Near-infrared luminescent bis-squaraine dyes linked by a thiophene or pyrene spacer for noncovalent protein labeling

Novel bis-squaraine fluorophores with near-infrared emissions have been synthesized, in which two squaraine moieties are conjugatively linked by a thiophene, oligothiophene or pyrene spacer. Bis-squaraine dyes linked by a monothiophenc or pyrene spacer exhibit enhanced fluorescence by the formation of 1:1 dye-protein complexes when bound as noncovalent labeling probes to human serum albumin (HSA) and bovine serum albumin (BSA).