Lateral charge migration in 1D semiconductor-metal hybrid photocatalytic systems.

Colloidal nanorods based on CdS or CdSe, functionalized with metal particles, have proven to be efficient catalysts for light-driven hydrogen evolution. Seeded CdSe@CdS nanorods have shown increasing performance with increasing rod length. This observation was rationalized by the increasing lifetime of the separated charges, as a large distance between holes localized in the CdSe seed and electrons localized at the metal tip decreases their recombination rate. However, the impact of nanorod length on the electron-to-tip localization efficiency or pathway remained an open question. Therefore, we investigated the photo-induced electron transfer to the metal in a series of Ni-tipped CdSe@CdS nanorods with varying length. We find that the transfer processes occurring from the region close to the semiconductor-metal interface, the rod region, and the CdSe seed region depend in different ways on the rods' length. The rate of the fastest process from excitonic states generated directly at the interface is independent of the rod length, but the relative amplitude decreases with increasing rod length, as the weight of the interface region is decreasing. The transfer of electrons to the metal tip from excitons generated in the CdS rod region depends strongly on the length of the nanorods, which indicates an electron transport-limited process, i.e., electron diffusion toward the interface region, followed by fast interface crossing. The transfer originating from the CdSe excitonic states again shows no significant length dependence in its time constant, as it is probably limited by the rate of overcoming the shallow confinement in the CdSe seed.

Curcuminoid-BF2 complexes: Synthesis, fluorescence and optimization of BF2 group cleavage.

Eight difluoroboron complexes of curcumin derivatives carrying alkyne groups containing substituents have been synthesized following an optimised reaction pathway. The complexes were received in yields up to 98% and high purities. Their properties as fluorescent dyes have been investigated. Furthermore, a strategy for the hydrolysis of the BF2 group has been established using aqueous methanol and sodium hydroxide or triethylamine.

Covalent Photosensitizer-Polyoxometalate-Catalyst Dyads for Visible-Light-Driven Hydrogen Evolution.

A general concept for the covalent linkage of coordination compounds to bipyridine-functionalized polyoxometalates is presented. The new route is used to link an iridium photosensitizer to an Anderson-type hydrogen-evolution catalyst. This covalent dyad catalyzes the visible-light-driven hydrogen evolution reaction (HER) and shows superior HER activity compared with the non-covalent reference. Hydrogen evolution is observed over periods >1 week. Spectroscopic, photophysical, and electrochemical analyses give initial insight into the stability, electronic structure, and reactivity of the dyad. The results demonstrate that the proposed linkage concept allows synergistic covalent interactions between functional coordination compounds and reactive molecular metal oxides.

Pyrimidinone: versatile Trojan horse in DNA photodamage?

(6-4) Photolesions between adjacent pyrimidine DNA bases are prone to secondary photochemistry. It has been shown that singlet excited (6-4) moieties form Dewar valence isomers as well as triplet excitations. We here report on the triplet state of a minimal model for the (6-4) photolesion, 1-methyl-2(1H)-pyrimidinone. Emphasis is laid on its ability to abstract hydrogen atoms from alcohols and carbohydrates. Steady-state and time-resolved experiments consistently yield bimolecular rate constants of ∼10(4) M(-1) s(-1) for the hydrogen abstraction. The process also occurs intramolecularly as experiments on zebularine (1-(ß-d-ribofuranosyl)-2(1H)-pyrimidinone) show.

Screening Cobalt-based Catalysts on Multicomponent CdSe@CdS Nanorods for Photocatalytic Hydrogen Evolution in Aqueous Media.

We present CdSe@CdS nanorods coated with a redox-active polydopamine (PDA) layer functionalized with cobaloxime-derived photocatalysts for efficient solar-driven hydrogen evolution in aqueous environments. The PDA-coating provides reactive groups for the functionalization of the nanorods with different molecular catalysts, facilitates charge separation and transfer of electrons from the excited photosensitizer to the catalyst, and reduces photo-oxidation of the photosensitizer. X-ray photoelectron spectroscopy (XPS) confirms the successful functionalization of the nanorods with cobalt-based catalysts, whereas the catalyst loading per nanorod is quantified by total reflection X-ray fluorescence spectrometry (TXRF). A systematic comparison of different types of cobalt-based catalysts was carried out, and their respective performance was analyzed in terms of the number of nanorods and the amount of catalyst in each sample [turnover number, (TON)]. This study shows that the performance of these multicomponent photocatalysts depends strongly on the catalyst loading and less on the specific structure of the molecular catalyst. Lower catalyst loading is advantageous for increasing the TON because the catalysts compete for a limited number of charge carriers at the nanoparticle surface. Therefore, increasing the catalyst loading relative to the absolute amount of hydrogen produced does not lead to a steady increase in the photocatalytic activity. In our work, we provide insights into how the performance of a multicomponent photocatalytic system is determined by the intricate interplay of its components. We identify the stable attachment of the catalyst and the ratio between the catalyst and photosensitizer as critical parameters that must be fine-tuned for optimal performance.

Covalent Functionalization of CdSe Quantum Dot Films with Molecular [FeFe] Hydrogenase Mimics for Light-Driven Hydrogen Evolution.

CdSe quantum dots (QDs) combined with [FeFe] hydrogenase mimics as molecular catalytic reaction centers based on earth-abundant elements have demonstrated promising activity for photocatalytic hydrogen generation. Direct linking of the [FeFe] hydrogenase mimics to the QD surface is expected to establish a close contact between the [FeFe] hydrogenase mimics and the light-harvesting QDs, supporting the transfer and accumulation of several electrons needed to drive hydrogen evolution. In this work, we report on the functionalization of QDs immobilized in a thin-film architecture on a substrate with [FeFe] hydrogenase mimics by covalent linking via carboxylate groups as the anchoring functionality. The functionalization was monitored via UV/vis, photoluminescence, IR, and X-ray photoelectron spectroscopy and quantified via micro-X-ray fluorescence spectrometry. The activity of the functionalized thin film was demonstrated, and turn-over numbers in the range of 360-580 (short linkers) and 130-160 (long linkers) were achieved. This work presents a proof-of-concept study, showing the potential of thin-film architectures of immobilized QDs as a platform for light-driven hydrogen evolution without the need for intricate surface modifications to ensure colloidal stability in aqueous environments.

The Other Dimension-Tuning Hole Extraction via Nanorod Width.

Solar-to-hydrogen generation is a promising approach to generate clean and renewable fuel. Nanohybrid structures such as CdSe@CdS-Pt nanorods were found favorable for this task (attaining 100% photon-to-hydrogen production efficiency); yet the rods cannot support overall water splitting. The key limitation seems to be the rate of hole extraction from the semiconductor, jeopardizing both activity and stability. It is suggested that hole extraction might be improved via tuning the rod's dimensions, specifically the width of the CdS shell around the CdSe seed in which the holes reside. In this contribution, we successfully attain atomic-scale control over the width of CdSe@CdS nanorods, which enables us to verify this hypothesis and explore the intricate influence of shell diameter over hole quenching and photocatalytic activity towards H2 production. A non-monotonic effect of the rod's diameter is revealed, and the underlying mechanism for this observation is discussed, alongside implications towards the future design of nanoscale photocatalysts.

Ultrafast Electron Transfer from CdSe Quantum Dots to an [FeFe]-Hydrogenase Mimic.

The combination of CdSe nanoparticles as photosensitizers with [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation with reported turnover numbers in the order of 104-106. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here, we investigate the time scale of light-induced electron transfer kinetics for a simple model system consisting of CdSe quantum dots (QDs) of 2.0 nm diameter and a simple [FeFe]-hydrogenase mimic adsorbed to the QD surface under noncatalytic conditions. Our (time-resolved) spectroscopic investigation shows that both hot electron transfer on a sub-ps time scale and band-edge electron transfer on a sub-10 ps time scale from photoexcited QDs to adsorbed [FeFe]-hydrogenase mimics occur. Fast recombination via back electron transfer is observed in the absence of a sacrificial agent or protons which, under real catalytic conditions, would quench remaining holes or could stabilize the charge separation, respectively.

Rhodium-Complex-Functionalized and Polydopamine-Coated CdSe@CdS Nanorods for Photocatalytic NAD+ Reduction.

We report on a photocatalytic system consisting of CdSe@CdS nanorods coated with a polydopamine (PDA) shell functionalized with molecular rhodium catalysts. The PDA shell was implemented to enhance the photostability of the photosensitizer, to act as a charge-transfer mediator between the nanorods and the catalyst, and to offer multiple options for stable covalent functionalization. This allows for spatial proximity and efficient shuttling of charges between the sensitizer and the reaction center. The activity of the photocatalytic system was demonstrated by light-driven reduction of nicotinamide adenine dinucleotide (NAD+) to its reduced form NADH. This work shows that PDA-coated nanostructures present an attractive platform for covalent attachment of reduction and oxidation reaction centers for photocatalytic applications.