Eight series of poly(alkyl methacrylate)s bearing different side chains and one series of poly(methyl acrylate) were randomly labeled with pyrene, and their ability to form pyrene excimer was characterized quantitatively by steady-state and time-resolved fluorescence to demonstrate that such measurements provide a quantitative measure of polymer chain dynamics (PCD) in solution. Each series of pyrene-labeled polymer showed increased excimer formation with increasing pyrene content, but the increase was more pronounced for the polymers known to be more flexible based on their reported glass transition temperature (T g ). In the case of the poly(alkyl methacrylate)s with a linear side chain, a shorter side chain resulted in increased excimer formation. Replacing a linear side chain with a more rigid one containing the same number of carbon atoms resulted in decreased mobility of the polymer. Fluorescence Blob Model (FBM) analysis of the fluorescence decays provided a more accurate representation of those pyrenes that formed excimer by diffusion and thus reflected PCD more precisely. Global FBM analysis of the pyrene monomer and excimer fluorescence decays yielded the blob size N blob and the product k blob × N blob which reflects PCD. For each series, both N blob and k blob × N blob remained constant with pyrene content. Their average value ⟨N blob ⟩ and ⟨k blob × N blob ⟩ decreased substantially with increasing side-chain length, addition of the α-methyl substituent to poly(methyl acrylate) to yield poly(methyl methacrylate), or increased rigidity of the side chain, demonstrating that an increase in bulkiness or stiffening of the side-or main chain is associated with a pronounced decrease in chain mobility. These experiments are the first to demonstrate that pyrene excimer formation can be used to characterize quantitatively PCD in solution in the same manner that T g is being used to characterize PCD in the bulk.

ABSTRACT

The fluorescent probe 1-pyrene­methoxy­ethanol (PyMe­EGOH) was designed to replace commercially available 1-pyrene­butanol (PyButOH) as an alternative fluorescent label to probe the internal dynamics and interior polarity of macro­molecules by steady-state and time-resolved fluorescence. While excimer formation and sensitivity to solvent polarity are two well-recognized properties of pyrene, much less known is that these properties are often mutually exclusive when a 1-pyrene­butyl derivative is used to prepare pyrene-labeled macro­molecules (PyLMs). As the sensitivity of pyrene to solvent polarity is a result of its symmetry, attaching a butyl group to pyrene breaks the symmetry of pyrene, so that the 1-pyrene­butyl derivatives are much less sensitive to the polarity of their environment compared to unmodified pyrene. This report demonstrates that replacement of a methylene group in the β-position of PyButOH by an oxygen atom, such as in PyMe­EGOH, restores the sensitivity of this pyrene derivative to the polarity of its local environment to the same level as that of molecular pyrene without impeding pyrene excimer formation upon incorporation into PyLMs.

Four different pyrene-labeled polymers were prepared by radical copolymerization of n-butyl methacrylate (BMA) and 1-pyrenemethyl methacrylate (PyEG0MA), 1-pyrenemethoxyethyl methacrylate (PyEG1MA), 1-pyrenemethoxyethoxyethyl methacrylate (PyEG2MA), and 1-pyrenemethoxydiethoxyethyl methacrylate (PyEG3MA) to yield PyEG0-PBMA, PyEG1-PBMA, PyEG2-PBMA, and PyEG3-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 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 (Nblob) of monomers constituting the volume in the polymer coil probed by an excited pyrene and the rate constant of excimer formation, kblob, inside a blob. Nblob increased more or less linearly with increasing spacer length reflecting a larger blob volume. kblob for PyEG0-PBMA was small due to steric hindrance while kblob 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.2

The long-range internal dynamics (LRID) of 74 pyrene-labeled macromolecules (PyLMs) were characterized in four solvents representing a broad range of dielectric constants equal to 2.4 (toluene), 7.6 (tetra­hydrofuran, THF), 37.8 (N,N-dimethyl­formamide, DMF), and 46.7 (dimethyl sulfoxide, DMSO). The LRID of the PyLMs were quantified based on the parameters retrieved from the global model free analysis (MFA) of the time-resolved fluorescence (TRF) decays of the pyrene monomer and excimer. These parameters were combined to yield ⟨k⟩, the average rate constant of pyrene excimer formation in the PyLMs, and (I E/I M)TRF(f free = 0), the ratio of excimer to monomer fluorescence intensity obtained in the absence of pyrene labels that do not form excimer. (I E/I M)TRF(f free = 0) was found to increase linearly with increasing ⟨k⟩ values over 3 orders of magnitude in the four solvents studied with a slope that equaled the average lifetime of the pyrene excimer (τE). The ⟨k⟩ values obtained to build these master lines could be correlated back to the expected LRID of the macromolecules. The lowest ⟨k⟩ values were obtained for the end-labeled linear chains holding the pyrene groups far apart, whereas the branched macromolecules bringing the pyrene labels close to each other yielded the larger ⟨k⟩ values. Furthermore, the fact that these master curves were observed for so many different PyLMs in the four solvents covering such a broad range of solvent polarity suggests that this relationship represents a general physical phenomenon that applies to all PyLMs. Considering the importance of characterizing the LRID of macromolecules in solution, the (I E/I M)TRF(f free = 0) vs ⟨k⟩ plots presented in this report can be viewed as calibration curves against which the LRID of any PyLM can now be compared. Thus, the 3 orders of magnitude range found for these master curves offers the scientific community an impressive analytical opportunity to gauge the LRID of macromolecules in solution.