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Pascal Jonkheijm

Publications and source records attributed to Pascal Jonkheijm.

At least 19 recordsLinked to original sources
PubMed2006

The importance of nanoscopic ordering on the kinetics of photoinduced charge transfer in aggregated pi-conjugated hydrogen-bonded donor-acceptor systems.

Aggregated complexes of diaminotriazine oligo(p-phenylene vinylene) (OPV) units hydrogen bonded to different complementary perylene bisimide (PERY) compounds have been investigated by means of absorption, circular dichroism, photoluminescence, and photoinduced absorption spectroscopy. These studies reveal that in the aggregated state an ultrafast photoinduced charge separation occurs via an intermolecular pathway in the J-type stack of hydrogen-bonded OPV-PERY arrays. The subsequent charge recombination reaction strongly depends on small structural differences within the J-type geometry as revealed by comparison of stacked supramolecular dimers, trimers, and covalently OPV-PERY linked systems. A coupled oscillator model is used to analyze absorption and circular dichroism spectra and to identify intermolecular arrangements that are consistent with the experimental spectra and the charge-transfer kinetics.

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PubMed2006

Control of ambipolar thin film architectures by co-self-assembling oligo(p-phenylenevinylene)s and perylene bisimides.

Control of thin film morphology by self-assembly of, respectively, p-type oligo(p-phenylenevinylene)s (OPV)s and n-type perylenebisimides (PBI)s in solution prior to processing, results in film architectures consisting of uniform rodlike domains as shown by atomic force microscopy. Such films from self-assembled molecules show superior charge-carrier mobility in comparison with films processed from molecular dissolved molecules. Moreover, connecting the OPV and PBI building blocks through hydrogen-bonding interactions creates dyad complexes that cofacially stack in apolar solvents. Ambipolar field-effect transistors constructed from these dyad complexes show two independent pathways for charge transport. In strong contrast, processing of OPV and PBI, that are not connected by hydrogen bonds, form charge transfer donor-acceptor complexes that show no mobility in field-effect transistors presumably due to an unfavorable supramolecular organization.

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PubMed2006

Probing the solvent-assisted nucleation pathway in chemical self-assembly.

Hierarchical self-assembly offers a powerful strategy for producing molecular nanostructures. Although widely used, the mechanistic details of self-assembly processes are poorly understood. We spectroscopically monitored a nucleation process in the self-assembly of p-conjugated molecules into helical supramolecular fibrillar structures. The data support a nucleation-growth pathway that gives rise to a remarkably high degree of cooperativity. Furthermore, we characterize a helical transition in the nucleating species before growth. The self-assembly process depends strongly on solvent structure, suggesting that an organized shell of solvent molecules plays an explicit role in rigidifying the aggregates and guiding them toward further assembly into bundles and/or gels.

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PubMed2006

Chiral amphiphilic self-assembled alpha,alpha'-linked quinque-, sexi-, and septithiophenes: synthesis, stability and odd-even effects.

The synthesis, characterization, and self-assembly in butanol of a series of well-defined alpha,alpha'-linked quinqui-, sexi-, and septithiophenes substituted, via ester links at their termini, by chiral oligo(ethylene oxide) chains carrying an alpha, beta, delta, and epsilon methyl, respectively, are reported. Studies of the self-assembly of these molecules using UV/visible absorption, luminescence, and circular dichroism spectroscopies reveal, for the sexithiophene case, that the magnitude of the observed Cotton effect in the aggregates diminishes progressively as the chiral substituent is moved away from the thiophene segment. The stability of the assemblies increases with the length of the oligothiophene and as the substituent chiral unit is moved away from the aromatic core, being greatest for the unsubstituted case. The sign of the Cotton effect alternates in an "odd/even" manner as the position of the chiral substituent is moved along the oligo(ethylene oxide) chain and on going from the quinquethiophene to the septithiophene having the same side chain. Atomic force microscopy on materials deposited from solution on an aluminum or glass surface and optical measurements show that capsules are formed from the oligothiophenes with H-type packing of the aromatic segments.

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PubMed2006

The influence of hydrogen bonding and pi-pi stacking interactions on the self-assembly properties of C3-symmetrical oligo(p-phenylenevinylene) discs.

Three C3-symmetrical discotics containing a 1,3,5-benzenetricarboxamide unit functionalized with pi-conjugated oligo(p-phenylenevinylene)s (OPV)s have been synthesized and fully characterized. For the two amide OPV discs a two-step transition from helical stacks to molecularly dissolved species was observed and surprisingly, the topology of the amide determines the stability and helicity of the fibers in solution and the length of the fibrils at a surface. In case of the bipyridine disc, aggregates were formed that show little chiral ordering while the stacks remain present over a large temperature range. At a surface, completely disordered structures exist probably as a result of competing types of pi-pi stacking interactions that differ in strength and orientation. The results show that the design of functional self-assembled architectures based on hydrogen bonding and pi-pi stacking interactions is an extremely delicate matter and reveal that special demands have to be taken into account to balance the topology, directionality and strength of multiple secondary interactions.

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PubMed2005

High anisotropy of the field-effect transistor mobility in magnetically aligned discotic liquid-crystalline semiconductors.

A magnetic field has been utilized for producing highly oriented films of a substituted hexabenzocoronene (HBC). Optical microscopy studies revealed large area HBC monodomains that covered the entire film, while wide-angle X-ray measurements showed that the HBC molecules are aligned with their planes along the applied field. On the basis of this method, solution-processed field-effect transistors (FET) have been constructed with charge carrier mobilities of up to 10(-3) cm2/V.s, which are significantly enhanced with respect to the unaligned material. Exceptionally high mobility anisotropies of 25-75 for current flow parallel and perpendicular to the alignment direction have been measured as a function of the channel length. Atomic force microscopy performed on the FET structures reveals fibril superstructures that are oriented perpendicularly to the magnetic field direction, consisting of molecular columns with a slippage angle of 40 degrees between the molecules. For channel lengths larger than 2.5 mum, the fibrils are smaller than the electrode spacing, which adversely affects the device performance.

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PubMed2005

Charge transport in self-organized pi-stacks of p-phenylene vinylene oligomers.

We have studied the mobility of charge carriers along self-organizing pi-stacks of hydrogen-bonded phenylene vinylene oligomers in solution, by time-resolved microwave conductivity measurements. The value deduced for the mobility along the stacks is 3 x 10(-3) and 9 x 10(-3) cm2/(V s) for holes and electrons, respectively. Additionally, we have calculated the mobility along the pi-stacks using a hopping model based on parameters from density functional theory. The mobility values obtained from these calculations are in good agreement with the experimental values if it is assumed that there are relatively large twist angles between neighboring molecules in the stack. It is shown that a significantly higher mobility can be attained if the twist angle between neighboring oligomers is reduced.

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PubMed2005

The effects of supramolecular assembly on exciton decay rates in organic semiconductors.

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene (OPV) materials that are functionalized with quadruple hydrogen-bonding groups. These form supramolecular assemblies with thermotropic reversibility. The morphology of the assemblies depends on the way that the oligomers are functionalized; monofunctionalized OPVs (MOPVs) form chiral, helical stacks while bifunctionalized OPVs (BOPVs) form less organized structures. These are therefore model systems to investigate the effects of supramolecular assembly, the effects of morphology, and the dependence of oligomer length on the radiative and nonradiative rates of pi-conjugated materials. The purpose of this work is to use MOPV and BOPV derivatives as model systems to study the effect of intermolecular interactions on the molecular photophysics by comparing optical properties in the dissolved phase and the supramolecular assemblies. A simple photophysical analysis allows us to extract the intrinsic radiative and nonradiative decay rates and to unravel the consequences of interchromophore coupling with unprecedented detail. We find that interchromophore coupling strongly reduces both radiative and intrinsic nonradiative rates and that the effect is more pronounced in short oligomers.

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PubMed2005

Polarized emission of individual self-assembled oligo(p-phenylenevinylene)-based nanofibers on a solid support.

We have prepared 5 nm diameter, micrometer long tetra(p-phenylenevinylene) (OPV)-based nanofibers on a graphite surface. The fluorescence emission of an individual fiber shows a profound polarization over its entire length that directly corresponds to its orientation on the substrate. Quantitative analysis of the fluorescence polarization, including the depolarizing effect of the underlying graphite, evidences the high degree of organization within chiral fibers with the OPV molecules perpendicular to the fiber axis. The control of the internal order within self-assembled fibers, and the ability to measure it, is a crucial step to obtain uniform organic fibers that can be applied in nanosized electronics at room temperature.

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PubMed2005

Magnetic alignment of self-assembled anthracene organogel fibers.

High magnetic fields are shown to be remarkably effective to orient self-assembled 2,3-bis-n-decyloxyanthracene (DDOA) fibers during organogel preparation. Magnetic orientation of DDOA results in a highly organized material displaying a fiber-orientation order parameter of 0.85, a large linear birefringence, and fluorescence dichroism. The aligned organogel is stable after removal of the magnetic field at room temperature and consists of fibers oriented perpendicular to the magnetic field direction, as shown by scanning electron microscopy. Models for the molecular organization within the gel fibers are discussed upon quantitative analysis of the birefringence. Prospectively, magnetic alignment can be used to improve specific properties of organogel materials.

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PubMed2005

Magnetic deformation of self-assembled sexithiophene spherical nanocapsules.

We report the experimental observation of magnetic field deformation of spherical nanocapsules, self-assembled from sexithiophene molecules, into oblate spheroids, confirming a long-standing theoretical prediction. The magnetically deformed objects can be trapped in a compatible organogel to make them suitable for further investigations and applications. Our results show that strong magnetic forces can be effectively used, in a contact-free manner, as a tool to control the self-organization of a whole class of functional organic molecules.

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PubMed2005

2D self-assembly of oligo(p-phenylene vinylene) derivatives: from dimers to chiral rosettes.

Enantiomerically pure oligo(p-phenylene vinylene) diaminotriazine derivatives and a short structurally related achiral diaminotriazine derivative, all having a rigid backbone in common, are studied to self-assemble at the solution-graphite interface by scanning tunneling microscopy. As a function of the length of the backbone, different two-dimensional motifs are formed (dimers and rosettes) that are rationalized in terms of the balance between different intermolecular interactions, in this case, intermolecular hydrogen bonding and the packing requirements of the alkyl chains on a graphite surface. In addition, the effect of molecular chirality on monolayer chirality is investigated, revealing molecular size-dependent expressions of the monolayer chirality.

Macromolecular Substances↗
PubMed2005

Two-dimensional self-assembly into multicomponent hydrogen-bonded nanostructures.

By means of scanning tunneling microscopy, we have explored the two-dimensional self-assembly of functional bicomponent hydrogen-bonding dye systems, leading to well-defined patterns, different from those of the individual components, and providing design rules to immobilize multicomponent systems at the liquid-solid interface.

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PubMed2004

Supramolecular p-n-heterojunctions by co-self-organization of oligo(p-phenylene vinylene) and perylene bisimide dyes.

Comparative studies on hydrogen-bonded versus covalently linked donor-acceptor-donor dye arrays obtained from oligo(p-phenylene vinylene)s (OPVs) as donor and bay-substituted perylene bisimides (PERYs) as acceptor dyes are presented. Both systems form well-ordered J-type aggregates in methylcyclohexane, but only hydrogen-bonded arrays afford hierarchically assembled chiral OPV-PERY dye superstructures consisting of left-handed helical pi-pi co-aggregates (CD spectroscopy) of the two dyes that further assemble into right-handed nanometer-scale supercoils in the solid state (AFM study). In the case of hydrogen-bonded arrays, the stability of the aggregates in solution increases with increasing conjugation length of the OPV unit. The well-defined co-aggregated dyes presented here exhibit photoinduced electron transfer on subpicosecond time scale, and thus, these supramolecular entities might serve as valuable nanoscopic functional units.

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