Stephan Kupfer

Protonation effects on the resonance Raman properties of a novel (terpyridine)Ru(4H-imidazole) complex: an experimental and theoretical case study

The optically active states in a novel (terpyridine)Ru(4H-imidazole) complex displaying an unusually broad and red-shifted absorption in the visible range are investigated experimentally and theoretically. Since this property renders the complex promising for an application as sensitizer in dye-sensitized solar cells, a detailed knowledge on the correlation between features in the absorption spectrum and structural elements is indispensable in order to develop strategies for spectroscopy/theory-guided design of such molecular components. To this aim, time-dependent density functional theory calculations, including solvent effects, are employed to analyze the experimental UV-vis absorption and resonance Raman (RR) spectra of the unprotonated and protonated forms of the complex. This provides a detailed photophysical picture for a complex belonging to a novel class of Ru-polypyridine black absorbers, which can be tuned by external pH stimuli. The complex presents two absorption maxima in the visible region, which are assigned by the calculations to metal-to-ligand charge transfer (MLCT) and intra-ligand states, respectively. RR simulations are performed in resonance with both bands and are found to correctly reproduce the observed effects of protonation. Finally, the examination of the molecular orbitals and of the RR spectra for the MLCT state shows that protonation favors a charge transfer excitation to the 4H-imidazole ligand.

Self-healing mechanism of metallopolymers investigated by QM/MM simulations and Raman spectroscopy

The thermally induced self-healing mechanisms in metallopolymers based on bisterpyridine complexes of iron(II) sulfate and cadmium(II) bromide, respectively, were studied by means of combined quantum mechanical/molecular mechanical (QM/MM) simulations and Raman spectroscopy. Two possible healing schemes, one based on a decomplexation of the cross-linking complexes and a second one relying on the dissociation of ionic clusters, have been addressed. Temperature-dependent Raman spectroscopy displayed bathochromic shifts of the Raman intensity pattern upon heating. QM/MM simulations on the polymer models assign these alterations to a partial decomplexation of the metal terpyridine complexes, i.e. signals originating from free terpyridine ligands increase upon heating. Thus, a healing mechanisms based on partial decomplexation of the cross-linking complexes is suggested. The possibility that the dissociation of ionic clusters, which are assumed to be present in this class of self-healing polymers, is also responsible for the self-healing process was investigated as well. However, such calculations on model clusters revealed relatively strong binding of the clusters, which renders reversible cluster breaking and reformation upon temperature cycling in the range up to 100 °C unlikely.

Resonance-Raman spectro-electrochemistry of intermediates in molecular artificial photosynthesis of bimetallic complexes

The sequential order of photoinduced charge transfer processes and accompanying structure changes were analyzed by UV-vis and resonance-Raman spectroscopy of intermediates of a Ru(ii) based photocatalytic hydrogen evolving system obtained by electrochemical reduction.

Trapped in Imidazole: How to Accumulate Multiple Photoelectrons on a Black‐Absorbing Ruthenium Complex

Ruthenium dyes incorporating a 4H-imidazole chromophore as a ligand exhibit a spectrally broad absorption in the UV/Vis region. Furthermore, they show the ability to store two electrons within the 4H-imidazole ligand. These features render them promising molecular systems, for example, as inter- or intramolecular electron relays. To optimize the structures with respect to their electron-storage capability, it is crucial to understand the impact of structural changes accompanying photoinduced charge transfer in the electronic intermediates of multistep electron-transfer processes. The photophysical properties of these (reactive) intermediates might impact the function of the molecular systems quite substantially. However, the spectroscopic study of short-lived intermediates in stepwise multielectron-transfer processes is experimentally challenging. To this end, this contribution reports on the electrochemical generation of anions identical to intermediate structures and their spectroscopic characterization by in situ resonance Raman and UV/Vis spectroelectrochemistry and computational methods. Thereby, an efficient two-electron pathway to the 4H-imidazole electron-accepting ligand is identified.

An Assessment of RASSCF and TDDFT Energies and Gradients on an Organic Donor–Acceptor Dye Assisted by Resonance Raman Spectroscopy

The excitation energies and gradients in the ground and the first excited state of a novel donor-(π-bridge)-acceptor 4-methoxy-1,3-thiazole-based chromophore were investigated by means of MS-RASPT2/RASSCF and TDDFT in solution. Within both methods, the excitation energies strongly depend on the employed equilibrium structures, whose differences can be rationalized in terms of bond length alternation indexes. It is shown that functionals with an increased amount of exact exchange provide the best estimation of the ground and excited state properties. While B3LYP fails to predict the excitation energies due to its intrinsic problems in describing charge transfer (CT) states, the long-range corrected CAM-B3LYP and M06-2X functionals deliver good agreement with the experimental UV/vis absorption spectrum. The calculation of resonance Raman intensity patterns is used to discern which ground and excited state gradients are best. The results clearly evidence that both CAM-B3LYP and RASSCF excited state gradients and energies in combination with CAM-B3LYP ground state gradients are appropriate to describe the CT state of this push-pull chromophore.

The Self-Healing Potential of Triazole-Pyridine-Based Metallopolymers

The development of artificial self-healing materials represents an emerging and challenging field in material science. Inspired by nature-for instance by the self-healing of mussel byssus threads-metallopolymers gain more and more attention as attractive self-healing materials. These compounds are able to combine the properties of both polymers and metal-ligand interactions. A novel metallopolymer is developed consisting of attached bidentate triazole-pyridine (TRZ-py) ligands and a low glass transition temperature (T g ) lauryl methacrylate backbone. The polymer is cross-linked with different Fe(II) and Co(II) salts. The resulting materials exhibit promising self-healing performance within time intervals of 5.5 to 26.5 h at moderate temperatures of 50 to 100 °C. The materials are characterized by X-ray scattering (SAXS), UV-Vis spectroscopy, and light microscopy.

Photophysics of Ru(II) Dyads Derived from Pyrenyl-Substitued Imidazo[4,5-f][1,10]phenanthroline Ligands

The photophysics of a series of Ru(II) dyads based on the 2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]-phenanthroline ligand was investigated. The ability of these metal complexes to intercalate DNA and induce cell death upon photoactivation makes them attractive photosensitizers for a range of photobiological applications, including photodynamic therapy. In the present study, time-resolved transient absorption and emission spectroscopy were used to interrogate the photoinduced processes that follow metal-to-ligand charge transfer excitation of the complexes in solution. It was found that energy transfer to pyrene-localized intraligand triplet states, facilitated by torsional motion of the pyrene moiety relative to the imidazo[4,5-f][1,10]phenanthroline ligand, was an important relaxation pathway governing the photophysical dynamics in this class of compounds. Biphasic decay kinetics were assigned to spontaneous (pre-equilibrium) and delayed emission, arising from an equilibrium established between (3)MLCT and (3)IL states. TDDFT calculations supported these interpretations.

Sensitization of NO‐Releasing Ruthenium Complexes to Visible Light

We report a combined spectroscopical-theoretical investigation on the photosensitization of transition metal nitrosyl complexes. For this purpose, ruthenium nitrosyl complexes based on tetradentate biscarboxamide ligands were synthesized. A crystal structure analysis of a lithium-based ligand intermediate is described. The Ru complexes have been characterized regarding their photophysical and nitric oxide (NO) releasing properties. Quantum chemical calculations have been performed to unravel the influence of the biscarboxamide ligand frame with respect to the molecular electronic properties of the NO-releasing pathway. A quantitative measure for the ligand design within photosensitized Ru complexes is introduced and evaluated spectroscopically and theoretically by using time-dependent density functional theory.

Influence of Protonation State on the Excited State Dynamics of a Photobiologically Active Ru(II) Dyad

The influence of ligand protonation on the photophysics of a ruthenium (Ru) dyad bearing the 2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]-phenanthroline (ippy) ligand was investigated by time-resolved transient absorption spectroscopy. It was found that changes in the protonation state of the imidazole group led to changes in the electronic configuration of the lowest lying excited state. Formation of the fully deprotonated imidazole anion resulted in excited state signatures that were consistent with a low-lying intraligand (IL) triplet state. This assignment was supported by time-dependent density functional theory (TDDFT) calculations. IL triplet states have been suggested to be potent mediators of photodynamic effects. Thus, these results are of interest in the design of Ru metal complexes as photosensitizers (PSs) for photodynamic therapy (PDT).

And yet they glow: thiazole based push–pull fluorophores containing nitro groups and the influence of regioisomerism

Reported is a study on the influence of regioisomerism on the photophysical properties in 4-hydroxy-1,3-thiazole-based push-pull-chromophores/fluorophores to evaluate the molecular structure-property relationship as a basic foundation for future design strategies concerning this class of dyes. Surprisingly, the nitro groups used as acceptors do not act as a fluorescence quencher, instead the derivatives synthesized exhibit quantum yields of 37-40%. Two 4-ethoxy-1,3-thiazole derivatives which differ only in the positioning of their electron donating (methoxy) and electron withdrawing (nitro) groups have been synthesized and examined in terms of their photophysical properties, i.e. UV/Vis absorption and fluorescence emission spectra. Additionally, quantum chemical calculations have been performed to unravel the underlying fundamental transitions and to explain the experimental results.