New halogenated diphenyl-2H-benzo[h]chromene derivatives: synthesis and optical properties

The synthesis and the optical properties of eleven new photochromic compounds of the diphenyl-chromene family containing different halogen substituents are reported. Kinetic studies under continuous irradiation indicate that for these compounds the bleaching of the merocyanines involves two first order processes. Fluoro-substitution of the phenyl ring(s) always leads to an increase in the fast rate constant, k1, that is also found to increase on changing the halogen in the naphthalenic fragment from fluorine to chlorine and to bromine. 19F NMR confirms the presence of isomers produced under irradiation. One of these isomers exhibits a great stability in solution. A single crystal X-ray diffraction study indicates that in 6-fluoro-2,2-bis(4′-fluorophenyl)-2H-benzo[h]chromene the two phenyl rings are orthogonal to each other and that the pyranic fragment deviates considerably from planarity.