Internal Dynamics of Pyrene-Labeled Polyols Studied Through the Lens of Pyrene Excimer Formation

Series of pyrene-labeled diols (Py2-DOs) and polyols (Py-POs) were synthesized by coupling a number (nPyBA) of 1-pyrenebutyric acids to diols and polyols to yield series of end-labeled linear (nPyBA = 2) and branched (nPyBA > 2) oligomers, respectively. Pyrene excimer formation (PEF) between an excited and a ground-state pyrene was studied for the Py2-DO and Py-PO samples by analyzing their fluorescence spectra and decays in tetrahydrofuran, dioxane, N,N-dimethylformamide, and dimethyl sulfoxide. Global model-free analysis (MFA) of the pyrene monomer and excimer fluorescence decays yielded the average rate constant (<k>) for PEF. After the calculation of the local pyrene concentration ([Py]loc) for the Py2-DO and Py-PO samples, the <k>-vs.-[Py]loc plots were linear in each solvent, with larger and smaller slopes for the Py2-DO and Py-PO samples, respectively, resulting in a clear kink in the middle of the plot. The difference in slope was attributed to a bias for PEF between pyrenes close to one another on the densely branched Py-PO constructs resulting in lower apparent [Py]loc and <k> values. This study illustrated the ability of PEF to probe how steric hindrance along a main chain affects the dynamic encounters between substituents in multifunctional oligomers such as diols and polyols.