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.