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 Ka = 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.