Conjugated Polyelectrolyte Dendrimers: Aggregation, Photophysics, and Amplified Quenching

Conjugated polyelectrolyte dendrimers (CPDs) are monodisperse macromolecules that feature a fully pi-conjugated dendrimer core surrounded on the periphery by ionic solubilizing groups. CPDs are soluble in water and polar organic solvents, and they exhibit photophysics characteristic of the pi-conjugated chromophores comprising the dendrimer core. Here we describe the synthesis and photophysical characterization of series of three generations of CPDs based on a phenylene ethynylene repeat unit structure that is surrounded by an array of anionic sodium carboxylate groups. Molecular dynamics simulations indicate that the first-generation CPD is flat while the second- and third-generation CPDs adopt oblate structures. Photophysical studies, including absorption, fluorescence spectroscopy, and lifetimes, show that the ester protected precursor dendrimers exhibit highly efficient blue fluorescence in THF solution emanating from the phenylene ethynylene chromophore that is in the dendrimer core. By contrast, the water-soluble CPDs have much lower fluorescence quantum yields and the absorption and fluorescence spectra exhibit features of strong chromophore chromophore interactions. The results are interpreted as suggesting that the CPDs exist as dimer or multimer aggregates, even in very dilute solution. Fluorescence quenching of the anionic CPDs with the dication electron acceptor N,N'-dimethylviologen (MV2+) is very efficient, with Stern-Volmer quenching constants (K-SV) increasing with generation number. The third-generation CPD exhibits highly efficient amplified quenching, with K-SV similar to 5 x 10(6) M-1.