Abstract: Compounds of formulae I and II ##STR1## wherein R is phenyl and R.sub.1 to R.sub.4 are H or a substituent such as phenylthio, and one of the substituents R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is a group of formula III--X--C(O)--CR.sub.5 .dbd.C(R.sub.6).sub.2 (III)or R is substituted by a group of formula III, wherein X is a linking group or --X--C(O)-- is a direct bond, and R.sub.5 and R.sub.6 are each independently of the other H, halogen, C.sub.1 -C.sub.12 alkyl or C.sub.6 -C.sub.10 aryl, can be polymerised to homopolymers or copolymers. The monomers and polymers are reversible photochromic systems which can be used as colour indicators, as photochemically modifiable colour filters, or for the optical storage of information.

Abstract: Compounds of formulae I and II ##STR1## wherein D is H or --OR wherein R is phenyl and R.sub.1 to R.sub.4 are H or a substituent such as phenylthio, and one of the substituents R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is a group of formula III--E-CR.sub.5 =C(R.sub.6).sub.2 (III)or R is substituted by a group of formula III, wherein E is a direct bond or --X-C(O)-- wherein X is a linking group, and R.sub.5 and R.sub.6 are each independently of the other H, halogen, C.sub.1 -C.sub.12 alkyl or C.sub.6 -C.sub.10 aryl, can be polymerised to homopolymers or copolymers. The monomers and polymers are reversible photochromic systems which can be used as color indicators, as photochemically modifiable color filters, or for the optical storage of information.

Abstract: Compounds of the formula I ##STR1## in which X is O, S, SO, SO.sub.2 or NR.sub.13, R.sub.13 is, for example, methyl, and R.sub.1 to R.sub.12, independently of one another, are, for example, H, C.sub.1 -C.sub.12 alkyl, C.sub.1 14 C.sub.12 alkoxy, halogen or --CN. The compounds are photosensitive and photochromic and as suitable are photosensitizers and simultaneously as colour indicators, and as photoswitchable colour filters.

Abstract: Compounds of the formula I ##STR1## in which X is O, S, SO, SO.sub.2 or NR.sub.13, R.sub.13 is, for example, methyl, and R.sub.1 to R.sub.12, independently of one another, are, for example, H, C.sub.1 -C.sub.12 alkyl, C.sub.1 -C.sub.12 Alkoxy, halogen or --CN. The compounds are photosensitive and photochromic and are suitable as photosensitizers and simultaneously as color indicators, and as photoswitchable color filters.

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

Abstract: Compounds of the formula I or V, or mixtures thereof, ##STR1## wherein R is unsubstituted C.sub.6 -C.sub.14 aryl or C.sub.6 -C.sub.14 aryl which is substituted by C.sub.1 -C.sub.12 alkyl, C.sub.1 -C.sub.12 alkoxy, C.sub.1 -C.sub.12 alkylthio, phenyl, benzyl, --CN, --CF.sub.3, halogen or --COOR.sub.5, and R.sub.5 is H, C.sub.16 l-C.sub.18 alkyl, cyclohexyl, cyclopentyl, phenyl, C.sub.1 -C.sub.12 alkylphenyl, benzyl or C.sub.1 -C.sub.12 alkylbenzyl, and at least one of the substituents R.sub.1 to R.sub.4 is an organic thio, sulfoxyl or sulfonyl group, and the other members R.sub.1 to R.sub.4 are H, F, Cl or Br, are reversible photochromic systems which can be used for contrast formation or light absorption.

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