A photostable polymeric tandem dye comprising: a water soluble light harvesting polymeric multichromophore comprising a conjugated segment comprising a fluorene co-monomer substituted with at least one water solubilizing group; and a luminescent ruthenium complex covalently linked to the polymeric multichromophore in energy-receiving proximity therewith, wherein the fluorene co-monomer is substituted with at least one non-ionic side group capable of imparting solubility in water in excess of 10 mg/mL, wherein the polymeric tandem dye is described by formula (X): wherein: F 1 is the fluorene co-monomer; each M 1 and M 2 are independently a co-monomer; L 1 is a linking co-monomer substituted with a linked luminescent metal complex Z 1 ; e is 1; a, b, c, d and f are each independently 0 or 1, wherein a+b+c+d+f ≥1; n is 0 or an integer from 1 to 10,000; m is 0 or an integer from 1 to 10,000; p is an integer from 1 to 100,000; and G 1 and G 2 are each independently selected from a terminal group, a π conjugated segment, a linker and a linked specific binding member, wherein the luminescent ruthenium complex comprises a substituted multidentate metal chelating ligand of the structure: wherein R 11 to R 18 are each independently selected from H, alkyl, substituted alkyl, heteroaryl, substituted heteroaryl, aryl, substituted aryl and linker, wherein at least one of R 11 to R 18 is covalently linked to the linking co-monomer, wherein the polymeric dye has an average molecular weight in the range of 10,000 to 100,000. a water soluble light harvesting polymeric multichromophore comprising a conjugated segment comprising a fluorene co-monomer substituted with at least one water solubilizing group; and a luminescent ruthenium complex covalently linked to the polymeric multichromophore in energy-receiving proximity therewith, wherein the fluorene co-monomer is substituted with at least one non-ionic side group capable of imparting solubility in water in excess of 10 mg/mL, wherein the polymeric tandem dye is described by formula (X): wherein: F 1 is the fluorene co-monomer; each M 1 and M 2 are independently a co-monomer; L 1 is a linking co-monomer substituted with a linked luminescent metal complex Z 1 ; e is 1; a, b, c, d and f are each independently 0 or 1, wherein a+b+c+d+f ≥1; n is 0 or an integer from 1 to 10,000; m is 0 or an integer from 1 to 10,000; p is an integer from 1 to 100,000; and G 1 and G 2 are each independently selected from a terminal group, a π conjugated segment, a linker and a linked specific binding member, wherein the luminescent ruthenium complex comprises a substituted multidentate metal chelating ligand of the structure: wherein R 11 to R 18 are each independently selected from H, alkyl, substituted alkyl, heteroaryl, substituted heteroaryl, aryl, substituted aryl and linker, wherein at least one of R 11 to R 18 is covalently linked to the linking co-monomer, wherein the polymeric dye has an average molecular weight in the range of 10,000 to 100,000. F 1 is the fluorene co-monomer; each M 1 and M 2 are independently a co-monomer; L 1 is a linking co-monomer substituted with a linked luminescent metal complex Z 1 ; e is 1; a, b, c, d and f are each independently 0 or 1, wherein a+b+c+d+f ≥1; n is 0 or an integer from 1 to 10,000; m is 0 or an integer from 1 to 10,000; p is an integer from 1 to 100,000; and G 1 and G 2 are each independently selected from a terminal group, a π conjugated segment, a linker and a linked specific binding member, wherein the luminescent ruthenium complex comprises a substituted multidentate metal chelating ligand of the structure: wherein R 11 to R 18 are each independently selected from H, alkyl, substituted alkyl, heteroaryl, substituted heteroaryl, aryl, substituted aryl and linker, wherein at least one of R 11 to R 18 is covalently linked to the linking co-monomer, wherein the polymeric dye has an average molecular weight in the range of 10,000 to 100,000. <EOS>