Synthesis of Alternate Copolymers Composed of Dithienosilole and π-Conjugated Units

Abstract

Stille cross-coupling reactions of 2,6-bis(tributylstannyl)-4,4-bis(4-butylphenyl)dithienosilole with dihaloarenes gave dithienosilole-arene alternate polymers. The resulting copolymers showed the UV absorption and emission maxima at 450-601 nm and 543-698 nm, respectively, depending on the arene units in the polymer backbone. Electroluminescence properties of the dithienosilole-pyridine polymer were studied in its spin-coated film.KEY WORDS: Silole / Electroluminescence / Conjugated Polymer / Thiophene / Recently, we demonstrated that dithienosilole having a silole ring condensed with a bithiophene system (DTS in Scheme 1) exhibits enhanced conjugation, as compared with silole-free bithiophene.1-3 The tricyclic system makes the thiophene rings retain high coplanarity, enhancing theconjugation in DTS. In addition, bonding interaction between the silicon à -orbital and the bithiophene2 lowers the LUMO energy, which allows the use of the DTS-based compounds as efficient electron transporting materials for multi-layered organic light emitting diodes (OLEDs).3 It was also found that DTS-based compounds bearing appropriate substituents, much as methylthio groups, are usable as hole-transport materials in OLEDs. 4 Recently, palladium-catalyzed oxidative coupling of 2,6-bis(tributylstannyl)dithienosilole proved to produce poly(dithienosilole-2,6-diyl)s cleanly. 5,6 These DTS homopolymers exhibit efficient conjugation in the polymer backbone. For example, UV absorption maximum of poly[4,4-bis(4-butylphenyl)dithienosilole-2,6-diyl] appears at 561 nm, red-shifted from that of regio-regular poly(3-hexylthiophene) (HT > 95%), by about 100 nm. It was also demonstrated that DTS-thiophene and bithiophene copolymers could be used as hole-transport emitters for OLEDs 5 and p-type semiconductors.7 DTS-fluorene 8,9 and DTS-dithienylenebenzothiadiazole 6 copolymers were also synthesized recently, as photoluminescence and photovoltaic-active materials, respectively.To know how the electronic states of DTS-containing -conjugated polymers are affected by the structure, we prepared alternate copolymers [DTS-Ar] n , by stille coupling reactions of bis(tributylstannyl)-substituted DTS and various dihaloarenes (ArX 2 ), dibromo-benzene, biphenyl, pyridine, and tetraphenylsilole, and diiodoquinoxaline. The present polymer with Ar = pyridine unit showed electroluminescence properties, although the efficiency was rather low. EXPERIMENTAL General ProcedureAll reactions were carried out in dry nitrogen. Toluene was distilled from Na and stored over activated molecular sieves at ambient temperature until use. Monomer DTS-Sn was prepared as reported in the literature.5 GPC analysis of the polymers was performed with Shodex columns, KF804 and KF806 connected in series, using a UV detector (240 nm). NMR spectra were measured on a JEOL model LA-400 spectrometer. Some 13 C NMR signals of the polymers are missing, because of the signal broadness and rather low solubility of the polymers. Preparation of DTS-containing PolymersIn a 30 mL two necked fla...

Conjugated Oligomers and Polymers Containing Dithienosilole Units

“…Recently, we prepared several copolymers by Stille coupling reactions, as shown in Scheme 4. [26,29] In these reactions, the appropriate combination of monomers (method a or b in Scheme 4) must be polymerized. For example, pDTS-Th and pDTS-Bt prepared by method b had much lower molecular weights than those of samples prepared by method a.…”