Selective Bromination of Dithienopyran and Its Application in Asymmetric Nonfused Ring Electron Acceptors

Pyran, considered as a crucial building block, has been widely used in organic semiconductor materials. In this work, we discovered the selective monobromination characteristic of the 5H-dithieno[3,2-b:2′,3′-d]pyran (DTP) unit. Based on this useful finding, we designed and synthesized three asymmetric nonfullerene electron acceptors (PBT-EH, PBT-BO, and PBT-HD) based on the DTP block combined with side-chain engineering. The intramolecular conformation locks ensure planar conjugated molecular backbones in all three molecules. Structure–performance studies revealed that electron acceptors utilizing the oxygen functionalized DTP unit could broaden the absorption spectra, thereby providing the premise to achieve a high short-circuit current density (JSC). Finally, careful selection of the side chain on the molecular center effectively fine-tunes the thin-film morphology, and the PBDB-T:PBT-BO-based device achieved the best photovoltaic performance among the three acceptors.