A series of semiconducting small molecules with bithiophene or bis‐3,4‐ethylenedioxythiophene cores are designed and synthesized. The molecules display stable reversible oxidation in solution and can be reversibly oxidized in the solid state with aqueous electrolyte when functionalized with polar triethylene glycol side chains. Evidence of promising ion injection properties observed with cyclic voltammetry is complemented by strong electrochromism probed by spectroelectrochemistry. Blending these molecules with high molecular weight polyethylene oxide (PEO) is found to improve both ion injection and thin film stability. The molecules and their corresponding PEO blends are investigated as active layers in organic electrochemical transistors (OECTs). For the most promising molecule:polymer blend (P4E4:PEO), p‐type accumulation mode OECTs with µA drain currents, μS peak transconductances, and a µC* figure‐of‐merit value of 0.81 F V−1 cm−1 s−1 are obtained.

Small-molecule organic semiconductors have displayed remarkable electronic properties with a multitude of π-conjugated structures developed and fine-tuned over recent years to afford highly efficient hole- and electron-transporting materials. Already making a significant impact on organic electronic applications including organic field-effect transistors and solar cells, this class of materials is also now naturally being considered for the emerging field of organic bioelectronics. In efforts aimed at identifying and developing (semi)­conducting materials for bioelectronic applications, particular attention has been placed on materials displaying mixed ionic and electronic conduction to interface efficiently with the inherently ionic biological world. Such mixed conductors are conveniently evaluated using an organic electrochemical transistor, which further presents itself as an ideal bioelectronic device for transducing biological signals into electrical signals. Here, we review recent literature relevant for the design of small-molecule mixed ionic and electronic conductors. We assess important classes of p- and n-type small-molecule semiconductors, consider structural modifications relevant for mixed conduction and for specific interactions with ionic species, and discuss the outlook of small-molecule semiconductors in the context of organic bioelectronics.

dominated by poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and closely related derivatives as the active material, [4][5][6][7] new semiconducting materials have started to enter the arena in recent years. [8][9][10][11][12] In particular, hole-transporting (p-type) mixed conductors have emerged with properties on par or even better than benchmark PEDOT:PSS formulations, [9,13,14] whereas electron-transporting (n-type) materials are still lacking considerably in terms of mixed conduction performance. [15][16][17][18][19] The design of semiconducting materials for bioelectronic applications is heavily influenced by the rich body of literature in the broader field of organic electronics, evidenced by the development of, for example, oligo-and polythiophenes, [8,11,20,21] fullerenes, [18] diketopyrrolopyrrole-, [12,[22][23][24] and naphthalene-diimide-based [15,16] systems as efficient OMIECs. With that in mind, another versatile and highly efficient charge-transporting moiety, isoindigo (IG), has received very little attention for bioelectronic applications. [25] The IG motif depicted in Figure 1a can be structurally modified not only by introducing various solubilizing Organic mixed ionic and electronic conductors are of significant interest for bioelectronic applications. Here, three different isoindigoid building blocks are used to obtain polymeric mixed conductors with vastly different structural and electronic properties which can be further fine-tuned through the choice of comonomer unit. This work shows how careful design of the isoindigoid scaffold can afford highly planar polymer structures with high degrees of electronic delocalization, while subtle structural modifications can control the dominant charge carrier (hole or electron) when probed in organic electrochemical transistors. A combination of experimental and computational techniques is employed to probe electrochemical, structural, and mixed ionic and electronic properties of the polymer series which in turn allows the derivation of important structure-property relations for this promising class of materials in the context of organic bioelectronics. Ultimately, these findings are used to outline robust molecular-design strategies for isoindigo-based mixed conductors that can support efficient p-type, n-type, and ambipolar transistor operation in an aqueous environment.

A series of copolymers containing a glycolated 1,4‐dithienyl‐2,3,5,6‐tetrafluorophenylene unit copolymerized with thiophene, bithiophene, thienothiophene and 1,2,4,5‐tetrafluorobenzene comonomer units were designed and synthesised by direct heteroarylation polymerisation. The optical, electrochemical, electrochromic and solid‐state structural properties of the copolymers were investigated. The copolymers exhibit stable redox properties in organic solvents and promising redox properties in thin film configuration with an aqueous electrolyte. Finally, the potential of the copolymers as active materials in organic electrochemical transistors (OECTs) was assessed, and promising performance was shown as an accumulation‐mode OECT material with a peak transconductance of 0.17 mS and a good on/off ratio of 105 for the thiophene copolymer.

The processability and optoelectronic properties of organic semiconductors can be tuned and manipulated via chemical design. The substitution of the popular alkyl side chains by oligoethers has recently been successful...