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Mercury (Hg) is a global pollutant with substantial risks to human and ecosystem health. By upward transport in tropical regions, mercury enters into the stratosphere, but the contribution of the stratosphere to global mercury dispersion and deposition remains unknown. We find that between 5 and 50% (passing through the 400-kelvin isentropic surface and tropopause, respectively) of the mercury mass deposited on Earth's surface is chemically processed in the lower stratosphere. Our results show the stratosphere as a unique chemical environment where elemental mercury is efficiently converted to long-lived oxidized species. Subsequent downward transport contributes substantially to the oxidized mercury burden in the troposphere. The results show that the stratosphere facilitates the global dispersion of large amounts of mercury from polluted source regions to Earth's remote environments. We find that stratospheric transport is as important as tropospheric transport in interhemispheric mercury dispersion. Future projections suggest that expected changes in atmospheric circulation will increase the transport of mercury into the stratosphere.

Among all amino acids, the three aromatic systems including phenylalanine, tyrosine, and tryptophan are known to manifest UV light absorption at the higher wavelengths. Their fluorescent properties are important for protein detection and determination of the spatial structure. Based on ab initio quantum chemical calculations, the absorption spectra in the 0-12 eV UV range of these aromatic amino acids were interpreted, and the photochemistry of the lowest-lying excited states was studied. For that, molecular ground-state geometries were determined using both the coupled-cluster single and double (CCSD) and complete active space self-consistent field (CASSCF) methods. The vertical electronic transition energies, associated oscillator strengths, and electric dipole moments of the lowest excited states were computed at the CASPT2//CASSCF level. The decay mechanisms of the lowest-energy excited states were investigated by performing minimum energy path (MEPs) computations. The results showed that the CCSD and CASSCF computations yielded similar structures. The lowest-energy S 1 state in phenylalanine and tyrosine, and S 1 and S 2 in tryptophan are mainly described by the π → π* excitation localized in the aromatic ring. Upon light absorption, the main photoresponse of the molecules is driven by the benzene, phenol, and indole chromophoric units., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Tetroxane derivatives are interesting drugs for antileishmaniasis and antimalaric treatments. The gas-phase thermal decomposition of 3,6,-dimethyl-1,2,4,5-tetroxane (DMT) and 3,3,6,6,-tetramethyl-1,2,4,5-tetroxane (acetone diperoxide (ACDP)) was studied at 493-543 K by direct gas chromatography by means of a flow reactor. The reaction is produced in the injector chamber at different temperatures. The resulting kinetics Arrhenius equations were calculated for both tetroxanes. Including the parent compound of the series 1,2,4,5-tetroxane (formaldehyde diperoxide (FDP)), the activation energy and frequency factors decrease linearly with the number of methyl groups. The reaction mechanisms of ACDP and 3,6,6-trimethyl-1,2,4,5-tetroxane (TMT) decomposition have been studied by means of the DFT method with the BHANDHLYP functional. Our calculations confirm that the concerted mechanism should be discarded and that only the stepwise mechanism occurs. The critical points of the singlet and triplet state potential energy surfaces (S- and T-PES) of the thermolysis reaction of both compounds have been determined. The calculated activation energies of the different steps vary linearly with the number of methyl groups of the methyl-tetroxanes series. The mechanism for the S-PES leads to a diradical O···O open structure, which leads to a C···O dissociation in the second step and the production of the first acetaldehyde/acetone molecule. This last one yields a second C···O dissociation, producing O 2 and another acetone/acetaldehyde molecule. The O 2 molecule is in the singlet state. A quasi-parallel mechanism for the T-PES from the open diradical to products is also found. Most of the critical points of both PES are linear with the number of methyl groups. Reaction in the triplet state is much more exothermic than the singlet state mechanism. Transitions from the singlet ground state, S 0 and low-lying singlet states S 1-3 , to the low-lying triplet excited states, T 1-4 , (chemical excitation) in the family of methyl tetroxanes are also studied at the CASSCF/CASPT2 level. Two possible mechanisms are possible here: (i) from S 0 to T 3 by strong spin orbit coupling (SOC) and subsequent fast internal conversion to the excited T 1 state and (ii) from S 0 to S 2 from internal conversion and subsequent S 2 to T 1 by SOC. From these experimental and theoretical results, the additivity effect of the methyl groups in the thermolysis reaction of the methyl tetroxane derivatives is clearly highlighted. This information will have a great impact for controlling these processes in the laboratory and chemical industries.

The photoinduced cycloreversion of oxetane derivatives is of considerable biological interest since these compounds are involved in the photochemical formation and repair of the highly mutagenic pyrimidine (6-4) pyrimidone DNA photoproducts ((6-4)PPs). Previous reports have dealt with the photoreactivity of heterodimeric oxetanes composed mainly of benzophenone (BP) and thymine (Thy) or uracil (Ura) derivatives. However, these models are far from the non-isolable Thy〈º〉Thy dimers, which are the real precursors of (6-4)PPs. Thus, we have synthesized two chemically stable homodimeric oxetanes through the Paternò-Büchi reaction between two identical enone units, i.e. 1,4-benzoquinone (BQ) and 1,4-naphthoquinone (NQ), that led to formation of BQ-Ox and NQ-Ox, respectively. Their photoreactivity has been studied by means of steady-state photolysis and transient absorption spectroscopy from the femtosecond to the microsecond time scale. Thus, photolysis of BQ-Ox and NQ-Ox led to formation of the monomeric BQ or NQ, respectively, through ring opening in a "non-adiabatic" process. Accordingly, the transient absorption spectra of the triplet excited quinones ( 3 BQ* and 3 NQ*) were not observed as a result of direct photolysis of the quinone-derived oxetanes. In the case of NQ-Ox, a minor signal corresponding to 3 NQ* was detected; its formation was ascribed to minor photodegradation of the oxetane during acquisitions of the spectra during the laser experiments. These results are supported by computational analyses based on density functional theory and multiconfigurational quantum chemistry (CASSCF/CASPT2); here, an accessible conical intersection between the ground and excited singlet states has been characterized as the main structure leading to deactivation of excited BQ-Ox or NQ-Ox. This behavior contrasts with those previously observed for heterodimeric thymine-derived oxetanes, where a certain degree of ring opening into the excited triplet state is observed.

DNA in living beings is constantly damaged by exogenous and endogenous agents. However, in some cases, DNA photodamage can have interesting applications, as it happens in photodynamic therapy. In this work, the current knowledge on the photophysics of 4-thiouracil has been extended by further quantum-chemistry studies to improve the agreement between theory and experiments, to better understand the differences with 2-thiouracil, and, last but not least, to verify its usefulness as a photosensitizer for photodynamic therapy. This study has been carried out by determining the most favorable deactivation paths of UV-vis photoexcited 4-thiouracil by means of the photochemical reaction path approach and an efficient combination of the complete-active-space second-order perturbation theory//complete-active-space self-consistent field (CASPT2//CASSCF), (CASPT2//CASPT2), time-dependent density functional theory (TDDFT), and spin-flip TDDFT (SF-TDDFT) methodologies. By comparing the data computed herein for both 4-thiouracil and 2-thiouracil, a rationale is provided on the relatively higher yields of intersystem crossing, triplet lifetime and singlet oxygen production of 4-thiouracil, and the relatively higher yield of phosphorescence of 2-thiouracil.

In this paper, we explore the molecular basis of combining photodynamic therapy (PDT), a light-triggered targeted anticancer therapy, with the traditional chemotherapeutic properties of the well-known cytotoxic agent gemcitabine. A photosensitizer prerequisite is significant absorption of biocompatible light in the visible/near IR range, ideally between 600 and 1000 nm. We use highly accurate multiconfigurational CASSCF/MS-CASPT2/MM and TD-DFT methodologies to determine the absorption properties of a series of gemcitabine derivatives with the goal of red-shifting the UV absorption band toward the visible region and facilitating triplet state population. The choice of the substitutions and, thus, the rational design is based on important biochemical criteria and on derivatives whose synthesis is reported in the literature. The modifications tackled in this paper consist of: (i) substitution of the oxygen atom at O2 position with heavier atoms (O → S and O → Se) to red shift the absorption band and increase the spin-orbit coupling, (ii) addition of a lipophilic chain at the N7 position to enhance transport into cancer cells and slow down gemcitabine metabolism, and (iii) attachment of aromatic systems at C5 position to enhance red shift further. Results indicate that the combination of these three chemical modifications markedly shifts the absorption spectrum toward the 500 nm region and beyond and drastically increases spin-orbit coupling values, two key PDT requirements. The obtained theoretical predictions encourage biological studies to further develop this anticancer approach., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations.

The quest for simple systems achieving the photoreductive splitting of four-membered ring compounds is a matter of interest not only in organic chemistry but also in biochemistry to mimic the activity of DNA photorepair enzymes. In this context, 8-oxoguanine, the main oxidatively generated lesion of guanine, has been shown to act as an intrinsic photoreductant by transferring an electron to bipyrimidine lesions and provoking their cycloreversion. But, in spite of appropriate photoredox properties, the capacity of guanine to repair cyclobutane pyrimidine dimer is not clearly established. Here, dyads containing the cyclobutane thymine dimer and guanine or 8-oxoguanine are synthesized, and their photoreactivities are compared. In both cases, the splitting of the ring takes place, leading to the formation of thymine, with a quantum yield 3.5 times lower than that for the guanine derivative. This result is in agreement with the more favored thermodynamics determined for the oxidized lesion. In addition, quantum chemistry calculations and molecular dynamics simulations are carried out to rationalize the crucial aspects of the overall cyclobutane thymine dimer photoreductive repair triggered by the nucleobase and its main lesion.

Chlorine radicals are strong atmospheric oxidants known to play an important role in the depletion of surface ozone and the degradation of methane in the Arctic troposphere. Initial oxidation processes of chlorine produce chlorine oxides, and it has been speculated that the final oxidation steps lead to the formation of chloric (HClO 3 ) and perchloric (HClO 4 ) acids, although these two species have not been detected in the atmosphere. Here, we present atmospheric observations of gas-phase HClO 3 and HClO 4 . Significant levels of HClO 3 were observed during springtime at Greenland (Villum Research Station), Ny-Ålesund research station and over the central Arctic Ocean, on-board research vessel Polarstern during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) campaign, with estimated concentrations up to 7 × 10 6 molecule cm -3 . The increase in HClO 3 , concomitantly with that in HClO 4 , was linked to the increase in bromine levels. These observations indicated that bromine chemistry enhances the formation of OClO, which is subsequently oxidized into HClO 3 and HClO 4 by hydroxyl radicals. HClO 3 and HClO 4 are not photoactive and therefore their loss through heterogeneous uptake on aerosol and snow surfaces can function as a previously missing atmospheric sink for reactive chlorine, thereby reducing the chlorine-driven oxidation capacity in the Arctic boundary layer. Our study reveals additional chlorine species in the atmosphere, providing further insights into atmospheric chlorine cycling in the polar environment., (© 2023. The Author(s).)

Excited-state absorption (ESA) spectra of π-conjugated compounds are frequently calculated by (quadratic response) time-dependent density functional theory, (QR) TD-DFT, often giving a reasonable representation of the experimental results despite the (known) incomplete electronic description. To investigate whether this is inherent to the method, we calculate here the ESA spectra of small-to-medium-sized oligophenylenevinylenes ( n PV) and oligothiophenes ( n T) using QR TD-DFT as well as CASPT2 based on CASSCF geometries. CASPT2 gives indeed a reliable, theoretically correct description of the ESA features for all compounds; the computational effort can be reduced without significant loss of accuracy using TD-DFT geometries. QR TD-DFT, based on BHandHLYP and CAM-/B3LYP functionals, fails on short n Ts but provides a reasonable description for spectral positions of n PVs and long n Ts . The failure on short n Ts is, however, only partly due to the incomplete configuration description but, in particular, related to an improper MO description, resulting in an asymmetric energy spacing of the occupied vs unoccupied MOs in the DFT scheme. Longer n Ts , on the other side, adapt approximately the MO scheme for alternant hydrocarbons just like in n PVs , while contributions by two triplet excitations combined to a singlet (which inhibits an accurate treatment of polyenes with standard TD-DFT) do not play a relevant role in the current case. For such "well-behaved" systems, a reasonable representation of ESA spectra is found at the QR TD-DFT level due to the rather small energy shifts when including higher-order excitations.

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) 2 isomers, cis-OSSO, directly yields S 2 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 3 , S 4 , and S 8 . We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S 2 is not a product in cis-OSSO photolysis. Instead, we establish a novel mechanism in which S 2 is formed in a two-step process. Firstly, the intermediate S 2 O 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, S 2 O reacts with SO. This modified chemistry yields S 2 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., (© 2022. The Author(s).)

Mercury, a global contaminant, enters the stratosphere through convective uplift, but its chemical cycling in the stratosphere is unknown. We report the first model of stratospheric mercury chemistry based on a novel photosensitized oxidation mechanism. We find two very distinct Hg chemical regimes in the stratosphere: in the upper stratosphere, above the ozone maximum concentration, Hg 0 oxidation is initiated by photosensitized reactions, followed by second-step chlorine chemistry. In the lower stratosphere, ground-state Hg 0 is oxidized by thermal reactions at much slower rates. This dichotomy arises due to the coincidence of the mercury absorption at 253.7 nm with the ozone Hartley band maximum at 254 nm. We also find that stratospheric Hg oxidation, controlled by chlorine and hydroxyl radicals, is much faster than previously assumed, but moderated by efficient photo-reduction of mercury compounds. Mercury lifetime shows a steep increase from hours in the upper-middle stratosphere to years in the lower stratosphere., (© 2022. The Authors.)

Sulfur trioxide is a critical intermediate for the sulfur cycle and the formation of sulfuric acid in the atmosphere. The traditional view is that sulfur trioxide is removed by water vapor in the troposphere. However, the concentration of water vapor decreases significantly with increasing altitude, leading to longer atmospheric lifetimes of sulfur trioxide. Here, we utilize a dual-level strategy that combines transition state theory calculated at the W2X//DF-CCSD(T)-F12b/jun'-cc-pVDZ level, with variational transition state theory with small-curvature tunneling from direct dynamics calculations at the M08-HX/MG3S level. We also report the pressure-dependent rate constants calculated using the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory. The present findings show that falloff effects in the SO 3 + HONO 2 reaction are pronounced below 1 bar. The SO 3 + HONO 2 reaction can be a potential removal reaction for SO 3 in the stratosphere and for HONO 2 in the troposphere, because the reaction can potentially compete well with the SO 3 + 2H 2 O reaction between 25 and 35 km, as well as the OH + HONO 2 reaction. The present findings also suggest an unexpected new product from the SO 3 + HONO 2 reaction, which, although very short-lived, would have broad implications for understanding the partitioning of sulfur in the stratosphere and the potential for the SO 3 reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.

The theoretical prediction of molecular electronic spectra by means of quantum mechanical (QM) computations is fundamental to gain a deep insight into many photophysical and photochemical processes. A computational strategy that is attracting significant attention is the so-called Nuclear Ensemble Approach (NEA), that relies on generating a representative ensemble of nuclear geometries around the equilibrium structure and computing the vertical excitation energies (Δ E ) and oscillator strengths ( f ) and phenomenologically broadening each transition with a line-shaped function with empirical full-width δ. Frequently, the choice of δ is carried out by visually finding the trade-off between artificial vibronic features (small δ) and over-smoothing of electronic signatures (large δ). Nevertheless, this approach is not satisfactory, as it relies on a subjective perception and may lead to spectral inaccuracies overall when the number of sampled configurations is limited due to an excessive computational burden (high-level QM methods, complex systems, solvent effects, etc.). In this work, we have developed and tested a new approach to reconstruct NEA spectra, dubbed GMM-NEA, based on the use of Gaussian Mixture Models (GMMs), a probabilistic machine learning algorithm, that circumvents the phenomenological broadening assumption and, in turn, the use of δ altogether. We show that GMM-NEA systematically outperforms other data-driven models to automatically select δ overall for small datasets. In addition, we report the use of an algorithm to detect anomalous QM computations (outliers) that can affect the overall shape and uncertainty of the NEA spectra. Finally, we apply GMM-NEA to predict the photolysis rate for HgBrOOH, a compound involved in Earth's atmospheric chemistry.

Until now, surface-deposited stilbenes have been much less studied than other photochromic systems. Here, an asymmetrically substituted styrene incorporating a redox-active ferrocene moiety and a terminal alkyne group has been synthesised to investigate its photoisomerization in solution, and upon the formation of chemisorbed self-assembled monolayers through a carbon-gold bond formation. Charge transport measurements across the monolayers reveal that upon chemical linkage to the gold substrate there is an alteration of the isomerization pathway, which favours the trans to cis conversion, which is not observed in solution. The experimental observations are interpreted based on quantum chemistry calculations.

State-of-the-art complete active space self-consistent field/complete active space second order perturbation theory (CASPT2) calculations are used to investigate the role of double excitations on the ground state absorption (GSA) and excited state absorption (ESA) spectra of distyrylbenzene, an important prototype medium-sized π-conjugated organic compound for optoelectronics. The multi-reference results are compared with linear and quadratic response time-dependent density functional theory (DFT) results, revealing an incomplete description of the electronic transitions in the latter. Careful selection of the active space and basis set in the CASPT2 approach allows for a reliable description of the GSA and ESA features; cost-effective DFT-based geometries can be utilized without a significant loss of accuracy. Double excitations are shown to play a pivotal role already for higher excited states in the GSA spectrum, however, without a relevant impact on the discernible spectral features. In the ESA, which shows a much more complex electronic situation, the crucial importance of double (and higher) excitations in all relevant electronic transitions, indeed, mandates a multiconfigurational treatment as done in the present benchmark study.

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., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Sulfur trioxide (SO 3 ) and the hydroxysulfonyl radical (HOSO 2 ) are two key intermediates in the production of sulfuric acid (H 2 SO 4 ) 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 HOSO 2 occurs primarily in the stratosphere through the ejection of hydroxyl radicals ( • OH) and sulfur dioxide (SO 2 ). This may decrease the production rate of H 2 SO 4 in atmospheric regions with low O 2 concentration. In contrast, the photostability of SO 3 under stratospheric conditions suggests that its removal efficiency, still poorly understood, is key to assess the H 2 SO 4 formation in the upper atmosphere.

Luminol is a prominent chemiluminescent (CL) agent, finding applications across numerous fields, including forensics, immunoassays, and imaging. Different substitution patterns on the aromatic ring can enhance or decrease its CL efficiency. We herein report a systematic study on the synthesis and photophysics of all possible 6,8-disubstituted luminol derivatives bearing H, Ph, and/or Me substituents. Their CL responses are monitored at three pH values (8, 10, and 12), thus revealing the architecture with the optimum CL efficiency. The most efficient pattern is used for the synthesis of a strongly CL luminol derivative, bearing a functional group for further, straightforward derivatization. This adduct exhibits an unprecedented increase in chemiluminescence efficiency at pH = 12, pH = 10, and especially at pH = 8 (closer to the biologically relevant conditions) compared to luminol. Complementary work on the fluorescence of the emissive species as well as quantum chemistry computations are employed for the rationalization of the observed results.

Hydroxysulfinyl radical (HOSO) is important due to its involvement in climate geoengineering upon SO 2 injection and generation of the highly hygroscopic H 2 SO 4 . 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 + SO 2 photoproducts. The efficient generation of SO is relevant due to its reactivity with O 3 and the consequent depletion of ozone in the stratosphere.

Theoretical determinations of absorption cross sections (σ) in the gas phase and molar extinction coefficients (ε) in condensed phases (water solution, interfaces or surfaces, protein or nucleic acids embeddings, etc.) are of interest when rates of photochemical processes, J = ∫ ϕ(λ) σ(λ) I (λ) dλ, are needed, where ϕ(λ) and I (λ) are the quantum yield of the process and the irradiance of the light source, respectively, as functions of the wavelength λ. Efficient computational strategies based on single-reference quantum-chemistry methods have been developed enabling determinations of line shapes or, in some cases, achieving rovibrational resolution. Developments are however lacking for strongly correlated problems, with many excited states, high-order excitations, and/or near degeneracies between states of the same and different spin multiplicities. In this work, we define and compare the performance of distinct computational strategies using multiconfigurational quantum chemistry, nuclear sampling of the chromophore (by means of molecular dynamics, ab initio molecular dynamics, or Wigner sampling), and conformational and statistical sampling of the environment (by means of molecular dynamics). A new mathematical approach revisiting previous absolute orientation algorithms is also developed to improve alignments of geometries. These approaches are benchmarked through the n π* band of acrolein not only in the gas phase and water solution but also in a gas-phase/water interface, a common situation for instance in atmospheric chemistry. Subsequently, the best strategy is used to compute the absorption band for the adduct formed upon addition of an OH radical to the C6 position of uracil and compared with the available experimental data. Overall, quantum Wigner sampling of the chromophore with molecular dynamics sampling of the environment with CASPT2 electronic-structure determinations arise as a powerful methodology to predict meaningful σ(λ) and ε(λ) band line shapes with accurate absolute intensities.

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.

Luminol is a popular molecule that is currently gaining further interest due to its potential role for non-invasive cancer treatments. Design of more efficient derivatives in this context would benefit from a clear knowledge on the origin of the distinct intensity and spectroscopic properties in protic and aprotic solvents observed experimentally, which are still not rationalized. By efficiently combining molecular dynamics, quantum methodologies based on density functional theory and multiconfigurational quantum chemistry and hybrid approaches, and developing herein a computational approach for accurately determining "molar negative extinction (or gain) coefficients of emission", we firstly demonstrate that the amino and imino forms of the 3-aminophthalate dianion are responsible for the chemiluminescence in protic and aprotic medium, respectively. Secondly, we show that the coupling between the adjacent amino and carboxylate groups of luminol existing in aprotic solvents must be kept in aqueous solution to increase the chemiexcitation and emission intensity. Thirdly, modifications of luminol are proposed and simulated showing improved performances as compared to the parent molecule (stronger emission electronic transition and longer emission wavelengths) under the physiological conditions of relevance in biological and medical applications.