Light-Driven Expulsion of the Sterically Hindering Ligand L in Tris-diimine Ruthenium(II) Complexes of the Ru(phen) 2 (L) 2+ Family: A Pronounced Ring Effect Jean-Paul Collin,* Damien Jouvenot, Masatoshi Koizumi, † and Jean-Pierre Sauvage* Laboratoire de Chimie Organo-Mine ´ rale, UMR 7513 du CNRS, UniVersite ´ Louis Pasteur, Faculte ´ de Chimie, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France Received February 17, 2005 Three new ruthenium(II) complexes have been prepared which contain two 1,10-phenanthroline units and a third sterically hindering chelate. In one case, the hindering ligand is a disubstituted 2,2′-bipyridine (bpy) attached to two very bulky manisyl groups. The two other systems are similar in terms of size of the hindering groups (anisyl sub- stituents) located close to the central metal. The complexes investigated in the Present Report are aimed at providing building blocks of future light-driven molecular machines. The photochemical expulsion of the sterically hindering chelate has thus been studied by UV-vis spectroscopy and 1 H NMR. Surprisingly, the manisyl-containing complex turned out to be photochemically inert, indicating that a too bulky group acts as a protecting function versus decom- plexation rather than as a destabilizing group. For the two other systems, a pronounced ring effect was observed: whereas the acyclic systems undergo fast photochemical expulsion of the bipy-based ligand, in the cyclic complex, the bipy-incorporating ring is decoordinated about 5 times less efficiently than the acyclic ligand of the previous case. These observations on the strong dependence of the photochemical behavior of the ruthenium(II) complexes on their structural properties are corroborated by X-ray diffraction studies on the three compounds investigated. Introduction Dynamic molecular systems in which a given part of the molecular system can be set in motion at will under the action of an external signal are often referred to as molecular machines or motors. 1 They are particularly promising in relation to nanomechanical devices and information storage and processing at the molecular level. 2 Among the many examples of such systems reported during the past decade, several examples of light-driven machines have been de- scribed. 3 Most of them contain a photoisomerizable group such as an azo benzene derivative. The light impulse converts the trans isomer to the cis isomer, leading to a significant change of the geometry of the photochemically active group and thus strongly modifying its ability to interact with a given part of the molecular system. As a consequence, a rear- rangement may occur. 4 Our group has proposed another approach of light-driven machines, on the basis of dissocia- tive excited states. 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