The smallest oligothiophene diradicals by asymmetric substitution of quinoidal cores

Abstract

Asymmetric conjugated diradicals possess fascinating electronic properties but are rarely found due to challenges in their synthesis and the delicate selection process of radical units. Herein, we report a series of asymmetric diradicals, PTI-n (n = 0 to 4), with different numbers of quinoidal thiophene units as π bridges and phenoxyl and imidazolyl moieties as radical centres. This conjunction provides dyads that exhibit excellent infrared absorption properties reaching beyond 1200 nm due to the transition from closed-shell to open-shell singlet ground states upon passing from the bithiophene to the trithiophene derivative. The quaterthiophene molecule features a vanishing singlet–triplet energy gap and a remarkable blue-shift (ca. 400 nm) in the electronic absorption spectrum. These modulations of the electronic and optical properties in a series of quinoidal oligothiophenes of only four units are unprecedented and highlight asymmetric substitution of these cores as a fertile niche in the area of developing conjugated molecules.