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DNA damage is ubiquitous in nature and is at the basis of emergent treatments such as photodynamic therapy, which is based on the activation of highly oxidative reactive oxygen species by photosensitizing O

Cytokines of the interleukin (IL)-1 family are widely expressed in epithelial surfaces, including the epidermis, where they play a key role in the maintenance of barrier integrity and host defense. A recent report associated the IL-1 family member IL-33 with stress granules (SGs) in epithelial cells. Formation of SGs is promoted by the aggregation of proteins harboring low complexity regions (LCRs). In this study, using computational analyses, we predicted the presence of LCRs in six of the eleven IL-1 family members. Among these, IL-38 contained a long LCR and localized to Ras GTPase-activating protein binding protein 1 (G3BP1) positive SGs, as well as to G3BP1 negative intracellular protein condensates in keratinocytes exposed to oxidative stress (OS). In addition, we identified two highly aggregation-prone amyloid core (AC) motifs in the IL-38 LCR and detected the formation of amyloid IL-38 aggregates in response to OS in cells andin vitro. Disulfide bond mapping,in silicomodelling and the analysis of specific cysteine mutants supported a model in which specific oxidation-sensitive cysteines act as redox switches to modify the conformation of IL-38 and thus the surface exposure of its ACs, shuttling it from a soluble state into biomolecular condensates. Finally, the presence of IL-38 granules in human epidermal layers highly exposed to environmental OS suggests that oxidation-induced formation of amyloid aggregates, as a previously unrecognized intrinsic biological property of IL-38, may be physiologically relevant at this epithelial barrier.

9 pags., 5 figs., Sulfur trioxide (SO3) and the hydroxysulfonyl radical (HOSO2) are two key intermediates in the production of sulfuric acid (H2SO4) on Earth's atmosphere, one of the major components of acid rain. Here, the photochemical properties of these species are determined by means of high-level quantum chemical methodologies, and the potential impact of their light-induced reactivity is assessed within the context of the conventional acid rain generation mechanism. Results reveal that the photodissociation of HOSO2 occurs primarily in the stratosphere through the ejection of hydroxyl radicals (•OH) and sulfur dioxide (SO2). This may decrease the production rate of H2SO4 in atmospheric regions with low O2 concentration. In contrast, the photostability of SO3 under stratospheric conditions suggests that its removal efficiency, still poorly understood, is key to assess the H2SO4 formation in the upper atmosphere., The project that gave rise to these results received the support of a fellowship for J.C.-G. from “la Caixa” Foundation (ID 100010434); the fellowship code is LCF/BQ/DR20/11790027. This work was supported by the Spanish “Ministerio de Ciencia e Innovación (MICINN)” (Project ref. CTQ2017-87054-C2-2-P) and Unit of Excellence María de Maeztu CEX2019-000919-M). D.R.-S. is grateful to the Spanish MICINN for the “Ramón y Cajal” grant (ref. RYC2015-19234). A.F.-M. is grateful to the Generalitat Valenciana and the European Social Fund for the postdoctoral contract APOSTD/2019/149 and the project GV/2020/226, and to the MICINN for the “Juan de la Cierva” contract IJC2019-039297-I. Computations have been partially conducted at the local QCEXVAL cluster and the Tirant v3 supercluster (Servei d’Informatica of the University of Valencia).

Cobaltabis(dicarbollide) anion ([o-COSAN]- ) is a well-known metallacarborane with multiple applications in a variety of fields. In aqueous solution, the cisoid rotamer is the most stable disposition in the ground state. The present work provides theoretical evidence on the possibility to photoinduce the rotation from the cisoid to the transoid rotamer, a conversion that can be reverted when the ground state is repopulated. The non-radiative decay mechanisms proposed in this work are coherent with the lack of fluorescence observed in 3D fluorescence mapping experiments performed on [o-COSAN]- and its derivatives. This phenomenon induced by light has the potential to destruct the vesicles and micelles cisoid [o-COSAN]- typically forms in aqueous solution, which could lead to promising applications, particularly in the field of nanomedicine., This work was supported by the Generalitat Valenciana (GV, project GV/2020/226), Generalitat de Catalunya (2017 SGR 1720), and the Spanish Ministerio de Ciencia e Innovación (MICINN, projects CTQ2017-87054-C2-2–304-P and PID2019-106832RB-I00). A.M.A.A. is grateful to the Erasmus+ Programme of the European Comission for his Erasmus Mundus TCCM scholarship. A.F.-M. is grateful to the GV and the European Social Fund for the postdoctoral contract APOSTD/2019/149 and to the MICINN for the Juan de la Cierva contract IJC2019-039297-I. D.R.-S. is grateful to the MICINN for the “Ramón y Cajal” grant (RYC-2015–19234). “Severo Ochoa” Program for Centers of Excellence in R&D 234 (SEV-2015-0496) is appreciated. J. A. M. Xavier acknowledges DOC-FAM programme under the Marie Sklodowska-Curie grant agreement No. 754397 and is enrolled in the PhD program of the UAB. Computations have been conducted at the local QCEXVAL and the LluisVives and Tirant III clusters of the Servei d'Informàtica (University of Valencia). We thank Dr. Jordi Faraudo (ICMAB-CSIC) for the scientific discussions., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

The photoreactivity of relatively large transition metal complexes is often limited to the description of the static potential energy surfaces of the involved electronic states. While useful to grasp some physical grounds of the photoinduced molecular responses, this approach does not statistically sample the multiple molecular degrees of freedom of the systems under investigation, which grow significantly if we consider the explicit coupling with the environment, and does not consider dynamic effects. The problem is even more complex if the reactivity takes place in the excited state. The present work uses state-of-the-art multiscale QM/MM dynamics to describe the photoactivation of a Pt(II)-unit of an

6 pags., 2 figs., Hydroxysulfinyl radical (HOSO) is important due to its involvement in climate geoengineering upon SO2 injection and generation of the highly hygroscopic H2SO4. Its photochemical behavior in the upper atmosphere is, however, uncertain. Here we present the ultraviolet-visible photochemistry and photodynamics of this species by simulating the atmospheric conditions with high-level quantum chemistry methods. Photocleavage to HO + SO arises as the major solar-induced channel, with a minor contribution of H + SO2 photoproducts. The efficient generation of SO is relevant due to its reactivity with O3 and the consequent depletion of ozone in the stratosphere., The project that gave rise to these results received the support of a fellowship for J.C.-G. from “La Caixa” Foundation (ID 100010434); the fellowship code is LCF/BQ/DR20/ 11790027. A.F.-M. is grateful to the Generalitat Valenciana and the European Social Fund for the postdoctoral contract APOSTD/2019/149 and the project GV/2020/226. This work was supported by the Spanish “Ministerio de Ciencia e Innovación (MICINN)” (Project ref CTQ2017-87054-C2-2-P and Unit of Excellence María de Maeztu CEX2019-000919- M). D.R.-S. is grateful to the Spanish MICINN for the “Ramón y Cajal” grant (ref RYC-2015-19234). Computations have been partially conducted at the local QCEXVAL cluster and the Tirant v3 supercluster (University of Valencia).

8 pags., 3 figs., Polysulfur species have been proposed to be the unknown near-UV absorber in the atmosphere of Venus. Recent work argues that photolysis of one of the (SO) isomers, cis-OSSO, directly yields S with a branching ratio of about 10%. If correct, this pathway dominates polysulfur formation by several orders of magnitude, and by addition reactions yields significant quantities of S, S, and S. We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S is not a product in cis-OSSO photolysis. Instead, we establish a novel mechanism in which S is formed in a two-step process. Firstly, the intermediate SO is produced by the coupling between the S and Cl atmospheric chemistries (in particular, SO reaction with ClS) and in a lesser extension by O-abstraction reactions from cis-OSSO. Secondly, SO reacts with SO. This modified chemistry yields S and subsequent polysulfur abundances comparable to the photolytic cis-OSSO mechanism through a more plausible pathway. Ab initio quantification of the photodissociations at play fills a critical data void in current atmospheric models of Venus., Calculations have been partially conducted at the local QCEXVAL cluster and the Lluis Vives v2 and Tirant v3 superclusters of the Servei d’Informàtica (University of Valencia), and at the local LPCT cluster and the regional ExpLor center (University of Lorraine, France). A.F.-M. thanks Generalitat Valenciana and the European Social Fund for the post-doctoral contract APOSTD/2019/149 and the project GV/2020/226, and the Ministerio de Ciencia e Innovación (MICINN) for the Juan de la Cierva contract IJC2019-039297-I. D.R.-S. is grateful to the MICINN for the project CTQ2017-87054-C2-2-P and the Ramón y Cajal grant RYC2015- 19234. The project that gave rise to these results received also the support of a fellowship for J.C.-G. from “la Caixa” Foundation (ID 100010434); the fellowship code is LCF/BQ/DR20/11790027.

Ru(II) complexes that undergo photosubstitution reactions from triplet metal-centered ((3)MC) excited states are of interest in photochemotherapy (PCT) for their potential to produce cytotoxic effects in hypoxia. Dual-action systems that incorporate this stoichiometric mode to complement the oxygen-dependent photosensitization pathways that define photodynamic therapy (PDT) are poised to maintain antitumor activity regardless of oxygenation status. Herein, we examine the way in which these two pathways influence photocytotoxicity in normoxia and in hypoxia using the [Ru(dmp)(2)(IP-nT)](2+) series (where dmp=2,9-dimethyl-1,10-phenanthroline and IP-nT=imidazo[4,5-f][1,10]phenanthroline tethered to n=0–4 thiophene rings) to switch the dominant excited state from the metal-based (3)MC state in the case of Ru-phen–Ru-1T to ligand-based (3)ILCT state for Ru-3T and Ru-4T. Ru-phen–Ru-1T, having dominant (3)MC states and the largest photosubstitution quantum yields, were inactive in both normoxia and hypoxia. Ru-3T and Ru-4T, with dominant (3)IL/(3)ILCT states and long triplet lifetimes (τ(TA)=20–25 μs), had the poorest photosubstitution quantum yields yet were extremely active. In the best instances, Ru-4T exhibited attomolar phototoxicity toward SKMEL28 cells in normoxia and picomolar in hypoxia, with PI values in normoxia of 10(5)–10(12) and 10(3)–10(6) in hypoxia. While maximizing excited state deactivation through photodissociative (3)MC states did not result in bonafide dual-action PDT/PCT agents, the study has produced the most potent photosensitizer we know of to date. The extraordinary photosensitizing capacity of Ru-3T and Ru-4T may stem from a combination of very efficient (1)O(2) production and possibly complementary Type I pathways via (3)ILCT excited states.

Polysulfur species have been proposed to be the unknown near-UV absorber in the atmosphere of Venus. Recent work argues that photolysis of one of the (SO)

International audience; Cyclocurcumin is a turmeric component that attracted much less attention compared to the well known curcumin. In spite of the less deep charcaterization of its properties, cyclocurcumin has shown promising anticancer effects when used in combination with curcumin. Especially, due to its peculiar molecular structure, cyclocurcumin can be regarded as an almost ideal photoswitch, whose capabilities can also be exploited for relevant biological applications. Here, by means of state-of-the-art computational methods for electronic excited-state calculations (TD-DFT, MS-CASPT2, XMS-CASPT2) we analyze in detail the absorption and photoisomerization pathways leading from the more stable trans isomer to the cis one. The different molecular surroundings, taken into account by means of electrostatic solvent effect and compared with available experimental data, have been found to be critical in describing the fate of irradiated cyclocurcumin: while in non-polar environments an excited state barrier prevents photoisomerization and favours fluorescence, in polar solvents an almost barrierless path results in a strikingly decrease of fluorescence, opening the way toward a crossing region with the ground state and thus funneling the photoproduction of the cis isomer.

[EN] Recently, sunscreen-based drug photocages have been introduced to provide UV protection to photoactive drugs, thus increasing their photosafety. Here, combined experimental and theoretical studies performed on a photocage based on the commercial UVA filter avobenzone (AB) and on the photosensitizing non-steroidal anti-inflammatory drug ketoprofen (KP) are presented unveiling the photophysical processes responsible for the light-triggered release. Particular attention is paid to solvent stabilization of the drug and UV filter excited states, respectively, which leads to a switching between the triplet excited state energies of the AB and KP units. Most notably, we show that the stabilization of the AB triplet excited state in ethanol solution is the key requirement for an efficient photouncaging. By contrast, in apolar solvents, in particular hexane, KP has the lowest triplet excited state, hence acting as an energy acceptor quenching the AB triplet manifold, thus inhibiting the desired photoreaction., Support from the Universite de Lorraine, CNRS, regional (Prometeo/2017/075) and Spanish Government (PGC2018-096684-B-I00, CTQ2017-87054-C2-2-P) is kindly acknowledged. A. F.-M. is grateful to Generalitat Valenciana and the European Social Fund (postdoctoral contract APOSTD/2019/149 and project GV/2020/226) for financial support. M. L.-R. acknowledges the Universitat Politecnica de Valencia for the FPI grant. All calculations have been performed on the local LPCT computer center and on the Explor regional center in the framework of the project "Dancing under the light".

International audience; Iron complexes are promising candidates for the development of sustainable molecular photoactive materials as an alternative to those based on precious metals such as Ir, Pt or Ru. These compounds possess metal-ligand charge transfer (MLCT) transitions potentially of high interest for energy conversion or photocatalysis applications if the ultrafast deactivation via lower-lying metal-centred (MC) states can be impeded. Following an introduction describing the main design strategies used so far to increase the MLCT lifetimes, we review some of our latest contributions to the field regarding bidentate Fe(II) complexes comprising N-heterocyclic carbene ligands. The discussion covers all aspects from their synthesis to their characterization via photophysical, electrochemical and computational techniques. The impact of bidentate coordination together with the configuration (facial and meridional isomers) is analysed, finally highlighting the current challenges in this promising area.

Sulfur trioxide (SO

Photocycloreversion plays a central role in the study of the repair of DNA lesions, reverting them into the original pyrimidine nucleobases. Particularly, among the proposed mechanisms for the repair of DNA (6-4) photoproducts by photolyases, it has been suggested that it takes place through an intermediate characterized by a four-membered heterocyclic oxetane or azetidine ring, whose opening requires the reduction of the fused nucleobases. The specific role of this electron transfer step and its impact on the ring opening energetics remain to be understood. These processes are studied herein by means of quantum-chemical calculations on the two azetidine stereoisomers obtained from photocycloaddition between 6-azauracil and cyclohexene. First, we analyze the efficiency of the electron-transfer processes by computing the redox properties of the azetidine isomers as well as those of a series of aromatic photosensitizers acting as photoreductants and photo-oxidants. We find certain stereodifferentiation favoring oxidation of the cis-isomer, in agreement with previous experimental data. Second, we determine the reaction profiles of the ring-opening mechanism of the cationic, neutral, and anionic systems and assess their feasibility based on their energy barrier heights and the stability of the reactants and products. Results show that oxidation largely decreases the ring-opening energy barrier for both stereoisomers, even though the process is forecast as too slow to be competitive. Conversely, one-electron reduction dramatically facilitates the ring opening of the azetidine heterocycle. Considering the overall quantum-chemistry findings, N,N-dimethylaniline is proposed as an efficient photosensitizer to trigger the photoinduced cycloreversion of the DNA lesion model., This work has been funded by the Generalitat Valenciana and the European Social Fund through the postdoctoral contract APOSTD/2019/149 and the project GV/2020/226. It also was funded by the Spanish Ministerio de Ciencia e Innovación (MICINN), projects CTQ2017-87054-C2-2-P and PGC2018-096684-B-I00, and a 2019 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. The Foundation takes no responsibility for the opinions, statements, and contents of this project, which are entirely the responsibility of its authors. D.R.-S. is grateful to the Spanish MICINN for the “Ramón y Cajal” grant (Ref. RYC-2015-19234). M.N.-M. acknowledges the Generalitat Valenciana for the predoctoral grant (Ref. ACIF/2020/075).

Photodynamic therapy is a promising approach to treat a variety of superficial tumors and other diseases. One of its major limitations arises from its dependence on molecular oxygen, which decreases the efficiency of the therapy in hypoxia conditions commonly developed by solid tumors. The present contribution reveals the molecular mechanism of a modified thymine bearing a nitroimidazole substituent, a photosensitizer able to produce highly harmful interstrand cross-links in the DNA double strand after irradiation selectively in absence of oxygen. The mechanism is resolved at a fully atomistic and electronic level relying on quantum mechanics (CASPT2, coupled-cluster, DFT, and TD-DFT methods), classical molecular dynamics, and advanced biased QM/MM simulations, revealing an energy penalty of ∼8 kcal/mol for the anionic nitromidazole release. Our findings indicate that the global interstrand cross-link production is driven by a combination of multiple factors, namely, the reverse energy penalty, the diffusion of the nitroimidazole anion, and the further reactivity of the formed thymine radical. On the basis of these results, we also suggest some possible strategies to improve the efficiency of interstrand cross-link production.

Treatment of the laser borane anti-B18H22 (compound 1) with iodine in ethanol gives the monoiodinated derivative 7-I-anti-B18H21 (compound 2) in 67% yield, or, by reaction with iodine or ICl in the presence of AlCl3 in dichloromethane, the diiodinated derivative 4,4'-I2-anti-B18H20 (compound 3) in 85% yield. On excitation with 360 nm light, both compounds 2 and 3 give strong green phosphorescent emissions (λmax = 525 nm, ΦL = 0.41 and λmax = 545 nm, ΦL = 0.71 respectively) that are quenched by dioxygen to produce O2(1Δg) singlet oxygen with quantum yields of ΦΔ = 0.52 and 0.36 respectively. Similarly strong emissions can be stimulated via the nonlinear process of two-photon absorption when exciting with 720 or 800 nm light. The high quantum yields of singlet-oxygen production, coupled with the option of two-photon excitation, make compounds 2 and 3 promising O2(1Δg) photosensitizers. The molecular structures of compounds 2 and 3 were determined by single-crystal X-ray crystallographic studies as well as multinuclear NMR spectroscopy and mass spectrometry. Time-resolved UV-vis spectroscopy was used to delineate their photophysical properties, and the electronic-structure properties of the emitting species were determined by means of multiconfigurational quantum-chemistry computations.

International audience

Decaborane(14), nido-B10H14, is the major commercially available molecular building block in boron cluster chemistry. The condensation of two such {nido-B10} blocks gives the known isomers of B18H22 – a molecule used in the fabrication of p-type semiconductors and capable of blue laser emission. Here, we computationally determine the structures and thermodynamic stabilities of 20 possible B26H30 regioisomers constructed from the fusion of three {nido-B10} blocks with the three subclusters conjoined by two-boron atom shared edges. In addition, density functional theory, time-dependent (TD)-DFT and multiconfigurational CASPT2 methods have been used to model and investigate the physical and photophysical properties of the three most stable of these isomers. Our findings predict these isomers to be potentially useful materials for the semiconductor industry, as high boron-content doping agents, and in the fabrication of new optical materials.

The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-to-ligand charge transfer (MLCT)-excited states deactivation pathways. The gap between the metal-centered states is maximal when the coordination sphere of the complex approaches an ideal octahedral geometry. Two new iron(II) complexes (C1 and C2), prepared from pyridylNHC and pyridylquinoline type ligands, respectively, have a near-perfect octahedral coordination of the metal. The photophysics of the complexes have been further investigated by means of ultrafast spectroscopy and TD-DFT modeling. For C1, it is shown that&mdash, despite the geometrical improvement&mdash, the excited state deactivation is faster than for the parent pseudo-octahedral C0 complex. This unexpected result is due to the increased ligand flexibility in C1 that lowers the energetic barrier for the relaxation of 3MLCT into the 3MC state. For C2, the effect of the increased ligand field is not strong enough to close the prominent deactivation channel into the metal-centered quintet state, as for other Fe-polypyridine complexes.

8 pags., 5 figs., 2 tabs., Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hgto the atmosphere where it is oxidized to reactive Hgcompounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized Hgand Hgspecies postulate their photodissociation back to Hgas a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg, Hg, and Hgspecies in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of Hgand Hgleads to insufficient Hg oxidation globally. The combined efficient photoreduction of Hgand Hgto Hgcompetes with thermal oxidation of Hg, resulting in a large model overestimation of 99% of measured Hgand underestimation of 51% of oxidized Hg and ∼66% of Hgwet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3-6-mo range based on observed atmospheric Hg variability. These results show that the Hgand Hgphotoreduction processes largely offset the efficiency of bromine-initiated Hgoxidation and reveal missing Hg oxidation processes in the troposphere., This study has received funding from the European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation programme (Project ERC-2016- COG 726349 CLIMAHAL) and the Spanish Ministerio de Economía y Competitividad (MINECO) /Fondo Europeo de Desarrollo Regional (FEDER) (Projects CTQ2017-87054-C2-2-P, RYC-2015-19234, and CEX2019-000919-M). This work was supported by the Consejo Superior de Investigaciones Científicas (CSIC) Spain. A.F.-M. acknowledges the Generalitat Valenciana and the European Social Fund (Contract APOSTD/2019/149 and Project GV/2020/ 226) for the financial support. J.C.-G. acknowledges the Universitat de València for his Masters Scholarship. M.J. acknowledges funding by the Swiss National Science Foundation (Grant PZ00P2_174101). The ETMEP measurements as well as ground-based measurements of the GMOS network were funded by the EU FP7-ENV-2010 project (GMOS, Grant Agreement 265113). J.S.F. acknowledges the H2020 ERA-PLANET (689443) Integrated Global Observing Systems for Persistent Pollutants (iGOSP) and Integrative and Comprehensive Understanding on Polar Environments (iCUPE) programs

[EN] Interaction of nucleic acids with light is a scientific question of paramount relevance not only in the understanding of life functioning and evolution, but also in the insurgence of diseases such as malignant skin cancer and in the development of biomarkers and novel light-assisted therapeutic tools. This work shows that the UVA portion of sunlight, not absorbed by canonical DNA nucleobases, can be absorbed by 5-formyluracil (ForU) and 5-formylcytosine (ForC), two ubiquitous oxidatively generated lesions and epigenetic intermediates present in living beings in natural conditions. We measure the strong propensity of these molecules to populate triplet excited states able to transfer the excitation energy to thymine-thymine dyads, inducing the formation of cyclobutane pyrimidine dimers (CPDs). By using steady-state and transient absorption spectroscopy, NMR, HPLC, and theoretical calculations, we quantify the differences in the triplet-triplet energy transfer mediated by ForU and ForC, revealing that the former is much more efficient in delivering the excitation energy and producing the CPD photoproduct. Although significantly slower than ForU, ForC is also able to harm DNA nucleobases and therefore this process has to be taken into account as a viable photosensitization mechanism. The present findings evidence a rich photochemistry crucial to understand DNA damage photobehavior., Support from the Universite de Lorraine, CNRS, regional (Prometeo/2017/075) and Spanish Government (PGC2018-096684-B-I00, CTQ2017-87054-C2-2-P) is kindly acknowledged. A. F.-M. is grateful to Generalitat Valenciana and the European Social Fund (postdoctoral contract APOSTD/2019/149 and project GV/2020/226) for financial support. M. L.-R. acknowledges the Universitat Politecnica de Valencia for the FPI grant. All calculations have been performed on the local LPCT computer center and on the Explor regional center in the framework of the project "Dancing under the light".

International audience; The emergence in late 2019 of the coronavirus SARS-CoV-2 has resulted in the breakthrough of the COVID-19 pandemic that is presently affecting a growing number of countries. The development of the pandemic has also prompted an unprecedented effort of the scientific community to understand the molecular bases of the virus infection and to propose rational drug design strategies able to alleviate the serious COVID-19 morbidity. In this context, a strong synergy between the structural biophysics and molecular modeling and simulation communities has emerged, resolving at the atomistic level the crucial protein apparatus of the virus and revealing the dynamic aspects of key viral processes. In this Review, we focus on how in silico studies have contributed to the understanding of the SARS-CoV-2 infection mechanism and the proposal of novel and original agents to inhibit the viral key functioning. This Review deals with the SARS-CoV-2 spike protein, including the mode of action that this structural protein uses to entry human cells, as well as with nonstructural viral proteins, focusing the attention on the most studied proteases and also proposing alternative mechanisms involving some of its domains, such as the SARS unique domain. We demonstrate that molecular modeling and simulation represent an effective approach to gather information on key biological processes and thus guide rational molecular design strategies.

International audience; Some bacterial species enter a dormant state in the form of spores to resist to unfavorable external conditions. Spores are resistant to a wide series of stress agents, including UV radiation, and can last for tens to hundreds of years. Due to the suspension of biological functions, such as DNA repair, they accumulate DNA damage upon exposure to UV radiation. Differently from active organisms, the most common DNA photoproducts in spores are not cyclobutane pyrimidine dimers, but rather the so‐called spore photoproducts. This noncanonical photochemistry results from the dry state of DNA and its binding to small, acid‐soluble proteins that drastically modify the structure and photoreactivity of the nucleic acid. Herein, multiscale molecular dynamics simulations, including extended classical molecular dynamics and quantum mechanics/molecular mechanics based dynamics, are used to elucidate the coupling of electronic and structural factors that lead to this photochemical outcome. In particular, the well‐described impact of the peculiar DNA environment found in spores on the favored formation of the spore photoproduct, given the small free energy barrier found for this path, is rationalized. Meanwhile, the specific organization of spore DNA precludes the photochemical path that leads to cyclobutane pyrimidine dimer formation.

DNA is constantly exposed to external and metabolic stress agents, including the solar radiation and in particular the UV portion of the electromagnetic spectrum. Such source of stress can induce photochemical modification of the structure of DNA and of its basic components, i.e. the nucleobases. DNA lesions may ultimately lead to genomic instability, mutations, and even to carcinogenesis. Hence, cells dispose of complex biochemical repair pathways in charge of remove the DNA lesions and avoid their accumulation. In this Chapter, we present the complexity of the DNA lesion chemical and structural space, also complicated by the intricate coupling with the biological relevant signaling pathways. Through some relevant examples, we will show how proper multiscale simulation protocols can provide a unified picture of the complex phenomena and hence answer biological relevant questions, paving the way to a veritable computational photobiology approach.

Since the end of 2019, the coronavirus SARS-CoV-2 has caused more than 180,000 deaths all over the world, still lacking a medical treatment despite the concerns of the whole scientific community. Human Angiotensin-Converting Enzyme 2 (ACE2) was recently recognized as the transmembrane protein serving as SARS-CoV-2 entry point into cells, thus constituting the first biomolecular event leading to COVID-19 disease. Here, by means of a state-of-the-art computational approach, we propose a rational evaluation of the molecular mechanisms behind the formation of the complex and of the effects of possible ligands. Moreover, binding free energy between ACE2 and the active Receptor Binding Domain (RBD) of the SARS-CoV-2 spike protein is evaluated quantitatively, assessing the molecular mechanisms at the basis of the recognition and the ligand-induced decreased affinity. These results boost the knowledge on the molecular grounds of the SARS-CoV-2 infection and allow to suggest rationales useful for the subsequent rational molecular design to treat severe COVID-19 cases.

Since the end of 2019, the coronavirus SARS-CoV-2 has caused more than 180,000 deaths all over the world, still lacking a medical treatment despite the concerns of the whole scientific community. Human Angiotensin-Converting Enzyme 2 (ACE2) was recently recognized as the transmembrane protein serving as SARS-CoV-2 entry point into cells, thus constituting the first biomolecular event leading to COVID-19 disease. Here, by means of a state-of-the-art computational approach, we propose a rational evaluation of the molecular mechanisms behind the formation of the complex and of the effects of possible ligands. Moreover, binding free energy between ACE2 and the active Receptor Binding Domain (RBD) of the SARS-CoV-2 spike protein is evaluated quantitatively, assessing the molecular mechanisms at the basis of the recognition and the ligand-induced decreased affinity. These results boost the knowledge on the molecular grounds of the SARS-CoV-2 infection and allow to suggest rationales useful for the subsequent rational molecular design to treat severe COVID-19 cases.

The possibility of achieving luminescent and photophysically active metal-organic compounds relies on the stabilization of charge transfer states and kinetically and thermodynamically blocking non-...

Interaction of nucleic acids with light is a scientific question of paramount relevance not only in the understanding of life functioning and evolution, but also in the insurgence of diseases such as malignant skin cancer and in the development of biomarkers and novel light-assisted therapeutic tools. This work shows that the UVA portion of sunlight, not absorbed by canonical DNA nucleobases, can be absorbed by 5-formyluracil (ForU) and 5-formylcytosine (ForC), two ubiquitous oxidative lesions and epigenetic intermediates present in living beings in natural conditions. We measure the strong propensity of these molecules to populate triplet excited states able to transfer the excitation energy to thymine-thymine dyads, inducing the formation of the highly toxic and mutagenic cyclobutane pyrimidine dimers (CPDs). By using steady-state and transient absorption spectroscopy, NMR, HPLC, and theoretical calculations, we quantify the differences in the triplet-triplet energy transfer mediated by ForU and ForC, revealing that the former is much more efficient in delivering the excitation energy and producing the CPD photoproduct. Although significantly slower than ForU, ForC is also able to harm DNA nucleobases and therefore this process has to be taken into account as a viable photosensitization mechanism. The present findings evidence a rich photochemistry crucial to understand DNA photodamage and of potential use in the development of biomarkers and non-conventional photodynamic therapy agents.

The novel SARS-CoV-2 coronavirus is causing a devastating pandemic in 2020, threatening public health in many countries. An unprecedented rapid and global response has been set in motion to identify efficient antiviral agents against SARS-CoV-2, mostly relying on the repurposing of drugs presenting or not previously known antiviral activity. Ivermectin is an approved drug used as antiparasitic in humans and animals with well documented broad-spectrum antiviral properties that emerge from host-directed effects. Recent results reported by Wagstaff and coworkers (Antiviral Research 2020, 178, 104787) show a potent inhibition of SARS-CoV-2 replication in vitro by ivermectin, and clinical trials with human volunteers have already started. However, the mode of action of ivermectin is still largely unknown, especially at the molecular level. Here, we employ advanced molecular dynamics simulations to assess the influence of ivermectin on several key viral protein targets, with the aim to reveal the molecular bases of antiviral mechanisms against SARS-CoV-2. Interestingly, we show that ivermectin could be regarded as a multitarget agent, inhibiting different viral functions. These include blocking the recognition by the SARS-CoV-2 Receptor Binding Domain (RBD) of the Angiotensin-Converting Enzyme 2 (ACE2), the interactions with the two viral proteases 3CLpro and PLpro, and the SARS Unique Domain (SUD) non-structural protein. Hence, the wide spectrum of actions involving i) the interference with cell infection, ii) the inhibition of viral replication, and iii) elusion of the host immune system, could point to an unprecedented synergy between host- and virus-directed effects explaining the high anti-SARS-CoV-2 activity observed for this compound.

13 pags., 7 figs., -- This article is part of the themed collection: Journal of Materials Chemistry C HOT Papers, In the search for innovative new light sources, the discovery that solutions of the boron hydride anti-B18H22 generate photostable blue laser emission stands out in its significance as the first laser borane. Surprisingly, though, the laser performance of anti-B18H22 (∼10% efficiency) does not match the expectations based on its exceptional photophysical properties (Φf = 0.97 and high photostability). To understand this contradiction, we herein present an investigation into the upper excited states of the anti-B18H22 photophysical system, which we suggest to be the most relevant factor to its laser performance. The use of computational quantum chemistry, laser and UV-vis spectroscopy, NMR spectroscopy, and mass spectrometry unveil the role of the upper excited states on the laser performance of anti-B18H22, showing that efficient excited state absorption (ESA) leads to the population of these states, and results not only in the loss of laser efficiency, but also in the activation of chemically reactive relaxation pathways and the formation of photochemically produced novel molecular species. The likely composition of these photoproducts, formed upon prolonged high intensity laser irradiation, is inferred from their molecular masses, NMR properties, and calculated natural orbitals. Together, these results are of key importance to the complete understanding of the anti-B18H22 photophysical system and provide valuable information to chemists and laser physicists working to mitigate deficiencies and enhance the performance of the next generation of borane lasers and borane-based photoactive materials., This work has been supported by the Czech Science Foundation (project No. 18-20286S). L. C. acknowledges financial support from the Spanish Ministerio de Economı´a y Competitividad (MINECO) through Grant number MAT2017-83856-C3-1. A. F.-M. is grateful to Generalitat Valenciana and the European Social Fund for a postdoctoral contract (APOSTD/2019/149). D. R.-S. is thankful to the Spanish MINECO/FEDER for financial support through the Ramon y Cajal fellowship (RYC-2015-19234) and the Unit of Excellence Maria de Maeztu (MDM-2015-0538). Support from the project CTQ2017-87054-C2-2-P (Ministerio de Ciencia e Innovacion) is gratefully acknowledged.

Recently synthetized iron complexes have achieved long-lived excited states and stabilities which are comparable, or even superior, to their ruthenium analogues, thus representing an eco-friendly and cheaper alternative to those materials based on rare metals. Most of computational tools which could help unravel the origin of this large efficiency rely on ab-initio methods which are not able, however, to capture the nanosecond time scale underlying these photophysical processes and the influence of their realistic environment. Therefore, it exists an urgent need of developing new low-cost, but still accurate enough, computational methodologies capable to deal with the steady-state and transient spectroscopy of transition metal complexes in solution. Following this idea, here we focus on the comparison between general-purpose transferable force-fields (FFs), directly available from existing databases, and specific quantum mechanical derived FFs (QMD-FFs), obtained in this work through the Joyce procedure. We have chosen a recently reported FeIII complex with nanosecond excited-state lifetime as a representative case. Our molecular dynamics (MD) simulations demonstrated that the QMD-FF nicely reproduces the structure and the dynamics of the complex and its chemical environment within the same precision as higher cost QM methods, whereas general-purpose FFs failed in this purpose. Although in this particular case the chemical environment plays a minor role on the photo physics of this system, these results highlight the potential of QMD-FFs to rationalize photophysical phenomena provided an accurate QM method to derive its parameters is chosen.

Some bacterial species enter a dormant state in the form of spores to resist to unfavorable external conditions. Spores are resistant to a wide series of stress agents, including UV radiation, and can last for tens to hundreds of years. Due to the suspension of biological functions such as DNA repair, they accumulate DNA damage upon exposure to UV radiation. Differently from active organisms, the most common DNA photoproduct in spores are not cyclobutane pyrimidine dimers, but rather the so-called spore photoproduct. This non-canonical photochemistry results from the dry state of DNA and the binding to small acid soluble proteins that drastically modify the structure and photoreactivity of the nucleic acid. In this contribution, we use multiscale molecular dynamics simulations including extended classical molecular dynamics and QM/MM biased dynamics to elucidate the coupling of electronic and structural factors leading to this photochemical outcome. In particular, we rationalize the well-described impact of the peculiar DNA environment found in spores on the favored formation of the spore photoproduct, given the small free energy barrier found for this path. Meanwhile, the specific organization of spore DNA precludes the photochemical path leading to cyclobutane pyrimidine dimers formation.TOC GRAPHICS

Since the end of 2019, the coronavirus SARS-CoV-2 has caused more than 180,000 deaths all over the world, still lacking a medical treatment despite the concerns of the whole scientific community. Human Angiotensin-Converting Enzyme 2 (ACE2) was recently recognized as the transmembrane protein serving as SARS-CoV-2 entry point into cells, thus constituting the first biomolecular event leading to COVID-19 disease. Here, by means of a state-of-the-art computational approach, we propose a rational evaluation of the molecular mechanisms behind the formation of the complex and of the effects of possible ligands. Moreover, binding free energy between ACE2 and the active Receptor Binding Domain (RBD) of the SARS-CoV-2 spike protein is evaluated quantitatively, assessing the molecular mechanisms at the basis of the recognition and the ligand-induced decreased affinity. These results boost the knowledge on the molecular grounds of the SARS-CoV-2 infection and allow to suggest rationales useful for the subsequent rational molecular design to treat severe COVID-19 cases.

[EN] The propensity of 5-formyluracil and 5-formylcytosine, i.e. oxidative lesions and epigenetic intermediates, in acting as intrinsic DNA photosensitizers is unraveled by using a combination of molecular modeling, simulation and spectroscopy. Exploration of potential energy surfaces and non-adiabatic dynamics confirm a higher intersystem crossing rate for 5-formyluracil, whereas the kinetic models evidence different equilibria in the excited states for both compounds., Support from the Universite de Lorraine, CNRS and Spanish Government (PGC2018-096684-B-I00) is kindly acknowledged. A. F.-M. is grateful to Generalitat Valenciana (CTQ2017-87054-C2-2-P) and the European Social Fund for a postdoctoral contract (APOSTD/2019/149), M. L.-R. acknowledges the Universitat Politecnica de Valencia for the FPI grant. Calculations have been performed on the local LPCT computer center and on the Explor regional center in the framework of the project "Dancing under the light''.

Coronaviruses may produce severe acute respiratory syndrome (SARS). As a matter of fact, a new SARS-type virus, SARS-CoV-2, is responsible of a global pandemic in 2020 with unprecedented sanitary and economic consequences for most countries. In the present contribution we study, by all-atom equilibrium and enhanced sampling molecular dynamics simulations, the interaction between the SARS Unique Domain and RNA guanine quadruplexes, a process involved in eluding the defensive response of the host thus favoring viral infection of human cells. Our results evidence two stable binding modes involving an interaction site spanning either the protein dimer interface or only one monomer. The free energy profile unequivocally points to the dimer mode as the thermodynamically favored one. The effect of these binding modes in stabilizing the protein dimer was also assessed, being related to its biological role in assisting SARS viruses to bypass the host protective response. This work also constitutes a first step of the possible rational design of efficient therapeutic agents aiming at perturbing the interaction between SARS Unique Domain and guanine quadruplexes, hence enhancing the host defenses against the virus.TOC GRAPHICS

Recent progress (2018–2019) in the field of quantum chemistry applied to the excited electronic state are presented in this book chapter. General developments of methods and theory are described first, followed by applications organised in three main topics, (i) photo-induced chemistry (photochemistry), (ii) chemically induced light emission (chemiluminescence) and (iii) chemically induced excited-state chemistry (dark photochemistry). We shall highlight in this occasion developments of machine learning, GPU and quantum computing algorithms for excited states, non-adiabatic new methodological approaches, sunlight chemistry of atmospheric mercury compounds, boranes and stilbenoids photochemistry, DNA spectroscopy and excited-state chemistry, the bio/chemiluminescence mechanism of luminol and distinct luciferin-luciferase complexes and retinal activation in the dark.

13 pags., 14 figs., 5 tabs., We present the first examples of alkylated derivatives of the macropolyhedral boron hydride, anti-B18H22, which is the gain medium in the first borane laser. This new series of ten highly stable and colorless organic-inorganic hybrid clusters are capable of the conversion of UVA irradiation to blue light with fluorescence quantum yields of unity. This study gives a comprehensive description of their synthesis, isolation, and structural characterization together with a delineation of their photophysical properties using a combined theoretical and experimental approach. Treatment of anti-B18H22 1 with RI (where R = Me or Et) in the presence of AlCl3 gives a series of alkylated derivatives, Rx-anti-B18H22-x (where x = 2 to 6), compounds 2-6, in which the 18-vertex octadecaborane cluster architectures are preserved and yet undergo a linear "polyhedral swelling", depending on the number of cluster alkyl substituents. The use of dichloromethane solvent in the synthetic procedure leads to dichlorination of the borane cluster and increased alkylation to give Me11-anti-B18H9Cl2 11, Me12-anti-B18H8Cl2 12, and Me13-anti-B18H7Cl2 13. All new alkyl derivatives are highly stable, extremely efficient (φF = 0.76-1.0) blue fluorophores (λems = 423-427 nm) and are soluble in a wide range of organic solvents and also a polystyrene matrix. The Et4-anti-B18H18 derivative 4b crystallizes from pentane solution in two phases with consequent multiabsorption and multiemission photophysical properties. An ultrafast transient UV-vis absorption spectroscopic study of compounds 4a and 4b reveals that an efficient excited-state absorption at the emission wavelength inhibits the laser performance of these otherwise remarkable luminescent molecules. All these new compounds add to the growing portfolio of octadecaborane-based luminescent species, and in an effort to broaden the perspective on their highly emissive photophysical properties, we highlight emerging patterns that successive substitutions have on the molecular size of the 18-vertex borane cluster structure and the distribution of the electron density within., This work was supported by the Czech Science Foundation (Project No. 18-20286S). We also acknowledge the working group Interactions of Inorganic Clusters, Cages, and Containers with Light within the AV21 Strategy, Czech Academy of Sciences. W.C. and P.G.W. thank Diamond Light Source for access to beamline I19 in remote-access mode (beam-time award CY22240). A.F.-M. is grateful to Generalitat Valenciana and the European Social Fund (postdoctoral contract APOSTD/2019/149 and project GV/2020/226) for financial support. A.F.-M. and D.R.-S. also acknowledge support from the Ministerio de Ciencia e Innovacioń (project CTQ2017- 87054-C2-2-P).

7 pags., 4 figs., Mercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg and its oxidized forms Hg and Hg. The efficient gas-phase photolysis of Hg and Hg has recently been reported. However, whether the photolysis of Hg leads to other stable Hg species, to Hg, or to Hg and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg by photolysis. Results are based on non-adiabatic dynamics simulations, in which the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time a complete quantitative mechanism of the photochemical and thermal conversion between atmospheric Hg, Hg, and Hg compounds. These results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle. Thus, photoreduction clearly competes with thermal oxidation, with Hg being the main photoproduct of Hg photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment., Ministerio de Economía y Competitividad. Grant Numbers: CTQ2017-87054-C2-2-P, RYC- 2015-19234, MDM-2015-0538 Generalitat Valenciana. Grant Number: APOSTD/2019/149 European Research Council. Grant Number: ERC-2016-COG 726349 CLIMAHAL

[EN] Nucleic acids are constantly exposed to external agents that can induce chemical and photochemical damage. In spite of the great advances achieved in the last years, some molecular mechanisms of DNA damage are not completely understood yet. A recent experimental report (I. Aparici-Espert et al., ACS Chem. Biol. 2018, 13, 542) proved the ability of 5-formyluracil (ForU), a common oxidatively generated product of thymine, to act as an intrinsic sensitizer of nucleic acids, causing single strand breaks and cyclobutane pyrimidine dimers in plasmid DNA. In the present contribution, we use theoretical methodologies to study the triplet photosensitization mechanism of thymine exerted by ForU in a model dimer and in DNA environment. The photochemical pathways in the former system are described combining the CASPT2 and TD-DFT methods, whereas molecular dynamics simulations and QM/MM calculations are employed for the DNA duplex. It is unambiguously shown that the (1)n,* state localised in ForU mediates the population of the triplet manifold, most likely the (3),* state centred in ForU, whereas the (3),* state localized in thymine can be populated via triplet-triplet energy transfer given the small energy barrier of, A. F. M. is grateful to Région Grand Est government (France) for the financial support. Spanish government (CTQ2015-70164P and CTQ2017-87054-C2-2-P projects) and Regional government (Prometeo/2017/075) are also acknowledged.

Reaction of anti-B18H221 with pyridine in neutral solvents gives sparingly soluble B16H18-3',8'-Py23a as the major product (ca. 53%) and B18H20-6',9'-Py22 (ca. 15%) as the minor product, with small quantities of B18H20-8'-Py 4 (ca. 1%) also being formed. The three new compounds 2, 3a and 4 are characterized by single-crystal X-ray diffraction analyses and by multinuclear multiple-resonance NMR spectroscopy. Compound 2 is of ten-vertex nido:ten-vertex arachno two-atoms-in-common architecture, long postulated for a species with borons-only cluster constitution, but previously elusive. Compound 3a is of unprecedented ten-vertex nido:eight-vertex arachno two-atoms-in-common architecture. The single-crystal X-ray diffraction analysis for the picoline derivative B16H18(NC5H4Me)23b, similarly obtained, is also presented. B18H20Py 4 is also previously unreported but is of known ten-vertex nido:ten-vertex nido two-atoms-in-common architecture of anti configuration, but now with the pyridine ligand positioned differently to other reported examples of B18H20L compounds. Factors behind the remarkably low solubility of 3a and 3b are elucidated in terms of electrostatic potential (ESP) calculations, polarity, and van der Waals complementarities. In view of contemporary developing high interest in the fluorescent properties of macropolyhedral boron-containing species, a detailed assessment of the photophysical characteristics of 3a and 4 is also presented. In contrast to the thermochromic fluorescence of 2 (from 620 nm brick-red at room temperature to 585 nm yellow at 8 K, quantum yield 0.15), compound 3a is only weakly phosphorescent in the yellow region (590 nm, quantum yield 0.01), whereas compound 4 exhibits no luminescence. The far more photoactive nature of compound 2 is associated with S1 excited-state minima structures that differ from each other only by the relative rotational positions of the pyridine substituents on its disubstituted ten-vertex {arachno-B10Py2}-subcluster. The wavelength and relative intensity of fluorescence from these structures depends on the rotational positions of the pyridine ligands, which in turn are influenced by temperature and/or rotational inhibition in the solid-state.

Almost all chemiluminescent and bioluminescent reactions involve cyclic peroxides. The structure of the peroxide and reaction conditions determine the quantum efficiency of light emission. Oxidizable fluorophores, the so-called activators, react with 1,2-dioxetanones promoting the former to their first singlet excited state. This transformation is inefficient and does not occur with 1,2-dioxetanes; however, they have been used as models for the efficient firefly bioluminescence. In this work, we use the SA-CASSCF/CASPT2 method to investigate the activated chemiexcitation of the parent 1,2-dioxetane and 1,2-dioxetanone. Our findings suggest that ground state decomposition of the peroxide competes efficiently with the chemiexcitation pathway, in agreement with the available experimental data. The formation of non-emissive triplet excited species is proposed to explain the low emission efficiency of the activated decomposition of 1,2-dioxetanone. Chemiexcitation is rationalized considering a peroxide/activator supermolecule undergoing an electron-transfer reaction followed by internal conversion.

In this study we reveal the detailed photocycle of a phenanthrene monomer. Phenanthrene serves as a popular building block for supramolecular systems and as an archetypal molecule to study the photochemistry of polycyclic aromatic hydrocarbons. By means of femtosecond time-resolved UV-vis transient absorption spectroscopy and molecular modeling, we found that the first bright transition involves the second excited singlet state, which relaxes toward the lowest excited singlet state with a biphasic internal conversion through a conical intersection region: a fast coherent branching followed by an exceptionally slow (∼ps) incoherent internal conversion. We succeeded to pinpoint the complete relaxation pathways and to extract the relevant parameters, e.g., the branching ratio at the conical intersection and internal conversion rates.

Treatment of the laser borane

The replacement of heavy metals used by industry to produce optical devices would considerably reduce the environmental and economic cost of man-made technology. A possible strategy relies on the employment of lighter and more abundant metals like iron. The exploitability of the photophysics of Fe(II) complexes is, however, generally limited by their short excited-state lifetimes and poor emission properties. The present work studies the impact of appending an electron acceptor (anthracene) to N-heterocyclic carbene (NHC) iron complexes with the aim to trap the excited-state energy and, therefore, delay the excited-state decay of the considered iron compounds. Hence, the photophysical properties of six prototypes (built with different spacers between the NHC ligand and the anthracene moieties) have been studied by using time-dependent density functional theory and by determining the natural transition orbitals of the excited states. The computational results suggest that ethynyl bridges induce dual absorption properties, covering red and infrared wavelengths in addition to the violet–blue absorption of the metal-to-ligand charge transfer band, already reported for the parent compound. The nature of the lowest lying triplet states indicates that, for all the considered prototypes, the excitation involves π* orbitals localized over anthracene, confirming its electron acceptor capabilities and suggesting a possible equilibrium between different excited states that might lead to enhanced excited-state lifetimes and/or boosted luminescence properties.

The control of photophysical properties of iron complexes and especially of their excited states decay is a great challenge in the search for sustainable alternatives to noble metals in photochemical applications. Herein we report the synthesis and investigations of the photophysics of mer and fac iron complexes bearing bidentate pyridyl-NHC ligands, coordinating the iron with three ligand-field-enhancing carbene bonds. Ultrafast transient absorption spectroscopy reveals two distinct excited state populations for both mer and fac forms, ascribed to the populations of the T1 and the T2 states, respectively, which decay to the ground state via parallel pathways. We find 3-4 ps and 15-20 ps excited-state lifetimes, with respective amplitudes depending on the isomer. The longer lifetime exceeds the one reported for iron complexes with tridentate ligands analogues involving four iron-carbene bonds. By combining experimental and computational results, a mechanism based on the differential trapping of the triplet states in spin-crossover regions is proposed for the first time to explain the impact of the fac/ mer isomerism on the overall excited-state lifetimes. Our results clearly highlight the impact of bidentate pyridyl-NHC ligands on the photophysics of iron complexes, especially the paramount role of fac/ mer isomerism in modulating the overall decay process, which can be potentially exploited in the design of new Fe(II)-based photoactive compounds.