Modulating crystallinity and mixed ionic–electronic conduction properties via terminal side chain engineering of n-type small molecules†

High-performance n-type bioinspired electronic devices are indispensable for advancing next-generation bionic electronics. However, the molecular design guidelines regarding their channel layer remain somewhat unclear in this burgeoning area. In this work, three n-type small molecular mixed conductors with precise structural alterations of the terminal side chain length were designed and synthesized using a straightforward, metal-free condensation method. These molecules were successfully applied as channels in high performance n-type accumulation-mode organic electrochemical transistors (OECTs) and OECT-based organic electrochemical neuronal synaptic (OENS) devices. Due to its highest crystallinity, gNR-Pr with n-propyl exhibits superior electron mobility of 6.5 × 10⁻² cm² V⁻¹ s⁻¹, along with the highest μC* value of 14.1 F V⁻¹ cm⁻¹ s⁻¹ and robust synaptic tunability, evidenced by a paired-pulse facilitation index of 204% at a pulse interval of 0.1 s, with non-volatile characteristics. Furthermore, gNR-Bu with n-butyl possessing weaker aggregation demonstrates the best ionic permeation and transport capabilities, reflected in an outstanding volumetric capacitance of 336 F cm⁻³. In contrast, gNR-Am with n-pentyl shows relatively higher para-crystalline disorder, resulting in the fastest switching on/off times in OECTs and volatile character in OENS devices. Through this versatile terminal side chain modification, this work provides a greater understanding of the structure–property guidelines necessary for high-performance n-type bio-inspired electronic devices.