Enhanced Brightness Emission-Tuned Nanoparticles from Heterodifunctional Polyfluorene Building Blocks

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Abstract

Three-coordinate complexes (bromo)[4-(2,2-dimethyl-1,3-dioxolan-4-yl)-phenyl](tri-tert-butyl-phosphine)palladium (1) and (bromo){4-[(tetrahydro-2H-pyran-2-yloxy)methyl]phenyl}(tri-tert-butyl-phosphine)palladium (2) were used to initiate Suzuki-Miyaura chain growth polymerization of 7'-bromo-9',9'-dioctyl-fluoren-2'-yl-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (3). The polymerization was optionally terminated by end-capping with red-emitting N-(2-ethylhexyl)-1,6-bis(4-tert-octylphenoxy)-9-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-perylene-3,4-dicarboximide. Heterodisubstituted polyfluorenes of adjustable molecular weights between 5 × 10(3) and 1.0 × 10(4) g mol(-1) and narrow molecular weight distribution (M(w)/M(n) < 1.2), bearing precisely one or two hydroxyl groups on one chain end and optionally a dye-label on the opposite end, were obtained virtually devoid of any side-products. Covalent attachment of polyethylene glycol (M(n) = 2 × 10(3) g mol(-1)) to the reactive end groups yielded amphiphilic block copolymer, which afforded stable nanoparticles with diameters in the range of 25-50 nm when dispersed in water. These particles exhibited a bright fluorescence emission with quantum yields as high as Φ = 84%, which could optionally be tuned to longer wavelengths by energy transfer to the perylene monoimide dye. The heterodifunctional nature of these polyfluorenes is crucial for a bright and enduring fluorescence brightness as revealed by comparison to nanoparticles containing physically mixed dye. Further addition of terrylene diimide dye to the nanoparticles of perylene-end-capped polyfluorene block copolymers allows for an energy cascade resulting in emission exclusively in the deep red and near-infrared regime.

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