Ultrabright and Serum-Stable Squaraine Dyes.

Highly stable symmetric and asymmetric squaraine fluorophores have been synthesized featuring an internal salt bridge between a quaternary ammonium cation and the central oxycyclobutenolate ring of the chromophore. Some of our newly synthesized symmetric and asymmetric compounds display increased molar absorptivity, quantum yield in serum, and thermal/photochemical stability over previously reported squaraine-based dyes. Consequently, both classes show great promise in resurfacing the normal environment-labile squaraine dyes as novel imaging agents and scaffolds for fluorescence sensing. Furthermore, incorporating a covalent attachment point away from the conjugated system allows for biological tagging applications without disturbing the optimum optical characteristics of the newly designed fluorophore.