Photodriven Molecular Systems for CO2 Reduction
Authors: Sinopoli Alessandro 
Published: June 2019

Alessandro Sinopoli, a Michael Wasielewski, b Muhammad Sohail. a aQatar Environment & Energy Research Institute, Hamad Bin Khalifa University, PO Box 5825, Doha, Qatar; bDepartment of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA. The use of fossil fuels, especially natural gas, which has a lower carbon burden than oil or coal, will continue to dominate the energy market for the next several decades. In order to mitigate the large amounts of CO2 released into the atmosphere, it is essential to have clean ways of converting the CO2 back into a high energy density liquid fuel. Benchmark catalysts for CO2 reduction typically employ second and third row transition metals like palladium, iridium, and rhenium. The rarity, cost, and environmental ramifications of mining large quantities of such precious metals deter scaling up the usage of such catalysts. Developing a CO2 catalytic system that uses earth abundant first row transition metals like manganese, will push the field away from rare earth metal catalysts. While still an emerging field, transition metal-based catalysts can be successfully integrated in electrochemical and photoelectrochemical devices for CO2 reduction. Inside this class of compounds, developing manganese-based systems remains an extremely promising strategy for the advent of low cost but at the same time efficient molecular systems for CO2 reduction. We will therefore investigate Mn(bpy)(CO)3Br and its related derivatives that have recently been shown to efficiently reduce CO2 to CO. More importantly, none of these Mn-bipyridine complexes have yet been integrated with a covalent photo-driven source of electrons and this will be the main synthetic and experimental challenge within this project. Aromatic organic anions (radical anions or closed shell dianions) are reactive species involved in various chemical transformations. The photochemistry and photophysics of these species have been the subject of extensive study since they have been implicated in many photo-induced electron transfer processes, and are suggested to be powerful reducing agents. Such properties were substantiated by the observation of rapid electron ejection to give solvated electrons, electron-cation pairs, or reduced electron acceptors upon photolysis of aromatic anions (e.g. sodium pyrenide or biphenylide). We envision that the high reducing power (up to − 3 V vs. SCE) of excited monoanions or dianions of aromatic diimides can be exploited for CO2 reduction catalysis. In particular, the anions of PDI perylene-3,4:9,10-bis(dicarboximide) and its homolog naphthalene-1,8:4,5-bis(dicarboximide) (NDI) will be used as the primary photosensitizers in this study. These planar, chemically robust, redox-active compounds are widely used as industrial pigments, supramolecular building blocks, photooxidants, and n-type semiconducting materials. In the effort of integrating NDI and PDI units in the design and development of manganese based photocatalysts, experiments will be carried out in two stages. In the first stage, we will synthesise photosensitizer systems designed specifically to act as catalysts, so that their structures and properties can be determined. Figure 1. Examples of Mn-based catalysts designed for this project. The structures depicted in Figure 1 are examples of PDI and NDI photosensitizer systems designed for this project. For example, in the molecule on the left (Figure 1), two easily reducible PDI molecules are covalently attached to the bpy ligand. One-electron reduction of PDI will produce the stable PDI-• radical anion. The structure of the designed manganese complexes, the dynamics of CO2 binding, and subsequent protonation of the bound Mn carboxylate will be studied using femtosecond transient absorption spectroscopies with both UV-Vis and mid-IR probing as well as and femtosecond stimulated Raman spectroscopy (FSRS). The second stage of the experiments will study the laser-generated Mn(0)(bpy)-1(CO)3-based catalyst followed by CO2 binding and protonation. For this we will perform steady-state photolysis experiments on the systems in DMF with varying amounts of HBF4 and CO2 up to the limit of a saturated solution of CO2 in DMF at 25 °C and 1 atm, which is about 0.14 M. These studies will look for products generated following one-electron reduction of the Mn catalysts. We will quantitate the yield of CO produced by the overall photoreaction using a 16-sample parallel system in which the samples are illuminated using filtered LEDs and a parallel, continuous gas sampling system connected to a gas chromatograph. The system will be calibrated with appropriate standard gas mixtures, and a full range of conditions and appropriate control experiments will be performed by varying the concentrations of catalyst, CO2, HBF4, and sodium ascorbate in the solution. Solar energy, an abundant resource in Qatar, can drive CO2 reduction to produce both liquid solar fuels and potentially high value chemicals. In this regard, our study will represent the birth of a new generation of efficient photocatalytic systems for CO2 reduction by coupling robust light harvesting molecules with earth abundant transition metals like manganese.