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Author: Micheel, Mathias × Type: Text ×

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Now showing 1 - 4 of 4
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    Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods
    (Basel : MDPI, 2020) Micheel, Mathias; Liu, Bei; Wächtler, Maria
    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.
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    Curcuminoid–BF2 complexes: Synthesis, fluorescence and optimization of BF2 group cleavage
    (Frankfurt a.M. : Beilstein-Institut, 2017) Weiß, Henning; Reichel, Jeannine; Görls, Helmar; Schneider, Kilian R.A.; Micheel, Mathias; Pröhl, Michael; Gottschaldt, Michael; Dietzek, Benjamin; Weigand, Wolfgang
    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.
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    Pyrimidinone: Versatile Trojan horse in DNA photodamage?
    (Heidelberg : Springer, 2015) Micheel, Mathias; Torres Ziegenbein, Christian; Gilch, Peter; Ryseck, Gerald
    (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.
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    The Other Dimension—Tuning Hole Extraction via Nanorod Width
    (Basel : MDPI, 2022) Rosner, Tal; Pavlopoulos, Nicholas G.; Shoyhet, Hagit; Micheel, Mathias; Wächtler, Maria; Adir, Noam; Amirav, Lilac
    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.