Internal Dynamics of Pyrene-Labeled Polyamines Probed by Pyrene Excimer Formation (PEF)

A series of pyrene-labeled diamines (Py2-DA) and polyethyleneamines (Py-PA) were synthesized with 1-pyrenebutyric acid to yield a set of linear Py2-DA samples end-labeled with 2 pyrenes and a set of branched Py-PA samples with more than 2 pyrenes. Fitting the fluorescence decays of dilute solutions of the Py2-DA and Py-PA samples in tetrahydrofuran (THF), dioxane, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) with the model-free analysis (MFA) yielded the experimental average rate constant (<k>exp) of pyrene excimer formation (PEF) for each sample. The local pyrene concentration ([Py]loc) was calculated for the Py2-DA and Py-PA samples assuming that the internal segments linking every pair of pyrene end groups obeyed Gaussian statistics. Plots of <k>exp-vs.-[Py]loc yielded a straight line that passed through the origin in the polar solvents DMF and DMSO but two straight lines in less polar THF and dioxane with a breakpoint between the linear Py2-DA and the branched Py-PA samples. Parameterizing the Py2-DA and Py-PA samples yielded the theoretical <k>th value of the Py-PA samples. Comparison of the <k>th and <k>exp values identified three main effects controlling PEF in the Py2-DA and Py-PA samples. First, shortening the spacer between two pyrenyl labels led to a significant increase in PEF due to a reduction in the possible pathways for pyrene–pyrene encounters. Second, steric hindrance by pyrenyl groups near an excited pyrene restricted access to pyrenyl labels further along the chain. Third, a transition from an sp3 to an sp2 hybridization of the amide nitrogens in the Py-PA backbone reduced the steric hindrance toward PEF. Accounting for these three effects enabled the complete description of the PEF process in these pyrene-labeled multifunctional oligomers. Consequently, this study allows the future use of the experimental observable <k>exp for determining [Py]loc reflecting local macromolecular density and conformation even for these macromolecules having a very high level of pyrene labeling and [Py]loc like the Py-PA samples.