Photocatalytic water splitting is a direct pathway to store solar energy in chemicals. Within the last decade, porous organic polymers (POPs) proved to be promising candidates to be applied as photocatalyst. Due to the variety of building blocks, the optical properties can be tuned towards the employment of visible light, which is mandatory achieving industrial relevant efficiencies. Monomers bearing electron-rich carbazole groups can easily be oxidative coupled yielding in carbazole-based porous organic polymers (CPOPs). Various CPOPs with varying monomer cores, specific for its certain application, were oxidative polymerized by applying iron(III) chloride as oxidative agent or by electro polymerization. Yet limited research has been conducted on the exact chemical structure of the resulting polymer or on alternative synthetic approaches. The first part of this thesis focuses on the coupling mechanism of the carbazole coupling applying various oxidative agents. With iron(III) chloride or with a mild organic oxidant the formed polymer is coupled via carbazole dimer formation. If a strong organic oxidant like DDQ is applied, further polymerization on a single carbazole group is possible. Depending on the amount and reaction time a polymer with higher rigidity and improved CO2 gas uptake results. Avoiding any metal-based oxidative agent, the dimer coupled polymer with improved surface areas can be synthesized via the metal-free synthesis method. The electro polymerization of carbazoles yielding thin polymer films is another metal-free synthesis approach of CPOPs. In the second part of the thesis such polymer films were applied in photocatalytic water splitting for the first time, showing excellent reproducibility, recyclability as well as stability after initial deposition of a co-catalyst. While the film thickness did not significantly change the photocatalytic activity it is beneficial to place several thin films adjacent or stack them to improve the easily accessible outer surface area. Another option is to employ a microstructured polymer film with increased now accessible vertical surface area in the photocatalysis. The results of this work were published in a research article entitled Carbazole‐Based Thin Microporous Polymer Films for Photocatalytic Hydrogen Evolution in the journal Advanced Materials. In the last part of this thesis a novel CPOP was introduced bearing an electron deficient triaryl borane core coupled to three carbazole groups. The resulting monomer proved to be highly emissive with a highly polarized excited state. Due to polymerization the absorption shifted from the UV region to the visible light region, which is beneficial for the application in photocatalysis. In the photocatalytic hydrogen evolution reaction (HER) both the monomer and the polymer showed photocatalytic activity in the absence of any metal-based co-catalyst. Theoretical and mechanistical studies showed that upon light absorption formed generated radical species can be transferred from the boron core to coordinated protons forming hydrogen radicals which couple to form hydrogen.
Veit Wilfried Dippold (Thu,) studied this question.