Optical and crystalline properties of benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene derivatives

When designing solution-processable semiconducting molecules and polymers, benzo[1,2-<i>b</i>:4,5-<i>b</i>']dithiophene (BDT) derivatives are widely used because of their planar structures, superior optical properties, ease of synthesis and ease of modification. In this work, four BDT derivatives-BDTT, BDTT-Et, BDTT-OMe and BDTT-CH₂-OMe-were designed and synthesized with different side chains, considering the important roles of side chains in the performance of organic semiconductors. Especially for BDTT-CH₂-OMe, with a new methoxymethyl chain, it exhibited excellent optical properties and the deepest highest occupied molecular orbital energy level (<i>E</i> _HOMO) among these derivatives. Moreover, it demonstrated strong intermolecular interactions and tight π-π stacking. The optical, electrochemical and crystalline properties suggested that BDTT-CH₂-OMe could be further modified as a potential building block for the design of electron-donating small molecules (SMs) or polymers when used in organic electronics, such as bulk heterojunction organic solar cells (BHJ-OSCs).