Temperature-Controlled Chemoselective Couplings of Alkyl Halides with Disulfides.

An unprecedented, transition metal-free -alkylation of disulfides with alkyl halides is developed for the first time, providing an efficient and green synthesis of thioethers and even thioesters. Notably, this new method allows the full utilization of both sulfur atoms of disulfides under chemical reductant-free conditions and can be easily scaled up in gram scale, showing good practical value. Control experiments suggested that water, unprecedentedly, serves as the terminal reductant of the whole reaction.

Quantum mechanics/molecular mechanics studies on the excited-state decay mechanisms of cytidine aza-analogues: 5-azacytidine and 2'-deoxy-5-azacytidine in aqueous solution.

The excited state properties and deactivation pathways of two DNA methylation inhibitors, , 5-azacytidine (5ACyd) and 2'-deoxy-5-azacytidine (5AdCyd) in aqueous solution, are comprehensively explored with the QM(CASPT2//CASSCF)/MM protocol. We systematically map the feasible decay mechanisms based on the obtained excited-state decay paths involving all the identified minimum-energy structures, conical intersections, and crossing points driving the different internal conversion (IC) and intersystem crossing (ISC) routes in and between the ππ*, nπ*, ππ*, nπ*, and S states. Unlike the nπ* state below the ππ* state in 5ACyd, deoxyribose group substitution at the N1 position leads to the ππ* state becoming the S state in 5AdCyd.

Mechanistic photophysics of tellurium-substituted cytosine: Electronic structure calculations and nonadiabatic dynamics simulations.

Previously, the MS-CASPT2 method was performed to study the static and qualitative photophysics of tellurium-substituted cytosine (TeC). To get quantitative information, we used our recently developed QTMF-FSSH dynamics method to simulate the excited-state decay of TeC. The CASSCF method was adopted to reduce the calculation costs, which was confirmed to provide reliable structures and energies as those of MS-CASPT2.

Quantum mechanics/molecular mechanics studies on mechanistic photophysics of cytosine aza-analogues: 2,4-diamino-1,3,5-triazine and 2-amino-1,3,5-triazine in aqueous solution.

The excited-state properties and photophysics of cytosine aza-analogues, , 2,4-diamino-1,3,5-triazine (2,4-DT) and 2-amino-1,3,5-triazine (2-AT) in solution have been systematically explored using the QM(MS-CASPT2//CASSCF)/MM approach. The excited-state nonradiative relaxation mechanisms for the initially photoexcited S(ππ*) state decay back to the S state are proposed in terms of the present computed minima, surface crossings (conical intersections and singlet-triplet crossings), and excited-state decay paths in the S, S, T, T, and S states. Upon photoexcitation to the bright S(ππ*) state, 2,4-DT quickly relaxes to its S minimum and then overcomes a small energy barrier of 5.

Quantum mechanics/molecular mechanics studies on excited state decay pathways of 5-azacytosine in aqueous solution.

In this work, we have used the QM(CASPT2//CASSCF)/MM approach to study the photophysical properties and relaxation mechanism of 5-azacytosine (5-AC) in aqueous solution. Based on the relevant minimum-energy structures and intersection structures, and excited-state decay paths in the S, S, T, T, and S states, several feasible excited-state nonradiative decay channels from the initially populated S(ππ*) state are proposed. Two major channels are singlet-mediated nonradiative pathways, in which the S system will internally convert (IC) to the S state directly or mediated by the nπ* state a ππ*/nπ* conical intersection.

Bifunctional 1,8-Diazabicyclo[5.4.0]undec-7-ene for Visible Light-Induced Heck-Type Perfluoroalkylation of Alkenes.

This article presents simple and efficient 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-promoted Heck-type alkene perfluoroalkylation under visible light irradiation.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution.

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the ππ*, nπ*, ππ*, nπ*, and S states, we have identified several feasible excited-state relaxation pathways for the initially populated S(ππ*) state to decay to the initial enol isomer' S state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless ππ* ESIPT process to generate the ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S → S internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution.

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S(ππ*) state and the lower-lying T and T states. In the major relaxation pathway, the photoexcited S keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S enol minimum, near which a favorable S/S conical intersection decays the system to the S state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S keto species.

Mechanistic photophysics and photochemistry of unnatural bases and sunscreen molecules: insights from electronic structure calculations.

Photophysics and photochemistry are basic subjects in the study of light-matter interactions and are ubiquitous in diverse fields such as biology, energy, materials, and environment. A full understanding of mechanistic photophysics and photochemistry underpins many recent advances and applications. This contribution first provides a short discussion on the theoretical calculation methods we have used in relevant studies, then we introduce our latest progress on the mechanistic photophysics and photochemistry of two classes of molecular systems, namely unnatural bases and sunscreens.

Unprecedented observation and characterization of sulfur-centred bifurcated hydrogen bonds.

Owing to the small electronegativity of the sulfur atom, it is commonly supposed that at most one weak H-bond can be formed between a sulfur atom and an H-bond donor. In this paper, an unprecedented 2 : 1 binding species generated from two molecules of phenol and a molecule of thioether was observed and characterized by various nuclear magnetic resonance (NMR) techniques, Fourier transform-infrared (FT-IR) techniques and density functional theory (DFT) calculations, revealing the formation of sulfur-centred O-H⋯S⋯H-O bifurcated H-bonds. This work may provide a simple and efficient method for the quantitative analysis of weak H-bonds between small organic molecules.

Mechanistic Photophysics of Tellurium-Substituted Uracils: Insights from Multistate Complete-Active-Space Second-Order Perturbation Calculations.

The photophysical mechanisms of tellurium-substituted uracils were studied at the multistate complete-active-space second-order perturbation level with a particular focus on how the position and number of tellurium substitutions affect their nonadiabatic relaxation processes. Electronic structure analysis reveals that the lowest several excited states are closely concerned with the n and π orbitals at the Te-C [Te-C] moiety of 2-tellurouracil (2TeU) [4TeU and 24TeU]. Both planar and twisted minima were optimized for 2TeU, whereas only planar ones were obtained for 4TeU and 24TeU, except for a twisted T minimum of 4TeU.

A high-performance voltammetric methodology for the ultra-sensitive detection of riboflavin in food matrices based on graphene oxide-covered hollow MnO spheres.

A high-performance voltammetric methodology was developed to achieve ultra-sensitive detection of riboflavin, employing an electrode modified by graphene oxide-covered hollow MnO spheres nanocomposite with high catalytic activity, large surface area, and hierarchical layered structure. Under the optimal conditions, the current responses of the oxidation peak located at -0.39 V showed a good linear relationship versus the concentration of riboflavin in the range of 1.

Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate.

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S, S, and T states, in a methanol solution.

Molecular Fluorescent Probes for Imaging and Evaluation of Hypochlorite Fluctuations during Diagnosis and Therapy of Osteoarthritis in Cells and in a Mouse Model.

The early diagnosis of osteoarthritis (OA) can halt or delay the progression of the disease, and it is essentially beneficial to its treatment. However, biomarkers with sufficient sensitivity for dynamically identifying early OA are still yet to be determined. The overproduced hypochlorous acid (HOCl) has been proposed as an obvious symptom in early OA.

QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model.

Methyl 2-cyano-3,3-diphenylacrylate (MCDPA) shares the same molecular skeleton with octocrylene (OCR) that is one of the most common molecules used in commercially available sunscreens. However, its excited-state relaxation mechanism is unclear. Herein, we have used the QM(CASPT2//CASSCF)/MM method to explore spectroscopic properties, geometric and electronic structures, relevant conical intersections and crossing points, and excited-state relaxation paths of MCDPA in methanol solution.

Multifunctional supramolecular self-assembly system for colorimetric detection of Hg, Fe, Cu and continuous sensing of volatile acids and organic amine gases.

A novel multifunctional gelator (1) based on an azobenzene derivative was designed and characterized. This compound could gelate some solvents including hexane, petroleum ether, DMSO, acetonitrile and ethanol through a heating-cooling procedure. The self-assembly process in different solvents was studied by means of UV-vis absorption and Fourier transform infrared (FTIR) spectra, field emission scanning electron microscopy (FESEM), rheological measurements, X-ray powder diffraction and water contact angle experiments.

KSO-Mediated Selective Trifluoromethylacylation and Trifluoromethylarylation of Alkenes under Transition-Metal-Free Conditions: Synthetic Scope and Mechanistic Studies.

A practical and efficient method for selective intramolecular radical trifluoromethylacylation and -arylation of alkenes with inexpensive CFSONa and KSO in aqueous media has been developed, respectively, affording the highly chemoselective synthesis of CF-functionalized chroman-4-ones and chromanes in satisfactory yields. Control experiments and DFT calculations indicate that the CFSONa/KSO system is capable of trifluoromethylating the substrate of alkenes without a transition metal catalyst and the oxidation of CFSONa to ·CF by KSO is involved in the rate-determining step.

Terminal Molecular Isomer-Effect on Supramolecular Self-Assembly System Based on Naphthalimide Derivative and Its Sensing Application for Mercury(II) and Iron(III) Ions.

A series of naphthalimide derivative gelators (G-o, G-m, and G-p) with three molecular isomers as their terminal groups were designed and synthesized. Only G-m and G-p could form stable organogels in some solvents including methanol, acetonitrile, n-hexane, toluene, ethanol, DMSO, DMF, and mixed solvents of acetonitrile/HO (1/1, v/v). The different self-assembly structures were obtained from the self-assembly process of G-o, G-m, and G-p such as structures like a Chinese chestnut formed by irregular micrometer pieces, microbelts, and microbelt structures mingled with the bird's nest structures which exhibited different surface hydrophobicity with water contact angles of 121-139° due to their different intermolecular noncovalent interactions.

Photophysics of a UV-B Filter 4-Methylbenzylidene Camphor: Intersystem Crossing Plays an Important Role.

4-Methylbenzylidene camphor (4MBC) is a frequently used ultraviolet (UV) filter in commercial sunscreens, which is experimentally found to undergo efficient intersystem crossing to triplet manifolds followed by predominant radiationless decay to the ground state. However, its photophysical mechanism is unclear. Herein, we have employed combined CASPT2 and CASSCF methods to study the spectroscopic properties, geometric and electronic structures, conical intersections and crossing points, and excited-state deactivation channels of 4MBC.

Quantum Mechanics/Molecular Mechanics Study on the Photoreactions of Dark- and Light-Adapted States of a Blue-Light YtvA LOV Photoreceptor.

The dark- and light-adapted states of YtvA LOV domains exhibit distinct excited-state behavior. We have employed high-level QM(MS-CASPT2)/MM calculations to study the photochemical reactions of the dark- and light-adapted states. The photoreaction from the dark-adapted state starts with an S →T intersystem crossing followed by a triplet-state hydrogen transfer from the thiol to the flavin moiety that produces a diradical intermediate, and a subsequent internal conversion that triggers a barrierless C-S bond formation in the S state.

A theoretical study of the light-induced cross-linking reaction of 5-fluoro-4-thiouridine with thymine.

In contrast to photophysics of thio-substituted nucleobases, their photoinduced cross-linking reactions with canonical nucleobases remain scarcely investigated computationally. In this work, we have adopted combined CASPT2/PCM//CASSCF and B3LYP-D3/PCM electronic structure methods to study this kind of photochemical reaction of 5-fluoro-4-thiouridine (truncated 5-fluoro-1-methyl-4-thiouracil used in calculations) and 1-methylthymine (referred to as thymine for clarity hereinafter). On the basis of CASPT2/PCM computed results, we have proposed two efficient excited-state relaxation pathways to populate the lowest T state of the complex of 5-fluoro-1-methyl-4-thiouracil and thymine from its initially populated S(ππ*) state.

Tuning excited-state-intramolecular-proton-transfer (ESIPT) process and emission by cocrystal formation: a combined experimental and theoretical study.

The formation of two-component molecular cocrystals can lead to the tunable excited state intramolecular proton transfer (ESIPT) process and emission, as first confirmed by both experimental and computational studies.

Excited-State Decay Paths in Tetraphenylethene Derivatives.

The photophysical properties of tetraphenylethene (TPE) compounds may differ widely depending on the substitution pattern, for example, with regard to the fluorescence quantum yield ϕ and the propensity to exhibit aggregation-induced emission (AIE). We report combined electronic structure calculations and nonadiabatic dynamics simulations to study the excited-state decay mechanisms of two TPE derivatives with four methyl substituents, either in the meta position (TPE-4mM, ϕ = 0.1%) or in the ortho position (TPE-4oM, ϕ = 64.

Photocycloaddition reaction of atropisomeric maleimides: mechanism and selectivity.

We report a density functional study on the mechanism of the [2+2] photocyclization of atropisomeric maleimides. Experimentally, the reaction is known to proceed through the triplet state. We have located all relevant S0 and T1 minima and transition states, as well as the T1/S0 crossing points, and mapped eight stepwise photocyclization pathways for four different conformers in the T1 state that lead to distinct regioisomers.

QM/MM Study on Mechanistic Photophysics of Alloxazine Chromophore in Aqueous Solution.

Compared with isoalloxazine, the core chromophore of biologically important flavins, alloxazine exhibits much lower fluorescence quantum yield and larger intersystem-crossing quantum yield. However, its efficient radiationless relaxation pathways are still elusive. In this work, we have used the QM(MS-CASPT2//CASSCF)/MM method to explore the mechanistic photophysics of alloxazine chromophore in aqueous solution.