Patent ID: 7559959

Claim:
A compound of formula (I) wherein R 1 , R′ 1 , R 2 and R′ 2 are independently hydrogen, unbranched C 1-6 alkyl or branched C 3-6 alkyl, unbranched C 1-6 alkoxy or branched C 3-6 alkoxy, halogen or —NHCO—(C 1-6 alkyl) with an unbranched C 1-6 alkyl group or —NHCO—(C 3-6 alkyl) with a branched C 3-6 alkyl group, R 3 and R′ 3 are independently unbranched C 1-6 alkyl or branched C 3-6 alkyl, R 4 and R′ 4 are independently unbranched C 1-6 alkylene or branched C 3-6 alkylene or arylene or —CH 2 -arylene optionally substituted by unbranched C 1-6 alkyl or branched C 3-6 alkyl, —OH, —CN or by further —SO 3 H groups, and X is a bridge or a direct bond.