Fluorescent dyes useful for detecting a target material in biological assays an electron donating moiety which is linked by a conjugated π-electron bridge to an electron accepting moiety. The π-electron bridge includes at least one carbocyclic ring structure or heterocyclic ring structure. The electron accepting moiety is a carboxylic acid, a salt of a carboxylic acid, or has the formula: wherein:X8 is selected from the group consisting of H, CH3, and (CH2)n6-Z2−, where Z2− is a monovalent anion; and n6 is an integer from 1 through 10; andQ3 is O or S.