Polyfluorene Nanoparticles and Quantum Dot Hybrids via Miniemulsion Polymerization

Suzuki-Miyam~polycondensatlbn of 2-( 4A,5;5tetramethyI -lJ3,2-dioxaborolan-2-yl)-7 -broino~9,9-dioctylfluor~ ene in aqueous miniemulsion With only tw'oequivalents of NaOH as~ base yields colloidally stable nanoparticles of polyfluorene With M;, ca. 2 X lQ4 g mol-1 and particle sizes of 40-85 nrn,depending on the surfactant concentration. Polymerization in the presen<:e of CdSe/CdS core/shell quantum dots affords hybrid nanoparticles of nonaggregated qllantum dots, in particular nanoparticles composed of a single quantllm dot embedded in a polyfluorene shell. Microphotolurninescence $pectros.copy on single hybrid. particles reveals an enhanced photostability of the quantllm dots and indicates an etIkient Forster energy transfer from the polyfluorene shell to the quantum dot. L uminescent nanoparticles based on conjugated polymers feature large absorption cross sections, high fluorescence intensities, photostability, and tOxicological safety. This combination compares favorably to other materials. Thus, conjugated polymer nanoparticles have emerged as luminescent probes.6 They are currently studied, for example, in live biological imaging9 or sensors. 12 Most frequently, studies of such luminescent particles have employed polyfluorene as the conjugated polymer. These particles were generated by postpolymerization methods, namely, secondary disperSion by emulsification of a polymer solution in a water-insoluble solvent• or a "reprecipitation" approach, in which a dilute solution of polyfluorene in a water-soluble organic solvent is injected manually into excess water. Here, we report a different approach, the direct synthesis of polyfluorene dispersions by Suzuki-Miyaura cross-coupling polycondensation in miniemulsion. This approach is further elaborated to the preparation of well-defined hybrid semiconductor quantum dot/polyfluorene particles, which are exceptionally suited for quantllm optics experiments With single nanoemitters. 19 Miniemulsions most commonly employ anionic surfactants, like sodium dodecyl sulfate (SDS), to stabilize the particles being formed. However, such a stabilization is very sensitive to the ionic strength of the aqueous phase?O In Suzuki polycondensation protocols, an excess of base of up to 30 wt % concentration of the aqueous phase is not unusual to yield high conversions and thus high molecular weights. According to the established mechanistic picture, the catalytic reaction requires two equivalents of base, one for the metathesis of the organopalladium halide species to the organopalladium hydroxide or alkoxide species and another one for the quatemization of the boronic acid.,23 Therefore, we employed a bimolar excess of NaOH only With respect to the monomer. Miniemulsions were generated by ultrasonication of a mixture of a basic (0.06 wt % NaOH, 14 mg) 1 wt % aqueous SDS 1343 solution, With a toluene solution of the bifunctional ABmonomer 2-( 4,4,5,5-tetramethyl-l,3,2-dioxaborolan-2-yI)-7bromo-9,9-dioctylfluorene, 1 mol % [Pd(PPh3)4]' and hexadecane as a hydrophobe as the oil phase. Heating to 85 °C for 20 h resulted in polymerization to afford a stable, virtually transparent dispersion. The number average particle sizes determined by dynamic light scattering (DLS) are typically on the order of 40 nm. GPC vs polystyrene standards revealed a polymer molecular weight (Mn) of ca. 2.3 X 10 g mol-I. This compares to molecular w~hts obtained With standard Suzuki polymerization protocols, indicating that very low base concentrations can sufficiently promote the Suzuki polycondensation in miniemulsion. Higher amounts of base (SO mg of NaOH, 0.2 wt % aqueous solution, 7-fold excess over monomer) did not afford stable dispersions, underlining the need of appropriately chosen reaction conditions compatible With colloidal stabilization. Only a 1:1 molar ratio of base:monomer (7 mg of NaOH) yielded dispersions With a polymer molecular weight (Mn) of 1.9 X 104 g mol-I, indicating that one equivalent of base is still adequate for an efficient catalysis, irrespective of the aforementioned mechanistic considerations. At this Iow base concentration, particles With a more uniform shape (Figure SlD in Supporting Information) are formed. This can be related to a better emulsion and particle stability at this lower concentration of added NaOH electrolyte. In the absence of any NaOH, no polymer was obtained, confirming the crucial role of the base for the polymerization. Steric stabilization is less sensitive than electrostatic stabilization toward additional electrolytes, like the added NaOH base. To this end, substitution of the anionic SDS for the nonionic surfactants Tween 80 or Span 80 afforded polyfluorene nanoparticles which precipitated within a few days. By comparison, SDS stabilized nanoparticles were stable for months or longer. Even at a Iow SDS concentration of 0.08 wt %, stable dispersions with particle sizes of 85 nm were obtained (Table 1). As expected, particle sizes decrease with Table L Effect of Surfactant Concentration on Particle Size and Molecular Weight in Suzuki Miniemulsion Polymerizations entry amount of SDS particle size b Mn< Mw/M/ I 20 mg 85 nm 1.3 X 10 2.1 2 50 mg 70 nm 9.0 X 103 2.5 3 125 mg 55 nm 1.7 X 10 2.3 4 250 mg 39 nm 2.3 X 10 2.5 aReaction conditions: 100 mg of 2-( 4,4,5,5-tetramethyl-l,3,2-dioxaborolan-2-yI)-7-bromo-9,9-dioctylfluorene, 1 mol % [Pd(PPh3)4], 0.05 mL ofhexadecane, 1 mL of toluene, 25 mL of H 20, 14 mg of NaOH, 2 min uitrasonication, and 20 h at 85 °c. bNumber average particle size determined by DLS CDetermined by GPC in THF vs polystyrene