Credit: © 2008 Wiley

Organic chromophores, where the difference between two molecular orbitals is in the visible light portion of the electromagnetic spectrum, have useful properties that can be widely tailored. Pyrene (analogous to four fused benzene rings) is an important example because it can form inter- and intramolecular excimers. These have been used to study the viscosity of micelles and biological membranes. Although pyrenes have been incorporated into dendrimers to investigate the interactions between branches, they have only been used at the core or the periphery.

Now, Klaus Müllen and Klaas Zachariasse and colleagues from the Max Planck Institutes for Polymer Research and Biophysical Chemistry have made1 first- and second-generation dendrimers consisting solely of pyrenes. The group used a step-wise strategy to build up the dendrimer, involving Suzuki–Miyaura coupling of the relevant bromides and boronic esters. Model compounds, with just two or three pyrenes compared with the five or 17 in the dendrimers, were also made for comparison. The steric hindrance between each pyrene unit results in each branch of the dendrimer being twisted up to 90° with respect to the previous one.

Absorption and fluorescence spectra revealed that the intramolecular interaction between pyrene units increased from the first- to second-generation dendrimers. Intermolecular excimer formation between dendrimers was ruled out by the low concentrations and short fluorescence lifetimes of around two nanoseconds. The large extinction coefficient — how strongly they absorb light — and high fluorescence quantum yield make these dendrimers good candidates for use as fluorescence labels.