Photochromism of 8-thienyl-naphthopyrans investigated by NMR spectroscopy

The photochromism of four 8-thienyl-naphthopyrans substituted either directly or via an acetylenic bridge to one or two thienyl groups, has been investigated by NMR spectroscopy. The two expected transoid photomerocyanines TC (transoid-cis) and TT (transoid-trans) have been characterised. From the numerical analysis of the kinetics recorded during photocolouration under UV irradiation and during thermal relaxation in the dark, several kinetic parameters related to the photochemical and thermal isomerization processes were determined. Their relative values are discussed and compared to two H-substituted parent compounds.