Investigations by NMR spectroscopy of a polyphotochromic system involving two entities, spirooxazine and naphthopyran, linked by a Z-ethenic bridge

Abstract

The biphotochromic molecule, made up of spirooxazine and naphthopyran entities linked by a Z-ethenic bridge, was synthesised to extend the conjugation and obtain photomerocyanines absorbing all the visible spectrum. Photochromic behaviour was studied by ¹H and ¹⁹F NMR spectroscopy, using previous NMR results obtained for spirooxazine alone and naphthopyran alone. At first, studies performed at low temperature (228 K) showed a clean conversion between closed and open structures with the demonstration of mono- and bi-opening of the biphotochromic molecule and with a Z/E isomery of the ethenic bridge. In contrast, studies at ambient temperature indicated intramolecular cyclisation stabilising the photomerocyanines. An unexpected thermal opening of the naphthopyran entity in the cyclised form was also observed. A thermal cyclisation–oxidation reaction occurred, implying an irreversible loss of photochromic properties. By carrying out experiments on degassed samples, a TC-TT isomery of open naphthopyran was obtained, leading to a consistent equilibrium between the different stereoisomers of the photomerocyanines.