Aggregation of Poly(p-phenylene ethynylene)s Containing Nonpolar and Amine Side Chains

Experimental and theoretical study of the n-doped successive polyanions of oligocruciform molecular wires: up to five units of charge

The electronic structure of polyanions of sterically encumbered triisopropylsilyl-substituted linear and cyclic oligo(phenyleneethynylene)s (Monomer, Trimer, Pentamer, and Triangle) is investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and UV/Vis-near-infrared (NIR) spectroscopies, cyclic voltammetry, and theoretical calculations (DFT). Increasing anion orders are generated sequentially in vacuo at room temperature by chemical reaction with potassium metal up to the pentaanion. The relevance of these compounds acting as electron reservoirs is thus demonstrated. Even-order anions are EPR silent, whereas the odd species exhibit different signatures, which are identified after comparison of the measured hyperfine couplings by ENDOR spectroscopy with those predicted by DFT calculations. With increasing size of the oligomers the electron spin density is first distributed over the backbone carbon atoms for the monoanions, and then further localized at the outer phenyl rings for the trianion species. Examination of the UV/Vis-NIR spectra indicates that the monoanions (T(.-) , P(.-) ) exhibit two transitions in the Vis-NIR region, whereas a strong absorption in the IR region is solely observed for higher reduced states. Electronic transitions of the neutral monoanions and trianions are redshifted with increasing oligomer size, whereas for a given oligomer a blueshift is observed upon increasing the charge, which suggests a localization of the spin density.

Controlled aggregation in conjugated polymer nanoparticles via organic acid treatments

Understanding and controlling aggregation structures of conjugated polymers (CPs) in aqueous solutions is critical to improving the physical and photophysical properties of CPs for biological applications. Here, we present spectroscopic evidence, including nuclear magnetic resonance (NMR) spectroscopic results, that different organic acid treatment induces different aggregation structures and photophysical properties of CPs in water. Conjugated polymer nanoparticles (CPNs) were fabricated by treating a non-aqueous soluble, primary amine-containing poly(phenylene ethynylene) (PPE-NH(2)) with organic acids followed by dialysis. CPNs formed by acetic acid (AA) treatment (CPN-AAs) exhibit characteristics of loose aggregation with minimal π-π stacking, while CPNs formed by tartaric acid (TA) treatment (CPN-TAs) exhibit a high degree of π-π stacking among PPE-NH(2) chains. The controlled aggregation for a specific application was demonstrated by comparing the fluorescence quenching abilities of the CPN-AAs and the CPN-TAs. A doubled Stern-Volmer constant was obtained from the densely packed CPN-TAs compared to that of the loosely aggregated CPN-AAs.

Strategy to control the chromism and fluorescence emission of a perylene dye in composite organogel phases

Composite organogels based on 1,3,5-tris(4-dodecyloxybenzoylamino)phenylbenzene (DBAPB), a known gelator, and N,N'-di(octadecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C18PTCDI), a nongelator dye, have been achieved, leading to controllable color and emitting color changes. SEM images and XRD patterns revealed that the packing of the DBAPB-based gelator could almost be maintained in the composite gels. The temperature-dependent UV-vis absorption and temperature-dependent fluorescence emission spectra illustrated that the color and emitting color of the composite gels could be controlled by the content of C18PTCDI as well as the temperature in the gel phases. When the content of C18PTCDI was 1 mol %, C18PTCDI could be isolated as unimolecules in the composite gel, which was yellow and gave bright greenish-yellow emission under 365 nm light. For the mixed systems containing 2-10 mol % C18PTCDI, the fresh gels, which were obtained after cooling the hot solutions for a short time, were yellow and produced greenish-yellow emission under 365 nm illumination. However, the corresponding stable composite gels, which were obtained via prolonging the cooling time, were red and emitted weak red emission excited by UV light as a result of the formation of C18PTCDI aggregates. The reversible color and emitting color changes could be realized in the gel phases over a narrow temperature range. Moreover, the excitation energy of DBAPB could be transferred to C18PTCDI in the composite gels, leading to obvious emission quenching of the former.

Solvation and Aggregation of Polyphenylethynylene Based Anionic Polyelectrolytes in Dilute Solutions

The absorption and fluorescence properties of a polyphenylethynylene based conjugated polyelectrolyte with sulfonate solubilizing groups (PP2) are shown to change dramatically with solution conditions because of the equilibrium between unaggregated and aggregated forms of the polymer. The fluorescence of PP2 is strongly quenched on addition of counterions such as Na+, K+, Li+, and TBA+, an effect which arises from the creation of salt stabilized aggregates. The formation of aggregates has been further corroborated by concentration and temperature studies in water and comparisons to dimethylsulfoxide solvent, in which the polymer does not aggregate. In aqueous solutions, the addition of the cationic surfactant, octadecyltrimethyl ammonium, causes the polymer aggregates to dissociate and creates polymer/surfactant aggregates that have spectral properties like that of the unaggregated polymer.

Synthesis and Self-Assembly of an Amphiphilic Poly(phenylene ethynylene) Ionomer

We have synthesized a conjugated amphiphilic polyelectrolyte, a poly(phenylene ethynylene) (PPE), and the structurally analogous neutral polymer. The solution-phase aggregation of the uncharged PPE can be reversibly controlled by varying the solvent polarity and concentration, while the charged polymer appears to self-assemble at any concentration in compatible solvents. These conclusions are based on a combination of absorption and photoluminescence spectroscopy and dynamic light scattering. Photoinduced absorption spectroscopy was also employed to investigate interchain electronic communication and the photoinduced production of free charge carriers. The uncharged PPE had a relatively high polaron yield, indicating pi-stacking of adjacent PPE chains and efficient exciton splitting, while the charged polymer did not produce polarons, indicating that the polymers are not pi-stacked despite their tendency to form aggregates. This is most likely due to the presence of the cationic trimethylammonium side chains which force neighboring polymer chains too far apart to achieve effective pi-orbital overlap. Polarons were observed in both polymers after chemical doping with iodine. The ability to control aggregation and interchain electronic communication could be a useful tool in designing nanostructured electronic materials.

Self-Assembled Nanotapes of Oligo(p-phenylene vinylene)s: Sol-Gel-Controlled Optical Properties in Fluorescent π-Electronic Gels

A rational approach to the design of supramolecular organogels of all-trans oligo(p-phenylene vinylene) (OPV) derivatives, a class of well-known organic semiconductor precursors, is reported. Self-assembly of these molecules induced gelation of hydrocarbon solvents at low concentrations (<1 mM), resulting in high aspect ratio nanostructures. Electron microscopy and atomic force microscopy (AFM) studies revealed twisted and entangled supramolecular tapes of an average of 50-200 nm in width, 12-20 nm in thickness, and several micrometers in length. The hierarchical growth of the entangled tapes and the consequent gelation is attributed to the lamellar-type packing of the molecules, facilitated by cooperative hydrogen bonding, pi stacking, and van der Waals interactions between the OPV units. Gelation of OPVs induced remarkable changes in the absorption and emission properties, which indicated strong electronic interaction in the aggregated chromophores. Comparison of the absorption and emission spectra in the gel form and in the solid film indicated a similar chromophore organization in both phases. The presence of self-assembled aggregates of OPVs was confirmed by solvent- and temperature-dependent changes in the absorption and emission properties, and by selective excitation experiments. This is the first detailed report of the gelation-induced formation of OPV nanotapes, assisted by weak, nondirectional hydrogen-bonding motifs and pi-pi stacking. These findings may provide opportunities for the design of a new class of functional soft materials and nanoarchitectures, based on pi-conjugated organic semiconductor-type molecules, thereby enabling the manipulation of their optical properties.