Charge-Transfer States in Organic Nanowires

Charge separation processes in organic semiconductors play a pivotal role in diverse applications ranging from photovoltaics to photocatalysis. Understanding these mechanisms, particularly the role of hybrid charge-transfer (CT) states, is essential for advancing these technologies. This thesis presents an extensive investigation into organic 1D aggregates, specifically dicyanoperylene bisimide derivatives imide-substituted with oligopeptide-polymer chains, and their emerging unusual charged states. Emphasis is placed on understanding the mechanisms leading to the formation of long-lived radicals. The study integrates experimental, computational, and theoretical approaches, offering a comprehensive insight into the photophysical properties of these nanowires.Initially, an in-depth examination of the structural and electronic properties of these nanowires is conducted. Notably, it reveals that the radicals within these systems are not only photogenerated but also exhibit an inherent stability. Investigations also confirm the self-doping character of these nanowires, hinting at a role of the oligopeptide substituent in this process. Further explorations into the magnetic properties of these nanowires uncover evidence of antiferromagnetic coupling among like-charged radicals, providing a potential explanation for the unexpected stabilization of these radical ions. The role of collective effects in the phenomenon is further supported by findings suggesting that while charge separation might not require extensive aggregation, the stabilization of polarons is dependent on it. Computational studies shed light on the CT interactions between the lateral substituents and the chromophores at the molecular level. This investigation demonstrates that the CT states in these systems are coupled with both the ground state and the local Frenkel excited state, facilitating the efficient formation of radical ion pairs through either continuous thermally-assisted pathways or photoexcitation. Finally, the thesis explores the kinetics of the decay process, following excitation and thus photopumping of the polaron population, across both short and long time scales. Combined with the computational results and theoretical insights, these investigations allow the derivation of a conceptual model explaining the properties of the radical anion population in these nanowires.This research establishes the oligopeptide-dicyanoperylene bisimide nanowires, and more generally, 1D donor-acceptor aggregates, as promising model systems for advancing the understanding of the CT processes at the molecular level.