A study of the dynamics of the branch ends of a series of pyrene-labeled dendrimers based on pyrene excimer formation.

A series of pyrene-labeled dendrimers were prepared from generation n = 1 to n = 4 where the pyrenes were attached to the end groups of the dendrimers. Pyrene excimer formation was monitored by steady-state and time-resolved fluorescence spectroscopy as a function of generation number and in terms of the I(E)/I(M) ratio and the average rate constant of excimer formation <k>. To account for the unconventional distribution of pyrene labels which were neither randomly distributed throughout the macromolecule nor limited to just two units which are the only two pyrene-labeling schemes that can be dealt with in a straightforward manner, a Model Free (MF) analysis was applied to the global analysis of the fluorescence decays. Within experimental error, the I(E)/I(M) ratios and <k> obtained from, respectively, the steady-state fluorescence spectra and the time-resolved fluorescence decays were found to increase linearly with increasing generation number. This result is inconsistent with the fact that both the I(E)/I(M) ratio and <k> are proportional to the local concentration of pyrene inside the dendrimer ([Py](loc)) which is not expected to increase with generation number if the excited pyrene is assumed to diffuse freely throughout the dendrimer interior. Since the core-dense model predicts that the dendrimer terminal ends can occupy any position throughout the dendrimer interior, these results suggest that excimer formation between the pyrene-labeled ends is enhanced due to the branched nature of the dendrimer.