@article{MieczkowskiSteinmetzgerBessietal.2021, author = {Mieczkowski, Mateusz and Steinmetzger, Christian and Bessi, Irene and Lenz, Ann-Kathrin and Schmiedel, Alexander and Holzapfel, Marco and Lambert, Christoph and Pena, Vladimir and H{\"o}bartner, Claudia}, title = {Large Stokes shift fluorescence activation in an RNA aptamer by intermolecular proton transfer to guanine}, series = {Nature Communications}, volume = {12}, journal = {Nature Communications}, doi = {10.1038/s41467-021-23932-0}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-270274}, year = {2021}, abstract = {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.}, language = {en} } @phdthesis{Namal2018, author = {Namal, Imge}, title = {Fabrication and Optical and Electronic Characterization of Conjugated Polymer-Stabilized Semiconducting Single-Wall Carbon Nanotubes in Dispersions and Thin Films}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-162393}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {In order to shrink the size of semiconductor devices and improve their efficiency at the same time, silicon-based semiconductor devices have been engineered, until the material almost reaches its performance limits. As the candidate to be used next in semiconducting devices, single-wall carbon nanotubes show a great potential due to their promise of increased device efficiency and their high charge carrier mobilities in the nanometer size active areas. However, there are material based problems to overcome in order to imply SWNTs in the semiconductor devices. SWNTs tend to aggregate in bundles and it is not trivial to obtain an electronically or chirally homogeneous SWNT dispersion and when it is done, a homogeneous thin film needs to be produced with a technique that is practical, easy and scalable. This work was aimed to solve both of these problems. In the first part of this study, six different polymers, containing fluorene or carbazole as the rigid part and bipyridine, bithiophene or biphenyl as the accompanying copolymer unit, were used to selectively disperse semiconducting SWNTs. With the data obtained from absorption and photoluminescence spectroscopy of the corresponding dispersions, it was found out that the rigid part of the copolymer plays a primary role in determining its dispersion efficiency and electronic sorting ability. Within the two tested units, carbazole has a higher π electron density. Due to increased π-π interactions, carbazole containing copolymers have higher dispersion efficiency. However, the electronic sorting ability of fluorene containing polymers is superior. Chiral selection of the polymers in the dispersion is not directly foreseeable from the selection of backbone units. At the end, obtaining a monochiral dispersion is found to be highly dependent on the used raw material in combination to the preferred polymer. Next, one of the best performing polymers due to high chirality enrichment and electronic sorting ability was chosen in order to disperse SWNTs. Thin films of varying thickness between 18 ± 5 to 755o±o5 nm were prepared using vacuum filtration wet transfer method in order to analyze them optically and electronically. The scalability and efficiency of the integrated thin film production method were shown using optical, topographical and electronic measurements. The relative photoluminescence quantum yield of the radiative decay from the SWNT thin films was found to be constant for the thickness scale. Constant roughness on the film surface and linearly increasing concentration of SWNTs were also supporting the scalability of this thin film production method. Electronic measurements on bottom gate top contact transistors have shown an increasing charge carrier mobility for linear and saturation regimes. This was caused by the missing normalization of the mobility for the thickness of the active layer. This emphasizes the importance of considering this dimension for comparison of different field effect transistor mobilities.}, subject = {Feldeffekttransistor}, language = {en} } @article{RyczkoMisiewiczHoflingetal.2017, author = {Ryczko, K. and Misiewicz, J. and Hofling, S. and Kamp, M. and Sęk, G.}, title = {Optimizing the active region of interband cascade lasers for passive mode-locking}, series = {AIP Advances}, volume = {7}, journal = {AIP Advances}, number = {1}, doi = {10.1063/1.4973937}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-181790}, year = {2017}, abstract = {The work proposes possible designs of active regions for a mode-locked interband cascade laser emitting in the mid infrared. For that purpose we investigated the electronic structure properties of respectively modified GaSb-based type II W-shaped quantum wells, including the effect of external bias in order to simultaneously fulfil the requirements for both the absorber as well as the gain sections of a device. The results show that introducing multiple InAs layers in type II InAs/GaInSb quantum wells or introducing a tensely-strained GaAsSb layer into "W-shaped" type II QWs offers significant difference in optical transitions' oscillator strengths (characteristic lifetimes) of the two oppositely polarized parts of such a laser, being promising for utilization in mode-locked devices.}, language = {en} } @article{SoaviScotognellaViolaetal.2015, author = {Soavi, Giancarlo and Scotognella, Francesco and Viola, Daniele and Hefner, Timo and Hertel, Tobias and Cerullo, Giulio and Lanzani, Guglielmo}, title = {High energetic excitons in carbon nanotubes directly probe charge-carriers}, series = {Scientific Reports}, volume = {5}, journal = {Scientific Reports}, number = {9681}, doi = {10.1038/srep09681}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143061}, year = {2015}, abstract = {Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S\(_{33}\) transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S\(_{33}\) excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S\(_{11}\). The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D.}, language = {en} } @article{TvingstedtMalinkiewiczBaumannetal.2014, author = {Tvingstedt, Kristofer and Malinkiewicz, Olga and Baumann, Andreas and Deibel, Carsten and Snaith, Henry J. and Dyakonov, Vladimir and Bolink, Henk J.}, title = {Radiative efficiency of lead iodide based perovskite solar cells}, series = {Scientific Reports}, volume = {4}, journal = {Scientific Reports}, doi = {10.1038/srep06071}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119360}, pages = {6071}, year = {2014}, abstract = {The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ~0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate.}, language = {en} } @phdthesis{Kullmann2013, author = {Kullmann, Martin Armin}, title = {Tracing Excited-State Photochemistry by Multidimensional Electronic Spectroscopy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-81276}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Light-induced excitation of matter proceeds within femtoseconds, resulting in excited states. Originating from these states chemical reaction mechanisms, like isomerization or bond formation, set in. Photophysical mechanisms like energy distribution and excitonic delocalization also occur. Thus, the reaction scheme has to be disentangled by assessing the importance of each process. Spectroscopic methods based on fs laser pulses have emerged as a versatile tool to study these reactions. Within this thesis time-resolved experiments with fs laser pulses on various molecular systems were performed. Novel photosystems, with possible applications ranging from ultrathin molecular wires to molecular switches, were extensively characterized. To resolve the complex kinetics of the investigated systems, time-resolved techniques had to be newly developed. By combining a visible excitation pulse pair with an additional pulse and a continuum probe electronic triggered-exchange two-dimensional spectroscopy (TE2D) was demonstrated for the first time. This goal was accomplished by combining a three-color transient-absorption setup with a pulse shaper. Hence, 2D spectroscopy with a continuum probe was also implemented. Using these methods two different molecular systems in solution were characterized in a comprehensive manner. (ZnTPP)2, a directly beta,beta'-linked Zn-metallated bisporphyrin, and a spiropyran-merocyanine photosystem, 6,8-dinitro BIPS, were characterized. (ZnTPP)2 is a homodimer, featuring strong excitonic effects. These manifest themselves in a twofold splitting of the Soret band (S2). 6,8-Dinitro BIPS exists in one of two possible conformations. The ring closed spiropyran absorbs only in the UV, while the ring open merocyanine also absorbs in the visible. For both molecular systems photodynamics upon illumination were monitored using transient-absorption. However, the obtained results were ambiguous, necessitating more complex methods. In the case of (ZnTPP)2 first the monomeric building block was characterized. There, population transfer from the S2 state into S1 within 2 ps was identified. Afterwards, intersystem crossing proceeds within 2 ns. For (ZnTPP)2 similar pathways were found, albeit the relaxation is faster. The intersystem crossing with 1.5 ns was not only indirectly deduced but directly measured by probing in the NIR spectral range. The excitonic influence of was investigated by coherent 2D spectroscopy in the Soret band. Population transfer within S2 was directly visualized on a time-scale of 100 fs. Calculation of the 2D spectra of a simple homodimer confirmed the results. After this analysis of the distinct excitonic character, this molecule may serve as a building block for larger porphyrin arrays with applications ranging from asymmetric catalysis over biomimicry of electron-transfer to organic optical devices. The second photosystem was the molecular switch 6,8-dinitro BIPS, existing in two conformations. Merocyanine is the more stable form in thermal equilibrium. Transient-absorption measurements uncovered that the sample consisted of a mixture of two merocyanine isomers, referred to as TTC and TTT. However, both isomers are capable of ring-closure forming spiropyran. The remaining excited molecules return to the ground state radiatively. Conducting 2D measurements utilizing a continuum probe the differing photochemistry of both isomers was examined in a single measurement. No isomerization between these conformations was detected. Therefore, 6,8-dinitro BIPS performs a concerted switching without long-living intermediates. This was confirmed by a pump-repump-probe scan. 6,8-DinitroBIPS can be closed by visible and opened by UV pulses using subsequent pulses and vice versa. These mechanisms via singlet pathways satisfy an important criterion for a unimolecular switching device. A second pump-repump-probe experiment showed that the sample is ionized, resulting in a merocyanine radical cation, when the first excited state is resonantly excited. Furthermore, by implementing TE2Dspectroscopy, it was elucidated that only TTC was ionized. Taking all this into account new techniques were developed and complex molecular systems were characterized within this thesis. Deeper insight into the photodynamics of (ZnTPP)2and 6,8-dinitro BIPS was gained by adapting transient absorption for the NIR spectral range, constructing a 2D setup in pump-probe geometry, and combining it with multipulse excitation to coherent TE2D. All techniques solved the questions for which they were constructed, but they are not limited to these cases. Especially TE2D opens new roads in photochemistry. By connecting reactant, product and the corresponding intermediates, a chemical reaction can be tracked through all stages, making unambiguous identification of the reactive states feasible. Thus, fundamental insight into the photochemistry of molecular compounds is gained.}, subject = {Femtosekundenspektroskopie}, language = {en} }