KINETIC AND STRUCTURAL STUDIES OF THE PHOTOCHROMIC PROCESS OF 3H-NAPHTHOPYRANS BY UV AND NMR-SPECTROSCOPY

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