Carboxylate Dyes

A carboxylate dye having the structure of Formula I:

• D-L-CO2M Formula I
• wherein D is a dye moiety selected from benzodifuranes, methines, triphenylmethanes, naphthalimides, pyrazoles, naphthoquinones, anthraquinones and mono- and di-azo dyes and mixtures thereof, azo dyes being particularly preferred, wherein the compound D-H, preferably has a maximum extinction coefficient greater than about 1000 liter/mol/cm at the λmax in the wavelength range from 400 nm to 750 nm in methanol solution;
• and L is an organic linking group preferably having a molecular weight from 14 to 1000 Daltons or 14 to 600 or 28 to 300, preferably consisting essentially only of C, H and optionally additionally O and/or N, and in the sequence of bonds starting from the carbonyl carbon of the C(O)OM group and ending at the dye moiety, any —(Ca(O)—Ob)— groups are incorporated in said L linking group such that the oxygen atom Ob is encountered prior to the carbonyl carbon Ca, preferably L is a C1-20 alkylene chain having optionally therein ether (—O—) and/or ester and/or amide links, the chain being optionally substituted for example with —OH, —CN, —NO2, —SO2CH3, —Cl, —Br;
• and M is any suitable counterion, typically hydrogen, sodium or potassium ion.