Photons as Reaction Initiator: Elucidation of Visible Light Activated Catalyst – Substrate and Dye – Dye Interactions

Abstract

Most probably, the rapidly advancing global mean temperature rise is the most pressing challenge, human society ever faced. In accordance to current scientific and technical knowledge, mean temperature increase is related to man-made emission of so called “greenhouse gases”. In order to deflate the massive negative impacts global warming causes, foremost energy generation as well as production sector has to steadily transform to become more sustainable. Hereby, two hand-in-hand going approaches are proposed: On the one hand, a change-over towards preferable utilization of renewable energy sources at the simultaneous renunciation of fossil fuels. On the other hand, traditional industry processes have to be organized more efficient in order to become less energy demanding at unvaryingly or even enhanced productivity. Amongst many other debatable possibilities, especially the energy demanding chemistry branch as well as raw material producing/processing heavy industry exhibit some remarkable entry points for far-reaching and innovative changes. In this context, catalysis in general and photocatalysis in particular provides some benefits: First of all, hereby a reduction in energy demand to drive chemical reactions is enabled. Up to now, only a few photochemical synthesis methods find their application in production chains on an industrial scale. The share of (visible) light-induced transformations will most likely rise in modern production facilities, since production is strived to become more energy efficient. To enable this goal, the necessity is given to further develop photocatalytic processes. To this end, systematic investigations may come into effect, which build upon established knowledge and expand the comprehension of the operating processes. Photochemistry, however, is known to spring some surprises already since its establishment as an independent field more than hundred years ago. As such, the outcome of photochemical driven reaction paths are still hard to predict. By the exploitation of (visible) light-induced transformations possibilities towards the formation of product structures, which are hitherto difficult to access or even unprecedented, is provided. Based on this, the synthesis of highly complex molecular structures may simplify by application of photochemical synthesis routes. Once more, the possibility is provided to pare resources down. Moreover, the utilization of visible light as reaction initiator serves as a mild alternative to established synthesis routes. But, how to make use of serendipity during the development of new synthesis routes? With the help of different screening approaches developed and optimized specifically for this purpose, a wide variety of reaction conditions and molecule combinations are tested for unforeseen interactions. Such screening methodologies, which constitute the starting point of many investigations described in the present dissertation, are based on the principle of knowledge-driven serendipity. For this purpose, systematically structured, combinations of different photocatalysts and substrate classes are used. High sample throughput is of utmost importance in these methods to identify novel reaction paths and/or product structures as efficiently as possible. Nevertheless, in order to fully implement novel (visible) light-induced transformations into industrial process chains and their further development it is essential to understand the processes ultimately leading to product formation as precisely as possible. For this purpose, the presented investigations link elaborated possibilities of (organic) analytics as well as physicochemical methods. Invigorated in such a way, the synergistically driven knowledge gain unfolds a completely new potential. The investigations described in course of this dissertation focus primarily on a detailed mechanism elucidation of novel reaction pathways using (time-resolved) optical spectroscopy techniques. With the help of these methods, pioneering work in step-by-step tracing of the underlying processes in the young field of triplet-triplet energy transfer catalysis is enabled. It is worth mentioning that most of the examined reactions were only discovered by means of recently established screening approaches. In a first attempt, mechanism elucidation of disulfide cleavage initiated by visible light and the subsequent addition of thereby resulting thiyl radicals to olefins lays the foundation for further investigations. Additionally, different [2+2]cycloadditions were enabled by triplet-triplet energy transfer catalysis. Decoding of their respective reaction sequences was henceforth subject of spectroscopically investigations. Furthermore, reaction pathways based on visible light-initiated photoredox-catalysis, were examined. These reactions originate in the reduction or oxidation of the substrate by an excited photocatalyst and are, therefore, based on electron transfer processes. From here, subsequent reactions ultimately resulting in product formation occur. Once more, a screening experiment attracted attention towards a promising reaction approach, namely the addition of alkyl radicals to either C=C or C=O double bonds of unsaturated carbonyls. Radicals are typically prone to undergo uncontrolled addition reactions. In the present case, however, radical addition is directed in selective way to form only one product. For this reason, the focus of the study is placed on the question about the factors that are responsible for the rather unexpected gain of selectivity control and how these mechanisms are applicable in future purposes. All of the aforementioned reactions are driven by iridium-centered complexes. Other dyes are also suitable as photocatalyst. As such, another innovative approach is based on the use of noble metal porphyrins. These are surface-immobilized onto the common semiconductor titanium dioxide. The system gives rise to hydrogen generation initiated by visible light. In this context, it is assumed that the photon-driven proton reduction is based on a series of electron transfer processes, simply by the nature of this reaction. Main objective of the described study is how the detected hydrogen generation proceeds. The last part of the present dissertation differs from mechanism elucidation of (organic) reactions. Albeit different dyes are immobilized onto metal oxide nanoparticle surfaces, too, the research interest is focused to their interactions between each other. For this purpose, electronically insulating aluminum oxide serves as supporting material. Pyrene and perylene bisimide derivatives are used as optically active probes to shed light on the respective surface morphologies. How surface attached organic molecules build up self-assembling monolayers is, however, still subject of various investigations. Research also focuses on well-defined surface structures possessing fine-tunable as well as customizable properties. Moreover, a profound understanding of the (electronic) interactions between the ordered organic monolayers is of utmost importance for future applications of these multifunctional materials. The knowledge and experience obtained from these projects pinpoint elaborated possibilities for the analysis of surface morphologies and thus the structure of self-assembling monolayers of newly developed multifunctional materials. This should pave the way for an overall simplified and predictable production of fine-tunable high-performance materials. Progress in energy-efficient photocatalytic driven synthesis approaches in combination with in-depth understanding of their underlying processes helps to establish large-scale application in a future-oriented and sustainable production sector. Moreover, the development of novel multifunctional materials will eventually enable energy savings, previously considered as impossible, in order to counteract the rise in average global temperature. Show more