Photochemical synthesis of dibenzothiophenes for the application in materials science

In the relatively young field of purely organic phosphorescence, the search for potent luminophores to substitute metal-containing emitters requires clever design of the molecules and their environment. This thesis focuses on dibenzothiophenes (DBTs) as promising candidates for high-performance organic phosphorescence. Specifically, the photochemical synthesis of this compound class in contrast to noble metal-catalyzed reactions is explored. On one hand, this approach gives access to high degrees of substitution of DBTs, which were hard to attain before. On the other hand, it exemplifies a change in chemical and photophysical properties as response to light as external stimulus, which can be utilized for material applications. The first project dealt with the influence of different substitution patterns on the emission behavior of tetrathioethers. These easily synthesized, regioisomeric compounds all displayed aggregation-induced phosphorescence (AIP). Although the variation of donor and acceptor positions significantly affected the emission color, lifetime and quantum yield, no direct correlation between structure and property was found. Still, the best performing luminophores were identified, giving valuable information for future design strategies. Secondly, the photocyclization of these tetrathioethers was examined thoroughly. Through experimental investigation of the scope, supported by quantum chemical calculations, it was derived that the reaction unfolds via the triplet state. The enhanced phosphorescence of the DBTs was studied in frozen matrices at 77 K and in polymer films at room temperature. Expectedly, long-lived phosphorescence was observed due to the beneficial structure of the photoproducts. As an application, a tetrathioether was incorporated into a polymer matrix and locally irradiated using a photo mask. Only the irradiated areas produced the DBT, permanently inscribing a phosphorescent image into the film. Besides this, in situ generation of the DBT in a 3D-printed object demonstrated a new approach to include roomtemperature phosphorescent compounds in additive manufacturing. In the last part, photocyclizable thioethers were merged with the motif of chalcogen-bridged ethers. The latter is known for prominent emission in solution and solid-state (SSSE). By means of photocyclization, the geometry of the starting materials was rigidified, transforming the compounds from aggregation-induced emitters (AIE) to SSSE luminophores. The resultant DBTs surpassed the phosphors from the second project in terms of lifetime and showed a significant shift between fluorescence and phosphorescence wavelengths. These results indicate the potential to further improve purely organic phosphorescence using these compounds. Overall, this work underscores the potential use of tetrathioethers and their photocyclization as multivalent platforms that are applicable in photoresponsive materials.