Probing Side Chain Dynamics of Branched Macromolecules by Pyrene Excimer Fluorescence

Abstract

Four different pyrene-labeled polymers were prepared by radical copolymerization of n-butyl methacrylate (BMA) and 1-pyrenemethyl methacrylate (PyEG(0)-MA), 1-pyrenemethoxyethyl methacrylate (PyEG(1)-MA), 1-pyrene-methoxyethoxyethyl methacrylate (PyEG(2)-MA), and 1-pyrene-methoxydiethoxyethyl methacrylate (PyEG(3)-MA) to yield PyEG(0)-PBMA, PyEG(1)-PBMA, PyEG(2)-PBMA, and PyEG(3)-PBMA, respectively. The only structural difference between the polymers was the length of the oligo(ethylene glycol) spacer separating the pyrene label from the main chain. Steady-state and time-resolved fluorescence were applied to investigate how the length of the spacer affected the photophysical properties of the pyrene-labeled polymers. Excimer formation between an excited-state and a ground-state pyrene was enhanced by a longer spacer which increased the probability of encounter between two pyrene labels. This conclusion was supported through the analysis of the fluorescence decays of the polymers according to the fluorescence blob model (FBM) which yielded the number (N-blob) of monomers constituting the volume in the polymer coil probed by an excited pyrene and the rate constant of excimer formation, k(blob), inside a blob. N-blob increased more or less linearly with increasing spacer length reflecting a larger blob volume. k(blob) for PyEG(0)-PBMA was small due to steric hindrance while k(blob) took a larger but similar value within experimental error for all polymers labeled with pyrene derivatives having oligo(ethylene glycol) spacers. These experiments demonstrate that for a branched macromolecule the volume probed by the tip of a side chain and its dynamics can be characterized quantitatively by monitoring pyrene excimer fluorescence. They are expected to provide important dynamic and structural information about the numerous highly branched macromolecules that are currently under intense scientific scrutiny.