Self-Assembly of Fluorescent Inclusion Complexes in Competitive Media Including the Interior of Living Cells

Anthracene-containing tetralactam macrocycles are prepared and found to have an extremely high affinity for squaraine dyes in chloroform (log K_a = 5.2). Simply mixing the two components produces highly fluorescent, near-infrared inclusion complexes in quantitative yield. An X-ray crystal structure shows the expected hydrogen bonding between the squaraine oxygens and the macrocycle amide NH residues, and a high degree of cofacial aromatic stacking. The kinetics and thermodynamics of the assembly process are very sensitive to small structural changes in the binding partners. For example, a macrocycle containing two isophthalamide units associates with the squaraine dye in chloroform 400 000 times faster than an analogous macrocycle containing two 2,6-dicarboxamidopyridine units. Squaraine encapsulation also occurs in highly competitive media such as mixed aqueous/organic solutions, vesicle membranes, and the organelles within living cells. The highly fluorescent inclusion complexes possess emergent properties; that is, as compared to the building blocks, the complexes have improved chemical stabilities, red-shifted absorption/emission maxima, and different cell localization propensities. These are useful properties for new classes of near-infrared fluorescent imaging probes.