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Über dieses Buch

This brief investigates the diradical character, which is one of the ground-state chemical indices for "bond weakness" or "electron correlation" and which allows researchers to explore the origins of the electron-correlation-driven physico-chemical phenomena concerned with electronic, optical and magnetic properties as well as to control them in the broad fields of physics and chemistry. It then provides the theoretical fundamentals of ground and excited electronic structures of symmetric and asymmetric open-shell molecular systems by using model molecular systems. Moreover, it presents the theoretical design guidelines for a new class of open-shell singlet molecular systems for nonlinear optics (NLO) and singlet fission.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

The concept of “diradical character based design” for efficient functional substances is introduced using the dissociation process of a

homodinuclear system

Homodinuclear system

. The

diradical character

Diradical character

, which is one of the quantum-chemically well-defined chemical indices and indicates the singlet open-shell nature, is employed for classification of arbitrary electronic structures into three categories, i.e., weak, intermediate and strong

electron correlation

Electron correlation

regions. In this book, we present a simple relationship between diradical character and the ground/excited electronic structures, and illuminate that the systems in the

intermediate diradical character

Intermediate diradical character

region have the advantage of exhibiting highly efficient optoelectronic functionality. As examples, we show the diradical character based molecular design principles for highly efficient nonlinear optical (NLO) and singlet fission (SF) properties.

Masayoshi Nakano

Chapter 2. Electronic Structures of Symmetric Diradical Systems

In general, the electronic structures of a molecular system is characterized by using the “diradical character”, which is well defined in quantum chemistry and implies a chemical index of a bond nature. In this chapter, we present analytical expressions for electronic energies and wavefunctions of the ground- and excited states as well as for the

excitation energies

Excitation energies

and transition properties based on symmetric two-site diradical models with different diradical characters using the valence configuration interaction method.

Masayoshi Nakano

Chapter 3. Electronic Structures of Asymmetric Diradical Systems

In this chapter, we present analytical expressions for electronic energies and wavefunctions of the ground and excited states as well as for the excitation energies and transition properties of asymmetric

two-site diradical models

Asymmetric two-site diradical model

as the functions of diradical character using the valence configuration interaction method. Such asymmetric diradical systems are realized by symmetric diradical molecules under

static electric fields

Static electric field

and/or by diradical molecules with asymmetric structures, e.g., donor-acceptor substituted diradicals. Several nondimensional physical factors concerned with “

asymmetricity

Asymmetricity

” are introduced in order to describe the electronic structures of these systems.

Masayoshi Nakano

Chapter 4. Diradical Character View of (Non)Linear Optical Properties

In this chapter, we clarify the diradical character dependences of (hyper)polarizabilities, which are molecular origins of (non)linear optical responses, in static and resonant cases based on the diradical character dependences of excitation energies and properties of two-site symmetric and

asymmetric diradical models

Symmetric diradical model

Asymmetric diradical model

with two electrons in two active orbitals. The analysis results highlight the differences of optical response properties between open-shell and closed-shell molecular systems, and contribute to the construction of novel

molecular design

Molecular design

guidelines for highly efficient nonlinear optical systems.

Masayoshi Nakano

Chapter 5. Diradical Character View of Singlet Fission

Singlet fission is one of the internal conversion process in which a

singlet exciton

Singlet exciton

Exciton

splits into two triplet excitons having long

lifetimes

Lifetime

. This phenomenon is expected to be useful for significantly improving the

photoelectric conversion

Photoelectric conversion

efficiency in organic photovoltaic cells. In this chapter, we present diradical character based

molecular design

Molecular design

guidelines for efficient singlet fission molecules based on the

energy level matching conditions

Energy level matching conditions

between the lowest singlet and triplet excited states, which are found to be described by the multiple diradical characters. A simple model, i.e., tetraradical hydrogen cluster, is investigated in order to reveal the multiple diradical character dependences of relative excitation energies and to build a diradical character based design guideline. On the basis of this guideline, several candidate molecules are proposed.

Masayoshi Nakano

Chapter 6. Summary and Future Prospects

The

diradical character based design principles

Diradical character based design principle

for efficient functional substances—highly efficient open-shell singlet nonlinear optical (NLO) systems and

singlet fission (SF)

Singlet fission (SF)

molecules—are summarized. The remaining problems to be solved for designing

open-shell

Open-shell

singlet NLO

Nonlinear optics (NLO)

and SF materials are presented together with future possible extension of the present concept.

Masayoshi Nakano

Backmatter

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