Iron sensitizer converts light to electrons with 92% yield Tobias C. B. Harlang 1 , Yizhu Liu 1,2 , Olga Gordivska 2 , Lisa A. Fredin 3 , Carlito S. Ponseca Jr 1 , Ping Huang 4 , Pavel Chábera 1 , Kasper S. Kjaer 1,5 , Helena Mateos 1 , Jens Uhlig 1 , Reiner Lomoth 4 , Reine Wallenberg 6 , Stenbjörn Styring 4 , Petter Persson 3 , Villy Sundström 1 and Kenneth Wärnmark 2 * Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron–nitrogen–heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the 3 MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements. T he sensitization of wide-bandgap semiconductors is the foun- dation of a number of photochemical processes related to solar energy use 1 . With carefully engineered thermodynamic and kinetic matching, the heterojunction formed at the sensitizer/semi- conductor interface facilitates photo-induced charge separation 2 and has thus led to tremendous success in dye-sensitized solar cells (DSCs) 3 , solar fuel cells 4,5 and even photocatalytic environ- mental applications 6 . A range of sensitizers, from zinc porphyrin molecules 7 to perovskite semiconductors 8 , have emerged in recent years, but ruthenium complexes remain the most widely used and investigated 9 . However, because of their preciousness and toxicity, scientists have never stopped searching for alternatives, among which Fe II complexes, as the lighter congener in the periodic table, are obvious candidates due to their shared properties as well as high abundance, environmental inertness and chemical stability 10 . Unfortunately, Fe II complexes suffer from extremely short-lived metal-to-ligand charge transfer (MLCT) states and are deactivated into the photo-inactive metal-centred (MC) states in a 100 fs regime 11–18 , preventing efficient photo-induced electron injection into a semiconductor. More than a decade ago, attempts to sensitize TiO 2 with Fe II complexes were encouraged by observations of ultrafast interfacial electron injection on sub-100 fs timescales in several dye-sensitized TiO 2 systems. Thus, ultrafast injection has been reported for a number of different sensitizers, including organic molecules 19,20 and transition-metal complexes such as RuN3 (cis-Ru(dcbpy) 2 (NCS) 2 , where dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid) 2,21 . Gregg and Ferrere 22–24 and later Meyer and collaborators 25,26 , used cis-Fe(dcbpy) 2 (CN) 2 and Na 2 [Fe(bpy)(CN) 4 ] (bpy = 2,2′-bipyri- dine), respectively, as well as their derivatives, as sensitizers. As rationalized by the theoretical work of Jakubikova and collaborators, sensitization from the lowest-energy MLCT state of these complexes is disfavoured both kinetically and thermodynamically 27,28 . Although direct metal-to-particle charge transfer (MPCT) from the Fe II centre to the Ti IV acceptor site through the μ-cyano ligand can potentially serve as an effective sensitization strategy, the generated Fe III cation may significantly attenuate the σ-donor strength of the cyano ligand, labilizing the attachment of the sensitizer to TiO 2 26 . To make the injection into TiO 2 competitive with the deactivation of the MLCT state, one can prolong the MLCT lifetime and/or accel- erate the injection step. For conventional Fe II polypyridyl complexes, however, the extremely short-lived MLCT manifold sets a rather harsh limit of <100 fs for the injection time, although ultrafast injec- tion has been reported for other sensitizers 2,19–21 and is also predicted to be feasible for Fe-based arrangements 27,28 . Therefore, research efforts have recently been directed to retarding the MLCT deactivation, mainly through the enhancement of the ligand field strength 29,30 , coupled with fine-tuning of the MLCT energy level, using conventional as well as cyclometallated ligands 17,31 . We have recently demonstrated that the N-heterocyclic carbene (NHC) ligands, as superior σ-donors, can effectively suppress the MLCT deactivation by significantly destabilizing the MC states 32–34 . Here, we take this work to a new level by demonstrating for the first time highly efficient photo-induced electron injection from the lowest-energy 3 MLCT state of a Fe II complex into a nanoporous TiO 2 film. This study makes use of our previously reported Fe II NHC complex 1 32 (Fig. 1a) where we functionalized it with COOH groups (2) for the sake of surface immobilization on TiO 2 (ref. 2). Note that, while we were completing the present work, Gros and co-workers published their work on the same Fe II NHC complex, including solution-based photophysical properties and photovoltaic performance in a DSC 35 . The low photovoltaic per- formance reported is not significantly different from that of the Fe(dcbpy) 2 (CN) 2 reported previously by Gregg et al. 22 and that 1 Department of Chemical Physics, Lund University, Box 124, Lund SE-22100, Sweden. 2 Centre for Analysis and Synthesis, Department of Chemistry, Lund University, Box 124, Lund SE-22100, Sweden. 3 Theoretical Chemistry Division, Chemistry Department, Lund University, Box 124, Lund SE-22100, Sweden. 4 Department of Chemistry – Ångström Laboratory, Uppsala University, Box 523, Uppsala SE-75120, Sweden. 5 Department of Physics, Technical University of Denmark, Kongens Lyngby DK-2800, Denmark. 6 nCHREM, Lund University, Box 124, Lund SE-22100, Sweden. *e-mail: kenneth.warnmark@chem.lu.se ARTICLES PUBLISHED ONLINE: 12 OCTOBER 2015 | DOI: 10.1038/NCHEM.2365 NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 1 © 2015 Macmillan Publishers Limited. All rights reserved