Detection and Mechanistic Studies of Multicomponent Assembly by Fluorescence Resonance Energy Transfer

The kinetics and thermodynamics of multicomponent assembly in organic solution are investigated using fluorescence resonance energy transfer (FRET). Calix[4]arenes functionalized with ureas on their wider rims dimerize in organic solution, via intermolecular hydrogen bonding, to form encapsulation complexes. When one calixarene component is outfitted with a donor fluorophore and the other with an acceptor, dimerization brings the chromophores within 20 Å, a distance suitable for efficient energy transfer to occur between them. Excitation of the donor results in two colors of emitted light:  one fluorescence band at the donor emission wavelength, and a second at the acceptor emission wavelength signaling the noncovalent union of three speciesdonor, acceptor, and small-molecule guest. By monitoring these wavelengths, assembly and dissociation processes are observed in real time at nanomolar concentrations. Rate and association constants for assembly processes are determined for the first time, and they reveal unexpected contributions from the structure and concentration of the monomer. Finally, the combination of FRET and the molecular recognition capabilities of the encapsulation complexes provides a sensitive and specific method for small-molecule sensing.