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Probing end-to-end cyclization beyond Willemski and Fixman
Shaohua Chen1, Jean Duhamel, Mitchell A Winnik
1Institute of Polymer Research, Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada.
The Journal of Physical Chemistry. B
|March 17, 2011
Summary
End-to-end cyclization in pyrene-labeled poly(ethylene oxide) chains was studied. Fluorescence excimer formation decreased with increasing chain length and solvent viscosity, challenging traditional models and supporting the fluorescence blob model.
Area of Science:
- Polymer Chemistry
- Photophysics
- Spectroscopy
Background:
- Poly(ethylene oxide) (PEO) chains labeled with pyrene were synthesized to investigate end-to-end cyclization (EEC).
- Intramolecular excimer formation is a key indicator of polymer chain dynamics and cyclization efficiency.
Purpose of the Study:
- To investigate the process of end-to-end cyclization (EEC) in pyrene-labeled poly(ethylene oxide) (PEO) chains.
- To understand the influence of polymer molecular weight (M(n)) and solvent viscosity (η) on EEC.
- To evaluate the applicability of the traditional Birks' scheme and the fluorescence blob model (FBM) to describe the observed phenomena.
Main Methods:
- Synthesis of PEO chains labeled at both ends with pyrene (PEO(X)-Py(2)) and one end with pyrene (PEO(2K)-Py).
- Investigation of intramolecular excimer formation using steady-state and time-resolved fluorescence spectroscopy.
- Analysis of fluorescence decay data using the Birks' scheme and the fluorescence blob model (FBM).
Main Results:
- Excimer formation of PEO(X)-Py(2) significantly decreased with increasing M(n) and solvent viscosity (η).
- Birks' scheme analysis revealed an increase in the fraction of non-excimer forming pyrenes (f(Mfree)) with increasing M(n) and η, contradicting the model's assumptions.
- The fluorescence blob model (FBM) successfully described the experimental data, particularly for longer PEO chains, by considering quenching within a finite volume probed by the excited chromophore.
Conclusions:
- The observed decrease in excimer formation is primarily due to an increased fraction of pyrene groups outside the fluorescence blob, as predicted by the FBM.
- The FBM provides a more accurate framework than the Birks' scheme for understanding EEC in end-labeled polymers under varying molecular weights and solvent viscosities.
- This study highlights the importance of polymer conformation and local environment in determining photophysical processes like excimer formation.
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