The proposal concerns the construction of ' molecular-based machinery' wherein mechanical movement within the molecule may control intermolecular magnetic interactions promoted by external stimuli. The molecular entities will be of the " rotaxanes" family which are molecules where a macro cycle is locked onto a linear " thread" by bulky " stoppers" at either end of the thread. Two types of rotaxane systems are proposed. The first involves a rotaxane where the stoppers are paramagnetic metal centers and the ring may either contain a stable organic radical or not. In this system, the thread portion will only have one position that the ring may occupy, and will be able to undergo photo-induced cis/trans isomerism. In the trans form there should be no interaction between the metal centers, whereas in the cis form they will be brought into close proximity with one another and will magnetically interact. If the ring does not contain a radical then the system will be a "light induced on/off magnetic switch", however, if there is a radical on the ring then there is a third state attainable by a second stimulus, namely a switch between a three-spin system to a two spin system. All these species will be identifiable by EPR. The second type of rotaxane system proposed will have a thread with two "docking sites" for a radical containing ring and heterometallic stoppers. An external stimulus will control whether the ring is sitting at one end of the rotaxane or the other. The magnetic interaction, and therefore the magnetic properties, will be different in either situation and may also be detected and studied by EPR. Therefore this system will be a case of a "two-state" switch promoted by external stimuli. The main outcome of this proposal will be innovation in the development of ' molecular machines' that feature mechanically controlled magnetic properties, and therefore are promising candidates for molecular memory storage devices.
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