Terminal Hydroxylated Side-Chains Enhance Ionic-Electronic Coupling Efficiency in Small-Molecule Semiconductors

Small-molecule semiconductors play a crucial role in organic electronics. However, when employed as organic mixed ionic-electronic conductors (OMIECs), their limited ionic transport capabilities will obstruct efficient ionic-electronic coupling. To address this issue, we propose a universal strategy by introducing terminal hydroxylated ethylene glycol (EG-OH) side chains onto small-molecule semiconductors. The 4Cl-PDI-EG-OH, synthesized using this strategy, exhibits higher transconductance, faster response time, and better stability than the common ethylene glycol (EG)-based 4Cl-PDI-EG in organic electrochemical transistors (OECTs). X-ray single-crystal diffraction and spectroscopic studies reveal that the terminal hydroxyl interlocking promotes the formation of ordered side-chain arrangements, broadening the ionic transport channels. Concurrently, this induces the formation of short-range charge-transfer (CT)-coupled J-aggregates (J_CT) within the backbones, maintaining efficient carrier mobility, thus achieving high-efficiency ionic-electronic coupling. Further applying this strategy to representative small-molecule skeletons yields significant performance improvements. Specifically, BTP-EG-OH exhibits a remarkable transconductance of 101.9 mS, which is comparable to that of many high-performance polymer-based OECTs. This study shows that the EG-OH side chain represents a superior choice compared to the currently common EG side chain for developing high-performance small-molecule OMIECs.