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 intra-molecular 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.