Photoinitators

A photoinitiator of the formula (I)

• in which R3, R4, R5 and R6 are selected from H or C1-C6 alkyl;
• wherein one of R1, R2, R7 or R8 has a structure of formula (Ia):
• in which:
• Z1 is a linker comprising one or more linker units selected from a single bond, —O—, —S—, optionally-substituted —(C1-C12 alkylene)-, optionally-substituted —(C2-C12 alkenylene)-, —SO2—, —CO—, —NR′—, —Si(R′)2—, optionally-substituted heterocyclyl and optionally-substituted aryl in which R′ is H or C1-C6 alkyl;
• wherein linker —Z1-, optionally in combination with —Z2-, comprises at least one sulfonamide, a sulfonic ester, a carboxamide or a carboxylate ester moiety;
• Z2 is a trivalent nitrogen atom or Z2 is C(R″), in which R″ is H or C1-C6 alkyl;
• provided that—when Z2 is a nitrogen atom—the linker unit in Z1 adjacent to Z2 is —SO2— or —CO—;
• each of X1 and X2 are independently selected from a single bond, or a linker comprising one or more linker units selected from —O—, —S—, optionally-substituted —(C1-C12 alkylene)-, optionally-substituted —(C2-C12 alkenylene)-, —SO2—, —CO—, —NR′—, optionally-substituted heterocyclyl and optionally-substituted aryl in which R′ is H or C1-C6 alkyl;
• and wherein X1 and X2 may be linked to one another or to Z1 to form one or more ring structures;
• and
• wherein the remaining R1, R2, R7 and R8 are selected from H or C1-C6 alkyl.