Guy Koeckelberghs

Interchromophoric interactions in chiral X-type π-conjugated oligomers: a linear and nonlinear optical study.

A concept of chiral, X-type organized π-conjugated oligomers, linked by means of a binaphthalene pincer, is presented. NMR spectroscopy and cyclic and differential pulse voltammetry indicate that these oligomers are in close proximity and influence each other in a through-space manner in their neutral as well as in their oxidized states. The interaction between the oligomers was also confirmed by UV-vis, CD, and emission spectroscopy. The synthetic versatility of this design also enables the development of heterocoupled binaphthalene derivatives BN1-2 and BN1-3, consisting of an electron-neutral oligothiophene or electron-rich oligomer and an electron-poor oligothiazole. Hyper-Rayleigh scattering data show a significant enhancement of the second-order nonlinear hyperpolarizability β for BN1-3 and BN1-2, in contrast with the homocoupled binaphthalene derivatives (BN1-1, BN2-2, and BN3-3). This enhancement provides direct proof for the through-space charge-transfer interaction between the p-type and the n-type oligomers within BN1-3 and BN1-2.

Transfer of Supramolecular Chirality in Block Copoly(thiophene)s

Conjugated block copolymers have received considerable attention in recent years, as they hold promise, for instance for solar cells and nanoelectronics. Block copolymers consisting of one conjugated block and one or more nonconjugated, usually coil-like blocks, have been shown to possess a variety of defined morphologies which differ from those of the parent conjugated homopolymers. However, since the nonconjugated segments do not contribute electronically to the material, their role remains restricted to the self-assembly. In contrast, different morphologies and unique electronic properties can be expected for conjugated block copolymers composed of two, electronically different blocks. For instance, Tu et al. showed that conjugated block copolymers can form morphologies which differ from those of a blend of their homopolymers and that the morphology and corresponding optical properties of amphiphilic block copolymers depend on the solvent conditions. Here, we show that the electronic properties and the supramolecular organization of one conjugated block can also be influenced by the other conjugated block.

Chirality in conjugated polymers: when two components meet

Chirality is a well-studied feature in the auspicious class of conjugated polymers. Proper use allows us to study and to control the behavior of the polymers which make them very valuable in many applications. Mostly chiral homopolymers are considered, but what happens when the chirality originates from the interplay between multiple components? This review summarizes different approaches to implement multicomponent chirality in conjugated polymers and their properties.

Poly(3-alkylthiophene) with tuneable regioregularity: synthesis and self-assembling properties

This work reports a synthetic strategy to generate poly(3-alkylthiophene)s (P3ATs) with joint-simultaneous control of the molar mass and the regioregularity. A series of chiral P3ATs with different regioregularities is synthesized using a Pd(RuPhos)-catalyzed chain-growth polymerisation. All polymers have molar masses and polydispersities (PDI) that lie within a narrow region. Furthermore, it is shown that the Pd-catalyst forms all kinds of couplings [head-to-tail (HT), tail-to-tail (TT) and head-to-head (HH)] to a similar extent, which allows insertion of predictable amounts of regio-irregularities into the polymer chain. This enables a thorough study of the influence of the regioregularity on the properties of P3AT, which was performed using UV-vis and circular dichroism (CD) spectroscopy, differential scanning calorimetry (DSC) and atomic force microscopy (AFM) measurements. Unexpectedly, it is found that under “kinetic” conditions the highest crystallinity, π-stacking, supramolecular organisation and chiral expression are not obtained for fully regioregular P3AT with 100% HT couplings, but that a small amount of regio-irregularity increases these properties and the chiral expression. Under “thermodynamic” conditions (after annealing, very slow solvent evaporation or very slow cooling from the melt), this effect is less pronounced or not found. This behaviour can be explained by a higher degree of motional freedom within the non-perfect polymer chains due to the increased steric repulsion from the HH-couplings, which leads to a more easy stacking under “kinetic” conditions.

Heterobifunctional PEG Ligands for Bioconjugation Reactions on Iron Oxide Nanoparticles

Ever since iron oxide nanoparticles have been recognized as promising scaffolds for biomedical applications, their surface functionalization has become even more important. We report the synthesis of a novel polyethylene glycol-based ligand that combines multiple advantageous properties for these applications. The ligand is covalently bound to the surface via a siloxane group, while its polyethylene glycol backbone significantly improves the colloidal stability of the particle in complex environments. End-capping the molecule with a carboxylic acid introduces a variety of coupling chemistry possibilities. In this study an antibody targeting plasminogen activator inhibitor-1 was coupled to the surface and its presence and binding activity was assessed by enzyme-linked immunosorbent assay and surface plasmon resonance experiments. The results indicate that the ligand has high potential towards biomedical applications where colloidal stability and advanced functionality is crucial.

Direct visualization of microphase separation in block copoly(3-alkylthiophene)s

A poly(3-octylthiophene)-block-poly(3-butylthiophene) block copolymer was synthesized in a one-pot block copolymerization reaction, starting from a functional o-tolyl initiator in order to maximize A–B diblock copolymer formation. First, the composition of this block copolymer is extensively studied using gel permeation chromatography (GPC) and 1H NMR measurements. A complete block copolymer formation is obtained with almost equal block length; B–A–B block copolymer contamination is shown to be very limited. In a second part, the self-assembly was analysed through differential scanning calorimetry (DSC), atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) measurements, focusing on the microphase separation. A direct visualization of the different microphases can be obtained with STM.

Amphiphilic chiral block-poly(thiophene)s: tuning the blocks

This report describes the synthesis and characterization of amphiphilic block-copoly(3-alkylthiophene)s with one hydrophobic and one hydrophilic block. Implementation of a chiral center in one of the two blocks enables a chiroptical evaluation of the materials in different media. Depending on the solvent conditions, phase-separation and selective aggregation of one of both blocks becomes feasible, which has implications on the aggregation behavior of the second block. The presented chiroptical and thermal study provides an insight into the self-assembly of these materials both in solution and in film.

Efficient Faraday rotation in conjugated polymers

Very efficient Faraday rotation has been observed in regioregular polyalkoxythiophene. These observations could be a first step towards a new type of application of conjugated polymers.

Synthesis and properties of ?,?-phenyl-capped bithiophene derivatives

Nine different α,ω-phenyl-endcapped bithiophenes were synthesised, and the effect of the different side chains on the liquid crystalline properties, alignment ability and charge carrier mobility have been studied. An increase in chain length leads to a decrease in the liquid crystalline–isotropic phase (clearing) transition temperature. Remarkably, introduction of an asymmetric carbon centre close to the π-conjugated segment resulted in the loss of the liquid crystalline phase. Alignment on rubbed polyimide was obtained for the liquid crystalline thiophene derivatives lacking heteroatoms in the side chain and for the chiral α,ω-phenyl-endcapped bithiophene. Some bithiophenes showed bipolar charge transport in time-of-flight (TOF) measurements, with mobilities up to 3 × 10−3 cm2 V−1 s−1 in the liquid crystalline state. Field effect transistors revealed mobility for holes up to 1 × 10−3 cm2 V−1 s−1 (crystalline state). From the data set obtained, it can be concluded that the use of linear hydrocarbon chains as solubilising tails in these types of π-conjugated building blocks gives the best overall electronic performance.

Influence of the regioregularity on the chiral supramolecular organization of poly(3-alkylsulfanylthiophene)s

The manuscript investigates the influence of the regioregularity (RR) of poly(3-alkylsulfanylthiophene)s (P3AST) on their properties. Therefore, a series of P3ASTs (P1–P5) with different RR were synthesized using a combination of a “reversed McCullough method” and the GRIM method. The degree of RR was determined by 1H NMR spectroscopy. A detailed chiroptical study in good solvent, poor solvent and film was performed, which revealed that the tendency to form chiral supramolecular aggregates clearly depends on the RR, but that the relation is not simply continuously increasing. Instead, the strongest effects were observed in P3ASTs with high, but not 100% RR. As a consequence, a similar behaviour can be expected for a regioregular and a particular irregular P3AST. This hypothesis was tested on a regioregular and an irregular P3AST that proved to have similar chiroptical properties in a previous study. Their properties in the solid state were investigated in detail using STM and AFM.