Photostable compounds, absorbing compounds and uses thereof

A compound of formula I, or a salt thereof:

• wherein, A is a six or seven-membered ring comprising at least one double bond, which ring may be further substituted or unsubstituted;
• W is selected from O, S, N or C;
• Ra, Rb and R3, when present, are independently selected from hydrogen, halo, C1 to C20 alkyl, C2 to C20 alkenyl, C2 to C20 alkynyl, aryl, heteroaryl, aroyl, C2 to C12 alkanone, C5 to C7 cycloalkyl, C4 to C7 cycloalkanone, C5 to C7 cycloalkenyl, C2 to C12 alkanoyl, C2 to C12 alkanoyloxy, C2 to C12 alkoxycarbonyl, C2 to C12 carbamoyl, C2 to C12 carboxyl, haloalkyl, N-alkyl, N-aryl, N-heterocyclyl or heterocyclic, all of which groups may be substituted or unsubstituted, or Rb and R3 may together form an alkene, an alkyne, a phenyl ring or a heteroaryl ring each of which is optionally substituted with at least one halogen or halogen-containing group;
• with the proviso that if Ra, Rb and R3 are all fluorine and A is a 6-membered nitrogen-containing alkene heterocycle then the nitrogen of that ring is not substituted with any of 4-dimethylaminophenyl, 4-methoxyphenyl, 4-methylphenyl, 4-tert-butylphenyl, isopropyl or 1-naphthyl.

A compound of formula II, or a salt thereof:

• wherein, the dotted line may be a bond;
• X is one or two carbon atoms forming part of the ring structure;
• R1 and R2 are independently selected from hydrogen, C1 to C10 alkyl, C1 to C10 alkenyl or C1 to C10 alkoxy, each of which groups may be substituted or unsubstituted;
• W is selected from O, S, N or C;
• Ra, Rb and R3, when present, are independently selected from hydrogen, halo, C1 to C20 alkyl, C2 to C20 alkenyl, C2 to C20 alkynyl, aryl, heteroaryl, aroyl, C2 to C12 alkanone, C5 to C7 cycloalkyl, C4 to C7 cycloalkanone, C5 to C7 cycloalkenyl, C2 to C12 alkanoyl, C2 to C12 alkanoyloxy, C2 to C12 alkoxycarbonyl, C2 to C12 carbamoyl, C2 to C12 carboxyl, haloalkyl, N-alkyl, N-aryl, N-heterocyclyl or heterocyclic, all of which groups may be substituted or unsubstituted, or Rb and R3 may together form an alkene, an alkyne, a phenyl ring or a heteroaryl ring each of which is optionally substituted with at least one halogen or halogen-containing group;
• R4 is selected from hydrogen, C1 to C12 alkyl, C2 to C12 alkenyl, aryl, heteroaryl, C5 to C7 cycloalkyl, C5 to C7 cycloalkenyl, C1 to C12 alkanoyl, C1 to C12 alkanoyloxy, C1 to C12 carboalkoxy or C1 to C12 alkanone all of which groups may be substituted or unsubstituted;
• R5 is selected from C1 to C12 alkyl, C2 to C12 alkenyl, C2 to C12 alkynyl, aryl, heteroaryl, C5 to C9 cycloalkyl, C5 to C7 cycloalkenyl, C2 to C9 alkanoyl, C1 to C12 alkanoyloxy or carbamoyl all of which groups may be substituted or unsubstituted;
• R6 is selected from hydrogen, oxo, substituted or unsubstituted to C6 alkyl, substituted or unsubstituted C2 to C6 alkenyl and/or substituted or unsubstituted C2 to C6 alkanoyl;
• and
• R18 and R19 are independently selected from hydrogen, C1 to C6 alkyl, C1 to C6 alkenyl or C1 to C6 alkoxy, each of which groups may be substituted or unsubstituted,
• with the proviso that when R1 and R2 are methyl, Ra, Rb and R3 are all fluorine, W is O and R4 is hydrogen then R5 is not any of 4-dimethylaminophenyl, 4-methoxyphenyl, 4-methylphenyl, 4-tert-butylphenyl, isopropyl or 1-naphthyl.