Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways

Oxidation-state sensitive light-induced dynamics of Ruthenium-4H-Imidazole complexes

Oxidized molecular states are key intermediates in photo-induced redox reactions, e. g., intermolecular charge transfer between photosensitizer and catalyst in photoredox catalysis. The stability and longevity of the oxidized photosensitizer is an important factor in optimizing the respective light-driven reaction pathways. In this work the oxidized states of ruthenium(II)-4H-imidazole dyes are studied. The ruthenium complexes constitute benchmark photosensitizers in solar energy interconversion processes with exceptional chemical stability, strong visible light absorption, and favourable redox properties. To rationalize the light-induced reaction in the oxidized ruthenium(III) systems, we combine UV-vis absorption, resonance Raman, and transient absorption spectroelectrochemistry (SEC) with time-dependent density functional theory (TDDFT) calculations. Three complexes are compared, which vary with respect to their coordination environment, i. e., combining an 4H-imidazole with either 2,2'-bipyridine (bpy) or 2,2';6'2"-terpyridine (tpy) coligands, and chloride or isothiocyanate ligands. While all oxidized complexes have similar steady state absorption properties, their excited state kinetics differ significantly; the study thus opens the doorway to study the light-driven reactivity of oxidized molecular intermediates in intermolecular charge transfer cascades.

Operando Spectroscopic Analysis of Axial Oxygen-Coordinated Single-Sn-Atom Sites for Electrochemical CO2 Reduction

Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO₂ reduction reaction (CO₂RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO₂RR. The selectivity and activity of CO₂ reduction to formic acid on Sn-single-atom sites are shown to be correlated with Sn(IV)-N₄ moieties axially coordinated with oxygen (O-Sn-N₄), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density (j_HCOOH) of 74.8 mA·cm^-2 at -1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and ¹¹⁹Sn Mössbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO₂RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn-O-CO₂ species over the O-Sn-N₄ sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn-N₄ sites, thereby greatly facilitating CO₂-to-HCOOH conversion.

Outpacing conventional nicotinamide hydrogenation catalysis by a strongly communicating heterodinuclear photocatalyst

Unequivocal assignment of rate-limiting steps in supramolecular photocatalysts is of utmost importance to rationally optimize photocatalytic activity. By spectroscopic and catalytic analysis of a series of three structurally similar [(tbbpy)2Ru-BL-Rh(Cp*)Cl]3+ photocatalysts just differing in the central part (alkynyl, triazole or phenazine) of the bridging ligand (BL) we are able to derive design strategies for improved photocatalytic activity of this class of compounds (tbbpy = 4,4´-tert-butyl-2,2´-bipyridine, Cp* = pentamethylcyclopentadienyl). Most importantly, not the rate of the transfer of the first electron towards the RhIII center but rather the rate at which a two-fold reduced RhI species is generated can directly be correlated with the observed photocatalytic formation of NADH from NAD+. Interestingly, the complex which exhibits the fastest intramolecular electron transfer kinetics for the first electron is not the one that allows the fastest photocatalysis. With the photocatalytically most efficient alkynyl linked system, it is even possible to overcome the rate of thermal NADH formation by avoiding the rate-determining β-hydride elimination step. Moreover, for this photocatalyst loss of the alkynyl functionality under photocatalytic conditions is identified as an important deactivation pathway.

Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes

This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.