LONG-LIVED, HIGHLY LUMINESCENT RHENIUM(I) COMPLEXES AS MOLECULAR PROBES - INTRAMOLECULAR AND INTERMOLECULAR EXCITED-STATE INTERACTIONS

A series of isonitrile complexes of the form fac-LRe(CO)3CNR+, where L is an alpha-diimine and R = t-Bu or n-alkyl, have been synthesized and characterized. These complexes can have extraordinarily high quantum yields (>0.7) and long excited-state lifetimes (>100 mus) in fluid solutions at room temperature. The lowest excited state (charge transfer or ligand localized) can be controlled by varying either L or the temperature. Intramolecular foldback can affect the luminescence properties. These complexes are bihydrophobic because they have two spacially separated hydrophobic binding sites (i.e. L and R). By suitably engineering the complex, one can control which group will bind to hydrophobic targets such as cyclodextrins. Since the luminescence is strongly localized in the alpha-diimine portion of the complex, the orientation of the metal complex in the binding site profoundly affects the luminescence and quenching properties that can be used to differentiate between the two binding modes. Since the systems have a near degeneracy of the lowest charge-transfer and ligand triplet state, the luminescence properties can depend strongly on temperature. A three-state model involving the lowest MLCT and pi-pi* states as well as higher energy d-d excited states must be evoked to explain the temperature dependencies of the luminescence. This model is used to assign the ordering and positions of the lowest MLCT and 3pi-pi* states as well as their relationship to the lowest deactivating d-d state.