KiMoPack: A python Package for Kinetic Modeling of the Chemical Mechanism

Reductive quenching of photosensitizer [Ru(bpy)3]2+ reveals the inhomogeneous distribution of sites in PAN polymer nanofibers for light-driven redox catalysis

Integration of molecular photocatalysts into redox-inert polymers constitutes a path towards photocatalytically active, lightweight materials. In particular, electrospun polymer fibers hold potential due to their favorable surface-to-volume ratio and their straightforward fabrication. This study focuses on the polyacrylonitrile (PAN) fibers, into which the prototype photosensitizer (PS) ruthenium tris(bipyridine) [Ru(bpy)3]2+, has been embedded by electrospinning. Studying the interaction between the optically excited [Ru(bpy)3]2+ with a non-redox inert solvent within the nanofibers, we resolve a distribution of microenvironments, which differ by the extent to which the photosensitizer is exposed to the solvent. This results in a non-exponential decay of the complex's emission and pronounced differences in the transient absorption signals.

A Tetracarbene Iron(II) Complex with a Long-lived Triplet Metal-to-Ligand Charge Transfer State due to a Triplet-Triplet Barrier

Mixed N-heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth-abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal-to-ligand charge transfer state (3MLCT), which typically decays via a triplet metal-centered (3MC) state back to the ground state. We reveal by variable-temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans- and cis-[Fe(pdmi)2]2+ with four NHC donors show 3MLCT→3MC population transfers with very different barriers and rationalize this by computational means. While trans-[Fe(pdmi)2]2+ possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm-1, which leads to a nanosecond 3MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi-classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

SpaiNN: equivariant message passing for excited-state nonadiabatic molecular dynamics

Excited-state molecular dynamics simulations are crucial for understanding processes like photosynthesis, vision, and radiation damage. However, the computational complexity of quantum chemical calculations restricts their scope. Machine learning offers a solution by delivering high-accuracy properties at lower computational costs. We present SpaiNN, an open-source Python software for ML-driven surface hopping nonadiabatic molecular dynamics simulations. SpaiNN combines the invariant and equivariant neural network architectures of SchNetPack with SHARC for surface hopping dynamics. Its modular design allows users to implement and adapt modules easily. We compare rotationally-invariant and equivariant representations in fitting potential energy surfaces of multiple electronic states and properties arising from the interaction of two electronic states. Simulations of the methyleneimmonium cation and various alkenes demonstrate the superior performance of equivariant SpaiNN models, improving accuracy, generalization, and efficiency in both training and inference.

Excited-State Dynamics of a CRABPII-Based Microbial Rhodopsin Mimic

Microbial rhodopsin, a pivotal photoreceptor protein, has garnered widespread application in diverse fields such as optogenetics, biotechnology, biodevices, etc. However, current microbial rhodopsins are all transmembrane proteins, which both complicates the investigation on the photoreaction mechanism and limits their further applications. Therefore, a specific mimic for microbial rhodopsin can not only provide a better model for understanding the mechanism but also can extend the applications. The human protein CRABPII turns out to be a good template for design mimics on rhodopsin due to the convenience in synthesis and the stability after mutations. Recently, Geiger et al. designed a new CRABPII-based mimic M1-L121E on microbial rhodopsin with the 13-cis, syn (13C) isomerization after irradiation. However, it still remains a question as to how similar it is compared with the natural microbial rhodopsin, in particular, in the aspect of the photoreaction dynamics. In this article, we investigate the excited-state dynamics of this mimic by measuring its transient absorption spectra. Our results reveal that there are two components in the solution of mimic M1-L121E at pH 8, known as protonated Schiff base (PSB) and unprotonated Schiff base (USB) states. In both states, the photoreaction process from 13-cis, syn(13C) to all-trans,anti (AT) is faster than that from the inverse direction. In addition, the photoreaction process in the PSB state is faster than that in the USB state. We compared the isomerization time of the PSB state to that of microbial rhodopsin. Our findings indicate that M1-L121E exhibits behaviors similar to those of microbial rhodopsins in the general pattern of PSB isomerization, where the isomerization from 13C to AT is much faster than its inverse direction. However, our results also reveal significant differences in the excited-state dynamics of the mimic relative to the native microbial rhodopsin, including the slower PSB isomerization rates as well as the unusual USB photoreaction dynamics at pH = 8. By elucidating the distinctive characteristics of mimics M1-L121E, this study enhances our understanding of microbial rhodopsin mimics and their potential applications.

Concentration-dependent aggregation of methylene blue acting as a photoredox catalyst

Hydroxylation reactions are important in biological processes and synthetic schemes. Many challenging hydroxylation reactions have been achieved using photoredox catalytic methods. For the oxidative hydroxylation of arylboronic acids, methylene blue has been used successfully as a photoredox catalyst to produce phenyl groups. Here we use broadband transient absorption spectroscopy to determine the mechanism of the photoredox catalytic reaction of methylene blue with phenylboronic acid in the presence of N,N-diisopropylethylamine. Our results show that the reaction proceeds through the triplet state of methylene blue in the presence of oxygen, generating superoxide radical anions. In addition, we observe dimerization of the methylene blue at typical catalytic loadings. As these dimers do not participate in the reaction, increasing the concentration of methylene blue is potentially detrimental to the overall yield.

Light-Induced Charge Separation in Covalently Linked BODIPY-Quinone-Alkyne Dyads

Visible light-induced charge separation and directional charge transfer are cornerstones for artificial photosynthesis and the generation of solar fuels. Here, we report synthetic access to a series of noble metal-free donor-acceptor dyads based on bodipy light-absorbers and redox-active quinone/anthraquinone charge storage sites. Peripheral functionalization of the quinone/anthraquinone units with alkynes primes the dyads for integration into a range of light-harvesting systems, e. g., by Cu-catalyzed cycloadditions (CLICK chemistry) or Pd-catalyzed C-C cross-coupling reactions. Initial photophysical, electrochemical and theoretical analyses reveal the principal processes during the light-induced charge separation in the reported dyads.

Intramolecular Energy Transfer Competing with Light-Driven Intermolecular Proton Transfer in an Iron(II)-NHC Complex? A Query into the Role of Photobasic Ligands and MLCT States

Inorganic photoacids and photobases comprising of photoactive transition metal complexes (TMCs) offer the ability to modulate proton transfer reactions through light irradiation, while utilizing the excellent optical properties of the latter. This provides a powerful tool for precise control over chemical reactions and processes, with implications for both fundamental science and practical applications. In this contribution, we present a novel molecular architecture amending an Fe-NHC complex with a pendant quinoline, as a prototypical photobase, as a representative earth-abundant TMC based inorganic photobase. We characterize the excited-state properties and proton-transfer dynamics using steady-state absorption and emission spectroscopy as well as pump wavelength dependent transient absorption spectroscopy in various protic solvents. The kinetics and thermodynamics of proton transfer in the quinoline moiety are influenced by both the presence of the metal center and the choice of the solvent. Furthermore, we see indications of intramolecular energy transfer from the quinoline to the MLCT state as a limiting factor for panchromatic photobasicity of the complex.

Oxidation-state sensitive light-induced dynamics of Ruthenium-4H-Imidazole complexes

Oxidized molecular states are key intermediates in photo-induced redox reactions, e. g., intermolecular charge transfer between photosensitizer and catalyst in photoredox catalysis. The stability and longevity of the oxidized photosensitizer is an important factor in optimizing the respective light-driven reaction pathways. In this work the oxidized states of ruthenium(II)-4H-imidazole dyes are studied. The ruthenium complexes constitute benchmark photosensitizers in solar energy interconversion processes with exceptional chemical stability, strong visible light absorption, and favourable redox properties. To rationalize the light-induced reaction in the oxidized ruthenium(III) systems, we combine UV-vis absorption, resonance Raman, and transient absorption spectroelectrochemistry (SEC) with time-dependent density functional theory (TDDFT) calculations. Three complexes are compared, which vary with respect to their coordination environment, i. e., combining an 4H-imidazole with either 2,2'-bipyridine (bpy) or 2,2';6'2"-terpyridine (tpy) coligands, and chloride or isothiocyanate ligands. While all oxidized complexes have similar steady state absorption properties, their excited state kinetics differ significantly; the study thus opens the doorway to study the light-driven reactivity of oxidized molecular intermediates in intermolecular charge transfer cascades.

New Twist on the Light-Switch Effect: Controlling the Fate of Excited States with pH in a 4-Hydroxy-thiazol-extended Ruthenium(II) Dppz Complex

We present a pH-dependent study of the excited state dynamics of a novel Ru complex bearing a 4-hydroxy thiazol-substituted dppz (dipyridophenazine) ligand (RuTzOH) and its deprotonated form (RuTzO-). We combine steady-state and time-resolved absorption and emission spectroscopy with electrochemical investigations to characterize the excited state relaxation, which upon photoexcitation at 400 nm is determined by a multitude of initially populated MLCT states for both complexes. Subsequently, for RuTzOH, two long-lived excited states are populated, leading to dual emission from the complexes, a feature that vanishes upon deprotonation. Upon deprotonation, the electron density on the dppz moiety increases significantly, leading to rapid energy populating ligand-centered states and thus deactivating the initially excited MLCT states.

Multifaceted Deactivation Dynamics of Fe(II) N-Heterocyclic Carbene Photosensitizers

Excited state dynamics of three iron(II) carbene complexes that serve as prototype Earth-abundant photosensitizers were investigated by ultrafast optical spectroscopy. Significant differences in the dynamics between the investigated complexes down to femtosecond time scales are used to characterize fundamental differences in the depopulation of triplet metal-to-ligand charge-transfer (3MLCT) excited states in the presence of energetically accessible triplet metal-centered (3MC) states. Novel insights into the full deactivation cascades of the investigated complexes include evidence of the need to revise the deactivation model for a prominent iron carbene prototype complex, a refined understanding of complex 3MC dynamics, and a quantitative discrimination between activated and barrierless deactivation steps along the 3MLCT → 3MC → 1GS path. Overall, the study provides an improved understanding of photophysical limitations and opportunities for the use of iron(II)-based photosensitizers in photochemical applications.

Pyglotaran: a lego-like Python framework for global and target analysis of time-resolved spectra

The dynamics of molecular systems can be studied with time-resolved spectroscopy combined with model-based analysis. A Python framework for global and target analysis of time-resolved spectra is introduced with the help of three case studies. The first study, concerning broadband absorption of intersystem crossing in 4-thiothymidine, demonstrates the framework's ability to resolve vibrational wavepackets with a time resolution of ≈10 fs using damped oscillations and their associated spectra and phases. Thereby, a parametric description of the "coherent artifact" is crucial. The second study addresses multichromophoric systems composed of two perylene bisimide chromophores. Here, pyglotaran's guidance spectra and lego-like model composition enable the integration of spectral and kinetic properties of the parent chromophores, revealing a loss process, the undesired production of a radical pair, that reduces the light harvesting efficiency. In the third, time-resolved emission case study of whole photosynthetic cells, a megacomplex containing ≈500 chromophores of five different types is described by a combination of the kinetic models for its elements. As direct fitting of the data by theoretical simulation is unfeasible, our global and target analysis methodology provides a useful 'middle ground' where the theoretical description and the fit of the experimental data can meet. The pyglotaran framework enables the lego-like creation of kinetic models through its modular design and seamless integration with the rich Python ecosystem, particularly Jupyter notebooks. With extensive documentation and a robust validation framework, pyglotaran ensures accessibility and reliability for researchers, serving as an invaluable tool for understanding complex molecular systems.

Triplet Formation in a 9,10-Bis(phenylethynyl)anthracene Dimer and Trimer Occurs by Charge Recombination Rather than Singlet Fission

We present an experimental study investigating the solvent-dependent dynamics of a 9,10-bis(phenylethynyl)anthracene monomer, dimer, and trimer. Using transient absorption spectroscopy, we have discovered that triplet excited state formation in the dimer and trimer molecules in polar solvents is a consequence of charge recombination subsequent to symmetry-breaking charge separation rather than singlet fission. Total internal reflection emission measurements of the monomer demonstrate that excimer formation serves as the primary decay pathway at a high concentration. In the case of highly concentrated solutions of the trimer, we observe evidence of triplet formation without the prior formation of a charge-separated state. We postulate that this is attributed to the formation of small aggregates, suggesting that oligomers mimicking the larger chromophore counts in crystals could potentially facilitate singlet fission. Our experimental study sheds light on the intricate dynamics of the 9,10-bis(phenylethynyl)anthracene system, elucidating the role of solvent- and concentration-dependent factors for triplet formation and charge separation.

Nanoelectromechanical Infrared Spectroscopy with In Situ Separation by Thermal Desorption: NEMS-IR-TD

We present a novel method for the quantitative analysis of mixtures of semivolatile chemical compounds. For the first time, thermal desorption is integrated directly with nanoelectromechanical infrared spectroscopy (NEMS-IR-TD). In this new technique, an analyte mixture is deposited via nebulization on the surface of a NEMS sensor and subsequently desorbed using heating under vacuum. The desorption process is monitored in situ via infrared spectroscopy and thermogravimetric analysis. The resulting spectro-temporal maps allow for selective identification and analysis of the mixture. In addition, the corresponding thermogravimetric data allow for analysis of the desorption dynamics of the mixture components. As a demonstration, caffeine and theobromine were selectively identified and quantified from a mixture with a detection limit of less than 6 pg (about 30 fmol). With its exceptional sensitivity, NEMS-IR-TD allows for the analysis of low abundance and complex analytes with potential applications ranging from environmental sensing to life sciences.

Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)-NHC Complexes with Long-Lived 2LMCT Excited States

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)2]PF6 (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]-, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]-, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]-. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]- by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (2LMCT) excited states feature nanosecond lifetimes (1.6-1.7 ns) and sizable emission quantum yields (1.7-1.9%) through spin-allowed transition to the doublet ground state (2GS), completely in line with the parent complex [Fe(phtmeimb)2]PF6 (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.

The Other Dimension-Tuning Hole Extraction via Nanorod Width

Solar-to-hydrogen generation is a promising approach to generate clean and renewable fuel. Nanohybrid structures such as CdSe@CdS-Pt nanorods were found favorable for this task (attaining 100% photon-to-hydrogen production efficiency); yet the rods cannot support overall water splitting. The key limitation seems to be the rate of hole extraction from the semiconductor, jeopardizing both activity and stability. It is suggested that hole extraction might be improved via tuning the rod's dimensions, specifically the width of the CdS shell around the CdSe seed in which the holes reside. In this contribution, we successfully attain atomic-scale control over the width of CdSe@CdS nanorods, which enables us to verify this hypothesis and explore the intricate influence of shell diameter over hole quenching and photocatalytic activity towards H₂ production. A non-monotonic effect of the rod's diameter is revealed, and the underlying mechanism for this observation is discussed, alongside implications towards the future design of nanoscale photocatalysts.