Abstract
This review exposes the current poor understanding of the internal segmental chain dynamics of dendrimers in solution probed by monitoring the process of excimer formation between pyrene labels covalently attached to the chain ends of dendrimers. The review begins by covering the bases of fluorescence and the kinetics of pyrene excimer formation before describing a procedure based on the Model Free (MF) analysis that is used to analyze quantitatively the fluorescence decays acquired for dendrimers, the ends of which have been fully and covalently labeled with pyrene. Comparison of the various trends obtained by different research groups describing the efficiency of pyrene excimer formation with the generation number of dendrimers illustrates the lack of consensus between the few studies devoted to the topic. One possible reason for this disagreement might reside in the presence of minute amounts of unattached pyrene labels which act as potent fluorescent impurities and affect the analysis of the fluorescence spectra and decays in an uncontrolled manner. The review points out that the MF analysis of the fluorescence decays acquired with pyrene-labeled dendrimers enables one to account for the presence of unattached pyrene and to retrieve information about the internal segmental dynamics of the dendrimer. It provides guidelines that should enable future studies on pyrene-labeled dendrimers to yield results that are more straightforward to interpret.
Highlights
The exquisite level of synthetic control achieved over the molecular architecture of dendrimers, which was demonstrated more than 25 years ago by the preparation of polyamide and poly(amido amine) (PAMAM) dendrimers by Newkome [1] and Tomalia [2], respectively, has made these macromolecules key elements in a broad range of applications such as for light harvesting devices [3,4], in the biomedical field [5,6], for catalysis [7,8], or as sensors [9,10]
Considering how complicated it already is to study the process of excimer formation in pyrene-labeled dendrimers [36,37,44,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134], studies should focus on dendrimers whose backbone is not expected to quench the excited pyrene or its excimer
Chemical motives that should be avoided at all cost include the tertiary amines such as those found in PAMAM [128,129,133] and PPI [127] dendrimers which are known quenchers of pyrene
Summary
The exquisite level of synthetic control achieved over the molecular architecture of dendrimers, which was demonstrated more than 25 years ago by the preparation of polyamide and poly(amido amine) (PAMAM) dendrimers by Newkome [1] and Tomalia [2], respectively, has made these macromolecules key elements in a broad range of applications such as for light harvesting devices [3,4], in the biomedical field [5,6], for catalysis [7,8], or as sensors [9,10]. Studies that probe τC for different segments of dendrimers demonstrate that the chain ends are locally mobile at their position inside the dendrimer, but provide little information about the range and dynamics of their translational displacement which enables the shuttle of the terminal groups from the dendrimer core to its surface. This information can be obtained with techniques that characterize the long range internal dynamics of a macromolecule. Clear guidelines will be provided on where the field should be heading and how further studies should be conducted
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