Third-order nonlinear optical properties of polymethine-based materials: a theoretical investigation
Gieseking, Rebecca Lynn
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Organic π-conjugated molecules and materials with large real parts and small imaginary parts of the third-order polarizability are of great interest for all-optical switching applications. In this dissertation, we use quantum-chemical and molecular-dynamics approaches to investigate the structure-property relationships that influence the nonlinear optical properties of π-conjugated molecules and materials. We begin with an overview of nonlinear optics, focusing in particular on the electronic properties of linear π-conjugated systems and some of the important problems that have limited device applications of these molecules to date. This is followed by a brief review of the computational methods employed in these studies. We then turn to the main results of the dissertation. Chapter 3 describes the structural dependence of the transition dipole moment between the first two polymethine excited states. Chapter 4 discusses the relationship between BLA, which depends on the geometric structure, and BOA, which probes electronic structure. Chapter 5 describes the benchmarking of computational methods to describe the symmetry-breaking of long polymethines and preliminary evidence regarding the role of vibrational modes in symmetry-breaking. Chapter 6 explains the negative third-order polarizability of tetraphenylphosphate and analogous systems. Chapter 7 focuses on molecular-dynamics studies of polymethine aggregation, particularly the relationships between chemical structure and the geometric and electronic structures of aggregates. Finally, Chapter 8 provides a synopsis of the work and discussion of further directions.