Lithographic printing plate precursor

1. A lithographic printing plate precursor comprising a support which includes a hydrophilic surface or which is provided with a hydrophilic layer and a coating thereon, said coating comprising an IR absorbing agent, a contrast enhancing compound and a polymeric binder that alters its solubility in an alkaline developing solution upon exposure, wherein said contrast enhancing compound has the structure of formula I:

wherein

R 1 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —NR 4 R 5 , —CO—NR 4 R 5 , —SO 2 —NR 4 R 5 , —COR 6 , —CN, —NO 2 , —COOR 6 , —OR 3 , —SR 3 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 , —NR 6 —CO—NR 4 R 5 , —O—COOR 6 , —NR 4 —COOR 5 , —NR 4 —CO—R 5 or a phosphoramidate group;

R 2 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —SO 2 —NR 4 R 5 , —CN, —NO 2 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 or a phosphoramidate group;

R 3 is an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group;

R 4 , R 5 and R 6 independently are hydrogen or one of the groups as defined for R 3 , or wherein two groups selected from the group consisting of R 4 , R 5 and R 6 together comprise the atoms necessary to form a ring;

Q is one of the following groups which forms an optionally substituted 6 membered heteroaromatic ring, said groups being selected from the group consisting of **—C(T 2 )-N—N—*, **—N—N—C(T 2 )-*, **—N—C(T 2 )-C(T 3 )-*, **—C(T 2 )-N—C(T 3 )-*, **—C(T 2 )-C(T 3 )-C(T 4 )-*, **—C(T 2 )-C(T 1 )-N—*, **—N—C(T 1 )-N—* and **—N—N—N—*, or Q is one of the following groups which forms an optionally substituted 5 membered heteroaromatic ring, said groups being selected from the group consisting of **—C(T 1 )-N(T 2 )-*, **—C(T 2 )-S—*, **—C(T 2 )-O—*, **—N—N(T 2 )-*, **—N—S—*, **—N—O—*, **—N(T 2 )-C(T 3 )-*, **—S—N—* and **—O—N—*,

wherein

* indicates a binding site to the carbon atom between the two nitrogen atoms and ** indicates a binding site to the carbon atom substituted by R 1 ; and

wherein

the symbol “O” in the middle of the ring comprising Q represents the pi-electrons necessary for the aromatic ring;

T 1 is one of the groups as defined for R 1 ;

T 2 , T 3 and T 4 independently are one of the groups as defined for R 2 ; or

wherein one of the groups of T 1 , T 2 , T 3 and T 4 together with one of the groups of R 1 comprise the atoms necessary to form a ring; or

wherein one of the groups of T 1 , T 2 , T 3 and T 4 together with one of the groups of R 2 comprise the atoms necessary to form a ring; or

wherein two groups, selected from the group consisting of T 1 , T 2 , T 3 and T 4 , comprise the atoms necessary to form a ring.

2. The lithographic printing plate precursor according to claim 1 , wherein said binder is a polymer comprising a sulphonamide group.

3. The lithographic printing plate precursor according to claim 2 , wherein said sulphonamide group is present in the side chain of said polymer.

4. The lithographic printing plate precursor according to claim 1 , wherein said binder is a phenolic resin.

5. A lithographic printing plate precursor comprising a support which includes a hydrophilic surface or which is provided with a hydrophilic layer and a coating thereon, said coating comprising an IR absorbing agent, a contrast enhancing compound and a polymeric binder that alters its solubility in an alkaline developing solution upon exposure, wherein said contrast enhancing compound has the structure of formula III:

wherein

Y is a nitrogen atom or a carbon atom;

X is the atom or group of atoms necessary to form an optionally-substituted five- or six-membered heteroaromatic ring;

Z is the atom or group of atoms necessary to form an optionally-substituted five- to eight-membered ring;

B 1 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —NR 4 R 5 , —CO—NR 4 R 5 , —SO 2 —NR 4 R 5 , —COR 6 , —CN, —NO 2 , —COOR 6 , —OR 3 , —SR 3 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 , —NR 6 —CO—NR 4 R 5 , —O—COOR 6 , —NR 4 —COOR 5 , —NR 4 —CO—R 5 or a phosphoramidate group; R 3 is an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group; R 4 , R 5 and R 6 independently are hydrogen or one of the groups as defined for R 3 , or wherein two groups selected from the group consisting of R 4 , R 5 and R 6 together comprise the atoms necessary to form a ring; and

the symbol “O” in the middle of the ring comprising X and Y represents a number of pi-electrons necessary for the aromatic ring.

6. A lithographic printing plate precursor comprising a support which includes a hydrophilic surface or which is provided with a hydrophilic layer and a coating thereon, said coating comprising an IR absorbing agent, a contrast enhancing compound and a polymeric binder that alters its solubility in an alkaline developing upon exposure, wherein said contrast enhancing compound has the structure of formula IV

wherein

K 1 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —NR 4 R 5 , —CO—NR 4 R 5 , —SO 2 —NR 4 R 5 , —COR 6 , —CN, —NO 2 , —COOR 6 , —OR 3 , —SR 3 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 , —NR 6 —CO—NR 4 R 5 , —O—COOR 6 , —NR 4 —COOR 5 , —NR 4 —CO—R 5 or a phosphoramidate group; R 3 is an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group; R 4 , R 5 and R 6 independently are hydrogen or one of the groups as defined for R 3 , or wherein two groups selected from the group consisting of R 4 , R 5 and R 6 together comprise the atoms necessary to form a ring; and

K 2 to K 5 independently are hydrogen, —NR 4 R 5 , —CO—NR 4 R 5 , —COR 6 , —COOR 6 , —OR 3 , —NR 6 —CO—NR 4 R 5 , —NR 4 —COOR 5 , or —NR 4 —CO—R 5 , or wherein two groups selected from the group consisting of K 2 , K 3 , K 4 and K 5 together comprise the atoms necessary to form a ring.

7. A lithographic printing plate precursor comprising a support which includes a hydrophilic surface or which is provided with a hydrophilic layer and a coating thereon, said coating comprising an IR absorbing agent, a contrast enhancing compound and a polymeric binder that alters its solubility in an alkaline developing solution upon exposure, wherein said contrast enhancing compound has the structure of formula V

wherein

M 1 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —NR 4 R 5 , —CO—NR 4 R 5 , —SO 2 —NR 4 R 5 , —COR 6 , —CN, —NO 2 , —COOR 6 , —OR 3 , —SR 3 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 , —NR 6 —CO—NR 4 R 5 , —O—COOR 6 , —NR 4 —COOR 5 , —NR 4 —CO—R 5 or a phosphoramidate group R 3 is an optionally substituted alkyl, alkenyl, alkenyl, aryl, alkaryl, aralkyl or heteroaryl group; R 4 , R 5 and R 6 independently are hydrogen or one of the groups as defined for R 3 , or wherein two groups selected from the group consisting of R 4 , R 5 and R 6 together comprise the atoms necessary to form a ring; and

M 2 to M 6 independently are hydrogen, —NR 4 R 5 , —CO—NR 4 R 5 , —COR 6 , —COOR 6 , —OR 3 , —NR 6 —CO—NR 4 R 5 , —NR 4 —COOR 5 , —NR 4 —CO—R 5 , or are one of R 3 , R 4 , R 5 and R 6 ; or wherein M 1 and M 2 together comprise the atoms necessary to form a ring; or wherein two groups, selected from the group consisting of M 2 to M 6 , together comprise the atoms necessary to form a ring.

8. A lithographic printing plate precursor comprising a support which includes a hydrophilic surface or which is provided with a hydrophilic layer and a coating thereon, said coating comprising an IR absorbing agent, a contrast enhancing compound and a polymeric binder that alters its solubility in an alkaline developing solution upon exposure, wherein said contrast enhancing compound has the structure of formula VI

wherein

V 1 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —NR 4 R 5 , —CO—NR 4 R 5 , —SO 2 —NR 4 R 5 , —COR 6 , —CN, —NO 2 , —COOR 6 , —OR 3 , —SR 3 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 , —NR 6 —CO—NR 4 R 5 , —O—COOR 6 , —NR 4 —COOR 5 , —NR 4 —CO—R 5 or a phosphoramidate group; R 3 is an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group; R 4 , R 5 and R 6 independently are hydrogen or one of the groups as defined for R 3 , or wherein two groups selected from the group consisting of R 4 , R 5 and R 6 together comprise the atoms necessary to form a ring; and

V 2 and V 3 independently are hydrogen or one of the groups as defined for R 3 ; and

V 4 is hydrogen or one of the groups as defined for R 3 ; or wherein two groups, selected from V 1 to V 3 , together comprise the atoms necessary to form a ring.

9. A lithographic printing plate precursor comprising a support which includes a hydrophilic surface or which is provided with a hydrophilic layer and a coating thereon, said coating comprising an IR absorbing agent, a contrast enhancing compound and a polymeric binder that alters its solubility in an alkaline developing solution upon exposure, wherein said contrast enhancing compound has the structure of formula I

wherein in formula I

R 1 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —NR 4 R 5 , —CO—NR 4 R 5 , —SO 2 —NR 4 R 5 , —COR 6 , —CN, —NO 2 , —COOR 6 , —OR 3 , —SR 3 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 , —NR 6 —CO—NR 4 R 5 , —O—COOR 6 , —NR 4 —COOR 5 , —NR 4 —CO—R 5 or a phosphoramidate group;

R 2 is hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group, halogen, —SO 2 —NR 4 R 5 , —CN, —NO 2 , —SOR 3 , —SO 2 R 6 , —SO 3 R 6 , —PO 4 R 4 R 5 , —PO 3 R 4 R 5 or a phosphoramidate group;

R 3 is an optionally substituted alkyl, alkenyl, alkynyl, aryl, alkaryl, aralkyl or heteroaryl group;

R 4 , R 5 and R 6 independently are hydrogen or one of the groups as defined for R 3 , or wherein two groups selected from the group consisting of R 4 , R 5 and R 6 together comprise the atoms necessary to form a ring;

Q is one of the following groups which forms an optionally substituted 6 membered heteroaromatic ring, said groups being selected from the group consisting of **—C(T 2 )—N—N—*, **—N—N—C(T 2 )-*, **—N—C(T 2 )-C(T 3 )-*, **—C(T 2 )-N—C(T 3 )-*, **—C(T 2 )-C(T 3 )-C(T 4 )-*, **—C(T 2 )-C(T 1 )-N—*, **—N—C(T 1 )-N—* and **—N—N—N—*, or Q is one of the following groups which forms an optionally substituted 5 membered heteroaromatic ring, said groups being selected from the group consisting of **—C(T 1 )-N(T 2 )-*, **—C(T 2 )-S—*, **—C(T 2 )-O—*, **—N—N(T 2 )-*, **—N—S—*, **—N—O—*, **—N(T 2 )-C(T 3 )-*, **—S—N—* and **—O—N—*,

wherein

* indicates a binding site to the carbon atom between the two nitrogen atoms and ** indicates a binding site to the carbon atom substituted by R 1 ; and

wherein

the symbol “O” in the middle of the ring comprising Q represents the pi-electrons necessary for the aromatic ring;

T 1 is one of the groups as defined for R 1 ;

T 2 , T 3 and T 4 independently are one of the groups as defined for R 2 ; or

wherein one of the groups of T 1 , T 2 , T 3 or T 4 together with one of the groups of R 1 comprise the atoms necessary to form a ring; or

wherein one of the groups of T 1 , T 2 , T 3 or T 4 together with one of the groups of R 2 comprise the atoms necessary to form a ring; or

wherein two groups, selected from T 1 , T 2 , T 3 or T 4 , comprise the atoms necessary to form a ring,

and wherein the contrast enhancing compound is linked to a polymer by a chemical bond formed between at least one atom of a group selected from the group consisting of R 1 , R 2 and T 1 to T 4 , and at least one atom of said polymer.

10. The lithographic printing plate precursor according to claim 9 , wherein said contrast enhancing compound is chemically bound to a side chain of said polymer and wherein said chemical bond is formed between at least one atom of a group selected from the group consisting of R 1 , R 2 and T 1 to T 4 and at least one atom of the side chain of said polymer, optionally by a linking group L between said side chain and said contrast enhancing compound.