Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. Here we present the iron complex [Fe(btz)] (where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (LMCT) state that is rarely seen for transition-metal complexes. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

Iron’s abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facialtris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}−, exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (2LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the2LMCT state of [Fe(phtmeimb)2]+in bimolecular quenching studies with methylviologen and diphenylamine.

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.

Electrons are the workhorses of solar energy conversion. Conversion of the energy of light to electricity in photovoltaics, or to energy-rich molecules (solar fuel) through photocatalytic processes, invariably starts with photoinduced generation of energy-rich electrons. The harvesting of these electrons in practical devices rests on a series of electron transfer processes whose dynamics and efficiencies determine the function of materials and devices. To capture the energy of a photogenerated electron-hole pair in a solar cell material, charges of opposite sign have to be separated against electrostatic attractions, prevented from recombining and being transported through the active material to electrodes where they can be extracted. In photocatalytic solar fuel production, these electron processes are coupled to chemical reactions leading to storage of the energy of light in chemical bonds. With the focus on the ultrafast time scale, we here discuss the light-induced electron processes underlying the function of several molecular and hybrid materials currently under development for solar energy applications in dye or quantum dot-sensitized solar cells, polymer-fullerene polymer solar cells, organometal halide perovskite solar cells, and finally some photocatalytic systems.

The iron carbene complex [Fe(btz)](PF) (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a MLCT state with a 528 ps excited-state lifetime in CHCN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported Fe complex. The low potential of the [Fe(btz)]/[Fe(btz)] redox couple makes the MLCT state of [Fe(btz)] a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [Fe(btz)] form of this complex, these results show that the Fe and Fe oxidation states of the same Fe(btz) complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.