Quantum dot gels as efficient and unique photocatalysts for organic synthesis.

ConspectusMetal chalcogenide quantum dots (QDs) are prized for their unique and functional properties, associated with both intrinsic (quantum confinement) and extrinsic (high surface area) effects, as dictated by their size, shape, and surface characteristics. Thus, they have considerable promise for diverse applications, including energy conversion (thermoelectrics and photovoltaics), photocatalysis, and sensing. QD gels are macroscopic porous structures consisting of interconnected QDs and pore networks in which the pores may be filled with solvent (i.e., wet gels) or air (i.e., aerogels). QD gels are unique because they can be prepared as macroscale objects while fully retaining the size-specific quantum-confined properties of the initial QD building blocks. The extensive porosity of the gels also ensures that each QD in the gel network is accessible to the ambient, leading to high performance in applications that require high surface areas, such as (photo)catalysis and sensing.Metal chalcogenide QD gels are conventionally prepared by chemical approaches. We recently expanded the toolbox for QD gel synthesis by developing electrochemical gelation methods. Relative to conventional chemical oxidation approaches, electrochemical assembly of QDs (1) enables the use of two additional levers for tuning the QD assembly process and gel structure: electrode material and potential, and (2) allows direct gel formation on device substrates to simplify device fabrication and improve reproducibility. We have discovered two distinct electrochemical gelation methods, each of which enables the direct writing of gels on an active electrode surface or the formation of free-standing monoliths. Oxidative electrogelation of QDs leads to assemblies bridged by dichalcogenide (covalent) linkers, whereas metal-mediated electrogelation proceeds via electrodissolution of active metal electrodes to produce free ions that link QDs by binding to pendant carboxylate functionalities on surface ligands (non-covalent linkers). We further demonstrated that the electrogel composition produced from the covalent assembly could be modified by controlled ion exchange to form single-ion decorated bimetallic QD gels, a new category of materials. The QD gels exhibit unprecedented performance for NO2 gas sensing and unique photocatalytic reactivities (e.g., the "cyano dance" isomerization and the reductive ring-opening arylation). The chemistry unveiled during the development of electrochemical gelation pathways for QDs and their post-modification has broad implications for guiding the design of new nanoparticle assembly strategies and QD gel-based gas sensors and catalysts.

Electrochemical Gelation of Metal Chalcogenide Quantum Dots: Applications in Gas Sensing and Photocatalysis.

Visible-light photocatalytic carboxylation with CO2 is highly important. However, it still remains challenging for reluctant substrates with low reduction potentials. Herein, we report a novel photocatalytic carboxylation of C-N bonds in cyclic amines with CO2 via consecutive photo-induced electron transfer (ConPET). It is also the first photocatalytic reductive ring-opening reactionof azetidines, pyrrolidines and piperidines.This strategyis practical to transform a variety of easily available cyclic amines to valuable β-, γ-, δ- and ε-amino acids in moderate-to-excellent yields. Moreover, themethodalso features mild and transition metal-free conditions, high selectivity, good functional group tolerance, facile scalability and productderivations. Mechanistic studies indicate that the ConPET might be the key to generating highly reactive photocatalysts, which enable the reductiveactivation of cyclic aminesto generatecarbon radicals and carbanions as the key intermediates.

Photocatalytic Carboxylation of C-N Bonds in Cyclic Amines with CO2 by Consecutive Visible-Light-Induced Electron Transfer.

Colloidal quantum dots (QDs) can photocatalyze diverse organic reactions. However, reported QD-photocatalysts often contain highly-toxic elements Cd or Pb, and have not surpassed prototypical transition-metal complexes in terms of their photoredox power or excited-state energy. Here we report low-toxicity ZnSe/ZnS core/shell QDs as potent visible photocatalysts to drive challenging organic transformations. To overcome the limitation of short excited-state lifetime of the QDs, we functionalize their surfaces with benzophenone ligands which can rapidly extract electrons from photoexcited QDs and sustain long-lived charge-separated states. The benzophenone anions function as potent electron relay to drive dehalogenation of aryl chlorides and additive-free polymerization of acrylates. Alternatively, the QDs are functionalized with biphenyl ligands to store energy in long-lived, energetic triplets, enabling [2+2] homo-cycloaddition of styrene and cycloaddition of carbonyls with alkenes.

Low-Toxicity ZnSe/ZnS Quantum Dots as Potent Photoreductants and Triplet Sensitizers for Organic Transformations.

We investigated the reduction of organic compounds photocatalyzed by Mn2+-doped CdS/ZnS quantum dot (Mn:CdS/ZnS QD) films. The incorporation of Mn2+ ion into the CdS nanocrystals as a dopant promotes Auger...

Mn(II)-doped CdS/ZnS core/shell quantum dot film photocatalyzes reductive organic transformations with a boost in efficiency from enhanced Auger processes

Strong reducing agents (<-2.0 V vs saturated calomel electrode (SCE)) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI2 are commonly employed for these reactions; however, considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li0 and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases, low stability of catalytic intermediates can limit turnover numbers. Herein, we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue light-emitting diodes (LEDs) and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs SCE are photoreductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small-molecule catalysts, QDs are stable under these conditions and turnover numbers up to 47 500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of benzyl-protected alcohols, and reductive ring opening of cyclopropane carboxylic acid derivatives.

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CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Processes.

Semiconductor quantum dots (QDs) offer many advantages as photocatalysts for synthetic photoredox catalysis, but no reports have explored the use of QDs with nickel catalysts for C–C bond formation. We show here that 5.7 nm CdS QDs are robust photocatalysts for photoredox-promoted cross-electrophile coupling (turnover number (TON) up to 40,000). These conditions can be utilized on a small scale (96-well plate) or adapted to flow. NMR studies show that triethanolamine (TEOA)-capped QDs are the active catalyst and that TEOA can displace native phosphonate and carboxylate ligands, demonstrating the importance of the QD surface chemistry. 

Jonas K. Widness

Papers

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Processes.

CdS Quantum Dots for Metallaphotoredox-Enabled Cross-Electrophile Coupling of Aryl Halides with Alkyl Halides