Light Induced CO2 2-Electron Activation and Mechanistic Insights – LOCO

The LOCO project aims to investigate the mechanistic events in the light-induced 2-electron activation of CO2 with a photocatalyst. LOCO covers all aspects concerning the design, synthesis and a full-fledged spectroscopic investigation of novel porphyrin-based photocatalysts to go beyond the usual performance of turnover numbers and frequencies.
We are set to employ complementary techniques to identify all intermediates formed during the catalytic cycle. In particular, we will make use of our novel pump-pump probe set up complemented by innovative time-resolved pump-pump resonance Raman probe spectroscopy to characterize short-lived intermediates resulting from photoexcitation. These studies will bring structural details of intermediates and inform on the kinetic changes in the presence of the substrate (CO2) by probing specific vibrational mode during a sequential 2-electron activation. Steady-state spectroscopic investigation of intermediates using electro- and chemical reduction will usefully complement time-resolved experiments to provide us a complete panorama of the processes governing CO2 reduction on multiple time scales.
For the molecular systems studied, we will build on the original findings in our recent reports on the design of metalloporphyrin families for CO2 reduction and juggle with three parameters to enhance the reactivity: electronic effects induced by suitable substitutions, second sphere hydrogen bonding interactions between the catalyst and the substrate and the presence of Lewis acids in the second coordination sphere.

A new time-resolved Raman spectrometer with two OPO lasers was set up in ISMO. The setup now can be operated in resonance mode for Raman probe in the whole visible range. A first experiment with a zinc porphyrin (ZnTPPF20) in the presence of an electron donor, clearly showed the formation of reduced state of the porphyrin both in transient absorption and time-resolved Raman.
Two of the first generation iron-porphyrin catalysts bearing four urea or four imidazolium groups were synthetized and fully characterized using different spectroscopic methods (NMR, UV-vis, MS, etc.). Their electrochemical properties as well as electrocatalytic performances were also determined. Three of the four second generation iron porphyrin catalysts were also successfully synthetized and characterized.
Reduced forms of the first generation catalyst bearing four urea have been chemically and electrochemically generated and their visible and mid-infrared spectroscopic features obtained to help in the interpretation of the time-resolved transient absorption as well as steady-state photo-accumulation studies.
The same catalyst has been also investigated for CO2 reduction photocatalysis in the presence of a chromophore, a sacrificial electron donor and a proton source. Results show selective formation of CO with excellent TOF and TON values.
Pump-probe and pump-pump-probe transient absorption studies were performed on the first generation of iron porphyrins. Single and doubly reduced states of the porphyrin were observed upon simple and double excitation of the photosystems containing a photosensitizer, iron porphyrin and an electron donor. The transient species obtained were compared to those observed in spectro-electrochemical and photoaccumulation studies, thus confirming the formation of these reduced states.

Second and third generation of catalyst will be prepared and characterized. TON, TOF and the selectivity of our catalysts will be determined by combining bulk electrocatalysis and photocatalysis with GC-MS analysis of the products. UV/VIS, IR and Raman signatures of chemical reduced species will be studied in absence and presence of substrate. The photophysical properties of our new catalytic systems will be studied by time-resolved spectroscopies (UV/VIS absorption, FTIR, Raman) in absence and in presence of CO2 substrate. Pump-pump-probe resonance Raman experiment will be set up to investigate the vibrational structures of charge accumulative states to characterize the structure of intermediate states. We will explore the dynamical and structural properties while varying different parameters such as the metal center, the periphery groups, concentration of CO2. The role of the functional groups (imidazolium, urea or metallic Lewis acid) present on the periphery of the porphyrin macrocycle on CO2 binding will be emphasized. In the light of experimental results obtained in this project, a mechanistic scheme can then be proposed for the photoreduction of CO2.

1. Gotico, P.; Tran, T.-T.; Baron, A.; Vauzeilles, B.; Lefumeux, C.; Ha-Thi, M.-H.; Pino, T.; Halime, Z.; Quaranta, A.; Leibl, W.; Aukauloo, A., ChemPhotoChem DOI : 10.1002/cptc.202100010

2. P. Gotico, L. Roupnel, R. Guillot, M. Sircoglou, W. Leibl, Z. Halime and A. Aukauloo, Angew. Chem. Int. Ed. Engl., 2020, 59, 22451-22455
3. A. Khadhraoui, P. Gotico, W. Leibl, Z. Halime and A. Aukauloo, ChemSusChem, 2021, 14, 1308-1315

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1. CEA 10 novembre 2020
Titre de l’article : Valorisation du CO2 : optimisation d’un catalyseur bio-inspiré
joliot.cea.fr/drf/joliot/Pages/Actualites/Scientifiques/2020/Valorisation-CO2-optimisation-catalyseur-bio-inspire.aspx
2. CNRS 26 Novembre 2020
Titre de l’article : Faciliter la transformation du CO2 par le contrôle des liaisons hydrogène
www.inc.cnrs.fr/fr/cnrsinfo/faciliter-la-transformation-du-co2-par-le-controle-des-liaisons-hydrogene
3. ChemistryViews 16 février 2021
Titre de l’article : Improved CO2 Reduction with Iron Porphyrins
www.chemistryviews.org/details/ezine/11284943/Improved_CO2_Reduction_with_Iron_Porphyrins.html

Scientists are urged to produce sustainable energy vectors based on the solar conversion of CO2 to energy-rich synthons. We have witnessed a prolific number of new electro- and photo-catalysts for the reduction of CO2. However, we are far from having in hands solutions to deploy on a large scale. Performing these reactions requires the coupling of multiple photoinduced one-electron transfer steps in multi-electron, multi-proton catalysis. Therefore chemists and physicists need to address urgently the problem of light-induced charge accumulation in photocatalytic systems. Understanding these early photophysical events is of primordial importance and stand as the bottleneck in the optimization process for efficient photochemical reduction of CO2. Whereas for electrocatalytic systems turnover numbers and frequencies together with Faradic efficiency are adequate parameters to evaluate the performance, in photocatalytic systems the wavelength dependent quantum (or photon-to-fuel) efficiency is a crucial parameter. The efficiency of the photoreduction is not only related to the thermodynamic requirements but also to other factors such as competing deactivation pathways such as unwanted electron or energy transfers or photo-degradation of the complexes. It is therefore urgent to interrogate the intrinsic, functional limitations in photocatalytic processes to develop viable solutions in the context of artificial photosynthesis. Despite numerous reports on new catalyst systems in the recent years, mechanistic studies on the elementary reactions involved in the photocatalytic cycle of CO2 reduction still remain scarce. In this context, the LOCO project aims to investigate the mechanistic events in the light-induced 2-electron activation of CO2 with a photocatalyst. LOCO covers all aspects concerning the design, synthesis and a full-fledged spectroscopic investigation of novel porphyrin-based photocatalysts to go beyond the usual performance of turnover numbers and frequencies. We are set to employ complementary techniques to identify all intermediates formed during the catalytic cycle. In particular, we will make use of our novel pump-pump probe set up (Angew. Chem. Int. Ed. 2017, 56, 15936 –15940) complemented by innovative time-resolved pump-pump resonance Raman probe spectroscopy to characterize short-lived intermediates resulting from photoexcitation. These studies will bring structural details of intermediates and inform on the kinetic changes in the presence of the substrate (CO2) by probing specific vibrational mode during a sequential 2-electron activation. Steady-state spectroscopic investigation of intermediates using electro- and chemical reduction will usefully complement time-resolved experiments to provide us a complete panorama of the processes governing CO2 reduction on multiple time scales. The insight gathered on constructive and unproductive reactions observed during charge-accumulation and catalysis will constitute a unique and powerful feedback to develop more efficient classes of metalloporphyrin photocatalysts for the activation of CO2. For the molecular systems studied, we will build on the original findings in our recent reports on the design of metalloporphyrin families for CO2 reduction (Chem. Commun., 2018,54, 11630-11633, Angew. Chem. Int. Ed 2019, 58, 4504-4509), and juggle with three parameters to enhance the reactivity: electronic effects induced by suitable substitutions, second sphere hydrogen bonding interactions between the catalyst and the substrate and the presence of Lewis acids in the second coordination sphere. The interdisciplinary LOCO project will enable us to gain a much deeper insight on the mechanistic events governing CO2 photoreduction. Understanding such fundamental processes in light-induced catalysis stands as the stepping stones for the development of more efficient catalytic systems towards artificial photosynthesis.