Polymers of 4-Thieno[3,2-b]thiophen-3-ylbenzonitrile with Anthracene and Biphenyl: Their electronic and optoelectronic properties

Abstract: Design and synthesis of conjugated organic polymers for electronic and optoelectronic applications, such as electrochromic devices (ECD), organic light emitting diodes (OLED), organic field effective transistors (OFET), lasers, photodiodes and solar cells, have an increasing momentum. Fused thiophenes, i.e. thieno[3,2-b]thiophenes (TT), have been widely accepted as important building blocks for such materials. In this work, 4-thieno[3,2-b]thiophen-3-ylbenzonitrile (TT-CN), having electron withdrawing cyano moiety, was polymerized with anthracene and biphenyl through Suzuki coupling to obtain the polymers. Electronic and optical properties of the resultant polymers were investigated. Effects of different molecular orientations of anthracene and biphenyl blocks in the polymer backbone were compared.


Introduction
Design and synthesis of conjugated organic polymers for electronic and optoelectronic applications, such as electrochromic devices (ECD), organic light emitting diodes (OLED), organic field effective transistors (OFET), lasers, photodiodes and solar cells, have an increasing momentum1-3. Conjugated organic semiconductors have been a subject of considerable interest, not only for the exploration of their fundamental structure-property relationships, specifically, for their electrical or optical properties. Fused thiophenes, i.e. thieno [3,2-b]thiophenes (TT), have been widely accepted as important building blocks for such polymeric materials4-7. Thienothiophenes, in general, have four isomers formed through the orientations of the sufur atoms of the thiophene rings, among which thieno [3,2-b]thiophene belongs to the most widely used TTs as it provides continuous conjugation through two fused thiophenes and polymer backbone. Moreover, presence of two sulfur atoms makes them electron-rich, enabling to be used as electron donating moieties in construction of polymeric materials [1][2][3][4][5][6][7] Compared with polymers, small molecule semiconductors own the advantages of high purity and ordered packing, which are key factors for electronic and optic properties. Among the small molecules, anthracene and biphenyl, with low C-H ratio and a highly conjugated planar structure have attracted the most attention for organic polymeric materials. Anthracene, consisting of three rigid fused benzene rings, the least conjugated but the most soluble and most stable member of the acenes, has been given great consideration for organic polymeric materials. In addition, the anthracene part is compatible with the π-electron-rich structure for good electron transport. Furthermore, chemical modifications of anthracene would tune the molecular packing and charge transport properties, and fortunately, aryl groups, used to extend the p-system, like phenyl, thiophene, and thienyl, could be very easily attached to anthracene at the active end-and peripositions by couplings in high yield. Another conjugated and ordered packing aromatic molecule is biphenyl. Biphenyl, by torsion angle Φ between the planes of the two phenyl rings that the degree of π-overlap in the two phenyl rings and the resulting extent of π-systems delocalization can be finetuned. In addition, biphenyls play important roles as π -conjugated bridge and as electron rich donor like anthracene [8,9]. These roles give them the ability to become UV and electro applicable properties. In this work, 4-thieno[3,2-b]thiophen-3-ylbenzonitrile (TT-CN), having electron withdrawing cyano moiety, was polymerized with anthracene and biphenyl through Suzuki coupling to obtain the polymers 1 and 2, respectively8,9. Reason for using CN group on TT is to obtain polymeric material suitable for donor-acceptor (D-A) model. Besides, cyano groups into the polymer backbone usually lowers its LUMO level, leading to an increased electron affinity anddecreased band gap energy. Electronic and optical properties of the resultant polymers were investigated. Effects of different molecular orientations of anthracene and biphenyl blocks in the polymer backbone were compared.

Synthesis of 4-[2-(Thiophen-3-ylsulfanyl)acetyl]benzonitrile (TAB)
To a solution of 3-bromothiophene (3 g, 18.40 mmol) in dry diethyl ether was added nbutyllithium (8.1 mL, 27 mmol) dropwise at −78 °C, under nitrogen atmosphere. After the reaction was stirred for 1 h, elemental sulfur (S8) (0.65g, 20.24 mmol) was added, and the mixture was further stirred for an additional 1 h. Then, the temperature was brought to 0 °C and 4-(2bromoacetyl)benzonitrile (4.53g, 20.24 mmol) was introduced portion wise into the mixture. Stirring was continued overnight, and the reaction was quenched with water. The solution was extracted with dichloromethane 3 times, and the organic layer was washed with NaHCO3 (10%) and water. The organic layer was dried over Na2SO4, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with nhexane:CH2Cl2 (3:1) to give the title compound as a white powder (4.02 g, 84%

Synthesis of 9,10-dibromoanthracene
To a solution of anthracene (10 g, 56 mmol) dissolved in CHCl3 (150 ml) and was added elemental bromine ( 5.8 ml, 2.01 eq) by using pipette. The reaction was heated at room temperature for 5h. After that, the mixture was waited for boiling of CHCl3. The mixture was heated in a beaker. Then, it was cooled in ice bath for crystallization. The precipitated solid particles were filtered and allowed to dry in vacuum oven. 15.2 g yellow product was obtained with yield of 81%. 1

Synthesis of 9,10-bis(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2-yl)anthracene
To a solution of 9,10-dibromoanthracene (2 g, 5.95 mmol) and bis(pinacolato)diboron (3.7 g, 14mmol) dissolved in DMF (45 ml). After the reactants completely dissolved, potassium acetate (1.8 g, 3 eq) was added onto the mixture. The mixture was treated with the gas of N2 for a while and Pd (II) was added as a catalyzer. It was allowed to mix at 80 0 C for all night. The mixture was filtered from celitte for holding the Pd (II). After the boiling of DMF, the mixture was extracted with CH2Cl2, sodium carbonate, and water. The organic layer was dried with Na2SO4, filtered and thr solvent was evaporated under reduced pressure. The mixture was purified by column chromatography eluting with CH2Cl2 : hexane ( 8:1). 1.53 g yellowish-white product was obtained with yield of 60%. 1

Optical Properties
The spectroscopic characterization of the two compounds was carried out in tetrahydrofuran solutions. While the p(TT-Biphenyl) P1 showed intense UV-visible absorption bands in the 380 nm region, the p(TT-Ant) P2 showed absorption maxima at the 260 and 400 nm (Figure 1, Table1).

Electrochemical Properties
Electrochemistry of the polymers P1 and P2 were conducted in a cyclic voltammetry (CV), using Pt wires as working and counter electrodes and Ag wire as a reference electrode. On the anodic scans, polymer reductions were observed at about -0.75 V for P1, -0.64 V and for P2 .Oxidation of the polymers formed a peak at + 1.28 V for P1 and + 1.42 V for P2.

Conclusion
In this work, two novel polymers, containing thienothiophene, anthracene and biphenyl groups, were designed and synthesized by Suzuki polymerization. Electronic and optical properties of the resultant polymers were investigated. UV, emission and CV values indicated that P1 and P2 are suitable materials for electronic and optical applications.