This thesis presents results on photophysics and spin dynamics of photoactive organic molecules that possess one unpaired electron spin in the ground state and two or three unpaired spins upon photoexcitation. The excited state dynamics of the systems were studied using transient optical absorption spectroscopies and non-Boltzmann population on the spin sublevels of the long-lived excited states of the systems were further interrogated using time-resolved electron paramagnetic resonance (EPR) spectroscopies and pulsed EPR spectroscopies. Despite the weak or moderate exchange couplings in these multi-spin systems, unique spin dynamics and intense spin polarization were still observed. The thesis consists of two parts, one concerning the charge transfer and spin dynamics in a covalently linked donor-acceptor-radical system and the second half concerning the excited state dynamics, spin multiplicity and spin polarization in chromophore-radical systems. In Chapter 2, a donor-acceptor-radical molecule that forms a triradical state upon photoexcitation is investigated. We studied charge transfer dynamics of the system in various dielectric environments and demonstrated the switching between a mono-radical state and a weakly coupled triplet-radical state in low temperature matrices. Chapter 3 and Chapter 4 discusses the spin dynamics of excited triplet states coupled with a radical in systems featuring an organic chromophore that is appended with a stable radical. In Chapter 3, we found that metalation of a porphyrin chromophore increased the exchange coupling between the excited porphyrin triplet state and 2,2,6,6-tetramethylpiperidinoxyl doublet state, forming excited quartet state that was confirmed by pulsed EPR measurements. Chapter 4 discusses results of spin polarization of α,γ-bisdiphenylene-β-phenylallyl (BDPA) doublet ground state by photogeneration of a C60 triplet state through weak exchange coupling in a series of covalently linked C60-BDPA dyad. Pulse EPR techniques unambiguously measured the spin polarization dynamics and enhancement factors in these systems. In chapter 5, a donor-acceptor molecule that serves as a candidate for two-qubit operation was investigated. Photoexcitation of the donor generates a long-lived radical pair that has distinct g factors for each radical component. Deuteration of the donor and aligning the molecule in a liquid crystal further simply the EPR spectrum of the radical pair, allowing for qubit specific operation and detection. Further, single-qubit and two-qubit CNOT gate was performed on this radical pair, demonstrating the feasibility of the complete set of qubit gates for molecular qubit.

Creator

• Mao, Haochuan

DOI

• https://doi.org/10.21985/n2-5vtv-jh34

Subject

• Physical chemistry
• Chemistry

Language

• en

Alternate Identifier

• etdadmin_upload_946833
• http://dissertations.umi.com/northwestern:16364

Date created

• 2022-01-01

Resource type

• Dissertation

Rights statement

• In Copyright