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Shuttling dynamics in an acid-base-switchable [2]rotaxane.
MedLine Citation:
PMID:  16208757     Owner:  NLM     Status:  PubMed-not-MEDLINE    
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 (-NH2+-)/amine (-NH-) center and a 4,4'-bipyridinium (bipy2+) unit--located on the dumbbell component in a [2]rotaxane. The affinity for DB24C8 decreases in the order -NH2+- > bipy2+ > -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 -NH2+- to the bipy2+ 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.
Sandrine Garaudée; Serena Silvi; Margherita Venturi; Alberto Credi; Amar H Flood; J Fraser Stoddart
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Publication Detail:
Type:  Journal Article    
Journal Detail:
Title:  Chemphyschem : a European journal of chemical physics and physical chemistry     Volume:  6     ISSN:  1439-4235     ISO Abbreviation:  Chemphyschem     Publication Date:  2005 Oct 
Date Detail:
Created Date:  2005-10-17     Completed Date:  2007-02-12     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  100954211     Medline TA:  Chemphyschem     Country:  Germany    
Other Details:
Languages:  eng     Pagination:  2145-52     Citation Subset:  -    
Dipartimento di Chimica G. Ciamician, Università di Bologna via Selmi 2, 40126 Bologna, Italy.
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