Thionation of the beta-oxo-delta-diimine (BODDI) 3 with P2S5 & centerdot;2Py affords the ditopic beta-thio-delta-diimine (BTDDI) 4. Based on H-1 NMR spectroscopy and single-crystal X-ray diffraction, 4 is best described as a bis(beta-enamine)-thione in both solution and the solid state. Furthermore, 4 features a thermochromic behavior in solution (c > 2 mmol l(-1)) and as a solid. Reaction with trimethyl aluminium readily affords the bimetallic dimethylaluminium BTDDI complex 5. In contrast to its oxygen relative, 5 is only weakly emissive and the emission is significantly red-shifted. Scalar-relativistic time-dependent density functional theory calculations suggest that introduction of the sulfur atom promotes an intersystem crossing pathway (similar to 1 ns) to low-lying and non-emissive triplet states, which competes with fluorescence (similar to 5 ns).

Heterogeneous molybdenum sulfides are widely used noble metal-free hydrogen evolution reaction (HER) catalysts. Thiomolybdates, their molecular analogues have been developed as viable minimal models to study reactivity at the molecular level. Here, we explore the light-driven HER reactivity and stability of the mixed thio-oxo-molybdate prototype [Mo2O2S6](2-) in homogeneous solution. In combination with the photosensitizer [Ru(bpy)(3)](2+), [Mo2O2S6](2-) shows promising HER performance (turnover number TON > 500), as well as strong reactivity dependence on the reaction conditions. Mechanistic experimental studies combined with density functional theory computations reveal complex speciation of the catalyst in solution, as well as light-induced and light-independent reaction pathways for catalyst and photosensitizer which are in line with disulfide-for-solvent ligand exchange reactions. These structure-reactivity insights outline design rules for more robust, solvent-tolerant thiomolybdate HER catalysts.

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.

The design of covalently linked photoactive donor-acceptor dyads offers major opportunities for photocatalysis and solar energy conversion. Here, we report a noble metal-free dyad obtained by covalent anchoring of a ferrocene moiety to a Dawson-type polyoxotungstate. The resulting dyad shows visible light photoinduced charge-separation and electron transfer from the ferrocene to the polyoxometalate. The separated charges can be used for the photoinduced, oxidative and reductive activation of organic peroxides to generate oxygen-based radicals. Structural and mechanistic studies using in situ spectroscopy, time-resolved spectroscopy and spectro-electrochemistry as well as quantum chemical calculations shed light on the underlying photoinduced reactivity. The study presents a blueprint for the design of photoactive covalent photosensitizer-polyoxometalate dyads based on earth-abundant elements.

The synthesis, spectroscopic characterization, computational analysis, and photoelectrochemical behavior of a new iron-based chromophore, [(Cpy)2Fe(deeb)](PF6)2 (Fe(Cpy)2(deeb)), where Cpy is 1-methyl-3-(2-pyridyl)imidazole and deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, is reported. Electrochemically reversible waves assigned to a metal-centered Eo(FeIII/II) = +0.48 and a ligand-centered Eo(Fe2+/+) = -1.47 V vs Fc+/0 reduction were evident in cyclic voltammetry measurements. The combination of a strong σ-donor and a π-acceptor lowered the energy of the metal-to-ligand charge-transfer (MLCT) excited state relative to the metal-centered state. Two MLCT transitions appear in the visible region at 424 and 580 nm. TDDFT calculations revealed that the lower-energy band was well formulated as Fe(II)→deeb, and the higher-energy transition was charge transfer to both the deeb and Cpy ligands. Resonance Raman spectroscopy supports these findings showing enhanced deeb vibrational modes with 532 nm excitation, both deeb and Cpy modes with 473 nm excitation, and exclusively Cpy with 405 nm excitation. Ultrafast spectroscopy reveals a short-lived (∼2 ps) MLCT excited state and a longer-lived (∼20 ps) metal-centered state. Efficient methods to deprotect the ester groups and anchor the complex to mesoporous TiO2 (anatase) thin films in high surface coverages, Fe(Cpy)2(dcb)|TiO2 σ = 3 × 10-8 mol/cm2, were established. Pulsed light excitation of Fe(Cpy)2(dcb)|TiO2 resulted in rapid excited state injection (kinj > 108 s-1) and formation of a charge-separated state, FeIII(Cpy)2(dcb)|TiO2(e), which persists on the millisecond time scale before returning cleanly to the ground state with second-order kinetics. Injection yields measured 50 ns after light excitation were found to double from Φ = 0.15 with green (532 nm) light to 0.30 with blue (457 nm) light excitation. Incident photon-to-current efficiency (% IPCE) measurements as a function of excitation wavelength in a 0.5 M LiI/I2/CH3CN electrolyte provide clear evidence for band-selective "hot carrier" injection from the remote Cpy-localized excited state. Collectively, the spectroscopic and photoelectrochemical data indicate that a semiconductor can intercept hot electrons from iron chromophores even when the excited-state dipole is oriented away from the surface-anchoring ligand.

The photo-induced dynamics of a redox-active dinuclear W(ii)/Ru(ii) complex, [Tp*W(CO)Br(PyC[triple bond, length as m-dash]CCH2)-Ru(bpy)2](PF6) (2-PF6), is revealed by femtosecond infrared and UV-vis pump-probe spectroscopy in combination with quantum chemical calculations. The use of the mononuclear tungsten alkyne complex [Tp*W(CO)Br(PyC[triple bond, length as m-dash]CCH3)] (1) as a benchmark allowed an in-depth analysis of the excited state kinetics of 2-PF6. Excitation of the dinuclear complex at 400 nm produces predominantly a triplet metal-to-ligand charge transfer state localised at the Ru(bpy)2 chromophore (3MLCTbpy) with a lifetime of 6 ps. The following transformation into a tungsten-centered triplet state (3MCW) is accompanied by significant charge transfer and rearrangement of the W-C-O geometry. Subsequent intersysten crossing back to the ground state on a timescale of 12 ps produces a vibrationally excited molecule with up to 3 quanta in the CO stretching vibration. A minor fraction of 10% of the population reacts to an intermediate exhibiting a lifetime of 140 ps and a CO stretching frequency of 1703 cm-1. Our quantum chemical calculations disclose that this species corresponds to an isomer trapped in a metastable state of the S0 potential surface, with the CO bound side-on to the W centre.

Molecular molybdenum sulfides, or thiomolybdates, are well-established noble-metal free molecular catalysts for the hydrogen evolution reaction (HER). To-date, there is a knowledge-gap regarding the impact of organo-functionalization on reactivity, stability and heterogenization of this compound class. Here, we report the development of synthetic routes for controlled introduction of organic N-donor ligands as a first step toward establishing structure-property-reactivity relationships in this new compound class. Photophysical studies combined with computational modelling are used to rationalize trends in the observed light-driven homogeneous HER activity. This work lays the foundation to develop organo-functionalized thiomolybdate HER catalysts suitable for covalent or supramolecular linkage to photosensitizers and for anchoring on heterogeneous supports, e.g. photocathodes.

Thermally activated delayed fluorescence (TADF) compounds have found great application as photocatalysts in recent years. Their use as photosensitizers (PS), however, is still limited although they have huge potential as abundant alternatives to traditional noble metal-based PSs, such as [Ru(bpy)3]2+. We herein investigate the TADF compound 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as a PS together with the catalyst cobaloxime (Co(dmgH)2(py)Cl) in the photochemical hydrogen evolution reaction (HER). Systematic optimization of the solvent composition, 4CzIPN and catalyst concentration, and sacrificial electron donor (SED) identified ammonium ascorbate as the most effective SED under the conditions tested. Under optimized conditions, the system reached a TON of 6451 ± 82 after 20 h irradiation. Stern-Volmer quenching experiments and scalar-relativistic quantum-chemical calculations provided rate constants for the different processes in the photosensitization mechanism, demonstrating that the quenching of the excited singlet state (S1) of 4CzIPN by ammonium ascorbate is by far more dominant than the quenching of its excited triplet state (T1). This work demonstrates that 4CzIPN can sensitize the cobaloxime-catalyzed light-driven HER with ammonium ascorbate as the SED and provides mechanistic insight into the role of singlet- and triplet-state quenching in TADF photosensitization.

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 (MV2+) and polymer-bound viologens by a factor of up to 2,200 as well as a rapid subsequent second reduction of MV•+ to MV0 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 (MV0). These results provide a toolbox of structurally simple compounds for the modular construction of efficient artificial photosynthesis schemes.

In contrast to well-studied 4d6 and 5d6 transition metal complexes such as the modern-day drosophila of photochemistry, Ru(ii)-tris(bipyridine), which often feature a typical triplet metal-to-ligand charge transfer emission in the nanosecond timescale, the photophysics of Cr(iii) complexes are drastically different. The 3d3 configuration of the chromium(iii) allows for an unusual spin-flip emission from the low-lying metal-centered (MC; 2T1 and 2E) states, exhibiting lifetimes up to the milliseconds to seconds timescale. In this fully computational contribution, the photophysical properties as well as the application of such long-lived excited states in the context of photoredox chemical transformations are investigated for the recently introduced [Cr(dqp)2]3+ [Cr(iii)-(2,6-bis(8'-quinolinyl)pyridine)2]3+, otherwise known as a type of molecular ruby. Our in-depth theoretical characterization of the complicated electronic structure of this 3d3 system relies on state-of-the-art multiconfigurational methods, i.e. the restricted active space self-consistent field (RASSCF) method followed by second-order perturbation theory (RASPT2). This way, the light-driven processes associated with the initial absorption from the quartet ground state, intersystem crossing to the doublet manifold as well as the spin-flip emission were elucidated. Furthermore, the applicability of the long-lived excited state in [Cr(dqp)2]3+ in photoredox chemistry, i.e. reductive quenching by N,N-dimethylaniline, was investigated by ab initio molecular dynamics (AIMD). Finally, the thermodynamics and kinetics of these underlying intermolecular electron transfer processes were analyzed in the context of semiclassical Marcus theory.

Light-driven electron transfer and subsequent multielectron storage is among the key aspects of photochemical reactions in artificial photosynthesis and molecular electronics. Following our previously introduced design and characterization of Ru(II)-based photosensitizers, four new Ru complexes with π-extended ligands featuring a flavin-inspired subunit were investigated via density functional theory in order to evaluate their electrochemical properties ahead of a time and resource-demanding synthesis. Two complexes, Ru-Me2alloxB and Ru-Me2deazaalloxB, with a bent ligand architecture, were identified as promising candidates for application in light-driven charge accumulation and subsequently synthesized. The electrochemical characterization of Ru-Me2alloxB confirmed the theoretical predictions and its photophysical properties were investigated using UV/Vis absorption, resonance Raman, time-resolved emission, and time-resolved absorption spectroscopy in combination with quantum chemical simulations. Furthermore, first insights into the electronic distribution in the singly reduced complex were modelled computationally and obtained by EPR and UV/Vis absorption spectroscopy and spectroelectrochemistry. These results underline the promising multielectron storage capacity of the newly designed π-extended alloxazine ligand.

Organic radical batteries (ORBs) based on the TEMPO (2,2,6,6-tetramethylpiperidin-1-yl oxyl) radical have drawn significant attention, owing to their unique redox properties. A key factor influencing ORB's redox properties, i.e., the kinetics of the electron transfer between the TEMPO-TEMPO+ pairs, is the communication between the underlying redox-active states as given by the electronic coupling. However, due to the complex structure, predicting accurate electronic couplings for these pairs is computationally expensive and challenging. In this study, we introduce a machine learning (ML) workflow to predict the electronic coupling for TEMPO-TEMPO+ pairs simply by their specific geometric orientations. For the ML models, a data set was generated through time-dependent density functional theory calculations coupled with the Generalized Mulliken Hush method to assess energies, (transition-)dipole moment, and couplings for specific TEMPO-TEMPO+ configurations obtained from classical molecular dynamics simulations that mimic a realistic electrolyte environment. Our results demonstrate that, among the three ML models-linear regression, kernel ridge regression (KRR), and random forest-the KRR model, with its kernel-based approach, most effectively handles the correlated orientation-based descriptors. Moreover, our SHapley Additive exPlanations (SHAP)-based feature importance analysis indicates that multiple orientation factors jointly influence electronic coupling, rather than any single distance or angle dominating, with each parameter's impact strongly contingent on the values of the others which is in agreement with previous studies computational by the consortium.

Using light energy to drive chemical transformations is of great relevance, with photosynthesis in nature as a grand example. In artificial light-driven catalysis, part of nature's complex supramolecular architecture can be mimicked through the so-called covalently linked photosensitizer-catalyst (PS-CAT) dyads. We herein report a dyad using an organic donor-acceptor PS, with dipyridophenazine as the acceptor and tert-butylcarbazole as the donor (2 t BuCzDPPZ), that contains a coordination site for a rhodium(iii)Cp* center as the catalyst. The organic PS shows a charge-transfer transition upon visible-light irradiation and has redox properties similar to typically used ruthenium-based PSs. The resulting PS-CAT dyad 2 t BuCzDPPZRhCp* shows - with methoxy-substituted 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH-OMe) as the sacrificial electron donor - photocatalytic activity in light-driven NAD+ reduction with a TON of 3.2 (after 4 h). Femtosecond transient absorption and resonance Raman spectroscopy, as well as time-dependent density functional theory (TDDFT) calculations, shed light on the photophysical properties of the PS and PS-CAT dyad and reveal a high dependency of the photoluminescence quantum yield and excited state properties on solvent polarity - in line with its donor-acceptor structure. This work presents a new design concept for PS-CAT dyads in artificial light-driven catalysis and provides important insight into the interplay between solvation dynamics of organic donor-acceptor systems and their photophysics, paving the way for future design strategies.

Supramolecular photocatalysts consisting of photosensitizer (PS), bridging ligand (BL), and catalytic center (CAT) have garnered significant attention in solar fuel applications. In this study, the photophysics and photocatalytic properties of two Ru(II)-based dinuclear complexes, specifically [(tbbpy)2Ru(p(Ph)np)Rh(Cp*)Cl]3+ (n = 0, 1; Ru(pp)Rh for n = 0 or Ru(p(Ph)p)Rh for n = 1; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, Cp* = pentamethylcyclopentadienyl, Ph = phenyl, p = 1,10-phenanthroline), are investigated. These complexes are studied as model complexes only differing by the distance between PS and CAT and thus allows a selective investigation of the influence of spacers in light-driven catalysis. A joint synthetic, spectroscopic, and theoretical approach, incorporating time-resolved absorption and emission spectroscopy, resonance Raman (rR) spectroscopy, density functional theory (DFT), and time-dependent (TD)DFT calculations, allows for comprehensive structural, electrochemical, photophysical, and photochemical characterization. Our findings suggest that minor structural variations in the intramolecular photocatalytic system significantly impact photocatalytic activity and system stability.

In this work, the synthesis and structural characterization of the smallest possible member of the family of bis-functionalized {MnMo6O24} Anderson–Evans polyoxometalates (POMs) is reported. The synthesis of the title compound TBA3{[HC(CH2O)3]2MnMo6O18} (1) was accomplished by using trimethylolmethane as the capping unit (TBA: tetra(n-butyl)ammonium, n-Bu4N+). The molecular structure of the organic–inorganic POM gave rise to yet undisclosed 1H-NMR features, which are discussed thoroughly. Single-crystal X-ray diffraction (XRD) analysis revealed a highly regular 3D packing of the polyoxoanions within a matrix of TBA cations. The hybrid POM is of particular interest regarding potential applications in photocatalysis (i.e., hydrogen evolution) and energy storage. Thus, the electrochemical and thermal properties of 1 are also analyzed.

The main obstacle in replacing well-established precious ruthenium photosensitizers with earth-abundant iron analogs is the short excited state lifetimes of metal-to-ligand charge transfer (MLCT) states due to relatively weak octahedral field splitting and relaxation via metal-centered (MC) states. In this study, we address the issue of short lifetime by using pentacyanoferrate(II) complexes and combat facile photodissociation by utilizing positively charged pyrazinium or bipyridinium ligands. We utilize femtosecond transient absorption spectroscopy alongside quantum chemical calculations to probe the excited states of three 4,4'-bipyridinium- or pyrazinium-based pentacyanoferrate(II) complexes. The 4,4'-bipyridinium-based complexes exhibit 3MLCT lifetimes of about 20 ps, while the pyrazinium-based complex exhibits a lifetime of 61 ps in an aqueous solution, setting a benchmark for cyanoferrate complexes. These results mark the foundation for a new group of easy-to-prepare iron photosensitizers that can be used for harvesting visible light.