Chinese Chemical Letters  2014, Vol.25 Issue (01):65-68   PDF    
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Chun Zhan, You-Yu Jiang, Ming-Yan Yang, Lu-Hua Lu, Sheng-Qiang Xiao.Synthesis and optoelectronic properties of a novel molecular semiconductor of dithieno[5,6-b：11,12-b']coronene-2,3,8,9-tetracarboxylic tetraester[J]. Chin. Chem. Lett., 2014,25(01): 65-68   
Synthesis and optoelectronic properties of a novel molecular semiconductor of dithieno[5,6-b：11,12-b']coronene-2,3,8,9-tetracarboxylic tetraester
Chun Zhana, You-Yu Jianga,b, Ming-Yan Yanga, Lu-Hua Lua,b, Sheng-Qiang Xiaoa,b,c     
* Corresponding authors at:a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China;
b WUT-USG Joint Laboratory of Advanced Optoelectronic Materials and Devices, Wuhan University of Technology, Wuhan 430070, China;
c Center for Chemical and Materials Engineering, Wuhan University of Technology, Wuhan 430070, China
Received:2013-7-6, Revised:2013-8-23, online:2013-10-20.
E-mail addresses:shengqiang@whut.edu.cn
Abstract: A facile procedure for the synthesis of dithieno[5,6-b:11,12-b']coronene-2,3,8,9-tetracarboxylic tetra(2-ethylhexyl)ester (DTCTTE-EH) from readily available perylene-3,4,9,10-tetracaroboxylic dianhydride is described. The electronic properties of DTCTTE-EH were elucidated on the basis of UV-vis spectra, emission spectrum and electrochemicalmeasurement, which demonstrate that DTCTTE is a new class of components for promising semiconducting materials.
Key words: Polycyclic aromatic molecules     Coronene tetracarboxylic tetraester     Semiconductor     Optoelectronic property    
1. Introduction

Polycyclic aromatic molecules (PCAs) capable of being processed as solutions are of great scientific and technological interest as semiconducting materials applied in thin-film transistors (OTFTs), light-emitting transistors (OLETs), and photovoltaic cells (OPVs). A number of such molecules with different optoelectronic properties have been synthesized, which has been important for developing high-performance organic electronic devices [1, 2]. Perylene dimides (PDIs) represent one of the most intensively investigated n-type organic semiconductors due to their strong electron accepting ability, high charge carrier mobility and photochemical stabilities [3, 4, 5]. The extension of the aromatic core of PDIs has emerged as one of the active topics in order to tune their optoelectronic properties and solid-state structures. In particular, with expanded π-systems at bay regions of PDIs, a variety of coronene diimides (CDIs), such as dibenzocoronene diimide [6], nitrogen heterocoronene diimide [7], dinaphthocoronene diimide [8], pyridinocoronene diimide [9], dithienocoronene diimide [10], are realized through reactions via palladiumcatalyzed ring annulation or photo-induced intramolecular cyclization. Distinctive optical and electronic behaviors were observed for these n-type semiconducting CDIs fused with different aromatic units.

Interestingly, closely related structures to PDI and CDI, but with four less electron deficient carboxylic ester groups attached to the perylene core, i.e., perylene tetracarboxylic tetraester (PTTE) and coronene tetracarboxylic tetraester (CTTE, see Fig. 1), have rarely been considered as efficient building blocks to construct semiconducting molecules. However, PTTEs and CTTEs do have interesting features when compared with the more commonly employed PDIs and recently developed CDIs. For example, attached four carboxylic ester groups to the perylene core will lead to an increased solubility of a PTTE or CTTE than that of corresponding PDI or CDI, respectively, which is crucial for solution processing. In addition, less electron deficient carboxylic ester groups can reduce the electron accepting ability of a PTTE or CTTE when compared to the corresponding PDI or CDI, respectively. The latter opens the way to tune a wide range of electronic properties of PCAs with tetracarboxylic tetraester or diimide groups from a weak electron donor to a strong electron acceptor. Since coronene has a planar, highly rigid and extended aromatic system with a perfect delocalization of aromaticity, semiconducting materials based on CTTE will be of great interest for electronic device applications. For the development of newmaterials based on CTTE, it is of primary importance to devise effective synthetic methods. To this end, little work has been done to develop new and convenient synthetic approaches to CTTE analogs. In addition, multi-thiophene fused PCAs are attracting current interest as promising organic electronic materials [11, 12]. Herein, we report a convenient synthetic approach of a coronene tetracarboxylic tetraester analog, dithieno[5,6-b:11,12-b']coronene-2,3,8,9-tetracarboxylic tetra(2- ethylhexyl)ester (DTCTTE-EH), as indicated in Fig. 1, starting from readily available perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). DTCTTE-EH integrates two electron rich thiophene units with coronene tetracarboxylic tetraester (CTTE). It is envisioned that incorporating two annulated thiophene units could make DTCTTE electron richer than CTTE andCDI and alloweasier functionalization through the two α positions of the fused thiophene moieties.

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Fig. 1.SXRD profile of the complex PANDAZO at room temperature. The inset is schematic representation of the layered architectures of the PANDAZO complex.
2. Experimental

General information on chemicals, instruments and experiments can be found in Supporting information.

2.1. Perylene-3,4,9,10-tetracarboxylic tetra(2-ethylhexyl)ester (PTTE-EH)

2-Ethylhexan-1-bromide (1.3 mL, 7.3 mmol), 2-ethylhexanol (1.4 mL, 8.8 mmol) and DBU (0.75 mL, 5.1 mmol) was added to a solution of PTCDA (0.21 g, 1.0 mmol) in 10 mL of acetonitrile in a 100-mL of two-necked round-bottom flask. After refluxing for 24 h, the solvent was evaporated under reduced pressure. The residue was washed with water and extracted with hexane. The organic layer was dried over anhydrous MgSO4 and the solvent was removed by a rotary evaporator. The excessive 2-ethylhexan-1- bromide and 2-ethylhexanol were removed by distillation under reduced pressure. The resulting mixture was poured slowly into 100 mL of methanol and filtered. The precipitate was further purified by flash silica gel column chromatography using hexane/ dichloromethane (1:1, v/v) to yield 0.74 g of red solid (84% yield). 1H NMR (400 MHz, CDCl3): δ 8.06 (d, 4H, J = 8.04 Hz), 7.89 (d, 4H, J = 7.88 Hz), 4.28 (m, 8H), 1.84-1.78 (m, 4H), 1.59-1.37 (m, 32H), 0.99 (t, 12H, J = 7.46 Hz), 0.92 (t, 12H, J = 6.80 Hz). 13C NMR (400 MHz, CDCl3): δ 168.58, 132.48, 130.27, 129.87, 128.63, 128.49, 121.12, 67.78, 38.84, 30.48, 28.98, 23.89, 23.03, 14.09, 10.98. MALDI-TOF MS for C56H76O8: calcd. 876.55; found 876.6 (M+).

2.2. 1,7-Dibromo-perylene-3,4,9,10-tetracarboxylic tetra(2-ethylhexyl)ester (1)

A mixture of PTTE-EH (0.43 g, 0.5 mmol), K2CO3M (0.55 g, 4.0 mmol), 12 mL of CH2Cl2 and 0.38 mL of bromine (8.0 mmol, 16 equiv.) was stirred at room temperature and monitored by TLC. After complete consumption of PTTE, the excessive bromine was removed by washing with aqueous Na2S2O3. The organic layer was washed with water and dried over anhydrous MgSO4 followed by removing the solvent using a rotary evaporator. The crude product was further purified by flash silica gel column chromatography using hexane/ethyl acetate (40:1, v/v) to afford 0.33 g of red solid (yield 64%) containing 0.18 equiv. of 1,6- dibromonated isomer. 1H NMR (400 MHz, CDCl3): δ 8.88 (d, 2H, J = 7.96 Hz), 8.24 (s, 2H), 8.03 (d, 2H, J = 4.80 Hz), 4.32-4.23 (m, 8H), 1.83-1.76 (m, 4H), 1.57-1.35 (m, 32H), 0.98 (t, 12H, J = 7.44 Hz), 0.92 (t, 12H, J = 5.78 Hz). 13C NMR (400MHz, CDCl3): δ 167.70, 167.02, 135.72, 131.21, 130.77, 130.51, 129.97, 129.87, 129.67, 128.17, 126.96, 125.99, 118.10, 68.22, 67.73, 38.75, 38.61, 31.44, 30.33, 28.95, 28.83, 23.74, 22.92, 22.50, 13.95, 10.87, 10.81. MALDI-TOF MS for C56H74Br2O8: calcd. 1034.39; found 1057.3 (M+Na+).

2.3. 1,7-Di(2-thienyl)-perylene-3,4,9,10-tetracarboxylic tetra(2-ethylhexyl)ester (2)

A mixture of 0.3 mL of tributyl(thiophen-2-yl)stannane (0.35 g, 0.84 mmol) and tetrakis(triphenylphosphine)palladium (14.2 mg, 1% equiv.) was added to the solution of compound 1 (0.414 g, 0.4 mmol) in 20 mL of dry toluene under nitrogen. The mixture was heated to reflux and reacted overnight. The mixture was allowed to cool to room temperature and was concentrated under reduced pressure. The resulting crude was purified by flash silica gel column chromatography using toluene as eluent to afford 0.79 g of the product (yield 76%), which was immediately used in the next step without further purification and characterization due to partial cyclization under light.

2.4. Dithieno[5,6-b:11,12-b']coronene-2,3,8,9-tetracarboxylic tetra(2-ethylhexyl)ester (DTCTTE-EH)

A catalytic amount of iodine (10 mg) was added to the solution of freshly synthesized compound 2 (0.26 g, 0.25 mmol) in 500 mL of CH2Cl2 at room temperature. The mixture was illuminated under sunlight for 2 h followed by removing of CH2Cl2 using a rotary evaporator. The crude product was purified by silica gel column chromatography with hexane/acetone (100:1, v/v) as eluent to obtain 0.2 g of the product (yield 80%). 1H NMR(400 MHz, CDCl3): δ 9.60 (s, 2H), 9.36 (s, 2H), 8.6 (d, 2H, J = 5.4 Hz), 8.1 (m, 2H, J = 5.32 Hz), 4.55-4.47 (m, 8H), 2.0-1.93 (m, 4H), 1.67-1.38 (m, 32H), 1.1-0.93 (m, 24H). 13C NMR (400 MHz, CDCl3): δ 168.90, 168.60, 137.12, 135.48, 128.46, 128.34, 126.84, 125.65, 124.64, 123.80, 123.53, 123.39, 121.38, 120.54, 119.94, 68.26, 68.16, 38.93, 38.88, 30.68, 29.11, 24.04, 23.19, 14.19, 11.15, 11.11. MALDI-TOF MS for C64H76O8S2: calcd. 1036.5; found 1036.3 (M+).

3. Results and discussion

Scheme 1 illustrates the synthetic approach to DTCTTE-EH. Firstly, perylene-3,4,9,10-tetracarboxylic tetra (2-hexyldecanyl) ester (PTTE-EH) was prepared from the esterification of PTCDA with 2-hexyl-1-decanol and 1-bromo-2-hexyldecane in acetonitrile. Alkyl chains of 2-ethylhexyl, as the ester chains, were introduced to improve the solubility of the target molecule. The subsequent bromination of PTTE-EH and purification led to the desirable 1,7-dibromo-perylene-3,4,9,10-tetracarboxylic tetra(2- hexyldecanyl)ester, mixed with ～0.18 equivalent of 1,6-isomer as indicated by 1H NMR (Fig. S1, Supporting information). Multiple attempts to separate the 1,6-isomer from the mixture were not successful, therefore we proceeded to the next step to prepare compound 2 via the Stille coupling reaction. During the purification of compound 2, partial cyclization was detected in the solution of CH2Cl2 under sunlight. The target molecule of DTCTTE-EH was then prepared via oxidative photocyclization by the irradiation of a diluted methylene dichloride solution of fresh compound 2 under ambient conditions in the presence of a catalytic amount of iodine [13]. DTCTTE-EH is soluble in various organic solvents, such as dichloromethane, chloroform, toluene and chlorobenzene. Thermal properties of DTCTTE-EH were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The decomposition temperatures (Td, referring to 5% weight loss) was above 350 ℃ (Fig. S2, Supporting information), indicating good thermal stability of the molecule. No obvious phase transition was detected from the DSC curve (Fig. S3, Supporting information).

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Scheme 1.Synthetic route of DTCTTE-EH.
Room temperature absorption and emission spectra of DTCTTEEH are shown in Fig. 2. DTCTTE-EH shows well-defined vibrational fine structure of π-π transition absorption bands - weak bands at 463 nm, 435 nm, 412 nm, 391 nm, 371 nm with extinction coefficients of 1.9 × 104, 1.5 × 104, 5.6 × 104, 4.1 × 104, 2.5 × 104 L mol-1 cm-1, respectively, and a strong band at 345 nm with high extinction coefficients of around 2.6 × 105 L mol-1 cm-1. The absorption bands of DTCTTE-EH in films show a slight bathochromic shift by about 10 nm with respect to those in solution. DTCTTE-EH fluoresces with an intensive green-yellow color with the maxima wavelength at 469 nm and 502 nm in CH2Cl2. The optical band gap of DTCTTE-EH was calculated from the onset of thin film absorption spectrum with the formula of Egopt = 1240/λonset, and was determined to be around 2.48 eV.

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Fig. 2.Normalized UV–vis absorption spectra of DTCTTE-EH in solution (CH2Cl2,8 × 10-6 mol L-1, gray solid) and in film (black dotted line) and emission spectrum (dark solid) of DTCTTE-EH in CH2Cl2 solution (1.6 × 10-7 mol L-1) excited at 413 nm.
Cyclic voltammetry (CV) of DTCTTE-EH in CH2Cl2 with tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte (0.1 mol L-1) was performed to examine the electrochemical behavior and to estimate the HOMO and LUMO energy levels. The experimental redox data were calibrated by a ferrocene-ferricenium couple (Fc/Fc+). The cyclic voltammogram of DTCTTE-EH, shown in Fig. 3, exhibits two reversible reduction waves at around -1.82 V and -2.07 V, respectively, implying the formation of electrochemically stable radical anion and dianions. Two quasi reversible oxidation waves from anodic scanning can be found at around 0.96 V and 1.31 V, respectively. The HOMO and LUMO energy levels of DTCTTE-EH relative to vacuum were then obtained by subtraction of -4.8 eV for Fc/Fc+ from the first redox potentials, and were estimated to be at -5.76 eV and -2.98 eV, respectively. The HOMO and LUMO levels of DTCTTE-EH were about 0.4 eV higher in energy than that of a coronene diimide (-3.4 and -6.1 eV for HOMO and LUMO, respectively) [14], indicating that DTCTTE derivatives are much less electron deficient and easier to be oxidized compared to coronene diimides. This result is consistent with the fact that the attached four carboxylic ester groups and the two annulated electron rich thiophene units increase the electrons of DTCTTE.

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Fig. 3.Cyclic voltammogram of DTCTTE-EH.
4. Conclusion

In summary, the synthesis of a novel semiconducting polycyclic aromatic molecule, namely dithieno[5,6-b:11,12-b']coronene- 2,3,8,9-tetracarboxylic tetra(2-ethylhexyl)ester (DTCTTE-EH), was reported via a facile route with an overall yield of 33% in four steps from readily available perylene-3,4,9,10-tetracarboxylic dianhydride. Investigations of the optical and electrochemical properties indicate that DTCTTE is a new class of components for promising semiconducting materials. Rational design and synthesis of new materials based on DTCTTE and their applications in organic electronics are currently underway in our laboratory.

Acknowledgments

This work was supported by NSFC (Nos. 51073124 and 21031006), Research Fund for the Doctoral Program of Higher Education of China (No. 20100143120002) and Natural Science Foundation of Hubei Province (No. 2011CDA102).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2013.09.006.

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