Flip-flopping aromaticity breaks fluorescence rule

Researchers have unravelled the reason azulene breaks a key fluorescence rule: the molecule’s switch to anti-aromaticity and back to aromaticity in its excited states.

Azulene is an aromatic molecule consisting of a fused seven-membered and five-membered ring. It’s an isomer of naphthalene (two fused six-membered rings) and even smells similar, but unlike colourless naphthalene it is dark blue. And it doesn’t obey Kasha’s rule, the principle that governs how molecules fluoresce.

According to the rule, fluorescence emission in molecular systems always comes from the first, lowest excited state. All higher excited states tend to be too short-lived for photon emission. Kasha’s rule says ‘that it doesn’t really matter to which excited state you excite the molecule, because it will always fall down to the lowest excited state and from there, it has a long enough lifetime to emit photons’, explains photochemist Tomáš Slanina from the Czech Academy of Sciences.