Quinine
An alkaloid compound obtained from cinchona bark, long used as a treatment for malaria and also used as the bitter flavoring in tonic water.
Tonic water glows blue under a blacklight because of quinine, its bitter ingredient. Quinine molecules absorb UV light, jump to an excited state, then release that energy as blue light (about 450nm) when they relax back down — the glow vanishes the instant the UV source is removed.
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