted the π-electron delocalization in the whole molecules. This also clarifies why the α-position linked oligomers have better π-conjugated degree than their β-position linked isomers which suggested by the optical and electrochemical measurements.

Fig. 3 Cyclic voltammetry of the naphthalene oligomers.

Table 1 Thermal, photophysical and electrochemical properties and calculated energy levels of the naphthalene oligomers

Compd	BBT2N	BBF2N	BBT3N	BBF3N
a Eg,op was determined from the onset of the UV-vis absorption spectra in solution.b HOMO = −(Eonset,ox + 4.4) eV (ref. 20 and 21).c LUMO = HOMO + Eg,op.d HOMO and LUMO is based on the DFT (B3LYP/6-31G(d,p)) calculations.
Td/°C	381	362	346	321
Eg,op^a/eV	3.1	3.15	3.51	3.64
Eonset,ox/V	1.35	1.24	1.30	1.15
HOMO^b/eV	−5.75	−5.64	−5.70	−5.55
LUMO^c/eV	−2.65	−2.49	−2.19	−1.91
HOMO^d/eV	−5.42	−5.25	−5.47	−5.50
LUMO^d/eV	−1.79	−1.83	−1.39	−1.36
Eg^d/eV	3.63	3.42	4.08	4.14

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Fig. 4 HOMO and LUMO orbitals of the naphthalene oligomers obtained by using DFT calculations.

OFETs performance

OFETs devices with these naphthalene oligomers were fabricated with the top-contact configuration. The organic semiconductor thin films were deposited onto the octadecyl-trichlorosilane (OTS) modified SiO₂/Si substrates by vacuum evaporation. Gold source and drain contacts were patterned through a shadow mask with the channel length and width of 31 μm and 273 μm, respectively. The transistors based on BBT2N and BBF2N exhibit typical p-type FET characteristics when fabricated at different substrate temperature (T_sub). Fig. 5 shows the typical transfer and output curves of the OFET devices based on BBT2N which obtained at T_sub = 50 °C. The FET performances of the devices are summarized in Table 2. From Table 2, we can see that FET performances depend on the substrate temperature. Both BBT2N and BBF2N exhibit highest mobility when deposited at T_sub = 50 °C, and the mobility of BBT2N reached up to 0.13 cm² v⁻¹ s⁻¹ as well as on/off ration over 10⁶. With the increase of substrate temperature to 80 °C, the mobility of BBF2N decreased slightly while the BBT2N decreased one order of magnitude. For the β-position linked BBT3N and BBF3N, no FET performance could be observed. This may be ascribed to the relatively low conjugated degree and poor film-forming characteristic since we could not obtain continuous films under the same fabrication condition.

Fig. 5 OFET characteristics of BBT2N on an OTS-treated substrate at T_sub = 50 °C: transfer characteristics (top) and output characteristics (down).

Table 2 OFET characteristics of BBT2N and BBF2N deposited at different substrate conditions

Compd	Tsub (°C)	μ (cm^2 V^−1 s^−1)	Vt (V)	Ion/Ioff
BBT2N	Rt	9.6 × 10^−2	−57	1 × 10^6
	50	1.3 × 10^−1	−67	2 × 10^6
	80	2.4 × 10^−2	−80	3 × 10^5
BBF2N	Rt	1.6 × 10^−4	−87	2 × 10^2
	50	9.4 × 10^−4	−48	2 × 10^4
	80	7.6 × 10^−4	−78	2 × 10^5

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Thin films microstructure

OFET device performance strongly depends on the morphology and crystallinity of the semiconductor thin films, both of which can be largely influenced by the deposition temperature of the substrate. To better understand the relationship between the thin film microstructure and the mobility, the thin films of BBT2N and BBF2N deposited simultaneously with the device fabrication were characterized by atomic force microscope (AFM) and X-ray diffraction (XRD). As shown in Fig. 6, the thin film of BBT2N deposited at room temperature presents good network interconnection between the microcrystallines with worm-like structure. With the increasing of T_sub, the grains boundary shrinked and the connectivity of the thin film improved. However, the distance of grains largen and the connectivity of the thin film become worse when the T_sub was increased further to 80 °C. For the BBF2N, the discontinuous semiconductor layer consists of small and disorder microcrystalline grains, which is probably responsible for poor charge-carrier mobility measured in OFETs.

Fig. 6 AFM images of BBT2N (left) and BBF2N (right) thin films at different substrate temperatures.

In spite of the significant difference in the morphology features, in both cases, as shown in Fig. 7, XRD measurements of the BBT2N and BBF2N thin films deposited at different temperatures exhibit similar diffraction peaks at 2θ = 4.4–4.5, 8.6–8.7° for BBT2N and 4.4–4.5, 9.0–9.1 and 13.5–13.6° for BBF2N. The corresponding d-spacing was determined to be ∼1.99 nm, which is consistent with the length of molecules (19.8 nm for BBT2N and 19.1 nm for BBF2N respectively).

Fig. 7 X-Ray diffraction of BBT2N (top) and BBF2N (down) thin films deposited at different substrate temperatures.

Experimental section

General

All reagents and chemicals were purchased from commercial sources and used as received. ¹H-NMR spectra were recorded on a Bruker DRX-400 spectrometer in deuterated chloroform or tetrachloroethane with tetramethylsilane as an internal reference.

Mass spectrometry was performed with MALDI-TOF spectrometer. Elemental analyses were performed on a vario MICRO CHN elemental analyzer. Thermal gravimetric analysis (TGA) was carried out on a PERKIN ELMER TGA7. The UV-vis spectrum was obtained on a JΛSCO V-570 UV-vis spectrometer. Fluorescence spectra were recorded on Hitachi F-4500 fluorescence spectrometer. Cyclic voltammeter (CV) was run on a CHI660C electrochemistry station in CH₂Cl₂ by using glass carbon as working electrode, Pt wire as counter electrode and Ag/AgCl as reference at a scan of 100 mV s⁻¹.

Device fabrication

The organic semiconductor materials were deposited to the OTS-treated SiO₂/Si substrate with about 50 nm of thickness by vacuum evaporation. Then the gold electrodes (source and drain) were fabricated by a shadow mask onto the semiconducting thin films as a top contact geometry. The characteristics of the OFETs were measured using a Keithley 4200 semiconductor parameter analyzer under ambient conditions. Atomic force microscopy (AFM) images of the film morphology were obtained using a Nanoscope IIIa atomic force microscope in tapping mode. X-Ray diffraction (XRD) measurements were carried out in the reflection mode using a 2 kW Rigaku X-ray diffraction system.

Synthesis

2,6-Bis(benzo[b]thiophen-2-yl)naphthalene (BBT2N). To a solution of 2,6-dibromonaphthalene (570 mg, 2 mmol) in THF (80 mL) was added 2-(benzo[b]thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (525 mg, 4.1 mmol) and Pd(PPh₃)₄ (200 mg). The mixture was stirred under argon for 10 min, and then aqueous Na₂CO₃ (2 M, 40 mL) was added. The mixture was heated to reflux for 24 h. The precipitate was collected by filtration, washed with water and methanol successively. After recrystallization from chlorobenzene pure BBT2N was obtained as a white solid (600 mg, 77%). MALID-TOF-MS: m/s 391; ¹H NMR (400 MHz, C₂D₂Cl₄) δ (ppm): 8.19 (s, 2H), 8.0–7.85 (m, 8H), 7.72 (s, 2H), 7.43–7.38 (m, 4H); anal. calcd for C₂₆H₁₆S₂: C, 79.56; H, 4.11; S, 16.33. Found: C, 79.54; H, 4.08.

2,6-Bis(benzo[b]furan-2-yl)naphthalene (BBF2N). The compound BBF2N was prepared according to the procedure used for BBT2N. BBF2N was isolated as a white solid in 41% yields. MALID-TOF-MS: m/s 361; ¹H NMR (400 MHz, C₂D₂Cl₄) δ (ppm): 8.40 (s, 2H), 8.02 (d, 4H), 7.68–7.61 (m, 4H), 7.39–7.28 (m, 4H), 7.20 (t, 2H); anal. calcd for C₂₆H₁₆O₂: C, 86.65; H, 4.47; O, 8.88. Found: C, 86.74; H, 4.44.

2,6-Bis(benzo[b]thiophen-3-yl)naphthalene (BBT3N). The compound BBT3N was prepared according to the procedure used for BBT2N. BBT3N was isolated as a white solid in 61% yields. MALID-TOF-MS: m/s 391; ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.13 (s, 2H), 8.04–7.96 (m, 6H), 7.78 (d, 2H), 7.55 (s, 2H), 7.45–7.43 (m, 4H); anal. calcd for C₂₆H₁₆S₂: C, 79.56; H, 4.11; S, 16.33. Found: C, 79.52; H, 4.12.

2,6-Bis(benzo[b]furan-3-yl)naphthalene (BBF3N). The compound BBF3N was prepared according to the procedure used for BBT2N. BBF3N was isolated as a white solid in 71% yields. MALID-TOF-MS: m/s 361; ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.18 (s, 2H), 8.00 (m, 4H), 7.95 (s, 2H), 7.81 (d, 2H), 7.60 (d, 2H), 7.43–7.36 (m, 4H); anal. calcd for C₂₆H₁₆O₂: C, 86.65; H, 4.47; O, 8.88. Found: C, 86.74; H, 4.49.

Conclusions

We have synthesized and characterized a series of new naphthalene oligomers end-capped with benzothiophenes and benzofurans through linking at the α- and β-position, respectively. The oligomers are highly thermally stable and the α-position linked oligomers have better π-conjugated degree than their β-position linked isomers which revealed by the optical, electrochemical measurements and DFT simulation of electronic structure. The naphthalene oligomer linking at the α-position of benzothiophene exhibits excellent field-effect performances, with mobility as high as 0.13 cm² V⁻¹ s⁻¹ and on/off ratio over 10⁶, which indicates that the α-position linked oligomer is better than the β-position linked isomer. The oligomers show distinct different OFETs performance and thin films microstructure which demonstrates that the substituted position have important effect on the properties of materials.

Acknowledgements

The present research was financially supported by the National Natural Science Foundation of China (21272049, 51003022), the Ministry of Science and Technology of China (2013CB933500) and HZNU (HSKQ0050). The authors thank Prof. Yongqiang, Ma from China Agricultural University for providing starting materials which prepared in the National S &T Pillar Program of China, 2012BAK25B03. Yingfeng Wang and Sufen Zou contributed equally to this work.

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