New Organic Materials for Bioelectronic Applications

The interface between electronic and biological systems and the ability to interconvert biological and electronical signals, that can be provided using an organic electrochemical transistor (OECT), can greatly affect our lives for the better. The possibility of direct detection of ions, metabolites or signalling molecules can ensure both better prevention and more effective treatment of many diseases. Synthesis and characterization of novel materials for OECTs is the primary focus of this thesis. In three research chapters, the basic design and gradual alternation of structures of novel polymers with gradual improvement of OECT performance and stability are discussed. The fifth chapter is devoted to the expansion of the currently small group of small molecules suitable for OECT applications. The second chapter of this thesis deals with the synthesis and characterization of new polymer materials with a high concentration of glycol side chains, suitable for OECT applications. The structures of the materials were chosen so that it was possible to investigate the influence of the electron density of the conjugated backbone on the overall performance. The design of the materials is based on materials with a central tetrafluorophenylene unit previously prepared by our group, the best representative of which showed figure of merit µC* = 10 F·cm−1·V−1·s−1. The new design brought a series of novel polymers, the best of which showed figure of merit µC* = 69 F·cm−1·V−1·s−1. The third chapter deals with further derivatization of the materials prepared in chapter two. Thanks to the newly developed synthetic strategy, both new materials and regioisomers of the materials from chapter two were prepared. Mentioned materials were optically and electrochemically characterized, and it was found that almost identical regioisomers can show up to a sixfold difference in the figure of merit, which was for the best material µC* = 268 F·cm−1·V−1·s−1, which was at the time a new record among glycolated polymers. The fourth chapter deals with the synthesis of building blocks suitable for better arrangement of polymer side chains in the solid phase. It also deals with the derivatization of the best performing polymers from the previous chapter. To improve their stability, derivatives with fluoride substitution, with a methylene linker between the conjugated backbone and the side chain, and subsequently a derivative combining both structural changes were prepared. Fluoride substitution of the best performing material from the previous chapter brought an increase in its OECT operational stability from t1/2 = 342 s to t1/2 = 2620 s. The fifth chapter deals with the synthesis of new small molecules, which are structurally based on the only known p-type small molecule that was able to function as an OECT. By changing the central units and introducing additional donor substituents, a new series of molecules with a gradually increasing HOMO level was prepared, which was tested in OECT applications

Role of oxygen within end group substituents on film morphology and charge carrier transport in thiophene/phenylene small-molecule semiconductors

In this study, the end group polarity of (5,5′)-biphenyl-(2,2’)-bithiophenes (PTTPs) was systematically varied from alkyl (1) to alkoxy (2) with one oxygen atom to glycol (3) with two oxygen atoms while the overall length of the end groups is kept constant. Thin films of the three compounds were sublimated at different substrate temperatures and their morphology, crystallinity and charge carrier transport in field-effect transistors was investigated to draw structure-property relationships for the PTTP derivatives. For all three compounds, the effective charge carrier mobility is improved with higher substrate temperatures at which films with higher crystallinity and larger grains are formed. The effective mobility decreases with higher polarity of the end groups from alkyl to alkoxy and glycol. The reliability factor of the alkyl (1) and alkoxy (2) substituted PTTPs decreases with higher substrate temperature, but at the same time this value is enhanced for the glycol substituted molecules (3). The transistors of 3 prepared at higher substrate temperatures also show a reduced threshold voltage and smaller hysteresis in the transfer characteristics. These insights are important for the understanding of the impact of oxygen incorporation into side chain/end group substituents of organic semiconductors and their implementation in organic electrochemical transistors, thermoelectrics and photovoltaics

Mixed Ionic and Electronic Conduction in Small-Molecule Semiconductors.

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