Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Energy Aspects of Thermal Molecular Switching: Molecular Thermal Hysteresis of Helicene Oligomers

Molecular switching is a phenomenon by which a molecule reversibly changes its structure and state in response to external stimuli or energy. Herein, molecular switching is discussed from thermodynamic and kinetic aspects in terms of energy supply with an emphasis on the thermal switching exhibited by helicene oligomers. It includes the inversion of relative thermodynamic stability induced by temperature changes and molecular thermal hysteresis in a closed system. The thermal phenomenon associated with the oligomers involves population/concentration changes between metastable states under nonequilibrium thermodynamic control.

Twisted arenes

The nonbonded steric interactions of substituents at the crowded C4 and C5 positions of phenanthrene cause the aromatic system to twist out of planarity. Similarly, the presence of substituents at the C1 and C12 positions of benzo[c]phenanthrene and at the C1 and C14 positions of dibenzo[c,g]phenanthrene are responsible for the helical twists of the aromatic frameworks. Highly substituted acenes, such as octamethylnaphthalene and decaphenylanthracene, also exhibit substantial end-to-end twists. The X-ray structures of these compounds allow direct measurements of the extent of the structural distortions. The configurational stabilities of many twisted arenes have also been determined.