Tuning the Electron Storage Potential of a Charge‐Photoaccumulating Ru^II Complex by a DFT‐Guided Approach

Abstract

Molecular photosensitizers that are able to store multiple reducing equivalents are of great interest in the field of solar fuel production, where most reactions involve multielectronic reduction processes. In order to increase the reducing power of a ruthenium tris‐diimine charge‐photoaccumulating complex, two structural modifications on its fused dipyridophenazine‐pyridoquinolinone ligand were computationally investigated. Addition of an electron‐donating oxime group was calculated to substantially decrease the reduction potentials of the complex, thus guiding the synthesis of a pyridoquinolinone‐oxime derivative. Its spectroscopic and (spectro)electrochemical characterization experimentally confirmed the DFT predictions, with the first and second reduction processes cathodically shifted by −0.24 and −0.14 V, respectively, compared to the parent complex. Moreover, the ability of this novel artificial photosynthetic system to store two photogenerated electrons at a more reducing potential, via a proton‐coupled electron‐transfer mechanism, was demonstrated.

A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π‐Extended dppz Ligand for Light‐Driven Accumulation of Multiple Reducing Equivalents

“…We conclude that the 3 ππ* states are non‐emissive regardless of the oxidation and protonation state, i.e ., for Ru , RuH + , and RuH 2 . In agreement with studies on the structurally closely related complex [(bpy) 2 Ru( oxo ‐ dppqp )] 2+ , we suggest this to be an inherent feature of the π‐extended ligand oxim ‐ dppqp [21,23] …”

“…Therefore, three doubly‐protonated species were considering a doubly‐protonated and doubly‐reduced species at the i) pq and pz (1.30 eV), ii) pq and N ‐oxime (0.00 eV), and iii) pz and N ‐oxime moiety (0.90 eV). As initially proposed, [23] the most stable doubly‐reduced and doubly‐protonated structure, i.e ., a formal hydrogenated species, is the aminophenylhydroxylamine derivative (Figure 1 and Table S10). A similar structure, namely an aminophenol species, was previously experimentally supported for the photogenerated doubly‐reduced doubly‐protonated derivative of [(bpy) 2 Ru( oxo ‐ dppqp )] 2+ [22] …”

“…S 7 at 2.66 eV in Ru , see CDDs in Figure 2), causing the appearance of new absorption features between 500 and 700 nm. Those features are blue‐shifted (by 0.23 eV) compared to the low‐lying absorption maximum of the doubly‐reduced form of Ru (655 nm) [23] . As the excess charge of the reduced complex is localized on the pq‐moiety [21,23] this blue‐shift indicates a charge‐compensation upon addition of protons, causing a stabilization of the pq‐centered molecular orbitals.…”

“…continuous illumination in the presence of triethylamine (TEA) as sacrificial agent (photoreaction yield of circa 24 %), according to our previous studies (cf. Figure S5) [21–23] . Likewise, as performed for Ru + and based on the most stable singly‐protonated species (pq), the equilibrium structure of RuH 2 was investigated by means of DFT.…”

“…Thus, we were able to reproduce the mechanism at work with the PSII – plastoquinone couple in the photosynthetic chain [21,22] . Furthermore, we have demonstrated that it is possible to use a DFT‐guided approach to design better performing systems, with the oxime derivative, namely [(bpy) 2 Ru( oxim ‐ dppqp )] 2+ ( Ru , bpy=2,2’‐bipyridine), storing electrons at a potential 200 to 300 mV more negative [23] . In this contribution, we study key intermediates involved in the 2e − /2H + storage in this oxime complex Ru .…”