Activation enthalpies and entropies of the atropisomerization of substituted butyl-bridged biphenyls.

A combined quantum chemical and experimental study of the atropisomerization energies of di-para-substituted butyl-bridged biphenyl cyclophanes is presented. We studied the influence of electron donor and electron acceptor substituents on the height of the enantiomerization barrier and examined the enthalpic and entropic contributions. The reaction pathway with minimum and transition state structures was established using density functional theory calculations. The Gibbs free activation energies derived from this pathway correspond well to the ones determined by temperature dependent high performance liquid chromatography (HPLC) measurements. Surprisingly, large discrepancies were found for the contributions of enthalpy and entropy. Thermodynamic data derived from circular dichroism (CD) measurements support the quantum chemical calculations for the distribution of enthalpy and entropy, contrary to the HPLC measurements. Rationalizations for this are given.

Molecular daisy chains: synthesis and aggregation studies of an amphiphilic molecular rod.

A water-soluble cyclophane as the loop subunit, monofunctionalized with a molecular rod, has been synthesized to introduce a new binding motif for mechanically interlinked oligomers. It has been demonstrated that this hermaphroditic compound forms [c2]daisy chains in polar solvent over a wide range of concentrations. Furthermore, evidence for the formation of higher mechanically interlinked oligomers above the critical aggregation concentration has been obtained.

Molecular daisy chains.

This tutorial review summarizes the progress made towards mechanically interlocked daisy chains. Such materials can be seen as a further development in polymer science, where the conventional covalent interlinking bonds are replaced by supramolecular binding concepts. Materials in which the mechanical bond is an integral part of the polymeric backbone are expected to possess unique macroscopic properties and are therefore the synthetic aim in an ever growing research community. After introducing general considerations about daisy chains, the most common analytic methods to get insight into the aggregation behaviour of such self-complementary monomers are presented. Cyclodextrins/aromatic rods, crown ethers/cationic rods and pillararenes/alkyl chains are systems used to achieve daisy chain-like molecular arrays. By comparison of the reported systems, conclusions about an improved structural design are drawn.

Atropisomerization of di-para-substituted propyl-bridged biphenyl cyclophanes.

The influence of electron donors and electron acceptors of variable strength in the 4 and 4' position of 2 and 2' propyl-bridged axial chiral biphenyl cyclophanes on their atropisomerization process was studied. Estimated free energies ΔG(‡)(T) of the rotation around the central biphenyl bond which were obtained from (1)H-NMR coalescence measurements were correlated to the Hammett parameters σ(p) as a measure for electron donor and acceptor strength. It is demonstrated that the resulting nice linear correlation is mainly based on the influence of the different substituents on the π-system of the biphenyl cyclophanes. By lineshape analysis the rate constants were calculated and by the use of the Eyring equation the enthalpic and entropic contributions were evaluated. Density functional theory calculations show a planar transition state of the isomerization process and the calculated energy barriers based on this reaction mechanism are in good agreement with the experimentally obtained free energies.

Multiscale charge injection and transport properties in self-assembled monolayers of biphenyl thiols with varying torsion angles.

This article describes the molecular structure-function relationship for a series of biphenylthiol derivatives with varying torsional degree of freedom in their molecular backbone when self-assembled on gold electrodes. These biphenylthiol molecules chemisorbed on Au exhibit different tilt angles with respect to the surface normal and different packing densities. The charge transport through the biphenylthiol self-assembled monolayers (SAMs) showed a characteristic decay trend with the effective monolayer thickness. Based on parallel pathways model the tunneling decay factor ß was estimated to be 0.27 Å(-1). The hole mobility of poly(3-hexylthiophene)-based thin-film transistors incorporating a biphenylthiol SAM coating the Au source and drain electrodes revealed a dependence on the injection barrier with the highest occupied molecular orbital (HOMO) level of the semiconductor. The possible role of the resistivity of the SAMs on transistor electrodes on the threshold voltage shift is discussed. The control over the chemical structure, electronic properties, and packing order of the SAMs provides a versatile platform to regulate the charge injection in organic electronic devices.

Conformationally controlled electron delocalization in n-type rods: synthesis, structure, and optical, electrochemical, and spectroelectrochemical properties of dicyanocyclophanes.

A series of dicyanobiphenylcyclophanes 1-6 with various π-backbone conformations and characteristic n-type semiconductor properties is presented. Their synthesis, optical, structural, electrochemical, spectroelectrochemical, and packing properties are investigated. The X-ray crystal structures of all n-type rods allow the systematic correlation of structural features with physical properties. In addition, the results are supported by quantum mechanical calculations based on density functional theory. A two-step reduction process is observed for all n-type rods, in which the first step is reversible. The potential gap between the reduction processes depends linearly on the cos(2) value of the torsion angle φ between the π-systems. Similarly, optical absorption spectroscopy shows that the vertical excitation energy of the conjugation band correlates with the cos(2) value of the torsion angle φ. These correlations demonstrate that the fixed intramolecular torsion angle φ is the dominant factor determining the extent of electron delocalization in these model compounds, and that the angle φ measured in the solid-state structure is a good proxy for the molecular conformation in solution. Spectroelectrochemical investigations demonstrate that conformational rigidity is maintained even in the radical anion form. In particular, the absorption bands corresponding to the SOMO-LUMO+i transitions are shifted bathochromically, whereas the absorption bands corresponding to the HOMO-SOMO transition are shifted hypsochromically with increasing torsion angle φ.

Racemisation dynamics of torsion angle restricted biphenyl push-pull cyclophanes.

The thermodynamics of the atropisomerisation of torsion angle restricted, axial chiral biphenyl-based push-pull cyclophanes were studied. Using (1)H NMR coalescence measurements the rotation barrier around the central C-C bond was determined to be 50 kJ mol(-1) for the propyl-bridged biphenyl derivative 1b, displaying only a negligible solvent dependence. By protonation of the piperidinyl nitrogen as electron donor, the free energy ΔG(‡)(T) of the rotation barrier increased, indicating that the tendency of the push-pull system to planarise may be considered as a driving force for the atropisomerisation. For the more restricted butyl-bridged cyclophane 1c a rotation barrier of ΔG(‡)(T) = 90 kJ mol(-1) was measured using dynamic chromatography. The difference in the free energy of rotation around the central C-C bond probably reflects the crowdedness of the transition states.