Nierengarten Group

Coordination compounds possessing low-lying metal-to-ligand charge transfer (MLCT) excited states with marked reducing character are also excellent partners for [60]fullerene in photoactive multicomponent hybrid systems. We have prepared a large number of dyads in which [60]fullerene is coupled with photoactive coordination compounds of Ru(II), Re(I), Ir(III) and Cu(I). For most of these systems, the energy of the lowest triplet MLCT level lies higher than that of the fullerene singlet and triplet, whereas the charge separated state is intermediate. Upon excitation of the metal-complexed moiety, charge separation followed by charge recombination to the fullerene triplet is generally observed. Practically, since direct excitation of the fullerene moiety results in regular deactivation without intercomponent interactions, the fullerene triplet level is the final energy sink of the dyad, whatever the excitation wavelength. The situation is rather different in the case of Cu(I)-bisphenanthroline fullerene hybrids. Cu(I) complexes are indeed stronger reducing agents than Ru(II), Re(I) or Ir(III) systems, thus the charge separated state is the lowest in the energy level diagram, originating a different pattern of photoinduced processes.

Stacks Image 8351

During the photophysical studies carried out on these dyads, we have also systematically investigated the electronic properties of the corresponding model compounds and thus became progressively involved in the field of phosphorescent metal complexes. In particular, we have developed strongly luminescent Cu(I) complexes and have been among the first to show the potential of such compounds for light emitting applications. Indeed, such non-toxic Cu(I) complexes are interesting alternatives to the heavy metal complexes (Ir(III) and Pt(II) derivatives) typically used as electrophosphorescent materials in LEDs.

Stacks Image 8363

More recently, we have investigated the preparation of heteroleptic Cu(I) complexes from various 2,9-disubstituted-1,10-phenanthrolines (phen) and different bis-phosphine (PP) ligands. Whereas the heteroleptic complexes are stable in the solid state, equilibrium between the homoleptic and the heteroleptic complexes is often observed in solution. This major limitation for the preparation of [Cu(phen)(PP)] derivatives results from the exceptionally high thermodynamic stability of the corresponding homoleptic [Cu(phen)2] complexes.

Stacks Image 8375

In order to prevent the formation of a stable homoleptic complex from the phen ligand, we have incorporated the phenanthroline subunit in a macrocyclic structure. The maximum site occupancy principle favors the formation of stable heteroleptic Cu(I)-complexed pseudo-rotaxanes. This is the case when the macrocycle is large and flexible enough to allow the threading of the bulky diphenylphosphino moieties of the PP building block. We have also shown that the complexation scenario is totally different when the phenanthroline-containing macrocycle does not allow the threading of the PP ligand. Indeed, the combination of this steric constraint with the topological restriction imposed by the macrocyclic ligand drives the complexation process and affords unprecedented trigonal Cu(I)-complexes with remarkable photophysical properties.

Stacks Image 8383