Michael Karnahl

Michael Karnahl
Technische Universität Braunschweig · Institut für Physikalische und Theoretische Chemie - Department of Energy Conversion

Dr.

About

65
Publications
7,146
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2,327
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Introduction
A sufficient and sustainable supply of energy for a growing world population is a major challenge. In this respect catalysts for the photo(electro)catalytic splitting of water have a high potential to contribute to solve this problem. Therefore, my research is focused on the design, application and spectroscopic study of different bioinspired photo- and electrocatalysts for solar fuels production. In particular, heteroleptic copper photosensitizers play a key role in our current studies. Furthermore, attention is given to modern synthetic methods for the direct formation of carbon-carbon bonds like CDC and ATRA reactions. see : www.karnahl-group.de
Additional affiliations
June 2014 - present
University of Stuttgart, Germany
Position
  • Akademischer Rat
August 2012 - May 2014
Leibniz Institute for Catalysis, Rostock, Germany
Position
  • Researcher: "Light2Hydrogen" - Energy for the future - Photocatalytic Splitting of Water to Hydrogen
Description
  • "Fuels Produced Regeneratively Through Light-Driven Water Splitting: Clarification of the Elemental Processes Involved and Prospects for Implementation in Technological Concepts"
June 2010 - June 2012
Uppsala University
Position
  • PostDoc Position
Description
  • “Hydrogen Production and Activation by Functional Bioinorganic Model Complexes of the [Fe] and [FeFe] Hydrogenase Active Sites”
Education
June 2010 - June 2012
Uppsala University, Uppsala, Sweden
Field of study
  • Chemistry
August 2006 - February 2010
Friedrich-Schiller-University, Jena, Germany
Field of study
  • Chemistry

Publications

Publications (65)
Article
Full-text available
Driven by the great potential of solar energy conversion this study comprises the evaluation and comparison of two different design approaches for the improvement of copper based photosensitizers. In particular, the distinction between the effects of a covalently linked and a directly fused naphthalimide unit was assessed. For this purpose, the two...
Preprint
Full-text available
The utilization of a fully noble-metal-free system for photocatalytic CO 2 reduction remains a fundamental challenge, demanding the precise design of photosensitizers and catalysts, as well as the exploitation of their intermolecular interactions to facilitate electron delivery. Herein, we have implemented triple modulations on catalyst, photosensi...
Article
Invited for the front cover of this issue are Stefanie Tschierlei, Sven Rau and co‐workers. The image shows the fusion of an organic chromophore with a RuII polypyridine moiety resulting in a unique bichromophoric photosensitizer. Read the full text of the article at 10.1002/chem.202103609. “The fusion of an organic chromophore with a RuII moiety c...
Article
Two are better than one: In a joint study, groups in two laboratories at TU Braunschweig and Ulm University present a novel bichromophoric RuII complex. This unique photosensitizer was obtained by fusing a ruthenium polypyridine chromophore with an additional perylene dye. This photosensitizer exhibits a strong absorptivity in the visible region an...
Article
Ru(II)- and Cu(I)-based photosensitizers featuring the recently developed biipo ligand (16H-benzo-[4',5']-isoquinolino-[2',1',:1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one) were comprehensively investigated by X-ray crystallography, electrochemistry, and especially several time-resolved spectroscopic methods covering all time scales from femto-...
Article
Multichromophoric systems based on a Ru(II) polypyridine moiety containing an additional organic chromophore are of increasing interest with respect to different light‐driven applications. Here, we present the synthesis and detailed characterization of a novel Ru(II) photosensitizer, namely [(tbbpy) 2 Ru((2‐(perylen‐3‐yl)‐1H‐imidazo[4,5‐ f ][1,10]‐...
Article
With the aims of increasing the antenna system and improving the photophysical properties of Cu(I)-based photosensitizers, the backbone of 2,9-dimethyl-1,10-phenanthroline was selectively extended in the 5,6-position. Applying specifically tailored Suzuki−Miyaura and “chemistry-on-the-complex” Sonogashira cross-coupling reactions enabled the develo...
Article
Full-text available
Photosensitizing units have already been applied to enable light‐driven catalytic reduction of CO 2 with mononuclear rhenium complexes. However, dinuclear catalytic systems that are able to activate CO 2 in a cooperative bimetallic fashion, have only rarely been combined with photosensitizers. We here present detailed studies on the influence of ad...
Article
Within the frame of the priority program SPP 2102 “Light‐controlled reactivity of metal complexes” and the collaborative research center TRR 234 “Catalight” a unique rylene type diimine ligand was developed. This ligand enabled a RuII photosensitizer with a strong and broad absorption in the visible and a very long‐lived excited state of up to 24.7...
Article
Full-text available
As a novel rylene type dye a diimine ligand with a fully rigid and extended π‐system in its backbone was prepared by directly fusing a 1,10‐phenanthroline building block with 1,8‐naphthalimide. The corresponding heteroleptic ruthenium photosensitizer bearing one biipo and two tbbpy ligands was synthesized and extensively analyzed by a combination o...
Article
To bind or not to bind: Driven by the motivation to increase the (photo)stability of traditional Cu(I) photosensitizers, multidentate diimine ligands, which contain two additional donor sites, were designed. To this end, a systematic series of four 1,10-phenanthroline ligands with either OR or SR (R = i Pr or Ph) donor groups at the 2 and 9 positi...
Article
Full-text available
A systematic series of four novel homo‐ and heteroleptic Cu(I) photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X = SMe or OMe) were designed. Their solid state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emi...
Article
Full-text available
A series of seven homoleptic Cu(I) complexes based on hetero‐bidentate P^N ligands was synthesized and comprehensively characterized. In order to study structure‐property relationships the type, size, number and configuration of substituents at the phosphinooxazoline (phox) ligands were systematically varied. To this end, a combination of X‐ray dif...
Article
The electrochemically singly-reduced Cu(I) photosensitizers of the type [Cu(xant)(N^N)]⁺ (with xant = xantphos and N^N = bidentate diimine ligand: dipyrido[3,2-a:2’,3’-c]phenazine = dppz or 3,6,11,12-tetramethyl-dipyrido[3,2-a:2’,3’-c]phenazine = tmdppz) exhibit a metal-to-ligand charge transfer (MLCT) transition from the Cu(I) center to the reduce...
Article
Full-text available
The capture and storage of solar energy is a promising option to overcome current energy issues. To put such systems into practice, molecular photosensitizers should be based on abundant metals and possess a strong absorption capability for visible light. Therefore, a systematic series of four novel heteroleptic Cu(I) complexes of the type [(PˆP)Cu...
Article
An ideal photosensitizer should strongly absorb visible light in a broad range and exhibit long-lived excited states. Therefore, an anthracene moiety attached to a bipyridine sphere was chosen to study the impact of a carbon extended π-system in the back on the structural, electrochemical and photophysical properties of the resulting copper photose...
Article
Heteroleptic Cu(I) complexes are a promising alternative towards traditional Ru(II) photosensitizers. In particular, Cu(I) complexes of the type [Cu(N^N)(P^P)]+, where N^N represents a diimine and P^P a bulky diphosphine ligand, are already successfully applied for photo(redox)catalysis, organic light-emitting diodes (OLEDs) or dye-sensitized solar...
Article
Full-text available
Titania is a promising material for numerous photocatalytic reactions such as water splitting and the degradation of organic compounds (e.g., methanol, phenol). Its catalytic performance can be significantly increased by the addition of co-catalysts. In this study, Au and Au/Ag nanoparticles were deposited onto mesoporous titania thin films using p...
Article
For efficient photocatalytic hydrogen production not only the photocatalytic system itself, but also the reactor design and appropriate catalytic conditions play a central role for achieving high overall activity and long lifetimes. To investigate these points further, a fully noble-metal-free system for the light-driven reduction of protons to hyd...
Article
Full-text available
We report the light-induced electronic and geometric changes taking place within a heteroleptic Cu(I) photosensitizer, namely [(xant)Cu(Me2phenPh2)]PF6 (xant = xantphos, Me2phenPh2 = bathocuproine), by time-resolved X-ray absorption spectroscopy in the ps-µs time-regime. Time-resolved X-ray absorption near edge structure (XANES) and extended X-ray...
Article
Photoactive Cu(I) complexes are considered as a promising alternative to traditional noble-metal complexes that are mainly based on Ru(II) and Ir(III). Therefore, in recent years heteroleptic Cu(I) complexes of the type [Cu(N^N)(P^P)]+ have been developed exhibiting impressive performance, e.g., long-lived excited-states, high emission quantum yiel...
Cover Page
In a modular approach conventional diphosphine P^P ligands were replaced by a heterobidentate P^N ligand in order to enhance stability and absorptivity of the resulting copper photosensitizers. The present study gives insights into their structure-property relationships by a combination of photophysical and electrochemical measurements as well as (...
Article
Driven by the intention to improve classic heteroleptic copper photosensitizers two novel Cu(I) complexes applying a hetero-bidentate P^N ligand were prepared. A combined photophysical, electrochemical and theoretical study gives insights into structure-activity-relationships and revealed an increased absorptivity. Both complexes were tested for th...
Article
Synthetic diiron compounds of the general formula Fe2(µ-S2R)(CO)n(L)6-n (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was charact...
Article
A series of novel heteroleptic copper photosensitizers was prepared and investigated with respect to their photocatalytic performance. In this context a strong impact of the extended π-system on the light-driven reduction of protons could be observed. The origin of this behavior was studied by a combination of different photophysical and electroche...
Article
A series of heteroleptic copper(I) photosensitizers of the type [(P^P)Cu(N^N)]+ with an extended π-system in the backbone of the diimine ligand have been prepared. The structures of all complexes are completely characterized by NMR spectroscopy, mass spectrometry and X-ray crystallography. These novel photosensitizers were assessed with respect to...
Article
Full-text available
A combination of UV-Vis and resonance Raman spectroscopy in conjunction with electrochemistry is employed to reveal the nature of a side procduct formed when using heteroleptic Cu(I)-photosensitizers [(P^P)Cu(N^N)]+ for photocatalytic hydrogen generation. It is shown that homoleptic [Cu(N^N)2]+ complexes are formed under oxidative conditions confir...
Article
Full-text available
Time-resolved spectroscopy was applied to investigate the excited state dynamics of two heteroleptic Ir(III) complexes with the general formula [Ir(C^N)2(N^N)]+. The excited state relaxation is influenced by the ligand substitution as well as the light polarisation. Vibrational relaxation occurs in the sub-ps timescale and interligand charge transf...
Article
No abstract is available for this article.
Article
Driven by an increasing demand for renewable energy great efforts have been made to design efficient photosensitizers, which can be applied for the photocatalytic splitting of water by using the energy of the sun. In order to bring such molecular systems into practice, conventional noble metal based complexes need to be replaced by cheaper and more...
Article
The activity of plasmonic Au-TiO2 catalysts for solar hydrogen production from H2O/MeOH mixtures was found to depend strongly on the support phase (anatase, rutile, brookite or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition or deposition-precipitation). Str...
Article
Four different heteroleptic [Cu(N^N)(P^P)]PF6 complexes, which combine classical bidentate diimine ligands and sterically demanding diphosphine ligands, are studied by a combination of ultrafast time-resolved spectroscopy and quantum chemical calculations. The light-induced excited state processes, accompanied by a structural change, are discussed...
Article
Although largely neglected, convenient and commercially available copper salts constitute appropriate precursors for efficient and stable water reduction catalysts (WRC). Various copper salts have been applied in the photocatalytic proton reduction together with [Ir(ppy)2(bpy)]PF6 as photosensitizer (PS) and triethylamine (TEA) as electron donor. A...
Article
This study provides detailed mechanistic insights into the light driven hydrogen production using an abundant copper-iron system. It focuses on the role of the heteroleptic copper photosensitizer [Cu(P^P)(N^N)]+, which can be oxidized or reduced after photoexcitation. By means of IR, EPR, UV/vis spectroscopy as well as computational studies and spe...
Article
Photocatalytic hydrogen evolution rates, structural properties as well as charge separation, electron transfer and stabilization have been analyzed in advanced sol-gel-derived carbon nitrides (SG-CN) pyrolyzed at different temperatures (350-600 °C) and in bulk polymeric carbon nitride reference samples (CN) by XRD, XPS, FTIR, UV-Vis, Raman and phot...
Article
Full-text available
A series of six mononuclear iron complexes of the type [Fe(X-bdt)(PR2NPh2)(CO)] (bdt = benzenedithiolate with X = H, Cl2 or Me and R = Ph, Bn, Cy or tert.-Bu) was prepared. This new class of penta-coordinate iron complexes contains a free coordination site and a pendant base as essential structural features of the [FeFe]-hydrogenase active site. Th...
Article
A series of heteroleptic copper photosensitizers [(P^P)Cu(N^N(SO3Na)2)]+ containing sulfonate anchor groups is described. In the presence of titanium dioxide they form composites, which are active photosensitizers in the light driven reduction of protons. Further stabilization of these systems is achieved by encapsulation within a plasma-polymerize...
Article
Hydrogen is considered to be the main energy carrier of the future and its carbon-free and efficient production is currently one of the main challenges in the development of sustainable energy technologies. In photocatalysis, sunlight is used to produce high-purity hydrogen from water. A core photocatalytic material is currently still titania, give...
Article
A series of heteroleptic copper(I) complexes with bidentate P^P and N^N chelate ligands was prepared and successfully applied as photosensitizers in the light-driven production of hydrogen, by using [Fe3(CO)12] as a water-reduction catalyst (WRC). These systems efficiently reduces protons from water/THF/triethylamine mixtures, in which the amine se...
Article
Golden electrons: Visible light excites conduction electron transfer from gold particles to support vacancies where they are taken up by protons to produce hydrogen. This transfer process was visualized by in situ EPR spectroscopy.
Article
Raus aus Gold! Sichtbares Licht regt den Transfer von Leitungsbandelektronen von Gold‐Nanopartikeln zu Leerstellen im Trägermaterial an, wo sie von Protonen unter Bildung von Wasserstoff aufgenommen werden. Dieser Transferprozess wurde mithilfe von In‐situ‐EPR‐Spektroskopie visualisiert.
Article
While inter-molecular hole-hopping along the surface of semiconductors is known, there are no previous examples of electron-hopping between molecules on a surface. Herein, we present the first evidence of electron transfer from the bound photosensitizer Coumarin-343 (C343) to a bound complex (1) on the surface of NiO. In solution, 1 has been shown...
Article
A series of [FeFe]-hydrogenase active site analogues, with the general formula [Fe(2)(dt)(CO)(4)(BC)] (dt = dithiolate, pdt = propyl-1,3-dt (), bdt = benzene-1,2-dt (), edt = ethyl-1,2-dt (); BC = 1,2-bisdiphenylphosphine-1,2-o-carborane), has been prepared and structurally characterized. While the electrochemical reductions of are largely invarian...
Article
Full-text available
A pair of tetranuclear iron complexes consisting of two Fe2(Cl2bdt)(CO)5 subunits (Cl2bdt = 3,6-dicholorobenzene-1,2-dithiolate) bridged by different cyclic 1,5-diaza-3,7-diphosphacyclooctane (P2N2) ligands were prepared and structurally characterized. In the solid state, the P2N2 ligands adopt a boat conformation, which results in rather short dis...
Article
Molecular photocatalysts, combining a light absorber and a catalytic active center via a bridging ligand, have received much attention during the last decade. Therefore, many new types of photocatalysts containing novel bridging ligands are reported. The bridging ligand is expected to play an important role and does not only represent the chemical...
Article
Molecular photocatalysts allow for selectively tuning their function on a molecular level based on an in-depth understanding of their chemical and photophysical properties. This contribution reports the synthesis and photophysical characterization of the novel molecular photocatalyst [(tbbpy)2Ru(tpac)PdCl2]2+ RutpacPd (with tpac = tetrapyrido[3,2-a...
Article
Ruthenium–polypyridine complexes play an important role as photosensitizers in supramolecular photocatalysis. Using multiple Ru-centers within a single supramolecular catalyst might be a promising path for improving its efficiency. The connection of several chromophores may, however, lead to direct interaction amongst individual photoactive centers...
Article
Mononuclear pentacoordinate iron complexes with a free coordination site were prepared as mimics of the distal Fe (Fe(d)) in the active site of [FeFe] hydrogenases. The complexes catalyze the electrochemical reduction of protons at mild overpotential.
Article
Artificial photosynthesis based on supramolecular photocatalysts offers the unique possibility to study the molecular processes underlying catalytic conversion of photons into chemical fuels in great detail and to tune the properties of the photocatalyst by alterations of the molecular framework. Herein we focus on both possibilities in studying th...
Article
The reactivity of [(μ-X2bdt)Fe2(CO)6] [(bdt)1, X2bdt = 3,6-disubstituted bezenedithiolate; X = H, Cl] with ligands of different donor strengths is investigated and compared to that of [(μ-pdt)Fe2(CO)6] [(pdt)1, pdt = propyldithiolate] and [(μ-edt)Fe2(CO)6] [(edt)1, edt = ethyldithiolate]. Strong donor ligands (L = CN–, PMe3) when added to (bdt)1 le...
Article
Ultrafast excited-state annihilation in a homodinuclear ruthenium complex is observed. This coordination compound constitutes a model system for approaches towards artificial photosynthetic systems. The observation of pump-intensity dependent triplet-triplet annihilation highlights the importance of considering various loss mechanisms in the design...
Article
Abstract We present a combined Raman-density functional theory (DFT) study of novel dipyridophenazine (dppz) derivatives and their Ru-bipyridine complexes. Our results show that the molecular architecture of dppz and its Ru complexes can be considered to consist of two independent moieties, the structural ground state properties of which can be tun...
Article
AIP Conf. Proc. 1267, pp. 881-882; doi:http://dx.doi.org/10.1063/1.3482867 XXII INTERNATIONAL CONFERENCE ON RAMAN SPECTROSCOPY Date: 8–10 August 2010, Location: Boston (MA) Keywords: Raman spectroscopy, catalysts, quantum chemistry, charge exchange
Article
Die Farbe des einfallenden Lichts bestimmt den Wirkungsgrad der Wasserstoffentwicklung mit einem supramolekularen Photokatalysator. Der Befund zeigt die Bedeutung des Franck-Condon-Punkts für künstliche Photosysteme auf und sollte in zukünftige Strategien für die Entwicklung solcher Systeme einfließen.
Article
The color of the incident light determines the efficiency of the hydrogen evolution mediated by a supramolecular photocatalyst. This finding demonstrates the importance of the Franck–Condon point in artificial photosynthesis systems and should have an impact on their design.
Article
A series of novel regioselective substituted tpphz ligands and two novel mononuclear ruthenium complexes of the type [(tbbpy)(2)Ru(tpphzR(n))](PF(6))(2) (where tbbpy = 4,4'-di-tert.-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, with n = 2 and R represents the bromine substituents at different positions) h...
Article
Full-text available
The complex [(dmcb)(2)Ru(dppz)](PF(6))(2) shows unexpected luminescence in water implying fundamentally different excited state relaxation pathways than are typically observed for complexes of this kind.
Article
Full-text available
Ru-dppz (dppz = dipyrido[3,2-a:2',3,3'-c]phenazine) complexes play an important role as environmentally sensitive luminescence sensors and building blocks for larger supramolecular compounds. Their photophysical properties are known to be highly sensitive to intermolecular solvent-solute interactions and solvent bulk-properties. Here, the synthesis...
Article
The palladium-catalysed cross-coupling reaction between 3,8-dibromo-1,10-phenanthroline with phenylacetylene or 3,5-bis(trifluoromethyl)phenylboronic acid gives good yields of the 3,8-disubstituted products. These 1,10-phenanthroline derivatives are used for the formation of novel ruthenium complexes of the type [(tbbpy)2Ru(phenR2)]2+ [where tbbpy...
Article
Photoinduced electron-transfer processes within a precatalyst for intramolecular hydrogen evolution [(tbbpy)(2)Ru(tpphz)PdCl(2)](2+) (RuPd; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'c:3'',2'',-h:2''',3'''-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By compar...
Article
Full-text available
The mononuclear compounds bis-(2,2'-bipyridine)ruthenium(ii)-(tris(2-pyridyl)triazine) [(bpy)(2)Ru(tpt)](PF(6))(2) and bis-(4,4'-di-tert-butyl-2,2'-bipyridine)ruthenium(ii)-(tris(2-pyridyl)triazine) [(tbbpy)(2)Ru(tpt)](PF(6))(2) have been synthesised and fully characterised. The attempted syntheses of heterodinuclear complexes with the tris(2-pyrid...
Article
Resonance Raman spectroscopy was utilized for the first time to characterize the nature of the 1metal-to-ligand charge transfer (MLCT) state of a ruthenium complex containing the redoxactive tetraazatetrapyrido-pentacene (tatpp) ligand. The presented results show that the long-wavelength absorption is originating from transitions involving the term...

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Projects (3)
Project
The design of the potential energy landscape of electronically excited states in discrete metal complexes is of utmost importance for unveiling and exploiting the photophysics and photochemistry of this class of compounds. The SPP 2102 aims at the development of rational design concepts for new functional photoactive metal complexes. At the core of the programme is the fundamental understanding of photoinduced metal-centred processes and the dynamics of electronically excited states of metal complexes.
Project
Characterization of light-induced processes in Cu(I) Complexes
Project
This project is part of Priority Program 2102 (Light controlled reactivity of metal complexes). It deals with the development of tailored heteroleptic Cu(I) photosensitizers and the detailed investigation of their (triplet) excited states as well as of their charge-transfer processes. For this purpose different design strategies and novel ligands will be applied to enhance stability and absorptivity. Furthermore, additional electron storage moieties will be introduced, which allow to affect the redox properties specifically as well as the efficiency and time constant of the photoinduced electron transfer. By the combination of synthesis, structural and spectroscopic characterization as well as application of novel photoactive Cu(I) complexes this project will establish heteroleptic copper photosensitizers as a valuable class of substances for the priority program SPP 2102. This will finally contribute to an increased utilization and conversion of sunlight as a promising and clean energy source.