An Organozinc Route for the Preparation of Functionalized Poly-3-alkylthiophenes

Abstract

In 1980, two groups reported the synthesis of unsubstituted polythiophene utilizing 2,5-dibromothiophene via a Ni-catalyzed method. Since then, several different types of catalysts were employed for the polymerization of 2,5dibromothiophene. Unfortunately, low molecular weight polymers generally resulted from these attempts. Even though these methods provided interesting materials, there was a big barrier to utilize this polymer due to the lack of processability. In order to overcome this difficulty, an alkyl chain was introduced on 3-position of the thiophene ring. Of the many methods, metal-catalyzed cross-coupling polymerization was most frequently used, and the synthesized polythiophenes containing an alkyl group equal or greater than C4 are soluble in common organic solvents. This wellestablished method, however, has significant drawbacks generating inconsistent results and regiorandom polymers. As is known, the regioregularity of the 3-substituted polythiophene plays a critical role in electronic and optical devices. Consequently, developing the efficient methods of preparing highly regioregular head-to-tail poly-3-alkylthiophenes is of high value. This goal was achieved by two research groups, seperately. Rieke and coworkers reported a synthetic approach to the preparation of regioregular poly-3alkylthiophene utilizing thienylzinc bromide. The other synthetic method, the McCullough method, was performed by using the Kumada cross-coupling method. Even though extensive studies have focused on the preparation and application of the linear alkyl-substituted polythiophenes, there is an increasing need to develop various types of polythiophenes because these polymers have a wide range of potential possibilities of being used in material chemistry. Recent reports also showed that the chemical composition of the P3HT significantly affect the device performance. Therefore, many efforts were devoted to finding new strategies for the preparation of functionalized P3HTs. One of the first attempts was to introduce a heteroatom on the thiophene ring. This has been accomplished wherein the thiophene has an oxygen atom directly attached to the ring. Furthermore, polythiophenes containing other functionalities such as an alcohol, ester, and urethane were prepared. In addition, thiophenes bearing a carboxylate or sulfonate functionality were introduced for the synthesis of water-soluble polythiophenes. More importantly, a recent report showed that P3HT-phosphonic ester was employed to improve the power conversion efficiencies in hybrid solar cells. We have developed a versatile synthetic method for polythiophenes bearing a phosphonic ester functionality on the 3-position. In addition, we wish to report more results obtained from the study on the preparation of polythiophenes bearing several different functional groups. In 2003, McCullough et al. reported the synthesis of poly[3-(6-diethylphosphorylhexyl)thiophene] utilizing the Stille method. This synthesis required a difficult monobromination as well as cryogenic condition (−70 C) for the preparation of the reactive organometallic intermediate. This study also pointed out that the GRIM and Kumada methods are not reliable for the preparation of polythiophenes containing the phosphonic ester moiety. Although poly[3(11-diethylphosphorylundecyl)thiophene] was also synthesized by Kowalik and Tolbert using an organozinc route, we have focused on a more practical synthetic method. It was accomplished starting with readily available 2,5-dibromo-3-(6-bromohexyl)thiophene (4), which is easily prepared by known literature methods. The use of this route provided a huge advantage over the previously reported studies requiring a regioselective monobromination of thiophene ring. As shown in Scheme 1, in our study, the phosphonate ester group was easily provided by the MichaelisArbuzov reaction with 4 which was readily obtained by the simple bromination of 3 with NBS. The oxidative addition of active zinc to 5 was completed in 20 min at 0 C showing 75:25 ratio regioselectivity. The resulting mixture of thienylzincs (6 and 7, Scheme 1) were polymerized in the presence of 0.3 mol % of Ni(dppe)Cl2 at room temperature yielding the title polymer (P1) in 93%. The UV-vis analysis exhibited a λmax of 440 nm in chloroform solution. This is compatible with the literature value (λmax 442 nm). 11a