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 α-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 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 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 α-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 α-CD through C 1 V 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.