Synthesis, characterization, and photophysical studies of new bichromophoric ruthenium(II) complexes

The photophysical properties of a series of prepared ruthenium tris(bipyridine) complexes, covalently linked to aromatic species, of type [Ru(bpy)(2)-(4-methyl-4'-(arylaminocarbonyl)-2,2'-bipyridine)](2+) ([Ru(bpy)(2)(mbpy-L)](2+), where bpy = 2,2'-bipyridine; mbpy = 4-methyl-4'-carbonyl-2,2'-bipyridine; and L = 2-aminonaphthyl (naph), 9-aminoanthryl (anth), 1-aminopyrenyl (pyr), or 9-aminoacridinyl (acrd)) were studied by electronic absorption spectroscopy and steady state and time resolved luminescence spectroscopies. The absorption spectra of the MLCT electronic transition of the complexes are similar, which is in agreement with a practically constant redox potential of Ru(III/II) close to 1.28 V versus Ag/AgCl. However, the luminescence spectra of the new complexes are red shifted compared to Ru(bpy)(3)(2+), and this effect is ascribed to solvation and inductive effects of the amide group which enhance the symmetry breakdown among the three bipyridyl ligands. The energy stabilization of the (MLCT)-M-3 state is in the range 2.1-8.4 kJ/mol. The triplet-triplet energy transfer between the Ru complex and the aromatic species linked by an amide spacer is a slow process with rate constants of 2.6 x 10(4), 3.6 x 10(4), and 4.9 x 10(4) s(-1) for anthracene, acridine, and pyrene as acceptors in methanol, respectively. The energy transfer rate constant increases with decreasing polarity of the solvent. In dichloromethane, the rate constants for anthracene, acridine, and pyrene acceptors are 2.6 x 10(5), 1.5 x 10(5), and 2.9 x 10(5) s(-1), respectively. The low efficiency of energy transfer is due to the small difference in triplet energy between donor and acceptor species, weak electronic coupling, and unfavorable Franck-Condon factors, despite the short separation distance between donor and acceptor species in an amide bridge.