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

Abstract

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)2RuII(tpphz)RhICp*] of [(tbbpy)2Ru(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.

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

“…Further, additional electronic states might contribute to light absorption leading to new relaxation or degradation pathways compared to the original species. [152][153][154][155][156] Furthermore, the interaction with the sacrificial agent can significantly alter the electron transfer process as is observed under inert conditions. Depending on the order of events, electron transfer before rereduction of the sensitizer (oxidative quenching) or first reduction of the photoexcited sensitizer followed by electron transfer (reductive quenching) can occur, and hence the mechanism might be completely different from what is observed in inert solvent environment.…”

“…167 A very intensely studied model system for intramolecular photoctalytic hydrogen generation of the types [(tbbpy) 2 M 1 -(tpphz)M 2 ] 2+ (M 1 = Ru, Os; M 2 = PdCl 2 RutpphzPd, PtCl 2 RutpphzPt, PtI 2 , RhCp*Cl RutpphzRhCp*; tbbpy = 4,4 0 -di-tertbutyl-2,2 0 -bipyridine; tpphz = tetrapyrido[3,2-a:2 0 ,3 0 c:3 00 ,2 00 ,-h: 2 0 0 0 ,3 0 0 0 -j]phenazine; Cp* = pentamethylcyclopentadienyl) [175][176][177][178][179] together with the model compounds [(tbbpy) 2 Ru(tpphz)] 2+ , and [(tbbpy) 2 Ru(dppz)] 2+ (dppz = dipyridophenazine) have been subject of intense spectroscopic investigations to understand the function underlying charge-transfer processes. 153,174,[180][181][182][183][184] One of the first questions, which was addressed is the character of the initial excitations and the direction of the connected initial charge transfer. The rR spectroscopic characterization of the non-reduced form revealed for RutpphzPd a correlation between the localization of the initial MLCT and the catalytic efficiency for hydrogen generation.…”

“…6), which is a photocatalyst for the hydrogenation of nicotinamide (a nicotinamide adenine dinucleotide (NAD) analogue) and proton reduction. 153 The catalytically active species, i.e., the doubly reduced [(tbbpy) 2 Ru II (tpphz)Rh I Cp*] is generated under catalytic conditions via two consecutive sequences of photoinduced electron transfers and rereduction of the light absorbing Ru-polypyridine unit by a sacrificial agent (triethylamine -TEA), but can also be generated by chemical reduction with cobaltocene. If the doubly reduced RutpphzRhCp* is generated by chemical reduction and tested for its reactivity in a ''dark'' reaction to react with benzylnicotinamide, it was observed that additional irradiation reduced the catalytic turnover significantly.…”

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

“…For related rhodium complexes, the photochemical conversion of chloro-to hydride complexes has been explored to some extent. [86][87][88] To probe this conversion under conditions resembling more closely those of the reactions in Section 2.2, 3.5 mM [Cp*Ir(phen)Cl] + in CD 3 CN containing 350 mM TEA (but no olen substrate) was irradiated at 470 nm. The 1 H-NMR spectra recorded (in the dark) aer photo-irradiation for 2 hours provide clear evidence for the formation of [Cp*Ir(phen)(H)] + (ESI, Section 4.2 †).…”

Ruthenium Assemblies for CO2 Reduction and H2 Generation: Time Resolved Infrared Spectroscopy, Spectroelectrochemistry and a Photocatalysis Study in Solution and on NiO

“…We and others have shown the importance of the energy levels of the peripheral and bridging ligand (Tschierlei et al, 2010), as directional electron transfer from the ruthenium centre via the bridging ligand to the catalytic centre is required to facilitate catalysis (Tschierlei et al, 2010;Karnahl et al, 2011;Singh Bindra et al, 2012;Zedler et al, 2019). For CO 2 reduction, the group of Ishitani has reported various suitable systems, showcasing the influence of the bridging ligand in particular on the catalysis (Sato et al, 2007;Nakada et al, 2015;Ueda et al, 2015;Tamaki and Ishitani, 2017).…”