Regina de Vivie-Riedle

Molecular dynamics (MD) simulations are a widely applied tool to investigate systems of varying complexity, from isolated molecules to biomolecules consisting of many thousands of atoms. Extracting mechanistic insights from the high-dimensional data sets frequently presents a challenge. Here, we introduce the publicly available MOlecules aNd Internal Cluster Analysis of Molecular Dynamics simulations (MonicaMD) program package, a versatile and efficient tool that targets the analysis of molecules and molecular clusters in atomistic classical and semiclassical trajectories. MonicaMD provides modular access to structural information, with a focus on internal and collective variables. A further key functionality is the extraction of electrostatic information. MonicaMD offers a user-friendly workflow including dimensionality reduction, automatic feature-space generation, and a templating functionality for generated grids in order to be readily used in conjunction with quantum chemical software and machine learning frameworks. The functionality of MonicaMD offers the user a convenient and efficient bridge between classical MD and higher-accuracy quantum mechanics simulations. This synergy enabled by MonicaMD is demonstrated by the investigation of conformational analysis in a protein–ligand complex, structural and electrostatic effects of DNA intercalation, and the excited-state isomerization of a photoswitch. Additional examples include reactive coordinates of a transition-metal-catalyzed C–N coupling reaction and of the light-initiated generation of free diazoalkane, as well as an analysis of chlorophyll binding sites in a photosynthetic complex.

An in‐depth mechanistic study for the photogeneration of free diazoalkanes from N‐tosylhydrazone precursors by combining observations from synthesis with spectroscopic and theoretical methods is presented. The N‐tosylhydrazones have been previously established as donors for alkyl diazo species upon light irradiation, but exact mechanistic details of this photodissociation have remained elusive. Investigations of cyclohexane tosylhydrazone (CyNNTsH) by time‐resolved FTIR spectroscopy proved the role of the deprotonated CyNNTs⁻ as the light‐harvesting species and revealed an intricate dependency of the thermal lifetime of the resulting diazoalkane on the deprotonating base. Computational studies including multiple approaches and levels of theory as well as rigorous benchmarking elucidated the dissociation mechanism via an allowed charge transfer state, a resulting destabilization of the dissociating bond, and a fast change of electronic character of the S₁. These insights allow to suggest specific reaction conditions for photolabile or previously incompatible reaction partners thus paving way towards photo‐orthogonal synthetic strategies.

The current interpretation of excitation energy transfer (EET) processes in natural photosynthesis generally relies on Kasha's rule, suggesting that internal conversion (IC) processes usually outpace any EET between higher excited states. It is, however, known from research on artificial systems that Kasha's rule does not apply to many dyes, especially when found in assembled clusters analogous to photosynthetic chlorophyll (Chl)-protein complexes. In this contribution, a semiempirical Förster-type model is applied to otherwise well-investigated pigments of natural photosynthesis (Chls a, b, c1 and various carotenoids). Strong potential for anti-Kasha processes is identified in all investigated pigments, based on their high Coulomb coupling elements, similar to compounds with already known anti-Kasha properties. The pigments are further found to form strongly delocalized excitons, especially between the higher excited states usually responsible for anti-Kasha pathways. Test calculations with different pigment compositions for various natural light harvesting complexes (LHCII, CP24, CP26, CP29, FCP) demonstrate how the higher band EET network and absorbance could be affected by the presence of accessory pigments: Chl a-only networks should perform anti-Kasha EET, but this is suppressed by the presence of accessory pigments via several mechanisms (exciton disruption, spectral competition, energy sinks and fast, non-Chl a IC). The apparent "special" behavior of photosynthetic systems is thus resolved as the result of pigment mixtures.

An in-depth mechanistic study for the photogeneration of free diazoalkanes from N-tosylhydrazone precursors by combining synthetic observations with spectroscopic and theoretical methods is presented. The N-tosylhydrazones have been previously established as donors for alkyl diazo species upon light irradiation, but exact mechanistic details of this photodissociation have remained elusive. Investigations of cyclohexane tosylhydrazone (CyTsH) by time-resolved FTIR spectroscopy proved the role of the deprotonated CyTs- as the light-harvesting species and revealed an intricate dependency of the thermal lifetime of the resulting diazoalkane on the deprotonating base. Computational studies including multiple approaches and levels of theory as well as rigorous benchmarking elucidated the dissociation mechanism via an allowed charge transfer state, a resulting destabilization of the dissociating bond, and a fast change of electronic character of the S1. These insights allow to suggest specific reaction conditions for photolabile or previously incompatible reaction partners thus paving way towards photo-orthogonal synthetic strategies.

Attosecond spectroscopy, especially time-resolved X-ray absorption spectra (XAS), enables direct observation of ultrafast molecular dynamics. The complementary and even preceding development of theoretical simulations can offer the necessary guidance and stimulate new experiments. In this work, we simulated high-level XAS for the magnesium and nitrogen K-edge of chlorophyll a. In our previous work on the ultrafast relaxation process in the Q-band, our quantum dynamics simulations found the Q_x and Q_y states to be energetically close and therefore strongly coupled. Here, we analyze the strong coupling between Q_x and Q_y via XAS, indicating promising possibilities for experimental observation. The excited-state energies, potential energy surfaces, and XAS are computed at the XMS-CASPT2 level of theory to capture the complex multireference character of chlorophyll excitations. In our simulated spectra, we could follow the ultrafast population transfer between Q_x and Q_y and thus draw conclusions about the strong vibrational coupling between them.

Chlorophylls are photoactive molecular building blocks essential to most photosynthetic systems. They have comparatively simple optical spectra defined by states with near-orthogonal transition dipole moments, referred to as B_x and B_y in the blue/green spectral region, and Q_x and Q_y in the red. Underlying these spectra is a surprisingly complex electronic structure, where strong electronic-vibrational interactions are crucial to the description of state characters. Following photoexcitation, energy-relaxation between these states is extremely fast and connected to only modest changes in spectral shapes. This has pushed conventional theoretical and experimental methods to their limits and left the energy transfer pathway under debate. In this work, we address the electronic structure and photodynamics of chlorophyll a using polarization-controlled static – and ultrafast – optical spectroscopies. We support the experimental data analysis with quantum dynamical simulations and effective heat dissipation models. We find clear evidence for B → Q transfer on a timescale of ∼100 fs and identify Q_x signatures within fluorescence excitation and transient spectra. However, Q_x is populated only fleetingly, with a lifetime well below our ∼30 fs experimental time resolution. Outside of these timescales, the kinetics are determined by vibrational relaxation and cooling. Despite its ultrashort lifetime, our theoretical analysis suggests that Q_x plays a crucial role as a bridging state in B → Q energy transfer. In summary, our findings present a unified and consistent picture of chlorophyll relaxation dynamics based on ultrafast and polarization-resolved spectroscopic techniques supported by extensive theoretical models; they clarify the role of Q_x in the energy deactivation network of chlorophyll a.

Lewis acid catalysis is a promising tool in photochemistry. Thus, the ultrafast dynamics of a substrate-Lewis acid complex is investigated in silico , which reveals an intriguing dissociation–reassociation mechanism.

Bio-nanohybrid devices featuring natural photocatalysts bound to a nanostructure hold great promise in the search for sustainable energy conversion. One of the major challenges of integrating biological systems is protecting them against harsh environmental conditions while retaining, or ideally enhancing, their photophysical properties. In this mainly computational work we investigate an assembly of cyanobacterial photosystem I (PS I) embedded in a metal-organic framework (MOF), namely the zeolitic imidazolate framework ZIF-8. This complex has been reported experimentally [Bennett et al., Nanoscale Adv., 2019, 1, 94.] but so far the molecular interactions between PS I and the MOF remained elusive. We show via absorption spectroscopy that PS I remains intact throughout the encapsulation-release cycle. Molecular dynamics (MD) simulations further confirm that the encapsulation has no noticeable structural impact on the photosystem. However, the MOF building blocks frequently coordinate to the Mg²⁺ ions of chlorophylls in the periphery of the antenna complex. High-level quantum mechanical calculations reveal charge-transfer interactions, which affect the excitonic network and thereby may reversibly change the fluorescence properties of PS I. Nevertheless, our results highlight the stability of PS I in the MOF, as the reaction center remains unimpeded by the heterogeneous environment, paving the way for applications in the foreseeable future.

Precious metal complexes remain ubiquitous in photoredox catalysis (PRC) despite concerted efforts to find more earth‐abundant catalysts and replacements based on 3d metals in particular. Most otherwise plausible 3d metal complexes are assumed to be unsuitable due to short‐lived excited states, which has led researchers to prioritize the pursuit of longer excited‐state lifetimes through careful molecular design. However, we report herein that the C−H arylation of pyrroles and related substrates (which are benchmark reactions for assessing the efficacy of photoredox catalysts) can be achieved using a simple and readily accessible octahedral bis(diiminopyridine) cobalt complex, [1‐Co](PF₆)₂. Notably, [1‐Co]²⁺ efficiently functionalizes both chloro‐ and bromoarene substrates despite the short excited‐state lifetime of the key photoexcited intermediate *[1‐Co]²⁺ (8 ps). We present herein the scope of this C−H arylation protocol and provide mechanistic insights derived from detailed spectroscopic and computational studies. These indicate that, despite its transient existence, reduction of *[1‐Co]²⁺ is facilitated via pre‐assembly with the NEt₃ reductant, highlighting an alternative strategy for the future development of 3d metal‐catalyzed PRC.

Precious metal complexes remain ubiquitous in photoredox catalysis (PRC) despite concerted efforts to find more earth‐abundant catalysts and replacements based on 3d metals in particular. Most otherwise plausible 3d metal complexes are assumed to be unsuitable due to short‐lived excited states, which has led researchers to prioritize the pursuit of longer excited‐state lifetimes through careful molecular design. However, we report herein that the C−H arylation of pyrroles and related substrates (which are benchmark reactions for assessing the efficacy of photoredox catalysts) can be achieved using a simple and readily accessible octahedral bis(diiminopyridine) cobalt complex, [1‐Co](PF₆)₂. Notably, [1‐Co]²⁺ efficiently functionalizes both chloro‐ and bromoarene substrates despite the short excited‐state lifetime of the key photoexcited intermediate *[1‐Co]²⁺ (8 ps). We present herein the scope of this C−H arylation protocol and provide mechanistic insights derived from detailed spectroscopic and computational studies. These indicate that, despite its transient existence, reduction of *[1‐Co]²⁺ is facilitated via pre‐assembly with the NEt₃ reductant, highlighting an alternative strategy for the future development of 3d metal‐catalyzed PRC.