Characterization of the Long-Range Internal Dynamics of Pyrene-Labeled Macromolecules by Pyrene Excimer Fluorescence

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 (<i>N</i>,<i>N</i>-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 ⟨<i>k</i>⟩, the average rate constant of pyrene excimer formation in the PyLMs, and (<i>I</i>_E/<i>I</i>_M)^TRF(<i>f</i>_free = 0), the ratio of excimer to monomer fluorescence intensity obtained in the absence of pyrene labels that do not form excimer. (<i>I</i>_E/<i>I</i>_M)^TRF(<i>f</i>_free = 0) was found to increase linearly with increasing ⟨<i>k</i>⟩ values over 3 orders of magnitude in the four solvents studied with a slope that equaled the average lifetime of the pyrene excimer (τ_E). The ⟨<i>k</i>⟩ values obtained to build these master lines could be correlated back to the expected LRID of the macromolecules. The lowest ⟨<i>k</i>⟩ 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 ⟨<i>k</i>⟩ 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>I</i>_E/<i>I</i>_M)^TRF(<i>f</i>_free = 0) vs ⟨<i>k</i>⟩ 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