Efficient Reverse Intersystem Crossing in Carbene-Copper-Amide TADF Emitters via an Intermediate Triplet State.

The mechanism behind reverse intersystem crossing (rISC) in metal-based TADF emitters is still under debate. Thermal rISC necessitates small singlet/triplet energy gaps as realized in donor-acceptor systems with charge-transfer excited states. However, their associated spin-orbit couplings are too small to account for effective rISC. Here, we report the first nonadiabatic dynamics simulation of the rISC process in a carbene-copper(I)-carbazolyl TADF emitter. Efficient rISC on a picosecond time scale is demonstrated for an initial triplet minimum geometry that exhibits a perpendicular orientation of the ligands. The dynamics involves an intermediate higher-lying triplet state of metal-to-ligand charge transfer character (3 MLCT), which enables large spin-orbit couplings with the lowest singlet charge transfer state. The mechanism is completed in the S1 state, where the complex can return to a co-planar coordination geometry that presents high fluorescence efficiency.

The importance of finite temperature and vibrational sampling in the absorption spectrum of a nitro-functionalized Ru(ii) water oxidation catalyst.

Consideration of finite temperature and vibrational motion can be an essential component for accurate simulations of absorption spectra. Here we use finite-temperature Wigner phase-space sampling to investigate the intense absorption of the water oxidation catalyst Ru(dppip-NO2) in the visible (vis) region. The influence of vibrational and torsional motions as well as temperature effects are addressed for the different protonation forms of the pH-sensitive dppip-NO2 ligand of the catalyst. Excitations to the nitrophenyl group and π-system of dppip-NO2, which characterize the absorption band in the equilibrium spectra, experience energy shifts and a significant decrease in oscillator strength when nuclear motion is considered. The importance of excitations to the nitrophenyl group for the vis band is reduced in the spectra computed from the 300 K ensembles, which feature broad distributions of the corresponding dihedral angles. The effects of vibrational sampling on the absorption spectra may be attributed to nitrophenyl and, in particular, to NO2 torsional motions. We expect finite temperature and vibrational sampling to be important for simulating the absorption spectra of other transition metal complexes with flexible ligands or nitro-aromatic motifs.

A Ruthenium(II) Water Oxidation Catalyst Containing a pH-Responsive Ligand Framework.

The synthesis of a new RuII-based water oxidation catalyst is presented, in which a nitrophenyl group is introduced into the backbone of dpp via a pH-sensitive imidazole bridge (dpp = 2,9-di-(2'-pyridyl)-1,10-phenanthroline). This modification had a pronounced effect on the photophysical properties and led to the appearance of a significant absorption band around 441 nm in the UV-vis spectrum upon formation of the monoprotonated species under neutral conditions. Theoretical investigations could show that the main contributions to this band arise from transitions involving the imidazole and nitrophenyl motif, allowing us to determine the pKa value (6.8 ± 0.1) of the corresponding, twofold protonated conjugated acid. In contrast, the influence of the nitrophenyl group on the electrochemical properties of the catalytic center was negligible. Likewise, the catalytic performance of Ru(dppip-NO2) and its parent complex Ru(dpp) was comparable over the entire investigated pH range (dppip-NO2 = 2-(4-nitrophenyl)-6,9-di(pyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline). This allowed the original catalytic properties to be retained while additionally featuring a functionalized ligand scaffold, which provides further modification opportunities as well as the ability to report the pH of the catalytic solution via UV-vis spectroscopy.

A theoretical study on trivalent europium: from the free ion to the water complex.

The energy levels within the (7)F and (5)D manifolds of trivalent europium were computed for the free ion and in the crystal field of 6-9 water molecules. Fully relativistic Kramers pairs configuration interaction (KRCI) calculations were performed with different correlation spaces for the free ion, and with the complete open-shell configuration interaction (COSCI) method including the 4f electrons in the active space for the water clusters. The best agreement with experimental data was found with the KRCI method or when including the spin-other-orbit effect in the COSCI calculations. For the free ion, the (7)F6 multiplet is found only 162 cm(-1) higher compared to experiment, while the (5)D(0-3) multiplets are approximately 3100 cm(-1) too high. In the crystal field of six water molecules, the multiplets with J > 0 split by 48-123 cm(-1). The energy separation (7)F0-(5)D0 of the unsplit multiplets is computed within 255 cm(-1)/247 cm(-1) compared to the experimental data for the free ion/in water. It has been found that the effect of higher coordination number or lower symmetry is small, increasing the transition energies by only about 40 cm(-1) by lowering the (7)F(J) states by the same amount. The hypersensitive transition (5)D0-(7)F2 is computed at 17079 cm(-1) with the KRCI method as compared to the experimental value of 16267 cm(-1).