Jeroen C. Gielen

Oligo(p-phenylenevinylene) peptide conjugates: Synthesis and self-assembly in solution and at the solid-liquid interface

Two oligo(p-phenylenevinylene)-peptide hybrid amphiphiles have been synthesized using solid- and liquid-phase strategies. The amphiliphiles are composed of a pi-conjugated oligo(p-phenylenevinylene) trimer (OPV) which is coupled at either a glycinyl-alanyl-glycinyl-alanyl-glycine (GAGAG) silk-inspired beta-sheet or a glycinyl-alanyl-asparagyl-prolyl-asparagy-alanyl-alanyl-glycine (GANPNAAG) beta-turn forming oligopeptide sequence. The solid-phase strategy enables one to use longer peptides if strong acidic conditions are avoided, whereas the solution-phase coupling gives better yields. The study of the two-dimensional (2D) self-assembly of OPV-GAGAG by scanning tunneling microscopy (STM) at the submolecular level demonstrated the formation of bilayers in which the molecules are lying antiparallel in a beta-sheet conformation. In the case of OPV-GANPNAAG self-assembled monolayers could not be observed. Absorption, fluorescence, and circular dichroism studies showed that OPV-GAGAG and OPV-GANPNAAG are aggregated in a variety of organic solvents. In water cryogenic temperature transmission electron microscopy (cryo-TEM), atomic force microscopy (AFM), light scattering, and optical studies reveal that self-assembled nanofibers are formed in which the helical organization of the OPV segments is dictated by the peptide sequence.

Dilution-induced self-assembly of porphyrin aggregates: A consequence of coupled equilibria

The self-assembly of organic molecules has attracted sub-stantialinterest asa bottom-upapproachto createnano-sizedobjects. Their properties depend strongly on the design,arrangement, and number of molecules in the aggregate. Forsupramolecular polymers, the monomers are entirely heldtogether by non-covalent interactions; these interactions aretypically weak, reversible, and highly sensitive to variablessuch as temperature, concentration, and solvent polarity.

Disk micelles from amphiphilic Janus gold nanoparticles

Janus gold nanoparticles are synthesised via a simple preparation method and behave as amphiphiles self-assembling in water into disk-shaped micelles.

Patterns of Diacetylene-Containing Peptide Amphiphiles Using Polarization Holography

The polarization dependence of a diacetylene polymerization was studied. For this purpose, peptide amphiphile fibers with a diacetylene moiety, which could only polymerize in the direction of the fiber, were employed. If nonaligned samples were illuminated with polarized light, only the fibers parallel to the polarization direction of the light were polymerized. With magnetically aligned fibers, spatially selective polymerization was accomplished using polarization holography.

Self-assembly of amphiphilic gold nanoparticles decorated with a mixed shell of oligo(p-phenylene vinylene)s and ethyleneoxide ligands

Amphiphilic gold nanoparticles decorated with ethyleneoxide and oligo(p-phenylene vinylene) ligands self-assemble into disc-shaped objects.

Intensive Chiroptical Properties of Chiral Polyfluorenes Associated with Fibril Formation

Thin films of chiral poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene} (1) were studied using circular dichroism (CD) spectroscopy. Films spin coated from chloroform solution, show CD with a degree of polarization g(abs) (= +4 x 10(-4) at 400 nm) that is independent of film thickness (50-290 nm). This implies that g(abs) is an intensive property of the material and related to the chiral organization of the molecules on a length scale less than 50 nm. Atomic force microscopy (AFM) on the films reveals fibrils. Addition of nonsolvent methanol to a solution of 1 in chloroform leads to fibril formation in solution and results in CD similar in band shape to that of the pristine spin coated films from chloroform solution and a g(abs) comparable in magnitude. Thus the chiral molecular arrangement leading to circular dichroism is part of the internal structure of these fibrils.

Tuning the self-assembly of a ditopic crown ether functionalized oligo(p-phenylenevinylene)

A chiral ditopic crownether functionalized oligo(p-phenylenevinylene), COPV, has been synthesized and fully characterized. The binding properties and the organization of COPV have been studied in solution and in the solid state. In chloroform, due to the benzo-15-crown-5 moieties, COPV is able to bind cations. In case of K+ a strong 2 : 2 complex is formed in which two COPVs are sandwiched between two potassium ions. Na+ is bound following a two-step negative cooperative process to form a 2 : 1 Na+/COPV complex. When COPV in the presence of potassium ions is drop-cast on a silicon substrate, fibers are formed. In waterCOPV forms stable helical aggregates as can be concluded from the Cotton effect, the UV-vis and fluorescence spectroscopy. AFM and light scattering studies show that ellipsoidal aggregates are present in water.

Molecular organization of cylindrical sexithiophene aggregates measured by X-ray scattering and magnetic alignment.

We have determined the internal organization of elongated sexithiophene aggregates in solution by combining small-angle X-ray scattering and magnetic birefringence experiments. The different aggregate axes can be probed independently by performing the experiments on magnetically aligned aggregates. We have found multiwalled cylindrical aggregates consisting of radially oriented sexithiophene molecules with pi-pi-stacking in the tangential direction, a structure that is considerably different from those previously found in other solvents. The aggregate morphology of this semiconducting material can thus be tuned by using different solvents, which offers the attractive perspective to steer chemical self-assembly toward nanostructures with desired functionalities, especially in combination with the alignment in a magnetic field.

Using magnetic birefringence to determine the molecular arrangement of supramolecular nanostructures

Abstract Supramolecular aggregates can be aligned in solution using a magnetic field. Because of the optical anisotropy of the molecular building blocks, the alignment results in an anisotropic refractive index of the solution parallel and perpendicular to the magnetic field. We present a model for calculating the magnetic birefringence, using solely the magnetic susceptibilities and optical polarizabilities of the molecules, for any molecular arrangement. We demonstrate that magnetic birefringence is a very sensitive tool for determining the molecular organization within supramolecular aggregates.

Aggregation Kinetics of Macrocycles Detected by Magnetic Birefringence

We have used magnetic-field-induced birefringence as a new sensitive technique to probe the aggregation kinetics of macrocyclic molecules in solution. We have found three consecutive aggregation stages: disordered objects, ordered fibers, and a network. The transition from disordered objects to ordered fibers is found to be slow, taking days or weeks to complete. We attribute this to the molecular tails of the macrocycles, which hamper fiber formation. We anticipate that linking aggregation kinetics to molecular properties will lead to a better understanding of the mechanisms by which molecules self-assemble, allowing for a more rational design of the molecular building blocks.