Electronic structure of the "molecular light switch" bis(bipyridine)dipyrido[3,2-a:2',3'-c]phenazineruthenium(2+). Cyclic voltammetric, UV/visible and EPR/ENDOR study of multiply reduced complexes and ligands

Abstract

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-ENDOR spectra of the intermediate anion radicals were obtained and analyzed. The results are interpreted using a HMO/McLachlan perturbation approach of x spin populations and orbital energies. Three low-lying unoccupied x molecular orbitals can be identified as phenazinetype (bi, lowest) and as the ^(b,) and xfo) orbitals of the a-diimine moiety. Complexes with the N(4),N(3)-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 x* orbital of (dm)dppz by a single electron, the stepwise reduction of the bpy ligands resulting in temperaturedependent intensity loss of that EPR signal. The very basic quadruply reduced state exhibits EPR characteristics which are typical for Ru(II)-bound a-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.

Influence of pH on the Photochemical and Electrochemical Reduction of the Dinuclear Ruthenium Complex, [(phen)2Ru(tatpp)Ru(phen)2]Cl4, in Water: Proton‐Coupled Sequential and Concerted Multi‐Electron Reduction

“…From both the current data and by comparison with the related Ru II complexes of dipyridophenazine (dppz) [31][32][33][34][35][36][37][38][39] and tetrapyridophenazine (tpphz), [28,32,40,41] it is clear that complex P can be viewed as three weakly coupled molecular components; the two independent "Ru II -trisbipyridine" chromophores and a central tetraazapenacene acceptor unit to form a chromophore-acceptor-chromophore triad. The additive nature of the absorption spectra for all three components, observed in Figure 1, reflects this nature.…”

“…2) Multiple reduction states are accessible at potentials more positive than the terminal phen ligands (e.g., compare with Ru II complexes of dppz [39] and tpphz [28] ).…”

An Experimental and Computational Study on Intramolecular Charge Transfer: A Tetrathiafulvalene‐Fused Dipyridophenazine Molecule

“…Upon one-electron reduction of dppz, two new absorption bands at about 330 nm and 580 nm are observed. [26] The spectrum is found to be similar to the singly reduced phenazine (maxima at 375 nm and 551 nm) and therefore the added electron is suggested to be located on the phenazine-based orbital. Thus, in the case of 1 À , the absorption bands at 29 500 cm À1 and at 16 000 cm À1 are most likely to be attributable to dppz À , while the band at 20 600 cm À1 is characteristic for fused (TTF-dppz) À , and it can tentatively be assigned to a dppz!TTF CT transition.…”

Design and spectroscopic study of new ruthenium(II) complexes containing ligands derived from terpyridine and dipyrido[3,2-a:2′,3′-c]phenazine: {Ru(4′-Rph-tpy) [dppz(COOH)]Cl}PF6 with R = NO2, Br, Cl

“…2) retains most of the features of the components of dppz(COOH), particularly the band centered at 364 nm, which essentially corresponds to the widely studied dppz fragment. 11 A broad transition band appears between 400 and 600 nm centered at 515 nm, which is solvent-dependent and can be assigned to a spinallowed 1 MLCT transition, while at ca. 278 nm and below that wavelength, the intense bands can be attributed to the overlap of Rph-tpy and dppz(COOH) π → π * transitions.…”