Iron hits the mark

Abstract

Solar energy can enable our society to thrive as we endeavor to reduce our dependence on fossil fuels. However, the Sun is an intermittent form of energy. Solar-cell technology is well suited for daytime electricity generation and usage, but our society uses energy around the clock. Thus, it is not only important to generate electricity for daytime use but also to store solar energy for nighttime use. Chemists see huge potential in molecules and materials that absorb light (that is, solar energy) and use that energy to generate electrons that then carry out chemical reactions to turn low-energy feedstocks into high-energy fuels. To date, the transition metal complex (TMC) photosensitzers that have sufficiently long excited-state lifetimes to enable this chemistry ([ 1 ][1]) contain expensive and scarce metals, such as complexes of ruthenium (Ru), osmium, and iridium. On page 249 of this issue, Kjær et al. ([ 2 ][2]) report an iron (Fe)–based photosensitizer with a quantum efficiency surpassing that of [Ru(bpy)3]2+ (where bpy is 2,2′-bipyridine), the historical standard bearer. Furthermore, the new iron-based photosensitizer has an excited-state lifetime of 2 ns, which is sufficiently long to transfer electrons to other compounds (see the figure). Ruthenium-based TMCs have received much attention as photosensitizers because of their long-lived metal-to-ligand charge-transfer (MLCT) excited state and general photo- and chemical stability ([ 3 ][3]). Despite their superior performance, ruthenium-based TMCs are limited in their usefulness to society because ruthenium is rare and expensive. Not only is there not enough ruthenium to go around, even if there were, it is harmful to the environment. A search for alternative earth-abundant and less expensive metals has been pursued for the past several decades. Iron, being in the same group of the periodic table as ruthenium, shares certain similarities that make it a potentially attractive target metal for use in TMCs. Iron-based TMCs would have several advantages, such as earth-abundancy, low toxicity, and strongly absorbing charge-transfer (CT) excited states ([ 4 ][4]). However, the photophysics of iron TMCs have made their solar-driven applications relatively rare ([ 5 ][5]). Molecular orbital ordering dictates the ability of TMC excited states to separate charge within the complex and maintain this separation long enough to carry out productive chemical reactions. Ruthenium(II)-based TMCs experience relatively large octahedral splitting that drives the eg and t2g metal-centered (MC) states of ruthenium(II)-based TMCs far enough apart that a ligand-centered (LC) state falls between them ([ 6 ][6]). This molecular orbital ordering generates a low-lying MLCT excited state that rapidly becomes populated upon photoexcitation of a ruthenium(II)-based TMC ([ 7 ][7]). By moving charge either away from or onto the ligands, the resulting excited state maintains the potential to drive a desired chemical reaction. ![Figure][8] Iron complexes that stay excited Most iron(II) complexes have short-lived, metal-centered (MC) photoexcited states that are unable to perform chemical reactions. A new iron(III) complex reported by Kjær et al. overcomes these limitations. GRAPHIC: A. KITTERMAN/ SCIENCE Iron(II)-based TMCs experience weak ligand-field splitting, such that the eg state falls below the LC state. The excited state is a low-energy MC state that cannot be used to carry out productive chemistry. The development of iron(II)-based TMCs that have long-lived CT excited states has been tackled by inorganic chemists who have purposefully designed ligands to overcome this issue. During the past several years, Wärnmark and colleagues have developed iron TMCs containing strong σ-donor ligands based on N -heterocyclic carbenes (NHCs). These ligands destabilize the low-lying eg MC state so that the MLCT excited state becomes the lowest-energy transition. Further research by the Gros and Wärnmark groups provided critical insight into molecular design for iron(II)-based TMCs. In parallel, they reported on iron(II) NHC complexes with the longest triplet 3MLCT lifetimes to that point, 16.5 and 18 ps, respectively ([ 8 ][9], [ 9 ][10]). In 2017, Wärnmark and co-workers reported an all-NHC coordination iron(III) complex, [Fe(btz)3]3+, where btz is 3,3′-dimethyl-1,1′- bis ( p -tolyl)-4,4′- bis (1,2,3-triazol-5-ylidene), that extended the CT lifetime to 100 ps ([ 10 ][11]). Kjær et al. build off these previous successes using a two-pronged approach. First, they used an exceptionally strongly σ-donating ligand that is also negatively charged and enforces a near perfect octahedral coordination sphere in order to substantially destabilize the otherwise low-lying eg MC state in the iron-containing complex ([ 7 ][7]), causing the LC state to become the lowest unoccupied molecular orbital. The switch in orbital ordering leads to a low-lying CT excited state that can be used to drive chemical reactions. Second, the newly reported iron-based TMC contains iron(III) rather than iron(II). The iron(III)-TMC contains an unpaired electron in the t2g ground state, so both the ground and ligand-to-metal charge transfer (LMCT) excited states are doublets. The advantages of the 2LMCT excited state are two-fold: There is no excited-state energy loss as a result of singlet-to-triplet conversion that is ubiquitous in many TMCs with singlet character, and the lower-lying MC scavenger states (4MC and 6MC) are less accessible than the scavenger states (3MC and 5MC) of the 3MLCT formed in iron(II)-based TMCs, which reduces nonradiative losses. For years, the goal of designing ligands for iron-based TMCs has been to separate charge in the excited state (forming LMCT or MLCT states) and to reduce the nonradiative decay through the MC scavenger states. This new ligand design hits both marks. Kjær et al. report that...