title: Linear Co-polymer Photocatalysts: From Fundamental Design to Organic/Inorganic Hybrids
Creators: name: Coates, Ian
Publisher: name: University of Liverpool Repository
Year: 2024

description: Hydrogen gas will likely play an important role in the decarbonisation of hard-to-abate sectors and the overall global race to net zero. Organic photocatalysts offer a route to cheap and sustainable production of green hydrogen by using solar energy to split water. Indeed, significant advances in using semiconducting conjugated polymers to produce hydrogen from water under sacrificial conditions have been made in recent years. However, examples of polymeric photocatalysts capable of achieving overall water splitting have been limited. Since the chemical space constructed from simple organic building blocks is almost infinite, there is enormous potential for further enhancements via the rational design of polymer structures. It is well known that small structural changes can enable the fine-tuning of physical, chemical, and optoelectronic properties of organic semiconductors. This thesis focuses on understanding the structure-activity relationships of linear organic co-polymer photocatalysts, designing new polymers based on these understandings, and then using these polymers to design hybrid organic/inorganic photocatalysts for overall water splitting.
First, an in-depth analysis of the structure-activity relationships of linear co-polymers is performed using data from previously published high-throughput experimental results. This leads to new insights into the detrimental effects of residual palladium, low synthesis yields, short fluorescence lifetimes, and small predicted oscillator strengths. Structural features, physical properties, and photocatalytic activities are correlated with computationally predicted properties, which are used to identify 78 new promising organic photocatalysts. Improved predictions for the optical bandgaps of these materials are also made based on newly devised methodology. A particular focus is placed on understanding the varied results obtained for structurally similar polymers upon adding a platinum co-catalyst.
A series of co-polymers based on dibenzothiophene sulfone (DBTS) and fluorinated phenylene (PhFx) monomers are synthesised via a microwave-assisted high-throughput reaction. These studies demonstrate that fine-tuning the reaction conditions and co-catalyst content can dramatically enhance photocatalytic activity. The resulting polymers consistently outperformed all 99 co-polymers reported in the previous high-throughput study. Some polymers outperformed the Pt-loaded DBTS-DBTS homopolymer under optimal conditions for the first time. Statistical ternary co-polymers combining the DBTS-PhFx co-polymers with the electron-accepting benzodithiophene (BDT) moiety are also reported. This work is reinforced with DFT calculations and demonstrates that fluorination can be used to fine-tune electronic structure and influence molecular packing.
Finally, organic/inorganic hybrid photocatalysts are prepared via in-situ polymerisation. This includes the first known attempts to form a direct heterojunction interface between a conjugated polymer and flux-mediated Al:SrTiO3. Attempts to form a polymer/BiVO4 hybrid S-scheme photocatalyst were also made. These initial experiments show promising signs as proof of concept and indicate that further experiments should be conducted.