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.

In this article, the nature of the “Z”-phase, which forms on charging many P2-type compounds to high voltages, is probed.

Operando mass spectroscopy demonstrates quantitatively that lithium extraction from Li2MnO3 is charge compensated by oxygen loss (O-loss) not oxidation of oxide ions which are retained within the structural framework (Oredox). This data is confirmed by X-ray absorption and emission spectroscopy. Li NMR shows that the two-phase coreshell structure, which forms on charging, is composed of an intact Li2MnO3 core and highly disordered shell containing no Li, with a composition close to MnO2. Discharge involves Li insertion into the disordered shell. CO2 and O2 are detected on charging at 15 mAg -1 , whereas charging by GITT forms only CO2; an observation in agreement with the previously described model of oxygen evolution from high voltage cathodes producing singlet O2 that reacts with the electrolyte forming CO2. The dominance of oxygen evolution over O-redox is in accord with the model of O-loss occurring when the oxide ions are undercoordinated; O in the shell devoid of Li is coordinate by only 2 Mn. ExperimentalSynthesis: Li2MnO3 was synthesized using a sol−gel method 14 . Stoichiometric amounts of LiCH3COO•2H2O (99.0%, Sigma-Aldrich), and Mn(CH3COO)2•4H2O (99.0%, Fluka) were dissolved in distilled water containing 0.1 mol of resorcinol (99.0%, Fluka), 0.15 mol of formaldehyde (Fluka 36.5% in water), and 0.25 mmol of Li2CO3 (99.0%, Sigma-Aldrich). The mixture was heated under vigorous stirring at 70 °C until an opaque uniform gel was formed. The gel was dried at 90 °C overnight, and finally heated in air first at 500 °C for 12 h and then at 800 °C for 12 h to obtain the final product. ICP-OES:Elemental analysis of the as-prepared sample was carried out after its complete dissolution in acidic solution by ion coupled plasma optical emission spectroscopy (ICP-OES) using a PerkinElmer Optima 7300DV ICP-OES. This allowed for the exact quantification of Li and Mn.Powder X-ray Diffraction (XRD): Powder X-ray diffraction analysis was carried out using a Rigaku SmartLab Xray powder diffractometer equipped with a 3 kW Cu anode.Soft X-ray Absorption and Resonant Inelastic X-ray Spectroscopy: SXAS and RIXS spectra were recorded at beamline BL27SU of the RIKEN/JASRI Spring8 synchrotron in Japan. Ex situ cathode samples were loaded onto adhesive copper tape and measured under 10 -6 Pa high-vacuum conditions. To obtain SXAS spectra at the O Kedge and Mn L-edge, partial fluorescence yield (PFY) method was employed and the fluorescence signal was recorded using silicon drift detectors (SSD). The O K-edge RIXS spectra were recorded using an XES monochromator composed of a varied-line-spacing cylindrical grating and a CCD detector with a resolution of 0.5 eV in the energy range of interest. Electrochemical Measurements:The working electrodes were prepared with a composition of 80 wt % active material, 10 wt % Super P carbon, and 10 wt % polytetrafluoroethylene (PTFE) binder. The components were combined with a pestle and mortar and then calendared to ~60 µm between two sheets of Al foil before being punched into 10 mm  pellets. T...

Lithium-rich materials, such as Li 1.2 Ni 0.2 Mn 0.6 O 2 , exhibit capacities not limited by transition metal redox, through the reversible oxidation of oxide anions. Here we offer detailed insight into the degree of oxygen redox as a function of depth within the material as it is charged and cycled. Energy-tuned photoelectron spectroscopy is used as a powerful, yet highly sensitive technique to probe electronic states of oxygen and transition metals from the top few nanometers at the near-surface through to the bulk of the particles. Two discrete oxygen species are identified, O nÀ and O 2À , where n < 2, confirming our previous model that oxidation generates localised hole states on O upon charging. This is in contrast to the oxygen redox inactive high voltage spinel LiNi 0.5 Mn 1.5 O 4 , for which no O nÀ species is detected. The depth profile results demonstrate a concentration gradient exists for O nÀ from the surface through to the bulk, indicating a preferential surface oxidation of the layered oxide particles. This is highly consistent with the already well-established core-shell model for such materials. Ab initio calculations reaffirm the electronic structure differences observed experimentally between the surface and bulk, while modelling of delithiated structures shows good agreement between experimental and calculated binding energies for O nÀ .