Hmadeh, Mohamad and Fahrenbach, Albert C. and Basu, Subhadeep and Trabolsi, Ali and Benítez, Diego and Li, Hao and Albrecht-Gary, Anne-Marie and Elhabiri, Mourad and Stoddart, J. Fraser (2011) Electrostatic Barriers in Rotaxanes and Pseudorotaxanes. Chemistry: a European Journal, 17 (22). pp. 6076-6087. ISSN 0947-6539 http://resolver.caltech.edu/CaltechAUTHORS:20110707-082821059
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The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY^(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY^(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT^(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature ^(1)H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT^(4+) ring over the central BIPY2+ unit is in excess of 17 kcal mol^(−1) at 343 K. Further information about the nature of the BIPY^(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY^(2+) moiety or a central DNP unit flanked by a BIPY2+ moiety. The threading and dethreading processes of the CBPQT^(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY^(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor–acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (Δequation image) of the CBPQT^(4+) for the case of the thread composed of a DNP flanked by two BIPY^(2+) units was found to be as high as 21.7 kcal mol^(−1) at room temperature. These results demonstrate that we can effectively employ the BIPY^(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT^(4+) ring in both mechanically interlocked and supramolecular systems.
|Additional Information:||© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Received: October 11, 2010; Revised: December 24, 2010; Published online: April 15, 2011. The authors acknowledge support from the Air Force Office of Scientific Research (AFOSR) under the Multidisciplinary Research Program of the University Research Initiative (MURI), Award FA9550-07-1-0534 entitled “Bioinspired Supramolecular Enzymatic Systems” in the United States as well as from the Non-Equilibrium Energy Research Center (NERC), an Energy Frontier Research Center funded by the US Department of Energy, Office of Basic Energy Sciences, under Award Number DE-SC0000989. This work was also supported by the Centre National de la Recherche Scientifique (CNRS) and the Université de Strasbourg (UMR 7177 CNRS-UdS) in France, as well as the French Ministry of Research and Education (Doctoral Fellowship to M.H.). A.C.F. acknowledges support from an NSF Graduate Research Fellowship.|
|Subject Keywords:||electrostatic barriers; host–guest chemistry; hydrophobic interactions; rotaxanes; spectrophotometric techniques; template synthesis|
|Official Citation:||Hmadeh, M., Fahrenbach, A. C., Basu, S., Trabolsi, A., Benítez, D., Li, H., Albrecht-Gary, A.-M., Elhabiri, M. and Stoddart, J. F. (2011), Electrostatic Barriers in Rotaxanes and Pseudorotaxanes. Chemistry - A European Journal, 17: 6076–6087. doi: 10.1002/chem.201002933|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||11 Jul 2011 16:21|
|Last Modified:||11 Jul 2011 16:21|
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