Elucidating of multielectron reaction pathways in, e.g., hydrogen evolving photocatalysis requires the time-resolved detection and identification of reactive intermediates. Due to the transient nature of these reactive intermediates, systematic spectroscopic studies of their (excited-state) properties are not straight-forward. To address this question we recently implemented ultrafast transient absorption spectroelectrochemistry^[1] complementing Rama-based methods to characterize electronically excited states in transient intermediates of multielectron transfer cascades.^[2] In this presentation I will review these experimental developments and focus specifically on recent work on a class of RuII-tetrapyridophenazine-based photocatalysts^[3] (see Figure 1). In-situ time-resolved femtosecond pump-probe data will be discussed to show the impact of catalyst inactivation on the intramolecular photoinduced processes underlying catalytic activity. Furthermore, in-situ transient absorption spectroelectrochemistry on the catalyst allows us to decipher the details of the excited-state dynamics in a catalytically competent intermediate of the photocatalytic cycle. We identify MLCT transitions from the fully activated (reduced) catalytic center back to the bridging ligand as one of the key processes limiting the catalytic activity. In addition, the key structure of the deactivation pathway, i.e., the long-lived radical anion [(tbbpy)2RuII(phen-phz^•--phen)RhIICp*], is prone to photoinduced hydrogenation at the reactive phz^•¯ moiety under catalytic conditions, might result in a loss of catalytic activity. The spectroelectrochemical data helps to rationalize the irradiation dependent catalytic turnover of the catalyst.

Figure: Substrate scope of photocatalytic activity of Ru(tpphz)RhCp^*, products are highlighted in red

1. S. Bold et al. Electron transfer in a covalent dye-cobalt catalyst assembly – a transient absorption spectroelectrochemistry perspective, Chemical Communications 2018, 54, 10594.
2. (a) L. Zedler et al. Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways, Molecules 2019, 24, 245; (b) J.F. Lefebvre et al. An artificial photosynthetic system for photoaccumulation of two electrons on a fused dipyridophenazine (dppz)-pyridoquinolinone ligans, Chem. Sci. 2018, 9, 4152.
3. (a) M.G. Pfeffer et al. Optimization of Hydrogen-evolving photochemical molecular devices, Angew. Chem. Int. Ed. 2015, 54, 6627-6631; (b) M.G. Pfeffer et al. Palladium versus Platinum: The metal in the catalytic center of a molecular photocatalyst determines the mechanism of the Hydrogen production with visible light, Angew. Chem. Int. Ed. 2015, 54, 5044.