Enhanced brightness emission-tuned nanoparticles from heterodifunctional polyfluorene building blocks

Article abstract of DOI:10.1021/ja311497e

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 × 10³ and 1.0 × 10⁴ 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 × 10³ 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.