| CPC C07F 5/00 (2013.01) [C01B 39/00 (2013.01); G06K 19/0614 (2013.01); G06K 7/10544 (2013.01); G06K 19/06084 (2013.01)] | 22 Claims |

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1. An optical tag comprising a rare earth metal-organic framework (REMOF), the REMOF comprising at least one high-energy donor rare earth and at least one lower-energy acceptor rare earth, wherein the rare earths are arranged in nonanuclear heterometallic clusters interconnected by carboxylic acid-based linkers.
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