Size Control of Spherical and Anisotropic Fluorescent Polymer Nanoparticles via Precise Rigid Molecules

Abstract

Highly monodisperse block copolymers allow for the preparation of luminescent particles with sizes reflecting the oligomer chain length. Self-organization in tetrahydrofuran/methanol leads to anisotropic nanorods with a thickness corresponding directly to the oligomer length (up to 14 nm for 21-mers). As a prerequisite for the realization of this concept, syntheses of defect-free poly(phenylene ethynylene)s with unprecedented chain length up to 43 repeat units were developed. Absorption and emission wavelength in solution continue to increase with increasing chain length, and do not converge until the 43-mer. ■ INTRODUCTIONConjugated polymer nanoparticles have received increased interest in the past few years due to their favorable photo-and electroluminescent properties that make them attractive candidates for e.g. optoelectronics, live cell imaging, and biosensing. 1−8 Gaining control over the particle size and shape is of fundamental interest yet remains a largely unresolved issue.The preparation of such nanoparticles most commonly involves postpolymerization dispersion techniques. 2 To obtain nanoparticles in the size regime below ca. 50 nm, which is particularly relevant e.g. for cell imaging, "nanoprecipitation" is frequently employed. A solution of the hydrophobic conjugated polymer in a water-miscible solvent (THF) is injected rapidly into an excess of water to afford highly diluted dispersions of nanoparticles. 9 Particle sizes can be varied to some extent by dilution of the polymer solution. For example, highly diluted dispersions of approximately spherical nanoparticles of high molecular weight substituted poly(phenylenevinylene) (MEH-PPV) were generated in this fashion. In these particles, the polymer chains are considered kinetically arrested and are probably randomly oriented. 10 Note that the mechanism of colloidal stabilization remains unclear here.As a concept to address the aforementioned problem of size control and to gain access to nonspherical shapes, we sought to encode particle dimensions directly via the length of the linear molecules with precise molecular dimension of these building blocks. In order to provide colloidal stability of the derived nanoparticles, these building blocks are employed in the form of block copolymers with a hydrophilic block. ■ RESULTS AND DISCUSSIONThe self-assembly behavior in solution of different classes of rod−coil molecules 11−13 with π-conjugated rod blocks has been reported, including polymeric or oligomeric poly(phenyl quinoline), 14−16 polythiophene, 17−20 polyfluorene, 21,22 poly-(phenylene vinylene), 23−25 poly(phenylene ethynylene), 26 and poly(p-phenylene). 27 However, syntheses of those polymers afford either polymers with rather broad molecular weight distributions, which is not favorable since different hydrophilicto-hydrophobic ratios in the polymer chain result in different supramolecular structures, or low molecular weight oligomers. An illustrative exception is Geng et al.'s synthesis of a polyfluorene 64-mer by Suzuki coupling in an iterativ...

Probing of chain conformations in conjugated polymer nanoparticles by electron spin resonance spectroscopy

“…43 Spin labeling was conducted via DCC coupling of the dihydroxy endgroup functionalized OPE n with 4-Carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, yielding the singly and doubly TEMPO-labeled oligomers, SL-OPE n and DL-OPE n , respectively (ESI †).…”

“…The synthesis of monodisperse HO-OPE n -OH (n = 5, 7, 9, 11, 21), HO-OPE 4 , HO-OPE 22 and PEG-OPE 9 has been reported previously. 43 DCC-coupling -general procedure for the preparation of TEMPO labeled DL-OPE n oligomers. 2-3 Eq.…”

Origin of the Anisotropy and Structure of Ellipsoidal Poly(fluorene) Nanoparticles

“…43,90,91 This aggregation-induced disorder occurs especially in case of nanoparticles generated rapidly, hence representing kinetically arrested structures. 3,93,94 Particles prepared over extended time periods show the tendency to form more ordered structures of stretched, aligned polymer chains and consequently exhibit a red-shift in absorption. 3 This was also observed for annealing or swelling of particles, indicating that distorted polymer chains can rearrange into a more ordered state.…”

One‐dimensional growth kinetics for formation of cylindrical crystalline micelles of block copolymers

“…By contrast, spherical crystalline micelles basically have no growth process and two‐dimensional lamellar micelles grow along four directions. The cylindrical micelles of BCPs also have a wide range of potential applications in various fields, including optoelectronic materials, catalysis and biomedical materials . The high aspect ratio of cylindrical micelles is advantageous to applications under some situations, as compared with the micelles with other morphologies.…”

“…The cylindrical micelles of BCPs also have a wide range of potential applications in various fields, including optoelectronic materials, catalysis and biomedical materials. [78][79][80][81][82][83][84][85][86][87] The high aspect ratio of cylindrical micelles is advantageous to applications under some situations, as compared with the micelles with other morphologies. For instance, worm-like micelles can toughen epoxy resin more effectively, [88] and conductive network can be formed more easily when cylindrical micelles with a conjugated core-forming block are used as filler.…”