The field of photophysics and photochemistry is a fascinating and intricate area of research that may hold the key to major technological advancements and help address some of the most pressing challenges of our time such as mitigating global warming and meeting the growing demand for sustainable energy through more efficient light harvesting, solar energy conversion, and energy storage. Despite the remarkable progress in quantum chemical computational methods and time-resolved spectroscopic techniques, accurately predicting and understanding photophysical deactivation dynamics remains a challenging and demanding task. In this regard, comprehensive and interdisciplinary studies that combine both theoretical and experimental approaches remain essential. This results in a somewhat paradoxical situation where even some of the oldest and most well-known molecular structures are still not fully understood from a photophysical perspective. Clearly, there is still much to uncover, and a deeper understanding of the structure–property relationships that underpin photophysical processes is urgently needed. Within this broader context, the present thesis aims to contribute to this understanding by investigating how subtle changes in molecular architecture, electronic structure, and intermolecular interactions influence excited-state behavior. By doing so, it seeks to lay the groundwork for the rational design of next-generation functional materials for light-driven technologies. Scope of the presented work is the photophysical investigation of chemical structures with unique closed-shell to open-shell character, so-called diradicaloids. These molecular moieties are generally known to excel through their intriguing electronic, magnetic and optical properties. However, chemical stabilization of diradicaloid structures is a challenging task and, thus, their photophysical aspects remain largely unexplored. In order to address this issue, molecules with a wide range of diradical character – ranging from pure closed-shell, moderate open-shell, up to the strong electron correlation regime – are photophysically characterized. To fulfill this mission, a whole spectroscopic arsenal, such as steady-state absorption and fluorescence spectroscopy, vibrational spectroscopy on both electronic ground state and excited states, or femtosecond as well as nanosecond transient absorption spectroscopy, was used to elucidate the underlying deactivation dynamics. The work begins with a detailed investigation of a partially chlorinated Thiele hydrocarbon, a diradicaloid dimer of the Gomberg radical, which is known for more than hundred years and referred to as one of the primary examples for diradicaloids. Main focus lies its efficient deep-red to near-infrared solvatochromic emission which originates from a sudden polarization mechanism within a formally dark singlet excited state. Subsequently, the excited-state dynamics of a series of aryl-capped odd ncumulenes with mild diradical character are examined, revealing chain length dependent photophysical behavior, shaped strongly by conformational effects through the attached endgroups. A third study focuses on carbene-derived diradicaloid with exceptionally high open-shell character, where efficient intermolecular singlet fission (SF) is achieved via π-stacked dimers in solution, demonstrating how non-covalent interactions modulate photophysics and electronic structure. Finally, 7superhelicenes composed of covalently fused hexa-peri-hexabenzocoronene units are introduced as tunable, chiral nanographenes. In contrast to the former projects, these platforms exhibit a pure closed-shell configuration. It is shown that minor structural modifications drastically affect their charge-transfer behavior, fluorescence, and triplet-state formation. Collectively, these studies highlight the fundamental limitations of simple additive reasoning in photophysical systems and underscore the importance of combined theoretical, synthetic, and spectroscopic efforts to decipher the complex structure–property relationships that govern excited-state processes in both open- and closed-shell molecular architectures. Moreover, these peculiar molecular building blocks allow valuable insights into mechanisms of some intriguing photophysical processes which is singlet fission and sudden polarization.
Tobias Ullrich (Thu,) studied this question.