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Pascal Jonkheijm

Publications and source records attributed to Pascal Jonkheijm.

24 records · Page 2Linked to original sources
PubMed2003

Photoinitiated polymerization of columnar stacks of self-assembled trialkyl-1,3,5-benzenetricarboxamide derivatives.

A disk-shaped molecule, N-(5-sorbyl-pentyl)-N',N"-di(n-octyl)benzene-1,3,5-tricarboxamide (1), has been synthesized and assembled into a columnar stack in cyclohexane. Using a photoinitiated process, we polymerized monomer 1 in its self-assembled state and analyzed the resulting poly-1. On cooling a boiling cyclohexane solution of 1, the molecules aggregate via amide hydrogen bonds, as supported by the position of the N-H stretching band in FT-IR spectroscopy. Evidence of helical columnar stacking of 1 in the aggregate is provided by an induced CD effect upon blending 1 with a chiral side-chain homologue 3 in a so-called "sergeants and soldiers" experiment. The columnar assembly in cyclohexane was polymerized by UV light (365 nm) irradiation in the presence of 2,2-dimethoxy-2-phenylacetophenone as radical photoinitiator. The polymerization occurs selectively to give 1,4-polymer with isolated trans C=C bonds, as shown by FT-IR, and (1)H and (13)C NMR spectroscopy. Lack of polymerization of methyl sorbate (4) under identical conditions, and low incorporation of 4 in copolymerizations with 1 in cyclohexane, suggests that the polymerization preferentially occurs within the columnar assembly. Size exclusion chromatography shows that the degree of polymerization (X(n)()) based on number average molecular weight is approximately 65. Incorporation of small amounts of nonpolymerizable derivatives 2 and 3 into the columns has no effect on X(n)() and conversion. Molecular models show that the polymerizable sidearm of 1 is long enough to span the distance between the monomers in the chiral stack. Under the atomic force microscope (AFM), purified samples of polymerized 1 displayed nanometer-sized fibrous morphologies with a high-axial-ratio (>150), uniform width (60 nm), and a thickness (1.0 nm) which corresponds to the width of the benzene tricarboxamide core of 1, whereas aggregates of nonpolymerized 1 yield a featureless image due to their instability.

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PubMed2003

Transfer of pi-conjugated columnar stacks from solution to surfaces.

Three hydrogen-bonded oligo(p-phenylenevinylene)s, OPV3, OPV4, and OPV5, that differ in conjugation length have been synthesized and fully characterized. All three compounds contain chiral side chains, long aliphatic chains, and a ureido-s-triazine hydrogen bonding unit. (1)H NMR and photophysical measurements show that the OPV oligomers grow hierarchically in an apolar solvent; initially, dimers are formed by hydrogen bonds that subsequently develop into stacks by pi-pi interactions of the phenylenevinylene backbone with induced helicity via the chiral side chains. SANS measurements show that rigid cylindrical objects are formed. Stacks of OPV4 have a persistence length of 150 nm and a diameter of 6 nm. OPV3 shows rigid columnar domains of 60 nm with a diameter of 5 nm. Temperature and concentration variable measurements show that the stability of the stacks increases with the conjugation length as a result of more favorable pi-pi interactions. The transfer of the single cylinders from solution to a solid support as isolated objects is only possible when specific concentrations and specific solid supports are used as investigated by AFM. At higher concentrations, an intertwined network is formed, while, at low concentration, ill-defined globular objects are observed. Only in the case of inert substrates (graphite and silicium oxide) single fibers are visible. In the case of the repulsive surfaces (mica and glass), clustering of the stacks occurs, while, at attractive surfaces (gold), the stacks are destroyed.

Journal Article↗
PubMed2003

Bias-dependent visualization of electron donor (D) and electron acceptor (A) moieties in a chiral DAD triad molecule.

The 2D crystal lattice structure and bias-dependent contrast of a chiral electron donor-acceptor-donor triad system, composed of two oligo(p-phenylene vinylene) electron donors and a perylenediimide electron acceptor (OPV4-PDI-OPV4), have been studied by means of scanning tunneling microscopy (STM) at the liquid-graphite interface. OPV4-PDI-OPV4 is ordered in rows and forms a well-ordered 2D crystal lattice structure. The electrical properties of the donor and acceptor parts are distinguished by the contrast in bias-dependent STM imaging.

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