Title: Elucidating the Nature of the Excited State of a Heteroleptic Copper Photosensitizer Using Time-Resolved X-ray Absorption Spectroscopy

We report the light-induced electronic and geometric changes taking place within a heteroleptic Cu(I) photosensitizer, namely [(xant)Cu(Me2phenPh2)]PF6 (xant = xantphos, Me2phenPh2 = bathocuproine), by time-resolved X-ray absorption spectroscopy in the ps-μs time-regime. Time-resolved X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis enabled the elucidation of the electronic and structural configuration of the copper centre in the excited state as well as its decay dynamics in different solvent conditions with and without triethylamine acting as a sacrificial electron donor. A threefold decrease in the decay lifetime of the excited state is observed in the presence of triethylamine showing the feasibility of the reductive quenching pathway in the latter case. A prominent pre-edge feature is observed in the XANES spectrum of the excited state upon metal to charge ligand transfer transition showing an increased hybridization of the 3d states with the ligand p orbitals in the tetrahedron around the Cu centre. EXAFS and Density Functional Theory illustrate a significant shortening of the Cu-N and an elongation of the Cu-P bonds together with a decrease in the torsional angle between the xantphos and bathocruproine ligand. This study provides mechanistic time-resolved understanding for the development of improved heteroleptic Cu(I) photosensitizers, which can be used for the light-driven production of hydrogen from water. Introduction The production of solar fuels by using the energy of the sun is one of the most urgent scientific challenges in view of the rapid depletion of fossil fuels, the resulting environmental pollution and climate change. Al o g od y’s s of technology has made considerable progress in producing electricity using sunlight and wind, these intermittent sources will only find limited applications without efficient energy storage. 2] One capable way to store solar energy is to convert it into chemical energy through fuel forming reactions inspired by natural photosynthesis, such as the light induced splitting of water into hydrogen and oxygen (2 H2O + 4 ʋ = O2 + 2 H2). [3] Indeed, the prospect of using molecular hydrogen has motivated the development and application of light harvesting units for solar H2 generation in both metal-complex chromophores and molecular photocatalysts. While most of the molecular photosensitizers are based on noble-metals, such as Ru(II), Ir(III), Pt(II), Au(III) and Re(I), some work on cheaper and more abundant 3d transition metals like Cu(I), Cr(III) and Zn(II) porphyrin complexes have also been reported. 4b, 5] In this respect, novel heteroleptic Cu(I) photosensitizers of the general type [(P^P)Cu(N^N)] composed of a diimine N^N and a bulky diphosphine P^P ligand have attracted particular interest as they allow for considerable photocatalytic activities in conjunction with an Fe-based water reduction catalyst. The design of these heteroleptic Cu(I) complexes is an ongoing area of research that has experienced a significant development during the last years with respect to their wellknown homoleptic counterparts of the type [Cu(N^N)2] + (Scheme 1A and 1B). 5d, 6a, 7] The properties of the homoleptic bisdiimine photosensitizers are already extensively studied since decades due to their tunable photophysical and electrochemical properties. These photosensitizers are found to undergo a structural reorganization upon photoexcitation from a pseudo tetrahedral geometry to a flattened state (flattening distortion) enabling the coordination of a solvent molecule to the Cu centre. As a result, a low-energy excited state is formed, known as an “ x pl x”, which exhibits a fast emission decay to the ground state with shortened excited state lifetimes in the order of ps-ns. 9a, . While significant advances have been made in the fine-tuning of the diimine ligands 8c, 8g, 10] and the improvement of the resulting bis-diimine complexes, these homoleptic Cu(I) compounds have only found restricted use in the photocatalytic reduction of protons from water. 11] In contrast, a large number of the heteroleptic ones exhibit increased excited state lifetimes, which even exceed some of the commonly applied Ru(II) photosensitizers. 6b, 7a, 12] In [a] Dr. D. Moonshiram*, Max Planck Institute for Chemical Energy Conversion, Stiftstr. 3436, Mülheim an der Ruhr, Germany, Email: dmoonshi@gmail.com [b] Dr. D. Moonshiram, Mr. P. Garrido-Barros, Dr. C. Gimbert , Prof. Dr. A. Llobet* Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16, 43007 Tarragona, Spain, E-mail: allobet@iciq.cat [c] Mr. P. Garrido-Barros Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Campus Sescelades, C/Marcellí Domingo, s/n, 43007, Tarragona, Spain [d] Dr. A. Picón Grupo de Investigacion en Aplicaciones del Laser y Fotonica, Universidad de Salamanca, 37008, Salamanca, Spain [e] Dr. A. Picón Departamento de Química, Modulo 13, Universidad Autónoma de Madrid, Cantoblanco 28049, Madrid, Spain [f] Dr. C.Liu, Dr. X.Zhang X-ray Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, IL, 60439, U.S.A [g] Dr. M. Karnahl* University of Stuttgart, Institute of Organic Chemistry, Pfaffenwaldring 55, 70569, Stuttgart, Germany, Email: michael.karnahl@uni-stuttgart.de [h] Prof. Dr. A. Llobet* Departament de Química, Universitat Autonoma de Barcelona, 08193 Cerdanyola del Valles, Barcelona, Spain Supporting information for this article is given via a link at the end of the document. This includes pre-edge XAS calculations, EXAFS simulations, EXAFS analysis, and DFT optimized methods and coordinates. Supporting information for this article is given via a link at the end of the document 10.1002/chem.201800330 A cc ep te d M an us cr ip t Chemistry A European Journal This article is protected by copyright. All rights reserved.