Dynamics of Molecular Excitons

Seogjoo J. Jang

• Publisher: Elsevier
• Year: 2020
• ISBN:
  9780081023365
• Topic: Naturwissenschaften
• Subtopic: Nanowissenschaft

Dynamics of Molecular Excitons provides a comprehensive, but concise description of major theories on the dynamics of molecular excitons, intended to serve as a self-contained resource on the topic. Designed to help those new to this area gain proficiency in this field, experts will also find the book useful in developing a deeper understanding of the subject.

The starting point of the book is the standard microscopic definition of molecular Hamiltonians presented in commonly accepted modern quantum mechanical notations. Major assumptions and approximations involved in constructing Frenkel-type exciton Hamiltonians, which are well established, but are often hidden under arcane notations and approximations of old publications, are presented in detail. This will help quantum chemists understand the major assumptions involved in the definition of commonly used exciton models.

Rate theories of exciton dynamics, such as Förster and Dexter theories and their modern generalizations, are presented in a unified and detailed manner. In addition, important aspects that are often neglected, such as local field effect and the role of fluctuating environments, are discussed. Various quantum dynamics methods allowing coherent dynamics of excitons are presented in a systematic manner in the context of quantum master equations or path integral formalisms. The author also provides a detailed theoretical explanation for the major spectroscopic techniques probing exciton dynamics, including modern two-dimensional electronic spectroscopy, with a critical assessment of the implications of these spectroscopic measurements. Finally, the book includes a brief overview of major applications including an explanation of organic photovoltaic materials and natural light harvesting complexes.

• Covers major theories of exciton dynamics in a consciously concise and easily readable way
• Bridges the gap between quantum dynamics working with phenomenological exciton Hamiltonian and quantum chemistry construct reliable models amenable for dynamics calculations from ab initio calculations
• Explores modern nonlinear electronic spectroscopy techniques to probe exciton dynamics, showing how it is applied

1

Introduction

Abstract

This chapter offers a general introduction of excitons, and outlines the scope and objectives of the book. Excitons are defined as electronic excited states or bound electron-hole pairs that can be formed across molecular boundaries. While their identity as unique quantum mechanical objects are well recognized and the areas of their application continue expanding, accurate and complete spatiotemporal information for many forms of excitons is still lacking. This chapter addresses how this can be addressed. For two well-known limiting forms of Frenkel and Wannier excitons, physical assumptions are clarified, and major features are illustrated based on simple models. Key issues concerning delocalization of excitons and disorder are discussed. The chapter concludes with remarks on a few key issues that need to be improved by more advanced computational and experimental studies.

Keywords

exciton; Frenkel; Wannier; spatiotemporal information; delocalization; disorder

Chapter Outline

• 1.1 Motivation and objective
• 1.2 Frenkel and Wannier excitons
  • 1.2.1 A brief overview
  • 1.2.2 Model Hamiltonians for Frenkel excitons
    • 1.2.2.1 Linear chain
    • 1.2.2.2 Circular chain
  • 1.2.3 Semiclassical model Hamiltonians for Wannier excitons
• 1.3 Disorder, fluctuations, and measure of delocalization
• 1.4 Utility and limitations of exciton models

1.1 Motivation and objective

An exciton represents a quantum mechanical state of electronic excitation or a bound pair of an electron and its hole.¹ Any el...