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

Abstract

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 SmI 2 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 Li 0 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.

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

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Redox Reactions at Colloidal Semiconductor Nanocrystal Surfaces

“…This area has advanced significantly over the past decade, including efforts in studying interfacial photoinduced charge transfer rates and the role of ligand shell permeability in several different NC systems . Colloidal NCs have also been used in more modern organic photoredox reactions. − Opportunities and challenges in this area have been discussed in recently published perspective articles. − …”

Gradient Alloyed Quantum Dots for Photocatalytic Lignin Valorization via Proton Coupled Electron Transfer

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