A compound for use as a fluorescent pH sensitive dye, the compound being of Formula VI: wherein the aniline moiety, when protonated, results in the compound being fluorescent, whilst the compound is quenched when the aniline moiety is not in the protonated state; wherein R 1, R 3 and R 6 are each independently H, Z, or an electron donating group, provided that a Z group is present; R 4 is selected from the group consisting of H, alkyl, substituted alkyl, acyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, cycloalkyl, substituted cycloalkyl, a reactive group, a carrier molecule, heterocyclyl, and substituted heterocyclyl; R 5 is independently selected from the group consisting of Y, alkyl, substituted alkyl, alkenyl, substituted alkenyl, acyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, cycloalkyl, substituted cycloalkyl, a reactive group, a carrier molecule, heterocyclyl, and substituted heterocyclyl; wherein R 4 and R 5 are either: i) the same and both unsubstituted hydrocarbyl; or ii) different; wherein: A is a five- or six-membered heteroaromatic ring; Y is =CR b R c or =CR b R d; Z is -OR c, -SR c, -NR b R c; R a is selected from the group consisting of H, alkyl, substituted alkyl, alkoxy, substituted alkoxy, acyl, acylamino, acyloxy, amino, substituted amino, aminocarbonyl, aminothiocarbonyl, aminocarbonylamino, aminothiocarbonylamino, aminocarbonyloxy, aminosulfonyl, aminosulfonyloxy, aminosulfonylamino, amidino, carboxyl, carboxyl ester, (carboxyl ester)amino, (carboxyl ester)oxy, cyano, halo, hydroxy, nitro, -SO 3 H, sulfonyl, substituted sulfonyl, sulfonyloxy, thioacyl, thiol, alkylthio, substituted alkylthio, aryl, substituted aryl, heteroaryl, substituted heteroaryl, cycloalkyl, substituted cycloalkyl, heterocyclyl, and substituted heterocyclyl; R b is H, alkyl, or substituted alkyl; R c is alkyl or substituted alkyl; R d is amino or substituted amino; and n is 0, 1, 2, or 3; or a stereoisomer, tautomer, or salt thereof. R 1, R 3 and R 6 are each independently H, Z, or an electron donating group, provided that a Z group is present; R 4 is selected from the group consisting of H, alkyl, substituted alkyl, acyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, cycloalkyl, substituted cycloalkyl, a reactive group, a carrier molecule, heterocyclyl, and substituted heterocyclyl; R 5 is independently selected from the group consisting of Y, alkyl, substituted alkyl, alkenyl, substituted alkenyl, acyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, cycloalkyl, substituted cycloalkyl, a reactive group, a carrier molecule, heterocyclyl, and substituted heterocyclyl; i) the same and both unsubstituted hydrocarbyl; or ii) different; A is a five- or six-membered heteroaromatic ring; Y is =CR b R c or =CR b R d; Z is -OR c, -SR c, -NR b R c; R a is selected from the group consisting of H, alkyl, substituted alkyl, alkoxy, substituted alkoxy, acyl, acylamino, acyloxy, amino, substituted amino, aminocarbonyl, aminothiocarbonyl, aminocarbonylamino, aminothiocarbonylamino, aminocarbonyloxy, aminosulfonyl, aminosulfonyloxy, aminosulfonylamino, amidino, carboxyl, carboxyl ester, (carboxyl ester)amino, (carboxyl ester)oxy, cyano, halo, hydroxy, nitro, -SO 3 H, sulfonyl, substituted sulfonyl, sulfonyloxy, thioacyl, thiol, alkylthio, substituted alkylthio, aryl, substituted aryl, heteroaryl, substituted heteroaryl, cycloalkyl, substituted cycloalkyl, heterocyclyl, and substituted heterocyclyl; R b is H, alkyl, or substituted alkyl; R c is alkyl or substituted alkyl; R d is amino or substituted amino; and n is 0, 1, 2, or 3;