Thermodynamics and kinetics of formation of orientationally isomeric [2]pseudorotaxanes between α-cyclodextrin and aliphatic chain-linked aromatic donor-viologen acceptor compounds

Abstract

This article reports the thermodynamics and kinetics of the formation of the [2]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 [2]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 [2]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 [2]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.