Multifunctional pi-expanded oligothiophene macrocycles

This tutorial review summarizes recent progress in the design, synthesis, and multifunctional properties of fully conjugated macrocyclic pi-systems. We focus on the pi-expanded oligothiophene macrocycles after a short survey of macrocyclic conjugated loops and belts such as [n]cycloparaphenylenes, cyclic[n]para-phenylacetylenes, [4]cyclo-2,8-crysenylenes, and cyclo[n]thiophenes. Fully conjugated pi-expanded oligothiophene macrocycles possess shape-persistent but sometimes pliable pi-frames, and the electronic and optoelectronic properties of the macrocycles largely depend on the pi-systems inserted into the oligothiophene macrocycles. Among them, the pi-expanded oligothiophene macrocycle composed of 2,5-thienylenes, ethynylenes, and vinylenes is one of the most widely applicable macrocycles for constructing multifunctional pi-systems. These pi-expanded oligothiophene macrocycles from small to very large ring sizes can be prepared via a short step procedure, and their various solid state structures can be determined by X-ray analysis. Since these macrocycles have inner and outer domains, specific information concerning structural, electronic, and optical properties is expected. Furthermore, pi-expanded oligothiophene macrocycles with alkyl substituents exhibit various morphologies depending on nanophase separation of molecules, and a morphological change is observed for the molecular switch.