Photocatalytic C-H Activation with Alcohol as a Hydrogen Atom Transfer Agent in a 9-Fluorenone Based Metal-Organic Framework

Visible light mediated photocatalysis by lanthanide metal-organic frameworks: enhanced specificity and mechanistic insights

The utilization of earth-abundant photosensitizers with visible light absorption to enable sustainable photocatalysis is a long-standing challenge. Overcoming such a challenge, in this work, two lanthanide (Ln3+ = Tb, Eu) based metal-organic frameworks (Ln-MOFs) have been synthesized utilizing a Co3+-based metalloligand. Both Ln-MOFs function as remarkable photocatalysts for the selective oxidation of assorted alcohols and sulfides to their corresponding aldehydes/ketones and sulfoxides using visible light. The photophysical behavior of both Ln-MOFs and mechanism of photocatalysis is comprehensively investigated using time-resolved transient absorption spectroscopy, electrochemical impedance spectroscopy, electron paramagnetic resonance spectroscopy, photoluminescence and phosphorescence studies. In both Ln-MOFs, a metalloligand acts as a light-harvester, being excited by visible light, while Ln3+ ions endow the resulting MOFs with long-lived triplet excited states. Ultrafast transient absorption spectroscopy, further supported by electron paramagnetic resonance spectra, revealed excited-state electron transfer from metalloligands to the Ln3+ ions and transient generation of Ln2+ sites alongside the facilitation of intersystem crossing. The excited Ln2+ ions transfer energy to the ground-state triplet oxygen (3O2) to generate singlet oxygen (1O2). The HOMO-LUMO positions of both Ln-MOFs support the generation of ˙O2 - and 1O2 but inhibit strongly-oxidizing yet non-selective ˙OH radicals. Scavenger experiments, 1O2 traps and electron paramagnetic resonance spectra confirmed the generation of singlet oxygen. The heavy-metal effect of a lanthanide ion in Ln-MOFs for the generation of triplet excitons is confirmed by the synthesis of a non-heavy-metal analogue involving a zinc ion via a single-crystal-to-single-crystal transformation strategy. The present results are noteworthy and may aid in the development of other earth-abundant metalloligand-based photocatalysts for challenging yet sustainable catalysis.

A bioactive sprite: Recent advances in the application of vinyl sulfones in drug design and organic synthesis

Vinyl sulfones, with their exceptional chemical properties, are known as the "chameleons" of organic synthesis and are widely used in the preparation of various sulfur-containing structures. However, their most alluring feature lies in their biological activity. The vinyl sulfone skeleton is ubiquitous in natural products and drug molecules and boasts a unique molecular structure and drug activity when compared to conventional drug molecules. As a result, vinyl sulfones have been extensively studied, playing a critical role in organic synthesis and pharmaceutical chemistry. In this review, we present a comprehensive analysis of the recent applications of vinyl sulfone structures in drug design, biology, and chemical synthesis. Furthermore, we explore the prospects of vinyl sulfones in diverse fields, offering insight into their potential future applications.

Photocatalytic Generation of H2O2 Via a Hydrogen-Abstraction Pathway by Bi2.15WO6 under Visible Light

Photocatalytic technology is a popular research area for converting solar energy into environmentally friendly chemicals and is considered the greenest approach for producing H2O2. However, the corresponding reactive oxygen species (ROS) and pathway involved in the photocatalytic generation of H2O2 by the Bi2.15WO6-glucose system are still not clear. Quenching experiments have established that neither •OH nor h+ contribute to the formation of H2O2, and show that the formed surface superoxo (≡Bi-OO•) and peroxo (≡Bi-OOH) species are the predominant ROS in H2O2 generation. In addition, various characterizations indicate the enhanced electron-transfer on the surface of Bi2.15WO6 with increasing contents of glucose via the ligand-to-metal charge transfer pathway, confirming H-transfer from glucose to ≡Bi-OO• or ≡Bi-OOH. The increased production of H2O2 with decreasing bond dissociation energy (BDEO-H) values of various phenolic compounds again supports the H-transfer mechanism from phenolic compounds to ≡Bi-OO• and then to ≡Bi-OOH. DFT calculations further reveal that on the Bi2.15WO6 surface, oxygen is sequentially reduced to ≡Bi-OO• and ≡Bi-OOH, while H-transfer from H2O or glucose to ≡Bi-OO• and ≡Bi-OOH, resulting in the production of H2O2. The lower energy barrier of H-transfer from adsorbed glucose (0.636 eV) than that from H2O (1.157 eV) indicates that H-transfer is more favorable from adsorbed glucose. This work gives new insight into the photocatalytic generation of H2O2 by Bi2.15WO6 in the presence of glucose/phenolic compounds via the H-abstraction pathway.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Truxene-to-Fluorenone Energy Transfer in a Robust Mesoporous Zn-MOF

A new metal-organic framework (MOF; [Zn4O(hett)4/3(fluo)1/2(bdc)1/2]n; TFT-MOF) constructed on chromophoric ligands 5,5',10,10',15,15'-hexaethyltruxene-2,7,12-triacetate (hett), 9-fluorenone-2,7-dicarboxylate (fluo), terephthalate (bdc), and the Zn4O node has been prepared and identified by powder X-ray diffraction. This luminescent MOF exhibits large mesoporous pores of 2.7 nm based on computer modeling using density functional theory (DFT) calculations. The steady-state and time-resolved fluorescence spectra and photophysical parameters of TFT-MOF have been investigated and compared with those of the free ligands and their basic chromophores. All in all, TFT-MOF exhibits particularly efficient singlet-singlet energy-transfer processes described as 1(hett)* → (fluo) and 1(bdc)* → (fluo), leading to fluorescence arising for the fluo lumophore operating only through Förster resonance energy transfer (FRET) with an efficiency of transfer of up to >95%. This experimental conclusion was corroborated by DFT and time-dependent DFT (TDDFT). For the 1(hett)* → (fluo) process, the approximated overall rate constant of energy transfer was evaluated to be at most 2.04 × 1010 s-1 (using a Stern-Volmer approach of solution data and the relationship between distance and concentration). This process was analyzed using the Förster theory, where two intrapore energy transfer paths of center-to-center distances of 13 and 25 Å have been identified. TFT-MOF photosensitizes the formation of singlet oxygen (1O2 (1Σg)) as detected by its phosphorescence signal at 1275 nm.

Blending Aryl Ketone in Covalent Organic Frameworks to Promote Photoinduced Electron Transfer

Aryl-ketone derivatives have been acknowledged as promising organic photocatalysts for photosynthesis. However, they are limited by their photostability and have been less explored for photoinduced electron transfer (PET) applications. Herein we demonstrate a novel strategy to cover the shortage of aryl-ketone photocatalysts and control the photoreactivity by implanting symmetric aryl ketones into the conjugated covalent organic frameworks (COFs). To prove the concept, three comparative materials with the same topology and varied electronic structures were built, adopting truxenone knot and functionalized terephthalaldehyde linkers. Spectroscopic investigation and excited carrier dynamics analysis disclosed improvements in the photostability and electronic transfer efficiency as well as the structure-performance relationships toward N-aryl tetrahydroisoquinoline oxidation. This system provides a robust rule of thumb for designing new-generation aryl-ketone photocatalysts.

Co-occurrence of polycyclic aromatic hydrocarbons and their oxygenated derivatives in indoor dust from various microenvironments in Guangzhou, China: levels, sources, and potential human health risk

For decades, the presence and potential health risk of polycyclic aromatic hydrocarbons (PAHs) in indoor dust have been extensively investigated while with limited attention to oxygenated PAHs (OPAHs). In this study, we collected 45 indoor dust from four microenvironments in Guangzhou City, China, and then focused on the co-occurrence of 16 PAHs and 8 OPAHs and their potential carcinogenic risk to humans. The ΣPAHs concentrations, dominated by 4-6 ring PAHs, ranged from 1761 to 14,290 ng/g (mean of 6058 ng/g) without significant difference in the different microenvironments (Tukey, p > 0.05). The OPAHs were observed with concentrations from 250 to 5160 ng/g (mean of 1646 ng/g), and anthraquinone (AQ) was identified as the main OPAHs with significantly high levels in the residential environment than in instrumental rooms. Notably, AQ dominated over the other target analytes in dust in this study. Our results indicated that PAHs and OPAHs in indoor dust were from outdoor environments, which mainly originated from vehicular exhaust and biomass/coal combustion. A potential cancer risk of PAHs and OPAHs to local adults and children was observed via inhalation, ingestion, and dermal absorption, with the main contribution from benzo[a]pyrene and dibenz[a,h]anthracene.

Application of Copper(II)-Organic Frameworks Bearing Dilophine Derivatives in Photocatalysis and Guest Separation

The functionalized design of metal-organic frameworks (MOFs) has been rapidly developed in the last 20 years, and its broad applicability has been demonstrated in many fields. MOFs with desired functions can be assembled using predesigned organic linkers with specific metal nodes, which possess the ordered functional sites and open structures. Although a large number of carboxylic acid junctions have been used to construct MOFs, it is still a great challenge to realize their multifunctionality. In particular, there is a relative lack of research on MOFs as direct photocatalysts, which require not only abundant active sites and open structures but also adsorption groups and effective electron-hole separation performance. To this end, MOFs constructed from the carboxylic acid ligands derived from lophine-based derivatives and copper ions were deliberately used as a photocatalyst, and then, their application in dye degradation and aromatic alcohol conversion was investigated. In addition, in combination with the abundant Lewis sites of copper ions and imidazole sites, the material shows not only the adsorption and separation of C₂ series and dyes but also the application of dye degradation and conversion of aromatic alcohols under illumination conditions. The corresponding results fully illustrate that the MOF constructed by using lophine derivatives can be an effective way to prepare photocatalysts. The subsequent research ideas will focus on designing a series of MOFs constructed with multilinked moieties of lophine groups and exploring their application strategies in the field of photocatalysis.

Ligand regulated metal–organic frameworks for synergistic photoredox and nickel catalysis

Synergistic photoredox and nickel catalytic cross-coupling systems have created a great attraction as a promising methodology to produce the aryl C−N bonds under mild conditions as well as extreme challenge,...

Post-Synthetic Modification of Metal-Organic Frameworks Bearing Phenazine Radical Cations for aza-Diels-Alder Reactions

Metal-organic frameworks (MOFs) consisting of organic radicals are of great interest because they have exhibited unique and intriguing optical, electronic, magnetic, and chemo-catalytic properties, and thus have demonstrated great potential applications in optical, electronic, and magnetic devices, and as catalysts. However, the preparation of MOFs bearing stable organic radicals is very challenging because most organic radicals are highly reactive and difficult to incorporate into the framework of MOFs. Herein we reported a post-synthetic modification strategy to prepare a novel MOF containing phenazine radical cations, which was used as heterogeneous catalyst for aza-Diels-Alder reaction. The zinc-based metal-organic framework Zn₂ (PHZ)₂ (dabco) (N) was successfully synthesized from 5,10-di(4-benzoic acid)-5,10-dihydrophenazine (PHZ), triethylene diamine (dabco) with Zn(NO₃ )₂  ⋅ 6H₂ O by solvothermal method. The as-synthesized MOF N was partially oxidized by AgSbF₆ to form MOF R containing ∼10% phenazine radical cation species. The resultant MOF R was found to keep the original crystal type of N and very persistent under ambient conditions. Consequently, MOF R was successfully employed in radical cation-catalyzed aza-Diels-Alder reactions with various imine substrates at room temperature with high reaction conversion. Moreover, heterogeneous catalyst MOF R was reusable up to five times without much loss of catalytic activity, demonstrating its excellent stability and recyclability. Therefore, the post-synthetic modification developed in this work is expected to become a versatile strategy to prepare radical-based MOFs for the application of heterogeneous catalysts in organic synthesis.