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This research elucidates the intricate nature of electronic coupling in the redox-active (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), commonly utilized in organic radical batteries. This study employs a combination of classical molecular dynamics and various electronic coupling calculation schemes. Within the context of the generalized Mulliken–Hush method, the electronic couplings are investigated via the complete active space self-consistent field approach, in combination with n-electron valence state perturbation theory, to provide an accurate description of both static and dynamic electron correlation as well as using (time-dependent) density functional theory simulations. Furthermore, the electronic communication between redox-active sites is studied using the cost-efficient density functional theory (DFT)-based frontier molecular orbital (FMO) approach. Our study reveals the dependence of the electronic coupling on the distance and the relative orientation of the redox pairs (TEMPO and TEMPO+). Apart from the expected exponential distance dependence, we found pronounced orientation dependence, with coupling values varying up to 0.2 eV, which is reflected by a substantial basis set dependency of the couplings, in particular at short distances. In addition, our study highlights the limitations of the DFT-based FMO method, in particular at short intermolecular distances between the redox-active sites, which may lead to a mixing of the involved molecular orbitals. This comparison will provide us with the most cost–accuracy-effective method for calculating electronic couplings in TEMPO–TEMPO+ systems. [ABSTRACT FROM AUTHOR]

In this work, a series of DFT and DFT‐D methods is combined with double‐ζ basis sets to benchmark their performance in predicting the structures of five newly synthesized hexacarbonyl diiron complexes with a bridging ligand featuring a μ‐S2C3 motif in a ring‐containing unit functionalized with aromatic groups. Such complexes have been considered as [FeFe] hydrogenase catalytic site models with potential for eco‐friendly energetic applications. According to this assessment, r2SCAN is identified as the density functional recommended for the reliable description of the molecular and crystal structures of the herein studied models. However, the butterfly (μ‐S)2Fe2 core of the models demonstrates a minor deformation of its optimized geometry obtained from both molecular and periodic calculations. The FeFe bond length is slightly underestimated while the FeS bonds tend to be too long. Adding the D3(BJ) correction to r2SCAN does not lead to any improvement in the calculated structures. [ABSTRACT FROM AUTHOR]

Molecular charge accumulating systems that act as both, photosensitizer and electron storage unit, are of interest in the context of multielectron redox processes, e. g. in solar fuel production. To this end, the photophysical properties of RuL1, a ruthenium tris‐diimine complex with an alloxazine‐based ligand as bioinspired structural motif, were investigated. The study includes absorption, emission, resonance Raman and transient absorption spectroscopy in combination with quantum chemical simulations to determine the light‐driven reactivity of the complex. Moreover, spectroelectrochemistry was employed for an in‐depth characterization of the optical properties of the reduced complex. Finally, a photolysis experiment using triethanolamine as electron source, in conjunction with redox titrations, demonstrated that visible light irradiation triggers the formation of the doubly‐reduced singly‐protonated derivative of RuL1, where both redox equivalents are stored on the alloxazine‐based ligand. [ABSTRACT FROM AUTHOR]

The activation step of Vaska‐type Rh(I) complexes, such as the photocleavage of the Rh−CO bond, plays an important role in the subsequent C−H activation. To elucidate the details of the photochemistry of Vaska‐type Rh(I) complexes, such as trans‐Rh(PMe3)2(CO)(Cl), we here present a computationally derived picture as obtained at the density functional level of theory in combination with multireference wavefunction‐based methods. We have identified that the photocleavage of CO proceeds via the metal‐centered excited state (3MC, dRhz2→dRhx2-y2 ${{d\left({\rm R}{\rm h}\right)}_{{z}^{2}}\to {d\left({\rm R}{\rm h}\right)}_{{x}^{2}-{y}^{2}}}$), which is populated through intersystem crossing from the dipole‐allowed excited state S1(dRhz2→pRhz ${{d\left({\rm R}{\rm h}\right)}_{{z}^{2}}\to {p\left({\rm R}{\rm h}\right)}_{z}}$ ‐π*CO ${{\pi }^{{\rm ^{\ast}}}\left({\rm C}{\rm O}\right)}$). Moreover, the present study unraveled the reasons for the low C−H activation efficiency when using Rh featuring the bidentate ligand 1,2‐bis(dimethylphosphino)ethane (dmpe), namely due to its unfavorable photochemical properties, i. e. the small driving force for light‐induced CO loss and the fast deactivation of 3MC state back to the singlet ground state. In this study, we provide theoretical insight into mechanistic details underlying the light‐induced CO dissociation process, for Rh complexes featuring PMe3 and dmpe ligands. [ABSTRACT FROM AUTHOR]

Design tactics and mechanistic studies both remain as fundamental challenges during the exploitations of earth-abundant molecular electrocatalysts for CO2 reduction, especially for the rarely studied Cr-based ones. Herein, a quaterpyridyl CrIII catalyst is found to be highly active for CO2 electroreduction to CO with 99.8% Faradaic efficiency in DMF/phenol medium. A nearly one order of magnitude higher turnover frequency (86.6 s-1) over the documented Cr-based catalysts (<10 s-1) can be achieved at an applied overpotential of only 190 mV which is generally 300 mV lower than these precedents. Such a high performance at this low driving force originates from the metal-ligand cooperativity that stabilizes the low-valent intermediates and serves as an efficient electron reservoir. Moreover, a synergy of electrochemistry, spectroelectrochemistry, electron paramagnetic resonance, and quantum chemical calculations allows to characterize the key CrII, CrI, Cr0, and CO-bound Cr0 intermediates as well as to verify the catalytic mechanism. [ABSTRACT FROM AUTHOR]

BODIPY‐perylene dyads have emerged as useful metal free sensitizers for triplet‐triplet annihilation upconversion (TTAUC), these dyads are capable of efficient triplet generation via spin‐orbit charge transfer intersystem crossing (SOCT‐ISC). This important route to triplet formation requires dyads in which two moieties are oriented perpendicular to each other. In this contribution, we give a deeper insight on the behavior of recently reported BODIPY‐perylene dyads, where BODIPY‐perylene dihedral exhibits a non‐orthogonal dyad geometry. The intersystem crossing of BODIPY‐perylene dyads with and without iodine are investigated using femtosecond transient absorption (fs‐TA) and nanosecond transient absorption (ns‐TA) spectroscopy. The concurrent decay of the singlet charge transfer state (1CT) and rise of triplet states in both the iodinated and non‐iodinated dyads confirms the SOCT‐ISC as the main intersystem crossing pathway despite the altered geometry of the dyads. The presence of an iodine atom on the BODIPY‐moiety enables intersystem crossing 2.6‐times faster and provides a higher triplet yield with respect to dyad without iodine. The upconversion quantum yield (φSUC ${{\phi{} }_{{\rm S}}^{{\rm U}{\rm C}}}$) is 8.4‐times higher in the sample containing iodinated dyad as sensitizer and perylene as annihilator. The triplet‐triplet energy transfer rate (kTTET) is ~8×108 M−1 s−1 for both iodinated and non‐iodinated sensitizer containing TTAUC systems in 1,4‐dioxane. [ABSTRACT FROM AUTHOR]