Mechanistic Investigation of Visible-Light-Induced Intermolecular [2 + 2] Photocycloaddition Catalyzed with Chiral Thioxanthone

The recent thioxanthone-sensitizer-catalyzed intermolecular [2 + 2] cycloaddition induced by visible-light irradiation set the stage for the future development of feasible photocycloadditions. Nonetheless, the mechanism of this reaction still remains under debate, especially on the activation mode of the thioxanthone photosensitizer (energy transfer, bielectron exchange, and hydrogen transfer are all possible mechanisms). To settle this issue, systematic density functional theory calculations have been carried out. The results indicate that the energy-transfer pathway is more favorable than the bielectron-exchange and the hydrogen-transfer pathways. Meanwhile, the overall transformations involve the complexation and excitation of photosensitizer, the first C–C bond formation, the dissociation of the sensitizer, the triplet-to-singlet electronic state crossing, and the second C–C bond formation. The first C–C bond formation is the rate- and selectivity-determining step, and synergistic energy and electron transfer from photosensitizer to substrate moieties takes place along this process. On this basis, the effect of olefin substrates (ethyl vinyl ketone vs vinyl acetate) on the stereoselectivity was finally analyzed

Mechanistic Investigation on Oxygen-Mediated Photoredox Diels–Alder Reactions with Chromium Catalysts

The recent dioxygen-mediated, Cr-complex-catalyzed photoredox Diels–Alder reaction between two electron-rich substrates represents the first example of first-row transition-metal photocatalysis. Motivated by the mechanistic ambiguity (such as the inherent interactions of O₂ with Cr complexes and the origin of the high selectivity), we performed systematic density functional theory (DFT) calculations. The calculation results show that O₂ shuttles an electron via an inner-sphere mechanism (in the form of [CrL₃-O₂] complexes, L = Ph₂phen). Meanwhile, the overall transformations involve the excitation and quenching of the CrL₃³⁺ complex, single-electron transfer from the dienophile to the excited state CrL₃³⁺ catalyst (SET-1), asynchronous cycloaddition (instead of synchronous), and the final single electron transfer from the quintet [CrL₃-O₂]²⁺ complex (instead of superoxide) to the radical cationic cycloadduct. The first C–C bond formation in the asynchronous cycloaddition is the rate-determining and selectivity-determining step. On this basis, the origins of the chemo-, regio-, and stereoselectivity have been identified

Mechanistic Investigation on Oxygen-Mediated Photoredox Diels–Alder Reactions with Chromium Catalysts

The recent dioxygen-mediated, Cr-complex-catalyzed photoredox Diels–Alder reaction between two electron-rich substrates represents the first example of first-row transition-metal photocatalysis. Motivated by the mechanistic ambiguity (such as the inherent interactions of O₂ with Cr complexes and the origin of the high selectivity), we performed systematic density functional theory (DFT) calculations. The calculation results show that O₂ shuttles an electron via an inner-sphere mechanism (in the form of [CrL₃-O₂] complexes, L = Ph₂phen). Meanwhile, the overall transformations involve the excitation and quenching of the CrL₃³⁺ complex, single-electron transfer from the dienophile to the excited state CrL₃³⁺ catalyst (SET-1), asynchronous cycloaddition (instead of synchronous), and the final single electron transfer from the quintet [CrL₃-O₂]²⁺ complex (instead of superoxide) to the radical cationic cycloadduct. The first C–C bond formation in the asynchronous cycloaddition is the rate-determining and selectivity-determining step. On this basis, the origins of the chemo-, regio-, and stereoselectivity have been identified

Nanocomposites of Spiropyran-Functionalized Polymers and Upconversion Nanoparticles for Controlled Release Stimulated by Near-Infrared Light and pH

Here a near-infrared light and pH responsive nanocomposite comprising spiropyran-functionalized amphiphilic polymers and upconversion nanoparticles (UCNPs) is reported, which is prepared through the self-assembly of the amphiphilic polymers and the encapsulation of the UCNPs in the core of the self-assemblies. Upon near-infrared light irradiation, the upconversion fluorescence can induce the hydrophobic spiropyran to be isomerized to the hydrophilic merocyanine and disrupt the spherical morphology of the nanocomposites. Meanwhile, at low pH, the hydrophobic spiropyran can be also protonated to become hydrophilic merocyanine, and the self-assemblies are swollen. Model molecules, hydrophobic Coumarin 102, are demonstrated to be released from the nanocomposites triggered by the near-infrared light and acidic pH. In addition, the cytotoxicity of the nanocomposites loaded with anticancer drugs Doxorubicin on cancer cells indicates that the loaded drugs can be released and kill the cells effectively and the efficiency can be enhanced significantly upon near-infrared light irradiation