| abstract |
Compounds of formula I or V, or mixtures thereof, ##STR1## wherein R is unsubstituted C 6 -C 14 aryl or C 6 -C 14 aryl which is substituted by C 1 -C 12 alkyl, C 1 -C 12 alkoxy, C 1 -C 12 alkylthiol, phenyl, benzyl, --CN,--CF 3 , halogen or --COOR 5 , and R 5 is H, C 1 -C 18 alkyl, cyclohexyl, cyclopentyl, phenyl, C 1 -C 12 alkylphenyl, benzyl or C 1 -C 12 alkylbenzyl, and at least one of the substituents R 1 to R 4 is an organic thiol, sulfoxyl or sulfonyl group, and the other members R 1 to R 4 are H, F, Cl or Br, are reversible photochromic systems which can be used for contrast formation or light absorption. |