Electron Donor-Acceptor Dyads and Triads Based on Tris(bipyridine)ruthenium(II) and Benzoquinone: Synthesis, Characterization, and Photoinduced Electron Transfer Reactions

Replacement of 2,2ʹ-bipyridine (bpy) by substituted 1,4-diazabutadiene (R-DAB) (alpha)-diimine ligands N(delta)N leads to a substantial hypsochromic shift of about 0.8 eV for the long-wavelength absorption band in compounds (N(delta)N)Ir(C5Me5) but to a bathochromic absorption shift of about 0.4 eV for the complex ions [(N(delta)N)IrCl(C5Me5)]T+. DFT calculations on model complexes based on experimental (R-DAB compounds) and geometry-optimized structures (bpy systems) reveal that the low-energy transitions of the cationic chloro complexes are largely of ligand-to-ligand charge-transfer character LʹLCT (L = (alpha)-diimine, Lʹ = Cl) whereas the neutral compounds exhibit (pi)(right arrow)(pi)* transitions between the considerably mixed metal d(pi) and (alpha)-diimine (pi)* orbitals. The much more pronounced metal-ligand orbital interaction for the R-DAB complexes causes the qualitatively different shifts on replacing the stronger basic bpy by the better -acceptors R-DAB. Only the LUMO of the neutral compounds is destabilized on replacement of bpy by R-DAB whereas the LUMO of [(N(delta)N)IrCl(C5Rʹ5)]^+ and both HOMOs are stabilized through this change.