Over the past 15 years, there have been significant developments in expanding the singlet fission (SF) library and understanding the SF mechanism. SF has been a topic of interest in recent years due to its potential applications in organic photovoltaics. During SF, a singlet exciton energetically down-converts into two triplet excitons. For this process to be spin-allowed, a multiexciton correlated triplet pair, (T1T1), state is formed. This (T1T1) state has interesting spin properties and thus makes SF relevant to quantum information science applications as well. This dissertation investigates the nature of the (T1T1) multiexciton state in covalent dimers and thin films. Terrylene-3,4:11,12-bis(dicarboximide) (TDI) is a member of the rylenediimide family that undergoes SF in covalent dimers and films. XanTDI2 is a covalently linked TDI dimer in which a xanthene bridge holds two TDI molecules in a π-stacked geometry and a biphenyl spacer forces a slip-stacked geometry. XanTDI2 undergoes rapid SF to form the 1(T1T1) state, which is a mixed state with contributions from the 1(S1S0), 1(T1T1), and charge transfer (CT) states. The mixed nature of this (T1T1) state was probed by femtosecond transient absorption and mid-infrared spectroscopies. The mixed nature of the multiexciton state was also observed in an orthogonal TDI dimer connected directly via the imides (TDI2). The electronic nature of the (T1T1) state of TDI2 could be tuned by modifying the dielectric environment. The (T1T1) state of TDI2 also has interesting spin properties. Time-resolved electron paramagnetic resonance spectroscopy demonstrated that the TDI2 (T1T1) state has mixed 5(T1T1) and 3(T1T1) character at room temperature. Mixing with the 3(T1T1) state promotes triplet-triplet annihilation of the (T1T1) state to form a single uncorrelated triplet exciton. The competition between intramolecular and intermolecular SF was studied in a TDI dimer attached through the imide position with a 2,5-di-tert-butyl phenyl spacer (TDI-Ph-TDI). The lack of through-bond coupling in TDI-Ph-TDI results in primarily intermolecular SF in the solid-state despite undergoing intramolecular SF in solution. New SF chromophores such as quaterrylene-3,4:13,14-bis(dicarboximide) (QDI) and zethrenebis(dicarboximide) (ZDI) were also explored. Although QDIs and ZDIs have low triplet energies, studying their SF dynamics provides valuable insights into the SF mechanism. A QDI derivative with aromatic imide substituents, ArQDI, undergoes ultrafast SF to form the (T1T1) due to the exoergicity of the system, but (T1T1) state dissociation is slow. When QDIs are substituted at the bay-positions, the energetics of the system change, resulting in isoergic and slower SF. A ZDI derivative, ZDIPh2, undergoes rapid SF to form the (T1T1) state, which eventually dissociates to form free triplet excitons. SF chromophores with longer-lived (T1T1) states due to slow dissociation are promising for applications in quantum information science.

Creator

• Chen, Michelle

DOI

• https://doi.org/10.21985/n2-8727-ga47

Subject

• Physical chemistry
• Chemistry

Language

• en

Alternate Identifier

• etdadmin_upload_768182
• http://dissertations.umi.com/northwestern:15313

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright