Ruthenium-terpyridine complexes with multiple ethynylpyrenyl or ethynyltoluyl subunits: X-ray structure, redox, and spectroscopic properties

Several heteroleptic and homoleptic ruthenium-terpyridine complexes bearing two and four ethynylpyrenyl or ethynyltoluyl residues have been prepared from complexes carrying reactive bromo functions. Cross-coupling promoted by low-valent palladium(0) on these preformed complexes has advantageously been used to prepare the target complexes. The structure of a bis-terpyridine complex carrying four ethynylpyrenyl subunits was determined by single-crystal X-ray diffraction, showing a distorted octahedral geometry around the metal center, with the ethynylpyrenyl fragment being slightly tilted (about 5 degrees) from the terpyridine plane. The molecular packing is characterized by intermolecular pi...pi interaction within dimers. The counteranions and the solvent molecules are entrapped in well-defined channels spanning along the a-axis. The complexes are redox active with a Ru oxidation overlapping the pyrene oxidation and two well-defined ligand-centered reduction processes. Pyrene reduction is irreversible and strongly cathodic. The new multichromophoric complexes are luminescent both in solution and in rigid matrix at 77 K, with room-temperature lifetimes and quantum yields significantly larger than those of [Ru(terpy)2]2+. At room temperature, the toluyl-substituted complexes are triplet metal-to-ligand charge-transfer (3MLCT) emitters, whereas for the pyrene-grafted complexes pyrene-centered emission is observed. For the latter complexes, the energy gap, DeltaTT, between higher 3MLCT levels and lower ligand-centered (3pipi*, 3LC) levels is in the 640-730 cm(-1) range, which results in the interstate dynamics at the basis of the observed luminescent behavior. At 77 K, for the pyrene-grafted complexes, the emission reveals features that are tentatively ascribed to intraligand interactions involving the pyrene and terpyridine units.