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Cucurbit[8]uril mediated donor-acceptor ternary complexes: a model system for studying charge-transfer interactions.
Frank Biedermann1, Oren A Scherman
1Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdon.
The Journal of Physical Chemistry. B
|February 8, 2012
Summary
This study introduces a supramolecular self-assembly method using cucurbit[8]uril (CB[8]) to create charge-transfer (CT) donor-acceptor complexes in water. The CB[8] host facilitates well-defined CT interactions, offering a practical approach for studying these phenomena.
Area of Science:
- Supramolecular Chemistry
- Physical Chemistry
- Materials Science
Background:
- Charge-transfer (CT) complexes are crucial in various chemical and physical processes.
- Studying CT phenomena in solution is often complicated by self-aggregation of donors and acceptors.
- Cucurbit[8]uril (CB[8]) is a macrocyclic host known for its ability to form stable complexes.
Purpose of the Study:
- To develop a convenient supramolecular self-assembly approach for preparing diverse CT donor-acceptor complexes in aqueous solutions.
- To systematically investigate and provide evidence for CB[8]-mediated CT interactions.
- To elucidate the driving forces and energetic relevance of CT in CB[8] ternary complexes.
Main Methods:
- Supramolecular self-assembly using cucurbit[8]uril (CB[8]) as a host.
- UV/Vis spectroscopy to characterize CT bands and determine CT λ(max).
- Computational methods (HOMO-LUMO energies) to correlate with experimental CT data.
- Isothermal Titration Calorimetry (ITC) and Electrospray Ionization Mass Spectrometry (ESI-MS) to evaluate binding thermodynamics.
Main Results:
- A facile method for generating heteroternary CT complexes within the CB[8] cavity was established.
- Systematic evidence for CB[8]-mediated CT interactions between various donor-acceptor pairs was presented.
- The CT process within the CB[8] cavity aligns with the Mulliken model, correlating with computed HOMO-LUMO energies.
- Charge-transfer was found not to be the primary energetic driver for complex formation; electrostatic and solvation effects are more significant.
- A significant red-shift in CT λ(max) within the CB[8] cavity indicates stabilization of the CT excited state.
Conclusions:
- The CB[8]-mediated supramolecular self-assembly offers a practical and synthetically advantageous method for creating CT donor-acceptor complexes in water.
- The unique electrostatic and polar environment of the CB[8] cavity stabilizes the CT excited state.
- Binding affinities are primarily governed by electrostatic interactions and solvation effects, rather than CT energy.
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