Improvement of the redox stability of dithieno[3,2-b:2′,3′-d]thiophene derivatives by using bulky substituents

With the goal of improving the long term redox stability of dithieno[3,2-b:2′,3′-d]thiophene (DTT), we synthesized derivatives with differently sized substituents. It is well known that substituents in the 2, 6 positions of DTT are active in this respect. We provide in this article more insight into mechanism. The present study provides the first support for a size effect. Large size substituents in the 2, 6 positions of the DTT are more effective than small size substituents. In these compounds, the DTT acts as a core of a molecule where the HOMO is located and the substituents work as a cage to protect the core from various attacks from the outside. This core-cage system is not only effective to prevent polymerization but also to increase the long term redox stability. The compounds with large size substituents, such as (1,1′:3′,1″-terphenyl)-2-yl and 4,4′-di-tert-butyl-(1,1′-biphenyl)-2-yl, showed excellent long term redox stability. The oxidation potential was maintained during repeated oxidation up to 17,000 and 20,000 times, respectively. The compounds with small size substituents, such as phenyl and mesityl, showed 1/10 or less of this durability. We show the substituent effects on repeated oxidation by investigating the electrochemical and photophysical properties.