Functionalization of nanoporous gold with macrocyclic metal complexes: a novel class of heterogeneous singlet oxygen photosensitizers

Abstract

In the present thesis, the usage of nanoporous gold as support material for immobilization of macrocyclic metal complexes resulting in highly active and truly heterogeneous hybrid photocatalysts was investigated. Such systems are promising photocatalysts for singlet oxygen driven photooxidation reactions as only an appropriate catalyst, oxygen and light are required, offering very mild and sustainable reaction conditions. After optimization of the preparation strategy, the detailed characterization of the obtained novel hybrid materials was a first major point. Including the determination of the pore sizes of the nanoporous gold support, the quantification of the amount of immobilized photosensitizer as well as the distribution over the entire porous structure were studied. By different energy dispersive X-ray mapping and line scanning experiments, a homogeneous distribution of the immobilized metal complex on the porous support was revealed. The photocatalytic investigations of the hybrid photocatalyst can be divided into three sections, which will be also represented in the structure of the present work. The first main focus was the investigation of the structure-activity relationship originating from immobilization onto the nanoporous gold support. Employing the photooxidation of 1,3-diphenylisobenzofuran as model reaction, the hybrid system was investigated in terms of the macroscopic and microscopic properties of the porous support. The macroscopic shapes included the functionalization and comparison of disks, powders and foil-based coatings. The major differences in photocatalytic activity were found as result of the differences in light exposed surface area. In addition, in case of the nanoporous gold coatings the variation of the pore sizes revealed a correlation of the specific surface area with the amount of immobilized photosensitizer. The differences of the pore sizes also allowed the investigation of mass transport as well as diffusion limitation phenomena. Regarding the linking self-assembled monolayer, the necessity of an additional spacer molecule was also investigated by comparison of hybrid systems based on monomolecular and bimolecular monolayers as linking units. The second and most extensive part of this work was the investigation of the underlying photophysical energy transfer processes and interactions between the gold support and the immobilized photosensitizer. For this purpose, the hybrid photocatalyst was irradiated with different excitation wavelengths resulting in action spectra for singlet oxygen formation indicating also singlet oxygen formation by irradiation of the nanoporous gold surface plasmon resonance. Based on those results a reaction mechanism was proposed including energy transfer from the gold support to the attached photosensitizer. The variation of the distance between the two chromophores confirmed the significant contribution of the plasmon resonance within the overall photocatalytic mechanism. Whereas for short distances photoinduced electron transfer from the excited photosensitizer to the support is dominant, for longer distances above 1 nm energy transfer becomes increasingly important. The ideal distance between the two chromophores was found to be around 1.5 nm. Within this study, the impact of the photosensitizer orientation was also investigated and was shown to play a crucial role for the overall distance. Based on those findings, the influence of peripheral and axial linkage was studied employing different classes of photosensitizers ranging from phthalocyanines and tetraphenylporphyrins to axial substituted subphthalocyanines. The variation of the immobilized macrocyclic metal complex can also be seen as first step towards a panchromatic hybrid photocatalyst absorbing light over the entire electromagnetic spectrum of visible light. The last part of the work was focussing on the versatility and predictability of the hybrid system in various photocatalytic oxidation reactions. Here, the novel hybrid photocatalyst was employed in photooxidation reactions of different furan, anthracene and 1,3-cyclohexadiene based derivatives. The results revealed a strong dependency on the used solvent as result of different singlet oxygen lifetimes and diffusion lengths within the porous network. Nevertheless, the entire reactivity of the scope could be predicted from homogeneous liquid-phase reaction rates available in literature considering the employed solvent. Finally, the reusability of the hybrid photocatalyst was examined to show the versatility of this novel system and to give a complete overview of all relevant properties and requirements addressed within the context of applicable truly heterogeneous catalysts.