The development of new solar-to-fuel scenarios is of great importance, but the construction of molecular systems that convert sunlight into chemical energy represents a challenge. One specific issue is that the molecular systems have to be able to accumulate redox equivalents to mediate the photodriven transformation of relevant small molecules, which mostly involves the orchestrated transfer of multiple electrons and protons. Disulfide/ dithiol interconversions are prominent 2e − /2H + couples and can play an important role for redox control and charge storage. With this background in mind, a new photosensitizer [Ru( S−S bpy)-(bpy) 2 ] 2+ (1 2+ ) equipped with a disulfide functionalized bpy ligand ( S−S bpy, bpy = 2,2′-bipyridine) was synthesized and has been comprehensively studied, including structural characterization by X-ray diffraction. In-depth electrochemical studies show that the S−S bpy ligand in 1 2+ can be reduced twice at moderate potentials (around −1.1 V vs Fc +/0 ), and simulation of the cyclic voltammetry (CV) traces revealed potential inversion (E 2 > E 1 ) and allowed to derive kinetic parameters for the sequential electron-transfer processes. However, reduction at room temperature also triggers the ejection of one sulfur atom from 1 2+ , leading to the formation of [Ru( S bpy)(bpy) 2 ] 2+ (2 2+ ). This chemical reaction can be suppressed by decreasing the temperature from 298 to 248 K. Compared to the archetypical photosensitizer [Ru(bpy) 3 ] 2+ , 1 2+ features an additional low energy optical excitation in the MLCT region, originating from charge transfer from the metal center to the S−S bpy ligand (aka MSCT) according to time-dependent density functional theory (TD-DFT) calculations. Analysis of the excited states of 1 2+ on the basis of ground-state Wigner sampling and using charge-transfer descriptors has shown that bpy modification with a peripheral disulfide moiety leads to an energy splitting between charge-transfer excitations to the S−S bpy and the bpy ligands, offering the possibility of selective charge transfer from the metal to either type of ligands. Compound 1 2+ is photostable and shows an emission from a 3 MLCT state in deoxygenated acetonitrile with a lifetime of 109 ns. This work demonstrates a rationally designed system that enables future studies of photoinduced multielectron, multiproton PCET chemistry.

Keywords:Heterometallic complexes / Metal-metal interactions / Mixed-valent compounds / Electronic structure / Singlemolecule conductanceThe study of metal string complexes with 1D transition-metal frameworks began in the early 1990s. As these complexes provide great insights into metal-metal multiple bonds and may have potential applications as molecular wires, this field of research has grown in the past 20 years. As such, the electronic structures of the simplest trinuclear complexes, the supporting ligand systems, and the single-molecular conduc-

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The transfer of multiple electrons and protons is of crucial importance in many reactions relevant in biology and chemistry. Natural redox-active cofactors are capable of storing and releasing electrons and protons under relatively mild conditions and thus serve as blueprints for synthetic proton-coupled electron transfer (PCET) reagents. Inspired by the prominence of the 2e − / 2H + disulfide/dithiol couple in biology, we investigate herein the diverse PCET reactivity of a Re complex equipped with a bipyridine ligand featuring a unique SH••• − S moiety in the backbone. The disulfide bond in fac-[Re( S−S bpy)(CO) 3 Cl] (1, S−S bpy = [1,2]dithiino[4,3-b:5,6-b′]dipyridine) undergoes two successive reductions at equal potentials of −1.16 V vs Fc +|0 at room temperature forming [Re( S2 bpy)(CO) 3 Cl] 2− (1 2− , S2 bpy = [2,2′-bipyridine]-3,3′-bis(thiolate)). 1 2− has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S−H••• − S unit, 1H − . The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (ΔG°H + = 34 kcal mol −1 , pK a = 24.7), an H atom (ΔG°H • = 59 kcal mol −1 ), or a hydride ion (ΔG°H − = 60 kcal mol −1 ) as demonstrated in the reactivity with various organic test substrates.

The novel photosensitizer [Ru( S–S bpy)(bpy) 2 ] 2+ harbors two distinct sets of excited states in the UV/Vis region of the absorption spectrum located on either bpy or S–S bpy ligands. Here, we address the question of whether following excitation into these two types of states could lead to the formation of different long-lived excited states from where energy transfer to a reactive species could occur. Femtosecond transient absorption spectroscopy identifies the formation of the final state within 80 fs for both excitation wavelengths. The recorded spectra hint at very similar dynamics following excitation toward either the parent or sulfur-decorated bpy ligands, indicating ultrafast interconversion into a unique excited-state species regardless of the initial state. Non-adiabatic surface hopping dynamics simulations show that ultrafast spin–orbit-mediated mixing of the states within less than 50 fs strongly increases the localization of the excited electron at the S–S bpy ligand. Extensive structural relaxation within this sulfurated ligand is possible, via S–S bond cleavage that results in triplet state energies that are lower than those in the analogue [Ru(bpy) 3 ] 2+ . This structural relaxation upon localization of the charge on S–S bpy is found to be the reason for the formation of a single long-lived species independent of the excitation wavelength.

Using the planar 1,8-naphthyridin-2(1H)-one (Hnpo) ligand, novel nonhelical HMSCs [MoM(npo)(NCS)] (M = Fe, Co, Ni) were synthesised and they exhibited high single-molecule conductance.

The heterotrimetallic complexes [FeMFe(dpa) Cl ] (M=Ni (1), Pd (2), and Pt (3); dpa =dipyridylamido) featuring two high-spin iron centers linked by Group 10 metals were synthesized and their physical properties were investigated. Oxidation of 1-3 with suitable oxidants in CH Cl solution yielded the mixed-valent species [1] -[3] . The solution properties of [1] -[3] were characterized by H NMR and UV/Vis/NIR spectroscopy as well as spectroelectrochemisty. The mixed-valent states of [1] -[3] obtained by electrochemical or chemical oxidation are classified as class II valence delocalization. The solid-state structures of 1-3, [1] , [3] , and [1] were determined by single-crystal X-ray diffraction analysis, exhibiting a linear metal framework with an approximate D symmetry. The spin states and magnetic properties were studied by using SQUID magnetometry, EPR and Mössbauer spectroscopy, and DFT calculations. Antiferromagnetic interactions between terminal high-spin iron centers are present within [1] -[3] and the |J| values increase with the central metal ion changing from Ni to Pt. The DFT calculations reproduce the antiferromagnetic coupling and ascribe it to a σ-type exchange pathway. The substitution of the central metal not only influences the spin-spin interactions but also the degree of electronic delocalization between the terminal iron sites along the Fe-M-Fe chains.

The CoRu(dpa)Cl (1) (dpa: 2,2'-dipyridylamide) is synthesized by the reaction of Ru(OAc)Cl and Co(dpa)Cl. By mixing 1 with NH, Co can be removed and result in the formation of unique binuclear complex 4,0-Ru(dpa)Cl (2) featuring one coordination pocket supported by free pyridine groups. Hence, this complex can act as an outstanding precursor for the formation of heterotrimetallic chains with MRu cores. A series of M-Ru complexes (M = Co (3), Ag (4), Mn (5), Fe (6), Zn (7), Cd (8), Pd (9), Rh (10), and Ir (11)) were prepared and isolated, representing the most complete series of heterotrimetallic chains to date. All these metal string complexes are in a linear trimetallic framework helically wrapped by four dpa ligands, characterized by X-ray diffraction measurements. The bending of the trinuclear metal cores in RhRu (10) and IrRu (11) (∠Ru-Ru-Rh: 167.58° and ∠Ru-Ru-Ir: 167.61°) indicates that a heterometallic metal-metal bonds (Ru-Rh; Ru-Ir) are generated. The studies from DFT calculation of 10 and 11 coincide with the experimental results. Furthermore, the MRu distances are regulated by the factors including the bonding force of M-pyridyl and the static repulsion between M and Ru unit. Interestingly, the trend for these distances is in line with that observed in trans-M(py)Cl complexes.