Photoinitators

1. A photoinitiator of the formula (I)

in which R3, R4, R5 and R6 are selected from H or C 1 -C 6 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 —(C 1 -C 12 alkylene)-, optionally-substituted —(C 2 -C 12 alkenylene)-, —SO 2 —, —CO—, —NR′—, —Si(R′) 2 —, optionally-substituted heterocyclyl and optionally-substituted aryl in which R′ is H or C 1 -C 6 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 C 1 -C 6 alkyl; provided that—when Z2 is a nitrogen atom—the linker unit in Z1 adjacent to Z2 is —SO 2 — 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 —(C 1 -C 12 alkylene)-, optionally-substituted —(C 2 -C 12 alkenylene)-, —SO 2 —, —CO—, —NR′—, optionally-substituted heterocyclyl and optionally-substituted aryl in which R′ is H or C 1 -C 6 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 C 1 -C 6 alkyl.

2. The photoinitiator according to claim 1 , wherein one of R2, R7 or R8, has a structure of formula (Ia).

3. The photoinitiator according to claim 1 , wherein R3, R4, R5 and R6 are H or methyl.

4. The photoinitiator according to claim 3 , wherein each R3, R4, R5 and R6 is a H.

5. The photoinitiator according to claim 1 , wherein R1 and R2 are both methyl.

6. The photoinitiator according to claim 1 , wherein at least one linker unit in Z1 is selected from —SO 2 —, —O— or —CO—.

7. The photoinitiator according to claim 6 , wherein the at least one linker unit in Z1 is-SO 2 — or —CO—.

8. The photoinitiator according to claim 1 , wherein —Z1-Z2- has a structure selected from —SO 2 —N—; —CO—N—; —CO—O—CR″—; —O—CO—CR″—; alkylene)-SO 2 —N—; —(C 1 -C 12 alkylene)-CO—N—; —(C 1 -C 12 alkylene)-CO—O—CR″—; alkylene)-O—CO—CR″—, alkylene)-SO 2 —O—(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-SO 2 —NR′—(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-O—SO 2 —(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-NR′—SO 2 —(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-CO—O—(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-O—CO—(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-CO—NR′—(C 1 -C 12 alkylene)-Z2-; or —(C 1 -C 12 alkylene)-NR′—CO—(C 1 -C 12 alkylene)-Z2.

9. The photoinitiator according to claim 1 , wherein —Z1-Z2- has a structure selected from —SO 2 —N—; —CO—N—; —CO—O—CR″—; —O—CO—CR″; —(C 1 -C 12 alkylene)-SO 2 —N—; —(C 1 -C 12 alkylene)-CO—N—; —(C 1 -C 12 alkylene)-CO—O—CR″—; alkylene)-O—CO—CR″—; alkylene)-SO 2 —O—(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-SO 2 —NR′—(C 1 -C 12 alkylene)-Z2-; —(C 1 -C 12 alkylene)-CO—O—(C 1 -C 12 alkylene)-Z2-; or —(C 1 -C 12 alkylene)-CO—NR′—(C 1 -C 12 alkylene)-Z2-.

10. The photoinitiator according to claim 1 , wherein —Z1-Z2- has a structure selected from —SO 2 —N—; —CO—N—; —(C 1 -C 12 alkylene)-SO 2 —N—; —(C 1 -C 12 alkylene)-CO—N—; —(C 1 -C 12 alkylene)-SO 2 —O—(C 1 -C 12 alkylene)-Z2-; or —(C 1 -C 12 alkylene)-CO—O—(C 1 -C 12 alkylene)-Z2-.

11. The photoinitiator according to claim 1 , wherein Z2 is N.

12. The photoinitiator according to claim 1 , wherein Z2 is C(R″), wherein R″ is H, methyl, ethyl or propyl.

13. The photoinitiator according to claim 1 , wherein the —(C 1 -C 12 alkylene)- linker unit in Z1 is a —(C 1 -C 6 alkylene)-.

14. The photoinitiator according to claim 13 , wherein the —(C 1 -C 6 alkylene)- linker unit in Z1 is propylene, ethylene or methylene.

15. The photoinitiator according to claim 1 , wherein X1 and X2 are each independently selected from —O—, —S—, optionally-substituted —(C 1 -C 12 alkylene)-, —SO 2 —, —CO—, and —NR′— in which R′ is H or C 1 -C 6 alkyl.

16. The photoinitiator according to claim 15 , wherein X1 and X2 are each independently selected from —O—, —S—, or an optionally-substituted —(C 1 -C 12 alkylene)-.

17. The photoinitiator according to claim 1 , wherein the —(C 1 -C 12 alkylene)- linker unit in X1 and X2 is a —(C 1 -C 6 alkylene)-.

18. The photoinitiator according to claim 17 , wherein the —(C 1 -C 6 alkylene)- linker unit in X1 and X2 is a propylene, ethylene or methylene.

19. The photoinitiator according to claim 1 , wherein X1 and X2 are the same.

20. The photoinitiator according to claim 1 , wherein one of R1, R2, R7 or R8 is selected from:

21. The photoinitiator according to claim 1 , selected from: