Pure Blue Emitting Poly(3,6-dimethoxy-9,9-dialkylsilafluorenes) Prepared via Nickel-Catalyzed Cross-Coupling of Diarylmagnesate Monomers

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Abstract

Polysilafluorenes (PSFs) are an important class of light-emitting conjugate polymers noted for their characteristic brilliant solid state blue luminescence, high quantum efficiency, excellent solubility, and improved thermal stability. These polymers are also reported to have superior electron conductivity to polyfluorenes. The higher electron affinity and conductivity, which are particularly promising for OLEDs, originate from σ*−π* conjugation between the σ* antibonding orbital of the exocyclic Si–C bond and the π* antibonding orbital of the butadiene fragment. In this paper, we present the synthesis and thorough characterization of several new derivatives of 2,7-dibromo-3,6-dimethoxy-9,9-dialkylsilafluorene monomers and demonstrate an efficient room temperature route to their polymerization. In addition to silafluorene monomers with simple alkyl side chains, we have increased the functionality of several of our monomers by incorporating vinyl, cyclohexenyl, and norbornenyl moieties into their side chains, which we believe is useful for postpolymerization modification. (i.e., adding pendant emitters to tune PL or cross-linking). The production of polymer was achieved using a nickel-catalyzed polycondenation of diarylmagnesate-type monomers in a mixed solvent system of THF and 1,4-dioxane (7:3). Using 1,4-dioxane as a cosolvent was discovered to significantly increase the Mg/Br exchange rate by a factor of 5, reducing the time required to achieve stiochiometric conversion of sterically hindered and electron rich 2,7-dibromo-3,6-dimethoxy-9,9-dialkylsilafluorene to 2 h. Also, relatively fast rates of polymerization were observed. Polymers reached their maximum molecular weight within 30 m. In many cases, Mn exceeds 50 kg/mol (PDI ∼ 1.5–2.0). The resultant polymers possess characteristic blue photoluminescence with solid state quantum yields (exceeding 80% in many cases). Polymer films have excellent transparency (with a measured Eg ∼ 3.0 eV) and thermal stability as demonstrated by TGA/DSC. Energy levels determined using CV were −5.62 and −2.62 eV for HOMO and LUMO, respectively.

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