Quantitative Characterization of the Molecular Dimensions of Flexible Dendritic Macromolecules by Pyrene Excimer Fluorescence

Abstract

Eight pyrene labeled dendrons (PyLDs) were prepared with a polyester backbone of bis(hydroxymethyl)propionic acid and the monomer and excimer fluorescence decays of the PyLDs were acquired and fitted according to the recently introduced model free analysis (MFA). The average rate constant of pyrene excimer formation <k> retrieved from the MFA of the decays was found to scale as (2N1)×l3/<LPy2>1.5 where N represents the number of ground-state pyrene labels in a dendrimer where one of the pyrene is already excited, l is the average bond length, and <LPy2> is the average squared end-to-end distance between every two pyrene labels. The remarkable agreement found between these two quantities, namely <k> and (2N1)×l3/<LPy2>1.5 which were determined experimentally and mathematically, respectively, provides strong support to the notion that pyrene excimer fluorescence (PEF) responds to <LPy2> for pyrene labels attached to macromolecular constructs. In turn, <LPy2>0.5 can be used as a measure of the dimension of the macromolecular object onto which the pyrene labels are covalently attached. Since (2N1)/<LPy2>1.5 is the local concentration of ground-state pyrenes in the PyLD, the ratio <k>/[(2N1)/<LPy2>1.5] yields the bimolecular rate constant for excimer formation kdiff which was found to equal 5.0 (±0.6)×109 s for the PyLDs. Consequently, this study demonstrates that PEF applied to macromolecules yields a quantitative measure of their dimension and internal dynamics and since the rate constant of excimer formation is not distance-dependent, provides a much simpler mathematical alternative to experiments based on fluorescence resonance energy transfer (FRET).