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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 the realm of photochemistry, the significance of double excitations (also known as doubly-excited states), where two electrons are concurrently elevated to higher energy levels, lies in their involvement in key electronic transitions essential in light-induced chemical reactions as well as their challenging nature from the computational theoretical chemistry point of view. Based on state-of-the-art electronic structure methods (such as high-order coupled-cluster, selected configuration interaction, and multiconfigurational methods), we improve and expand our prior set of accurate reference excitation energies for electronic states exhibiting a substantial amount of double excitations [http://dx.doi.org/10.1021/acs.jctc.8b01205; Loos et al. J. Chem. Theory Comput. 2019, 15, 1939]. This extended collection encompasses 47 electronic transitions across 26 molecular systems that we separate into two distinct subsets: (i) 28 "genuine" doubly-excited states where the transitions almost exclusively involve doubly-excited configurations and (ii) 19 "partial" doubly-excited states which exhibit a more balanced character between singly- and doubly-excited configurations. For each subset, we assess the performance of high-order coupled-cluster (CC3, CCSDT, CC4, and CCSDTQ) and multiconfigurational methods (CASPT2, CASPT3, PC-NEVPT2, and SC-NEVPT2). Using as a probe the percentage of single excitations involved in a given transition ($\%T_1$) computed at the CC3 level, we also propose a simple correction that reduces the errors of CC3 by a factor of 3, for both sets of excitations. We hope that this more complete and diverse compilation of double excitations will help future developments of electronic excited-state methodologies.
To enrich and enhance the diversity of the \textsc{quest} database of highly-accurate excitation energies [\href{https://doi.org/10.1002/wcms.1517}{V\'eril \textit{et al.}, \textit{WIREs Comput.~Mol.~Sci.}~\textbf{11}, e1517 (2021)}], we report vertical transition energies in transition metal compounds. Eleven diatomic molecules with singlet or doublet ground state containing a fourth-row transition metal (\ce{CuCl}, \ce{CuF}, \ce{CuH}, \ce{ScF}, \ce{ScH}, \ce{ScO}, \ce{ScS}, \ce{TiN}, \ce{ZnH}, \ce{ZnO}, and \ce{ZnS}) are considered and the corresponding excitation energies are computed using high-level coupled-cluster (CC) methods, namely CC3, CCSDT, CC4, and CCSDTQ, as well as multiconfigurational methods such as CASPT2 and NEVPT2. In some cases, to provide more comprehensive benchmark data, we also provide full configuration interaction estimates computed with the \textit{``Configuration Interaction using a Perturbative Selection made Iteratively''} (CIPSI) method. Based on these calculations, theoretical best estimates of the transition energies are established in both the aug-cc-pVDZ and aug-cc-pVTZ basis sets. This allows us to accurately assess the performance of CC and multiconfigurational methods for this specific set of challenging transitions. Furthermore, comparisons with experimental data and previous theoretical results are also reported.
The complex photoisomerization mechanism of the dihydropyrene (DHP) photochromic system is revisited using spin-flip time-dependent density functional theory (SF-TD-DFT). The photoinduced ring-opening reaction of DHP into its cyclophanediene isomer involves multiple coupled electronic states of different character. A balanced treatment of both static and dynamic electron correlations is required to determine both the photophysical and photochemical paths in this system. The present results provide a refinement of the mechanistic picture provided in a previous complete active space self-consistent field plus second-order perturbation theory (CASPT2//CASSCF) study based on geometry optimizations at the CASSCF level. In particular, the nature of the conical intersection playing the central role of the photochemical funnel is different. While at the CASSCF level, the crossing with the ground state involves a covalent doubly excited state leading to a three-electron/three-center bond conical intersection, SF-TD-DFT predicts a crossing between the ground state and a zwitterionic state. These results are supported by multi-state CASPT2 calculations. This study illustrates the importance of optimizing conical intersections at a sufficiently correlated level of theory to describe a photochemical path involving crossings between covalent and ionic states.
Metal complexes with a 3d6 electron count are emerging as an alternative to 4d6-based photosensitizers, emitters, or photoredox catalysts. In recent years several Fe(II) potential emitters have been proposed, based on strongly donating ligand sets. Those tend to facilitate oxidation to their 3d5 species, whose photophysics is based on low-lying ligand-to-metal charge transfer (LMCT) states. The geometry and electronic structure of 2LMCT states are unveiled in this work.
Sujets
Photoluminescence
Redox reactions
Sulphate
Photochemistry
Chimie inorganique
Photochromism
Ruthenium polypyridine complex
Coordination compounds
Photochromisme
Sulfite
INFRARED-SPECTRUM
Groundwaters
Quantum mechanics
Diarylethenes
Ruthenium complexes
Phosphorescence
Metalloporphyrin
Crystal structure
Nitrosyl Ruthenium Complexes
Inorganic chemistry
Mechanoresponsive luminescence
Photophysique
Organic semiconductor
Mécanisme de Photolibération
PERTURBATION-THEORY APPROACH
KOHN-SHAM ORBITALS
Electrochemical reduction
Ruthenium
Nudged elastic band
Rhenium
Hydrolysis
Photosolvolysis mechanism
Mathematical methods
Photoisomerization
3MC
Solid state luminescence enhancement SLE
Insertion reaction
Density Functional Theory DFT
Aggregation induced emission AIE solid state luminescence enhancement SLE ESIPT photoluminescence crystal structure SF-TD-DFT
Photoisomerization Mechanism
Carbonate
Chimie théorique
Etats Excités
Nitric oxide
Ruthénium
Density functional theory
Excited states
Electrochemical properties
RASPT2
Molecular orbitals
Ion-molecule reactions
SF-TD-DFT
Ab initio calculations
Photorelease Mechanism
MOLECULES
DFT computations
Mécanisme de Photoisomérisation
Metal-centered excited states
Complexes de Ruthénium à Ligand Nitrosyle
Photochimie
Photodissociation
Multiple bonds
DFT
Electrochemistry
Photochromes
ICP-MS
Orbitales moléculaires
Excited States
DENSITY-FUNCTIONAL THEORY
Chimie Théorique
DIMER
NBO
DER-WAALS COMPLEXES
Computational Photochemistry
CROSS-SECTIONS
Aggregation induced emission AIE
ACETYLENE
Quinones
3MLCT
Photoisomérisation
Actinides
ESIPT
TD-DFT computations
SPECTROSCOPY
Complexe de coordination
Photosubstitution
Lanthanides
Oxidation
Ab initio
Photochimie Computationnelle
Chimie Théorique et Computationnelle
Modeling
Photophysics
Density functional calculations
Iron
Photorelease
Crystal
IPEA
Computational photochemistry
Dithienylethene