Georgina E. Shillito

Achieving multiple photo‐induced charge accumulations on the same acceptor is one option to propel photochemical reactions that demand highly reducing or oxidizing conditions. By switching the initial light‐driven process from an oxidative to a reductive quenching pathway via mutual redox potential control over Ru(II) polypyridyl chromophores and sacrificial electron donors (SEDs), acceleration of complete single electron reduction of methyl viologen (MV ²⁺ ) and polymer‐bound viologens by a factor of up to 2,200 as well as a rapid subsequent second reduction of MV ^•+ to MV ⁰ was achieved in homogeneous solution. Ab initio molecular dynamics (AIMD) allowed the conformer space to be mapped and to shed light on the thermodynamics of the photochemical intermediates involved in the formation of the two‐electron reduced methyl viologen (MV ⁰ ). These results provide a toolbox of structurally simple compounds for the modular construction of efficient artificial photosynthesis schemes.

Organic batteries represent a promising class of energy storage materials, due to their mechanical flexibility and sustainability. Typically, stable radicals, lacking intrinsic conductivity, are utilized as redox‐active materials. A recently introduced strategy to overcome this shortcoming is to incorporate stable radicals, i.e., (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO), into a polythiophene backbone. Thereby, an electrode material was obtained which does not require conductive additives. The current computational study aims to elucidate the functionality of this material by drawing in‐depth structure–property relationships utilizing a hierarchical molecular and quantum mechanical approach. Initially, structural properties of the electrode material's macroenvironment—containing the functionalized polythiophene, electrolyte, and solvent—were assessed in various charging states by molecular dynamics simulations. Subsequently, electronic properties were investigated by time‐dependent density functional theory for 564 microenvironments. Via this computational setup, the electronic communication within the material was assessed along intrastrand and interstrand CT processes involving the respective TEMPO and polythiophene units. Thereby, our hierarchical computational approach reveals that the intrinsic conductivity and charge storage capacity of the electrode material stems from efficient intrastrand TEMPO‐polythiophene CT processes along short and rigid amid linkers. These insights help to tailor improved conductive organic electrode materials with higher charging and discharging rate capabilities.

Photophysical and kinetic processes in a Cr( iii ) spin-flip luminescent complex were elucidated using in-depth computational modelling. Use of the excited state in photoredox applications was also investigated using ab initio molecular dynamics.

Bridged photosensitizer‐catalyst systems are promising models to study photocatalytic hydrogen evolution. However, the systems in the literature structurally diverse and therefore hard to compare. Many systems show highly complex photophysics including several accepting orbitals for the excited state, as a result catalytic activity is hard to predict. Here we present a bimetallic Ru−Pt photocatalyst bearing peripheral spectator ligands at the ruthenium(II) photocenter as a member of the Ru‐tpphz‐Pt family. Consequently, it features a single acceptor tpphz ligand and so‐called unidirectional electron transfer, i. e., electron transfer without co‐occurring transfer to peripheral ligands, from the excited state. Thus – and in contrast to recently used peripheral ligands – the new spectator ligands do not disrupt electron transfer towards the catalytic center. By comparison to known systems, this facilitates unprecedented insight into the importance of electron transfer from the bridge to the catalytic center moving towards more rational design of oligonuclear photocatalysts.

Excited states are the key species in photocatalysis, while the critical parameters that govern their applications are (i) excitation energy, (ii) accessibility, and (iii) lifetime. However, in molecular transition metal-based photosensitizers, there is a design tension between the creation of long-lived excited (triplet), e.g., metal-to-ligand charge transfer (3MLCT) states and the population of such states. Long-lived triplet states have low spin–orbit coupling (SOC) and hence their population is low. Thus, a long-lived triplet state can be populated but inefficiently. If the SOC is increased, the triplet state population efficiency is improved—coming at the cost of decreasing the lifetime. A promising strategy to isolate the triplet excited state away from the metal after intersystem crossing (ISC) involves the combination of transition metal complex and an organic donor/acceptor group. Here, we elucidate the excited state branching processes in a series of Ru(II)-terpyridyl push–pull triads by quantum chemical simulations. Scalar-relativistic time-dependent density theory simulations reveal that efficient ISC takes place along 1/3MLCT gateway states. Subsequently, competitive electron transfer (ET) pathways involving the organic chromophore, i.e., 10-methylphenothiazinyl and the terpyridyl ligands are available. The kinetics of the underlying ET processes were investigated within the semiclassical Marcus picture and along efficient internal reaction coordinates that connect the respective photoredox intermediates. The key parameter that governs the population transfer away from the metal toward the organic chromophore either by means of ligand-to-ligand (3LLCT; weakly coupled) or intra-ligand charge transfer (3ILCT; strongly coupled) states was determined to be the magnitude of the involved electronic coupling.

A hexa-peri-hexabenzocoronene (HBC)-substituted dipyridophenazine (dppz) ligand (dppz-HBC) and its corresponding rhenium [Re(CO)₃Cl] and ruthenium [Ru(bpy)₂]²⁺ complexes were synthesized and characterized. The interplay of their various excited states was investigated using spectroscopic and computational techniques. Perturbation of the HBC was seen through a broadening and decreased intensity of the HBC absorption bands that dominate the absorption spectra. A delocalized, partial charge transfer state was shown through emission (520 nm) in the ligand and rhenium complex and is supported by time-dependent density functional theory calculations. Transient absorption measurements revealed the presence of dark states with a triplet delocalized state populated in the ligand, while in the complexes, longer-lived (2.3-2.5 μs) triplet HBC states could be accessed. The properties of the studied ligand and complexes provide insight into the future design of polyaromatic systems and add to the rich history of dppz systems.

A review. In recent decades, mimicking natural photosynthesis by artificial photocatalysis represented a major research direction with the ultimate goal of reducing fossil fuel consumption through efficient solar energy harvesting. To transfer molecular photocatalysis from the lab scale to an industrially relevant process, it is important to overcome instability problems of the catalysts during light‐driven operation. As it is well‐known that many of the typically utilized noble metal‐based catalytic centres (e. g. Pt and Pd) undergo particle formation during (photo)catalysis and thus switch the whole process from a homogeneous into a heterogeneous one, an understanding of the factors governing particle formation is crucially needed. The review therefore focuses on di‐ and oligonuclear photocatalysts bearing a range of different bridging ligand architectures for drawing structure‐catalyst‐stability relationships in light‐driven intramolecular reductive catalysis. In addition, ligand effects at the catalytic centre and their implications for catalytic activity in intermolecular systems will be discussed, as will important insights into the future design of operationally stable catalysts.

Quantum chemical methods have been utilised to explore the kinetics and thermodynamics of a prominent charge recombination pathway in a series of Ru^II based molecular photocatalysts. Selective tuning of the Ru^II coordination sphere, replacing the tbbpy ligands of the hydrogen evolving parent photocatalyst with electron rich, biimidazole based ligands, promotes unidirectional charge transfer towards the bridging ligand during initial photoexcitation. These electronic effects are also significant in the triplet manifold, where the predicted rate of the undesired deactivation process from the ³MLCT state on the bridging ligand to a ³MC state on the ruthenium centre, is decreased relative to the parent complex, by 1–2 orders of magnitude, alongside a decrease in electronic coupling. This design methodology could be utilised to promote targeted (light‐driven) electron transfer pathways, as well as to potentially reduce ³MC deactivation pathways in commonly used polypyridyl‐based Ru^II photocentres, thus enhancing the quantum efficiency of light driven catalysis.

The ability to identify the contents of a sealed container, without the need to extract a sample, is desirable in applications ranging from forensics to product quality control. One technique suited to this is inverse spatially offset Raman spectroscopy (ISORS) which illuminates a sample of interest with an annular beam of light and collects Raman scattering from the center of the ring, thereby retrieving the chemical signature of the contents while suppressing signal from the container. Here we explore in detail the relative benefits of a recently developed variant of ISORS, called focus-matched ISORS. In this variant, the Fourier relationship between the annular beam and a tightly focused Bessel beam is exploited to focus the excitation light inside the sample and to match the focal point of excitation and collection optics to increase the signal from the contents without compromising the suppression of the container signal. Using a flexible experimental setup which can realize both traditional and focus-matched ISORS, and Monte-Carlo simulations, we elucidate the relative advantages of the two techniques for a range of optical properties of sample and container.