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 - JOUR A1 - Verma, Pramod Kumar A1 - Steinbacher, Andreas A1 - Schmiedel, Alexander A1 - Nuernberger, Patrick A1 - Brixner, Tobias T1 - Excited-state intramolecular proton transfer of 2-acetylindan-1,3-dione studied by ultrafast absorption and fluorescence spectroscopy JF - Structural Dynamics N2 - We employ transient absorption from the deep-UV to the visible region and fluorescence upconversion to investigate the photoinduced excited-state intramolecular proton-transfer dynamics in a biologically relevant drug molecule, 2-acetylindan-1,3-dione. The molecule is a ß-diketone which in the electronic ground state exists as exocyclic enol with an intramolecular H-bond. Upon electronic excitation at 300 nm, the first excited state of the exocyclic enol is initially populated, followed by ultrafast proton transfer (≈160 fs) to form the vibrationally hot endocyclic enol. Subsequently, solvent-induced vibrational relaxation takes place (≈10 ps) followed by decay (≈390 ps) to the corresponding ground state. KW - time resolved spectroscopy KW - ground states KW - fluorescence spectra KW - absorption spectra KW - ultraviolet light KW - hydrogen bonding KW - excited states KW - reaction mechanisms KW - fluorescence KW - solvents Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-181301 VL - 3 ER - TY - JOUR A1 - Wächtler, Maria A1 - Kübel, Joachim A1 - Barthelmes, Kevin A1 - Winter, Andreas A1 - Schmiedel, Alexander A1 - Pascher, Torbjörn A1 - Lambert, Christoph A1 - Schubert, Ulrich S. A1 - Dietzek, Benjamin T1 - Energy transfer and formation of long-lived \(^3\)MLCT states in multimetallic complexes with extended highly conjugated bis-terpyridyl ligands JF - Physical Chemistry Chemical Physics N2 - Multimetallic complexes with extended and highly conjugated bis-2,2':6',2''-terpyridyl bridging ligands, which present building blocks for coordination polymers, are investigated with respect to their ability to act as light-harvesting antennae. The investigated species combine Ru(II)- with Os(II)- and Fe(II)-terpyridyl chromophores, the latter acting as energy sinks. Due to the extended conjugated system the ligands are able to prolong the lifetime of the \(^3\)MLCT states compared to unsubstituted terpyridyl species by delocalization and energetic stabilization of the \(^3\)MLCT states. This concept is applied for the first time to Fe(II) terpyridyl species and results in an exceptionally long lifetime of 23 ps for the Fe(II) \(^3\)MLCT state. While partial energy (>80%) transfer is observed between the Ru(II) and Fe(II) centers with a time-constant of 15 ps, excitation energy is transferred completely from the Ru(II) to the Os(II) center within the first 200 fs after excitation. KW - polypyridyl complexes KW - bis-terpyridyl ligands KW - multimetallic complexes KW - metal-to-ligand charge transfer (MLCT) KW - RU-(II) complexes KW - Ru(II)–Fe(II)–Ru(II) complex Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-191041 VL - 18 IS - 4 ER - 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 - JOUR A1 - Hattori, Yohei A1 - Michail, Evripidis A1 - Schmiedel, Alexander A1 - Moos, Michael A1 - Holzapfel, Marco A1 - Krummenacher, Ivo A1 - Braunschweig, Holger A1 - Müller, Ulrich A1 - Pflaum, Jens A1 - Lambert, Christoph T1 - Luminescent Mono-, Di-, and Tri-radicals: Bridging Polychlorinated Triarylmethyl Radicals by Triarylamines and Triarylboranes JF - Chemistry - A European Journal N2 - Up to three polychlorinated pyridyldiphenylmethyl radicals bridged by a triphenylamine carrying electron withdrawing (CN), neutral (Me), or donating (OMe) groups were synthesized and analogous radicals bridged by tris(2,6‐dimethylphenyl)borane were prepared for comparison. All compounds were as stable as common closed‐shell organic compounds and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and emission properties were examined in detail, and experimental results were interpreted using DFT calculations. Oxidation potentials, absorption and emission energies could be tuned depending on the electron density of the bridges. The triphenylamine bridges mediated intramolecular weak antiferromagnetic interactions between the radical spins, and the energy difference between the high spin and low spin states was determined by temperature dependent ESR spectroscopy and DFT calculations. The fluorescent properties of all radicals were examined in detail and revealed no difference for high and low spin states which facilitates application of these dyes in two‐photon absorption spectroscopy and OLED devices. KW - density functional calculations KW - fluorescence KW - NIR OLED KW - radical KW - two-photon absorption Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-208162 VL - 25 IS - 68 ER - TY - JOUR A1 - Lübtow, Michael M. A1 - Marciniak, Henning A1 - Schmiedel, Alexander A1 - Roos, Markus A1 - Lambert, Christoph A1 - Luxenhofer, Robert T1 - Ultra-high to ultra-low drug loaded micelles: Probing host-guest interactions by fluorescence spectroscopy JF - Chemistry - A European Journal N2 - Polymer micelles are an attractive means to solubilize water insoluble compounds such as drugs. Drug loading, formulations stability and control over drug release are crucial factors for drug‐loaded polymer micelles. The interactions between the polymeric host and the guest molecules are considered critical to control these factors but typically barely understood. Here, we compare two isomeric polymer micelles, one of which enables ultra‐high curcumin loading exceeding 50 wt.%, while the other allows a drug loading of only 25 wt.%. In the low capacity micelles, steady‐state fluorescence revealed a very unusual feature of curcumin fluorescence, a high energy emission at 510 nm. Time‐resolved fluorescence upconversion showed that the fluorescence life time of the corresponding species is too short in the high‐capacity micelles, preventing an observable emission in steady‐state. Therefore, contrary to common perception, stronger interactions between host and guest can be detrimental to the drug loading in polymer micelles. KW - curcumin KW - drug delivery KW - fluorenscence KW - poly(2-oxazine) KW - pol(2-oxazoline) KW - Polymer-drug interaction KW - upconversion Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-206128 VL - 25 IS - 54 ER - TY - JOUR A1 - Mieczkowski, Mateusz A1 - Steinmetzger, Christian A1 - Bessi, Irene A1 - Lenz, Ann-Kathrin A1 - Schmiedel, Alexander A1 - Holzapfel, Marco A1 - Lambert, Christoph A1 - Pena, Vladimir A1 - Höbartner, Claudia T1 - Large Stokes shift fluorescence activation in an RNA aptamer by intermolecular proton transfer to guanine JF - Nature Communications N2 - Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO+ and DMHBI+ ligands revealed a G-quadruplex and a trans-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer. KW - Fluorogenic RNA Aptamers KW - Synthetic Functional RNAs KW - Chili RNA Aptamer KW - Co-Crystal Structures of Chili RNA KW - RNA KW - Optical Spectroscopy KW - Structural Biology KW - X-ray Crystallography Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-254527 VL - 12 ER - TY - JOUR A1 - Mieczkowski, Mateusz A1 - Steinmetzger, Christian A1 - Bessi, Irene A1 - Lenz, Ann-Kathrin A1 - Schmiedel, Alexander A1 - Holzapfel, Marco A1 - Lambert, Christoph A1 - Pena, Vladimir A1 - Höbartner, Claudia T1 - Large Stokes shift fluorescence activation in an RNA aptamer by intermolecular proton transfer to guanine JF - Nature Communications N2 - Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO+ and DMHBI+ ligands revealed a G-quadruplex and a trans-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer. KW - RNA KW - optical spectroscopy KW - structural biology KW - X-ray crystallography Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-270274 VL - 12 ER - TY - JOUR A1 - Bold, Kevin A1 - Stolte, Matthias A1 - Shoyama, Kazutaka A1 - Holzapfel, Marco A1 - Schmiedel, Alexander A1 - Lambert, Christoph A1 - Würthner, Frank T1 - Macrocyclic donor-acceptor dyads composed of a perylene bisimide dye surrounded by oligothiophene bridges JF - Angewandte Chemie Internationale Edition N2 - Two macrocyclic architectures comprising oligothiophene strands that connect the imide positions of a perylene bisimide (PBI) dye have been synthesized via a platinum-mediated cross-coupling strategy. The crystal structure of the double bridged PBI reveals all syn-arranged thiophene units that completely enclose the planar PBI chromophore via a 12-membered macrocycle. The target structures were characterized by steady-state UV/Vis absorption, fluorescence and transient absorption spectroscopy, as well as cyclic and differential pulse voltammetry. Both donor–acceptor dyads show ultrafast Förster Resonance Energy Transfer and photoinduced electron transfer, thereby leading to extremely low fluorescence quantum yields even in the lowest polarity cyclohexane solvent. KW - organic chemistry KW - photoinduced electron transfer KW - donor–acceptor dyads KW - macrocycles KW - oligothiophenes KW - perylenebisimide Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-256569 VL - 61 IS - 1 ER - TY - JOUR A1 - Bold, Kevin A1 - Stolte, Matthias A1 - Shoyama, Kazutaka A1 - Krause, Ana‐Maria A1 - Schmiedel, Alexander A1 - Holzapfel, Marco A1 - Lambert, Christoph A1 - Würthner, Frank T1 - Macrocyclic Donor‐Acceptor Dyads Composed of Oligothiophene Half‐Cycles and Perylene Bisimides JF - Chemistry – A European Journal N2 - A series of donor‐acceptor (D−A) macrocyclic dyads consisting of an electron‐poor perylene bisimide (PBI) π‐scaffold bridged with electron‐rich α‐oligothiophenes bearing four, five, six and seven thiophene units between the two phenyl‐imide substituents has been synthesized and characterized by steady‐state UV/Vis absorption and fluorescence spectroscopy, cyclic and differential pulse voltammetry as well as transient absorption spectroscopy. Tying the oligothiophene strands in a conformationally fixed macrocyclic arrangement leads to a more rigid π‐scaffold with vibronic fine structure in the respective absorption spectra. Electrochemical analysis disclosed charged state properties in solution which are strongly dependent on the degree of rigidification within the individual macrocycle. Investigation of the excited state dynamics revealed an oligothiophene bridge size‐dependent fast charge transfer process for the macrocyclic dyads upon PBI subunit excitation. KW - donor-acceptor dyad KW - macrocycle KW - oligothiophene KW - perylene bisimide KW - photoinduced electron transfer Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-276435 VL - 28 IS - 30 ER -