Ultra bright dimeric or polymeric dyes with spacing linker groups

A compound having the following structure (IA):

• or a stereoisomer, salt or tautomer thereof, wherein:
• M is, at each occurrence, independently a fluorescent dye, wherein at least one occurrence of M has absorbance for use in fluorescence resonance energy transfer (FRET), and at least one occurrence of M has emissions for use in FRET;
• L1 is at each occurrence, independently either i) an alkylene linker;
• ii) a heteroalkylene linker including at least one nitrogen atom;
• or iii) a linker comprising a triazolyl functional group;
• L2 and L3 are, at each occurrence, independently an alkylene or heteroalkylene linker;
• R1 is, at each occurrence, H;
• R2 is Q, or a protected form thereof, or a linker comprising a covalent bond to Q;
• R3 comprises a deoxythymidine (dT) group;
• R4 is, at each occurrence, OH, O− or ORd;
• R5 is, at each occurrence, oxo;
• Rd is a counter ion;
• Q is a moiety comprising a sulfhydryl, disulfide, activated ester, isothiocyanate, azide, alkyne, alkene, diene, dienophile, acid halide, sulfonyl halide, phosphine, «-haloamide, biotin, amino or maleimide functional group, wherein the activated ester is an N-succinimide ester, imidoester or polyflourophenyl ester, and wherein the azide is an alkyl azide or acyl azide;
• m is, at each occurrence, independently an integer of one to 10;
• n is an integer of one to 32;
• and
• z is an integer from 2 to 25.