Femtochemie pericyclischer Reaktionen und Fortschritte in Richtung einer chiralen Kontrolle

Pericyclic reactions possess changed reactivities in the excited state compared to the ground state which complement each other, as can be shown by simple frontier molecular orbital analysis. Hence, most molecules that undergo pericyclic reactions feature two different photochemical pathways. In this thesis an investigation of the first nanoseconds after excitation of Diazo Meldrum’s acid (DMA) is presented. The time-resolved absorption change in the mid-infrared spectral region revealed indeed two reaction pathways after excitation of DMA with at least one of them being a pericyclic reaction (a sigmatropic rearrangement). These two pathways most probably start from different electronic states and make the spectroscopy of DMA especially interesting. Femtochemistry also allows the spectroscopy of very short-lived intermediates, which is discussed in context of the sequential mechanism of the Wolff rearrangement of DMA. An interesting application of pericyclic reactions are also molecular photoswitches, i.e. molecules that can be switched by light between two stable states. This work presents a photoswitch on the basis of a 6-pi-electrocyclic reaction, whose reaction dynamics after excitation are unravelled with transient-absorption spectroscopy for both switching directions. The 6-pi-electrocyclic reaction is especially attractive, because of the huge electronic changes and subsequent absorption changes upon switching between the ring-open and ring-closed form. Fulgides, diarlyethenes, maleimides as well as spiropyrans belong to this class of switches. Despite the popularity of spiropyrans, the femtochemistry of the ring-open form (“merocyanine”) is still unknown to a great extent. The experiments in this thesis on this system combined with special modeling algorithms allowed to determine the quantum efficiencies of all reaction pathways of the system, including the ring-closure pathway. With the knowledge of the reaction dynamics, a multipulse control experiment showed that bidirectional full-cycle switching between the two stable states on an ultrafast time scale is possible. Such a controlled ultrafast switching is a process which is inaccessible with conventional light sources and may allow faster switching electronics in the future. Theoretical calculations suggest an enantioselective photochemistry, i.e. to influence the chirality of the emerging molecule with the chirality of the light, a field called “chiral control”. The challenges that need to be overcome to prove a successful chiral control are extremely hard, since enantiosensitive signals, such as circular dichroism, are inherently very small. Hence, chiral control calls for a very sensitive detection as well as an experiment that cancels all effects that may influence the enantiosensitive signal. The first challenge, the sensitive detection, is solved with a polarimeter, which is optimized to be combined with femtosecond spectroscopy. This polarimeter will be an attractive tool for future chiral-control experiments due to its extreme sensitivity. The second challenge, the design of an artefact-free experiment, gives rise to a variety of new questions. The polarization state of the light is the decisive property in such an experiment, because on the one hand the polarization carries the chiral information of the excitation and on the other hand the change of the polarization or the intensity change dependent on the polarization is used as the enantiosensitive probing signal. A new theoretical model presented in this thesis allows to calculate the anisotropic distribution of any given pump-probe experiment in which any pulse can have any polarization state. This allows the design of arbitrary experiments for example polarization shaped pump-probe experiments. Furthermore a setup is presented and simulated that allows the shot-to-shot switching between mirror-images of light polarization states. It can be used either for control experiments in which the sample is excited with mirror-images of the pump polarization or for spectroscopy purposes, such as transient circular dichroism or transient optical rotatory dispersion. The spectroscopic results of this thesis may serve as a basis for these experiments. The parallel and sequential photochemical pathways of DMA and the feasibility of the bidirectional switching of 6,8-dinitro BIPS in a pump–repump experiment on the one hand offer a playground to test the relation of the anisotropy with the polarization of the pump, repump and probe pulse. On the other hand control experiments with varying pump and repump polarization may be able to take influence on the dynamics after excitation. Especially interesting is the combination of the 6,8-dinitro BIPS with the polarization-mirroring setup, because the closed form (spiropyran) is chiral. Perhaps in the future it will be possible to prove a cumulative circular-dichroism effect or even a chiral control with this system.Pericyclische Reaktionen besitzen unterschiedliche Reaktivitäten im elektronischen Grund- und angeregten Zustand, wie anhand einfacher Grenzorbitalbetrachtungen gezeigt werden kann. Deswegen weisen Moleküle die eine pericyclische Reaktion eingehen meist mehrere photochemische Reaktionspfade auf. In dieser Arbeit wird die Femtochemie von Diazo-Meldrumssäure (DMA) utnersucht. Die zeitaufgelösten Absorptionsänderungen im mittleren Infrarotbereich zeigen tatsächlich zwei Reaktionspfade nach Anregung der DMA, von denen zumindest einer eine pericyclische Reaktion ist (eine sigmatrope Umlagerung). Diese zwei Pfade starten vermutlich von unterschiedlichen elektronischen Zuständen, was die Spektroskopie von DMA besonders interessant macht. Besonders kurzlebige Intermediate oder transiente Zustände können mit Hilfe der Femtochemie auch beobachtet werden, was in Zusammenhang mit der Wolff Umlagerung von DMA gezeigt wird. Eine weitere interessante Anwendung pericyclischer Reaktionen sind die molekularen Schalter, also Moleküle die mit Licht zwischen zwei stabilen Zuständen hin und hergeschaltet werden können. In dieser Arbeit wird ein Photoschalter, 6,8-dinitro BIPS, auf Basis einer 6-pi elektrocyclischen Reaktion vorgestellt, dessen Reaktionsdynamiken nach Anregung mit Hilfe transienter Absorption sichtbar gemacht werden. Die 6-pi elektrocyclische Reaktion ist besonders attraktiv, da mit ihr große elektronische Änderungen und somit auch starke Absorptionsänderungen einhergehen beim Schaltvorgang. Fulgide, Diarylethene, Maleimide und Spiropyrane gehören zu dieser Klasse von Schaltern. Trotz der großen Verbreitung der Spiropyrane ist jedoch bisher die Femtochemie der offenen Form ("Merocyanin") zum großen Teil unbekannt. Die Experimente und Modellierungen an diesem System in dieser Arbeit erlauben die Bestimmung der Quanteneffizienzen aller beteiligten Reaktionspfade beider Schaltrichtungen. Mit diesem Wissen ausgestattet konnte ein Multipulse-Kontroll Experiment durchgeführt werden in dem bidirektional zwischen den beiden Zuständen des Photoschalters auf Pikosekunden Zeitskala hin und hergeschaltet wurde. Dieser Prozess ist mit konventionellen Lichtquellen nicht möglich. Laut theoretischen Rechnungen ist eine enantionselektive Photochemie, also die Beeinflussung der Chiralität von gebildeten Produkten einer Photoreaktion, möglich. Dieses Feld wird "chirale Kontrolle" genannt. Die Herausforderungen eine erfolgreiche chirale Kontrolle zu beweisen sind extrem anspruchsvoll, da enantiosensitive Signale, wie zum Beispiel der Zirkulardichroismus, sehr klein sind. Deswegen ist einerseits eine sehr genaue Detektionsmethode notwendig sowie eine experimentelle Anordnung in der Artefakte direkt ausgeschlossen werden. Für die sehr genaue Detektion wurde in dieser Arbeit ein Polarimeter entwickelt, das zudem für die Kombination mit Femtosekundenlaserpulsen optimiert ist. Dieses Polarimeter wird in Zukunft eine attraktive Detektionsmethode für chirale-Kontrollexperimente sein auf Grund seiner extrem guten Sensitivität. Die zweite Herausforderung eine artefaktfreie experimentelle Anordnung zu finden, eröffnet eine Fülle neuer Fragen. Der Polarisationszustand in diesen Experimenten ist die entscheidende Eigenschaft, da einerseits die Polarisation die chirale Information der Anregung trägt und andererseits die Änderung des Polarisationszustands oder der Intensität benutzt wird als enantiosensitives Abfragesignal. Ein neues theoretisches Modell ist in dieser Arbeit präsentiert, das es ermöglicht die anisotropen Verteilungen beliebiger Anrege-Abfrage Experimente mit beliebigen Polarisationszuständen aller beteiligten Pulse zu berechnen. Das ermöglicht den Aufbau beliebiger Anrege-Abfrage Experimente, z.B. polarisationsgeformte Anrege-Abfrage Experimente. Außerdem wird ein Setup vorgestellt und simuliert, das es ermöglicht Schuss-zu-Schuss zwischen spiegelbildlichen Polarisationszuständen des Lichts hin und herzuschalten. Mit diesem Setup können zum Beispiel Kontrollexperimente durchgeführt werden in denen die Probe mit spiegelbildlichen Polarisationszuständen angeregt wird. Des weiteren können mit dem Setup auch Spektroskopieexperimente durchgeführt werden, wie z.B. transienter Zirkulardichroismus oder transiente optische Rotationsdisperision. Die spektroskopische Ergebnisse dieser Arbeit können als Basis dienen für solche Experimente. Die parallelen und sequentiellen photochemischen Pfade des DMA sowie das bidirektionale Schalten des 6,8-dinitro BIPS in einem Anrege-Wiederanrege Experiment bieten viele Möglichkeiten die neuen Zusammenhänge der Anisotropie mit den Polarisationszuständen des Anrege, Wiederanrege oder Abfragestrahls zu überprüfen. Andererseits könnte man mit Kontrollexperimenten mit variierender Anrege und Wiederanregepolarisation Einfluss nehmen auf die induzierten Dynamiken. Besonders interessant ist hier die Kombination des 6,8-dinitro BIPS mit der Polarisationsspiegelungssetups, weil die geschlossene Form (Spiropyran) chiral ist. Vielleicht ist es mit diesem System tatsächlich in der Zukunft möglich einen kumulativen Zirkulardichroismuseffekt oder sogar eine chirale Kontrolle zu zeigen

Precise and rapid detection of optical activity for accumulative femtosecond spectroscopy

We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 μm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment

Termination and Transfer Kinetics of Sodium Acrylate Polymerization in Aqueous Solution

Rate coefficients of termination and backbiting of secondary chain-end radicals (SPRs) and of propagation of tertiary midchain radicals (MCRs) have been measured for 20 wt % sodium acrylate (NaA), i.e., fully ionized acrylic acid, in aqueous solution at temperatures from 0 to 60 °C. Highly time-resolved SPR and MCR concentration vs time traces, measured after applying an intense laser pulse at <i>t</i> = 0, were analyzed via PREDICI simulation. Cross- and homotermination of MCRs have no major impact on the kinetics. The termination rate coefficient of SPRs in dilute solution of NaA is by more than 1 order of magnitude below the associated value of nonionized AA. The reduction of backbiting rate coefficient upon full ionization is close to the one associated with enhancing the concentration of nonionized AA from 10 to 50 wt %

Determination of ATRP Equilibrium Constants under Polymerization Conditions

Atom transfer radical polymerization (ATRP) equilibrium constants (<i>K</i>_ATRP) were measured during polymerization of methyl acrylate (MA) with Cu^IBr/Cu^IIBr₂ in either dimethyl sulfoxide (DMSO) or acetonitrile (MeCN) in the presence of either tris­(2-pyridylmethyl)­amine (TPMA) or tris­[2-(dimethylamino)­ethyl]­amine (Me₆TREN) as the ligand and with ethyl 2-bromopropionate as the initiator. The ln­(<i>K</i>_ATRP) values changed linearly with the volume fraction of solvents in the reaction medium, allowing extrapolation of the values for <i>K</i>_ATRP to bulk conditions, which were 2 × 10^–9 and 3 × 10^–8 for TPMA and Me₆TREN ligands at 25 °C, respectively. The temperature effect on <i>K</i>_ATRP values was studied in MA/MeCN = 1/1 (v/v) with TPMA as the ligand in the temperature range from 0 to 60 °C. The <i>K</i>_ATRP values increased with temperature providing Δ<i>H</i> = 36 kJ mol^–1 in MeCN

The Borderline between Simultaneous Reverse and Normal Initiation and Initiators for Continuous Activator Regeneration ATRP

Multiple methods for initiation and selecting catalyst concentration exist in atom transfer radical polymerization (ATRP). Among them, simultaneous reverse and normal initiation (SR&NI) ATRP and initiators for continuous activator regeneration (ICAR) ATRP are phenomenologically very similar. In both methods, thermal radical initiators are employed to reduce the catalyst in the higher oxidation state and generate Cu^I activator <i>in situ</i>. SR&NI and ICAR ATRP generally differ in the amount of catalyst used and in the rate of catalyst reduction. Commonly, SR&NI ATRP requires high catalyst loadings and a quick initial reduction of Cu^II, while ICAR ATRP relies on slow and continuous reduction of smaller catalyst loadings. However, these criteria might not be sufficient to universally distinguish among both techniques. This article investigates both methods through kinetic simulations and establishes a borderline kinetic criterion. If the polymerization rate depends on the rate of decomposition of the radical initiator, the system follows ICAR ATRP kinetics, and if it depends on the ATRP equilibrium constant, it follows SR&NI ATRP. The transition from one to the other mechanism occurred continuously with an inflection point at a ratio of rate coefficients of radical initiator decomposition to propagation of about <i>k</i>_dc/<i>k</i>_p ≈ 10^–7 M under typical conditions. For faster initiator decomposition and slower propagation ATRP follows SR&NI ATRP, and for slower decomposition and faster propagation it obeys ICAR ATRP kinetics. The analysis to verify which mechanism is in operation is helpful for designing reaction conditions in order to obtain well-defined products

SP-PLP-EPR Study into Termination and Transfer Kinetics of Non-Ionized Acrylic Acid Polymerized in Aqueous Solution

Polymerization of nonionized acrylic acid (AA) in aqueous solution has been studied via single pulse–pulsed laser polymerization in conjunction with electron paramagnetic resonance spectroscopy. Termination of two types of radicals, secondary chain-end radicals (SPRs) and midchain radicals (MCRs), as well as intramolecular chain transfer (backbiting) of SPRs have been studied between 5 and 40 °C at initial AA concentrations of 10 and 50 wt % in the presence of 15 wt % poly­(AA). Predici modeling of the measured SPR and MCR concentration vs time traces after single-pulse initiation at <i>t</i> = 0 yields rate coefficients for backbiting, <i>k</i>_bb, and for propagation from an MCR, <i>k</i>_p^t. These rate coefficients increase toward lower AA-in-water concentration. Estimates for termination of two SPRs, <i>k</i>_t^s,s, and of an SPR and an MCR, <i>k</i>_t^s,t have been obtained by assuming the reported composite-model behavior for acrylates to also hold for acrylic acid polymerization

Speciation Analysis in Iron-Mediated ATRP Studied via FT-Near-IR and Mössbauer Spectroscopy

The structure of [Fe^IIBr_<i>u</i>(Solv)_<i>v</i>] and [Fe^IIIBr_<i>w</i>(Solv)_<i>x</i>] complexes to be used as catalysts for atom-transfer radical polymerization (ATRP) was determined in several solvents via FT-near-IR in conjunction with Mössbauer spectroscopy. The relative amounts of these complexes are sensitive to both the type of solvent and pressure. The [Fe^IIBr₃(Solv)]⁻/[Fe^IIIBr₄]⁻ redox pair essentially governs the ATRP equilibrium. The properties of these complexes are correlated with the measured ATRP activation rate coefficients, <i>k</i>_act, for monomer-free model systems. In weakly polar solvents such as esters, ketones, and substituted benzenes, <i>k</i>_act was found to be larger than in strongly polar solvents, such as NMP, MeCN, and DMF, due to stabilization of the [Fe^IIBr₃(Solv)]⁻ complex

Visible Light and Sunlight Photoinduced ATRP with ppm of Cu Catalyst

Photochemically induced ATRP was performed with visible light and sunlight in the presence of parts per million (ppm) copper catalysts. Illumination of the reaction mixture yielded polymerization in case of 392 and 450 nm light but not for 631 nm light. Sunlight was also a viable source for the photoinduced ATRP. Control experiments suggest photoreduction of the Cu^II complex (ligand to metal charge transfer in the excited state), yielding a Cu^I complex, and a bromine radical that can initiate polymerization. No photoactivation of Cu^I complex was detected. This implies that the mechanism of ATRP in the presence of light is a hybrid of ICAR and ARGET ATRP. The method was also used to synthesize block copolymers and polymerizations in water

SP-PLP-EPR Measurement of ATRP Deactivation Rate

The ATRP deactivation rate coefficient is measured by applying laser single pulses for radical production in conjunction with subsequent time-resolved detection of the decay of radical concentration. The novel method is illustrated for copper-mediated dodecyl methacrylate (DMA) polymerization with 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) as the ligand in the presence of 15 wt % acetonitrile. The experiment consists of two parts: measurement of chain-length-dependent termination on the copper-free monomer system followed by monitoring the decay of propagating radicals after laser pulsing applied to the copper–HMTETA system. The rate coefficient of deactivation by the Cu^II complex is about 2 orders of magnitude below the one of termination of two small propagating DMA-type radicals, which confirms that deactivation is chemically controlled

Substituted Tris(2-pyridylmethyl)amine Ligands for Highly Active ATRP Catalysts

The synthesis and application of a very active catalyst for copper-catalyzed atom transfer radical polymerizations (ATRP) with tris­([(4-methoxy-2,5-dimethyl)-2-pyridyl] methyl)­amine (TPMA*) ligand is reported. Catalysts with TPMA* ligands are approximately 3 orders of magnitude more active than those with tris­(2-pyridylmethyl)­amine (TPMA). Catalyst activity was evaluated by cyclic voltammetry, stopped-flow, and ATRP kinetics. Catalysts with TPMA* ligands perform better than those with TPMA ligands, especially at low catalyst concentrations