Molecular and Nanoscale Architectures for Solar Energy Conversion and Beyond

Over recent decades, the Earth has significantly warmed due to anthropogenic climate change caused by substantial greenhouse gas emissions linked to industrialization. Although international efforts to mitigate warming gained momentum toward the late 20th and early 21st centuries, recent political developments suggest stagnation or even regression of such initiatives. Current data suggest that a long-term global temperature increase of 1.5 °C may be reached within the next few years, with a rise exceeding 2 °C highly likely by the end of the century. Irrespective of the exact warming scenario, severe consequences such as biodiversity loss, intensified extreme weather events, and significant ice melt will ensue, disproportionately impacting vulnerable populations. Higher warming further exacerbates these effects, increasing the risk of irreversible tipping points. The largest contributor to greenhouse gas emissions is the energy sector, driven predominantly by fossil fuel combustion for electricity and heat generation. Therefore, transitioning from fossil fuels to renewable energy sources, along with the electrification of related sectors such as transport, offers significant potential for effective decarbonization. This transition is not only vital from ecological, economic, and social perspectives but also critical geopolitically, particularly for resource-scarce regions like the European Union, as it promises energy independence, thereby mitigating risks arising from increasing global tensions and protectionist tendencies. Among renewable energy sources, solar energy is especially promising due to its abundance and inexhaustibility. However, with conventional silicon-based solar cells nearing their efficiency limits, there is growing interest in next-generation solar technologies, including those based on organic materials, which can provide improved light-harvesting capabilities and potentially higher conversion efficiencies. Within this context, this thesis aims to elucidate the photophysical properties of several molecular and nanoscale architectures, thus enhancing the fundamental understanding of their structure-property relationships. Such insights are crucial for future developments in solar energy conversion technologies and may further contribute to advancements in optoelectronic and quantum-information applications. To achieve these goals, the materials were extensively characterized using a variety of photophysical techniques, including steady-state absorption and emission spectroscopy, time-correlated single-photon counting, and transient absorption spectroscopy to examine their optical properties. These were complemented by spectroelectrochemical analyses to investigate the nature of photo-generated charged species. This thesis is structured into two main work packages. The first focuses on pentacene-based molecular architectures and their potential to undergo singlet fission (SF). Four sub-projects were conducted, aimed at improving the SF response across the entire visible spectrum and deepening the mechanistic understanding of the SF process, specifically regarding intermediate triplet pair evolution and deactivation. The first sub-project involved covalently attaching a known SF-active pentacene dimer to a subphthalocyanine (SubPc) moiety which acts as a light-harvesting energy donor. The influence of different linking motifs on the excited-state dynamics, particularly energy transfer rates and efficiencies, was thoroughly investigated. In the second sub-project, multiple SubPc energy donors were connected to the pentacene dimer, inducing structural modifications that allowed for a precise modulation of interpentacene couplings and, consequently, effects on SF dynamics. To address the shortcomings identified in these initial studies, the third sub-project combined the advantages of discrete dimers and solid-state systems. To this end, three pentacene dimers were employed, attached to a central SubPc, which now acts not only as an energy donor but also as a scaffold to allow for a hexameric arrangement of the pentacenes. Comprehensive spectroscopic analysis and kinetic modeling revealed unusual excited-state dynamics and enabled formulation of design guidelines for more efficient covalent SF systems. The fourth sub-project featured two isomeric pentacene dimers with minimal interchromophore coupling, achieved by tailored linker design and substitution patterns. Extensive spectroscopic characterization, including magnetic-field-dependent measurements, explored the fundamental relationships between coupling strength, topology, and available deactivation pathways. The second work package addresses nanoscale systems, that is, nanographenes (NGs) and graphene nanoribbons (NRs). These materials exhibit highly tunable properties closely governed by their structural characteristics. However, since synthetic progress has outpaced structural and functional characterization, two sub-projects were conducted to help bridge this gap. The first one investigated nitrogen-doped twisted molecular NRs of varying lengths. In-depth photophysical characterization provided insights into their rich deactivation dynamics in the excited state, highlighting their potential as novel SF-active materials with elevated triplet excited-state energies. The second sub-project examined a NG embedding non-hexagonal rings. Strictly speaking, four heptagonal rings were incorporated which induce a pronounced double curvature, endowing the compound with distinctive electronic and optical properties. This project primarily aimed to clarify the structure-property relationship, particularly the electron-donating capabilities facilitated by the incorporation of heptagonal rings. Additionally, host-guest chemistry with electron-accepting fullerenes was explored, further elucidating the potential of such twisted NGs for applications in energy conversion technologies.