Charge-Transfer-Induced Self-Assembly of Doped Conjugated Block Copolymer Nanofibers.

Here, we report charge-transfer-driven self-assembly of conjugated block copolymers (BCP) into highly doped conjugated polymer nanofibers. The ground-state integer charge transfer (ICT) between a BCP composed of poly(3-hexylthiophene) and poly(ethylene oxide) (P3HT-b-PEO) and electron-deficient 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) induced spontaneous self-assembly of the donor and the acceptor into well-defined one-dimensional nanofibers. The presence of the PEO block plays an important role for the self-assembly by providing a polar environment that can stabilize nanoscale charge transfer (CT) assemblies. The doped nanofibers were responsive to various external stimuli such as heat, chemical, and light and exhibited efficient photothermal properties in the near-IR region. The CT-driven BCP self-assembly reported here provides a new platform for the fabrication of highly doped semiconductor nanostructures.

Magnetic Field-Induced Self-Assembly of Conjugated Block Copolymers and Nanoparticles at the Air-Water Interface.

Here, we report the magnetic field-induced self-assembly of a conjugated block copolymer, poly(3-hexylthiopene)-block-poly(ethylene glycol) (P3HT-b-PEG), and iron oxide nanoparticles (IONPs) at the air-water interface. Binary self-assembly of P3HT-b-PEG and IONPs at the interface results in nanoparticle-embedded P3HT-b-PEG nanowire arrays with a micrometer-scale domain size. Under the magnetic field, the field-induced magnetic interaction significantly improves the degree of order, generating long-range ordered, direction-controlled nanoarrays of P3HT-b-PEG and IONPs, where IONPs are aligned in the direction of the magnetic field over a sub-millimeter scale. The size of IONPs is an important factor for the formation of an ordered assembly structure at the nanometer scale, as it dictates the magnetic dipole interaction and the entropic interaction between nanoparticles and polymers. The consideration of magnetic dipole interactions suggests that the field-induced self-assembly occurs through the formation of intermediate magnetic subunits composed of short IONP strings along the semirigid P3HT nanowires, which can be aligned through the magnetic interactions, ultimately driving the long-range ordered self-assembly. This study demonstrates for the first time that the magnetic field-induced self-assembly can be used to generate macroscopically ordered polymer films with a nanometer-scale order in low fields.

Responsive Thin-Film Interference Colors from Polaronic Conjugated Block Copolymers.

Colors responsive to the chemical environment can form the basis for simple and highly accessible diagnostic tools. Herein, the charge modulation of conjugated polymers is demonstrated as a new mechanism for chemically responsive structural colors based on thin-film interference. A liquid-liquid interfacial self-assembly is employed to create a conjugated block copolymer film that is flexible, transferable, and highly homogeneous in thickness over a large area. Gold (Au) complexes are introduced in the self-assembly process for in situ oxidation of conjugated polymers into a hole-polaronic state that renders the polymer film responsive to the chemical environment. When transferred onto a reflective substrate, the film shows thickness-dependent tunable reflective colors due to the optical interference. Furthermore, it experiences drastic changes in its dielectric behavior upon switching of the polaronic state, thereby enabling large modulations to the interferometric colors. Such responsive thin-film colors, in turn, can be used as a simple and intuitive multicolor readout for the recognition of reductive vapors including biological decomposition products.

Direct evidence for secondary interactions in planar and nonplanar aromatic π-conjugates and their photophysical characteristics in solid-state assemblies.

Direct evidence for non-covalent secondary interactions in planar and nonplanar aromatic π-conjugates and their solid-state assemblies is established. A series of horizontally, vertically, and radially expanded oligo(phenylenevinylene)s (H-OPVs, V-OPVs, and R-OPVs, respectively) were designed with a fixed π-core and variable alkyl chain lengths on the periphery. Single-crystal structures of the OPVs were resolved to trace the secondary interactions that direct the solid-state self-organization and molecular packing of the chromophores. The H-OPVs were found to be planar, and they did not show any secondary interactions in the crystal lattices. The V-OPVs and R-OPVs were found to be nonplanar and to exhibit multiple CH/π hydrogen-bonding interactions among aryl hydrogen donors and acceptors. The enthalpies of the melting and crystallization transitions revealed that the planar H-OPVs are highly crystalline compared with the nonplanar R-OPVs and V-OPVs. Polarized light microscopy studies revealed the formation of one-dimensional nematic mesophases in H-OPVs. The absolute solid-state photoluminescence quantum yields (PLQYs) of the OPVs were determined using an integrating sphere setup. The highly packed H-OPVs showed low PLQYs compared with those of the weakly packed V-OPVs and R-OPVs. Time-resolved fluorescence decay measurements revealed that the excited-state decay dynamics of highly packed H-OPVs was much faster with respect to their low PLQYs. The decay profiles were found to be relatively slow (with higher life time (τ)) in the V-OPVs and R-OPVs. A field-effect transistor (FET) device was constructed for an OPV sample that showed a hole carrier mobility in the range of 10(-5) cm(2) V(-1) s(-1). The present investigation thus provides a new opportunity to trace the role of secondary interactions on π-conjugated mesophase self-assemblies and their solid-state emission and FET devices, more specifically based on OPV chromophores.

π-Conjugated polymer anisotropic organogel nanofibrous assemblies for thermoresponsive photonic switches.

The present work demonstrates one of the first examples of π-conjugated photonic switches (or photonic wave plates) based on the tailor-made π-conjugated polymer anisotropic organogel. New semicrystalline segmented π-conjugated polymers are designed with rigid aromatic oligophenylenevinylene π-core and flexible alkyl chain along the polymer backbone. These polymers are found to be self-assembled as semicrystalline or amorphous with respect to the number of carbon atoms in the alkyl units. These semicrystalline polymers produce organogels having nanofibrous morphology of 20 nm thickness with length up to 5 µm. The polymer organogel is aligned in a narrow glass capillary, and this anisotropic gel device is further demonstrated as photonic switches. The glass capillary device behaves as typical λ/4 photonic wave plates upon the illumination of the plane polarized light. The λ/4 photonic switching ability is found to be maximum at θ = 45° angle under the cross polarizers. The orthogonal arrangements of the gel capillaries produce dark and bright spots as on-and-off optical switches. Thermoreversibility of the polymer organogel (also its xerogel) was exploited to construct thermoresponsive photonic switches for the temperature window starting from 25 to 160 °C. The organic photonic switch concept can be adapted to large number of other π-conjugated materials for optical communication and storage.