Photochemical Deracemization of 3-Substituted Oxindoles

Racemic 3-substituted oxindoles were successfully converted into enantiomerically pure or enriched material (up to 99 % ee) upon irradiation at λ=366 nm in the presence of a chiral benzophenone catalyst (10 mol %). The photochemical deracemization process allows predictable editing of the stereogenic center at carbon atom C3. Light energy compensates for the associated loss of entropy and enables the decoupling of potentially reversible reactions, i.e. a hydrogen atom transfer to (photochemical) and from (thermal) the carbonyl group of the catalyst. The major enantiomer is continuously enriched in several catalytic cycles. The obtained oxindoles were shown to be valuable intermediates for further transformations, which proceeded with complete retention at the stereogenic center.     

Blocked Phosphates as Photolatent Catalysts for Dynamic Photopolymer Networks

While latent catalysts are a well-established strategy for initiating and controlling the rate of polymerization reactions, their use in dynamic polymer networks is still in its infancy. The ideal latent catalyst should be thermally stable and release a highly active species in response to an external trigger. Here, we have synthesized a temperature resistant (>200 °C) organic phosphate with a photolabile o-nitrobenzyl protecting group that can be cleaved by UV light. Introduced in a visible light curable thiol-click photopolymer, the sequence-dependent λ-orthogonality of the curing and cleavage enables an efficient network formation at 451 nm, without premature release of the catalyst. Once cured, irradiation at 372 nm spatiotemporally activates the phosphate, which catalyzes transesterifications at elevated temperature. The formed catalyst has no effect on the thermal stability of the polymeric network and allows the activation of bond exchange reactions in selected domains of printed 3D objects.     

Carbene Formation or Reduction of the Diazo Functional Group? An Unexpected Solvent-Dependent Reactivity of Cyclic Diazo Imides

The control of the reactivity of diazo compounds is commonly achieved by the choice of a suitable catalyst, e.g. via stabilization of singlet carbenes or radical intermediates. Herein, we report on the light-promoted reactivity of cyclic diazo imides with thiols, where the choice of solvent results in two fundamentally different reaction pathways. In dichloromethane (DCM), a carbene is formed initially and engages in a cascade C−H functionalization/thiolation reaction to deliver indane-fused pyrrolidines in good to excellent yields. When switching to acetonitrile solvent, the carbene pathway is shut down and an unusual reduction of the diazo compound occurs under otherwise identical reaction conditions, where the aryl thiol acts as reductant. A combined set of experimental and computational studies was carried out to obtain mechanistic understanding and to support that indane formation proceeds via the insertion of a triplet carbene, while the reduction of diazo imides proceeds via an electron transfer process.     

A Supramolecular Naphthalene Diimide Radical Anion with Efficient NIR-II Photothermal Conversion for E. coli-Responsive Photothermal Therapy

We report a supramolecular naphthalene diimide (NDI) radical anion with efficient NIR-II photothermal conversion for E. coli-responsive photothermal therapy. The supramolecular radical anion (NDI-2CB[7])⋅⁻, which is obtained from the E. coli-induced in situ reduction of NDI-2CB[7] neutral complex, formed by the host–guest interaction between an NDI derivative and cucurbit[7]uril (CB[7]), exhibits unexpectedly strong NIR-II absorption and remarkable photothermal conversion capacity in aqueous solution. The NIR-II absorption is caused by the self-assembly of NDI radical anions to form supramolecular dimer radicals in aqueous solution, which is supported by theoretically predicted spectra. The (NDI-2CB[7])⋅⁻ demonstrates excellent NIR-II photothermal antimicrobial activity (>99 %). This work provides a new approach for constructing NIR-II photothermal agents and non-contact treatments for bacterial infections.     

Synthesis,Characterization, and Singlet Oxygen Sensitization by Antimony (III/V) Corrole Complexes

The synthesis, structural, spectroscopic characterization, and DFT/TD-DFT calculations of antimony corroles are reported herein. The studied complexes can be described as [(Corr)Sb^III] and [(Corr)(oxo)Sb^V]₂, where Corr is the trianion of corrole. All these complexes are diamagnetic in nature as is evident from sharp peaks with normal chemical shifts in the ¹H NMR spectra. Single crystal XRD analysis reveals that the antimony(V) corrole complex is the bis-μ-oxo-bridged dinuclear antimony(V). Both the tetra and hexa-coordinated [(Corr)Sb^III] and [(Corr)(oxo)Sb^V]₂ antimony complexes adopt domed-structure with weak d-π electron coupling. The Sb−O bond distances in the co-facial dimer of [(Corr)(oxo)Sb^V]₂ are 1.9802(16) Å (DFT: 2.0141 Å ) (for Sb1−O1), and 1.9639(17) Å (DFT: 1.9957 Å ) (for Sb2−O2) respectively. We observed that even though iodosobenzene is frequently used to oxidize [(Corr)Sb^III] species, the oxidation of [(Corr)Sb^III] is indeed very facile in nature and it even occurred in the air-equilibrated CHCl₃ solution while storing for few days. Excitation of these antimony (III/V) corrole complexes in DCM/MeOH (1 : 1) at 77 K results in red emission with maxima at 640–720 nm. The singlet oxygen production of [(Corr)(oxo)Sb^V]₂ has a quantum yield of 69 % and is two times higher than the analogous [(Corr)Sb^III] derivatives.     

Do The Twist: Efficient Heavy-Atom-Free Visible Light Polymerization Facilitated by Spin-Orbit Charge Transfer Inter-system Crossing

The use of visible light to drive polymerizations with spatiotemporal control offers a mild alternative to contemporary UV-light-based production of soft materials. In this spectral region, photoredox catalysis represents the most efficient polymerization method, yet it relies on the use of heavy-atoms, such as precious metals or toxic halogens. Herein, spin-orbit charge transfer intersystem crossing from boron dipyrromethene (BODIPY) dyads bearing twisted aromatic groups is shown to enable efficient visible light polymerizations in the absence of heavy-atoms. A ≈5–15× increase in polymerization rate and improved photostability was achieved for twisted BODIPYs relative to controls. Furthermore, monomer polarity had a distinct effect on polymerization rate, which was attributed to charge transfer stabilization based on ultrafast transient absorption and phosphorescence spectroscopies. Finally, rapid and high-resolution 3D printing with a green LED was demonstrated using the present photosystem.     

Synthesis of 2-Azanorbornanes via Strain-Release Formal Cycloadditions Initiated by Energy Transfer

Rigid bicycles are becoming more popular in the pharmaceutical industry because they allow for expansion to new and unique chemical spaces. This work describes a new strategy to construct 2-azanorbornanes, which can act as rigid piperidine/pyrrolidine scaffolds with well-defined exit vectors. To achieve the synthesis of 2-azanorbornanes, new strain-release reagent, azahousane, is introduced along with its photosensitized strain-release formal cycloaddition with alkenes. Furthermore, new reactivity between a housane and an imine is disclosed. Both strategies lead to various substituted 2-azanorbornanes with good selectivities.     

Singlet-Triplet Energy Inversion in Carbon Nitride Photocatalysts

Time-resolved EPR (TR-EPR) demonstrates the formation of well-defined spin triplet excitons in carbon nitride. This permits to experimentally probe the extent of the triplet wavefunction which delocalizes over several tri-s-triazine units. Analysis of the temperature dependence of the TR-EPR signal reveals the mobility of the triplet excitons. By employing monochromatic light excitation in the range 430–600 nm, the energy of the spin triplet is estimated to be ≈0.2 eV above the conduction band edge, proving that the triplet exciton lies above the corresponding singlet. Comparison between amorphous and graphitic forms establishes the singlet-triplet inversion as a general feature of carbon nitride materials.     

Peptide Conjugated Dihydroazulene/Vinylheptafulvene Photoswitches in Aqueous Environment

Light-responsive molecules have seen a major advance in modulating biological functions in recent years. Especially photoswitches are highly attractive building blocks due to the reversible nature of their light-mediated reactivity. They are frequently used to affect both the properties of small bioactive compounds and biomacromolecules if incorporated suitably. Despite their success in a plethora of applications, only a limited set of photochromic core structures is routinely employed and a large number of photochromic couples are under-investigated in biological context. Broadening the toolbox of photoswitches available to modulate biological activity would open new avenues and unlock the full potential of photoswitchable molecules for biological studies. In this work, we explore the photochemical and thermal properties of the dihydroazulene/vinylheptafulvene photochromic couple as peptide conjugates in aqueous environment.     

Photocatalytic Enantioselective Hydrosulfonylation of α,β-Unsaturated Carbonyls with Sulfonyl Chlorides

This research explores the enantioselective hydrosulfonylation of various α,β-unsaturated carbonyl compounds via the use of visible light and redox-active chiral Ni-catalysis, facilitating the synthesis of enantioenriched α-chiral sulfones with remarkable enantioselectivity (exceeding 99 % ee). A significant challenge entails enhancing the reactivity between chiral metal-coordinated carbonyl compounds and moderate electrophilic sulfonyl radicals, aiming to minimize the background reactions. The success of our approach stems from two distinctive attributes: 1) the Cl-atom abstraction employed for sulfonyl radical generation from sulfonyl chlorides, and 2) the single-electron reduction to produce a key enolate radical Ni-complex. The latter process appears to enhance the feasibility of the sulfonyl radical's addition to the electron-rich enolate radical. An in-depth investigation into the reaction mechanism, supported by both experimental observations and theoretical analysis, offers insight into the intricate reaction process. Moreover, the versatility of our methodology is highlighted through its successful application in the late-stage functionalization of complex bioactive molecules, demonstrating its practicality as a strategy for producing α-chiral sulfones.