Annulated Thienyl-Vinylene-Thienyl Building Blocks for π-Conjugated Copolymers: Ring Dimensions and Isomeric Structure Effects on π-Conjugation Length and Charge Transport

A series of annulated thienyl-vinylene-thienyl (ATVT) building blocks having varied ring sizes, isomeric structures, and substituents was synthesized and characterized by spectroscopic, electrochemical, quantum chemical, and crystallographic methods. It is found that ATVT ring size and isomeric structure critically affect the planarity, structural rigidity, optical absorption, and redox properties of these new π-units. Various solubilizing substituents can be introduced on the annulated hydrocarbon fragments, preserving the ATVT planarity and redox properties. The corresponding π-conjugated copolymers comprising ATVT units and electron-deficient units were also synthesized and characterized. The solubility, redox properties, and carrier transport behavior of these copolymers also depend remarkably on the annulated ring size and the ATVT unit isomeric structure. One of the copolymers composed of an ATVT with five-membered rings (1), (E)-4,4′,5,5′-tetrahydro-6,6′-bi(cyclopenta[b]thiophenylidene), and a naphthalenediimide (NDI) unit exhibits a broad UV-vis-NIR absorption with an onset beyond 1100 nm both in solution and in the film state, and thin films exhibit n-type semiconducting properties in field-effect transistors. These results are ascribed to the extended main chain π-conjugation length and the low HOMO-LUMO bandgap. Other π-conjugated copolymers containing unit 1 also exhibit characteristic red-shifted UV-vis-NIR absorption. A diketopyrrolopyrrole-based copolymer with unit 1 serves as an electron donor material in organic photovoltaic devices, exhibiting broad-range external quantum efficiencies from the UV to beyond 1000 nm.