Hydrochromic fluorescence of organo-boronium-avobenzone complexes

Commercially available avobenzone readily reacts with boron trichloride or tribromide to afford boron-bisavobenzone salts. The complexes were found to exhibit unusual solvent-dependent fluorescence in solution and hydrochromic fluorescence in the solid state: a blue-to-orange emission colour change was observed upon water vapour exposure. The phenomenon was likely caused by interference of the boron coordination sphere by water molecules.