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Shuttling Dynamics in an Acid–Base‐Switchable [2]Rotaxane
Author(s) -
Garaudée Sandrine,
Silvi Serena,
Venturi Margherita,
Credi Alberto,
Flood Amar H.,
Stoddart J. Fraser
Publication year - 2005
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500295
Subject(s) - deprotonation , rotaxane , chemistry , counterion , cationic polymerization , protonation , amine gas treating , acetonitrile , stereochemistry , polymer chemistry , molecule , supramolecular chemistry , organic chemistry , ion
Molecular shuttles are an intriguing class of rotaxanes which constitute prototypes of mechanical molecular machines and motors. By using stopped‐flow spectroscopic techniques in acetonitrile solution, we investigated the kinetics of the shuttling process of a dibenzo[24]crown‐8 ether (DB24C8) macrocycle between two recognition sites or “stations”—a secondary ammonium (NH 2 + )/amine (NH) center and a 4,4′‐bipyridinium (bipy 2+ ) unit—located on the dumbbell component in a [2]rotaxane. The affinity for DB24C8 decreases in the order NH 2 + >bipy 2+ >NH. Hence, shuttling of the DB24C8 macrocycle can be obtained by deprotonation and reprotonation of the ammonium station, reactions which are easily accomplished by addition of base and acid to the solution. The rate constants were measured as a function of temperature in the range 277–303 K, and activation parameters for the shuttling motion in both directions were determined. The effect of different counterions on the shuttling rates was examined. The shuttling from the NH 2 + to the bipy 2+ station, induced by the deprotonation of the ammonium site, is considerably slower than the shuttling in the reverse direction, which is, in turn, activated by reprotonation of the amine site. The results show that the dynamics of the shuttling processes are related to the change in the intercomponent interactions and structural features of the two mutually interlocked molecular components. Our observations also indicate that the counterions of the cationic rotaxane constitute an important contribution to the activation barrier for shuttling.