On the Association and Structure of Radicals Derived from Dipyridil[3,2-a:2‘3‘-c]phenazine. Contrast between the Electrochemical, Radiolytic, and Photochemical Reduction Processes

Unraveling the Light-Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis

Understanding photodriven multielectron reaction pathways requires the identification and spectroscopic characterization of intermediates and their excited-state dynamics, which is very challenging due to their short lifetimes. To the best of our knowledge, this manuscript reports for the first time on in situ spectroelectrochemistry as an alternative approach to study the excited-state properties of reactive intermediates of photocatalytic cycles. UV/Vis, resonance-Raman, and transient-absorption spectroscopy have been employed to characterize the catalytically competent intermediate [(tbbpy)2 RuII (tpphz)RhI Cp*] of [(tbbpy)2 Ru(tpphz)Rh(Cp*)Cl]Cl(PF6 )2 (Ru(tpphz)RhCp*), a photocatalyst for the hydrogenation of nicotinamide (NAD-analogue) and proton reduction, generated by electrochemical and chemical reduction. Electronic transitions shifting electron density from the activated catalytic center to the bridging tpphz ligand significantly reduce the catalytic activity upon visible-light irradiation.

Excited-state dynamics of heteroleptic copper(i) photosensitizers and their electrochemically reduced forms containing a dipyridophenazine moiety – a spectroelectrochemical transient absorption study

Heteroleptic copper(i) dipyridophenazine complexes were investigated by transient absorption spectroelectrochemistry to examine their multi-electron photoaccumulation properties.

Electrocatalytic and photocatalytic conversion of CO(2) to methanol using ruthenium complexes with internal pyridyl cocatalysts

The ruthenium complexes [Ru(phen)2(ptpbα)](2+) (Ruα) and [Ru(phen)2(ptpbβ)](2+) (Ruβ), where phen =1,10-phenanthroline ; ptpbα = pyrido[2',3':5,6]pyrazino[2,3-f][1,10]phenanthroline; ptpbβ = pyrido[3',4':5,6]pyrazino[2,3-f][1,10]phenanthroline, are shown as electrocatalysts and photocatalysts for CO2 reduction to formate, formaldehyde, and methanol. Photochemical activity of both complexes is lost in water but is retained in 1 M H2O in DMF. Controlled current electrolysis of a solution of Ruβ in CO2 saturated DMF:H2O (1 M) yields predominantly methanol over a 6 h period at ∼ -0.60 V versus Ag/AgCl, with traces of formaldehyde. After this time, the potential jumped to -1.15 V producing both methanol and CO as products. Irradiation of Ruβ in a solution of DMF:H2O (1 M) containing 0.2 M TEA (as the sacrificial reductant) yields methanol, formaldehyde, and formate. Identifications of all of the relevant redox and protonated states of the respective complexes were obtained by a combination of voltammetry and differential reflectance measurements. Spectroelectrochemistry was particularly useful to probe the photochemical and electrochemical reduction mechanisms of both complexes as well as the complexes speciation in the absence and presence of CO2.

Intercalation of fac-[(4,4'-bpy)ReI(CO)3(dppz)]+, dppz = dipyridyl[3,2-a:2'3'-c]phenazine, in polynucleotides. On the UV-vis photophysics of the Re(I) intercalator and the redox reactions with pulse radiolysis-generated radicals

The intercalation of fac-[(4,4'-bpy)Re(I)(CO)3(dppz)]+ (dppz = dipyridyl[3,2-a:2'3'-c]phenazine) in polynucleotides, poly[dAdT]2 and poly[dGdC]2, where A = adenine, G = guanine, C = cytosine and T = thymine, is a major cause of changes in the absorption and emission spectra of the complex. A strong complex-poly[dAdT]2 interaction drives the intercalation process, which has a binding constant, Kb approximately 1.8 x 10(5) M(-1). Pulse radiolysis was used for a study of the redox reactions of e(-)(aq), C*H(2)OH and N3* radicals with the intercalated complex. These radicals exhibited more affinity for the intercalated complex than for the bases. Ligand-radical complexes, fac-[(4,4'-bpy*)Re(I)(CO)3(dppz)] and fac-[(4,4'-bpy)Re(I)(CO)3(dppz *)], were produced by e(-)(aq) and C*H(2)OH, respectively. A Re(II) species, fac-[(4,4'-bpy)Re(II)(CO)3(dppz)](2+), was produced by N3* radicals. The rate of annihilation of the ligand-radical species was second order on the concentration of ligand-radical while the disappearance of the Re(II) complex induced the oxidative cleavage of the polynucleotide strand.