Organic long persistent luminescence

Long persistent luminescence (LPL) materials - widely commercialized as 'glow-in-the-dark' paints - store excitation energy in excited states that slowly release this energy as light. At present, most LPL materials are based on an inorganic system of strontium aluminium oxide (SrAl 2 O 4) doped with europium and dysprosium, and exhibit emission for more than ten hours. However, this system requires rare elements and temperatures higher than 1,000 degrees Celsius during fabrication, and light scattering by SrAl 2 O 4 powders limits the transparency of LPL paints. Here we show that an organic LPL (OLPL) system of two simple organic molecules that is free from rare elements and easy to fabricate can generate emission that lasts for more than one hour at room temperature. Previous organic systems, which were based on two-photon ionization, required high excitation intensities and low temperatures. By contrast, our OLPL system - which is based on emission from excited complexes (exciplexes) upon the recombination of long-lived charge-separated states - can be excited by a standard white LED light source and generate long emission even at temperatures above 100 degrees Celsius. This OLPL system is transparent, soluble, and potentially flexible and colour-tunable, opening new applications for LPL in large-area and flexible paints, biomarkers, fabrics, and windows. Moreover, the study of long-lived charge separation in this system should advance understanding of a wide variety of organic semiconductor devices.