Terahertz field interaction and ultrafast dynamics in donor-acceptor systems

Developing electronic components from small collections of molecules, or even a single molecule, is the primary goal of molecular electronics. To achieve this, donor-acceptor systems are being explored as potential molecular switches, which function by using intramolecular charge transfer. Paracyclophanes (PCPs) serve as one of the proposed systems for studying and controlling through-space charge transfer. Achieving ultrafast control over these molecular states requires a fundamental understanding of both the interaction between the control field (e.g., terahertz (THz) electric field) and the molecule, and the excited state dynamics of the target molecules. This thesis addresses these prerequisites through foundational spectroscopic investigations. First, time-resolved THz Stark spectroscopy was employed to gain essential insights into the interaction between intense, sub-picosecond THz electric fields and donor-acceptor molecules. Studies on a donor-acceptor system in non-polar solvent successfully demonstrated the extraction of changes in dipole moment and polarizability under ambient conditions, providing crucial data on the THz-driven response mechanism. The technique’s robustness was further confirmed by extending measurements to model dyes in water as a highly polar solvent. Second, the excited state dynamics of novel PCPs were characterized using femtosecond transient absorption spectroscopy, complemented by theoretical modeling (TD-DFT/CC2). Homo-PCPs were investigated to validate the theoretical framework, confirming its capability in reproducing the experimental data. Meanwhile, investigations of hetero-PCPs revealed ultrafast charge transfer dynamics, whose characteristics could be tuned through different linkers. Together, these results provide essential groundwork for achieving THz field manipulation of molecular states on surfaces. The time-resolved THz Stark spectroscopy experiments help to understand the interaction of the THz field and donor-acceptor, while the transient absorption spectroscopy provides quantitative insight on charge transfer state lifetimes and relaxation pathways. Both experimental techniques were validated by theoretical studies. This comprehensive foundation is valuable for future experiments aimed at depositing these PCPs onto surfaces and controlling their molecular states with THz electric field.

2025

dissertation

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