p-carborane-bridged bipyridine ligands for energy transfer between two iridium centers

Two iridium(III) complexes displaying for one a high HOMO-LUMO gap and for the other a weaker gap were linked in a controlled and logical manner to closo-p-carborane spacers. The bridging ligand is composed of 5-ethynyl-2,2'-bipyridine units, and the peripherical Ir-ligands are orthometalated 2',4'-difluoro-2-phenylpyridine (dfppy) (λ(abs) at 400 nm for the "Ir(dfppy)2(bpy')") for the energy donor fragment and dibenzo[a,c]phenazine (dbpz) (λ(abs) at 525 nm for "Ir(dbpz)2(bpy')") for the energy acceptor fragment.Redox, spectroscopic, and photophysical properties for models and the donor-carborane-acceptor complex were determined. Efficient energy transfer from the "Ir(dfppy)2(bpy')" moiety to the "Ir(dbpz)2(bpy')" fragment is occurring with a rate constant of 3.3 × 10(8) s(-1) despite weak electronic coupling through the inert p-carborane spacer. From flash photolysis experiments it is shown that, by excitation of the donor, a low lying triplet state localized on the acceptor bridging ligand side is formed which decays by conversion to the (3)MLCT of the acceptor fragment which phosphoresces at 644 nm.