8-Mercaptoquinoline as a Ligand for Enhancing the Photocatalytic Activity of Pt(II) Coordination Complexes: Reactions and Mechanistic Insights

A family of quinoline-platinum(II) complexes as efficient photocatalysts is presented. Their key characteristic is their easy preparation by coordination of the readily available 8-hydroxy- or 8-thio-quinoline ligands, which are well known for their strong chelating ability to different metal ions. In the different photochemical transformations investigated, such as cross-dehydrogenative coupling, oxidation of arylboronic acids, and asymmetric alkylation of aldehydes, 8-mercaptoquinoline-Pt(II) complex proved to be the most general catalyst. Moreover, quenching experiments showed that, contrary to related methods reported in the literature, these complexes followed an oxidative quenching mechanism in all transformations studied. Besides, simulations performed with high-level ab initio methods of the complexes have helped to understand their photocatalytic activity.

Radical alkylation of para-quinone methides with 4-substituted Hantzsch esters/nitriles via organic photoredox catalysis

A novel photocatalytic protocol is herein described for the preparation of functionalized phenols via radical alkylation of para-quinone methides under transition-metal-free conditions. The reaction is external oxidant free and performed at ambient temperature upon visible light irradiation, allowing the access to various desired products in satisfactory yields. The readily available 4-alkyl-1,4-dihydropyridines serve as the effective alkyl radical precursors.

Unified Mechanism of Oxygen Atom Transfer and Hydrogen Atom Transfer Reactions with a Triflic Acid-Bound Nonheme Manganese(IV)-Oxo Complex via Outer-Sphere Electron Transfer

Outer-sphere electron transfer from styrene, thioanisole, and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complex, [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ (N4Py = N, N-bis(2-pyridylmethyl)- N-bis(2-pyridyl)methylamine), has been shown to be the rate-determining step of different types of redox reactions such as epoxidation, sulfoxidation, and hydroxylation of styrene, thioanisole, and toluene derivatives, respectively, by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂. The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)Mn^IV(O)]²⁺ and electron transfer from electron donors to [(N4Py)Mn^V(O)]²⁺ exhibit a remarkably unified correlation with the driving force of outer-sphere electron transfer in light of the Marcus theory of electron transfer. The same electron-transfer driving force dependence is observed in the oxygen atom transfer from [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂. Thus, mechanisms of oxygen atom transfer (epoxidation and sulfoxidation) reactions of styrene and thioanisole derivatives and hydrogen atom transfer (hydroxylation) reactions of toluene derivatives by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ have been unified for the first time as the same reaction pathway via outer-sphere electron transfer, followed by the fast bond-forming step, which exhibits the singly unified electron-transfer driving force dependence of the rate constants as outer-sphere electron-transfer reactions. In the case of the epoxidation of cis-stilbene by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂, the isomerization of cis-stilbene radical cation to trans-stilbene radical cation occurs after outer-sphere electron transfer from cis-stilbene to [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ to yield trans-stilbene oxide selectively, which is also taken as evidence for the occurrence of electron transfer in the acid-catalyzed epoxidation.

Efficient photocatalytic proton-coupled electron-transfer reduction of O2 using a saddle-distorted porphyrin as a photocatalyst

Photocatalytic O2 reduction reactions proceeded to produce H2O2 using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst. The quantum yield (12%), the turnover number (3000 for 6 h), and the turnover frequency (500 h-1) are achieved in photocatalytic systems based on free-base porphyrins for the first time. The photocatalytic reaction mechanism has been revealed by ns-laser flash photolysis and kinetic analysis.

Combining Flavin Photocatalysis and Organocatalysis: Metal-Free Aerobic Oxidation of Unactivated Benzylic Substrates

We report a system with ethylene-bridged flavinium salt 2b which catalyzes the aerobic oxidation of toluenes and benzyl alcohols with high oxidation potential ( E_ox > +2.5 V vs SCE) to give the corresponding benzoic acids under visible light irradiation. This is caused by the high oxidizing power of excited 2b ( E(2b*) = +2.67 V vs SCE) involved in photooxidation and by the accompanying dark organocatalytic oxygenation provided by the in situ formed flavin hydroperoxide 2b-OOH.

Organic Dye-Catalyzed, Visible-Light Photoredox Bromination of Arenes and Heteroarenes Using N-Bromosuccinimide

A variety of arenes and heteroarenes are brominated in good to excellent yields using N-bromosuccinimide (NBS) under mild and practical conditions. According to mechanistic investigations described within, the reaction is speculated to proceed via activation of NBS through a visible-light photoredox pathway utilizing erythrosine B as a photocatalyst. A photo-oxidative approach effectively amplifies the positive polarization on the bromine atom of the NBS reagent. This increase in the electrophilic nature of NBS results in drastically reduced reaction times and diversion from competing light-promoted reactive pathways.

Supramolecular Organic Photocatalyst Containing a Cubanelike Water Cluster and Donor-Acceptor Stacks: Hydrogen Evolution and Dye Degradation under Visible Light

Supramolecular organic photocatalysts are scarcely explored for the generation of sustainable energy as well as for environmental remediation purposes. An organic photocatalyst, containing a cubanelike water cluster and donor-acceptor stacks, was efficiently developed through a supramolecular approach. The material exhibited remarkable photocatalytic hydrogen generation, in the absence of any cocatalyst, with excellent stability and recyclability. The photoactivity was further assessed through time-resolved photoluminescence and electrochemical impedance spectroscopy. The material also exhibited highly efficient sunlight-driven photocatalytic activity through the degradation of harmful organic dye methylene blue.

Intramolecular Energy and Electron Transfers in Bodipy Naphthalenediimide Triads

Borondipyrromethene (BDP) naphthalenediimide (NDI) triads (BDP-NDI) and diiodo-BDP derivative (DiiodoBDP-NDI)) were synthesized to study the Förster resonance energy transfer (FRET) and its impact on the triplet state formation and dynamics. In these triads, diiodo-BDP and BDP are the energy donors and NDI is the energy acceptor. Nanosecond transient absorption spectra of triads indicated that triplet state is localized on NDI moiety, either by selective photoexcitation of the Diiodo-BDP or NDI unit. The intersystem crossing (ISC) is attributed to intramolecular heavy atom effect. The triplet state quantum yield was found to be 54% with a lifetime of 38 μs. However, no triplet state is observed for BDP-NDI system either by exciting BDP or NDI unit. Thus, we confirmed that charge recombination does not produce a triplet state. Interestingly, DiiodoBDP-NDI can be used as broadband excitable (500-620 nm) triplet photosensitizer, and high triplet-triplet annihilation (TTA) upconversion quantum yield of Φ_UC = 2.8% was observed with 9,10-bis(phenylethynyl)-anthracene (BPEA) as a triplet acceptor/emitter.

Photocatalytic Generation of Nitrenes for Rapid Diaziridination

A blue LED, an organic photocatalyst (rose bengal), and the Lewis acid like oxidant PhI(OAc)₂ were utilized to generate nitrene intermediates through reactions of 1,2-diols and aliphatic amines under mild reaction conditions. A versatile and rapid diaziridination strategy was established to construct functionalized 1,2-disubstituted diaziridines, diaziridines with chiral substituents, and 1,2,3-trisubstituted analogues with excellent reaction rates, yields, and stereoselectivities. Control and labeling experiments to elucidate the mechanism of this elegant metal-free photocatalyzed cyclization reaction were performed.

Exploiting the benefit of S0→ T1 excitation in triplet-triplet annihilation upconversion to attain large anti-stokes shifts: tuning the triplet state lifetime of a tris(2,2'-bipyridine) osmium(ii) complex

Os(ii) complexes are particularly interesting for triplet-triplet annihilation (TTA) upconversion, due to the strong direct S₀→ T₁ photoexcitation, as in this way, energy loss is minimized and large anti-Stokes shift can be achieved for TTA upconversion. However, Os(bpy)₃ has an intrinsic short T₁ state lifetime (56 ns), which is detrimental for the intermolecular triplet-triplet energy transfer (TTET), one of the crucial steps in TTA upconversion. In order to prolong the triplet state lifetime, we prepared an Os(ii) tris(bpy) complex with a Bodipy moiety attached, so that an extended T₁ state lifetime is achieved by excited state electronic configuration mixing or triplet state equilibrium between the coordination center-localized state (³MLCT state) and Bodipy ligand-localized state (³IL state). With steady-state and time-resolved transient absorption/emission spectroscopy, we proved that the ³MLCT is slightly above the ³IL state (by 0.05 eV), and the triplet state lifetime was prolonged by 31-fold (from 56 ns to 1.73 μs). The TTA upconversion quantum yield was increased by 4-fold as compared to that of the unsubstituted Os(ii) complex.

Bifunctional organic sponge photocatalyst for efficient cross-dehydrogenative coupling of tertiary amines to ketones

A novel bifunctional organic sponge photocatalyst can enable the efficient coupling of tertiary amines with ketones in water. The asymmetric transformation can be also achieved by using this sponge photocatalyst.

Redox-neutral tri-/difluoromethylation of para-quinone methides with sodium sulfinates

A redox-neutral, mild, and simple protocol is developed for the synthesis of functionalized phenols from para-quinone methides upon visible-light irradiation.

Visible light induced tetramethylethylenediamine assisted formylation of imidazopyridines

A metal-free visible light induced C-3 formylation of imidazo[1,2-a]pyridine has been developed using tetramethylethylenediamine (TMEDA) as a one carbon source under ambient air.

Spin–orbit charge transfer intersystem crossing in perylenemonoimide–phenothiazine compact electron donor–acceptor dyads

Efficient triplet excited state production (57%) was observed for perylenemonoimide–phenothiazine compact electron donor–acceptor dyads based on spin–orbit charge transfer ISC.

Visible-light initiated polymerization of dopamine in a neutral environment for surface coating and visual protein detection

Mussel-inspired polydopamine (PDA) coating is a promising avenue for surface modification.

Effect of thiophene substitution on the intersystem crossing of arene photosensitizers

Thiophene substitution gives energy level-matched S₁/T₂ states and the ISC is enhanced, which was not observed with phenyl substitution.

Visible-light-induced photocatalytic benzene/cyclohexane cross-coupling utilizing a ligand-to-metal charge transfer benzene complex adsorbed on titanium oxides

High selectivity was achieved in the photocatalytic cross-coupling of benzene and cyclohexane by photoexcitation of a surface benzene complex.

Electron donor-free photoredox catalysis via an electron transfer cascade by cooperative organic photocatalysts

Electron transfer cascade in cooperative organic photocatalysts can prevent the use of sacrificial reagent for photoredox catalysis.

Recent progress in heavy atom-free organic compounds showing unexpected intersystem crossing (ISC) ability

The intersystem crossing (ISC) of the heavy atom-free triplet photosensitizers was summarised, including the spin–orbit charge transfer ISC mechanism.

Selective Photooxidation Reactions using Water-Soluble Anthraquinone Photocatalysts

The aerobic organocatalytic oxidation of alcohols was achieved by using water‐soluble sodium anthraquinone sulfonate. Under visible‐light activation, this catalyst mediated the aerobic oxidation of alcohols to aldehydes and ketones. The photo‐oxyfunctionalization of alkanes was also possible under these conditions.

Detection of Fleeting Amine Radical Cations and Elucidation of Chain Processes in Visible-Light-Mediated [3 + 2] Annulation by Online Mass Spectrometric Techniques

Visible-light-mediated photoredox reactions have recently emerged as a powerful means for organic synthesis and thus have generated significant interest from the organic chemistry community. Although the mechanisms of these reactions have been probed by a number of techniques such as NMR, fluorescence quenching, and laser flash photolysis and various degrees of success has been achieved, mechanistic ambiguity still exists (for instance, the involvement of the chain mechanism is still under debate) because of the lack of structural information about the proposed and short-lived intermediates. Herein, we present the detection of transient amine radical cations involved in the intermolecular [3 + 2] annulation reaction of N-cyclopropylaniline (CPA, 1) and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradiation of the reaction mixture. In particular, the reactive CPA radical cation 1^+•, the reduced photocatalyst Ru(I)(bpz)₃⁺, and the [3 + 2] annulation product radical cation 3^+• are all successfully detected and confirmed by high-resolution MS. More importantly, the post-irradiation reaction with an additional substrate, isotope-labeled CPA, following photolysis of 1, 2, and Ru catalyst provides strong evidence to support the chain mechanism in the [3 + 2] annulation reaction. Furthermore, the key step of the proposed chain reaction, the oxidation of CPA 1 to amine radical cation 1^+• by product radical cation 3^+• (generated using online electrochemical oxidation of 3), is successfully established. Additionally, the coupling of ESI-MS with online laser irradiation has been successfully applied to probe the photostability of photocatalysts.