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Wed Oct 18, 2017, 08:39 PM

Long Persisting Organic luminescent Systems.

Last edited Thu Oct 19, 2017, 08:32 PM - Edit history (1)

Here's another cool paper: 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 light1. At present, most LPL materials are based on an inorganic system of strontium aluminium oxide (SrAl2O4) doped with europium and dysprosium, and exhibit emission for more than ten hours2. However, this system requires rare elements and temperatures higher than 1,000 degrees Celsius during fabrication, and light scattering by SrAl2O4 powders limits the transparency of LPL paints1. 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 temperatures3. 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 devices4.

...Upon exposure to ultraviolet or visible light, some substances absorb the excitation energy and release it as a differently coloured light either quickly, as fluorescence, or slowly, in the form of either phosphorescence or long persistent luminescence. Exploiting the slowest emission process, LPL materials have been widely commercialized as glow-in-the-dark paints for watches and emergency signs1, and are being explored for application in in vivo biological imaging because their long-lived emission makes it possible to take time-resolved images long after excitation5. In the mid-1990s, a highly efficient LPL system was developed that uses SrAl2O4 doped with europium and dysprosium, and this inorganic system forms the basis of most commercial glow-in-the-dark paints because of its long emission (more than ten hours) and high durability2. However, this system requires not only rare elements for long-lived emission but also very high fabrication temperatures of more than 1,000 °C. Moreover, the manufacturing of paints from the insoluble SrAl2O4 requires many steps, including grinding of the compounds into micrometre-scale powders for dispersion into solvents or matrices, and light scattering by the powders prevents the formulation of a transparent paint1. The realization of LPL from organic molecules would solve many of these problems.

Their glow in the dark organic molecules can stay lit up for about an hour.

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