A water-soluble compound of the formula: wherein R 1 to R 8 are each independently selected from hydrogen; optionally substituted alkyl groups; optionally substituted cycloalkyl groups; optionally substituted heterocyclyl groups; optionally substituted alkenyl groups; optionally substituted alkynyl groups; optionally substituted aryl groups; optionally substituted heteroaryl groups; alkoxyl groups; ketone and aldehyde groups; carboxylic acids and carboxylate ions; carboxylate esters; -SO 3 H; -SO 3 -;-OSO 3 H; -OSO 3 -; -PO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge; -OPO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge; amines; amides; halo groups; -CN; -NO 2; -OH; and imino and imido groups, provided that in any one or more of the pairs R 1 R 2, R 3 R 4, R 5 R 6 and R 7 R 8, the two substituents may be joined together to form part of an optionally substituted cyclic group; and R 9 and R 10 are each independently selected from hydrogen; optionally substituted alkyl groups; optionally substituted cycloalkyl groups; optionally substituted heterocyclyl groups; optionally substituted alkenyl groups; optionally substituted alkynyl groups; optionally substituted aryl groups; optionally substituted heteroaryl groups; alkoxyl groups; ketone and aldehyde groups; carboxylic acids and carboxylate ions; carboxylate esters; -SO 3 H; -SO 3 -; -OSO 3 H; -OSO 3 -;-PO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge;-OPO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge; amines; amides; halo groups; -CN; -NO 2; -OH; and imino and imido groups; or R 9 and R 10 are each independently selected from hydrogen or hydrophilic substituents containing one or more hydrophilic functional groups selected from polar groups; and wherein one or more of the substituents R 1 to R 10 is a hydrophilic substituent containing one or more hydrophilic functional groups selected from polar groups. wherein R 1 to R 8 are each independently selected from hydrogen; optionally substituted alkyl groups; optionally substituted cycloalkyl groups; optionally substituted heterocyclyl groups; optionally substituted alkenyl groups; optionally substituted alkynyl groups; optionally substituted aryl groups; optionally substituted heteroaryl groups; alkoxyl groups; ketone and aldehyde groups; carboxylic acids and carboxylate ions; carboxylate esters; -SO 3 H; -SO 3 -;-OSO 3 H; -OSO 3 -; -PO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge; -OPO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge; amines; amides; halo groups; -CN; -NO 2; -OH; and imino and imido groups, provided that in any one or more of the pairs R 1 R 2, R 3 R 4, R 5 R 6 and R 7 R 8, the two substituents may be joined together to form part of an optionally substituted cyclic group; and R 9 and R 10 are each independently selected from hydrogen; optionally substituted alkyl groups; optionally substituted cycloalkyl groups; optionally substituted heterocyclyl groups; optionally substituted alkenyl groups; optionally substituted alkynyl groups; optionally substituted aryl groups; optionally substituted heteroaryl groups; alkoxyl groups; ketone and aldehyde groups; carboxylic acids and carboxylate ions; carboxylate esters; -SO 3 H; -SO 3 -; -OSO 3 H; -OSO 3 -;-PO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge;-OPO 3 XY where X and Y are independently hydrogen, alkyl or a negative charge; amines; amides; halo groups; -CN; -NO 2; -OH; and imino and imido groups; or R 9 and R 10 are each independently selected from hydrogen or hydrophilic substituents containing one or more hydrophilic functional groups selected from polar groups; and wherein one or more of the substituents R 1 to R 10 is a hydrophilic substituent containing one or more hydrophilic functional groups selected from polar groups.