Forschungsbereiche

Wir entwickeln und verwenden theoretische Methoden, um aggregationsbedingte Funktionen in supramolekularen Strukturen im Grund- und im angeregten Zustand zu untersuchen. Insbesondere untersuchen wir, wie supramolekulare Systeme (photo-)katalytische Reaktionen erleichtern können und wie strukturelle Motive den Ladungs- und Exzitonentransport beeinflussen. Wir wenden neue Simulationsmethoden der atomistischen Dynamik an, die uns einen Einblick in die mechanistischen Details dieser Prozesse ermöglichen. Darüber hinaus wollen wir verstehen, wie molekulare Aggregate gebildet werden und wie die Bildung von gewünschten strukturellen Motiven kontrolliert werden kann.

Veröffentlichungen

2024[ to top ]

Paschold, A., Schäffler, M., Miao, X., Gardon, L., Krüger, S., Heise, H., Röhr, M. I. S., Ott, M., Strodel, B., & Binder, W. H. (2024). Photocontrolled Reversible Amyloid Fibril Formation of Parathyroid Hormone-Derived Peptides. Bioconjugate Chemistry, 35(7), 981–995. https://doi.org/10.1021/acs.bioconjchem.4c00188
- [ DOI ]
Greiner, J. E., Singh, A., & Röhr, M. I. S. (2024). Functionality optimization for effective singlet fission coupling screening in the full-dimensional molecular and intermolecular coordinate space. Phys. Chem. Chem. Phys., 26(28), 19257-19265. https://doi.org/10.1039/D4CP01274G
- [ Abstract ]
- [ DOI ]
In computational chemistry{,} accurately predicting molecular configurations that exhibit specific properties remains a critical challenge. Its intricacies become especially evident in the study of molecular aggregates{,} where the light-induced functionality is tied to highly structure-dependent electronic couplings between molecules. Here{,} we present an efficient strategy for the targeted screening of the structural space employing a “functionality optimization” technique{,} in which a chosen descriptor{,} constrained by the ground state energy expression{,} is optimized. The chosen algorithmic differentiation (AD) framework allows one to automatically obtain gradients without its tedious implementation. We demonstrate the effectiveness of the approach by identifying perylene bisimide (PBI) dimer motifs with enhanced effective SF coupling. Our findings reveal that certain structural modifications of the PBI monomer{,} such as helical twisting and bending as well as slipped-rotated packing arrangements{,} can significantly increase the effective SF coupling.
Singh, A., & Röhr, M. I. S. (2024). Configuration Interaction in Frontier Molecular Orbital Basis for Screening the Spin-Correlated, Spatially Separated Triplet Pair State 1(T···T) Formation. Journal of Chemical Theory and Computation, 20(19), 8624–8633. https://doi.org/10.1021/acs.jctc.4c00473
- [ DOI ]
Greiner, J. E., Singh, A., & Röhr, M. I. S. (2024). Functionality optimization for effective singlet fission coupling screening in the full-dimensional molecular and intermolecular coordinate space. Phys. Chem. Chem. Phys., 26(28), 19257-19265. https://doi.org/10.1039/D4CP01274G
- [ Abstract ]
- [ DOI ]
In computational chemistry{,} accurately predicting molecular configurations that exhibit specific properties remains a critical challenge. Its intricacies become especially evident in the study of molecular aggregates{,} where the light-induced functionality is tied to highly structure-dependent electronic couplings between molecules. Here{,} we present an efficient strategy for the targeted screening of the structural space employing a “functionality optimization” technique{,} in which a chosen descriptor{,} constrained by the ground state energy expression{,} is optimized. The chosen algorithmic differentiation (AD) framework allows one to automatically obtain gradients without its tedious implementation. We demonstrate the effectiveness of the approach by identifying perylene bisimide (PBI) dimer motifs with enhanced effective SF coupling. Our findings reveal that certain structural modifications of the PBI monomer{,} such as helical twisting and bending as well as slipped-rotated packing arrangements{,} can significantly increase the effective SF coupling.
Sturm, F., Bühler, M., Stapper, C., Schneider, J. S., Helten, H., Fischer, I., & Röhr, M. I. S. (2024). Impact of isoelectronic substitution on the excited state processes in polycyclic aromatic hydrocarbons: a joint experimental and theoretical study of 4a{,}8a-azaboranaphthalene. Phys. Chem. Chem. Phys., 26(9), 7363-7370. https://doi.org/10.1039/D3CP05508F
- [ Abstract ]
- [ DOI ]
Substituting CC with the isoelectronic BN units is a promising approach to modify the optoelectronic properties of polycyclic aromatic hydrocarbons. While computational studies have already addressed trends in the electronic structure of the various isosteres{,} experimental data are still scarce. Here{,} the excited state spectroscopy and dynamics of 4a{,}8a-azaboranaphthalene were studied by picosecond time-resolved photoionization in a supersonic jet and analyzed with the aid of XMS-CASPT2 and time-dependent DFT calculations. A resonance-enhanced multiphoton ionization spectrum (REMPI) reveals the S1 origin at  = 33 830 ± 12 cm−1. Several vibrational bands were resolved and assigned by comparison with the computations. A [1+1] photoelectron spectrum via the S1 origin yielded an adiabatic ionization energy of 8.27 eV. Selected vibrational bands were subsequently investigated by pump–probe photoionization. While the origin as well as several low-lying vibronic states exhibit lifetimes in the ns-range{,} a monoexponential decay is observed at higher excitation energies{,} ranging from 400 ps at +1710 cm−1 to 13 ps at +3360 cm−1. The deactivation is attributed to an internal conversion of the optically excited S1 state via a barrier that gives access to a conical intersection (CI) to the S0 state. The doping significantly changes the energetic ordering of CIs and lowers the corresponding energy barrier for the associated deactivation pathway{,} as revealed by nudged elastic band (NEB) calculations.

2023[ to top ]

Singh, A., & Röhr, M. I. S. (2023). Formation Pathways of the Spin-Correlated, Spatially Separated $^{1}$(T...T) State in the Singlet Fission Process of Perylene Diimide Stacks. https://arxiv.org/abs/2312.13966

2022[ to top ]

Hong, Y., Rudolf, M., Kim, M., Kim, J., Schembri, T., Krause, A.-M., Shoyama, K., Bialas, D., R{ö}hr, M. I. S., Joo, T., Kim, H., Kim, D., & W{ü}rthner, F. (2022). Steering the multiexciton generation in slip-stacked perylene dye array via exciton coupling. Nature Communications, 13(1), 4488. https://doi.org/10.1038/s41467-022-31958-1
- [ Abstract ]
- [ DOI ]
Dye arrays from dimers up to larger oligomers constitute the functional units of natural light harvesting systems as well as organic photonic and photovoltaic materials. Whilst in the past decades many photophysical studies were devoted to molecular dimers for deriving structure-property relationship to unravel the design principles for ideal optoelectronic materials, they fail to accomplish the subsequent processes of charge carrier generation or the detachment of two triplet species in singlet fission (SF). Here, we present a slip-stacked perylene bisimide trimer, which constitutes a bridge between hitherto studied dimer and solid-state materials, to investigate SF mechanisms. This work showcases multiple pathways towards the multiexciton state through direct or excimer-mediated mechanisms by depending upon interchromophoric interaction. These results suggest the comprehensive role of the exciton coupling, exciton delocalization, and excimer state to facilitate the SF process. In this regard, our observations expand the fundamental understanding the structure-property relationship in dye arrays.

2021[ to top ]

Schäfer, N., Bühler, M., Heyer, L., Röhr, M. I. S., & Beuerle, F. (2021). Endohedral Hydrogen Bonding Templates the Formation of a Highly Strained Covalent Organic Cage Compound**. Chemistry – A European Journal, 27(19), 6077–6085. https://doi.org/10.1002/chem.202005276
- [ DOI ]
Bialas, D., Kirchner, E., Röhr, M. I. S., & Würthner, F. (2021). Perspectives in Dye Chemistry: A Rational Approach toward Functional Materials by Understanding the Aggregate State. Journal of the American Chemical Society, 143(12), 4500–4518. https://doi.org/10.1021/jacs.0c13245
- [ DOI ]
Singh, A., Humeniuk, A., & Röhr, M. I. S. (2021). Energetics and optimal molecular packing for singlet fission in BN-doped perylenes: electronic adiabatic state basis screening. Phys. Chem. Chem. Phys., 23(31), 16525-16536. https://doi.org/10.1039/D1CP01762D
- [ Abstract ]
- [ DOI ]
Singlet fission has the potential to increase the efficiency of photovoltaic devices{,} but the design of suitable chromophores is notoriously difficult. Both the electronic properties of the monomer and the packing motif in the crystal have a big impact on the singlet fission efficiency. Using perylene as an example{,} it is shown that doping with boron and nitrogen not only helps to align the energy levels but also shifts the stacking position that is optimal for singlet fission. Among all perylene derivatives doped with one or two BN groups{,} we identify the most suitable isomer for singlet fission with the help of TD-DFT and CASPT2 calculations. The optimal relative disposition of the two monomer units in a cofacially stacked homodimer is explored using two semiempirical models for the singlet fission rate: The first one is the well-known diabatic frontier orbital model{,} while the second treats singlet fission as a non-adiabatic transition and approximates the rate as the length squared of the non-adiabatic coupling vector between eigenfunctions of the diabatic Hamiltonian.
Rausch, R., Röhr, M. I. S., Schmidt, D., Krummenacher, I., Braunschweig, H., & Würthner, F. (2021). Tuning phenoxyl-substituted diketopyrrolopyrroles from quinoidal to biradical ground states through (hetero-)aromatic linkers. Chem. Sci., 12(2), 793-802. https://doi.org/10.1039/D0SC05475E
- [ Abstract ]
- [ DOI ]
Strongly fluorescent halochromic 2{,}6-di-tert-butyl-phenol-functionalised phenyl-{,} thienyl- and furyl-substituted diketopyrrolopyrrole (DPP) dyes were deprotonated and oxidised to give either phenylene-linked DPP1˙˙ biradical (y0 = 0.75) with a singlet open shell ground state and a thermally populated triplet state (ΔEST = 19 meV; 1.8 kJ mol−1; 0.43 kcal mol−1) or thienylene/furylene-linked DPP2q and DPP3q compounds with closed shell quinoidal ground states. Accordingly{,} we identified the aromaticity of the conjugated (hetero-)aromatic bridge to be key for modulating the electronic character of these biradicaloid compounds and achieved a spin crossover from closed shell quinones DPP2q and DPP3q to open shell biradical DPP1˙˙ as confirmed by optical and magnetic spectroscopic studies (UV/vis/NIR{,} NMR{,} EPR) as well as computational investigations (spin-flip TD-DFT calculations in combination with CASSCF(4{,}4) and harmonic oscillator model of aromaticity (HOMA) analysis). Spectroelectrochemical studies and comproportionation experiments further prove the reversible formation of mixed-valent radical anions for the DPP2q and DPP3q quinoidal compounds with absorption bands edging into the NIR spectral region.
Schmitt, H.-C., Fischer, I., Ji, L., Merz, J., Marder, T. B., Hoche, J., Röhr, M. I. S., & Mitric, R. (2021). Isolated 2-hydroxypyrene and its dimer: a frequency- and time-resolved spectroscopic study. New J. Chem., 45(33), 14949-14956. https://doi.org/10.1039/D0NJ02391D
- [ Abstract ]
- [ DOI ]
We investigated isolated 2-hydroxypyrene and its dimer in the gas phase by time- and frequency-resolved photoionisation with picosecond time-resolution. The experiments are supported by simulations that include an extensive conformational search based on the machine learning ANI-1ccx neural network potential combined with automatic structure classification using a data clustering algorithm. Vibrationally resolved spectra of the S1 ← S0 and S2 ← S0 transitions are reported which are in very good agreement with the simulated spectra at the TDDFT level. As expected from the molecular orbitals involved in the transitions{,} the red-shifts of the transitions are more pronounced for the S1 state compared to those of unsubstituted pyrene. While a ns-lifetime is observed for the S1 state{,} the lifetime decreases to 3 ps or less for the origin of the S2 state{,} indicating a strong interaction between the two states. For the dimer{,} a slightly V-shaped structure was computed{,} and intermolecular interactions are dominated by dispersion rather than hydrogen-bonding. The highest oscillator strength was computed for the transition to the S4 state{,} which deactivates within 4 ps to a lower-lying excited state.
Röhr, M. I. S. (2021). New theoretical methods for the exploration of functional landscapes. International Journal of Quantum Chemistry, 121(24), Article 24. https://doi.org/10.1002/qua.26747
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2020[ to top ]

Meza-Chincha, A.-L., Lindner, J. O., Schindler, D., Schmidt, D., Krause, A.-M., Röhr, M. I. S., Mitrić, R., & Würthner, F. (2020). Impact of substituents on molecular properties and catalytic activities of trinuclear Ru macrocycles in water oxidation. Chem. Sci., 11(29), 7654-7664. https://doi.org/10.1039/D0SC01097A
- [ Abstract ]
- [ DOI ]
Herein we report a broad series of new trinuclear supramolecular Ru(bda) macrocycles bearing different substituents at the axial or equatorial ligands which enabled investigation of substituent effects on the catalytic activities in chemical and photocatalytic water oxidation. Our detailed investigations revealed that the activities of these functionalized macrocycles in water oxidation are significantly affected by the position at which the substituents were introduced. Interestingly{,} this effect could not be explained based on the redox properties of the catalysts since these are not markedly influenced by the functionalization of the ligands. Instead{,} detailed investigations by X-ray crystal structure analysis and theoretical simulations showed that conformational changes imparted by the substituents are responsible for the variation of catalytic activities of the Ru macrocycles. For the first time{,} macrocyclic structure of this class of water oxidation catalysts is unequivocally confirmed and experimental indication for a hydrogen-bonded water network present in the cavity of the macrocycles is provided by crystal structure analysis. We ascribe the high catalytic efficiency of our Ru(bda) macrocycles to cooperative proton abstractions facilitated by such a network of preorganized water molecules in their cavity{,} which is reminiscent of catalytic activities of enzymes at active sites.
Wehner, M., R{ö}hr, M. I. S., Stepanenko, V., & W{ü}rthner, F. (2020). Control of self-assembly pathways toward conglomerate and racemic supramolecular polymers. Nature Communications, 11(1), 5460. https://doi.org/10.1038/s41467-020-19189-8
- [ Abstract ]
- [ DOI ]
Homo- and heterochiral aggregation during crystallization of organic molecules has significance both for fundamental questions related to the origin of life as well as for the separation of homochiral compounds from their racemates in industrial processes. Herein, we analyse these phenomena at the lowest level of hierarchy -- that is the self-assembly of a racemic mixture of (R,R)- and (S,S)-PBI into 1D supramolecular polymers. By a combination of UV/vis and NMR spectroscopy as well as atomic force microscopy, we demonstrate that homochiral aggregation of the racemic mixture leads to the formation of two types of supramolecular conglomerates under kinetic control, while under thermodynamic control heterochiral aggregation is preferred, affording a racemic supramolecular polymer. FT-IR spectroscopy and quantum-chemical calculations reveal unique packing arrangements and hydrogen-bonding patterns within these supramolecular polymers. Time-, concentration- and temperature-dependent UV/vis experiments provide further insights into the kinetic and thermodynamic control of the conglomerate and racemic supramolecular polymer formation.

2019[ to top ]

Lindner, J. O., Sultangaleeva, K., Röhr, M. I. S., & Mitrić, R. (2019). metaFALCON: A Program Package for Automatic Sampling of Conical Intersection Seams Using Multistate Metadynamics. Journal of Chemical Theory and Computation, 15(6), 3450–3460. https://doi.org/10.1021/acs.jctc.9b00029
- [ DOI ]
Schreck, M. H., Röhr, M. I. S., Clark, T., Stepanenko, V., Würthner, F., & Lambert, C. (2019). A Self‐Assembled Unit Comprising 12 Squaraine Dyes Built Up from Two Star‐Shaped Hexasquarainyl‐Benzene Molecules. Chemistry – A European Journal, 25(11), 2831–2839. https://doi.org/10.1002/chem.201805685
- [ DOI ]
Lindner, J. O., & Röhr, M. I. S. (2019). Metadynamics for automatic sampling of quantum property manifolds: exploration of molecular biradicality landscapes. Phys. Chem. Chem. Phys., 21(44), 24716-24722. https://doi.org/10.1039/C9CP05182A
- [ Abstract ]
- [ DOI ]
We present a general extension of the metadynamics allowing for an automatic sampling of quantum property manifolds (ASQPM) giving rise to functional landscapes that are analogous to the potential energy surfaces in the frame of the Born–Oppenheimer approximation. For this purpose{,} we employ generalized electronic collective variables to carry out biased molecular dynamics simulations in the framework of quantum chemical methods that explore the desired property manifold. We illustrate our method on the example of the “biradicality landscapes”{,} which we explore by introducing the natural orbital occupation numbers (NOONs) as the electronic collective variable driving the dynamics. We demonstrate the applicability of the method on the simulation of p-xylylene and [8]annulene allowing to automatically extract the biradical geometries. In the case of [8]annulene the ASQPM metadynamics leads to the prediction of biradical scaffolds that can be stabilized by a suitable chemical substitution{,} leading to the design of novel functional molecules exhibiting biradical functionality.
Auerhammer, N., Schulz, A., Schmiedel, A., Holzapfel, M., Hoche, J., Röhr, M. I. S., Mitric, R., & Lambert, C. (2019). Dynamic exciton localisation in a pyrene–BODIPY–pyrene dye conjugate. Phys. Chem. Chem. Phys., 21(18), 9013-9025. https://doi.org/10.1039/C9CP00908F
- [ Abstract ]
- [ DOI ]
The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad{,} the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.
Löhner, A., Kunsel, T., Röhr, M. I. S., Jansen, T. L. C., Sengupta, S., Würthner, F., Knoester, J., & Köhler, J. (2019). Spectral and Structural Variations of Biomimetic Light-Harvesting Nanotubes. The Journal of Physical Chemistry Letters, 10(11), 2715–2724. https://doi.org/10.1021/acs.jpclett.9b00303
- [ DOI ]
Wehner, M., Röhr, M. I. S., Bühler, M., Stepanenko, V., Wagner, W., & Würthner, F. (2019). Supramolecular Polymorphism in One-Dimensional Self-Assembly by Kinetic Pathway Control. Journal of the American Chemical Society, 141(14), 6092–6107. https://doi.org/10.1021/jacs.9b02046
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2018[ to top ]

Röhr, M. I. S., Marciniak, H., Hoche, J., Schreck, M. H., Ceymann, H., Mitric, R., & Lambert, C. (2018). Exciton Dynamics from Strong to Weak Coupling Limit Illustrated on a Series of Squaraine Dimers. The Journal of Physical Chemistry C, 122(15), 8082–8093. https://doi.org/10.1021/acs.jpcc.8b00847
- [ DOI ]
Lindner, J. O., Röhr, M. I. S., & Mitrić, R. (2018). Multistate metadynamics for automatic exploration of conical intersections. Physical Review A, 97(5), Article 5. https://doi.org/10.1103/physreva.97.052502
- [ DOI ]
B{ö}hnke, J., Dellermann, T., Celik, M. A., Krummenacher, I., Dewhurst, R. D., Demeshko, S., Ewing, W. C., Hammond, K., Heß, M., Bill, E., Welz, E., R{ö}hr, M. I. S., Mitri{{{\’c}}}, R., Engels, B., Meyer, F., & Braunschweig, H. (2018). Isolation of diborenes and their 90{\textdegree}-twisted diradical congeners. Nature Communications, 9(1), 1197. https://doi.org/10.1038/s41467-018-02998-3
- [ Abstract ]
- [ DOI ]
Molecules containing multiple bonds between atoms---most often in the form of olefins---are ubiquitous in nature, commerce, and science, and as such have a huge impact on everyday life. Given their prominence, over the last few decades, frequent attempts have been made to perturb the structure and reactivity of multiply-bound species through bending and twisting. However, only modest success has been achieved in the quest to completely twist double bonds in order to homolytically cleave the associated $\pi$ bond. Here, we present the isolation of double-bond-containing species based on boron, as well as their fully twisted diradical congeners, by the incorporation of attached groups with different electronic properties. The compounds comprise a structurally authenticated set of diamagnetic multiply-bound and diradical singly-bound congeners of the same class of compound.

2017[ to top ]

Kunz, V., Schmidt, D., Röhr, M. I. S., Mitrić, R., & Würthner, F. (2017). Supramolecular Approaches to Improve the Performance of Ruthenium‐Based Water Oxidation Catalysts. Advanced Energy Materials, 7(16), Article 16. https://doi.org/10.1002/aenm.201602939
- [ DOI ]
Hoche, J., Schmitt, H.-C., Humeniuk, A., Fischer, I., Mitrić, R., & Röhr, M. I. S. (2017). The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer. Phys. Chem. Chem. Phys., 19(36), 25002-25015. https://doi.org/10.1039/C7CP03990E
- [ Abstract ]
- [ DOI ]
The understanding of excimer formation in organic materials is of fundamental importance{,} since excimers profoundly influence their functional performance in applications such as light-harvesting{,} photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet{,} which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Franck–Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for several picoseconds. The second excimer forming pathway involves large amplitude oscillations along the parallel shift coordinate with a period of ≈900 fs that after intramolecular vibrational energy redistribution leads to the formation of a perfectly stacked dimer. The electronic relaxation within the excitonic manifold is mediated by the presence of intermolecular conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6–7 ps{,} in good agreement with theory. Finally{,} in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the long-range corrected tight-binding TDDFT. In contrast to the isolated dimer{,} the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by the interaction with the surrounding molecules leading to the slow excimer stabilization on a picosecond time scale.
Nowak‐Król, A., Röhr, M. I. S., Schmidt, D., & Würthner, F. (2017). A Crystalline π‐Stack Containing Five Stereoisomers: Insights into Conformational Isomorphism, Chirality Inversion, and Disorder. Angewandte Chemie, 129(39), 11936–11940. https://doi.org/10.1002/ange.201705445
- [ DOI ]
Kunz, V., Lindner, J. O., Schulze, M., Röhr, M. I. S., Schmidt, D., Mitrić, R., & Würthner, F. (2017). Cooperative water oxidation catalysis in a series of trinuclear metallosupramolecular ruthenium macrocycles. Energy Environ. Sci., 10(10), 2137-2153. https://doi.org/10.1039/C7EE01557G
- [ Abstract ]
- [ DOI ]
A series of trinuclear metallosupramolecular Ru(bda) macrocycles has been synthesized and the impact of the ring size on the catalytic activity in homogeneous water oxidation has been elucidated (bda = 2{,}2′-bipyridine-6{,}6′-dicarboxylate). Kinetic experiments revealed that for all macrocyclic catalysts a proton-coupled oxidation event is rate-determining. Distinct H/D kinetic isotope effects (KIE) have been observed for the individual macrocycles{,} which follow the same tendency like the catalytic activity where an intermediate ring size exhibits the best performance. To get insight into the different molecular properties{,} molecular dynamics simulations using a QM/MM approach have been performed for all reaction steps{,} revealing that the size of the cavity strongly influences the hydrogen-bonded water network inside the macrocyclic structure and thus{,} important proton-coupled reaction steps of the catalytic cycle. For the most active catalysts{,} all reactive sites point into the interior of the cavity{,} which is supported by a single crystal X-ray analysis of the largest macrocycle{,} and a cooperative effect between the individual Ru(bda) centers facilitating proton abstraction has been substantiated. On the other hand{,} different structural conformations predominate the smaller macrocycles{,} which prevent efficient interactions between the Ru(bda) centers.

2016[ to top ]

Röhr, M. I. S., Mitrić, R., & Petersen, J. (2016). Vibrationally resolved optical spectra and ultrafast electronic relaxation dynamics of diamantane. Phys. Chem. Chem. Phys., 18(12), 8701-8709. https://doi.org/10.1039/C6CP00137H
- [ Abstract ]
- [ DOI ]
We present theoretical simulations of the vibrationally resolved photoabsorption and photoemission spectra of diamantane combined with nonadiabatic dynamics simulations in order to identify the state responsible for the measured photoluminescence of diamantane and to determine the mechanism and the time-scales of the electronic state relaxation. Diamantane is a prototype representative of the diamondoid class of hydrocarbons which have recently gained significant interest due to their unique electronic properties. This molecule is characterised by an almost dark first excited state{,} which therefore cannot be directly excited. Moreover{,} the calculated vertical transition from the geometrically relaxed first excited state to the ground state also bears no intensity. However{,} recent experiments suggest that the observed photoluminescence originates from the lowest excited state. We have performed spectral simulations in the frame of the Herzberg–Teller approximation for vibronic transitions{,} which goes beyond the Franck–Condon approximation of constant transition dipole moments and takes into account their linear dependence on the geometrical deformations. In this way{,} the available experimental spectrum could be fully reproduced{,} resolving the issue about the origin of the photoluminescence. Moreover{,} the photoemission from the first excited state also implies that ultrafast nonradiative processes have to take place after the initial excitation of the bright electronic states. We have determined the mechanism and time-scales of these relaxation processes by performing nonadiabatic dynamics simulations in the manifold of s- and p-type Rydberg excited states. The simulations demonstrate that the lowest excited electronic state of diamantane gains significant population from higher-lying states already after several hundreds of femtoseconds. Thus{,} our dynamics simulations combined with spectra calculated using the Herzberg–Teller approximation allow us to fully explain the observed photoabsorption and photoemission properties of diamantane.
Lisinetskaya, P. G., R{ö}hr, M. I. S., & Mitri{{{\’c}}}, R. (2016). First-principles simulation of light propagation and exciton dynamics in metal cluster nanostructures. Applied Physics B, 122(6), 175. https://doi.org/10.1007/s00340-016-6436-6
- [ Abstract ]
- [ DOI ]
We present a theoretical approach for the simulation of the electric field and exciton propagation in ordered arrays constructed of molecular-sized noble metal clusters bound to organic polymer templates. In order to describe the electronic coupling between individual constituents of the nanostructure we use the ab initio parameterized transition charge method which is more accurate than the usual dipole--dipole coupling. The electronic population dynamics in the nanostructure under an external laser pulse excitation is simulated by numerical integration of the time-dependent Schr{ö}dinger equation employing the fully coupled Hamiltonian. The solution of the TDSE gives rise to time-dependent partial point charges for each subunit of the nanostructure, and the spatio-temporal electric field distribution is evaluated by means of classical electrodynamics methods. The time-dependent partial charges are determined based on the stationary partial and transition charges obtained in the framework of the TDDFT. In order to treat large plasmonic nanostructures constructed of many constituents, the approximate self-consistent iterative approach presented in (Lisinetskaya and Mitri{{{\'c}}} in Phys Rev B 89:035433, 2014) is modified to include the transition-charge-based interaction. The developed methods are used to study the optical response and exciton dynamics of {\$}{\$}{\backslash}hbox{\{}Ag{\}}{\_}{\{}3{\}}^{\{}+{\}}{\$}{\$}and porphyrin-Ag4 dimers. Subsequently, the spatio-temporal electric field distribution in a ring constructed of ten porphyrin-Ag4 subunits under the action of circularly polarized laser pulse is simulated. The presented methodology provides a theoretical basis for the investigation of coupled light-exciton propagation in nanoarchitectures built from molecular size metal nanoclusters in which quantum confinement effects are important.
Röhr, M. I. S., Lisinetskaya, P. G., & Mitric, R. (2016). Excitonic Properties of Ordered Metal Nanocluster Arrays: 2D Silver Clusters at Multiporphyrin Templates. The Journal of Physical Chemistry A, 120(26), 4465–4472. https://doi.org/10.1021/acs.jpca.6b04243
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2015[ to top ]

Lambert, C., Koch, F., Völker, S. F., Schmiedel, A., Holzapfel, M., Humeniuk, A., Röhr, M. I. S., Mitric, R., & Brixner, T. (2015). Energy Transfer Between Squaraine Polymer Sections: From Helix to Zigzag and All the Way Back. Journal of the American Chemical Society, 137(24), 7851–7861. https://doi.org/10.1021/jacs.5b03644
- [ DOI ]
Richter, R., Röhr, M. I. S., Zimmermann, T., Petersen, J., Heidrich, C., Rahner, R., Möller, T., Dahl, J. E., Carlson, R. M. K., Mitric, R., Rander, T., & Merli, A. (2015). Laser-induced fluorescence of free diamondoid molecules. Phys. Chem. Chem. Phys., 17(6), 4739-4749. https://doi.org/10.1039/C4CP04423A
- [ Abstract ]
- [ DOI ]
We observe the fluorescence of pristine diamondoids in the gas phase{,} excited using narrow band ultraviolet laser light. The emission spectra show well-defined features{,} which can be attributed to transitions from the excited electronic state into different vibrational modes of the electronic ground state. We assign the normal modes responsible for the vibrational bands{,} and determine the geometry of the excited states. Calculations indicate that for large diamondoids{,} the spectral bands do not result from progressions of single modes{,} but rather from combination bands composed of a large number of Δv = 1 transitions. The vibrational modes determining the spectral envelope can mainly be assigned to wagging and twisting modes of the surface atoms. We conclude that our theoretical approach accurately describes the photophysics in diamondoids and possibly other hydrocarbons in general.
Megow, J., Röhr, M. I. S., Schmidt am Busch, M., Renger, T., Mitrić, R., Kirstein, S., Rabe, J. P., & May, V. (2015). Site-dependence of van der Waals interaction explains exciton spectra of double-walled tubular J-aggregates. Phys. Chem. Chem. Phys., 17(10), 6741-6747. https://doi.org/10.1039/C4CP05945J
- [ Abstract ]
- [ DOI ]
The simulation of the optical properties of supramolecular aggregates requires the development of methods{,} which are able to treat a large number of coupled chromophores interacting with the environment. Since it is currently not possible to treat large systems by quantum chemistry{,} the Frenkel exciton model is a valuable alternative. In this work we show how the Frenkel exciton model can be extended in order to explain the excitonic spectra of a specific double-walled tubular dye aggregate explicitly taking into account dispersive energy shifts of ground and excited states due to van der Waals interaction with all surrounding molecules. The experimentally observed splitting is well explained by the site-dependent energy shift of molecules placed at the inner or outer side of the double-walled tube{,} respectively. Therefore we can conclude that inclusion of the site-dependent dispersive effect in the theoretical description of optical properties of nanoscaled dye aggregates is mandatory.

2013[ to top ]

Gell, L., Kulesza, A., Petersen, J., Röhr, M. I. S., Mitrić, R., & Bonačić-Koutecký, V. (2013). Tuning Structural and Optical Properties of Thiolate-Protected Silver Clusters by Formation of a Silver Core with Confined Electrons. The Journal of Physical Chemistry C, 117(28), 14824–14831. https://doi.org/10.1021/jp402931w
- [ DOI ]
Röhr, M. I. S., Petersen, J., Wohlgemuth, M., Bonačić‐Koutecký, V., & Mitrić, R. (2013). Nonlinear Absorption Dynamics Using Field‐Induced Surface Hopping: Zinc Porphyrin in Water. ChemPhysChem, 14(7), 1377–1386. https://doi.org/10.1002/cphc.201300053
- [ DOI ]

2012[ to top ]

Röhr, M. I. S., Petersen, J., Brunet, C., Antoine, R., Broyer, M., Dugourd, P., Bonačić-Koutecký, V., O’Hair, R. A. J., & Mitrić, R. (2012). Synthesis and Spectroscopic Characterization of Diphenylargentate, [(C6H5)2Ag]−. The Journal of Physical Chemistry Letters, 3(9), 1197–1201. https://doi.org/10.1021/jz300280f
- [ DOI ]

2011[ to top ]

Brunet, C., Antoine, R., Broyer, M., Dugourd, P., Kulesza, A., Petersen, J., Röhr, M. I. S., Mitrić, R., Bonačić-Koutecký, V., & O’Hair, R. A. J. (2011). Structural and Photochemical Properties of Organosilver Reactive Intermediates MeAg2+and PhAg2+. The Journal of Physical Chemistry A, 115(33), 9120–9127. https://doi.org/10.1021/jp205716a
- [ DOI ]
Mitrić, R., Petersen, J., Kulesza, A., Röhr, M. I. S., Bonačić-Koutecký, V., Brunet, C., Antoine, R., Dugourd, P., Broyer, M., & O’Hair, R. A. J. (2011). Gas-Phase Synthesis and Vibronic Action Spectroscopy of Ag2H+. The Journal of Physical Chemistry Letters, 2(6), 548–552. https://doi.org/10.1021/jz101733v
- [ DOI ]

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