Towards novel photoswitches for applications in organic electronics

Abstract

Central to the development of new optoelectronic devices based on organic materials are novel photoswitches. These are molecules that are able to undergo a reversible structural change under a light stimulus which gives rise to unique properties. Thus, it allows the control of properties via the use of light in a variety of applications. The interplay of switches with organic semiconducting polymers allows for the manipulation of the polymers through light thus provides a new level of control. In this thesis we will thus discuss the incorporation of novel photoswitches within polymers and how we can potentially achieve novel photochromic polymers for use in future optoelectronic applications. In Chapter 1, we discuss the basic principles of photoswitches with examples and move on to the key concepts within organic electronics. This introduces the importance of conjugated materials and their interests to both industry and research. This leads onto the interplay between photoswitches and conjugated materials, with select examples showcasing the applications and methods for the photoswitches to be incorporated. This motivates our investigation of exploring new novel photoswitches and incorporating them into new photochromic organic polymers. Chapter 2 starts with an introduction to excited state intramolecular proton transfer (ESIPT) molecules and the background to the concept. This leads into a review on the applications of the material and how this molecule can potential be used as photoswitch. This work is centred around the incorporation of the ultra-fast ESIPT within a conjugated polymer backbone. We synthesise a range of these ESIPT fluorene-based polymers. This is based on the concept of excited state aromaticity where the loss/gain of aromaticity is a driving force towards the barrierless switching. We report the synthesis, photophysical characterisation and DFT on the polymers produced. Although stable switching effects were not realised, the ESIPT cycle showed potential within polymers via a strong stability to UV irradiation and thus potential use as UV stabiliser. Chapter 3 focuses on a more stable photoswitch dihydropyrene (DHP) after focussing on the less well known ultrafast photoswitches. This is in hopes of being able to produce a photoswitch that will be possible to incorporate into a polymer. This starts with a mini-introduction to the DHP and the applications that have been derived thus far. We propose a new set of DHP molecules which will exploit the switching effect of opening and closing the conjugated centre to achieve single molecule applications. This includes a thiol based DHP for use in a nano particle on a mirror system and also a diradical photoswitchable system. This lays down key groundwork for the final chapter as the photoswitching is studied and optimised in chapter 3. In the final chapter, we propose the incorporation of the DHP molecule to be directly incorporated into a DPP based polymer. This polymer has potential in organic electrochemical transistors (OECTs) thus can be used in the field of neuromorphics. The DHP was successfully incorporated into the DPP molecules via a Stille random copolymerisation which allowed varying levels of incorporation. We study the photoswitching of these polymers which is reported in the chapter. This paves the way to further studies into the interplay of photoswitches with polymers for use in next generation optoelectronic devices.