Named for chemist Michael Kasha, who proposed it in 1950, Kasha's rule holds that when light is shined on a molecule, the molecule will only emit light (fluorescence or phosphorescence) from its lowest energy excited state. This is why photoluminescent molecules emit light at a lower energy than the excitation light. While there have been examples of organic molecules, such as azulene, that break Kasha's rule, these examples are rare. Highly luminescent molecular systems crafted from quantum dots that break Kasha's rule have not been reported – until now.
"We have demonstrated a semiconductor nanocrystal molecule, in the form of a tetrapod consisting of a cadmium-selenide quantum dot core and four cadmium sulfide arms, that breaks Kasha's rule by emitting light from multiple excited states," says Paul Alivisatos, director of Berkeley Lab and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley. "Because this nanocrystal system has much higher quantum yield and is relatively more photostable than organic molecules, it holds promising potential for optical sensing and light emission-based applications, such as LEDs and imaging labels."
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