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.

A series of modular biphenyl building blocks with stepwise adjusted torsion angles and terminally functionalized with leaving groups have been synthesized. The two phenyl rings of the biphenyl synthon are clamped by alkyl chains of various lengths. The desired building blocks 3 and 4 were obtained by copper-mediated C-C biaryl coupling reactions followed by the construction of interlinking alkyl bridges. The key intermediates 14 and 15 were transformed into the corresponding cycloheptadienones 16 and 17, which were reduced to the desired propyl-bridged biphenyls 3b and 3c. The butyl-bridged derivatives 4b and 4c were obtained from

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.

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.

Terminal piperidinyl‐ and nitro‐functionalized biphenyls, bridged between the 2 and 2′ positions by a variable number of methylene groups, are synthesized and fully characterized. These push‐pull systems with defined and restricted torsion angles between their phenyl rings are ideal model compounds to investigate the influence of the chromophore's conjugation in nonlinear optic (NLO) responses. A general synthetic route that can be implemented to access these model compounds is reported, starting from dibromo or ditriflate biphenyls. Hartwig–Buchwald cross‐coupling, a selective azacycloalkylation of diaminobiphenyls and a mild oxidation of primary amines to nitro groups in the presence of a tertiary amine summarizes the synthetic pathway towards the desired model compounds. NLO properties of the series of torsionally constrained push‐pull biphenyls are collected by electric‐field‐induced second‐harmonic generation (EFISH) experiments. The results agree qualitatively with semi‐empirical simulations based on the AM1 Hamiltonian. A linear dependence of the quadratic response on the cos2(Φ) of the inter‐aryl dihedral angle is observed, which points to oscillator strength loss as the dominant effect of increasing backbone twist.