A ππ* State Enables Photoaccumulation of Charges on a π-Extended Dipyridophenazine Ligand in a Ru(II) Polypyridine Complex

Abstract

The π-extended dipyrido[3,2-a:2′,3′-c]phenazine (dppz) ligand of the Ru(II) complex [Ru(bpy) 2 (oxo-dppqp)]- 3″-4,5,6]quinolino[2,3-h]phenazin-15-one, bpy = 2,2′-bipyridine) enables the mononuclear complex for visible-light-driven accumulation of two electrons on a single ligand structure. Although this has been shown before, the excited-state physics underlying this promising feature are exploited in this work. The photophysics of the complex was investigated by excitation-wavelengthdependent resonance Raman and transient absorption spectroscopy in combination with time-dependent density functional theory. The results show that excitation with visible light leads to the population of the two excited-state branches: (i) the population of a short-lived 3 MLCT state in which the excess electronic density is localized on the pyridoquinolinone moiety of the extended ligand (τ = 105 ps) and (ii) the population of a more long-lived 3 ππ* state (τ = 9 ns). Notably, the long-lived 3 ππ* state rather than a 3 MLCT state is prone to reductive quenching by the sacrificial electron donor and, hence, presents the critical excited-state intermediate in the photochemical charge accumulation experiments.

Tuning the Electron Storage Potential of a Charge‐Photoaccumulating RuII Complex by a DFT‐Guided Approach

“…In addition to the intense ligand‐centered transition (bpy ligands) at 280 nm, it features a broad band in the visible region, tailing at 600 nm, thus slightly red‐shifted with respect to [1] 2+ . According to our previous studies, metal‐to‐ligand‐charge‐transfer (MLCT) and ligand‐centered π–π* transitions (on the π‐extended ligand) both contribute to this relatively intense absorption (molar absorptivity of 25 000 L mol −1 cm −1 ) …”

“…In addition to the intense ligand-centered transition (bpy ligands) at 280 nm, it features ab road band in the visible region, tailing at 600 nm, thus slightly redshifted with respectt o[1] 2 + .A ccording to our previous studies, metal-to-ligand-charge-transfer (MLCT) and ligand-centered p-p*t ransitions (on the p-extended ligand)b oth contribute to this relativelyi ntensea bsorption (molar absorptivity of 25 000 Lmol À1 cm À1 ). [50,51] Cyclic (CV) and differentialp ulse (DPV) voltammetries were used to investigate the electrochemical properties of [2](PF 6 ) 2 (Figure 2). In Figure 2a,t he first reduction of [2] 2 + is compared to that of [1] 2 + ;i ti sa pparent that the first reduction of [2] 2 + occurs at as ubstantially lower potentialt han that of [1] 2 + .A ccordingt ot he DPV results ( Figure 2c), this reduction occurs at À1.11Vvs.…”

“…In 2018, we reported a mononuclear ruthenium tris‐diimine complex with an extended phenazine‐based ligand [1] (PF 6 ) 2 (Scheme ) which falls into the second category of charge‐accumulating PS . The phenazine ligand features a tetracyclic pyridoquinolinone subunit inspired by the quinone‐based electron storage and relay systems involved in photosynthesis .…”

“…[42][43][44][45] In 2018, we reported am ononuclear ruthenium tris-diimine complex with an extended phenazine-based ligand [1](PF 6 ) 2 (Scheme 1) which falls into the second category of charge-accumulating PS. [50,51] The phenazine ligand features at etracyclic pyridoquinolinone subunit inspired by the quinone-based electron storage and relay systems involved in photosynthesis. [52] We demonstrated that [1](PF 6 ) 2 is capable of storing two photogenerated electrons upon excitation of the complexw ith visible light in the presence of as acrificial electron donor.I n the absence of as ource of protons, the first reduction of [1] 2 + occurs at À0.87 Vand the second at À1.32 Vv ersus the ferrocenium/ferrocene (Fc + /Fc) redox couple;t hese potentials are low enought od onate electrons to many of the best-performing catalysts for H 2 production.I nt he presence of proton donors, however,t he reduction processes become coupled to proton transfers, yielding the stabilized doubly reduced doubly Scheme1.Synthesis of [2](PF 6 ) 2 from [1](PF 6 ) 2 .…”

Phenanthroline‐Fused Pyrazinacenes: One‐Pot Synthesis, Tautomerization and a RuII(2,2′‐bpy)2 Derivative

“…In particular, the addition of extended aromatic ligands has been extensively studied for the corresponding Ru II complexes. Those works have given insight into the structure parameters that might affect the materials' performances of the compounds, and the compounds themselves remain highly relevant , . Beyond the already well‐studied π‐extended phenanthroline ligands, the introduction of the pyrazinacene‐type ligands to these dyes present some prospects for developing a series of highly fluorescent compounds whose substituents can be easily modified either at (bpy) 2 Ru II moiety (as has already been done) or at the pyrazinacene (here a fluorubine) moiety where lateral substitution at nitrogen atoms could be advantageous for tuning the interaction of the complexes with TiO 2 .…”