Optical and Transport Properties of Organic Molecules: Methods and Applications

Organic molecules are versatile and tunable building blocks for technology, in nanoscale and bulk devices. In this dissertation, I will consider some important applications for organic molecules involving optical and transport properties, and develop methods and software appropriate for theoretical calculations of these properties. Specifically, we will consider second-harmonic generation, a nonlinear optical process; photoisomerization, in which absorption of light leads to mechanical motion; charge transport in junctions formed of single molecules; and optical excitations in pentacene, an organic semiconductor with applications in photovoltaics, optoelectronics, and flexible electronics. In the Introduction (Chapter 1), I will give an overview of some phenomenology about organic molecules and these application areas, and discuss the basics of the theoretical methodology I will use: density-functional theory (DFT), time-dependent density-functional theory (TDDFT), and many-body perturbation theory based on the GW approximation. In the subsequent chapters, I will further discuss, develop, and apply this methodology./n2. I will give a pedagogical derivation of the methods for calculating response properties in TDDFT, with particular focus on the Sternheimer equation, as will be used in subsequent chapters. I will review the many different response properties that can be calculated (dynamic and static) and the appropriate perturbations used to calculate them./n3. Standard techniques for calculating response use either integer occupations (as appropriate for a system with an energy gap) or fractional occupations due to a smearing function, used to improve convergence for metallic systems. I will present a generalization which can be used to compute response for a system with arbitrary fractional occupations./n4. Chloroform (CHCl3) is a small molecule commonly used as a solvent in measurements of nonlinear optics. I computed its hyperpolarizability for second-harmonic generation with TDDFT with a real-space grid, finding good agreement with calculations using localized bases and with experimental measurements, and that the response is very long-ranged in space./n5. N@C60 is an endohedral fullerene, a sphere of carbon containing a single N atom inside, which is weakly coupled electronically. I show with TDDFT calculations that a laser pulse can excite the vibrational mode of this N atom, transiently turning on and off the system's ability to undergo second-harmonic generation. The calculated susceptibility is as large as some commercially used frequency-doubling materials./n6. A crucial question in understanding experimental measurements of nonlinear optics and their relation to device performance is the effect of the solution environment on the properties of the isolated molecules. I will consider possible explanations for the large enhancement of the hyperpolarizability of chloroform in solution, demonstrate an ab initio method of calculating electrostatic effects with local-field factors, and derive the equations necessary for a full calculation of liquid chloroform./n7. Many-body perturbation theory, in the GW approximation for quasiparticle bandstructure and Bethe-Salpeter equation for optical properties, is a powerful method for calculations in solids, nanostructures, and molecules. The BerkeleyGW code is a freely available implementation of this methodology which has been extensively tested and efficiently parallelized for use on large systems./n8. Molecular junctions, in which a single molecule is contacted to two metallic leads, are interesting systems for studying nanoscale transport. I will present a method called DFT+Sigma which approximates many-body perturbation theory to enable accurate and efficient calculations of the conductance of these systems./n9. Azobenzene is a molecule with the unusual property that it can switch reversible between two different geometries, cis and trans, upon absorption of light. I have calculated the structures of these two forms when absorbed on the Au(111) surface, to understand scanning tunneling microscope studies and elucidate the switching mechanism on the surface. I have also calculated the conductance of the two forms in a molecular junction./n10. The Seebeck and Peltier thermoelectric effects can interconvert electricity and heat, and are parametrized by the Seebeck coefficient. Standard methods in quantum transport for computing this quantity are problematic numerically. I will show this fact in a simple model and derive a more robust and efficient approach./n11. Pentacene is an organic semiconductor which shows exciton self-trapping in its optical spectra. I will present a method for calculation of excited-state forces with the Bethe-Salpeter equation that can be applied to study the geometrical relaxation that occurs upon absorption of light by pentacene.

Advisor: Louie, Steven G.