The design and application of heavy-atom-free and redox-active organic triplet photosensitizers

Wright, Anna 2024. The design and application of heavy-atom-free and redox-active organic triplet photosensitizers. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Triplet photosensitizers are a class of molecule that can absorb light and transfer the absorbed energy to another molecule, inducing a chemical reaction. This phenomenon has resulted in triplet photosensitizers being utilized as powerful tools in medical therapies, materials development, and industrial catalysis. Heavy-atom-free triplet photosensitizers are promising alternatives to traditional transition-metal-containing triplet photosensitizers due to their atomic abundance, versatility, and ease of processing. However, heavy-atom-free or ‘organic’ molecules for photosensitization have remained largely unexplored due to the ambiguous relationship between their molecular structure and facile triplet formation, that is required for sensitization. This thesis begins with the development of a molecular design suitable for organic triplet photosensitizers. Inspired by various design motifs for intersystem crossing (ISC) in organic molecules documented in the literature, a series of compact donor-acceptor (D-A) chromophores was prepared with varying molecular sterics, which were then further modified using thiocarbonylation (Chapter 2). It was determined using spectroscopic and electrochemical measurements that each thionated chromophore could be utilized as amphoteric photosensitizers to catalyze reductive and oxidative photoreactions with efficiencies comparable to the popular inorganic photosensitizer Ru(bpy)3 2+. The photocatalytic ability of the photosensitizers demonstrates that thiocarbonylation of amide/imide D-A chromophores makes them suitable redox-active organic triplet photosensitizers. In Chapter 3, our established molecular design for organic triplet photosensitizers was modified, instead using phenothiazine as the electron donor with the objective of exploring the physical effects of highly rigid, strong electron donors in D-A chromophores. Using spectroscopic measurements, we determined that both the unthionated and thionated chromophores display facile triplet formation, for which the mechanism was determined using DFT calculations. Both the unthionated and thionated D-A chromophores were then analyzed as photosensitizers for triplet-triplet annihilation up-conversion, where the unthionated D-A chromophores pose as excellent candidates for solar energy harvesting materials. xii The feasibility of utilizing thiocarbonylation in commercial amide/imide-containing chromophores was then assessed (Chapter 4). A typical ‘push-pull’ dye comprised of a barbituric acid electron acceptor group with either one or two 4-(dimethylamino)phenyl electron donor groups was chosen and the thionated counterparts were also prepared for comparison. It was determined that each thionated dye displayed facile triplet formation and amphoteric electrochemical properties using spectroscopic and electrochemical measurements. So, the dyes were utilized as photosensitizers in both reductive and oxidative photolysis reactions, which revealed that the presence of two donors limits the desired effects of thiocarbonylation for facile triplet formation when compared to the single donor. Subsequent DFT calculations determined that the two-donor system hinders the necessary effects of thionation for ISC due to the lowest unoccupied and highest occupied molecular orbitals (LUMO and HOMO, respectively) being localized across the two donor moieties. This demonstrates a previously unknown limitation of thiocarbonylation in amide/imide D-A chromophores for developing organic triplet photosensitizers. In addition to developing a renovated molecular design for organic triplet photosensitizers, a series of thienodipyrimidine-2,4,5,7-tetra(thi)ones was synthesized (Chapter 5). The mechanism of formation was probed using various reaction conditions, where it was proposed that the molecules formed via a one-pot, two-step photocyclization mechanism. The physical properties of each molecule were analyzed, and it was determined that the thienodipyrimidine-2,4,5,7-tetra(thi)ones oxidation potential is significantly lowered when there are two neighboring sulfur atoms in close proximity. Further investigation using DFT calculations suggested that the presence of an intramolecular two-center three-electron interaction was responsible for the low oxidation potential that has not previously been observed in multiple thiocarbonyl-containing systems. Finally, a series of D-π-A chromophores were prepared with varying alkyl substituents and electron donors (Chapter 6) based on early findings, where N-substituted naphthalene monoimides displayed triplet formation that was attributed to the charge separation and/or twisting between the D/A moieties. Further DFT calculations revealed that the series of chromophores have small, excited singlet/triplet state energy gaps (ΔEST) and a mixture of excited local and charge transfer triplet states suitable for reverse ISC and, therefore, delayed fluorescence. Time-resolved emission measurements demonstrated that the donor variation xiii modified the delayed fluorescence properties, allowing for either in-solution or solid-state delayed emission, and showed that alkyl substituents can help to enhance delayed fluorescence without affecting the molecules’ physical properties.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Date of First Compliant Deposit:	19 November 2024
Last Modified:	19 Nov 2024 11:15
URI:	https://orca.cardiff.ac.uk/id/eprint/174134

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