Photogenerated reactive intermediates from thiophene ylides: thiophenes, oxenes and carbenes, oh my

Substituted thiophene-S-oxides and dimethylmalonate thiophene-S,C-ylides were photolytically degraded under a variety of conditions. The thiophene-S-oxides produced complex mixtures of products, including furan. The dimethylmalonate thiophene-S,C-ylides cleanly produced a mixture of singlet and triplet dicarbomethoxycarbene. The reactivity of the photogenerated carbenes were studied, and the characteristics of the chromophores manipulated in order to change the spin characteristics of the carbene

Highly Constrained Dithienylethenes

Diarylethene sind molekulare Schalter, welche sich unter Einwirkung von Licht zwischen einem offenen und einem geschlossenen Isomer umwandeln. Die Effizienz dieser beiden Photoreaktionen ist von verschiedenen Parametern abhängig, welche bisher nur unzureichend verstanden sind. Ein entscheidender Faktor für die Hinreaktion ist das Verhältnis zweier Konformere, von denen jedoch nur eines photochemisch aktiv ist. In der vorliegenden Arbeit wird eine neue Klasse von Diarylethenen beschrieben, in welcher die aktive Konformation durch kovalente Verbrückungen unterschiedlicher Länge stabilisiert wird. Gleichzeitigeröffnet sich ein zusätzlicher Reaktionspfad in Form einer Doppelbindungsisomerisierung. Es stellte sich heraus, dass bei geeigneter Verbrückungslänge das zyklisierte Isomer mit ungewöhnlich hoher Effizienz gebildet wird, während die Effizienz der Ringöffnung nicht beeinflusst wird. Der Mechanismus und die Dynamik der Photoreaktion wurden anhand ausgewählter Vertreter durch Ultrakurzzeitspektroskopie untersucht. Weiterhin konnte gezeigt werden, dass der Ringschluss auch durch elektrochemische Oxidation oder Reduktion erfolgen kann. Die vorgestellten Systeme agieren bei direkter photochemischer Anregung wie herkömmliche Diarylethene nur im Ringschluss/Ringöffnungsregime. Durch Tripletsensibilisierung kann jedoch eine selektive Z→E Isomerisierung erzielt werden, was diese Diarylethenklasse zu reversiblen 3-Zustandssystemen erweitert. In Erweiterung des Projektes wurde die Struktur des Diarylethens noch stärker fixiert. Nach vielseitigen Syntheseversuchen konnten zwei Vertreter dieser Klasse erhalten und photochemisch untersucht werden, wobei ein Umsatz zu etwa 60% zyklisiertem Isomer bei der Bestrahlung mit UV-Licht gefunden wurde. Zusammengefasst stellt die kovalente Verbrückung der Diarylethenstruktur eine erfolgreiche Strategie dar, um sowohl die Effizienz der Ringschlussreaktion zu steigern als auch photochrome Verbindungen mit drei Schaltzuständen zu kreieren.Diarylethenes are molecular switches that interconvert reversibly between an open and a closed isomer by irradiation with light. The efficiency of both photochemical reactions depends on several parameters, which, so far, are only insufficiently understood. One important factor in the cyclization reaction is the presence of two conformations of the open isomer of which only one is photochemically active. In the current work, a new class of diarylethenes is presented, in which the active conformation is covalently stabilized by alkyl chains of different lengths. As the central double bond is not fixated, double bond isomerization emerges as an additional pathway in these annulated diarylethenes. In dependence of the chosen ring size both open isomers convert with increased efficiency to the closed isomer upon irradiation. The efficiency of the cycloreversion process remains unaffected. The mechanism and dynamic of the photoreaction were investigated for selected compounds using transient absorption spectroscopy. Furthermore, electrochemical studies revealed that both the E- and the Z-isomer cyclize rapidly upon anodic oxidation or cathodic reduction. In general, the photochemical reactivity of annulated diarylethenes parallels that of normal diarylethenes and takes place exclusively in the cyclization/cycloreversion regime if irradiated directly. However, it was demonstrated that a selective Z→E double bond isomerization is possible, thus implementing a 3-state photoswitchable system. In extension of the project, the structure of diarylethenes was further stiffened. Using diverse synthetic approaches, two members of this class could be obtained. Photochemical investigation showed a conversion to the closed isomer of 60% upon irradiation with UV-light. In brief, the covalent fixation of diarylethenes represents an attractive strategy to increase the efficiency of the photochemical cyclization and extent the scope of diarylethenes to 3-state photochromic systems

Photochemistry of transition metal hydrides

Photochemical reactivity associated with metal-hydrogen bonds is widespread among metal hydride complexes and has played a critical part in opening up C-H bond activation. It has been exploited to design different types of photocatalytic reactions and to obtain NMR spectra of dilute solutions with a single pulse of an NMR spectrometer. Because photolysis can be performed on fast time scales and at low temperature, metal-hydride photochemistry has enabled determination of the molecular structure and rates of reaction of highly reactive intermediates. We identify five characteristic photoprocesses of metal monohydride complexes associated with the M-H bond, of which the most widespread are M-H homolysis and R-H reductive elimination. For metal dihydride complexes, the dominant photoprocess is reductive elimination of H2. Dihydrogen complexes typically lose H2 photochemically. The majority of photochemical reactions are likely to be dissociative, but hydride complexes may be designed with equilibrated excited states that undergo different photochemical reactions, including proton transfer or hydride transfer. The photochemical mechanisms of a few reactions have been analyzed by computational methods, including quantum dynamics. A section on specialist methods (time-resolved spectroscopy, matrix isolation, NMR, and computational methods) and a survey of transition metal hydride photochemistry organized by transition metal group complete the Review

Schiff Bases and Their Metallic Derivatives: Highly Versatile Molecules with Biological and Abiological Perspective

1998 onwards, a span reporting thousands of research articles describes the ever-increasing applicability of Schiff bases and their metallic complexes; this chapter comprehensively examines the literature of the last 20 years. The structural diversity of these molecules made them available for a very wide range of biological and abiological applications. Schiff bases are excellent chelators and due to this unique property have found their place in qualitative and quantitative determination of metals in aqueous media. The structural diversity of metal chelates proved these to be outstanding catalysts and displayed interesting fluorescence effect. Finally, Schiff base moieties have found a unique position during the in vitro and in vivo experiments for drug development against a huge number of biological entities including bacteria, fungi, cancer cells, viruses, parasites, etc

Light-controlled reversible modulation of frontier molecular orbital energy levels in trifluoromethylated diarylethenes

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their pelectronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge-as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energy levels. Here, we present a comprehensive study detailing how variation of the substitution pattern of DAE compounds, in particular using strongly electron-accepting and chemically stable trifluoromethyl groups either in the periphery or at the reactive carbon atoms, allows for the precise tuning of frontier molecular orbital levels over a broad energy range and the generation of photoinduced shifts of more than 1 eV. Furthermore, the effect of different DAE architectures on the transduction of these shifts to an adjacent functional group is discussed. Whereas substitution in the periphery of the DAE motif has only minor implications on the photochemistry, trifluoromethylation at the reactive carbon atoms strongly disturbs the isomerization efficiency. However, this can be overcome by using a nonsymmetrical substitution pattern or by combination with donor groups, rendering the resulting photoswitches attractive candidates for the construction of remote-controlled functional systems

Light-induced molecular processes in organic-based energy conversion and biomimetic synthesis of natural products

Processes initiated by sunlight are fundamental steps in photovoltaic devices as well as in biosyntheses. The present work investigates the photoinduced processes in organic-based energy conversion materials and biomimetic synthesis of natural products by quantum chemical calculations. The work is performed in close collaboration with experimental groups and enables a deeper understanding of the observations. The detailed knowledge allows to predict the optimal conditions to initiate the photochemical syntheses and the chemical substitution to achieve the desired properties. In the first and second part of the thesis, two classes of molecules commonly used in organic-based optoelectronic devices are considered and potential factors influencing the performance of the optical devices are revealed. In the third part, the photochemical and biomimetic syntheses of two natural products and the details of the complex reaction mechanisms are elucidated. In the first part of the present work the deactivation pathways from the first excited singlet state S1 of thiophene and of small oligothiophenes containing up to four rings are investigated by state-of-the-art quantum chemical methods. For thiophene a low-lying S1/S0 conical intersection seam is easily accessible and drives the fast internal conversion. In the oligothiophenes barriers in combination with fast intersystem crossing channels inhibit this passage. The calculated spin-orbit coupling strength together with the singlet-triplet energy gaps can explain the decreasing triplet and increasing fluorescence quantum yields for growing chain length. The present theoretical results allow a deeper understanding of the deactivation pathways of thiophene and small oligothiophenes and are of potential interest for the photophysics of longer oligothiophenes and polythiophenes used in optoelectronic devices. In the second part the photoinduced dynamics of perylene diimide dyads based on a donor-spacer-acceptor motif are considered. The dyads based on pyridine spacer undergo energy transfer from the donor to the acceptor with near-unity quantum efficiency. In contrast in the dyads with phenyl spacers the energy transfer decreases below 50%, suggesting the presence of a competing electron transfer from the spacer to the donor. However, the measurements indicate that the spacer itself mediates the energy transfer dynamics. Ab initio calculations reveal the existence of bright charge transfer states which enable the energy transfer. This new energy transfer represents a first example that show how electron transfer can be connected to energy transfer for the use in novel photovoltaic devices. Additional experiments and calculations of subsystems demonstrate that the solvation time and not the polarity of the solvent is surprisingly the crucial property of the solvent for the charge and energy transfer dynamics. In the last part the photochemical syntheses of the two natural products intricarene and aplydactone are studied. Intricarene was isolated from a Carribbean coral and according to its proposed biosynthesis it arises from an oxidopyrylium intermediate via an intramolecular 1,3-dipolar cycloaddition. By a combination of experiments and theory it is shown that oxidopyrylium indeed forms under biomimetic and photochemical conditions and that it represents the key intermediate in the complex reaction cascade leading to intricarene. Triplet states as well as conical intersections enable the formation of intricarene and of an intriguing by-product which may constitute a new natural product. In the second part of the last chapter a quantum chemical study of the [2+2] photocycloaddition of dactylone to aplydactone is performed. Both compounds were isolated from a Madagascan sea hare and especially aplydactone exhibits an unprecedented molecular structure. However, for both compounds no total syntheses have been reported yet. According to the proposed biosynthesis, aplydactone is formed by a photochemical [2+2] cycloaddition out of dactylone but attempts to synthesize aplydactone through irradiation of dactylone failed. In the present work quantum chemical calculations elucidate the optimal biomimetic conditions to initiate the photochemical reaction and the different reaction pathways on the excited state potential energy surface are revealed. Overall, the last chapter highlights the importance of weak absorption bands and long-lived triplet states for the photochemical synthesis of natural products

Polymer sensors and actuators: synthesis and characterization of polythiophenes modified with benzospiropyran derivatives

The aim of this thesis is to understand the basic physico/chemical behaviour of novel nitrospiropyran-substituted-(poly)terthiophene materials. Attractive characteristics that distinguish these materials include electrochemical/photochemical-switching processes. Understanding the fundamental behaviour of these new derivatives is the key to taking advantage of these characteristics. The stimuli induced isomerisation between the colourless spiropyran and the highly coloured merocyanine is the key-process studied in this research. Chapter 2 reports the electrochromic behaviour of spiropyran, through irreversible electrooxidative isomerisation in at least two merocyanine isomers. The molecular photophysical processes of the electrochemically-generated polymer are reported and the properties studied. In Chapter 3 a new monomer, which differs by a single methylene group in the linking ester-linker functionality from the previous one, will be introduced. A nitrospiropyran was used as a reference, and a detailed spectroscopy analysis will help in the description of the thermodynamic properties. Chapter 4 highlights how the polymeric molecular switches can bind an important extracellular protein, fibronectin, and explores by means of microscopy the on-demand tuning of the protein adhesion. A new derivative is presented in Chapter 5, wherein a spiropyran moiety is intercalated between two thiophenes. The electrochemical and optical switching of the spiropyran and thiophene groups were investigated. In Chapter 6 three new spiropyran-based derivatives are presented, and routes to multi-modal switching behavior are explored via photochemical, electrochemical and chemical stimuli, such as pH. Their reversibility and their properties are investigated using both spectroscopic and electroanalytical methods. The final chapter outlines future work and initial progress related to the integration of spiropyran-thiophene polymers into photo-electro active portable analytical devices