Studies of polyphotochromic behaviour of supermolecules by NMR spectroscopy. Part 2. A bis-[3H]naphthopyran with a (Z)-ethenic bridge between each moiety

Abstract

The behaviour of a new biphotochromic molecule (CC–Z), formed by two naphthopyran moieties linked through a (Z)-ethenic double bond, has been studied by ¹H and ¹⁹F NMR spectroscopy. To facilitate the investigations, a naphthopyran (CHR) and a model constituted by a naphthopyran entity linked through a double bond spacer to a methoxynaphthalene nucleus (C–Z) were also studied. At first, the studies were performed at low temperature (228 K) to extract the ¹H and ¹⁹F NMR chemical shifts characterising photomerocyanines. Thereafter, the behaviour at ambient temperature was checked, identifying a major reaction of (Z–E) isomerisation as well as an intramolecular cyclisation reaction. An unexpected thermal opening of the naphthopyran entities in the cyclised form was also observed, leading to an equilibrium between the different stereoisomers of the photomerocyanines.