Electro-optical π-radicals: design advances, applications and future perspectives

The unique open-shell electronic structure of neutral π-radicals has inspired research on novel molecular materials for over a century. In recent years, rapid development in carbon-centred π-radicals has resulted in new molecular electronics technologies, record breaking organic light-emitting diodes and other efficient energy applications. With the rapidly expanding literature in hand, it is not always straightforward to draw conclusions from results across different fields to aid further materials development. In this review, classification and design of neutral π-radicals are discussed and clarified in relation to their electro-optical properties and different target applications. Factors such as type of radical centre, steric effects and extent of conjugation, delocalisation and additional electron-donating and electron-withdrawing groups synergistically affect the stability, electronic structure, energetics, intra- and intermolecular spin-spin interactions as well as charge transport, optical and magnetic properties of π-radicals. Control of these properties and successful use of previously reported materials are exemplified in optics and electronics applications. Future potential and research directions are discussed in the light of remaining challenges that require critical consideration. The aim of this review is to provide an overview of the advantages and pitfalls of π-radicals and in that way help future design advances in the field. Perspectives on carbon-centred neutral π-radicals are discussed in terms of materials design, optical and electrochemical properties, target applications and intriguing research directions that deserve further attention.