The synthesis and properties of vinyl substituted naphthopyrans and their styrene copolymers

A comprehensive series of mono-, di- and tri-bromo-3H-naphtho[2,1-b]pyrans was synthesised by the traditional union of a 2-naphthol with an alkynol. The bromine atom(s) in these naphthopyrans was readily replaced by a vinyl group using a Suzuki coupling reaction with vinyl boronic anhydride pyridine complex. The efficiency of the Suzuki vinylation reaction decreases with the increasing number of bromine atoms to be substituted. The vinylnaphthopyrans served as styrene analogues and readily underwent a thermally initiated free radical addition co-polymerisation with styrene to efficiently afford low molecular weight poly(styrene-co-naphthopyrans).

The photochromic response of the bromonaphthopyrans and vinylnaphthopyrans followed the established colour-structure relationships for photochromic naphthopyrans. The photochromic response of toluene solutions of the poly(styrene-co-naphthopyrans) was characterised by a hypsochromic shift of the λmax relative to that recorded for the vinylnaphthopyran monomers. Increased half-lives were noted for the copolymers derived from the bis- and tris-vinylnaphthopyrans where some degree of crosslinking was expected. A further hypsochromic shift in λmax resulted for the thin films of the poly(styrene-co-naphthopyrans). Of particular note was the behaviour of the 3-(2-vinylphenyl)-3-phenylnaphthopyran which gave the typical yellow colour of the photomerocyanine coupled with the predictable large half-life. However, all photochromism of this naphthopyran was lost upon polymerisation suggesting that an alternative pathway supervenes in the expected polymerisation sequence.