Preparation of poly(fluorene)s using trans‐bis(dicyclohexylamine)palladium diacetate as a catalyst: Scope and limitations

The 9,9-disubstituted fluorene moiety represents a very favorable building block for obtaining processable, conjugated polymers of the poly(para-phenylene) type. Its excellent optical and electrical properties make semiconducting poly(fluorene)s (PFs) and their analogues an important class of blue-light-emitting dyes in polymer light-emitting diodes (PLEDs). Pioneering work on the synthesis of soluble PFs was published in the late 1980s by Yoshino et al. using FeCl3-mediated oxidative coupling. Regioregular PFs were prepared first by nickel-catalyzed Yamamoto coupling of 2,7-dibromofluorene derivatives and shortly afterwards by palladium-catalyzed Suzuki coupling reactions of 2,7-dibromofluorene and 2,7diboronylfluorene derivatives. These two methodologies represent the most important routes toward PFs and PF-based polymers today. Among other ways, Stille coupling, a reductive aryl–aryl cross-coupling of 2,7-distannyl derivatives and 2,7-dibromofluorene derivatives, represents a third methodology that is applied occasionally, particularly when thiophene-based materials are used as comonomers. Although Yamamoto coupling of two different dibromoaryl derivatives provides statistical copolymers, Suzuki coupling offers convenient access to alternating copolymers and is therefore the preferred polymerization methodology today. Alternating copolymers are an important class of semiconducting materials, as they offer the possibility of designing their optoelectronic properties to maximize their application potential in areas such as organic photovoltaics, field-effect transistors, and light-emitting diodes. For the broad scope of such polymers, endgroup modification provides another tool to fine-tune material properties by adding appropriate monofunctionalized comonomers. In this way, an attachment of defined functional groups, which are often chemically, chromophorically, or electronically active, can be achieved. Even when a functional end group is not desired, halogen or boronic acid substituents should be removed from the polymer chain because boron and bromine units could quench emissions and contribute to a redshift of the electroluminescence emission in PLEDs. A typical Suzuki coupling is carried out with dibromide and diboronate derivatives with tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] as the catalyst in a mixture of toluene and aqueous K2CO3 in the presence of a phase-transfer reagent such as Aliquate 336. Typically, the reaction mixture is refluxed under Correspondence to: C. Slugovc (E-mail: slugovc@tugraz.at)