Photochemistry in 3D: Porphyrin Cage Nanomaterials

In this thesis, an emerging class of materials known as porous organic cages, specifically those bearing light absorbing porphyrin-based building blocks, is studied. These unique molecules, colloquially known as porphyrin cages, exhibit permanent porosity owing to their internal cavities. The three-dimensional structures of these molecules allow their incorporation into a variety of nanomaterials. Three different porphyrin cages with different morphologies were synthesised in the thesis. A detailed study on their steady state photophysical properties and excited state dynamics was conducted. These studies showed that highly rigid and conjugated cages show increased molar absorptivity and photoluminescence quantum yields values. Next, the synthesis of a composite material consisting of plasmonic silver nanoparticles decorated with porphyrin cages is shown. The porphyrin cages act as both the stabilizing, and energy acceptor ligand which is hypothesized to allow diffusion and trapping of small molecules close to the metallic surface where photochemistry can take place. The optical spectra show spectral doublets due to resonant interaction between the porphyrin Soret band and silver nanoparticle plasmon resonance band. These phenomena, in addition to boosted light absorption of the porphyrin-acceptor, leads to enhancements in photoelectrochemical water splitting efficiency. This synthesis was then extended to incorporate gold nanoparticles, demonstrating the cages' ability to stabilize gold nanoparticles both at the cage periphery and within its cavity. The former process is referred to as 'confinement' synthesis. Then, the successful encapsulation of metal chalcogenide nanocrystals within the largest purely organic cage reported to date, was demonstrated. The confinement synthesis yielded nanocrystals below Bohr exciton radius of cadmium sulfide and cadmium selenide. Optical and electrochemical measurements revealed the cage transfers its excited state energy to the encapsulated nanocrystal core, resulting in marked increases in photoelectrochemical water splitting efficiency. The encapsulated CdSe nanocrystals also showed enhanced E isomer selectivity in photocatalytic olefination for alkene formation, likely due to the three-dimensional hierarchical assembly of the cage encapsulated nanocrystals. Lastly, the porphyrin cages were integrated into plasmonic near-perfect absorbers which exhibited near perfect absorption of visible light. The porphyrin containing macromolecule placed within the particle-mirror gap shows significantly enhanced emission over macro sized areas, primarily through increases in excitation rate. These near-perfect absorbers were then used directly for photoelectrochemical water splitting measurements. This thesis successfully demonstrates the harnessing of sunlight, which holds the most theoretical potential energy of any source known to man, via the improvement of conventional porphyrin and metalloporphyrin photocatalytic systems. Through the hybridisation of these three-dimensional porphyrin cages with nanocrystals, both plasmonic and semiconductor, the development of new photocatalysts with the ability to molecularly “sort” desired intermediates and stereo-products, is envisioned.