Singlet Fission (SF) is a down-conversion process in organic chromophores where one high-energy singlet exciton is converted into two low-energy triplet excitons. Such carrier multiplication aspects of SF are promising in increasing the solar cell efficiency. The generation of two bound spin triplet states from the same singlet exciton also provides an interesting platform to study spin entanglement, which is an important concept for quantum information science and spintronics. As such, SF process necessitates fundamental understanding of exciton and electron spin dynamics. The first half of this thesis investigates important design principles for anthracene-based SF chromophores. Although SF was first discovered in single crystals of anthracene, it has ~3% efficiency because the SF process needs to occur from the second excited singlet state, where internal conversion to the first excited singlet state is a dominant process. We increase π conjugation of anthracene by adding phenylethynyl groups at the 9,10 positions to form 9,10-bis(phenylethynyl)anthracene (BPEA). This lowers both the singlet and the triplet excited state energies. Unlike anthracene, BPEA has the correct energetics for SF to occur from the first singlet excited state, allowing fast and efficient SF. Since SF requires two or more coupled chromophores, we first synthesized the BPEA covalent dimer and further explored SF dynamics in solid state films. These results are summarized in chapters 2 and 3. The second half of the thesis focuses on electron spin dynamics of SF by directly probing the spin sublevels using time-resolved electron paramagnetic resonance spectroscopy. Understanding how the triplet spin sublevel is populated provides information on how triplets in organic chromophores are generated. In chapter 4, we investigate the triplet polarization pattern in various anthanthrone and anthanthrene compounds to identify triplets produced via intersystem crossing versus SF. In chapter 5, we directly probe the quintet sublevel population and verify the recent theoretical work that discusses how the orientation of the magnetic field with respect to the crystal axis alters quintet sublevel state mixing.

Creator

• Bae, Youn Jue

DOI

• https://doi.org/10.21985/n2-smz0-kd87

Subject

• Physical chemistry

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:15306
• etdadmin_upload_766668

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright