NMR structural and kinetic assignment of fluoro-3H-naphthopyran photomerocyanines

The kinetic and structural behavior of a photochromic compound, 3-(2-fluoro phenyl)-3-phenyl-3H-naphtho[2,1-b]pyran (F-Py), was investigated using H-1 and F-19 nuclear magnetic resonance (NMR) spectroscopy. Upon irradiation, t he four theoretically predicted photomerocyanines appear along with a fifth form X, whose final structure has not been elucidated. This last form and two of the photomerocyanines are thermally labile, whereas the other two do not show any signs of decay. The system has been analyzed by NMR spectrosc opy. This led to the structural assignment of each photomerocyanine. The ki netics of the thermal bleaching were monitored by directly and separately m easuring the concentrations of each species at regular time intervals using 19F NMR spectroscopy. We therefore propose a plausible reaction mechanism. On the basis of this mechanism, the mathematical treatment and the study o f the effects of temperature led to the determination of the kinetic and th ermodynamic parameters (rate coefficients, enthalpy and entropy of activati on) of this photochromic system. The leading role of the labile intermediat e X on the formation of traps-transoid-cis (TTC) and cis-transoid-cis (CTC) photomerocyanines is pointed out.