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Thermodynamics and kinetics of formation of orientationally isomeric pseudorotaxanes between α-cyclodextrin and aliphatic chain-linked aromatic donor-viologen acceptor compounds
- Thermodynamics and kinetics of formation of orientationally isomeric pseudorotaxanes between α-cyclodextrin and aliphatic chain-linked aromatic donor-viologen acceptor compounds
- Park J.W.; Song H.J.; Cho Y.J.; Park K.K.
- Ewha Authors
- SCOPUS Author ID
- Issue Date
- Journal Title
- Journal of Physical Chemistry C
- vol. 111, no. 50, pp. 18605 - 18614
- SCI; SCIE; SCOPUS
- This article reports the thermodynamics and kinetics of the formation of the pseudorotaxanes between α-CD and decamethylene chain-linked aromatic donor-viologen acceptor threads 1-4 in aqueous media, studied by 1H NMR, absorption, and fluorescence spectroscopic methods. The aromatic donor units of threads are carbazole (Cz) for 1, 1-naphthoxy (1-Nap) group for 2 and 4, and 2-naphthoxy (2-Nap) group for 3. The viologen units are 1-methyl-4,4′-bipyridinium (C 1V 2+) for 1-3 and 1-(3,5-dimethoxybenzyl)-4,4′-bipyridinium for 4. The threads form intramolecular charge-transfer (CT) complexes having folded structure and the order of the stability of the CT complexes is 1 > 4 > 2 ≥ 3. The overall equilibrium constant for the pseudorotaxane formation reactions with the open conformer of 1 is 68 000 M -1, whereas the corresponding values with 2-4 are about 7000 M -1. The reactions of α-CD with 1-4 give two orientationally isomeric pseudorotaxanes. In all of those, the isomers (P II) with the secondary side of α-CD facing toward the viologen unit are thermodynamically more favored by > 10 times than the isomers (P I) having the opposite α-CD orientation. The microscopic directional rate constants of the threading and the dethreading reactions for the pseudorotaxane isomers were obtained by analyzing the kinetic traces and 1H NMR spectra. The reactions proceed mostly through C 1V 2+ for 1 and 2, through 2-Nap for 3, and through 1-Nap unit for 4. Little kinetic selectivity for the orientation of α-CD is shown for the threading reaction through C 1V 2+, while the primary side is favored by four times for 1-Nap and the secondary side is preferred by about 1.5 times for 2-Nap group. The rates of the threading and dethreading of α-CD through C 1V 2 are more than 10 2 times faster than the corresponding rates through 1-Nap group but are slower by more than 10 2 times than those through 2-Nap. The thermodynamic preference of the P II isomer to the P I isomer is mostly due to the slower dethreading rates of the P II isomer than the P I isomer. © 2007 American Chemical Society.
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