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

52 Publikationen filtern

Die Publikationen filtern

Highlighted authors are members of the University of Jena.

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

    Year of publicationPublished in:Journal of the American Chemical Society P. Yaltseva, T. Maisuradze, A. Prescimone, S. Kupfer, O. Wenger
    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

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

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

  3. DFT-Guided Synthesis, Electrochemical, and Photophysical Properties of Ruthenium(II) Polypyridyl Complexes Featuring Flavin-Inspired π-Extended Ligands

    Year of publicationPublished in:Chemistry: a European Journal N. Hagmeyer, N. Mroweh, A. Schwab, C. McManus, M. Varghese, J. Mouesca, S. Gambarelli, S. Kupfer, B. Dietzek-Ivanšić, M. Chavarot-Kerlidou
    University Bibliography Jena:
    fsu_mods_00025279External link

  4. Evaluating the contribution of electromagnetic nearfield gradients in TERS

    Year of publicationPublished in:Optics communications : a journal devoted to the rapid publication of contributions in the field of optics and interaction of light with matter A. Khodadadi, K. Fiederling, S. Kupfer, S. Gräfe
    University Bibliography Jena:
    fsu_mods_00025747External link

  6. A Heterodox Approach for Designing Iron Photosensitizers: Pentacyanoferrate(II) Complexes with Monodentate Bipyridinium/Pyrazinium-Based Acceptor Ligands

    Year of publicationPublished in:Inorganic chemistry: including bioinorganic chemistry H. Schmidt, R. Oglou, H. Tunçer, T. Ghobadi, Ş. Tekir, K. Sertcelik, A. Ibrahim, L. Döhler, S. Özçubukçu, S. Kupfer, B. Dietzek-Ivanšić, F. Karadaş
    The main obstacle in replacing well-established precious ruthenium photosensitizers with earth-abundant iron analogs is the short excited state lifetimes of metal-to-ligand charge transfer (MLCT) states due to relatively weak octahedral field splitting and relaxation via metal-centered (MC) states. In this study, we address the issue of short lifetime by using pentacyanoferrate(II) complexes and combat facile photodissociation by utilizing positively charged pyrazinium or bipyridinium ligands. We utilize femtosecond transient absorption spectroscopy alongside quantum chemical calculations to probe the excited states of three 4,4′-bipyridinium- or pyrazinium-based pentacyanoferrate(II) complexes. The 4,4′-bipyridinium-based complexes exhibit ³MLCT lifetimes of about 20 ps, while the pyrazinium-based complex exhibits a lifetime of 61 ps in an aqueous solution, setting a benchmark for cyanoferrate complexes. These results mark the foundation for a new group of easy-to-prepare iron photosensitizers that can be used for harvesting visible light.
    University Bibliography Jena:
    fsu_mods_00024183External link

  7. Role of Spacers in Molecularly Linked RuRh Dyads: A Comparative Synthetic and Ultrafast Spectroscopic Investigation

    Year of publicationPublished in:Inorganic chemistry: including bioinorganic chemistry M. Semwal, M. Lämmle, E. Brohmer, S. Volk, L. Zedler, S. Kupfer, A. Mengele, G. Shillito, S. Rau, B. Dietzek-Ivanšić
    Supramolecular photocatalysts consisting of photosensitizer (PS), bridging ligand (BL), and catalytic center (CAT) have garnered significant attention in solar fuel applications. In this study, the photophysics and photocatalytic properties of two Ru(II)-based dinuclear complexes, specifically [(tbbpy)₂Ru(p(Ph)np)Rh(Cp*)Cl]³⁺ (n = 0, 1; Ru(pp)Rh for n = 0 or Ru(p(Ph)p)Rh for n = 1; tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, Cp* = pentamethylcyclopentadienyl, Ph = phenyl, p = 1,10-phenanthroline), are investigated. These complexes are studied as model complexes only differing by the distance between PS and CAT and thus allows a selective investigation of the influence of spacers in light-driven catalysis. A joint synthetic, spectroscopic, and theoretical approach, incorporating time-resolved absorption and emission spectroscopy, resonance Raman (rR) spectroscopy, density functional theory (DFT), and time-dependent (TD)DFT calculations, allows for comprehensive structural, electrochemical, photophysical, and photochemical characterization. Our findings suggest that minor structural variations in the intramolecular photocatalytic system significantly impact photocatalytic activity and system stability.
    University Bibliography Jena:
    fsu_mods_00024179External link

  8. Excited State Branching Processes in a Ru(II)-Based Donor–Acceptor–Donor System

    Year of publicationPublished in:Chemistry: a European Journal G. Yang, L. Blechschmidt, L. Zedler, C. Zens, K. Witas, M. Schmidt, B. Esser, S. Rau, G. Shillito, B. Dietzek-Ivanšić, S. Kupfer
    Excited state properties such as excitation energy, accessibility of the respective excited state either by direct or indirect population transfer, and its lifetime govern the application of these excited states in light-driven reactions, for example, photocatalysis using transition metal complexes. Compared with triplet metal-to-ligand charge transfer (³MLCT) states, charge-separated (³CS) excited states involving organic moieties, such as triplet intra-ligand or ligand-to-ligand charge transfer (³ILCT and ³LLCT) states, tend to possess longer-lived excited states due to the weak spin-orbit coupling with the closed-shell ground state. Thus, the combination of inorganic and organic chromophores enables isolating the triplet states onto the organic chromophore. In this study, we aim to elucidate the excited-state relaxation processes in a Ru(II)-terpyridyl donor–acceptor–donor system (RuCl) in a joint spectroscopic-theoretical approach combining steady-state and time-resolved spectroscopy as well as quantum chemical simulations and dissipative quantum dynamics. The electron transfer (ET) processes involving the low-lying ³MLCT, ³ILCT, and ³LLCT excited states were investigated experimentally and computationally within a semiclassical Marcus picture. Finally, dissipative quantum dynamical simulations—capable of describing incomplete ET processes involving all three states—enabled us to unravel the competitive relaxation channels at short and long timescales among the strongly coupled ³MLCT-³ILCT states and weakly coupled ³MLCT-³LLCT and ³ILCT-³LLCT states.
    University Bibliography Jena:
    fsu_mods_00024671External link

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

    Year of publicationStatusReview pendingPublished in:Journal of Physical Chemistry C S. Mitra, C. Zens, S. Kupfer, A. Heuer, D. Diddens
    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

  10. Unraveling the photoredox chemistry of a molecular ruby

    Year of publicationStatusReview pendingPublished in:Chemical science / Royal Society of Chemistry G. Yang, G. Shillito, P. Seeber, O. Wenger, S. Kupfer
    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

  11. Silver dithiocarbamates derived from amino acid esters

    Year of publicationStatusReview pendingPublished in:Dalton transactions : an international journal of inorganic chemistry / the Royal Society of Chemistry V. Behling, J. Heinrich, D. Díaz, E. Brohmer, J. Heinrich, N. Schlörer, S. Kupfer, N. Kulak, P. Köhler
    We report herein a series of new silver compounds with dithiocarbamate ligands derived from amino acid esters (AAE-DTCs). Compounds [Ag{SSC-N(R′)(CH₂R′′COOR)}]n (Ag(L1)-Ag(L5); N-dithioato-diethyliminodiacetate (L1), -ethyl-sarcosinate (L2), -tert-butyl-sarcosinate (L3), -methyl-l-prolinate (L4), -ethyl-N-benzylglycinate (L5)) were synthesised from in situ generated AAE-DTCs by salt metathesis with silver nitrate. The isolated products were characterised by different analytical techniques. Ag(L1), Ag(L4), and Ag(L5) were accessible to single-crystal X-ray structure determination, comprising hexameric subunits linked by dimeric units into a 1D-polymeric structure (Ag(L1)) and more homogeneous ribbon-like polymeric structures (Ag(L4) and Ag(L5)). DOSY NMR measurements and supporting DFT calculations were carried out to elucidate the structure of these compounds in solution, showing evidence for smaller agglomerates like dimers and tetramers. Additionally, as first evaluation of the biological activity of these complexes, ethidium bromide displacement assays and DNA melting curve experiments were carried out, the results showing moderate DNA binding abilities.
    University Bibliography Jena:
    fsu_mods_00027152External link

  12. Exploring the Potential of Al(III) Photosensitizers for Energy Transfer Reactions

    Year of publicationPublished in:Inorganic chemistry: including bioinorganic chemistry V. Caliskanyürek, A. Riabchunova, S. Kupfer, F. Ma, J. Wang, M. Karnahl
    Three homoleptic Al(III) complexes (Al1-Al3) with different degrees of methylation at the 2-pyridylpyrrolide ligand were systematically tested for their function as photosensitizers (PS) in two types of energy transfer reactions. First, in the generation of reactive singlet oxygen (¹O₂), and second, in the isomerization of (E)- to (Z)-stilbene. ¹O₂ was directly evidenced by its characteristic NIR emission at around 1276 nm and indirectly by the reaction with an organic substrate [e.g. 2,5-diphenylfuran (DPF)] using in situ UV/vis spectroscopy. In a previous study, the presence of additional methyl groups was found to be beneficial for the photocatalytic reduction of CO₂ to CO, but here Al1 without any methyl groups exhibits superior performance. To rationalize this behavior, a combination of photophysical experiments (absorption, emission and excited state lifetimes) together with photostability measurements and scalar-relativistic time-dependent density functional theory calculations was applied. As a result, Al1 exhibited the highest emission quantum yield (64%), the longest emission lifetime (8.7 ns) and the best photostability under the reaction conditions required for the energy transfer reactions (e.g. in aerated chloroform). Moreover, Al1 provided the highest rate constant (0.043 min⁻¹) for the photocatalytic oxygenation of DPF, outperforming even noble metal-based competitors such as [Ru(bpy)₃]²⁺. Finally, its superior photostability enabled a long-term test (7 h), in which Al1 was successfully recycled seven times, underlining the high potential of this new class of earth-abundant PSs.
    University Bibliography Jena:
    fsu_mods_00016275External link
Pagination Page 1

List of other publications

2023

Contact

Stephan Kupfer, Dr
PostDoc
Professorship of Theoretical Chemistry
Dr. Stephan Kupfer
Image: Dr. Stephan Kupfer
Room 102
Lessingstraße 4
07743 Jena Google Maps site planExternal link