Thiophene based A-D-A small molecules with a dithienosilole core: synthesis, theoretical calculations and optoelectronic properties

Polythiophenes are conjugated materials finding application in many optoelectronic devices.1 Recently, much attention has been devoted to oligothiophenes, the shorter analogous compounds of these polymers. These semiconductor materials are structurally well-defined, monodisperse, free of defects and often possess higher polarizability and charge mobility with respect to the corresponding polymers.2 Thiophene-based small molecules3 are well studied as photoactive materials in organic solar cells. In particular, molecules with A-π-D-π-A architecture, formed by a central electron donating building block (D) and two terminal electron accepting groups (A) linked by π-conjugated bridges, show a low energy absorption band making them suitable as active layers in organic solar cells.4,5 Little modifications of the backbone, i.e. by changing the length of the alkyl substituents or the type of the functional groups, will often result in remarkable modification of the band gap and other optical properties. Here, we present the theoretical calculations, the synthesis and the characterization of two A-π-D-π-A small molecules E1 and E2, where the central unit is a dithienosilole, the terminal units are methyldicyanovinyl functionalized thiophene rings and the π-bridges are alkyl or alkylsulfanyl functionalized thiophene rings, respectively. The aim of this research is to study the structural, electronic and optical properties of these molecules to better understand the role of the sulfur atom of the alkylsulfanyl chains and to gain insight on the properties of these materials in view of their application in optoelectronic and photovoltaic devices