Novel Norbornadiene-Based Photoswitches for Molecular Energy and Data Storage

To maintain the possibility of a carefree and comfortable life in a developed society, tackling the challenges of climate change is crucial. A way to mitigate its impact is to reduce global greenhouse gas emissions. In this endeavor, over the last decades, the search for reliable use of renewable energy harvesting technologies has increased tremendously. Besides conventional wind- and water-based energy powerplants, the implementation of the limitless source of sunlight has been extensively researched recently, primarily for solar cells and other solar-to-energy technologies. So-called molecular solar thermal systems (MOST) exploit tailored molecules to collect solar energy and directly store it in strained molecular scaffolds. This thesis investigated new ways to fine-tune the photophysical and chemical properties of the norbornadiene/quadricyclane (NBD/QC) photoswitch couple to make it suitable for future energy storage applications. Here, the energy of incoming irradiation is stored upon conversion of NBD to QC and can be released catalytically on demand by various stimuli, initiating the inverse rearrangement. Although this system has already been extensively researched for use in MOST devices, certain challenges still remain. Hereby, the detailed understanding of the relationship between the photophysical properties and structural modifications of the parent hydrocarbon scaffold is significant. Therefore, in the first part of this thesis, a synthetic roadmap was developed to generate a variety of new NBD derivatives, which were then systematically investigated regarding their properties, characteristics, and conversion features. The introduced tosyl (Ts) functionality served as a recurring electron-withdrawing functionality, while different electron-donating groups (EDG) were attached. The applied modular approach, comprising a central tosyl-halide-bearing NBD precursor suitable for further functionalization via cross-coupling chemistry, enabled easy and variable substitution with tailored EDGs. The synthesized library was compared to literature-established nitrile or ester bearing analogues, rendering the investigated Ts functionality as a promising alternative as an electron-acceptor. Furthermore, comprehensive analysis and conversion studies in different solvents and variable concentrations were conducted to get detailed understanding of the switching behavior of the investigated molecules. Here, specific photophysical key-features such as half-lives, quantum yields, and storage capacities are crucial for properly classifying the potential of photoswitches in terms of MOST systems. All identified properties were in alignment with literature known analogues, which proves the suitability of the newly developed candidates for potential application implementation. The best-performing derivatives were further examined in collaborative work analyzing the electrochemically driven energy-release process. The applied stimulus initiated an oxidation reaction, leading to an autocatalytic reconversion from QC to NBD. Due to the unique properties of this technique, precise control of the energy release process is possible by carefully adjusting the applied onset voltage, making this method particularly useful for device implementation. Here, the influence of the EDG groups was found to be crucial on the related electrochemical features. Another possibility of altering the characteristics of the NBD/QC couple is by the implementation of heteroatoms in the fundamental carbon framework. This transition from isocyclic to heterocyclic NBDs offers promising opportunities but simultaneously presents specific additional challenges. Based on the synthetic knowledge gained during the synthesis of the isocyclic derivatives, again, cross-coupling-based modification of oxygen and nitrogen-containing hetero-NBD was performed in the second part of this thesis. Therefore, first, the properties of halide-tosyl functionalized oxa- and aza-NBDs precursors, and afterwards, electron donor functionalized analogues were compared to the isocyclic counterparts. However, for the heterocyclic derivatives, significantly more complex forward and backward-conversion processes were identified, as analyzed by comprehensive spectroscopic investigations. Especially the variety of different back-conversion processes led to the conduction of further collaborative examinations. With simple bis-methyl-ester and bis-benzyl-ester oxa-QC derivatives, the energy release on a defined Pt(111)-surface was monitored by X-ray photoelectron spectroscopy under ultrahigh vacuum conditions. Based on the different lengths and rotational freedom of the ester groups, various surface alignments of the molecules and intermediates are possible throughout the experiments, resulting in variable reconversion mechanisms. Depending on the spatial prerequisites, differing initial conversion temperatures as well as fragmentation patterns were observed, leading to either the regeneration of the parent NBD species or decomposition. The last part of this thesis describes the synthesis and investigation of triazine-based multi-NBD derivatives. Here, symmetric and asymmetric bis- and tris-NBDs were generated by anchoring various NBDs to a central electron-withdrawing triazine core. Due to the different EDG substituted periphery, selective switching of the individual moieties within the single molecule was rationalized. Exploiting acid/base-sensitive amine functionalities as an additional switch further extended the amount of available forms. With this concept, the number of different states generated within a single and low-weight molecule is unprecedented to date for similar small multi-switch molecules. Combined with the determination of the photophysical key features mentioned above, comprehensive switching studies incorporating different input sequences led to a precise understanding and control of the investigated molecules. With this, most of the theoretically achievable states could also be accessed practically. Furthermore, quantum chemical calculations corroborated the analyzed observations.