A macrocyclic tetralactam host is threaded by a highly fluorescent squaraine dye that is flanked by two polyethylene glycol (PEG) chains with nanomolar dissociation constants in water. Furthermore, the rates of bimolecular association are very fast with k(on) approximate to 10(6)-10(7) M-1 s(-1). The association is effective under cell culture conditions and produces large changes in dye optical properties including turn-on near-infrared fluorescence that can be imaged using cell microscopy. Association constants in water are similar to 1000 times higher than those in organic solvents and strongly enthalpically favored at 27 degrees C. The threading rate is hardly affected by the length of the PEG chains that flank the squaraine dye. For example, macrocycle threading by a dye conjugate with two appended PEG2000 chains is only three times slower than threading by a conjugate with triethylene glycol chains that are 20 times shorter. The results are a promising advance toward synthetic mimics of streptavidin/biotin.