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

This article reports the thermodynamics and kinetics of the formation of the [2]pseudorotaxanes between alpha-CD and decamethylene chain-linked aromatic donor-viologen acceptor threads 1-4 in aqueous media, studied by (1)H 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(1)V(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 alpha-CD with 1-4 give two orientationally isomeric [2]pseudorotaxanes. In all of those, the isomers (P(II)) with the secondary side of (alpha-CD facing toward the viologen unit are thermodynamically more favored by >10 times than the isomers (P(I)) having the opposite alpha-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 (1)H NMR spectra. The reactions proceed mostly through C(1)V(2+) for 1 and 2, through 2-Nap for 3, and through 1-Nap unit for 4. Little kinetic selectivity for the orientation of alpha-CD is shown for the threading reaction through C(1)V(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 alpha-CD through C(1)V(2) are more than 10(2) times faster than the corTesponding 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, isomer is mostly due to the slower dethreading rates of the Pit isomer than the P(I) isomer.