541 Physikalische Chemie
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- Fraunhofer-Institut für Silicatforschung ISC (3)
- Institut für Optik und Atomare Physik, Technische Universität Berlin, 10623 Berlin, Germany (2)
- Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan (2)
- Center for Nanosystems Chemistry (CNC), Universität Würzburg (1)
- Center for Nanosystems Chemistry (CNC), Universität Würzburg, Am Hubland, 97074 Würzburg, Germany (1)
- Center of Excellence for Science and Technology - Integration of Mediterranean region (STIM), Faculty of Science, University of Split, Poljička cesta 35, 2100 Split, Croatia (1)
- Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Prague, Czech Republic (1)
- Departamento de Química, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain (1)
- Department of Chemistry, Humboldt Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany (1)
- Department of Chemistry, Sungkyunkwan University, 440-746 Suwon, Republic of Korea (1)
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- M-1240-2017 (1)
Im Rahmen dieser Arbeit werden unterschiedliche Aspekte der korrelierten Elektronen-Kerndynamik, anhand verschiedener Modellsysteme untersucht. Dabei wird vor allem auf den Vergleich numerisch exakter und approximativer Methoden zur Beschreibung der Wellenpaketdynamik eingegangen, wobei bei letzterem das Augenmerk auf der Born-Oppenheimer (BO) Näherung liegt. Die verwendeten Modellsysteme erlauben es, die gekoppelte Elektronen-Kern-Dynamik exakt zu beschreiben. Die daraus gewonnenen Ergebnisse dienen als Referenz für den Vergleich mit den Näherungsmethoden.
Im ersten Teil der Arbeit wird die Dynamik eines Wellenpakets in der Umgebung einer Konischen-Durchschneidung (CI) untersucht, wobei die Beschreibung des Wellenpakets quantenmechanisch und durch die klassische Mechanik im Phasenraum erfolgt.
Im zweiten Teil wird die Wahrscheinlichkeitsflussdichte untersucht. Zuerst wird ein Fall konstruiert, in welchem die Bewegung im elektronischen Grundzustand stattfindet, sodass die Bedingungen der BO Näherung erfüllt sind. Dabei wird vor allem auf das Verschwinden der elektronischen Wahrscheinlichkeitsflussdichte innerhalb der BO Näherung eingegangen. Im weiteren Verlauf werden die Flussdichten in der Umgebung einer CI untersucht, wobei unterschiedliche Situationen modelliert werden.
Im dritten Teil wird die Berechnung des elektronischen Impulserwartungswerts innerhalb der BO Näherung untersucht. Dieser verschwindet innerhalb der BO Näherung, wenn man diesen direkt berechnet (Geschwindigkeitsform), während man über das Ehrenfest Theorem (Längenform) sehr gute Werte erhält.
Im vierten Teil wird eine neue Flussdichte, die Translationsflussdichte, vorgestellt. Diese ergibt sich aus der Überlegung, dass die Geschwindigkeitsform des Impulserwartungswerts durch die Wahrscheinlichkeitsflussdichte ausgedrückt werden kann. Demnach muss auch die Längenform einer Flussdichte entsprechen und man erhält die Translationsflussdichte.
While the field of electrochromic (EC) materials and devices (ECDs) continues to advance in terms of color palette and understanding the underlying mechanism, several scientific and technological challenges need to be addressed by optimizing the materials and understanding the electrochemical interplay of these materials in full cells. The main issue here is to further improve the EC profile for color neutrality and cycling stability in order to commercialize dimmable EC products. The transparent conductive substrates used in this work (FTO and ultra-thin ITO glass) have high visible light transmittance (τv > 85%) and low sheet resistance (< 25 Ω·sq-1). In addition, the Li+-containing gel electrolyte has sufficient ionic conductivity (2.8·10-4 S·cm-1 at 25 °C), so the investigated ECDs could achieve a fast response (required ionic conductivity is between 10−3 and 10−7 S·cm-1).
This work shows that the combination of cathodically-coloring Fe-MEPE with anodically-coloring non-stoichiometric nickel oxide (Ni1-xO) electrodes (prepared by the National Institute of Chemistry in Ljubljana, Slovenia) can be used in neutral-coloring type III ECDs. The Fe-MEPE/Ni1-xO ECD with the underbalanced CE (ECD1-1, 2: 1) and the balanced configuration (ECD1-2, 1: 1) are both nearly neutrally-colored (ECD1-1: a* = -6.7, b* = 8.8; ECD1-2: a* = -9.0, b* = 10.1) in the bright state with a τv of almost 70%. Due to the overbalancing of the CE (ECD1-3, 1:3), a deviation (a* = -2.8, b* = 19.9) from the neutral coloration occurred here. The balanced as well as the overbalanced ECD configurations show high electrochemical cycling stability (over 1,000 potentiostatic switching cycles). In general, the overbalanced configuration offers the advantage of a smaller operating voltage range (-1 V ↔ 2.5 V to -1 V ↔ 1.5 V), i.e., avoiding possible electrochemical degradation of the EC materials, electrolyte, or conductive layers. By using a Li RE in the full cell, insights into the optimal matching of electrochemical and optical properties between the two electrodes are obtained to achieve more stable ECDs. Thereby, the redox potentials of both EC electrodes (Fe-MEPE and Ni1-xO) can be measured during operation. The incomplete decolorization of ECD1-1 can be explained by the measured electrode potentials (below the required 4 V vs. Li/Li+), excluding side reactions and degradation at both electrodes. The results demonstrate the importance of using balanced and (slightly) overbalanced ECD configurations with complementary-coloring EC electrodes to achieve high cycling stability and fast switching at low operating voltages. Therefore, this three-electrode configuration provides an excellent method for in situ electrochemical characterization of the individual EC electrodes to better understand the redox processes during device operation and to further improve the optical contrast and cycle stability of ECDs.
The Fe-MEPE/Ni1-xO combination was tested on flexible ultrathin ITO glass (ECD1-4). Here, by applying a low voltage of -1 V ↔ 2.5 V, the MEPE/Ni1-xO ECDs can be reversibly switched from a colored (L* = 35.6, a* = 19.4, b* = -26.7) to a nearly colorless (L* = 78.5, a* = -14.0, b* = 21.3) state. This is accompanied by a change in τv from 6% to 53%. The ECDs exhibit fast response and good cycling stability (5% loss of optical contrast over 100 switching cycles).
To further improve color neutrality and cycling stability, ECDs combining Fe-MEPE and mixed metal oxides as ion storage layers were investigated. Titanium manganese oxide (TMO, Fraunhofer IST) and titanium vanadium oxide (TiVOx, EControl-Glas GmbH & Co. KG) electrodes are compared for use as optically-passive ion storage layers. TiVOx with a maximum charge density of approx. 27 mC·cm-2 and a coloration efficiency of η = 2 cm·C-1 at 584 nm shows a color change from yellow to light gray at 2 V vs. Ag/AgCl, while the slightly anodically-coloring Ti-rich TMO (10.5 mC·cm-², η584 nm = -4 cm·C-1) switches from light yellow to colorless at -2.5 V vs. Ag/AgCl. These materials show only a slight change in τv value from 85% to 75% and from 72% to 81%, respectively, thus reaching the requirements for highly transmissive optical-passive ion storage layers. The ECDs with Fe-MEPE in combination with TiVOx (ECD2-1) and TMO-1 (ECD2-2) are blue-purple in the dark state (0 V) and turn colorless by applying a voltage of 1.5 V, changing the τv value from 28% to 69% and from 21% to 57% in 3 s and 13 s, respectively. The ECDs show fast responses and high cyclability over more than 100 cycles.
In the last section, the simplification of cell architecture by using redox mediators shows that different redox mediators (KHCF(III), Fc-PF6, Fc-BF4, and TMTU) can be used in type II ECDs (4 instead of 5 layers) consisting of Fe-MEPE or Ni1-xO thin film electrodes. The combination of KHCF(III) with Fe-MEPE has a low cycling stability due to the electrochemical formation of Prussian blue (PB). This side reaction is undesirable as it decreases the optical contrast. It can be avoided by using Fc+- (ECD3-5/6) or TMTU-based (ECD3-7) redox mediators, which exhibit reversible redox behavior. A high τv value of 72% is obtained for the use of TMTU. Low concentrations (<0.1 M) of redox mediators decrease the cell voltage for complete switching without affecting the optical properties of the ECDs. The redox couple TMTU/TMFDS2+ (molar ratio of 1:0.1 in 1 M LiClO4/PC as electrolyte) works well in combination with
Ni1-xO electrodes (ECD3-10), with a change in τv value from 38% (colored at 2 V, L* = 67.1, a* = 3.9, b* = 17.2) to 70% at (decolored at -2 V, L* = 86.6, a* = -0.6, b* = 17.2). This result implies that incorporating redox mediators into the electrolyte is an effective means to simplify the cell assembly and color neutrality can be obtained with one optically active WE and a color-neutral redox mediator. Moreover, the combination of Ni1-xO and the colorless TMTU/TMFDS2+ redox mediator is a potential candidate to obtain neutrally colored ECDs.
It is shown that the lab-sized FTO- and ultra-thin ITO-glass-based ECDs are very attractive for energy-efficient EC applications, e.g., in architectural or automotive glazing, aircraft, ships, home appliances and displays. To monitor the EC performance and to prevent diverging electrode potentials during the switching process, the studied three-electrode configuration can help to extend the cycle stability as well as to improve the charge balancing of dimmable applications. The studied ECDs display a route towards neutral tint, e.g., EC active Ni1-xO, optically-inactive mixed metal oxides, and colorless redox mediators. Nevertheless, color neutrality should be further improved to meet the requirements for industrial applications. For future work, a scale-up process from lab-sized (few cm²) to prototype (few m²) ECDs will be necessary.
Reactive hydrocarbon species are important in a multitude of different scientific areas. In this thesis, the vibrational spectra of hydrocarbon radicals, biradicals and their reaction product have been studied in a gas-phase environment. The specific molecules investigated here, are of particular importance in the field of combustion and astrochemistry. They were produced from suitable precursors in a pyrolytically heated micro-reactor and subsequently seeded in an appropriate carrier gas. As methodology, IR/UV ion dip spectroscopy has been utilized, which delivers massselected gas-phase IR spectra of all ionizable species detectable in the molecular beam. These, with the help of DFT calculations, allow for determination of the fingerprint IR spectra, identification of mass carriers and formulation of potential reaction mechanisms. All studies have been conducted in collaboration with the group of Prof. Dr. Anouk M. Rjis and the necessary potent IR radiation has been provided by the free-electron laser FELIX. Thus, the IR/UV measurements have been executed at the FELIX Laboratory of the Radboud University in Nijmegen. The first study presented in this thesis is the investigation of ortho-benzyne in Chapter 3.1. This molecule is of particular interest due to its uncommon electronic structure and its role in high-temperature reactions. Although, the infrared spectrum of o-C6H4 was not accessible, a number of reaction products were identified via their fingerprint spectra. Masses in the range from 78 - 228 were assigned to their respective carrier. The identified species include typical PAHs like naphthalene, phenanthrene, up to triphenylene. The identified masses further suggest a PAH growth heavily influenced by diradical 1,4-cycloaddition followed by fragmentation, as well as by classical HACA- and PAC-like mechanisms. These results were augmented by threshold photoionization measurements from Engelbert Reusch, who identified lighter reaction products, which have insufficient IR absorption or unsuitable ionization characteristics to be identified in the IR/UV experiment. An interesting observation is the identification of m/z = 152. This carrier has been assigned differently by the IR and TPES experiments. Whereas the IR spectrum clearly identifies the species as 2-ethynylnaphthalene, the TPES evidently is in great agreement with biphenylene. This is a good example how different experimental methodologies can benefit from each other to gain a deeper insight into the actual science of a particular system. Probably, the prime example for an aromatically resonance stabilized radical is benzyl. This radical is of high importance for many combustion studies, as it represents the primary high-temperature decomposition product of toluene. The goal of the study was the identification of the benzyl self reaction products and the results are discussed in Section 3.2. The radical was pyrolytically produced by its respective nitrite precursor. The mass spectrum showed that the benzyl self reaction formed two products with C11 and three with C14 constitution. All mass peaks were evenly spaced by two mass units, respectively, which suggests a close relation in formation. Indeed, the C11 products were identified as diphenylmethane and fluorene, which are simply connected via cyclization. The heaviest product was identified as phenanthrene, which is formed via the cyclization of bibenzyl to 9,10-dihydrophenanthrene and subsequent elimination of hydrogen. This result was quiet interesting as the intermediate of this reaction was often assumed to be stilbene, which was not observed in the study. Hence, the reaction seems to undergo cyclization first before phenanthrene is finally formed via hydrogen elimination. Expanding the molecular frame of benzyl by an additional methyl group leads to the xylyl radicals and its decomposition product the xylylenes. Also important in combustion research, xylyl radicals represent the preferred decomposition products of xylene, a frequently used anti-knock agent in modern gasoline blends. After further hydrogen elimination the xylyl radicals can then form their respective xylylenes. The results of the xylyl experiments are discussed in Section 3.3. Here the gas-phase vibrational spectrum in the fingerprint region for all three isomers has been recorded for the first time in isolation. Although, all isomers have a very similar structure and symmetry, and consequently similar vibrational bands, the resolution of the experimental data was exceedingly sufficient for a clear assignment. Additionally, the dimerization products of meta- and para-xylyl could also be identified. A similar approach was taken to determine the fingerprint spectra for the xylylenes. Here, only para-xylylene could be unambiguously identified as the carrier of mass 104. For both ortho- and meta-xylylene precursors, only isomerization products were observed as the carriers of mass 104; benzocyclobutene and styrene, respectively. A possible explanation is elaborated upon in the troubleshooting Sec- tion 3.4.3.5. In the final experimental section a study on the decomposition of phthalide is presented. The objective of this experiment was mainly focused around the formation of C7 species, particularly the fulvenallenyl radical C7H5. In fact, the first experimental fingerprint spectrum of isolated C7H5 in the gas-phase was measured and is displayed in Fig. 3.45. Furthermore, the experiment demonstrates that the pyrolysis products of phthalide are excellent soot precursors, as many heavier reaction products have been identified. These include typical PAH species like naphthalene and phenanthrene as well as their methylated isomers. A large number of molecules with terminal ethynyl moieties indicate a strong influence of HACA growth in the experimental environment. However, many formation pathways of products have been discussed, which are formed involving experiment specific species, like C5H5 and C7H5, and often include expansion steps from 5- to 6-membered rings.
Die Lehre von physikalisch-chemischen Inhalten in der universitären Lehramtsausbildung und im gymnasialen Chemieunterricht ist herausfordernd. Mögliche Ursachen hierfür sind das teils hohe Abstraktionsniveau und fehlende Messgeräte. Im Rahmen dieser Arbeit wurden kostengünstige Messgeräte entwickelt, mit denen Lernende in typische physikochemische Methoden und deren Anwendungen experimentell eingeführt werden können. Durch offen gestaltete und kontextbezogene Experimente zu Themenfeldern der Spektroskopie, Thermodynamik und Kinetik sollen Lernende einen phänomenologischen Zugang zur physikalischen Chemie finden. Durch eine entsprechende didaktische und experimentelle Aufarbeitung der Konzepte sollen insbesondere Schülerinnen und Schüler ohne größeres Vorwissen für physikalisch-chemische Inhalte im Sinne eines modernen und experimentell orientierten Chemieunterrichts begeistert werden.
The present work builds on a conjugated electrochromic polymer with a highly transmissive and colorless bright state and its application in electrochromic devices. The main body of this work focuses on the investigation of the influence of moisture on electrochromic devices and solutions to overcome possible degradation of these devices due to moisture ingress.
Firstly, a series of EDOT derivatives with a terminal double bond in the lateral sidechain to potentially achieve a highly transmissive and fully colorless bright state was investigated. All of the EDOT derivatives were electrochemically polymerized and characterized by means of (in-situ) spectroelectrochemistry. The results highlight the dramatic influence of the terminal double bond on the improved visible light transmittance and color neutrality in the bright state. After detailed evaluation and comparison, the best performing compound, which contains a hexenyl sidechain (PEDOT-EthC6), was scaled-up by changing the deposition technique from an electrochemical to a chemical in-situ polymerization process on a R2R-pilot line in an industrially relevant environment. The R2R-processed PEDOTEthC6 half-cells were characterized in detail and provide enhanced electrochromic properties in terms of visible light transmittance and color neutrality in the bright state as well as short response times, improved contrast ratio, coloration efficiency and cycling stability (10 000 cycles).[21]
In a second step, the novel PEDOT-EthC6 electrochromic polymer was combined with a Prussian Blue counter electrode and a solid polymer electrolyte to form an all-solid-sate ECDs based on complementary switching electrodes and PET-ITO as flexible substrates. The fabricated ECDs were optically and spectroelectrochemically characterized. Excellent functionality of the S2S-processed flexible ECDs was maintained throughout 10 000 switching cycles under laboratory conditions. The ECDs offer enhanced electrochromic properties in terms of visible light transmittance change and color neutrality in the bright state as well as contrast ratio, coloration efficiency, cycling stability and fast response times. Furthermore, the final device assembly was transferred from a S2S-process to a continuous R2R-lamination process.[238]
In a third step, the PEDOT-EthC6/PB-based ECDs were submitted to conscious environmental aging tests. The emphasis of the research presented in this work, was mainly put at the influence of moisture and possible failure mechanisms regarding the PEDOT-EthC6/PB based ECDs. An intense brown coloration of the electrodes was observed while cycling the ECDs in humid atmospheres (90% rH) as a major degradation phenomenon. The brown coloration and a thereby accompanied loss of conductivity of the PET-ITO substrates was related to significant degradation of the ITO layers, inserted as the conductive layers in the flexible ECDs. A dissolution of the ITO thin films and formation of metallic indium particles on the surface of the ITO layers was observed that harmed the cycling stability enormously. The conductive layers of the aged ECDs were investigated by XRD, UV-Vis, SEM and spectroelectrochemical measurements and validated the supposed irreversible reduction of the ITO layers.[279]
In the absence of reasonable alternatives to PET-ITO for flexible (R2R-processed) ECDs, it is also important to investigate measures to avoid the degradation of ECDs. This is primarily associated with the avoidance of appropriate electrode potentials necessary for ITO reduction in humid atmospheres. As an intrinsic action point, the electrode potentials were investigated via electrochemical measurements in a three-electrode setup of an all-solid-state ECD. Extensive knowledge on the electrode potentials allowed the voltage-induced degradation of the ITO in flexible ECDs to be avoided through the implementation of an unbalanced electrode configuration (charge density ratio of working and counter electrode). It was possible to narrow the overall operational voltage window to an extent in which irreversible ITO reduction no longer occurs. The unbalanced electrode configuration lead to an improved cycling stability without harming other characteristics such as response time and light transmittance change and allows ECD operation in the presence of humidity.[279]
The avoidance of the mentioned degradation phenomena is further associated with appropriate sealing methods and materials as well as appropriate electrode and device fabrication processes. Since a variety of sealing materials is commercially available, due to the commercial launch of organic photovoltaic (OPV) and light emitting diodes (OLEDs), the focus in the present work was put to water-free electrode fabrication. As an extrinsic action point, a novel preparation method of a nanoscale PEDOT-EthC6 dispersion based on organic solvents is presented here in a final step. The water-free processing method gives access to straightforward printing and coating processes on flexible PET-ITO substrates and thus represents a promising and simplified alternative to the established PEDOT:PSS. The resulting nano-PEDOT-EthC6 thin films exhibit enhanced color neutrality and transmissivity in the bright state and are comparable to the properties of the in-situ polymerized PEDOT-EthC6 thin films.[280]
Die vorliegenden Arbeit behandelt VUV Valenz-Photoionisations-Experimente in der Gasphase. Zunächst wird die Photoionisation von stickstoffhaltigen Radikalen und deren Pyrolyseprodukten untersucht. Im Anschluss werden molekulare Biradikale betrachtet. Da in der Literatur bislang nur wenige solcher Biradikale als Intermediate experimentell zugänglich waren, war es das Ziel dieser Arbeit, neue reaktive Spezies dieser Substanzklassen in der Gasphase zu isolieren und deren Struktur, Eigenschaften und Reaktivität besser zu verstehen. Im Mittelpunkt stehen dabei Intermediate, die als echte Biradikale, Biradikaloide oder Triplett Carbene auftreten. Zu letzteren zählen das Methylbismut sowie die Pentadiinylidene. Biradikale bilden in Verbrennungsprozessen sehr effizient Ruß(vorläufer), was anhand des ortho-Benz-ins dargelegt wurde, indem dessen Pyrolyseprodukte charakterisiert und mögliche PAH-Bildungswege aufgezeigt wurden. Vakuum Flash Pyrolyse wurde verwendet, um in situ aus den geeigneten Vorläufermolekülen die radikalischen und biradikalischen Intermediate zu erzeugen. Während für biradikalische Zwischenstufen meist spezielle Verbindungen als Vorläufer synthetisiert werden müssen, waren die verwendeten Vorläufer für die stickstoffhaltigen Radikale kommerziell erhältlich. Die reaktiven Spezies wurden alle mittels monochromatischer VUV Synchrotronstrahlung an der Swiss Light Source in Villigen/ Schweiz ionisiert. Die Ionisationsereignisse wurden mit der Schwellenphotoelektronen-Photoionen-Koinzidenz (TPEPICO) Technik detektiert und ausgewertet. Anhand der resultierenden massenselektiven Schwellenphotoelektronenspektren wurden die Ionisierungsenergien der (Bi)radikale bestimmt und die Schwingungsstruktur der jeweiligen Kationen analysiert. Die erhaltenen Spektren und Daten wurden in Zusammenarbeit mit der theoretischen Chemie interpretiert.
Wichtige Erkenntnisse
• Es wurde die Ionisierungsenergie der 2-, 3- und 4-Picolylradikale auf 7.70\pm
0.02 eV, 7.59\pm
0.01 eV und 8.01\pm
0.01 eV bestimmt. Diese wurden in der Pyrolyse selektiv aus ihren zugehörigen Picolylaminen erzeugt. Zudem wurde analog zum Benzyl-Radikal für alle drei Radikale eine ausgeprägte Schwingungsprogression ermittelt, die der totalsymmetrischen Deformationsmode des aromatischen Rings entspricht.
• Die Picolyl-Radikale dissoziieren in der Pyrolyse thermisch zu weiteren Produkten. Die Fragmentierung verläuft dabei isomerenunabhängig über ein stickstoffhaltiges Siebenringintermediat, dem Azepinyl-Radikal. Der Fragmentierungsmechanismus wurde mit dem von Benzyl verglichen. Die gewonnenen Erkenntnisse haben Relevanz für Verbrennungsprozesse, beispielsweise von Biokraftstoffen.Im ersten Schritt entstehen vier Isomere, das Cyclopenta-1,4-dien-1-carbonitril, das Cyclopenta-1,3-dien-1-carbonitril, das 2-Ethynyl-1H-pyrrol und das3-Ethynyl-1H-pyrrol mit den zugehörigen Ionisierungsenergien von 9.25\pm
0.02 eV, 9.14\pm
0.02 eV, 7.99\pm
0.02 eV und 8.12\pm
0.02 eV. Durch einen zweiten H-Verlust konnte das Cyanocyclopentadienyl-Radikal mit einer Ionisierungsenergie für die zwei niedrigsten Zustände im Kation mit 9.07\pm
0.02 eV (T0) und 9.21\pm
0.02 eV (S1) untersucht werden. Weitere Pyrolyseprodukte, deren Ionisierungsenergien bereits literaturbekannt sind und die bestätigt wurden, sind das Cyclopentadienyl-Radikal, das Cyclopenta-1,3-dien, das Propargyl-Radikal, das Penta-1,3-diin und das Cyanopropenyl.
• Das ortho-Benz-in wurde pyrolytisch aus dem selbst synthetisierten Benzocyclobutendion erzeugt und ein Schwellenphotoelektronenspektrum frei von Störsignalen konnte aufgenommen werden. Mit Hilfe von Rechnungen aufCASPT2(11,14) Niveau, die neben dem elektronischen Übergang in den kationischen Grundzustand noch die Übergänge in zwei weitere angeregte kationische Zustände beinhalten, wurde die Ionisierungsenergie im Vergleich zu früheren Experimenten auf 9.51 eV revidiert. Eine verdrillte Geometrie für den kationischen Grundzustand konnte erstmals nachgewiesen werden. Zusätzlich wurden die offenkettigen Isomere cis- und trans-Hexa-1,5-diin-3-en im Spektrum detektiert und zugeordnet.
• Die Auftrittsenergien aus der DPI des Vorläufermoleküls Benzocyclobutendion betragen für den ersten CO-Verlust 9.62\pm
0.05 eV und für den zweiten CO-Verlust 12.14\pm
0.10 eV. Damit konnte über einen thermochemischen Kreisprozess eine Bindungsdissoziationsenergie für die Ph-CO Bindung im Benzoylkation von 2.52 eV berechnet werden.
• Verschiedenen Pyrolyseprodukte des ortho-Benz-ins, wie Ethin, Buta-1,3-diin, Benzol, Biphenylen und 2-Ethinylnaphthalin, werden entweder in bimolekularen Reaktionen gebildet oder ortho-Benz-in fragmentiert unimolekular zu diesen. Die beiden kompetitiven Reaktionspfade tragen zur PAH-Bildung des ortho-Benz-ins bei.
• Die Triplett-Carbene Pentadiinyliden, Methylpentadiinyliden und Dimethylpentadiinyliden wurden als Pyrolyseprodukt aus ihren zugehörigen Diazovorläufern identifiziert und die Ionisierungsenergien mit 8.36\pm
0.03 eV, 7.77\pm
0.04 eV und 7.27\pm
0.06 eV bestimmt. Jede Methylierung stabilisiert folglich das Carben. Zusätzlich konnte ein weiteres C5H2 Isomer, das 3-(Didehydrovinyliden)cyclopropen, mit einer Ionisierungsenergie von 8.60\pm
0.03 eV charakterisiert werden.
• Zwei bismuthaltige, reaktive Spezies, das Dimethylbismut-Radikal\cdot
BiMe2 (IE = 7.27\pm
0.04 eV) und das Methylbismut-Carben :BiMe(IE = 7.88\pm
0.02 eV) wurden als Pyrolyseprodukte aus dem BiMe3 identifiziert. Beide Verbindungen zeigen eine ausgeprägte Schwingungsstruktur, die der Bi-C Streckschwingung zugeordnet wurde. Weiterhin wurden elementares Bismut Bi und das Bismut-Dimer Bi2 nachgewiesen.
• Die homolytische Dissoziation der ersten Me2Bi-CH3 Bindung im BiMe3 wurde untersucht und eine BDE von 210\pm
7 kJ/ mol bestimmt. Sie liegt um +15 % bzw. +28 kJ/ mol über dem aus der Literatur abgeschätzten Wert.
Photochemical reactions in solution often proceed via competing reaction pathways
comprising intermediates that capture a solvent molecule. A disclosure of the underlying
reaction mechanisms is challenging due to the rapid nature of these processes and the
intricate identification of how many solvent molecules are involved. Here combining
broadband femtosecond transient absorption and quantum mechanics/molecular mechanics
simulations, we show for one of the most reactive species, diphenylcarbene, that the
decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding
dynamics determine which reaction channels are accessible in binary solvent mixtures at
room temperature. In-depth analysis of the amount of nascent intermediates corroborates
the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking
analogy to complexes found at cryogenic temperatures. Our results show that adjacent
solvent molecules take the role of key abettors rather than bystanders for the fate of the
reactive intermediate.
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.
Nowadays, computational-aided investigations become an essential part in the chemical, biochemical or pharmaceutical research. With increasing computing power, the calculation of larger biological systems becomes feasible. In this work molecular mechanical (MM) and quantum mechanical approaches (QM) and the combination of both (QM/MM) have been applied to study several questions which arose from different working groups. Thus, this work comprises eight different subjects which deals with chemical reactions or proton transfer in enzymes, conformational changes of ligands or proteins and verification of experimental data.
This work firstly deals with reaction mechanisms of aromatic inhibitors of cysteine proteases which can be found in many organisms. These enzymes are responsible for various cancer or diseases as for example Human African Trypanosomiasis (HAT) or the Chagas disease. Aromatic SNAr-type electrophiles might offer a new possibility to covalently modify these proteases. Quantum mechanical calculations have been performed to gain insights into the energetics and possible mechanisms.
The next chapter also deals with Trypanosomiasis but the focus was set on a different enzyme. The particularity of Trypanosomiasis is the thiol metabolism which can also be modified by covalent inhibitors. In this context, the wild type and point mutations of the enzyme tryparedoxin have been investigated via molecular dynamic (MD) simulations to examine the influence of specific amino acids in regard to the inhibitor. Experimental data showed that a dimerization of the enzyme occurs if the inhibitor is present. Simulations revealed that the stability of the dimer decreases in absence of the inhibitor and thus confirms these experiments.
Further investigations concerning cysteine proteases such as cruzain and rhodesain have been conducted with respect to experimental kinetic data of covalent vinylsulfone inhibitors. Several approaches such as QM or QM/MM calculations and docking, MD or MMPBSA/MMGBSA simulations have been applied to reproduce these data. The utilization of force field approaches resulted in a qualitatively accurate prediction.
The kinase AKT is involved in a range of diseases and plays an important role in the formation of cancer. Novel covalent-allosteric inhibitors have been developed and crystallized in complex with AKT. It was shown that depending on the inhibitor a different cysteine residue is modified. To investigate these differences in covalent modification computational simulations have been applied.
Enoyl-(acyl carrier) (ENR) proteins are essential in the last step of the fatty acid biosynthesis II (FAS) and represent a good target for inhibition. The diphenylether inhibitor SKTS1 which was originally designed to target the ENR’s of Staphylococcus aureus was also crystallized in InhA, the ENR of Mycobacterium tuberculosis (TB). Crystal structures indicate a change of the inhibitor's tautomeric form. This subject was investigated via MD simulations. Results of these simulations confirmed the tautomerization of the inhibitor.
This work also deals with the development of a covalent inhibitor originating from a non-covalent ligand. The target FadA5 is an essential enzyme for the degradation of steroids in TB and is responsible for chronic tuberculosis. This enzyme was crystallized in complex with a non-covalent ligand which served as starting point for this study. Computations on QM or QM/MM level and docking and MD simulations have been applied to evaluate potential candidates.
The next chapter focuses on the modification of the product spectrum of Bacillus megaterium levansucrase, a polymerase which catalyzes the biosynthesis of fructans. The covalent modification of the wild type or mutants of the enzyme lead to an accumulation of oligosaccharides but also to polymers with higher polymerization degree. To understand these changes in product spectra MD simulations have been performed.
Finally, the proton transfer in catalytic cysteine histidine dyads was investigated. The focus was set on the influence of the relaxation of the protein environment to the reaction. Calculations of the enzymes FadA5 and rhodesain revealed that the preferred protonation state of the dyade depends on the protein environment and has an impact on the reaction barrier. Furthermore, the adaptation of the environment to a fixed protonation state was analyzed via MD simulations.
Ziel dieser Dissertation war es zu einem besseren Verständnis hinsichtlich folgender Themen beizutragen und Möglichkeiten aufzuzeigen, mit welchen die Voraussetzungen für Anwendungen von einzelnen, funktionalisierten Kohlenstoffnanoröhren, wie u.a. Einzelphotonenquellen, erfüllt werden können.
Eine wesentliche Voraussetzung für die Funktionalisierung von einzelnen Kohlenstoffnanoröhren ist zunächst eine Probenpräparation, welche SWNT-Suspensionen mit einem hohen Anteil an vereinzelten SWNTs hoher PL-Intensität bereitstellen kann. Um solche SWNT-Suspensionen herstellen zu können, wurden drei verschiedene Rohmaterialien und Dispergiermittel auf deren Entbündelungseffizienz- und relativer Photolumineszenzquantenausbeute untersucht. Anhand von photolumineszenzspektroskopischen Untersuchungen und Messungen der Extinktion stellte sich heraus, dass in Kombination des unaufbereiteten CVD-Kohlenstoffnanorohrrußes mit dem Copolymer PFO:BPy als Dispergiermittel und einem speziell in dieser Dissertation entwickelten Herstellungsverfahren für die Mikroskopieproben, stabile (6,5)-SWNT-Suspensionen mit einem großen Anteil an einzelnen SWNTs hoher PL-Intensität, hergestellt werden können.
Letztere Suspension diente als Ausgangsmaterial für die, in dieser Dissertation neuartige, entwickelte Methodik zur Differenzierung zwischen einzelnen SWNTs und Aggregaten mittels PL- und Ramanmessungen an einem PL-Mikroskopie-Aufbau, welche eine weitere Voraussetzung für Einzelpartikelstudien darstellt. Hierbei wurden im Rahmen einer statistischen Messreihe PL- und Ramanspektren von 150 SWNT-Objekten aufgenommen und hieraus resultierend die Parameter FWHM, Energie des S1-Emissions-Zustands und relative Photolumineszenzquantenausbeute ermittelt. Schließlich konnten die zwischen einer einzelnen SWNT und einem Aggregat charakteristischen Differenzen anhand der Korrelationen zwischen den drei Parametern dargestellt werden. Zudem erfolgte eine statistische Analyse zur Bestimmung der statistischen Signifikanz dieser Korrelationen. Hierbei wurde anhand der nicht-parametrischen Spearman-Korrelationskoeffizienten und der p-Werte gezeigt, dass in Kombination dieser drei Messparameter mit einer hohen Wahrscheinlichkeit zwischen einer einzelnen SWNT und einem Aggregat differenziert werden kann. Demnach konnte eine neuartige, im Vergleich zur Literatur, praktikable Methodik zur Differenzierung zwischen einzelnen SWNTs und Aggregaten, etabliert werden, welche die Voraussetzung für Einzelrohrstudien ist.
Der Fokus dieser Dissertation ist die Entschlüsselung der Reaktionsmechanismen der Arylierung und reduktiven Alkylierung von (6,5)-SWNTs im Ensemble und auf Einzelrohrbasis. Durch diese kovalenten Funktionalisierungsverfahren entstehen neue fluoreszierende Defekt-Zustände, deren zeitabhängiges Intensitätsverhalten in der vorliegenden Arbeit näher untersucht wurde. Hinsichtlich der Arylierung von SWNTs mit Diazoniumsalzen postulieren Studien einen zweistufigen Reaktionsmechanismus, welcher durch eine kombinatorische, spektroskopische Gesamtbetrachtung im Rahmen dieser Dissertation bestätigt werden konnte. Auch konnte erstmalig in der Literatur gezeigt werden, dass die Reaktion in hohem Maße reproduzierbar ist.
Reproduzierbarkeitsstudien wurden auch im Falle der reduktiven Alkylierung unternommen, wobei erstmalig festgestellt wurde, dass diese Reaktion lediglich im hohen Maße reproduzierbar ist, sofern die Reduktionslösung mindestens 17 Stunden vor Reaktionsstart angesetzt wird. Basierend auf diesem Resultat, wurden reproduzierbare Messreihen zur Untersuchung der Reaktionsbedingungen und des Reaktionsmechanismus unternommen, da diesbezüglich unzureichend Kenntnis in der Literatur vorhanden ist.
Zur Klärung des Reaktionsmechanismus, von welchem lediglich Annahmen existieren, wurde zum einen der Einfluss der Laseranregung auf die Reaktion untersucht. Da lediglich für den Falle des Ansetzens der Reduktionslösung unmittelbar vor Messbeginn, wobei die reaktiven SO2- -Radikale erzeugt werden, ein Einfluss der Laseranregung festgestellt werden konnte, nicht jedoch im weiteren Reaktionsverlauf, ist von keiner radikalischen Reaktion im Funktionalisierungsschritt auszugehen. Dies konnte durch den Einsatz von Konstitutionsisomeren des Iodbutans bestätigt werden, wobei das Iodbutanisomer, welches im Fall einer radikalischen Reaktion die höchste Reaktivität zeigen sollte, zu keiner Funktionalisierung der SWNTs führte. Im Gegensatz hierzu, konnte durch das 1-Iodbutan, mit dem primären C-Atom, eine hohe PL-Intensität der defekt-induzierten Zustände E11- und T- verzeichnet werden, was die weitere Annahme einer SN2-Reaktion stützt.
Im Rahmen dieser Dissertation konnte zudem erstmalig entdeckt werden, dass unter deren alkalischen, reduktiven Bedingungen, eine Funktionalisierung mit Acetonitril erfolgen kann, was durch die Durchstimmung der PL-Intensität des Defektzustands bei Variation des Volumenanteils von Acetonitril bestätigt werden konnte. Hierbei gilt es jedoch weiter zu analysieren, auf welche Art die Koordination bzw. Funktionalisierung von Acetonitril an den SWNTs erfolgt, was u.a. durch Ramanmessungen untersucht werden könnte.
Auch konnten neuartige Kenntnisse bezüglich der Reaktionskinetik basierend auf den Studien dieser Dissertation erhalten werden, wobei festgestellt wurde, dass das Reaktionsprofil mit dem einer komplexen Folgereaktion angenähert werden kann.
Zudem konnten neuartige Kenntnisse aus der Thermodynamik, wie die Ermittlung der Aktivierungsenergie der Adsorption von DOC-Molekülen auf der SWNT-Oberfläche, durch die Zugabe des Tensids DOC zum Reaktionsansatz und dem hieraus resultierenden Reaktionsabbruch, erhalten werden.
Schließlich fand eine Übertragung der Ergebnisse aus den Ensemblestudien der reduktiven Alkylierung auf Einzelpartikeluntersuchungen statt, wobei letztere erstmalig im Rahmen dieser Arbeit durchgeführt wurden. Aus der statistischen Analyse, welche von Martina Wederhake durchgeführt wurde, resultierte durch Erhöhung des Stoffmengenverhältnisses von 1-Iodbutan zu Kohlenstoff eine inhomogene Steigerung des Funktionalisierungsgrades. Ausblickend gilt es nun zu prüfen, ob die zeitlichen Reaktionsverläufe der photolumineszierenden Zustände, welche aus den Ensemble-Studien erhalten wurden, auf Einzelrohrbasis reproduziert werden können.
Es lässt sich demnach festhalten, dass mithilfe der Studien dieser Dissertation ein Probenherstellungsverfahren, welches stabile SWNT-Suspensionen mit einem großen Anteil an einzelnen Kohlenstoffnanoröhren, hoher PL-Intensität ermöglicht, etabliert werden konnte. Zudem wurde eine neuartige, praktikable und statistisch signifikante Methodik zur Differenzierung zwischen einzelnen Kohlenstoffnanoröhren und Aggregaten entwickelt. Schließlich konnten neue, essentielle Informationen bezüglich des Reaktionsmechanismus und den Reaktionsbedingungen der Arylierung und reduktiven Alkylierung von halbleitenden (6,5)-SWNTs erhalten werden. Wie in der Einleitung bereits erwähnt, sind sowohl der Erhalt einer stabilen SWNT-Suspension mit einem großen Anteil an einzelnen Nanoröhren hoher PL-Intensität, die Möglichkeit der Identifizierung einzelner SWNTs, als auch ein ausgiebiges Verständnis der Reaktionsmechanismen der Funktionalisierungsreaktionen, essentielle Voraussetzungen für die Verwirklichung von Einzelphotonenquellen auf Basis einzelner, funktionalisierter Kohlenstoffnanoröhren. Diese können aufgrund derer geeigneter Emissionseigenschaften als vielversprechende Kandidaten für das Ausgangsmaterial von Einzelphotonenquellen in der Quanteninformationstechnologie angesehen werden.
We present a theoretical study on exciton–exciton annihilation (EEA) in a molecular dimer. This process is monitored using a fifth-order coherent two-dimensional (2D) spectroscopy as was recently proposed by Dostál et al. [Nat. Commun. 9, 2466 (2018)]. Using an electronic three-level system for each monomer, we analyze the different paths which contribute to the 2D spectrum. The spectrum is determined by two entangled relaxation processes, namely, the EEA and the direct relaxation of higher lying excited states. It is shown that the change of the spectrum as a function of a pulse delay can be linked directly to the presence of the EEA process.
For the rational design of new fluorophores, reliable predictions of fluorescence quantum yields from first principles would be of great help. However, efficient computational approaches for predicting transition rates usually assume that the vibrational structure is harmonic. While the harmonic approximation has been used successfully to predict vibrationally resolved spectra and radiative rates, its reliability for non-radiative rates is much more questionable. Since non-adiabatic transitions convert large amounts of electronic energy into vibrational energy, the highly excited final vibrational states deviate greatly from harmonic oscillator eigenfunctions. We employ a time-dependent formalism to compute radiative and non-radiative rates for transitions and study the dependence on model parameters. For several coumarin dyes we compare different adiabatic and vertical harmonic models (AS, ASF, AH, VG, VGF, VH), in order to dissect the
importance of displacements, frequency changes and Duschinsky rotations. In addition we analyze the effect of different broadening functions (Gaussian, Lorentzian or Voigt). Moreover, to assess the qualitative influence of anharmonicity on the internal conversion rate, we develop a simplified anharmonic model. We adress the reliability of these models considering the potential errors introduced by the harmonic approximation and the phenomenological width of the broadening function.
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.
Comparison of moving and fixed basis sets for nonadiabatic quantum dynamics at conical intersections
(2020)
We assess the performance of two different types of basis sets for nonadiabatic quantum dynamics at conical intersections. The basis sets of both types are generated using Ehrenfest trajectories of nuclear coherent states. These trajectories can either serve as a moving (time-dependent) basis or be employed to sample a fixed (time-independent) basis. We demonstrate on the example of two-state two-dimensional and three-state five-dimensional models that both basis set types can yield highly accurate results for population transfer at intersections, as compared with reference quantum dynamics. The details of wave packet evolutions are discussed for the case of the two-dimensional model. The fixed basis is found to be superior to the moving one in reproducing nonlocal spreading and maintaining correct shape of the wave packet upon time evolution. Moreover, for the models considered, the fixed basis set outperforms the moving one in terms of computational efficiency.
Comparison of moving and fixed basis sets for nonadiabatic quantum dynamics at conical intersections
(2020)
We assess the performance of two different types of basis sets for nonadiabatic quantum dynamics at conical intersections. The basis sets of both types are generated using Ehrenfest trajectories of nuclear coherent states. These trajectories can either serve as a moving (time-dependent) basis or be employed to sample a fixed (time-independent) basis. We demonstrate on the example of two-state two-dimensional and three-state five-dimensional models that both basis set types can yield highly accurate results for population transfer at intersections, as compared with reference quantum dynamics. The details of wave packet evolutions are discussed for the case of the two-dimensional model. The fixed basis is found to be superior to the moving one in reproducing true nonlocal spreading and maintaining correct shape of the wave packet upon time evolution. Moreover, for the models considered, the fixed basis set outperforms the moving one in terms of computational efficiency.
The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes
(2019)
Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM),i.e., merocyanine 4-(dicyanomethylene)-2-tert-butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4H-pyran (DCBT). We measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law.This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.
Collective Response in DNA-Stabilized Silver Cluster Assemblies from First-Principles Simulations
(2019)
We investigate fluorescence resonant energy transfer and concurrent electron dynamics in a pair of DNA-stabilized silver clusters. For this purpose we introduce a methodology for the simulation of collective optoelectronic properties of coupled molecular aggregates starting from first-principles quantum chemistry, which can be further applied to a broad range of coupled molecular systems to study their electro-optical response. Our simulations reveal the existence of low-energy coupled excitonic states, which enable ultrafast energy transport between subunits, and give insight into the origin of the fluorescence signal in coupled DNA-stabilized silver clusters, which have been recently experimentally detected. Hence, we demonstrate the possibility of constructing ultrasmall energy transmission lines and optical converters based on these hybrid molecular systems.
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.
Excitons in the molecular aggregates of chromophores are key participants in important processes such as photosynthesis or the functioning of organic photovoltaic devices. Therefore, the exploration of exciton dynamics is crucial. Here we report on exciton localization during excited-state dynamics of the recently synthesized tetracene trimer [Liu et al., Org. Lett., 2017, 19, 580]. We employ the surface hopping approach to nonadiabatic molecular dynamics in conjunction with the long-range corrected time-dependent density functional tight binding (LC-TDDFTB) method [Humeniuk and Mitrić, Comput. Phys. Commun., 2017, 221, 174]. Utilizing a set of descriptors based on the transition density matrix, we perform comprehensive analysis of exciton dynamics. The obtained results reveal an ultrafast exciton localization to a single tetracene unit of the trimer during excited-state dynamics, along with exciton transfer between units.