Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

In this study, the impact of the type of ligand at the surface of colloidal CdSe@CdS dotin-rod nanostructures on the basic exciton relaxation and charge localization processes is closely examined. These systems have been introduced into the field of artificial photosynthesis as potent photosensitizers in assemblies for light driven hydrogen generation. Following photoinduced exciton generation, electrons can be transferred to catalytic reaction centers while holes localize into the CdSe seed, which can prevent charge recombination and lead to the formation of longlived charge separation in assemblies containing catalytic reaction centers. These processes are in competition with trapping processes of charges at surface defect sites. The density and type of surface defects strongly depend on the type of ligand used. Here we report on a systematic steadystate and time-resolved spectroscopic investigation of the impact of the type of anchoring group (phosphine oxide, thiols, dithiols, amines) and the bulkiness of the ligand (alkyl chains vs. poly(ethylene glycol) (PEG)) to unravel trapping pathways and localization efficiencies. We show that the introduction of the widely used thiol ligands leads to an increase of hole traps at the surface compared to trioctylphosphine oxide (TOPO) capped rods, which prevent hole localization in the CdSe core. On the other hand, steric restrictions, e.g., in dithiolates or with bulky side chains (PEG), decrease the surface coverage, and increase the density of electron trap states, impacting the recombination dynamics at the ns timescale. The amines in poly(ethylene imine) (PEI) on the other hand can saturate and remove surface traps to a wide extent. Implications for catalysis are discussed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Photophysical characterization of dynamically linked polymers for self-healing applications

The thesis at hand deals with the spectroscopic and photophysical characterization of dynamically linked polymers, so called dynamers. Dynamers have already found extensive use in the fields of sensor systems or self-healing materials, but the combination of their dynamic chemistry with optical properties such as absorption or emission is still an open field of research. The present thesis is divided in three main parts. First, dynamers that rely on the Diels-Alder functionality were investigated by steady-state and time-resolved, i.e. transient absorption and time-resolved emission, spectroscopies. The chromophores embedded in the polymer scaffold belonged to the class of oligo(arylene ethynylene)s, which are known for their pronounced emission properties. In particular, the influence of the dynamer structure on their ground- and excited state properties both in solution as well as in thin films was studied. Additionally, energy transfer experiments in different polymer compositions were conducted. Second, an imine based polymer was probed with regards to its photostability. Utilizing different excitation energies and solvent properties, different photochemical deactivation pathways were found. Last, an imine-based polymer system was investigated that could partially self-heal its absorption properties after photodamage. Different polymer sructures which affected polymer mobility in films were tested and general rules for the design of optically active self-healing polymers were derived. Keywords: dynamic polymers, dynamic chemistry, self-healing, time-resolved spectroscopy, transient absorption spectroscopy, energy transfer, arylene ethynylene, phenylene ethynylene, photoisomerization, photooxidation

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 ∼104 M−1 s−1 for the hydrogen abstraction. The process also occurs intramolecularly as experiments on zebularine (1-(β-D-ribofuranosyl)-2(1H)-pyrimidinone) show

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

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

Psycho-social stress and social support systems - an empirical reconstruction of the diversity of young people\u27s need for help and support

Angesichts gesellschaftlicher Modernisierung und der Entstrukturierung der Jugendphase steht die Jugendhilfe vor der Herausforderung, sich ihrer Bezugspunkte in den konkreten Lebensbedingungen ihrer Adressaten zu vergewissern. Insbesondere in den neuen Bundesländern sind, vor dem Hintergrund der gesellschaftlichen Transformationsprozesse, die Bedingungen des Aufwachsens einer Dynamik ausgesetzt, die die traditionellen Referenzpunkte verschwinden läßt. Anhand einer empirischen Rekonstruktion unterschiedlicher Lebenslagen Jugendlicher und junger Erwachsener in den neuen Bundesländern werden dabei die vielschichtigen und komplexen Anforderungen an die Lebensplanung und -gestaltung sowie die diversifizierten kulturellen und sozialen Bedürfnisse junger Menschen herausgearbeitet. (DIPF/Text übernommen)In view of the social modernization and the destructuring of the phase of adolescence, youth welfare is confronted with the need to ensure itself of its points of reference in the concrete living conditions of its addressees. Especially in the new Laender and against the background of the processes of social transformation, the conditions of growing up are subject to a dynamics which causes the traditional points of reference to disappear. On the basis of an empirical reconstruction of different living conditions of adolescents and young adults in the new Laender, the authors analyze the complex requirements concerning the planning and the design of individual lives as well as the diversified cultural and social needs of young people. (DIPF/Orig.

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 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 towards the interface region followed by fast interface crossing. The transfer originating from 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

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

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

The combination of CdSe nanoparticles as photosensitizers and [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here we investigate the timescale of light-induced electron transfer between CdSe quantum dots and a simple [FeFe]-hydrogenase mimic adsorbed on the surface of the quantum dot under non-catalytic conditions. Our time-resolved spectroscopic investigation shows that hot electron transfer on a sub-ps timescale and band-edge electron transfer on a sub-10-ps timescale occurs. Fast recombination 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. <br /

The Other Dimension&mdash;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&rsquo;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&rsquo;s diameter is revealed, and the underlying mechanism for this observation is discussed, alongside implications towards the future design of nanoscale photocatalysts