Dr. Stephan Kupfer

Dr. Stephan Kupfer

Dr. Stephan Kupfer
Image: Dr. Stephan Kupfer

Stephan Kupfer received his PhD in 2013 from the Friedrich-Schiller-University Jena, Germany, where he is currently a group leader in the Physical Chemistry Department. His research is focused on the theoretical modeling of photo-induced processes, i.e., in the fields of solar energy conversion and plasmonic hybrid systems. For light-harvesting applications, either in the scope of (dye-sensitized) solar cells or light-driven water-splitting, detailed understanding of the fundamental photophysics and photochemistry is of uttermost importance. Therefore, he aims to elucidate as well as to tune excited state relaxation dynamics in (supra)molecular photocatalysts and light-harvesting antenna associated to electron and energy transfer processes, i.e., charge separation, charge recombination and photodegradation.

Photo-induced electron transfer

Photo-induced electron transfer

Image: Dr. Stephan Kupfer

Furthermore, his research involves the theoretical description of plasmon-enhanced spectroscopy, i.e., in the scope of tip-enhanced Raman spectroscopy. Therefore, he aims for an holistic approach combining the electromagnet effect as well as the chemical effect including non-resonant, resonant and charge transfer contributions.

Tip

Image: Dr. Stephan Kupfer

List of publications in peer-reviewed journals

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Highlighted authors are members of the research group.

  1. Time-resolved spectroscopy of a photoactive dinuclear W/Ru complex: spectroscopic evidence for a metastable intermediate with side-on coordinated carbonyl ligand

    Authors
    J. Borter, S. Kangsa Banik, K. Kunze, S. Kupfer, D. Schwarzer, W. Seidel
    Year of publication
    Published in:
    Chemical science
    The photo-induced dynamics of a redox-active dinuclear W(ii)/Ru(ii) complex, [Tp*W(CO)Br(PyC 00000000000000000 00000000000000000 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 00000000000000000 00000000000000000 CCH ₂ )–Ru(bpy) ₂ ](PF ₆ ) (2-PF ₆ ), is revealed by femtosecond infrared and UV-vis pump-probe spectroscopy in combination with quantum chemical calculations. The use of the mononuclear tungsten alkyne complex [Tp*W(CO)Br(PyCCCH ₃ )] (1) as a benchmark allowed an in-depth analysis of the excited state kinetics of 2-PF ₆ . Excitation of the dinuclear complex at 400 nm produces predominantly a triplet metal-to-ligand charge transfer state localised at the Ru(bpy) ₂ chromophore ( ³ MLCT bpy ) with a lifetime of 6 ps. The following transformation into a tungsten-centered triplet state ( ³ MC W ) is accompanied by significant charge transfer and rearrangement of the W–C–O geometry. Subsequent intersysten crossing back to the ground state on a timescale of 12 ps produces a vibrationally excited molecule with up to 3 quanta in the CO stretching vibration. A minor fraction of 10% of the population reacts to an intermediate exhibiting a lifetime of 140 ps and a CO stretching frequency of 1703 cm −¹ . Our quantum chemical calculations disclose that this species corresponds to an isomer trapped in a metastable state of the S ₀ potential surface, with the CO bound side-on to the W centre.
    University Bibliography Jena:
    fsu_mods_00035844External link

  2. Synthesis and Characterization of a β-Thio-δ-Diimine (BTDDI) and the Related Bimetallic Dimethylaluminium(III) Compound

    Authors
    J. Kowalke, T. Rathsack, T. Rüffer, S. Kupfer, R. Kretschmer
    Year of publication
    Published in:
    Zeitschrift für anorganische und allgemeine Chemie
    Thionation of the β-oxo-δ-diimine (BODDI) 3 with P ₂ S ₅ ·2Py affords the ditopic β-thio-δ-diimine (BTDDI) 4. Based on ¹ H NMR spectroscopy and single-crystal X-ray diffraction, 4 is best described as a bis(β-enamine)-thione in both solution and the solid state. Furthermore, 4 features a thermochromic behavior in solution (c > 2 mmol l −¹ ) and as a solid. Reaction with trimethyl aluminium readily affords the bimetallic dimethylaluminium BTDDI complex 5. In contrast to its oxygen relative, 5 is only weakly emissive and the emission is significantly red-shifted. Scalar-relativistic time-dependent density functional theory calculations suggest that introduction of the sulfur atom promotes an intersystem crossing pathway (∼1 ns) to low-lying and non-emissive triplet states, which competes with fluorescence (∼5 ns).
    University Bibliography Jena:
    fsu_mods_00035653External link

  3. Probing Metal Tip-Induced Bond Weakening of a Reactive Alkyne Center Aligned via a Rigid Triphenylmethane-Based Tripod on Au(111) by TERS and DFT

    Authors
    G. Li, S. Mennicken, L. Zhu, S. Ehtesabi, T. Reichenauer, S. Kupfer, D. Schäfer, S. Mehrparvar, G. Haberhauer, Y. Zhang, S. Gräfe, S. Schlücker, Z. Dong
    Year of publication
    Published in:
    Journal of Raman spectroscopy
    The chemical reactivity of molecules can be controlled by a variety of effects, ranging from chemical reagents to purely physical stimuli. Metal tips employed in scanning probe microscopy are an elegant tool to manipulate reactive centers in single molecules. However, to achieve excellent control over distance and orientation, it is crucial to immobilize the reactive center and align it along the direction of the tip. Here, we aligned a reactive alkyne center via a rigid triphenylmethane-based tripod for upright adsorption on Au(111) for inducing bond weakening in the alkyne moiety by approaching a silver tip. Single-molecule ultrahigh vacuum low-temperature tip-enhanced Raman scattering was employed for probing tip-induced bond weakening in the gap distance range from 550 to 250 pm. Both the ≡C–H stretching at ~3330 cm−¹ and the dominant –C≡C– stretching peak at ~2130 cm−¹ exhibit a shift to smaller wavenumbers due to tip-induced bond weakening and an exponential increase in Raman intensity originating from the increased local electric field in the nanogap. To rationalize the underlying physical contributions and chemical effects of tip-induced bond weakening, density functional theory calculations for gap distances in the range 800 to 100 pm were performed. The computational results confirmed the presence of different gap distance regimes including the onset of Pauli repulsion for short distances; for the latter, the calculations additionally predict structural distortions of the terminal alkyne induced by the nearby metal tip. These findings allow us to set a lower limit for the tip–tripod gap distance in studies requiring an intact upright configuration of the alkyne-tripod, for example, electric field-induced chemistry.
    University Bibliography Jena:
    fsu_mods_00029568External link

  4. Hot-Carrier Injection and Millisecond Charge Separation from a Robust Heteroleptic Iron(II) Chromophore Immobilized on TiO2

    Authors
    T. Whittemore, M. Schmalle, E. Ryndin, M. Spitler, E. Brohmer, S. Rau, L. Zedler, E. Danilov, F. Castellano, S. Kupfer, G. Meyer, D. Sorsche
    Year of publication
    Published in:
    Journal of the American Chemical Society
    The synthesis, spectroscopic characterization, computational analysis, and photoelectrochemical behavior of a new iron-based chromophore, [(Cpy) ₂ Fe(deeb)](PF ₆ ) ₂ (Fe(Cpy) ₂ (deeb)), where Cpy is 1-methyl-3-(2-pyridyl)imidazole and deeb is 4,4’-(CO ₂ CH ₂ CH ₃ ) ₂ -2,2’-bipyridine, is reported. Electrochemically reversible waves assigned to a metal-centered E o (Fe III/II ) = +0.48 and a ligand-centered E o (Fe ²⁺/⁺ ) = −1.47 V vs Fc ⁺/⁰ reduction were evident in cyclic voltammetry measurements. The combination of a strong σ-donor and a π-acceptor lowered the energy of the metal-to-ligand charge-transfer (MLCT) excited state relative to the metal-centered state. Two MLCT transitions appear in the visible region at 424 and 580 nm. TDDFT calculations revealed that the lower-energy band was well formulated as Fe(II)→deeb, and the higher-energy transition was charge transfer to both the deeb and Cpy ligands. Resonance Raman spectroscopy supports these findings showing enhanced deeb vibrational modes with 532 nm excitation, both deeb and Cpy modes with 473 nm excitation, and exclusively Cpy with 405 nm excitation. Ultrafast spectroscopy reveals a short-lived (∼2 ps) MLCT excited state and a longer-lived (∼20 ps) metal-centered state. Efficient methods to deprotect the ester groups and anchor the complex to mesoporous TiO ₂ (anatase) thin films in high surface coverages, Fe(Cpy) ₂ (dcb)|TiO ₂ σ = 3 × 10 –⁸ mol/cm ² , were established. Pulsed light excitation of Fe(Cpy) ₂ (dcb)|TiO ₂ resulted in rapid excited state injection (k inj > 10 ⁸ s –¹ ) and formation of a charge-separated state, Fe III (Cpy) ₂ (dcb)|TiO ₂ (e), which persists on the millisecond time scale before returning cleanly to the ground state with second-order kinetics. Injection yields measured 50 ns after light excitation were found to double from Φ = 0.15 with green (532 nm) light to 0.30 with blue (457 nm) light excitation. Incident photon-to-current efficiency (% IPCE) measurements as a function of excitation wavelength in a 0.5 M LiI/I ₂ /CH ₃ CN electrolyte provide clear evidence for band-selective “hot carrier” injection from the remote Cpy-localized excited state. Collectively, the spectroscopic and photoelectrochemical data indicate that a semiconductor can intercept hot electrons from iron chromophores even when the excited-state dipole is oriented away from the surface-anchoring ligand.
    University Bibliography Jena:
    fsu_mods_00036374External link

  5. Unraveling Charging and Discharging Processes in Organic Radical-Based Electrodes : A Hierarchical Molecular and Quantum Mechanical Approach

    Authors
    C. Zens, G. Shillito, C. Friebe, S. Kupfer
    Year of publication
    Published in:
    ChemSusChem :: chemistry & sustainability, energy & materials
    Organic batteries represent a promising class of energy storage materials, due to their mechanical flexibility and sustainability. Typically, stable radicals, lacking intrinsic conductivity, are utilized as redox-active materials. A recently introduced strategy to overcome this shortcoming is to incorporate stable radicals, i.e., (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), into a polythiophene backbone. Thereby, an electrode material was obtained which does not require conductive additives. The current computational study aims to elucidate the functionality of this material by drawing in-depth structure–property relationships utilizing a hierarchical molecular and quantum mechanical approach. Initially, structural properties of the electrode material's macroenvironment—containing the functionalized polythiophene, electrolyte, and solvent—were assessed in various charging states by molecular dynamics simulations. Subsequently, electronic properties were investigated by time-dependent density functional theory for 564 microenvironments. Via this computational setup, the electronic communication within the material was assessed along intrastrand and interstrand CT processes involving the respective TEMPO and polythiophene units. Thereby, our hierarchical computational approach reveals that the intrinsic conductivity and charge storage capacity of the electrode material stems from efficient intrastrand TEMPO-polythiophene CT processes along short and rigid amid linkers. These insights help to tailor improved conductive organic electrode materials with higher charging and discharging rate capabilities.
    University Bibliography Jena:
    fsu_mods_00034972External link

  6. A donor–acceptor photosensitizer-catalyst dyad for light-driven nicotinamide hydrogenation

    Authors
    A. Tombrink, M. Semwal, T. Maisuradze, A. Mengele, D. Straub, A. Kuehne, S. Rau, S. Kupfer, B. Dietzek-Ivanšić, B. Esser
    Year of publication
    Published in:
    Chemical science
    Using light energy to drive chemical transformations is of great relevance, with photosynthesis in nature as a grand example. In artificial light-driven catalysis, part of nature's complex supramolecular architecture can be mimicked through the so-called covalently linked photosensitizer-catalyst (PS-CAT) dyads. We herein report a dyad using an organic donor–acceptor PS, with dipyridophenazine as the acceptor and tert-butylcarbazole as the donor (2 t BuCzDPPZ), that contains a coordination site for a rhodium(iii)Cp* center as the catalyst. The organic PS shows a charge-transfer transition upon visible-light irradiation and has redox properties similar to typically used ruthenium-based PSs. The resulting PS-CAT dyad 2 t BuCzDPPZRhCp* shows – with methoxy-substituted 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH-OMe) as the sacrificial electron donor – photocatalytic activity in light-driven NAD ⁺ reduction with a TON of 3.2 (after 4 h). Femtosecond transient absorption and resonance Raman spectroscopy, as well as time-dependent density functional theory (TDDFT) calculations, shed light on the photophysical properties of the PS and PS-CAT dyad and reveal a high dependency of the photoluminescence quantum yield and excited state properties on solvent polarity – in line with its donor–acceptor structure. This work presents a new design concept for PS-CAT dyads in artificial light-driven catalysis and provides important insight into the interplay between solvation dynamics of organic donor–acceptor systems and their photophysics, paving the way for future design strategies.
    University Bibliography Jena:
    fsu_mods_00034669External link

  7. Light-driven hydrogen evolution reactivity of molecular thio-oxomolybdate catalysts

    Authors
    L. Schleicher, S. Kolbinger, A. Edwards, K. Sellmann, L. Senz, S. Kupfer, M. Schmitt, J. Popp, C. Streb
    Year of publication
    Published in:
    Sustainable Energy and Fuels
    Heterogeneous molybdenum sulfides are widely used noble metal-free hydrogen evolution reaction (HER) catalysts. Thiomolybdates, their molecular analogues have been developed as viable minimal models to study reactivity at the molecular level. Here, we explore the light-driven HER reactivity and stability of the mixed thio-oxo-molybdate prototype [Mo ₂ O ₂ S ₆ ] ²− in homogeneous solution. In combination with the photosensitizer [Ru(bpy) ₃ ] ²⁺ , [Mo ₂ O ₂ S ₆ ] ²− shows promising HER performance (turnover number TON > 500), as well as strong reactivity dependence on the reaction conditions. Mechanistic experimental studies combined with density functional theory computations reveal complex speciation of the catalyst in solution, as well as light-induced and light-independent reaction pathways for catalyst and photosensitizer which are in line with disulfide-for-solvent ligand exchange reactions. These structure–reactivity insights outline design rules for more robust, solvent-tolerant thiomolybdate HER catalysts.
    University Bibliography Jena:
    fsu_mods_00035842External link

  8. Impact of Organo-Functionalization on the Light-Driven Hydrogen Evolution Reactivity of Molecular Molybdenum Sulfides

    Authors
    M. Jahn, L. Kammerer, L. Schleicher, A. Meyer, M. Puthanagady, A. Mengele, S. Rau, S. Kupfer, B. Dietzek-Ivanšić, C. Streb
    Year of publication
    Status
    Review pending
    Published in:
    Chemistry - A European Journal
    Molecular molybdenum sulfides, or thiomolybdates, are well-established noble-metal free molecular catalysts for the hydrogen evolution reaction (HER). To-date, there is a knowledge-gap regarding the impact of organo-functionalization on reactivity, stability and heterogenization of this compound class. Here, we report the development of synthetic routes for controlled introduction of organic N-donor ligands as a first step toward establishing structure-property-reactivity relationships in this new compound class. Photophysical studies combined with computational modelling are used to rationalize trends in the observed light-driven homogeneous HER activity. This work lays the foundation to develop organo-functionalized thiomolybdate HER catalysts suitable for covalent or supramolecular linkage to photosensitizers and for anchoring on heterogeneous supports, e.g. photocathodes.
    University Bibliography Jena:
    fsu_mods_00036502External link

  9. Structural Control of Metal-Centered Excited States in Cobalt(III) Complexes via Bite Angle and π–π Interactions

    Authors
    P. Yaltseva, T. Maisuradze, A. Prescimone, S. Kupfer, O. Wenger
    Year of publication
    Published in:
    Journal of the American Chemical Society
    CoIIIcomplexes have recently become an important focus in photophysics and photoredox catalysis due to metal-centered excited states with strong oxidizing properties. Optimizing chelate ligand bite angles is a widely used strategy to strengthen metal–ligand interactions in coordination complexes, with the resulting enhanced ligand fields often contributing to extended excited-state lifetimes that are advantageous for photochemical applications. We demonstrate that bite-angle optimization exerts the opposite effect on CoIIIpolypyridines compared to previously studied transition metal complexes, as polypyridine ligands function as π-donors to CoIIIrather than π-acceptors. Our findings reveal two counterintuitive paradigms: while bite-angle optimization weakens the ligand field in CoIIIcomplexes, the resulting lower-energy metal-centered excited states can be accompanied by extended excited-state lifetimes, driven by increased rigidification through intramolecular π–π interactions. These insights, along with additional experiments investigating the possibility of photoreactions from higher excited states, advance the current understanding of the photophysics and photochemistry of first-row transition metal complexes and highlight key distinctions from the more extensively studied photoactive complexes of second- and third-row transition metals.
    University Bibliography Jena:
    fsu_mods_00027038External link

  10. Machine Learning Models for Predicting Electronic Coupling in TEMPO/TEMPO+ Systems

    Authors
    S. Mitra, C. Zens, S. Kupfer, A. Heuer, D. Diddens
    Year of publication
    Published in:
    The journal of physical chemistry C
    Organic radical batteries (ORBs) based on the TEMPO (2,2,6,6-tetramethylpiperidin-1-yl oxyl) radical have drawn significant attention, owing to their unique redox properties. A key factor influencing ORB’s redox properties, i.e., the kinetics of the electron transfer between the TEMPO–TEMPO⁺pairs, is the communication between the underlying redox-active states as given by the electronic coupling. However, due to the complex structure, predicting accurate electronic couplings for these pairs is computationally expensive and challenging. In this study, we introduce a machine learning (ML) workflow to predict the electronic coupling for TEMPO–TEMPO⁺pairs simply by their specific geometric orientations. For the ML models, a data set was generated through time-dependent density functional theory calculations coupled with the Generalized Mulliken Hush method to assess energies, (transition-)dipole moment, and couplings for specific TEMPO–TEMPO⁺configurations obtained from classical molecular dynamics simulations that mimic a realistic electrolyte environment. Our results demonstrate that, among the three ML models─linear regression, kernel ridge regression (KRR), and random forest─the KRR model, with its kernel-based approach, most effectively handles the correlated orientation-based descriptors. Moreover, our SHapley Additive exPlanations (SHAP)-based feature importance analysis indicates that multiple orientation factors jointly influence electronic coupling, rather than any single distance or angle dominating, with each parameter’s impact strongly contingent on the values of the others which is in agreement with previous studies computational by the consortium.
    University Bibliography Jena:
    fsu_mods_00027182External link

  11. Probing the performance of DFT in the structural characterization of [FeFe] hydrogenase models

    Authors
    P. Matczak, P. Buday, S. Kupfer, H. Görls, G. Mlostoń, W. Weigand
    Year of publication
    Published in:
    Journal of computational chemistry : organic, inorganic, physical, biological
    University Bibliography Jena:
    fsu_mods_00017792External link

  12. Unraveling the photoredox chemistry of a molecular ruby

    Authors
    G. Yang, G. Shillito, P. Seeber, O. Wenger, S. Kupfer
    Year of publication
    Published in:
    Chemical science
    In contrast to well-studied 4d⁶ and 5d⁶ transition metal complexes such as the modern-day drosophila of photochemistry, Ru(ii)-tris(bipyridine), which often feature a typical triplet metal-to-ligand charge transfer emission in the nanosecond timescale, the photophysics of Cr(iii) complexes are drastically different. The 3d³ configuration of the chromium(iii) allows for an unusual spin-flip emission from the low-lying metal-centered (MC; ²T₁ and ²E) states, exhibiting lifetimes up to the milliseconds to seconds timescale. In this fully computational contribution, the photophysical properties as well as the application of such long-lived excited states in the context of photoredox chemical transformations are investigated for the recently introduced [Cr(dqp)2]3+ [Cr(iii)-(2,6-bis(8′-quinolinyl)pyridine)₂]³⁺, otherwise known as a type of molecular ruby. Our in-depth theoretical characterization of the complicated electronic structure of this 3d³ system relies on state-of-the-art multiconfigurational methods, i.e. the restricted active space self-consistent field (RASSCF) method followed by second-order perturbation theory (RASPT2). This way, the light-driven processes associated with the initial absorption from the quartet ground state, intersystem crossing to the doublet manifold as well as the spin-flip emission were elucidated. Furthermore, the applicability of the long-lived excited state in [Cr(dqp)2]3+ in photoredox chemistry, i.e. reductive quenching by N,N-dimethylaniline, was investigated by ab initio molecular dynamics (AIMD). Finally, the thermodynamics and kinetics of these underlying intermolecular electron transfer processes were analyzed in the context of semiclassical Marcus theory.
    University Bibliography Jena:
    fsu_mods_00028334External link
Pagination Page 1

List of other publications

2023

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Stephan Kupfer, Dr

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Professorship of Theoretical Chemistry
Dr. Stephan Kupfer
Image: Dr. Stephan Kupfer
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