Dynamics of Singlet Fission and Electron Injection in Self-Assembled Acene Monolayers on Titanium Dioxide

We employ a combination of linear spectroscopy, electrochemistry, and transient absorption spectroscopy to characterize the interplay between electron transfer and singlet fission dynamics in polyacene-based dyes attached to nanostructured TiO2. For triisopropyl silylethynyl (TIPS)-pentacene, we find that the singlet fission time constant increases to 6.5 ps on a nanostructured TiO2 surface relative to a thin film time constant of 150 fs, and that triplets do not dissociate after they are formed. In contrast, TIPS-tetracene singlets quickly dissociate in 2 ps at the molecule/TiO2 interface, and this dissociation outcompetes the relatively slow singlet fission process. The addition of an alumina layer slows down electron injection, allowing the formation of triplets from singlet fission in 40 ps. However, the triplets do not inject electrons, which is likely due to a lack of sufficient driving force for triplet dissociation. These results point to the critical balance required between efficient singlet fission and appropriate energetics for interfacial charge transfer

Multielectron Dynamics of Singlet Fission in the Condensed Phase

Elementary energy and electron transfer processes are ubiquitous in the renewable energy science of the last half of the 20th century. As global energy demands increase, researchers are inclined to explore chemical physics that is outside the scope of the single electron paradigm using new theoretical concepts and methods. This thesis advances theories of two specific condensed phase phenomena: singlet fission, and energy transfer in photosynthetic light harvesting complexes. Some photoactive organic molecules relax through a multielectron process known as singlet fission, where a photon excites a chromophore that can down-convert the energy of a singlet excitation by relaxing to two triplet excitations. Singlet fission may lead to unprecedented solar power conversion efficiencies, but its many-body chemical physics can be challenging to model. We explore the fundamental role of thermal energy in singlet fission in liquids and solids over multiple timescales. Using quantum master equations and diabatic representations of the single and double electronic excitations, we study the scope of the Markovian approximation for the chemical environment\u27s response to singlet fission. To better understand how singlet delocalization and triplet localization impact quantum yields in molecular crystals, we develop a theory for delocalized singlets interacting with a dense band of two triplet excitations that includes biexciton interactions. We use the Bethe Ansatz for the two triplets and calculate an entanglement for indistinguishable bipartite systems to analyze the triplet-triplet entanglement born out of singlet fission

Polymer-Solvent Phase Separation and Criticality in the Formation of Porous Polymeric Solids: A Modeling Study

This manuscript describes a model for simulating the properties of polymer-solvent mixtures and the nonequilibrium processes that lead to their demixing and gel formation. In this model, the description of the system is based on a coarse grained lattice model of the polymer density field whose parameters can be chosen to reflect the microscopic characteristics of specific chemical systems. By using a lattice model, we are able to perform block renormalization and thereby access study the critical behavior of these systems upon demixing. We relate this critical behavior to the hierarchical structure of the resulting extended gel-like polymer network.</p

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Nanoscale Probing of Dynamics in Local Molecular Environments

Vibrational spectroscopy can provide information about structure, coupling, and dynamics underlying the properties of complex molecular systems. While measurements of spectral line broadening can probe local chemical environments, the spatial averaging in conventional spectroscopies limits insight into underlying heterogeneity, in particular in disordered molecular solids. Here, using femtosecond infrared scattering scanning near-field optical microscopy (IR <i>s</i>-SNOM), we resolve in vibrational free-induction decay (FID) measurements a high degree of spatial heterogeneity in polytetrafluoroethylene (PTFE) as a dense molecular model system. In nanoscopic probe volumes as small as 10³ vibrational oscillators, we approach the homogeneous response limit, with extended vibrational dephasing times of several picoseconds, that is, up to 10 times the inhomogeneous lifetime, and spatial average converging to the bulk ensemble response. We simulate the dynamics of relaxation with a finite set of local vibrational transitions subject to random modulations in frequency. The combined results suggest that the observed heterogeneity arises due to static and dynamic variations in the local molecular environment. This approach thus provides real-space and real-time visualization of the subensemble dynamics that define the properties of many functional materials

Der Einfluss von Bewehrungsstoessen auf die Feuerwiderstandsdauer von Stahlbetonbalken Forschungsbericht

TIB: AC 8679 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Untersuchung ueber die Wirkung von Blenden auf die Feuerwiderstandsdauer von Stahltraegern

SIGLETIB: RN 5905 (896) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Der Einfluss von Bewehrungsstoessen auf die Feuerwiderstandsdauer von Stahlbetonbalken Forschungsbericht

TIB: RN 5905 (1449) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman