Multicomponent Microporous Polymers as Photosystems for Everlasting CO2 Reduction

Heterogeneous catalysis allows to circumvent the problem of separation of the catalyst from the products and to simplify its recyclability. The integration of the catalytically active centers into a solid support without loss of performance compared to the homogeneous analog is still a major challenge. In this context, a molecularly defined support as macroligand, i.e. a solid acting like the ligand in the corresponding molecular complex, can be considered as a key to bridge the gap between molecular and heterogeneous catalysis. In particular, porous frame-works made by the repetition of a coordinating motif, like the bipyridine motif are of a high interest as bipyridines are widely used as chelating ligand for molecular catalysts.[1–4] Amongst the catalytic applications, photochemical reactions such as CO2 reduction are of great interest as routes to different value-added C1-molecules, which have high potential as renewable energy sources. Although many efforts on increasing the catalytic efficiency have been achieved, typical catalytic photosystems suffer from low long-term stability or selectivity. Here, we will present a set of fully heterogeneous photosystems for the photocatalytic reduction of CO2 using visible light in the absence of any external photosensitizer. In these materials, the light harvesting moieties are directly enchased in the material structure and tethered to the catalytically active rhodium complexes. These new types of heterogeneous photocatalysts allow a constant and efficient transformation of CO2 into formate with production rates of up to 700 µmol formate per gram of catalyst per hour without any deactivation for at least 4 days, a superior performance with respect to state of the art. We explain their superior performance by a combination of stable organic dyes used as photosensitizer under the visible light, a well-adapted electron density on the active site by a rational choice of the porous framework and an optimized charge separation. We will demonstrate, using in-situ transient spectroscopy, that upon metalation, the excited states of the fully heterogeneous photosystems are effectively quenched resulting in a decrease of their life-times. DFT calculations show that the LUMO of the fully heterogeneous photosystem is centered on the Rh-based catalyst. This allows for an efficient electron transfer from the photosensitizer to the catalytically active Rh and, as a result, helps to stabilize the charge-separated states. References [1]A. Corma, H. Garcia, F. X. Llabres i Xamena, Chem. Rev. 2010, 110, 4606–4655. [2]C. Kaes, A. Katz, M. W. Hosseini, Chem. Rev. 2000, 100, 3553–3590. [3]F. M. Wisser, P. Berruyer, L. Cardenas, Y. Mohr, E. A. Quadrelli, A. Lesage, D. Farrusseng, J. Canivet, ACS Catal. 2018, 8, 1653–1661. [4]F. M. Wisser, Y. Mohr, E. A. Quadrelli, D. Farrusseng, J. Canivet, ChemCatChem 2018, 10, 1778–1782.