Molecular Devices DOI: 10.1002/ange.201200555 Photoactivated Directionally Controlled Transit of a Non-Symmetric Molecular Axle Through a Macrocycle** Massimo Baroncini, Serena Silvi, Margherita Venturi,* and Alberto Credi* Dedicated to Professor Vincenzo Balzani on the occasion of his 75th birthday The control of motion on the molecular scale is of funda- mental importance for living organisms, [1] and one of the most fascinating challenges in nanoscience. [2, 3] Artificial molecular machines have been realized on the laboratory scale, [4] and utilization of such systems to construct responsive materials [5] and surfaces, [6] control catalytic processes, [7] and develop test structures for information storage devices [8] and drug deliv- ery [9] has been investigated. Nevertheless, the construction of synthetic nanoscale motors capable of showing directionally controlled linear or rotary movements still poses a consider- able challenge to chemists. [10] Moreover, the use of such systems to perform the tasks that natural molecular motors do, [11] namely active transport of substrates over long distances or across membranes, remains a very difficult endeavor that is further complicated by the fact that most currently available synthetic molecular motors are based on sophisticated chemical structures and/or operation proce- dures. [4, 10] In this context, the development of (supra)molec- ular systems that exhibit directionally controlled relative motions of their components based on a minimalist design and activated by convenient inputs is of the highest impor- tance. Herein we describe the construction and operation of a simple supramolecular assembly in which a molecular axle passes unidirectionally through the cavity of a molecular ring in response to photochemical and chemical stimulation. A system of this kind constitutes a first step towards the construction of an artificial molecular pump; it can also lead to the realization of molecular linear motors based on rotaxanes and rotary motors based on catenanes. [12] The strategy that we have tackled is illustrated in Figure 1. The system is composed of a molecular ring and a non- symmetric molecular axle that comprises 1) a passive pseudo- stopper (D) at one end; 2) a central recognition site (S) for the ring; and 3) a bistable photoswitchable unit (P) at the other end. Under the conditions employed, the axle pierces the ring exclusively with the photoactive gate in its initial form (a) for kinetic reasons, [13] affording a pseudorotaxane in which the molecular ring encircles the recognition site S. Subsequently, light irradiation converts the a-P end group into the bulkier b form, a process which also causes a destabilization of the supramolecular complex. Therefore, a dethreading of the system is expected, which should occur by extrusion of the D moiety of the axle. Reset is obtained by photochemical or thermal conversion of the b-P gate back to the a state, thereby regenerating the starting form of the axle. Overall, the photoinduced directionally controlled transit of the axle through the ring would be obtained according to a flashing energy ratchet mechanism (Figure 1 b). [4b, 14] Clearly, the success of such a strategy relies on two basic requirements: 1) the kinetic barriers for the slippage of the axle end groups through the ring [15] should follow the order: E ° a-P < E ° D < E ° b–P ; and 2) the ring should form a more Figure 1. a) Strategy for the photoinduced unidirectional transit of a non-symmetric axle through a molecular ring. b) Simplified potential energy curves (free energy versus ring–axle distance) for the states shown in (a) describing the operation of the system in terms of a flashing ratchet mechanism. Slower processes that do not take place under our conditions are represented by shaded cartoons and dashed lines. [*] Dr. M. Baroncini, Dr. S. Silvi, Prof. M. Venturi, Prof. A. Credi Dipartimento di Chimica “G. Ciamician”, Università di Bologna Via Selmi 2, 40126 Bologna (Italy) E-mail: margherita.venturi@unibo.it alberto.credi@unibo.it Homepage: http://www.credi-group.it [**] We thank the University of Bologna, MIUR (PRIN 2008), and Fondazione CARISBO for financial support. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201200555. A ngewandte Chemi e 4299 Angew. Chem. 2012, 124, 4299 –4302  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim