Benzodithiophene-Cored Small Optoelectronic Molecules: Influence of Extension Direction of Conjugated Segments

Conjugated extension of an organic optoelectronic moiety along different directions is of significance for exploring its usage in material design and understanding its structure-property relationships. In this work, three benzo[1,2-b:4,5-b']dithiophene-cored molecules were designed and synthesized by attaching donor-acceptor conjugated arms composed of thiophene and benzothiadiazole via the direction of its 2,6-position (horizontal), 4,8-position (vertical), and the both (cross), respectively. Compared with vertical linkage, the horizontal modification benefits the achievement of a red-shifted and wider absorption spectrum, a narrow bandgap and a more regular crystalline property, but a higher highest occupied molecular orbital (HOMO) energy level. Cross-shaped compound exhibits both a wide absorption spectrum and large absorption coefficients, and thus gave the largest short-circuit current density for its solar cell among three so-prepared devices.