TY - INPR A1 - Auerhammer, Nina A1 - Schulz, Alexander A1 - Schmiedel, Alexander A1 - Holzapfel, Marco A1 - Hoche, Joscha A1 - Röhr, Merle I. S. A1 - Mitric, Roland A1 - Lambert, Christoph T1 - Dynamic exciton localisation in a pyrene-BODIPY-pyrene dye conjugate T2 - Physical Chemistry Chemical Physics N2 - 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. KW - Exciton localization dynamics Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-198718 UR - https://doi.org/10.1039/C9CP00908F N1 - Accepted manuscript ER - TY - INPR A1 - Böhnke, Julian A1 - Dellermann, Theresa A1 - Celik, Mehmet Ali A1 - Krummenacher, Ivo A1 - Dewhurst, Rian D. A1 - Demeshko, Serhiy A1 - Ewing, William C. A1 - Hammond, Kai A1 - Heß, Merlin A1 - Bill, Eckhard A1 - Welz, Eileen A1 - Röhr, Merle I. S. A1 - Mitric, Roland A1 - Engels, Bernd A1 - Meyer, Franc A1 - Braunschweig, Holger T1 - Isolation of diradical products of twisted double bonds T2 - Nature Communications N2 - 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 π 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. KW - diradicals KW - diborenes KW - carbenes KW - boron Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-160248 N1 - Submitted version of Julian Böhnke, Theresa Dellermann, Mehmet Ali Celik, Ivo Krummenacher, Rian D. Dewhurst, Serhiy Demeshko, William C. Ewing, Kai Hammond, Merlin Heß, Eckhard Bill, Eileen Welz, Merle I. S. Röhr, Roland Mitrić, Bernd Engels, Franc Meyer & Holger Braunschweig: Isolation of diborenes and their 90°-twisted diradical congeners. Nature Communications. Volume 9, Article number: 1197 (2018) doi:10.1038/s41467-018-02998-3 ER - TY - INPR A1 - Hoche, Joscha A1 - Schmitt, Hans-Christian A1 - Humeniuk, Alexander A1 - Fischer, Ingo A1 - Mitrić, Roland A1 - Röhr, Merle I. S. T1 - The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer T2 - Physical Chemistry Chemical Physics N2 - 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. KW - exciton dynamics KW - pyrene dimer Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-159656 UR - http://dx.doi.org/10.1039/C7CP03990E N1 - Submitted version ER - TY - INPR A1 - Lambert, Christoph A1 - Völker, Sebastian F. A1 - Koch, Federico A1 - Schmiedel, Alexander A1 - Holzapfel, Marco A1 - Humeniuk, Alexander A1 - Röhr, Merle I. S. A1 - Mitric, Roland A1 - Brixner, Tobias T1 - Energy Transfer Between Squaraine Polymer Sections: From helix to zig-zag and All the Way Back T2 - Journal of the American Chemical Society N2 - Joint experimental and theoretical study of the absorption spectra of squaraine polymers in solution provide evidence that two different conformations are present in solution: a helix and a zig-zag structure. This unique situation allows investigating ultrafast energy transfer processes between different structural segments within a single polymer chain in solution. The understanding of the underlying dynamics is of fundamental importance for the development of novel materials for light-harvesting and optoelectronic applications. We combine here femtosecond transient absorption spectroscopy with time-resolved 2D electronic spectroscopy showing that ultrafast energy transfer within the squaraine polymer chains proceeds from initially excited helix segments to zig-zag segments or vice versa, depending on the solvent as well as on the excitation wavenumber. These observations contrast other conjugated polymers such as MEH-PPV where much slower intrachain energy transfer was reported. The reason for the very fast energy transfer in squaraine polymers is most likely a close matching of the density of states between donor and acceptor polymer segments because of very small reorganization energy in these cyanine-like chromophores. KW - energy transfer dynamics KW - squaraine polymer Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-159607 UR - http://dx.doi.org/10.1021/jacs.5b03644 N1 - This document is the unedited Author's version of a Submitted Work that war subsequently accepted for publication in Journal of the American Chemical Society, copyright American Chemical Society after peer review. To access the final edited and published work see doi:10.1021/jacs.5b03644. ER - TY - INPR A1 - Lindner, Joachim O. A1 - Sultangaleeva, Karina A1 - Röhr, Merle I. S. A1 - Mitric, Roland T1 - metaFALCON: A program package for automatic sampling of conical intersection seams using multistate metadynamics T2 - Journal of Chemical Theory and Computation N2 - The multistate metadynamics for automatic exploration of conical intersection seams and systematic location of minimum energy crossing points in molecular systems and its implementation into the software package metaFALCON is presented. Based on a locally modified energy gap between two Born–Oppenheimer electronic states as a collective variable, multistate metadynamics trajectories are driven toward an intersection point starting from an arbitrary ground state geometry and are subsequently forced to explore the conical intersection seam landscape. For this purpose, an additional collective variable capable of distinguishing structures within the seam needs to be defined and an additional bias is introduced into the off-diagonal elements of an extended (multistate) electronic Hamiltonian. We demonstrate the performance of the algorithm on the examples of the 1,3-butadiene, benzene, and 9H-adenine molecules, where multiple minimum energy crossing points could be systematically located using the Wiener number or Cremer–Pople parameters as collective variables. Finally, with the example of 9H-adenine, we show that the multistate metadynamics potential can be used to obtain a global picture of a conical intersection seam. Our method can be straightforwardly connected with any ab initio or semiempirical electronic structure theory that provides energies and gradients of the respective electronic states and can serve for systematic elucidation of the role of conical intersections in the photophysics and photochemistry of complex molecular systems, thus complementing nonadiabatic dynamics simulations. KW - Computational Chemistry KW - Metadynamics KW - Hydrogen KW - Hydrocarbons KW - Chemical Structure Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-199258 UR - https://doi.org/10.1021/acs.jctc.9b00029 N1 - This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Chemical Theory and Computation, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see Journal of Chemical Theory and Computation 2019, 15, 6, 3450-3460. https://doi.org/10.1021/acs.jctc.9b00029. ER -