Dendrimers functionalized with a single fluorescent dansyl group attached "off center": Synthesis and photophysical studies

A series of three new fluorescent dendrimers containing a single, focally located dansyl group and 3 (1), 9 (2), and 27 (3) carboxylic acid groups in their peripheries were synthesized and characterized. The photophysical properties of these dendrimers were investigated in aqueous solution. The host-guest interactions of the dendrimers through their dansyl subunits with beta-cyclodextrin and polyclonal anti-dansyl antibodies were also investigated by various methods. Photophysical measurements on the dendrimers demonstrate that the dansyl residue is progressively shielded from the solvent as the dendrimer generation increases, resulting in marked changes in spectral features, fluorescence quantum yields, excited-state fluorescence lifetimes, radiative and nonradiative decay rates, and rotational reorientation times. The excited-state intensity decay kinetics for 1-3 are well described by a single exponential. Contrary to the popularly held belief that lower generation dendrimers are "floppy" species in solution, the molecular motions of 1-3 are described by a single rotational reorientation time. Access to the dansyl moiety is impeded with increasing dendrimer size as the dendrimer mass affords a significant degree of protection from binding by nonselective (beta-cyclodextrin (beta-CD)) and selective (anti-dansyl antibody) hosts for the dansyl residue. The equilibrium constant for beta-CD binding of the dansyl residue in 1 is similar to 2.5-fold lower than that for binding to dansylamine (DA). Dendrimers 2 and 3 do not associate with beta-CD at all. Anti-dansyl antibodies can bind to the dansyl residue in dendrimers 1-3 with remarkably large binding affinities. The equilibrium constant for the antibody complex decreases systematically from 5.0 x 10(7) M-1 for DA to 1.5 x 10(6) M-1 for 3.