Volume 16, Issue 38 p. 11580-11587
Full Paper

Reversible Photoswitching of Rotaxane Character and Interplay of Thermodynamic Stability and Kinetic Lability in a Self-Assembling Ring–Axle Molecular System

Dr. Massimo Baroncini

Dr. Massimo Baroncini

Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456

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Dr. Serena Silvi

Dr. Serena Silvi

Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456

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Prof. Margherita Venturi

Prof. Margherita Venturi

Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456

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Prof. Alberto Credi

Prof. Alberto Credi

Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456

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First published: 14 September 2010
Citations: 65

Graphical Abstract

By night one way, by day another: Clear-cut, facile, and reversible switching between the thermodynamically stable pseudorotaxane form and the kinetically inert rotaxane form (see figure) is obtained by light irradiation of a self-assembling system composed of a macrocycle and a thread molecule equipped with azobenzene end groups.

Abstract

We have designed, synthesized, and investigated a self-assembling system that can be reversibly interconverted between thermodynamically stable (pseudorotaxane) and kinetically inert (rotaxane) forms by light irradiation. The system is composed of a dibenzo[24]crown-8 ring and an axle comprised of a dibenzylammonium recognition site and two azobenzene end groups. The isomeric form of the azobenzene units of the axle has a little influence on the stability constants of the respective pseudorotaxanes but greatly affects the threading–dethreading rate constants. In fact, equilibration of the ring and the axle in its EE isomeric form occurs within seconds in acetonitrile at room temperature, whereas the ZZ axle threads–dethreads the ring at least four orders of magnitude slower. Moreover, we show that a change in the stability of the complex, achieved by deprotonating the dibenzylammonium recognition site on the axle, affects its kinetic behavior. We compare the results of these experiments with those observed upon dethreading the (pseudo)rotaxane by using a competitive guest for the ring, an approach which does not inherently destabilize the ring–axle interaction. This study outlines a general strategy for the reversible photochemical control of motion kinetics in threaded and interlocked compounds and constitutes a starting point for the construction of multicomponent structures that can behave as photochemically driven nanomachines.