Ultralong organic afterglow from small molecular host-guest materials: state of the art

Ultralong organic afterglow materials are being actively explored as attractive candidates for a wide range of applications such as data storage, security inks, emergency lighting, etc., due to their unique long-lived excited state properties and inherent advantages of low cost, appreciable functionality and ease of preparation. In the last three years, much effort has been devoted to achieving efficient ultralong afterglow from organic small molecules, which possess controllable intermolecular interactions and defined energy levels, making them a good platform to suppress the non-radiative decays, hence stabilizing the excitons for efficient afterglow emissions at room temperature. Nevertheless, there has been a lack of reviews on how efficient ultralong organic afterglow can be systematically achieved from small molecular host-guest materials, which is not conducive to the development of the field. In this review, we have outlined and summarized small-molecule ultralong organic afterglow materials based on different emission mechanisms. We have included emission mechanisms involving ultralong room-temperature phosphorescence (URTP), ultralong thermally activated delayed fluorescence (UTADF) and organic long persistent luminescence (OLPL), where the latter two mechanisms have rarely been reported. In addition, challenges and future perspectives are discussed to emphasize the future directions.