Polymethine compounds with long stokes shifts and their use as fluorescent labels

A compound of formula (I′) or mesomeric forms thereof:

• wherein mCat+ or mAn− is an organic or inorganic positively/negatively charged counterion;
• m is an integer 0-3;
• x is an integer 0-2;
• Ra1 is H, SO3−, sulfonamide, halogen, hydroxy, alkoxy, amino or a further ring fused to an adjacent carbon atom where the ring is optionally substituted with one or more SO3−, sulfonamide, or halogen;
• Rb1 is SO3−, sulfonamide, halogen, hydroxy, alkoxy, amino, COOH or an amide or ester thereof;
• n is 0-3;
• each of Rc1 and Rc2 is independently alkyl or substituted alkyl;
• each of Rd1 and Rd2 is independently H, alkyl, aryl, substituted alkyl, or substituted aryl;
• and
• Re1 is alkyl, substituted alkyl, aryl or substituted aryl;
• wherein either Rc1, Rb1 or Re1 comprises a COOH or COO− or an amide or ester thereof.