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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. [ABSTRACT FROM AUTHOR]

The effects of backbone amine functionalization in three new homoleptic C^N^C type ruthenium(II) complexes bearing a tridentate bis‐imidazole‐2‐ylidene pyridine ligand framework are characterized and studied by single crystal diffraction, electrochemistry, optical spectroscopy and transient absorption spectroscopy in combination with ab initio DFT calculations. Functionalization by dimethylamine groups in 4‐position of the pyridine backbone significantly influences the properties of the complexes as revealed by comparison with the unfunctionalized references. As a result of the amine functionalization, a higher molar absorption coefficient of the MLCT bands, a decreased photoluminescence quantum yield at room temperature together with a shortened excited state lifetime but an improved photostability is observed. Introduction of electron donating and withdrawing groups at the NHC unit modifies the electronic and optical properties, such as the oxidation potential, absorption and emission properties, and the lifetimes of the excited states. [ABSTRACT FROM AUTHOR]

The main aim of the present study was to determine if synapses from the exceptionally small brain of the Etruscan shrew show any peculiarities compared to the much larger human brain. We analyzed the cortical synaptic density and a variety of structural characteristics of 7,239 3D reconstructed synapses, using using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM). We found that some of the general synaptic characteristics are remarkably similar to those found in the human cerebral cortex. However, the cortical volume of the human brain is about 50,000 times larger than the cortical volume of the Etruscan shrew, while the total number of cortical synapses in human is only 20,000 times the number of synapses in the shrew, and synaptic junctions are 35% smaller in the Etruscan shrew. Thus, the differences in the number and size of synapses cannot be attributed to a brain size scaling effect but rather to adaptations of synaptic circuits to particular functions. [ABSTRACT FROM AUTHOR]

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero‐field splittings qualify as qubits with optical read out thanks to potentially luminescent spin‐flip states. We identified two V2+ complexes [V(ddpd)2]2+ with the strong field ligand N,N'‐dimethyl‐N,N'‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine (ddpd) in two isomeric forms (cis‐fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm−1 allowing pulsed EPR experiments at conventional Q‐band frequencies (35 GHz). Both isomers possess spin‐lattice relaxation times T1 of around 300 μs and a phase memory time TM of around 1 μs at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd)2]3+ are emissive in the near‐IR‐II region, the [V(ddpd)2]2+ complexes are non‐luminescent due to metal‐to‐ligand charge transfer admixture to the spin‐flip states. [ABSTRACT FROM AUTHOR]

Cr(ppy)3, a structural analog of the green phosphorescent Ir(ppy)3, emits even in solution at room temperature from a weakly distorted spin–flip state at 910 nm (Hppy = 2-phenylpyridine). The low energy arises from an enhanced covalence of the Cr–C bonds as compared to Cr–N bonds. Lower temperature reduces thermally activated decay increasing the emission intensity. [ABSTRACT FROM AUTHOR]