Design and Synthesis of Acene Oligomers for the Study of Photophysical Processes
Creators: Hou, Yuxuan
Publisher: name: University of Alberta Library
Year: 2024

<p>A variety of organic molecules have been used to study photophysical processes, such as singlet fission (SF) and triplet-triplet annihilation upconversion (TTA-UC), which are proposed strategies to improve the power conversion efficiencies of photovoltaics. Despite extensive study on these photophysical concepts over the past decades, the underlying molecular mechanisms are not well defined yet, and applications to photovoltaics remain to be achieved. Of the chromophores suited for photophysical studies, acene-based molecules have been the most common candidates to answer specific underlying questions about the mechanisms of SF and TTA-UC.<br>
This thesis focuses on the design, synthesis, and characterization of tetracene and pentacene derivatives. Chapter 1 introduces acenes and the synthesis of acene derivatives, in particular peri-alkyne-functionalized acenes. This chapter also gives a brief introduction of the photophysical processes that are key to the studies of molecules in the thesis, SF and TTA UC. Then, the state-of-the-art designs of acene-based molecules using the heavy-atom effect (HAE) and chromophore multiplication to promote more efficient materials for SF are described. Furthermore, the representative spacers used in dimeric acenes are presented, particularly with respect to the influence of electronic coupling on SF and TTA-UC.<br>
Chapter 2 focuses on the synthesis of sterically hindered pyridyl ligands either with or without a pentacene moiety. Subsequently, these ligands are used to coordinate with Pt(II) and Pd(II) for the construction of pentacene dimers. The synthesis of tetramers with Ru(II) is presented using the same ligands. The successful formation of these complexes is supported by mass spectrometric and NMR spectroscopic analyses. These compounds serve as model compounds for our collaborators to study the electronic influence of the heavy atoms and the impact on the mechanism of SF. Besides the study of the HAE, the pentacene tetramer is used to determine the prospect of triplet diffusion via chromophore multiplication.<br>
Chapter 3 presents a stepwise and modular synthesis of covalently linked tetracene dimers featuring tunable electronic coupling using a common building block, which is obtained by mono-addition of acetylides into 5,12-naphthacenequinone. The electronic coupling of the tetracene derivatives has been examined with UV-vis spectroscopy analysis and photophysical analysis by our collaborators. These compounds are designed to unravel the interplay between SF and TTA-UC in dimeric systems.<br>
Chapter 4 presents the synthesis of two sterically hindered pyridyl-endcapped tetracene dimers with identical structures, except that the intramolecular distance between the tetracene moiety and a pendent photosensitizer is varied via the spacer. These dimers are accessed through stepwise nucleophilic addition reactions using the common building blocks developed in Chapter 3. Moreover, pyridyl-endcapped tetracene dimers linked by a butadiyne or a meta-diethynylphenylene spacer have been synthesized. These dimers have been coordinated to a ruthenium phthalocyanine to form complexes with the potential to achieve intramolecular sensitization in addition to intramolecular upconversion. All molecules have been sent to our collaborators in Germany to study the process of TTA-UC.<br>
Chapter 5 describes several syntheses as part of projects that are in various stages of exploration. Attempts to synthesize a twisted pentacene dimer are presented, followed by the synthesis of an azobenzene linked pentacene dimer, and finally, the synthesis of a tetracenyl pyridyl ligand for the formation of tetracene dimers with Pt(II) and Pd(II). While the attempts to synthesize the twisted pentacene dimer are, to date, unsuccessful, a [5]cumulene endcapped with pentacenequinone has been obtained. The pentacene dimer linked by azobenzene has been synthesized successfully and preliminary studies of photoswitching have been conducted on both the unaromatized precursor and targeted pentacene dimer. With irradiation, the unaromatized precursor shows trans to cis isomerization. Studies of the pentacene dimer, however, reveal that the decomposition of the pentacene chromophore effectively competes with switching. In the final section of this chapter, the successful synthesis of tetracene dimers via the coordination of the sterically hindered pyridyl ligand with a tetracene moiety to Pt(II) and Pd(II) is outlined. These compounds serve as models for our collaborators for additional studies of the influence of the HAE on SF.<br>
Chapter 6 gives a summary and an outlook for the design of acene-based molecules for the study of diffusion of triplets in SF and more efficient TTA-UC. Chapter 7 provides the synthetic procedures and the spectroscopic characterization data of the compounds discussed in this thesis.</p>