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

48 Publikationen filtern

Die Publikationen filtern

Highlighted authors are members of the University of Jena.

  1. 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

  2. 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

  3. 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

  4. 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

  5. Evaluating the contribution of electromagnetic nearfield gradients in TERS

    Year of publicationStatusReview pendingPublished 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. Synthesis and Electrochemical Investigations of a Binuclear [FeFe]-Hydrogenase Mimic

    Year of publicationStatusReview pendingPublished in:ChemElectroChem B. Callies, K. Kuessner, S. Stripp, S. Kupfer, P. Köhler, H. Görls, I. Siewert, W. Weigand
    University Bibliography Jena:
    fsu_mods_00025147External link

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

    Year of publicationStatusReview pendingPublished 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

  8. Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, MnI, and FeII

    Year of publicationPublished in:Journal of the American Chemical Society C. Wegeberg, D. Häussinger, S. Kupfer, O. Wenger
    Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.
    University Bibliography Jena:
    fsu_mods_00010728External link

  9. 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

  10. Controlling Excited State Localization in Bichromophoric Photosensitizers via the Bridging Group

    Year of publicationPublished in:Inorganic chemistry: including bioinorganic chemistry G. Shillito, D. Preston, J. Crowley, P. Wagner, S. Harris, K. Gordon, S. Kupfer
    A series of photosensitizers comprised of both an inorganic and an organic chromophore are investigated in a joint synthetic, spectroscopic, and theoretical study. This bichromophoric design strategy provides a means by which to significantly increase the excited state lifetime by isolating the excited state away from the metal center following intersystem crossing. A variable bridging group is incorporated between the donor and acceptor units of the organic chromophore, and its influence on the excited state properties is explored. The Franck-Condon (FC) photophysics and subsequent excited state relaxation pathways are investigated with a suite of steady-state and time-resolved spectroscopic techniques in combination with scalar-relativistic quantum chemical calculations. It is demonstrated that the presence of an electronically conducting bridge that facilitates donor-acceptor communication is vital to generate long-lived (32 to 45 μs), charge-separated states with organic character. In contrast, when an insulating 1,2,3-triazole bridge is used, the excited state properties are dominated by the inorganic chromophore, with a notably shorter lifetime of 60 ns. This method of extending the lifetime of a molecular photosensitizer is, therefore, of interest for a range of molecular electronic devices and photophysical applications.
    University Bibliography Jena:
    fsu_mods_00011383External link

  11. Highly efficient electrocatalytic CO₂ reduction by a CrIII quaterpyridine complex

    Year of publicationPublished in:Proceedings of the National Academy of Sciences of the United States of America : PNAS J. Wang, Z. Luo, G. Yang, M. Gil-Sepulcre, S. Kupfer, O. Rüdiger, G. Ouyang
    Design tactics and mechanistic studies both remain as fundamental challenges during the exploitations of earth-abundant molecular electrocatalysts for CO2 reduction, especially for the rarely studied Cr-based ones. Herein, a quaterpyridyl CrIII catalyst is found to be highly active for CO2 electroreduction to CO with 99.8% Faradaic efficiency in DMF/phenol medium. A nearly one order of magnitude higher turnover frequency (86.6 s-1) over the documented Cr-based catalysts (<10 s-1) can be achieved at an applied overpotential of only 190 mV which is generally 300 mV lower than these precedents. Such a high performance at this low driving force originates from the metal-ligand cooperativity that stabilizes the low-valent intermediates and serves as an efficient electron reservoir. Moreover, a synergy of electrochemistry, spectroelectrochemistry, electron paramagnetic resonance, and quantum chemical calculations allows to characterize the key CrII, CrI, Cr0, and CO-bound Cr0 intermediates as well as to verify the catalytic mechanism.
    University Bibliography Jena:
    fsu_mods_00012063External link

  12. Toward robust electronic coupling predictions in redox-active TEMPO/TEMPO⁺ systems

    Year of publicationPublished in:The Journal of Chemical Physics: bridges a gap between journals of physics and journals of chemistry S. Mitra, C. Zens, S. Kupfer, D. Diddens
    This research elucidates the intricate nature of electronic coupling in the redox-active (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), commonly utilized in organic radical batteries. This study employs a combination of classical molecular dynamics and various electronic coupling calculation schemes. Within the context of the generalized Mulliken-Hush method, the electronic couplings are investigated via the complete active space self-consistent field approach, in combination with n-electron valence state perturbation theory, to provide an accurate description of both static and dynamic electron correlation as well as using (time-dependent) density functional theory simulations. Furthermore, the electronic communication between redox-active sites is studied using the cost-efficient density functional theory (DFT)-based frontier molecular orbital (FMO) approach. Our study reveals the dependence of the electronic coupling on the distance and the relative orientation of the redox pairs (TEMPO and TEMPO⁺). Apart from the expected exponential distance dependence, we found pronounced orientation dependence, with coupling values varying up to 0.2 eV, which is reflected by a substantial basis set dependency of the couplings, in particular at short distances. In addition, our study highlights the limitations of the DFT-based FMO method, in particular at short intermolecular distances between the redox-active sites, which may lead to a mixing of the involved molecular orbitals. This comparison will provide us with the most cost-accuracy-effective method for calculating electronic couplings in TEMPO-TEMPO⁺ systems.
    University Bibliography Jena:
    fsu_mods_00018602External link
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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