A Comparison of Shuttling Mechanisms in Two Constitutionally Isomeric Bistable Rotaxane-Based Sunlight-Powered Nanomotors

Vincenzo Balzani; Miguel Clemente-León; Alberto Credi; Monica Semeraro; Margherita Venturi; Hsian-Rong Tseng; Sabine Wenger; Sourav Saha; J. Fraser Stoddart

doi:10.1071/CH06019

RESEARCH FRONT

A Comparison of Shuttling Mechanisms in Two Constitutionally Isomeric Bistable Rotaxane-Based Sunlight-Powered Nanomotors

Vincenzo Balzani ^A , Miguel Clemente-León ^A ^C , Alberto Credi ^A , Monica Semeraro ^A , Margherita Venturi ^A ^E , Hsian-Rong Tseng ^B ^D , Sabine Wenger ^B , Sourav Saha ^B and J. Fraser Stoddart ^B ^E

+ Author Affiliations

- Author Affiliations

^A Dipartimento di Chimica ‘G. Ciamician’, Università di Bologna, 40126 Bologna, Italy.

^B The California NanoSystems Institute and the Department of Chemistry and Biochemistry, University of California, Los Angeles CA 90095-1569, USA.

^C Current address: Instituto de Ciencia Molecular, Universidad de Valencia, 46100 Burjassot, Spain.

^D Current address: Department of Molecular and Medical Pharmacology and The Crump Institute for Molecular Imaging, University of California, Los Angeles CA 90095, USA.

^E Corresponding authors. Email: margherita.venturi@unibo.it; stoddart@chem.ucla.edu

Australian Journal of Chemistry 59(3) 193-206 https://doi.org/10.1071/CH06019
Submitted: 15 January 2006 Accepted: 23 February 2006 Published: 24 March 2006

Abstract

To find out how best to optimize shuttling of the macrocycle in a particular class of photochemically driven molecular abacus, which has the molecular structure of BR-I⁶⁺ in its Mark I prototype (Ashton et al., Chem. Eur. J. 2000, 6, 3558), we have synthesized and characterized a Mark II version of this kind of two-station rotaxane comprised of six molecular modules, namely (a) a bisparaphenylene[34]crown-10 electron donor macrocycle M and its dumbbell-shaped component which contains (b) a Ru(ii)-polypyridine photoactive unit P²⁺ as one of its stoppers, (c) a p-terphenyl-type ring system as a rigid spacer S, (d) 4,4′-bipyridinium (A₁²⁺) and (e) 3,3′-dimethyl-4,4′-bipyridinium (A₂²⁺) electron acceptor units that can play the role of stations for the macrocycle M, and (f) a tetraarylmethane group T as the second stopper. This Mark II version is identical with BR-I⁶⁺ in the Mark I series that works as a sunlight-powered nanomotor (Balzani et al., Proc. Natl. Acad. Sci. USA 2006, 103, 1178), except for the swapping of the two stations A₁²⁺ and A₂²⁺ along the dumbbell-shaped component, i.e. the Mark I and II bistable rotaxanes are constitutionally isomeric. We have found the closer the juxtaposition of the electron transfer photosensitizer P²⁺ to the better (A₁²⁺) of the two electron acceptors, namely the situation in BR-II⁶⁺ compared with that in BR-I⁶⁺ results in an increase in the rate — and hence the efficiency — of the photoinduced electron-transfer step. The rate of the back electron transfer, however, also increases. As a consequence, BR-II⁶⁺ performs better than BR-I⁶⁺ in the fuel-assisted system, but much worse when it is powered by visible light (e.g. sunlight) alone. By contrast, when shuttling is electrochemically driven, the only difference between the two bistable rotaxanes in the Mark I and Mark II series is that the macrocycle M moves in opposite directions.

Acknowledgments

A part of this research was supported by a National Science Foundation (NSF) Nanoscale Interdisciplinary Research Team fund (NIRT ECS-0103559), an Office of Naval Research grant (contract number N00014-00-1-0216), an NSF Equipment Grant (CHE-9974928), an NIH Shared Instrument Grant (S10RR1S9S2), and the Center for Cell Mimetic Space Exploration (CMISE) — a NASA University Research, Engineering, and Technology Institute (URETI) award (number NCC2-1364). The European Union is gratefully acknowledged for support under the auspices of the Molecular-Level Devices and Machines RTN Network, the Biomach STREP project, and a Marie Curie Individual Fellowship to M.C.-L. The Italian authors complain that they cannot thank any Italian agency for financial support.

References

A Comparison of Shuttling Mechanisms in Two Constitutionally Isomeric Bistable Rotaxane-Based Sunlight-Powered Nanomotors

Abstract

Acknowledgments

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