publications

2024

1. ACS Appl. Nano Mater.

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

   Marcel Boecker, Sarah Lander, Riccarda Müller , and 10 more authors

   ACS Applied Nano Materials, Jun 2024

   Publisher: American Chemical Society

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   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.

2. ChemCatChem

   Molecular Dyad vs Multi-component Approach – Photocatalytic Hydrogen Evolution by Combining Oligothiophene Photosensitizers with [FeFe]-hydrogenase Mimics

   Chizuru Kasahara, Katharina Rediger, Mathias Micheel , and 5 more authors

   ChemCatChem, Jun 2024

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/cctc.202400247

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   A molecular dyad (PS-CAT) consisting of a metal-free photosensitiser (PS) and a [FeFe]-hydrogenase-inspired [Fe2S2] cluster (CAT) has been synthesized for hydrogen evolution under visible light irradiation. The electrochemical properties of PS-CAT in the absence and presence of acid were investigated by CV and IR-SEC. The ground state and excited state photophysical properties of PS and PS-CAT, and the non-chemically connected reference system [PS, CAT] were investigated to elucidate the effect of a covalent linkage between PS and CAT on electronic communication and hence performance. The photo-relaxation pathway of PS-CAT was revealed computationally by scalar-relativist time-dependent density functional theory simulations. These studies are complemented by comparative analyses of photocatalytic hydrogen evolution between PS-CAT and [PS, CAT] under visible light irradiation.

2023

1. J. Chem. Phys.

   Lateral charge migration in 1D semiconductor–metal hybrid photocatalytic systems

   Mathias Micheel, Kaituo Dong, Lilac Amirav , and 1 more author

   The Journal of Chemical Physics, Apr 2023

   Publisher: American Institute of Physics

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   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.

2. ACS Appl. Mater. Interfaces

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

   Stefan Benndorf, Alexander Schleusener, Riccarda Müller , and 9 more authors

   ACS Applied Materials & Interfaces, Apr 2023

   Publisher: American Chemical Society

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   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.

2022

1. Nanomaterials

   The Other Dimension—Tuning Hole Extraction via Nanorod Width

   Tal Rosner, Nicholas G. Pavlopoulos, Hagit Shoyhet , and 4 more authors

   Nanomaterials, Jan 2022

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   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.

2. Adv. Mater. Interfaces

   Assembly, Properties, and Application of Ordered Group II–VI and IV–VI Colloidal Semiconductor Nanoparticle Films

   Mathias Micheel, Raktim Baruah, Krishan Kumar , and 1 more author

   Advanced Materials Interfaces, Jan 2022

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/admi.202201039

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   Colloidal semiconductor nanocrystals, in particular cadmium and lead chalcogenide particles, are pivotal materials in many optoelectronic applications. While synthetic advances in the past decades now allow precise control of their size, shape, and composition, they must be processed from dispersion into functional films for many applications, which presents additional challenges. In this review, recent progresses in the field of nanoparticle self-assembly into functional thin films with precise control over nanoparticle order and orientation are discussed. A comprehensive overview on characterization methods used in the study of such ordered materials is provided and the gas–liquid interfacial self-assembly and electric field driven deposition as the two most promising ways to generate superstructures with defined thickness, order, and orientation are discussed. The superstructure–property relationships are then reviewed with emphasis on charge migration through the film and radiative processes. Last, challenges and open questions in the field are addressed.

2021

1. J. Phys. Chem. Lett.

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

   Alexander Schleusener, Mathias Micheel, Stefan Benndorf , and 3 more authors

   The Journal of Physical Chemistry Letters, May 2021

   Publisher: American Chemical Society

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   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.

2. NadC

   Konferenzen und Corona - Routineaufgaben, digitale Kaffeepausen, Chatdiskussionen

   Mathias Micheel, Julian Hniopek, Ludwig Schwiedrzik , and 4 more authors

   Nachrichten aus der Chemie, May 2021

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/nadc.20214106721

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   AbstractIn der fortdauernden Covid-19-Pandemie versuchen junge Wissenschaftler und Wissenschaftlerinnen, den akademischen Austausch digital aufrechtzuerhalten. Open-Source-Programme bieten dabei oft eine schnelle und machbare Alternative , um sich zu vernetzen.

3. ACS Appl. Nano Mater.

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

   Marcel Boecker, Mathias Micheel, Alexander K. Mengele , and 10 more authors

   ACS Applied Nano Materials, Dec 2021

   Publisher: American Chemical Society

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   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.

4. ChemRxiv

   How to Organize a Photocatalysis Conference Online (on a Budget)

   Mathias Micheel, Julian Hniopek, Elisabeth Hofmeister , and 4 more authors

   ChemRxiv, Jan 2021

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   Originally planned as an on-site meeting, the inaugural CataLight Young Scientist Symposium (CYSS) took place as a fully online conference in November 2020. Dedicated to various aspects of photocatalysis, namely synthesis, theory, characterization, and application, CYSS aimed to provide a stage for early-career scientists to connect to each other and present their research to peers in the field. While still keeping a traditional on-site conference format including both plenary and poster sessions, several minor and major changes had to be applied to the format to deliver a full experience. In this report, we highlight key steps in the organization of such an online conference, laying a focus on using mostly open source software to minimize costs, and discuss differences to both on-site and other online conference formats.

2020

1. Catalysts

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

   Mathias Micheel, Bei Liu, and Maria Wächtler

   Catalysts, Oct 2020

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   In this study, the impact of the type of ligand at the surface of colloidal CdSe@CdS dot-in-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 long-lived 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 steady-state 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.

2019

1. ChemPhotoChem

   Photophysics of a Bis-Furan-Functionalized 4,7-bis(Phenylethynyl)-2,1,3-benzothiadiazole: A Building Block for Dynamic Polymers

   Mathias Micheel, Johannes Ahner, Martha Frey , and 3 more authors

   ChemPhotoChem, Oct 2019

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   Dynamically linked polymers (dynamers) have attracted attention as a versatile building block for self-healing polymers, for example. However, research on combining optically active materials with dynamic chemistry is still in its infancy. Here, we report a highly emissive arylene ethynylene oligomer bearing a bis-furan functionalization, which makes it suitable for utilization in Diels−Alder dynamers. By combining steady-state and time-resolved absorption and emission spectroscopy from the femto- to the nanosecond timescale, the molecular dynamics of the excited state of the monomeric building block and the corresponding Diels−Alder polymer have been investigated in detail. Although in solution no pronounced differences in their photophysics were observed, thin films of the polymer exhibit a distinct dual emission with single chromophore and excimer character, respectively.

2. Thesis

   Photophysical characterization of dynamically linked polymers for self-healing applications

   Mathias Micheel

   Oct 2019

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   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.

2017

1. Macromol. Chem. Phys.

   Directed Orientation of Oligo(phenylene ethynylene)s Using Ureas or Urethanes in Rod–Coil Copolymers

   Johannes Ahner, Mathias Micheel, Marcel Enke , and 4 more authors

   Macromolecular Chemistry and Physics, Oct 2017

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/macp.201700343

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   Interchromophoric interactions between oligo(phenylene ethynylene)s incorporated in copolymers in solution as well as in solid state are investigated using UV/Vis and IR spectroscopy. One promising strategy to enable strong interchromophoric interactions is the introduction of hydrogen-bonding moieties. Together with the tendency of the chromophores to self-assemble via π–π-stacking a dense packing of the chromophores within a polymer film can be achieved. In order to demonstrate this strategy, oligo(phenylene ethynylene)s are used as monomers for copolymers using the highly efficient blocked isocyanate route and polymer films are prepared by spin coating technique. The copolymers are prepared by an in situ deprotection of a pyrazol protected bis-isocyanate functionalized rigid chromophore followed by subsequent polyaddition using a bis-amine or bis-alcohol linker unit. The specific introduction of urea as well as urethane moieties enhances coplanarization of the chromophores in the polymer films into well-organized structures, which can be characterized based on their specific optical properties.

2. Macromolecules

   Self-healing Functional Polymers: Optical Property Recovery of Conjugated Polymer Films by Uncatalyzed Imine Metathesis

   Johannes Ahner, Mathias Micheel, Robert Geitner , and 4 more authors

   Macromolecules, May 2017

   Publisher: American Chemical Society

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   The implementation of a self-healing functionality into materials has become a prevalent approach for materials which require long-term reliability. As of today, the restoration of mechanical properties has dominated the research on self-healing materials, whereas research on healing of other functionalities (e.g., conductivity or optical properties) is still in its infancy. Here, the first conjugated polymer, which can restore its optical properties after photodamage is reported. The proposed self-healing mechanism relies on a thermally triggered imine metathesis between the conjugated polymer and additional macromolecular healing agents with no catalyst needed.

3. Beilstein J. Org. Chem.

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

   Henning Weiss, Jeannine Reichel, Helmar Görls , and 6 more authors

   Beilstein Journal of Organic Chemistry, Oct 2017

   Publisher: Beilstein-Institut

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   Beilstein Journal of Organic Chemistry

2016

1. RSC Adv.

   Thermally triggered optical tuning of π-conjugated graft copolymers based on reversible Diels–Alder reaction

   J. Ahner, M. Micheel, J. Kötteritzsch , and 2 more authors

   RSC Advances, Oct 2016

   Publisher: The Royal Society of Chemistry

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   In order to design a π-conjugated polymer film with tunable optical properties by thermally triggered activation of energy transfer after processing, two monodisperse phenylene ethynylene based oligomers with different optical properties were synthesized and attached to aliphatic polymers as π-conjugated side chains. Subsequently, the exchange of the side chain chromophores between the prepared donor and acceptor graft polymers in the solid state based on a reversible Diels–Alder reaction was studied in detail. The resulting donor–acceptor graft copolymer exhibits intra polymer energy transfer upon excitation of the donor moiety. The photophysical properties of the original and exchanged graft copolymers were investigated by means of absorption and emission spectroscopy. This novel concept opens the possibility for optical tuning of π-conjugated polymer films after processing as well as applications as thermally triggered sensor systems.

2. Chem. Eur. J.

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

   Stefanie Schönweiz, Sebastian A. Rommel, Joachim Kübel , and 4 more authors

   Chemistry – A European Journal, Oct 2016

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   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 \textgreater1 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.

2015

1. Book Chapter

   Self-Healing Functional Polymeric Materials

   Johannes Ahner, Stefan Bode, Mathias Micheel , and 2 more authors

   In Self-healing Materials , Oct 2015

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   Self-healing materials have been intensively investigated in recent decades, whereby the healing process was mostly based on the restoration of mechanical properties after mechanical damage. However, self-healing functional polymeric materials have now become the focus of research. In recent years, several approaches have been developed for self-healing of conductivity as well as the restoration of optical properties. In contrast to the healing of mechanical properties, such as stiffness and strength, the self-healing of functional materials focuses on the restoration of functionalities after damage caused by harmful environments (e.g., high temperatures or irradiation). The ultimate goal is the investigation or mimicking of a multifunctional self-healing system (e.g., biological material). In this review, the current state of the art in self-healing functional polymeric materials is summarized. In particular, we discuss self-healing conductive materials, healable optoelectronics, and functional coatings.

2. Photochem. Photobiol. Sci.

   Pyrimidinone: versatile Trojan horse in DNA photodamage?

   Mathias Micheel, Christian Torres Ziegenbein, Peter Gilch , and 1 more author

   Photochemical & Photobiological Sciences, Sep 2015

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   (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.