Kinetic and structural studies of the photochromic process of 3H-naphthopyrans by UV and NMR spectroscopy

Abstract

The photomerocyanines resulting from the UV irradiation of closed colourless 3,3-diphenyl-3H-naphtho[2,1-b]pyran and its 3,3-bis(4-fluorophenyl) derivative have been studied by UV–VIS and NMR spectroscopy. Kinetic bleaching studies have been carried out on a UV–VIS spectrometer at the photostationary state in acetonitrile solution. A thermal biexponential back-isomerization is observed. These two different kinetics can be attributed to two different isomers of the photomerocyanine. NMR spectroscopy allows us to obtain a ¹H, ¹³C and ¹⁹F (where present) structural identification of these two forms: complete assignment for the predominant isomer (trans–cis) and partial assignment for the second one (trans–trans). The presence of a third form (which quickly decays) is also observed. Each open form follows a mono-exponential decay, with very different bleaching rate coefficients. This is the first time that ¹⁹F NMR has been used to identify the number and structure of isomers produced by irradiation, illustrating the advantages of this technique.