| Cucurbit[8]uril Mediated Donor-Acceptor Ternary Complexes: A Model System for Studying Charge-Transfer Interactions. | |
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MedLine Citation:
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PMID: 22309573 Owner: NLM Status: Publisher |
Abstract/OtherAbstract:
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A supramolecular self-assembly approach is described which allows for the convenient preparation of a wide range of charge transfer (CT) donor-acceptor complexes in aqueous solutions. When one equivalent of the macrocyclic host cucurbit[8]uril (CB[8]) is added to an aqueous donor and acceptor solution, a hetero-ternary complex forms inside the host's cavity with a well-defined face-to-face π-π stacking geometry of the donor and acceptor. This heteroternary, CB[8] mediated, complex offers the opportunity to study the CT phenomena at low concentrations and free from complications arising from any donor-donor and acceptor-acceptor interactions as a result of the large binding affinities and the very high selectivity over the formation of these homo-ternary complexes. Thus, this supramolocular self-assembly strategy is a practical donor-acceptor mix-and-match approach with synthetic advantages over much more cumbersome tethering schemes. While the characteristic UV/vis features of a few CB[8] ternary systems had been described as a CT band, we present for the first time systematic evidence for the existence of CT interactions between several donor-acceptor pairs that are mediated by the host CB[8]. Correlation of the experimentally obtained CT λ<sub>max</sub> to computed HOMO-LUMO energies demonstrated that the CT process in the host's cavity can be described by the Mulliken-model. Furthermore, the literature claim of a "CT driving force" for the formation of CB[8] ternary complexes was scrutinized and evaluated by calorimetric (ITC) and ESI-MS measurements. The findings indicated that neither in the aqueous medium nor in the "gas-phase" is CT of energetic relevance to the Gibbs free binding energy. In contrast, electrostatic considerations combined with solvation effects are much better suited to rationalize the observed trends in binding affinities. Additionally, the CT λ<sub>max</sub> was found to be much more red-shifted (≥75 nm) inside the CB[8] cavity than in any polar organic solvents or water, indicating a significant stabilization of the CT excited state within the host cavity, further demonstrating the unique electrostatic, polar properties of the host-cavity. |
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Authors:
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Frank Biedermann; Oren A Scherman |
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Publication Detail:
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Type: JOURNAL ARTICLE Date: 2012-2-6 |
Journal Detail:
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Title: The journal of physical chemistry. B Volume: - ISSN: 1520-5207 ISO Abbreviation: - Publication Date: 2012 Feb |
Date Detail:
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Created Date: 2012-2-7 Completed Date: - Revised Date: - |
Medline Journal Info:
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Nlm Unique ID: 101157530 Medline TA: J Phys Chem B Country: - |
Other Details:
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Languages: ENG Pagination: - Citation Subset: - |
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