Reduced Excimer Formation in Polyfluorenes by Introducing Coil-like Poly[penta(ethylene glycol) methyl ether methacrylate] Block Segments

Abstract

Conjugated polymers are of special interest due to their excellent optical and optoelectrical properties both in the solid state and in selective solvents. They have delocalized, conjugated electronic structures in their main chains, which results in unique material characteristics such as photoluminescence (PL), electroluminescence (EL), conductivity, and oxidative stability. Typical examples include the oligomers and polymers of thiophene, p-phenylene, phenylenevinylene, phenylquinoline, and fluorene, which have been extensively used in molecular electronics and electronic devices. The molecular chains of these polymers tend to aggregate by strong interchain interactions between neighboring aromatic moieties, which causes an undesirable red-shift fluorescence and reduced emission intensity upon thermal annealing or with the passage of current. In addition, because of the strong, interchain interactions between the highly anisotropic, rigid polymer backbones, most of these compounds are insoluble or infusible. Recent attempts have been made to enhance the performance of conjugated compounds by controlling their strong interchain interactions through introducing structural irregularities within the polymer backbone or copolymerizing with conjugated or nonconjugated comonomers. One attractive approach is to combine heterogeneous polymer segments to the conjugated polymer backbones in the forms of block copolymers. Interesting results have been reported on the incorporation of conjugated, rod-like block segments such as polythiophene, poly(p-phenylene), poly(phenylenevinylene), phenylquinoline, vinylenic biphenyls, and polyfluorene into a heterogeneous polymer chain. Amphiphilic rod-coil diblock and triblock copolymers consisting of poly(phenylenevinylene) or polyfluorenic block were also synthesized to form self-assembling nanostructures in selective solvents. In the case where rod-coil block copolymers are used, coil-like blocks tend to be segregated from rod-like blocks due to a conformational dissymmetry between component blocks to generate a series of supramolecular nanostructures. These block copolymers exhibit almost the same EL and PL properties as those observed for the corresponding conjugated homopolymers. The spatial confinement of the conjugated blocks in the geometrically well-defined nanoscopic structures may reduce the dimensions of the molecular clusters and decrease the intermolecular aggregation between the luminescent, conjugated molecules. Polyfluorene and its derivatives are one of promising materials for light-emitting diodes because of its high PL quantum efficiencies and thermal stability. However, rigid-rodlike polyfluorene chains tend to stack cofacially with each other due to the favorable interchain π-π interactions between aromatic moieties, which is considered to be troublesome in light-emitting devices because it may enhance nonradiative decay both in solution and in the solid state. Here, we attempt to synthesize a novel, amphiphilic symmetric triblock copolymer containing a conjugated rod-like midblock of poly(9,9-didodecylfluorene-2,7-diyl) (PF) and coil-like endblocks of poly[penta(ethylene glycol) methyl ether methacrylate]s (PEGMA) by using the atom transfer radical polymerization (ATRP) technique and demonstrate the effect of the coil-like PEGMA endblocks on the aggregation and excimer formation of the conjugated PF segments during annealing at high temperature. PEGMA was chosen as a hydrophilic, coil-like endblock due to its excellent solubility in both hydrophobic and hydrophilic media. Because of the dissimilarities in the molecular conformation and hydrophilicity between the rod-like, conjugated PF and coil-like PEGMA blocks, they are expected to be phaseseparated to display a self-assembled nanostructure.