Ketones as Molecular Co-catalysts for Boosting Exciton-Based Photocatalytic Molecular Oxygen Activation

Excitonic processes in semiconductors open up the possibility for pursuing photocatalytic organic synthesis. However, the insufficient spin relaxation and robust nonradiative decays in semiconductors place restrictions on both quantum yield and selectivity of these reactions. Herein, by taking polymeric carbon nitride (PCN)/acetone as a prototypical system, we propose that extrinsic aliphatic ketones can serve as molecular co-catalysts for promoting spin-flip transition and suppressing non-radiative energy losses. Spectroscopic investigations indicate that hot excitons in PCN can be transferred to ketones, while triplet excitons in ketones can be transferred to PCN. As such, the PCN/ketone systems exhibit considerable triplet-exciton accumulation and extended visible-light response, leading to excellent performance in exciton-based photocatalysis, such as singlet oxygen generation. This work provides a fundamental understanding of energy harvesting in semiconductor/molecule systems, and paves the way for optimizing exciton-based photocatalysis via molecular co-catalyst design.     

Molten‐Salt‐Mediated Synthesis of an Atomic Nickel Co‐catalyst on TiO2 for Improved Photocatalytic H2 Evolution

Atomic co‐catalysts offer high potential to improve the photocatalytic performance, of which the preparation with earth‐abundant elements is challenging. Here, a new molten salt method (MSM) is designed to prepare atomic Ni co‐catalyst on widely studied TiO₂ nanoparticles. The liquid environment and space confinement effect of the molten salt leads to atomic dispersion of Ni ions on TiO₂, while the strong polarizing force provided by the molten salt promotes formation of strong Ni?O bonds. Interestingly, Ni atoms are found to facilitate the formation of oxygen vacancies (OV) on TiO₂ during the MSM process, which benefits the charge transfer and hydrogen evolution reaction. The synergy of atomic Ni co‐catalyst and OV results in 4‐time increase in H₂ evolution rate compared to that of the Ni co‐catalyst on TiO₂ prepared by an impregnation method. This work provides a new strategy of controlling atomic co‐catalyst together with defects for efficient photocatalytic water splitting.     

Metallophthalocyanine‐Based Conjugated Microporous Polymers as Highly Efficient Photosensitizers for Singlet Oxygen Generation

Singlet oxygen (¹O₂) is of great interest because of its potential applications in photodynamic therapy, photooxidation of toxic molecules, and photochemical synthesis. Herein, we report novel metallophthalocyanine (MPc) based conjugated microporous polymers (MPc‐CMPs) as photosensitizers for the generation of ¹O₂. The rigid microporous structure efficiently improves the exposure of the majority of the MPc units to oxygen. The MPc‐CMPs also exhibit an enhanced light‐harvesting capability in the far‐red region through their extended π‐conjugation systems. Their microporous structure and excellent absorption capability for long‐wavelength photons result in the MPc‐CMPs showing high efficiency for ¹O₂ generation upon irradiation with 700 nm light, as evident by using 1,3‐diphenylisobenzofuran as an ¹O₂ trap. These results indicate that MPc‐CMPs can be considered as promising photosensitizers for the generation of ¹O₂.     

Photocatalytic Oxygen Evolution from Functional Triazine‐Based Polymers with Tunable Band Structures

Conjugated polymers (CPs) are emerging and appealing light harvesters for photocatalytic water splitting owing to their adjustable band gap and facile processing. Herein, we report an advanced mild synthesis of three conjugated triazine‐based polymers (CTPs) with different chain lengths by increasing the quantity of electron‐donating benzyl units in the backbone. Varying the chain length of the CTPs modulates their electronic, optical, and redox properties, resulting in an enhanced performance for photocatalytic oxygen evolution, which is the more challenging half‐reaction of water splitting owing to the sluggish reaction kinetics. Our results could stimulate interest in these functional polymers where a molecular engineering strategy enables the production of suitable semiconductor redox energetics for oxygenic photosynthesis.     

Visible‐Light‐Mediated 1,2‐Acyl Migration: The Reaction of Secondary Enamino Ketones with Singlet Oxygen

Secondary enaminones were oxidized by photochemically generated singlet oxygen, followed by nucleophilic addition of alcohol and an unexpected 1,2‐acyl migration to afford quaternary amino acid derivatives. An ene‐type reaction pathway is proposed. It is distinctively different from the typical [2+2] addition of singlet oxygen to a C?C bond pathway.     

Phase‐Contact Engineering in Mono‐ and Bimetallic Cu‐Ni Co‐catalysts for Hydrogen Photocatalytic Materials

Understanding how a photocatalyst modulates its oxidation state, size, and structure during a photocatalytic reaction under operando conditions is strongly limited by the mismatch between (catalyst) volume sampled by light and, to date, the physicochemical techniques and probes employed to study them. A synchrotron micro‐beam X‐ray absorption spectroscopy study together with the computational simulation and analysis (at the X‐ray cell) of the light‐matter interaction occurring in powdered TiO₂‐based monometallic Cu, Ni and bimetallic CuNi catalysts for hydrogen production from renewables was carried out. The combined information unveils an unexpected key catalytic role involving the phase contact between the reduced and oxidized non‐noble metal phases in all catalysts and, additionally, reveals the source of the synergistic Cu‐Ni interaction in the bimetallic material. The experimental method is applicable to operando studies of a wide variety of photocatalytic materials.     

Gadolinium(III) Porpholactones as Efficient and Robust Singlet Oxygen Photosensitizers

Construction of Gd^III photosensitizers is important for designing theranostic agents owing to the unique properties arising from seven unpaired f electrons of the Gd³⁺ ion. Combining these with the advantages of porpholactones with tunable NIR absorption, we herein report the synthesis of Gd^III complexes Gd‐1 – 4 ( 1 , porphyrin; 2 , porpholactone; 3 and 4 , cis‐ and trans‐porphodilactone, respectively) and investigated their function as singlet oxygen (¹O₂) photosensitizers. These Gd complexes displayed ¹O₂ quantum yields (Φ_Δs) from 0.64–0.99 with the order Gd‐1 < Gd‐2 < Gd‐3 < Gd‐4 . The gradually enhanced ¹O₂ sensitization after β‐oxazolone moiety replacement was ascribed to the narrowing of the energy gap (ΔE) between the lowest triplet states (T₁) of the ligand and the energy level of the ¹Δ_g→³Σ_g transition of ¹O₂. In particular, Gd‐4 is capable of excitation in the visible to NIR region (400–700 nm) with a quantum yield near unity. These Gd complexes were first demonstrated as efficient photosensitizers in photocatalysis such as oxidative C?H bond functionalization of secondary or tertiary amines, and the oxygenation of the natural product cholesterol. Finally, after glycosylation, these water‐soluble Gd complexes showed potential applications in photodynamic therapy (PDT) in HeLa cells. This work revealed that Gd^III complexes of “bioinspired” β‐modified porpholactones are efficient NIR photosensitizers and form a chemical basis to construct appealing photocatalysts and theranostic agents based on lanthanides.     

Unravelling the Synergy between Oxygen Vacancies and Oxygen Substitution in BiO2−x for Efficient Molecular‐Oxygen Activation

Defects in nanomaterials often lead to properties that are absent in their pristine counterparts. To date, most studies have focused on the effect of single defects, while ignoring the synergy of multiple defects. In this study, a model of photocatalytic O₂ activation was selected to unravel the role of dual defects by decorating bismuth oxide with surface O vacancies and bulk O substitution simultaneously. The introduction of dual defects led to a spatial and electronic synergistic process: i) O substitution induced a local electric field in the bulk of BiO_2?x, which promoted bulk separation of electrons and holes immediately after their generation; ii) O vacancies efficiently lowered the conduction band, served as the capture center for electrons, and thus facilitated the adsorption and activation of O₂. This effect was greatly promoted by the coexistence of bulk O substitution, and DFT calculations showed that only O substitution near an O vacancy could have this effect.     

Visible‐Light Photocatalytic Hydrogen Production Activity of ZnIn2S4 Microspheres Using Carbon Quantum Dots and Platinum as Dual Co‐catalysts

ZnIn₂S₄ microspheres (ZIS MSs) were for the first time decorated with carbon quantum dots (CQDs) and platinum nanoparticles (NPs) as dual co‐catalysts of for photocatalytic H₂ production. The ZIS MSs co‐loaded with CQDs and Pt exhibited a high photocatalytic H₂ production rate of 1032.2 μmol h^?1 g^?1 with an apparent quantum efficiency of 2.2 % (420 nm) in triethanolamine aqueous solution under visible‐light irradiation, which was much higher than the respective photocatalytic rates of pure ZIS, Pt loaded ZIS, and CQDs‐decorated ZIS. Such a great enhancement was attributed to the integrative effect of good crystallization, enhanced light absorption, high electrical conductivity of CQDs, and the vectorial electron transfer from ZIS to CQDs and Pt NPs (ZIS→CQDs→Pt).     

苝衍生物光催化氧化硫醚的研究

本文通过对苝酐进行化学修饰,合成了两种具有较好溶解性和强可见光吸收的苝衍生物:PMI-(iPr)2An(PSⅡ)和PMI-((iPr)2An)2(PSⅢ)。在可见光照射下,以PSⅡ和PSⅢ作为光敏剂,氧气为氧化剂,实现了高转化率和高选择性的硫醚到亚砜的氧化。在优化反应条件下,PSⅢ对苯基硫醚衍生物光催化氧化的转化率和选择性均接近100%。活性中间体的清除实验和电子顺磁共振实验表明,该体系中单重态氧(~1O_2)和超氧自由基同时对硫醚的氧化起作用。     

Conversion of Olefins into Ketones by an Iron‐Catalyzed Wacker‐type Oxidation Using Oxygen as the Sole Oxidant

We describe a mild and operationally simple procedure for the oxidation of olefins into ketones. The reaction is catalyzed by the hexadecafluorinated iron–phthalocyanine complex FePcF₁₆ with stoichiometric amounts of triethylsilane as an additive under oxygen atmosphere to give ketones in good to high yields with excellent chemoselectivity and functional group tolerance. Ketone formation proceeds in up to 95 % yield and with 100 % regioselectivity while the corresponding alcohols were observed as side products.     

Remazol‐Catalyzed Hydroperoxyarylation of Styrenes

A mild photocatalytic hydroperoxyarylation of styrenes has been developed, in which a novel photocatalyst, remazol brilliant blue R (RBBR), is employed at low catalytic loading (1 mol %). The operationally easy procedure uses air as the dioxygen source. Simple mono‐substituted styrenes react with aryl hydrazines in moderate‐to‐good yields. RBBR is proposed to act as a photosensitizer for the generation of singlet oxygen.     

Sunlight‐Driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal‐Free Photocatalysts

Design of green, safe, and sustainable process for the synthesis of hydrogen peroxide (H₂O₂) is a very important subject. Early reported processes, however, require hydrogen (H₂) and palladium‐based catalysts. Herein we propose a photocatalytic process for H₂O₂ synthesis driven by metal‐free catalysts with earth‐abundant water and molecular oxygen (O₂) as resources under sunlight irradiation (λ>400 nm). We use graphitic carbon nitride (g‐C₃N₄) containing electron‐deficient aromatic diimide units as catalysts. Incorporating the diimide units positively shifts the valence‐band potential of the catalysts, while maintaining sufficient conduction‐band potential for O₂ reduction. Visible light irradiation of the catalysts in pure water with O₂ successfully produces H₂O₂ by oxidation of water by the photoformed valence‐band holes and selective two‐electron reduction of O₂ by the conduction band electrons.     

A Self‐Assembled Metallomacrocycle Singlet Oxygen Sensitizer for Photodynamic Therapy

Although metal‐ion‐directed self‐assembly has been widely used to construct a vast number of macrocycles and cages, it is only recently that the biological properties of these systems have begun to be explored. However, up until now, none of these studies have involved intrinsically photoexcitable self‐assembled structures. Herein we report the first metallomacrocycle that functions as an intracellular singlet oxygen sensitizer. Not only does this Ru₂Re₂ system possess potent photocytotoxicity at light fluences below those used for current medically employed systems, it offers an entirely new paradigm for the construction of sensitizers for photodynamic therapy.     

Double‐Side Co‐Catalytic Activation of Anodic TiO2 Nanotube Membranes with Sputter‐Coated Pt for Photocatalytic H2 Generation from Water/Methanol Mixtures

Self‐standing TiO₂ nanotube layers in the form of membranes are fabricated by self‐organizing anodization of Ti metal and a potential shock technique. The membranes are then decorated by sputtering different Pt amounts i) only at the top, ii) only at the bottom or iii) at both top and bottom of the tube layers. The Pt‐decorated membranes are transferred either in tube top‐up or in tube top‐down configuration onto FTO slides and are investigated, after crystallization, as photocatalysts for H₂ generation using either front or back‐side light irradiation. Double‐side Pt‐decoration of the tube membranes leads to higher H₂ generation rates (independently of tube and light‐irradiation configuration) compared to membranes decorated at only one side with similar overall Pt amounts. The results suggest that this effect cannot be only ascribed to the overall amount of Pt co‐catalyst as such but also to its distribution at both tube extremities. This leads to optimized light absorption and electron diffusion/transfer dynamics: the central part of the membranes acts as light‐harvesting zone and electrons therein generated can diffuse towards the Pt/TiO₂ active zones (tube extremities) where they can react with the environment and generate H₂.     

A Glutathione‐Activated Phthalocyanine‐Based Photosensitizer for Photodynamic Therapy

A zinc(II) phthalocyanine substituted with a 2,4‐dinitrobenzenesulfonate group has been prepared. Its fluorescence emission and reactive oxygen species generation can be greatly enhanced by glutathione in phosphate‐buffered saline and inside MCF‐7 cells. This compound thus functions as a highly efficient molecular‐based activatable photosensitizer.