Dendrimers functionalized with a single pyrene label: Synthesis, photophysics, and fluorescence quenching

We have used a wide variety of molecular (i.e., nitromethane, acrylamide, N,N'-dimethylaniline, and methyl iodide) and ionic (iodide and cupric ions) quenchers to assess the relative structural permeabilities of a single, pyrenyl residue attached to the tertiary amine within a series of asymmetric poly(amido) dendrimers possessing carboxylate moieties at their periphery. From these quenching experiments, chain segmental densities and pyrene accessibility are probed as a function of dendrimer generation number (Pn, n = 1, 2, or 3), providing insight into the roles of size and electrostatics in this process. With the exception of dendrimer quenching by Cu²⁺, we observe classic Stern-Volmer behavior for Pn fluorescence quenching by all quenching agents. The recovered Stern-Volmer quenching constants (K_sv) and bimolecular quenching rates (k_q) generally decrease as n increases. This result is explained by a blocking of the pyrenyl residue by the growing dendrimer network. The decrease is particularly dramatic for the anionic heavy atom quencher I^-. This observation is rationalized in terms of pronounced electrostatic repulsion between the I^- quencher and the terminal COO^- residues of the dendrimer combined with an increase in the molecular network density surrounding the pyrenyl moiety as n increases. The Cu²⁺ quenching of the dendrimers is inconsistent with a diffusion-controlled reaction. Binding between the dendrimer and the Cu²⁺ is demonstrated.