A crystal that glows blue but changes colour to purple and then orange within a second could make banknotes more resistant to counterfeiting.

Fluorescence and phosphorescence - a type of delayed fluorescence - make materials glow a single colour. Now, a team of chemists in China has discovered the first single compound that has acolour-changing afterglow. Currently, to get a similar effect, researchers needed to build materials out of several different phosphorescent molecules.

The molecule in question is 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP), a symmetric, linear aromatic. Under ultraviolet light, it is faint blue in colour. But after the UV light isswitched off, it glows blue, changing to purple and then orange before fading.

In combination with a diphenylanthracene derivative, CBP might make for more secure anti-counterfeiting marks on bank notes and other items that need a security tag. Under UV light, one set ofsymbols is visible as the anthracene interferes with CBP's fluorescence. When the UV light is switched off, the anthracene becomes inactive and CBP's afterglow shows off another, colour-changing setof symbols.

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First colour-changing crystal glow could make for safer banknotes(https://www.chemistryworld.com/news/first-colour-changing-crystal-glow-could-make-for-safer-banknotes/4011246.article)