Donor-Acceptor Derivatives of Indolo[3,2-b]indole and Benzothieno[3,2-b]benzothiophene: Similar Annulated Structures but Divergent Properties

Annulated organic molecular structures with planar, fused backbones exhibit superior properties compared to non-fused systems, including high crystallinity, strong π–π stacking, and excellent charge transport characteristics. The rational design of annulated compounds with targeted characteristics presents a significant challenge that requires a comprehensive understanding of structure–property relationships. This work addresses this by synthesizing a series of novel push–pull systems featuring benzothieno[3,2-b]benzothiophene (BT) or its nitrogen-rich analogue, indolo[3,2-b]indole (ID), as electron-donating units, connected via a phenylene π-spacer to two distinct electron-accepting groups (carbonyl or dicyanovinyl). The thermal, structural, optical and electrochemical properties of these compounds were thoroughly investigated. Computational studies of the optical and electrochemical properties, including those of unsubstituted ID and BT model cores, showed excellent agreement with experimental data, validating the theoretical models. Notably, ID-based derivatives exhibited remarkably high photoluminescence quantum yield and enhanced solubility compared to their BT counterparts, along with thermal properties that are more favorable for device fabrication. This work provides the first systematic comparison of these annulated cores, offering novel structure–property insights that may support the rational design of organic functional materials and contribute to the further development of organic electronics.