Application of Time-Resolved Fluorescence Anisotropy To Probe Quinoline-Based Foldamers Labeled with Oligo(phenylene vinylene)
Abstract
A series of oligoquinoline carboxamides (Q(n) with n = 1-32) were prepared with an oligo(phenylene vinylene) (OPV) fluorophore covalently attached at one end via a rigid amide bond. The fluorescence decays of the OPV-O-n solutions in chloroform were acquired with a vertically polarized excitation and an emission that was either vertically or horizontally polarized to yield the fluorescence decays I-VV(t) and I-VH(t), respectively. The I-VV(t) and I-VH(t) decays were fitted globally assuming a monoexponential anisotropy. The fits were good, indicating that the theoretical triexponential anisotropy of these rigid symmetric top macromolecules was well represented by a single exponential over the range of Q(n) lengths studied. The rotational time retrieved from the global analysis of the I-VV(t) and I-VH(t) fluorescence decays was found to increase linearly with increasing oligoquinoline chain length in agreement with the notion that these macromolecules were foldamers that adopted a rigid helical conformation in solution. Furthermore, the hydrodynamic volume of the OPV-Q(n) constructs determined from their rotational times perfectly matched that expected from the known dimensions of the Q(n) samples obtained from single-crystal X-ray diffraction. Unfortunately, the small aspect ratio of the foldamers prevented the resolution of the separate rotational times that would be expected from symmetric top macromolecules whose geometry could be described as ellipsoids or cylinders. Consequently, reliable values for the D-parallel to and D-perpendicular to diffusion coefficients representing the tumbling of the foldamers along and perpendicular to the main axis of the symmetric top macromolecule could not be obtained for the OPV-Q(n) foldamers. Nevertheless, the excellent correlation found between the foldamer size and rotational time suggests that time-resolved fluorescence anisotropy is a robust experimental technique to characterize the size and conformation of rigid foldamers in solution.