ELECTRONIC-STRUCTURE OF THE MOLECULAR LIGHT SWITCH [RU(BPY)2(DPPZ)]2+ (DPPZ = DIPYRIDO[3,2-A-2',3'-C]PHENAZINE) - CYCLIC VOLTAMMETRIC, UV VIS, AND EPR ENDOR STUDY OF MULTIPLY REDUCED COMPLEXES AND LIGANDS

The chelate ligand dipyrido[3,2-a:2',3'-c]phenazine(dppz), its 11,12-dimethyl derivative dmdppz, and corresponding complexes with [Ru(bpy)2]2+ were studied in multiply reduced states by low-temperature cyclic voltammetry and UV/vis and EPR spectroscopy. The (dm)dppz ligands are reduced in two reversible steps, followed by a very moisture-sensitive third step. Highly resolved EPR and H-1-ENDOR spectra of the intermediate anion radicals were obtained and analyzed. The results are interpreted using a HMO/McLachlan perturbation approach of pi spin populations and orbital energies. Three low-lying unoccupied pi molecular orbitals can be identified as phenazine-type (b1, lowest) and as the psi(b1) and chi(a2) orbitals of the alpha-diimine moiety. Complexes with the N(4),N(5)-bound [Ru(bpy)2]2+ fragment show at least six reversible one-electron reduction steps in rigorously dried DMF at 200 K; the first four persistent reduced states were characterized by EPR and UV/vis spectroscopy. The EPR spectra of the first three reduced states of the complexes show a signal which proves the occupation of the phenazine-localized pi* orbital of (dm)dppz by a single electron, the stepwise reduction of the bpy ligands resulting in temperature-dependent intensity loss of that EPR signal. The very basic quadruply reduced state exhibits EPR characteristics which are typical for Ru(II)-bound alpha-diimine anion radicals. All assignments are supported by UV/vis spectra and analyses of redox potential values. Because the very easily protonated higher reduced states are not sufficiently persistent for EPR and UV/vis characterization, further assignments could thus be based only on the analysis of redox potential values. The particular composite electronic structure of the complexes with differing redox and ''optical'' orbitals is related to their ''light switch'' behavior, i.e. to the absence of luminescence quenching in a nonaqueous environment.