Synthesis of Soluble Polythiophene Partially Containing 3 , 4-Ethylenedioxythiophene and 3-Hexylthiophene by Polycondensation

A novel poly(quinquethiophene) partially containing 3,4-ethylenedioxythiophene (EDOT) and 3hexylthiophene, poly(3,3''''-dihexyl-3',4',3''',4'''-diethylenedioxy-2,2':5',2'':5'',2''':5''',2''''-quinquethiophene), was synthesized by three types of polycondensations. Among them, direct C-H coupling reaction gave the polymer with the highest molecular weight. The resulting polymer was soluble in common organic solvents. Absorption and fluorescence spectra of the polymer showed a remarkable red-shift compared with the corresponding monomer due to the expansion of effective π-conjugation length.


Introduction
Since the late 1970s, π-conjugated polymers have attracted a great deal of attention from many researchers, especially in the field of plastic electronics [1]- [5].Among them, polythiophenes have been well investigated as the most significant class of π-conjugated polymers.Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most popular polythiophene derivatives and has excellent properties such as stability of the oxidized state, moderate band-gap energy, low oxidation potential, and optical transparency in the visible spectral region [5]- [11].These properties result from the synergistic associations of the strong donor effect of the ethylenedioxy groups with the propensity of 3,4-ethylenedioxythiophene (EDOT) to develop noncovalent intramolecular S-O interaction [12]- [17].This S-O interaction, on the other hand, leads to a rigid polymer backbone to reduce the solubility in solvents.Thus, PEDOT obtained by electrochemical oxidation of EDOT is insoluble in common organic solvents.
Until now, several types of polythiophenes partially containing EDOT have been synthesized and successfully applied to electrochromic devices because of the stable nature of the oxidized state [18]- [24].However, there are few reports on the characterization of these polymers and their charge transport properties.Recently, we have synthesized a polythiophene partially containing EDOT unit in the repeat unit, poly(3',4'-ethylenedioxy-2,2':5',2''terthiophene) (polyTET), by electrolytic polymerization of the EDOT-containing terthiophene, 3',4'-ethylenedioxy-2,2':5',2''-terthiophene (TET) (Scheme 1) [25].The resulting polymer was found to be soluble in dimethylsuldoxide (DMSO), tetrahedrofuran (THF) and dimethylformamide (DMF), and could be characterized by 1 H NMR and mass spectroscopies, and gel permeation chromatography (GPC).The electrical conductivities of polyTET were measured as a function of oxidation levels, and the highest conductivity was found to be comparable to that of polythiophene obtained by the electrolytic polymerization of unsubstituted thiophene.However, the molecular weight of the polymer was not so high because the cationic species formed by electrochemical oxidation were stable and could not be polymerized, and the film-forming property was very poor.
Poly(arylene)s including polythiophenes have been synthesized not only by electrolytic polymerizations [26], but also by chemically oxidative polymerizations [27] [28] and organometallic polycondensations [29].Since the polycondensations afford the polymers with well-controlled bonding between monomeric units, many types of polycondensations have been developed.
1 H NMR spectra were recorded by a 500 MHz spectrometer (Varian Inc., NMR System 500).IR spectra were taken on a Perkin Elmer Spectrum One FT-IR spectrophotometer by ATR method.Molecular weights of the resulting polymers were analyzed by a GPC coupled with an UV detector (Shimadzu Corp., SPD-10A).Combination of Shodex KF-801 (30 cm, exclusion limit: M n = 1.5 × 10 3 , polystyrene) KF-802 (30 cm, exclusion limit: M n = 5.0 × 10 3 , polystyrene) and KF-803L (30 cm, exclusion limit: M n = 7.0 × 10 4 , polystyrene) columns (linear calibration down to M n = 100) were used for molecular weight analysis with THF (1.0 dm 3 •min −1 ) as an eluent.Measurements of mass spectroscopy and the elemental analysis were made using a High Performance Gas Chromatograph-Time-of-Flight Mass Spectrometer (JEOL, JMS-T100GCV (Accu TOFGCv 4G)) and an Elemental Analyzer (PerkinElmer, 2400 Series II CHNS/O), respectively.UV-vis absorption and fluorescence spectra were measured by a Shimadzu UV-3150 spectrophotometer and a Hitachi F-4500 spectrophotometer, respectively.Fluorescence quantum yields (Φ) were determined by a Hamamatsu C9920-01 equipped with CCD by using a calibrated integrating sphere system.

Monomer Synthesis
The synthetic routes of the monomers are shown in Scheme 2 and the detailed synthetic processes are described below.

Polymer Synthesis
PolyHE5T was synthesized by three types of polycondensation reactions shown in Scheme 3 and the detailed synthetic processes are described below.

Polycondensation Methods
To choose the polycondensation method which gives polyHE5T with the highest molecular weight, three types of polycondensations using (1) Suzuki, (2) Stille, and (3) direct C-H coupling reactions were investigated (Scheme 3). Figure 2 shows GPC curves of as-prepared polymers obtained by three types of polycondensations.Four peaks were observed at 23.4, 21.0, 19.8, and 19.0 min, whose retention times were ascribed to the molecular weights of monomer, dimer, trimer, and tetramer of HE5T unit.A broad shoulder peak due to component of higher molecular weight of polyHE5T was also observed from 16 to 19 min.The intensity of the shoulder peak of a polymer prepared by direct C-H coupling was much stronger than those of polymers prepared by Suzuki and Stille coupling reactions.In the case of direct C-H coupling, the threshold value of retention time (RT th ), where GPC curve raise up, was 16 min corresponding to the molecular weight (MW) of 63,000 (polystyrene standard).On the other hand, RT th s in the case of Suzuki and Stille couplings were 16.9 min (MW = 22,000) and 16.7 min (MW = 28,000), respectively.These results suggest that the molecular weight of polyHE5T prepared by direct C-H coupling is higher than those of Suzuki and Stille couplings.In the case of Suzuki and Stille couplings, bis(dioxaborolanyl)-and/or bis(trialkylstannyl)-substituted HE5Ts (BBpinHE5T and BSnHE5T in Scheme 3, respectively) were synthesized from HE5T as monomers, and coupled with DBrHE5T in stoichiometric amount.Since the stabilities of these bifunctional HE5Ts were low, their purifications were difficult.This is the reason why the molecular weights of polymers synthesized by Suzuki and Stille coupling reactions were lower than that of the polymer synthesized by direct C-H coupling reaction.

Reaction Condition of Direct C-H Coupling
Polycondensation reactions using direct C-H coupling were carried out under various conditions.Table 1 summarizes the reaction conditions, and Figure 3 shows the GPC curves of as-prepared polymers obtained by these conditions.
Because PCy 3 •HBF 4 was reported as an effective ligand for Pd-catalyzed direct C-H coupling reaction of thiophene derivatives [33] [34], the catalytic system of Pd(OAc) 2 with PCy 3 •HBF 4 was examined first (entry 1-3).When DMAc was used as a solvent, polyHE5T with the highest molecular weight was obtained (entry 2).It is known that PtBu 2 Me•HBF 4 was the best ligand for the polycondensation of 1,2,4,5-tetrafluorobenzene with 2,7-dibromo-9,9-dioctylfluorene [35], while this catalytic system did not promote the polycondensation reaction of 3,3',4,4'-tetramethylbithiophene and 2,7-dibromo-9,9-dioctylfluorene [36].In our case, the use of PtBu 2 Me•HBF 4 (entry 4) did not improve the molecular weight of polyHE5T compared with the case of PCy 3 •HBF 4 (entry 2).Herrmann's catalyst is also known as an effective catalyst for direct C-H coupling [37]- [39], but polycondensation did not proceed in the current system (entry 5).Very recently, Ozawa et al. reported that the palladium complex with dibenzylideneacetone and P(o-MeOPh) 3 , in conjunction with pivalic acid and Cs 2 CO 3 exhibit high catalyst performance in polycondensation [40] [41].Using this catalytic system (entry 6), the molecular weight of polyHE5T was slightly improved in comparison with the catalytic system of Pd(OAc) 2 and PCy 3 •HBF 4 (entry 2).Since the resulting polymer was found to be soluble in common organic solvents such as THF, CHCl 3 , DMF, and o-dichlorobenzene, the polymer could be purified by reprecipitation.GPC analysis of polyHE5T obtained by the reaction condition of entry 6 revealed that the molecular weight of the polymer was M n = 8600, M w = 14,000 and M w /M n = 1.6.

Physicochemical Properties of PolyHE5T
The normalized UV-vis absorption and fluorescence spectra of HE5T and polyHE5T in dilute THF solution are depicted in Figure 4, and their optical properties are summarized in Table 2.Both absorption and fluorescence spectra of polyHE5T were red-shifted compared with those of HE5T due to the expansion of effective π-conjugation length by the polymerization.It was also found that smooth films of polyHE5T were successfully obtained by spin-coating of its solution in o-dichlorobenzene on ITO and/or glass substrate.Using the good film-forming property, it is expected that electrochemical and electrical properties of polyHE5T in film state can be investigated.Their detailed properties will be discussed in the near future.

Conclusion
Polythiophene containing 3,4-ethylenedioxythiophene in a repeat unit was firstly synthesized by polycondensation.Among polycondensation reactions which were tried in this study, direct C-H coupling reaction gave a polymer with the highest molecular weight.The molecular weight of the polymer was further increased by selecting the appropriate condition of catalytic system.Absorption and fluorescence spectra of the resulting polymer were red-shifted in comparison with those of the corresponding monomer reflecting that the effective π-conjugation length was expanded by the polymerization.

Figure 2 .
Figure 2. GPC curves of as-prepared polyHE5Ts obtained by three types of polycondensations.

Figure 3 .
Figure 3. GPC curves of as-prepared polyHE5Ts obtained by polycondensation reactions using direct C-H coupling with different reac tion conditions.

Table 1 .
Reaction conditions of polycondensations using direct C-H coupling a) .