Four homo- and heteroleptic complexes bearing both polypyridyl units and N-heterocyclic carbene (NHC) donor functions are studied as potential noble metal-free photosensitizers. The complexes [Fe(L1)(terpy)][PF], [Fe(L2)][PF], [Fe(L1)(L3)][PF], and [Fe(L3)][PF] (terpy = 2,2':6',2″ terpyridine, L1 = 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine, L2 = 2,6-bis[3-isopropylimidazol-2-ylidene]pyridine, L3 = 1-(2,2'-bipyridyl)-3-methylimidazol-2-ylidene) contain tridentate ligands of the C^N^C and N^N^C type, respectively, resulting in a Fe-NHC number between two and four. Thorough ground state characterization by single crystal diffraction, electrochemistry, valence-to-core X-ray emission spectroscopy (VtC-XES), and high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) in combination with ab initio calculations show a correlation between the geometric and electronic structure of these new compounds and the number of the NHC donor functions. These results serve as a basis for the investigation of the excited states by ultrafast transient absorption spectroscopy, where the lifetime of the MLCT states is found to increase with the NHC donor count. The results demonstrate for the first time the close interplay between the number of NHC functionalities in Fe(II) complexes and their photochemical properties, as revealed in a comparison of the activity as photosensitizers in photocatalytic proton reduction.

Keywords: Nitrogen donor ligands / Guanidine ligands / Ligand designA series of bis-guanidine ligands designed for use in biomimetic coordination chemistry has been extended to a library matrix combining unprecedented substitutional flexibility within the guanidyl residues with a wide range of aliphatic and aromatic spacers connecting these functionalities. The underlying protocol can be used with predefined ureas as well as secondary amines to build up these units by reaction with phosgene if the ureas are otherwise unavailable. In the latter case, the resulting urea intermediates do not have to be isolated as the reaction proceeds further with additional phosgene to yield a chloroformamidinium chloride which is transformed into the bis-guanidine functionality by subsequent reaction with a suitable primary diamine in the presence of triethylamine as an auxiliary base. This concept has been used to synthesise and characterise more then two dozen different bis-guanidines based on 12 discrete monoguanidine units and seven different spacers. These spacers have been chosen such that the most important phenotypes have been dealt with and which range from rigid to more flexible scaffolds. In addition to spacers with no metal-bind-

For the first time, the potential of noble‐metal‐free iron‐based photosensitizers for photocatalytic water reduction by using homo‐ and heteroleptic carbene complexes was proven. The development of a facile synthesis route for such carbene complexes was delineated. The complexes were investigated by UV/Vis and fluorescence spectroscopy, DFT calculations, cyclic voltammetry, and X‐ray absorption spectroscopy. Their catalytic performance was compared to that of established noble‐metal‐containing photosensitizers such as [Ir(ppy)2(bpy)][PF6] (Hppy = 2‐phenylpyridine, bpy = 2,2′‐bipyridine).

One to two and back again: A disulfide–thiolate interconversion process in a dinuclear copper complex is induced by chloride ions (see scheme). Within this compound, the coordinating copper atoms serve either as electron source or as electron sink and thus oscillate between the oxidation states +I and +II.

The central role of cupric superoxide intermediates proposed in hormone and neurotransmitter biosynthesis by noncoupled binuclear copper monooxygenases like dopamine-β-monooxygenase has drawn significant attention to the unusual methionine ligation of the Cu M ("Cu B ") active site characteristic of this class of enzymes. The copper−sulfur interaction has proven critical for turnover, raising still-unresolved questions concerning Nature's selection of an oxidizable Met residue to facilitate C−H oxygenation. We describe herein a model for Cu M , [( TMG N 3 S)Cu I ] + ([1] + ), and its O 2 -bound analog [The latter is the first reported cupric superoxide with an experimentally proven Cu−S bond which also possesses demonstrated hydrogen atom abstraction (HAA) reactivity. Introduction of O 2 to a precooled solution of the cuprous precursor [1]B(C 6 F 5 ) 4 (−135 °C, 2-methyltetrahydrofuran (2-MeTHF)) reversibly forms [1•O 2 ]B(C 6 F 5 ) 4 (UV/vis spectroscopy: λ max 442, 642, 742 nm). Resonance Raman studies (413 nm) using 16 O 2 [ 18 O 2 ] corroborated the identity of [1•O 2 ] + by revealing Cu−O (446 [425] cm −1 ) and O−O (1105 [1042] cm −1 ) stretches, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed a Cu−S interatomic distance of 2.55 Å. HAA reactivity between [1•O 2 ] + and TEMPO−H proceeds rapidly (1.28 × 10 −1 M −1 s −1 , −135 °C, 2-MeTHF) with a primary kinetic isotope effect of k H /k D = 5.4. Comparisons of the O 2 -binding behavior and redox activity of [1] + vs [2] + , the latter a close analog of [1] + but with all N atom ligation (i.e., N 3 S vs N 4 ), are presented.

A new chelating mesoionic carbene ligand, derived from 1,2,3-triazoles, with two redox-active tert-butyl-phenolate linkers has been synthesized and explored towards its reactivity and electrochemical properties in early transition metal chemistry.

Perylene derivatives with a unilaterally and bilaterally extended core show luminescence in the visible wavelength range (500−600 nm) that can be excited by absorption in the ultraviolet range (360−370 nm). This unusual behavior is investigated by means of excitation spectroscopy, cyclic voltammetry, and calculations based on (time-dependent) density functional theory. The results indicate that the extended compounds show promising features for optoelectronic applications and even might be used as fluorescent dyes in lasing. This is supported by nonadiabatic ab initio molecular dynamics. With respect to applications in organic optoelectronic nanostructures, nanofilms were prepared via spin-coating and thermal vapor deposition and demonstrated the formation of excimers. The relationship between the excimer-induced bathochromic shift and the interplanar distance of the molecules opens up the possibility to vary the perceived color of a nanofilm via tempering.

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