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Two kinds of nitrogen-bridged star-shaped fused-ring electron acceptors based on benzo[1,2-b:3,4-b’-5,6-b’’]tri(4-H-dithieno[3,2-b:2’,3’-d]pyrrole) (BTDTP) core with linear and branched alkyl side chains on bridging nitrogen atoms are developed. Both electron acceptors exhibit strong intramolecular charge transfer absorption bands at 500–800 nm (molar extinction coefficient of 3.0 × 105 M −1 cm−1) with narrow optical bandgaps around 1.58 eV, but reveal different molecular packing and morphology. Compared to acceptor with linear side chains, the acceptor bearing branched side chains exhibits ordered face-on orientation with tight π-π stacking, as well as more favorable phase separation morphology and balanced charge transport when blended with polymer donor PM6. Organic solar cell device based on acceptors bearing branched side chains achieves a power conversion efficiency (PCE) of 9.95% with a fill factor of 74.98% and a short-circuit current density of 14.90 mA cm−2, which is much superior to the device using acceptors bearing linear side chains (a PCE of 6.74%).

Abstract Halogen bonding, as a kind of intermolecular interaction, has rarely been used to tune solid-state emission properties of luminescent materials, especially fluorescent materials. Herein, three trans-enaminone (TE) derivatives (nonbrominated TE, monobrominated BrTE, and tribrominated Br3TE) with aggregation-induced emission property have been designed and synthesized. Two types of BrTE crystals (BrTE-B and BrTE-G) with different fluorescence properties were obtained. It was observed that their solid-state fluorescence has been enhanced by the formation of halogen bonding. In particular, the crystal BrTE-G containing Br…π interactions exhibits a fluorescence quantum yield (9.6%) nearly sevenfold higher than BrTE-B, the crystal without halogen bonding (1.4%), and fivefold higher than the nonbrominated TE derivative (2.1%). By careful inspection of the single-crystal data and theoretical calculations, the high fluorescence quantum yield of BrTE-G appears to be due to halogen-bonding interactions as well as multiple stronger intermolecular interactions which may restrain molecular motions, leading to the reduced nonradiative decay rate and the enhanced radiative decay rate. Additionally, increasing the number of bromine substituents may further promote the radiative decay rate, explaining therefore the higher fluorescence quantum yield (12.5%) of Br3TE.