From photoinduced charge separation to light-driven molecular machines

The photochemical properties of transition metal complexes, such as those of iridium(III) or ruthenium(II), can be exploited in various ways to generate charge-separated (CS) states, in relation to the mimicry of the natural photosynthetic reaction centres, or to set multicomponent compounds or assemblies in motion. The first part of the present chapter summarizes the work carried out in our groups (Bologna and Strasbourg) in recent years with iridium(III)-terpy complexes (terpy: 2,2′,6′,6″-terpyridine). The synthesis of multicomponent iridium(III) complexes in reasonable yields has been achieved and their photochemical properties have been investigated. Unexpectedly, the excited state lifetimes of some of these compounds are very long at room temperature (several microseconds) in fluid solution, making the Ir(terpy)₂ ³⁺ fragment an interesting chromophore. Once attached to electron donor (D) groups, dyads of the Ir(terpy)₂ ³⁺ -D type undergo fast photoinduced electron transfer. In addition Ir(terpy)₂ ³⁺ in the ground state is a relatively good electron acceptor, displaying interesting properties as electron relay in porphyrinic triads. A triad, consisting of an Ir(terpy)₂ ³⁺ central core, a Zn porphyrin as the primary donor on one side and a gold(III) porphyrin as the terminal acceptor on the other side, leads to a relatively long-lived CS state (close to the microsecond). The other section of the present chapter deals with light-driven molecular machines built around Ru(bpy) ₃ ²⁺ derivatives, including catenanes and rotaxanes. In order to set the system in motion, a dissociative ligand field (LF) state is generated from the light-absorbing metal-to-ligand charge transfer (MLCT) state, originating in the expulsion of a given ligand in a perfectly controlled fashion. This step is rapidly followed by coordination of another ligand to afford a kinetically stable new complex. The process can be inverted by thermal energy, so as to regenerate the starting state of the system.