Photoinduced Charge Transfer from Quantum Dots Measured by Cyclic Voltammetry

Interfacial Charge Transfer Regulates Photoredox Catalysis

Photoredox catalytic processes offer the potential for precise chemical reactions using light and materials. The central determinant is identified as interfacial charge transfer, which simultaneously engenders distinctive behavior in the overall reaction. An in-depth elucidation of the main mechanism and highlighting of the complexity of interfacial charge transfer can occur through both diffusive and direct transfer models, revealing its potential for sophisticated design in complex transformations. The fundamental photophysics uncover these comprehensive applications and offer a clue for future development. This research contributes to the growing body of knowledge on interfacial charge transfer in photoredox catalysis and sets the stage for further exploration of this fascinating area of research.

Ultrafast Electron Transfer from CuInS2 Quantum Dots to a Molecular Catalyst for Hydrogen Production: Challenging Diffusion Limitations

Systems integrating quantum dots with molecular catalysts are attracting ever more attention, primarily owing to their tunability and notable photocatalytic activity in the context of the hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). CuInS2 (CIS) quantum dots (QDs) are effective photoreductants, having relatively high-energy conduction bands, but their electronic structure and defect states often lead to poor performance, prompting many researchers to employ them with a core-shell structure. Molecular cobalt HER catalysts, on the other hand, often suffer from poor stability. Here, we have combined CIS QDs, surface-passivated with l-cysteine and iodide from a water-based synthesis, with two tetraazamacrocyclic cobalt complexes to realize systems which demonstrate high turnover numbers for the HER (up to >8000 per catalyst), using ascorbate as the sacrificial electron donor at pH = 4.5. Photoluminescence intensity and lifetime quenching data indicated a large degree of binding of the catalysts to the QDs, even with only ca. 1 μM each of QDs and catalysts, linked to an entirely static quenching mechanism. The data was fitted with a Poissonian distribution of catalyst molecules over the QDs, from which the concentration of QDs could be evaluated. No important difference in either quenching or photocatalysis was observed between catalysts with and without the carboxylate as a potential anchoring group. Femtosecond transient absorption spectroscopy confirmed ultrafast interfacial electron transfer from the QDs and the formation of the singly reduced catalyst (CoII state) for both complexes, with an average electron transfer rate constant of ≈ (10 ps)-1. These favorable results confirm that the core tetraazamacrocyclic cobalt complex is remarkably stable under photocatalytic conditions and that CIS QDs without inorganic shell structures for passivation can act as effective photosensitizers, while their smaller size makes them suitable for application in the sensitization of, inter alia, mesoporous electrodes.

Ligand Desorption and Fragmentation in Oleate-Capped CdSe Nanocrystals under High-Intensity Photoexcitation

Semiconductor nanocrystals (NCs) offer prospective use as active optical elements in photovoltaics, light-emitting diodes, lasers, and photocatalysts due to their tunable optical absorption and emission properties, high stability, and scalable solution processing, as well as compatibility with additive manufacturing routes. Over the course of experiments, during device fabrication, or while in use commercially, these materials are often subjected to intense or prolonged electronic excitation and high carrier densities. The influence of such conditions on ligand integrity and binding remains underexplored. Here, we expose CdSe NCs to laser excitation and monitor changes in oleate that is covalently attached to the NC surface using nuclear magnetic resonance as a function of time and laser intensity. Higher photon doses cause increased rates of ligand loss from the particles, with upward of 50% total ligand desorption measured for the longest, most intense excitation. Surprisingly, for a range of excitation intensities, fragmentation of the oleate is detected and occurs concomitantly with formation of aldehydes, terminal alkenes, H2, and water. After illumination, NC size, shape, and bandgap remain constant although low-energy absorption features (Urbach tails) develop in some samples, indicating formation of substantial trap states. The observed reaction chemistry, which here occurs with low photon to chemical conversion efficiency, suggests that ligand reactivity may require examination for improved NC dispersion stability but can also be manipulated to yield desired photocatalytically accessed chemical species.

EPR spin trapping of nucleophilic and radical reactions at colloidal metal chalcogenide quantum dot surfaces

The participation of the surfaces of colloidal semiconductor nanocrystal quantum dots (QDs) in QD-mediated photocatalytic reactions is an important factor that distinguishes QDs from other photosensitizers (e.g. transition metal complexes or organic dyes). Here, we probe nucleophilic and radical reactivity of surface sulfides and selenides of metal chalcogenide (CdSe, CdS, ZnSe, and PbS) QDs using chemical reactions and NMR spectroscopy. Additionally, the high sensitivity of EPR spectroscopy is adapted to study these surface-centered reactions through the use of spin traps like 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) under photoexcitation and thermal conditions. We demonstrate that DMPO likely adds to CdSe QD surfaces under thermal conditions by a nucleophilic mechanism in which the surface chalcogenides add to the double bond, followed by further oxidation of the surface-bound product. In contrast, CdS QDs more readily form surface sulfur-centered radicals that can perform reactions including alkene isomerization. These results indicate that QD surfaces should be an important consideration for the design of photocatalysis beyond simply tuning QD semiconductor band gaps.

Lewis Acids and Electron-Withdrawing Ligands Accelerate CO Coordination to Dinuclear CuI Compounds

A series of dinuclear molecular copper complexes were prepared and used to model the binding and Lewis acid stabilization of CO in heterogeneous copper CO₂ reduction electrocatalysts. Experimental studies (including measurement of rate and equilibrium constants) and electronic structure calculations suggest that the key kinetic barrier for CO binding may be a σ-interaction between Cu^I and the incoming CO ligand. The rate of CO coordination can be increased upon the addition of Lewis acids or electron-withdrawing substituents on the ligand backbone. Conversely, K_eq for CO coordination can be increased by adding electron density to the metal centers of the compound, consistent with stronger π-backbonding. Finally, the electrochemically measured kinetic results were mapped onto an electrochemical zone diagram to illustrate how these system changes enabled access to each zone.

Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it.

Photoinduced Ligand-to-Metal Charge Transfer of Cobaltocene: Radical Release and Catalytic Cyclotrimerization

Irradiation of cobalt metallocenes at the ligand-to-metal charge transfer energies results in the labilization of the cyclopentadienyl-cobalt bond and radical release. The cyclopentadienyl radical is detected by electron paramagnetic resonance (EPR) spectroscopy using a spin trap and can also be chemically trapped using hydrogen-atom-donating reagents. This reaction presents a new photochemical method of generating new cobalt complexes or of forming cyclopentadienyl cobalt(I) species that are active for catalytic [2 + 2 + 2] cyclotrimerization reactions. More importantly, these results also show that cobaltocene should not be considered as a photostable redox reagent under many conditions, including those relevant to photovoltaics or photocatalysis.