Jan Klett

Switching the electronic spin state in molecules by light is achieved via chemical bond reshuffling or light-induced excited spin state trapping. These procedures either require large-amplitude nuclear motions or cryogenic temperatures limiting their application in data storage or quantum computing devices. Here we show that a carbene manganese(II) complex undergoes light-induced excited spin state trapping at 141 K in solution and 112.5 K in the solid state. Irradiation quantitatively switches from low- to high-spin in solution and in the solid. The lifetime of the metastable high-spin state extrapolated from experimental data amounts to over 1 million years at 77 K. Key to success for the very long lifetime is the high reorganization energy due to high metal–ligand bond covalency, and hence, a large barrier height and width for the spin relaxation, paving the way for photo-switchable magnetic materials operating at ambient temperature.

Transition-metal complexes with earth-abundant metal centers hold significant potential for light-induced processes. Pseudo-octahedral vanadium(III) and vanadium(II) complexes with d² and d³ electron configurations are promising candidates, yet their redox and excited-state properties are only insufficiently understood. Here, we combine quantum chemistry and experimental data (structure, cyclic voltammetry, spectroelectrochemistry, electron-transfer dissociation, collision-induced dissociation, ultraviolet photodissociation mass spectrometry, and ultrafast transient absorption spectroscopy) to elucidate redox and photostability as well as excited-state lifetimes of the vanadium(III/II) redox couple cisfac-[V(dgpy)₂]^3+/2+ (dgpy = 2,6-diguanidylpyridine). Ground-state splitting, mixing of spin-flip excited states with metal-to-ligand charge transfer states, and multiphonon relaxation are identified as the key challenges for excited-state lifetimes in these d²/d³ systems.

Switching the electronic spin state in molecules by light is achieved via chemical bond reshuffling or light-induced excited spin state trapping (LIESST). These procedures either require large-amplitude nuclear motions or cryogenic temperatures limiting their application in data storage or quantum computing devices. A carbene manganese(II) complex shows LIESST at unusually high temperature. Irradiation quantitatively switches from low- to high-spin in solution and in the solid. The lifetime of the metastable high-spin state extrapolated from experimental data amounts to over one million years at 77 K. Key to success for the very long lifetime is the high reorganization energy due to strong metal-ligand bond covalency and hence a large barrier height and width for the spin relaxation, paving the way for photo-switchable magnetic materials operating at ambient temperature.

Application of photoactive transition metal complexes in lighting, imaging, sensing, and photocatalysis is usually based on the triplet metal-to-ligand charge transfer (³MLCT) excited state of precious metal complexes with 4/5d⁶ valence electron configurations. These photocatalysts exhibit excited state lifetimes exceeding hundreds of nanoseconds. Simple 3d⁶ transition metal complexes containing abundant metals exhibit lifetimes below 1–2 nanoseconds, and they require multistep ligand syntheses mitigating large-scale implementation. We report that a commercially available bis(imidazolium) pyridine pro-ligand [H₂pbmi]²⁺ and a manganese(II) salt yield the tetracarbene manganese(I) complex [Mn(pbmi)₂]⁺. This complex phosphoresces at room temperature in fluid solution from a ³MLCT state with a lifetime of 190 ns. In combination with the reversible [Mn(pbmi)₂]^2+/+ process, this translates to an excited state capable of reducing benzophenone. Combination of manganese(I) with rigid carbene/pyridine ligands expands key strategies for photoactive 3d⁶ metal complexes of earth-abundant metals with ³MLCT lifetimes rivalling those of precious metals and providing a conceptual starting point for a sustainable photochemistry.

Ruthenium and iridium are key components in the most important applications of photoactive complexes, namely, light-emitting devices, photocatalysis, bioimaging, biosensing, and photodynamic therapy. Especially, near-infrared (NIR) emissive materials are required in fiber-optic telecommunications, anticounterfeit inks, night-vision readable displays, and bioimaging. Replacing rare and expensive precious metals with more abundant first-row transition metals is of great interest; however, photophysical properties and the chemical stability of 3d metal complexes are often insufficient. Here, we tackle these challenges with a nonprecious metal polypyridine vanadium(II) complex that shows emission above 1300 nm with excited state lifetimes of up to 760 ns. Strong light absorption in the visible spectral region and exceptional stability in the presence of oxygen enable photocatalysis in water and acetonitrile using green to orange-red light for excitation. This study unravels a new design principle for NIR-II luminescent and photoactive complexes based on the abundant first-row transition metal vanadium.

The successful application of transition metal based photoactive complexes in lighting, imaging, sensing, and photocatalysis is usually based on the triplet metal-to-ligand charge transfer (3MLCT) excited state of precious metal complexes with 4d6 and 5d6 valence electron configuration. These photocatalysts exhibit excited state lifetimes exceeding hundreds of nanoseconds. Simple transition metal complexes with 3d6 valence electron configuration containing abundant metals exhibit lifetimes of less than 1-2 nanoseconds and they require multistep ligand syntheses mitigating large-scale implementation. We report that a commercially available bis(imidazolium) pyridine pro-ligand [H2pbmi]2+ and a manganese(II) salt give access to the tetracarbene manganese(I) complex [Mn(pbmi)2]+. This purple colored complex phosphoresces at room temperature in fluid solution. Its 3MLCT excited state lifetime of 190 ns exceeds those of simple 3MLCT photoactive 3d6 complexes by more than one to two orders of magnitude. In combination with its reversible ground state MnII/I redox chemistry, this translates to a 3MLCT excited state capable of reducing an organic substrate by bimolecular quenching. The combination of manganese(I) with rigid carbene and pyridine chelate ligands expands key strategies for photoactive 3d6 metal complexes of earth-abundant metals with 3MLCT lifetimes rivalling those of precious metals and providing a conceptual starting point for a sustainable photochemistry.

Mixed N‐heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth‐abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal‐to‐ligand charge transfer state (³MLCT), which typically decays via a triplet metal‐centered (³MC) state back to the ground state. We reveal by variable‐temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans‐ and cis‐[Fe(pdmi)₂]²⁺ with four NHC donors show ³MLCT→³MC population transfers with very different barriers and rationalize this by computational means. While trans‐[Fe(pdmi)₂]²⁺ possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm⁻¹, which leads to a nanosecond ³MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi‐classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

Mixed N‐heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth‐abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal‐to‐ligand charge transfer state (³MLCT), which typically decays via a triplet metal‐centered (³MC) state back to the ground state. We reveal by variable‐temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans‐ and cis‐[Fe(pdmi)₂]²⁺ with four NHC donors show ³MLCT→³MC population transfers with very different barriers and rationalize this by computational means. While trans‐[Fe(pdmi)₂]²⁺ possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm⁻¹, which leads to a nanosecond ³MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi‐classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

Treatment of silyl compounds with excess sodium tert -butoxide results in encapsulation of the released anion. While the BF ₄ ⁻ anion just fits in a spherical sodium alkoxide hull, the butadiyenediide dianion pierces its hull resulting in dimerisation.

Alkali metal alkoxides are widely used in chemistry due to their Brønsted basic and nucleophilic properties. Potassium alkoxides assist alkyllithium in the metalation of hydrocarbons in Lochmann‐Schlosser‐bases. Both compounds form mixed aggregates, which enhance the thermal stability, solubility, and the basic reactivity of these mixtures. A very unusual spherical mixed alkoxy aggregate was discovered by Grützmacher et al., where a central dihydrogen phosphide anion is surrounded by a highly dynamic shell of thirteen sodium atoms and a hull of twelve tert‐butoxide groups. This structural motif can be reproduced by a reaction of trimethylsilyl compounds of methane, halogens, or pseudo‐halogens with excess sodium tert‐butoxide. A nucleophilic substitution releases the corresponding anion, which is then encapsulated by the sodium alkoxide units. The compounds are soluble in hydrocarbon solvents, enabling studies of solutions by high‐resolution NMR spectroscopy and IR/Raman studies of the crystalline materials.