Metal-Organic frameworks (MOFs) have recently gained interest as an innovative class of photocatalytic materials owing to their catalytically active oxo-metal clusters, functional organic linkers, easily tunable photophysical/chemical properties and a large surface area along with well-ordered microporous structure. In dynamic processes and especially in liquid phase catalysis, the accessibility of active sites becomes a critical parameter as the reactant diffusion is often limited by the inherently small micropores of MOFs. One promising strategy to overcome this challenge involves the synthesis of mixed-ligand MOFs, followed by selective ligand removal (e.g. via selective oxidation, thermal degradation or dissolution) to design novel hierarchical microporous-mesoporous MOFs that facilitate reactant diffusion as well as induce uncoordinated centers as potential catalytic sites. These are truly novel materials that combine hierarchical porosity with the multifunctionality of the inorganic-organic hybrid system. We developed a strategy to introduce additional mesopores by selectively removing one ligand in mixed-ligand MOFs via thermolysis. Accordingly, photoactive MOFs of the MIL-125-Ti family were designed with two distinct hierarchical pore architectures resembling either large cavities or branching fractures. The ligand removal follows a 2-step process tunable by temperature and time. Both pore geometries significantly improved the HER rates of the MOFs by up to 400% by providing better access to newly created adsorption sites. This strategy provides a powerful tool for the purposeful engineering of hierarchical MOFs with advanced applicability in liquid media. We also investigated the reaction mechanism in various MOFs (e.g. COK-47, MIL-125) towards photocatalytic HER and OER for different light Irradiation, sacrificial agents and process conditions. The results show that methanol follows a direct charge transfer pathway, while TEOA prefers oxidation via radicals. The presence of amino-groups in the organic linker enables light absorption in the visible range, however it is detrimental to the catalytic conversion. Moreover, the 2D-arranged Ti-SBU in COK-47 facilitates charge separation and transport to adsorbed reactants. Finally, we describe new strategies to introduce isolated co-catalyst species within the pore framework for enhaced photocatalytic performance.