Diradicaloids

Description

π-Conjugated molecules with an even number of π-electrons usually have a closed-shell ground state. However, recent studies have demonstrated that a certain type of molecules could show open-shell singlet ground state and display diradical-like (diradicaloid) behavior. Their electronic structure can be understood in terms of the “diradical character” and “aromaticity” concepts. They display very different electronic properties from traditional closed-shell π-conjugated molecules and could be used as next-generation molecular materials. This book provides a comprehensive review on the chemistry, physics, and material applications of open-shell singlet diradicaloids. Particularly, it elaborates the fundamental structure–diradical character–electronic property relationships both theoretically and experimentally. The book has been written by leading scientists in the field from Japan, Germany, Spain, Italy, China, and Singapore. Cover Half Title Title Page Copyright Page Table of Contents Preface Chapter 1: Excitation Energies and Optical Properties of Open-Shell Singlet Diradicaloids 1.1: Introduction 1.2: Electronic Structures of Singlet Diradicaloids 1.2.1: VCI Model of Symmetric Diradicaloids 1.2.2: Diradical Character Dependences of Excitation Energies and Properties of Symmetric Diradicaloids by VCI Model 1.2.3: VCI Model of Asymmetric Diradicaloids 1.2.4: Diradical Character Dependences of Excitation Energies and Properties of Asymmetric Diradicaloids by VCI Model 1.2.5: Novel Definition of Diradical and Ionic Characters 1.2.5.1: Diradical and ionic characters and their densities for arbitrary states within thetwo-site model 1.2.5.2: Application to π-stacked dimer of phenalenyl-derivatives with varying intermonomer distance 1.2.6: Experimental Estimation of Diradical Character 1.3: BS Approach to Diradicaloids 1.3.1: Spin-Projected BS Approach to Diradical Character 1.3.2: Relationship between Diradical Character and Aromaticity 1.3.3: Estimation of Singlet Excitation Energies and ST Energy Gap for Diradicaloids 1.4: Functionalities of Singlet Diradicaloids 1.4.1: NLO Property 1.4.2: Singlet Fission 1.5: Summary Chapter 2: Electronic Structure Characterization of Diradicaloids with Spin-Flip (SF) Methods 2.1: Introduction 2.2: Theory of SF Methods 2.2.1: SF Methods to Describe Diradicals 2.2.2: SF in CC and CI 2.2.3: SF in TDDFT 2.2.4: SF with Active Space 2.2.5: Characterization of Diradical Character 2.2.5.1: Singlet–triplet energy gap 2.2.5.2: Diradical index 2.2.5.3: Natural orbitals and occupation numbers 2.2.5.4: Density of unpaired electrons 2.2.5.5: Number of unpaired electrons 2.3: Application of SF Methods to the Study of Diradicaloids 2.3.1: Ethylene Torsion with SF-TDDFT 2.3.2: Radical Character of Triangulenes 2.3.3: Diradical Character of Linear Acenes and Zethrenes 2.3.4: Diradical Character of Fluorenofluorenes 2.3.5: Diradical Character of Cyclic Acenes and Carbon Nanobelts 2.3.6: Diradical Character in Nanographene Induced by Pressure 2.4: Summary Chapter 3: Spectroscopy of Open-Shell Singlet Ground-State Diradicaloids: A Computational Perspective 3.1: Introduction 3.2: Cost-Effective Computational Approaches for Description of SE and DE States of Diradicaloids 3.2.1: Descriptors of Diradical Character 3.2.2: 2e-2o Model 3.2.3: TDUDFT 3.2.4: SF-TDDFT 3.2.5: DFT/MRCI 3.3: Results 3.3.1: Diradical Character from y0 and NFOD Descriptors 3.3.2: DE State from High-Level Computational Studies 3.3.3: Bright SE Excited State from DFT-Based Approaches 3.3.4: DE Excited State from DFT-Based Approaches 3.4: Concluding Remarks Chapter 4: Vibrational Raman Spectroscopy of Diradicaloids: Revealing Their Physical Origin 4.1: Introduction to Vibrational Spectroscopy of Hydrocarbon Molecules 4.2: Fundamental Physics on the Raman Spectra of Poly-Conjugated Molecules and Diradicaloids 4.3: Tetracyano Quinoidal Oligothiophenes: The Oligomer Approach to Diradicaloid Molecules by Raman Spectroscopy 4.4: Tetracyano Oligoperylenes: Ground Electronic State Triplets Detected by Raman Spectroscopy 4.5: Planar Aromatic Oligorylenes Diradicaloids: Raman Spectra beyond Peierls Restrictions 4.6: Planar Zethrenes and Indenoacene Diradicaloids 4.7: Conclusions Chapter 5: Phenalenyl- and Anthene-Based Diradicaloids 5.1: Introduction 5.2: Phenalenyl-Based Diradicaloids 5.2.1: Thermodynamic Stability of Phenalenyl Radical 5.2.2: Design and Synthesis of Bisphenalenyl Diradicaloid 5.2.3: Physical Properties of Bisphenalenyl Diradicaloid 5.2.4: Tuning of Diradical Character of Bisphenalenyl Diradicaloids 5.2.5: Non-linear Optical Property of Bisphenalenyl Diradicaloids 5.2.6: Cycloaddition Reactions of Bisphenalenyl Diradicaloid having o-Quinodimethane Scaffold 5.2.7: Electrocyclization of Bisphenalenyl Diradicaloid 5.2.8: Through-Space Conjugated Bisphenalenyl Diradicaloids 5.3: Anthene-Based Diradicaloids 5.3.1: Unique Electronic Properties of Graphene 5.3.2: Model System for Investigating Edge State 5.3.3: Synthesis of Anthenes 5.3.4: Molecular Structure of Anthenes 5.3.5: Magnetic Properties of Anthenes 5.3.6: Optical Properties of Anthenes 5.3.7: Mechanism of Diradical Character in Anthenes 5.3.8: Lateral Extension from Anthenes 5.3.9: Other Model Systems of Graphene 5.4: Summary Chapter 6: Zethrenes and Related Molecules 6.1: Pioneers of Zethrene Chemistry 6.2: Modern Syntheses of Zethrenes and Discovery of Open-Shell Diradical Character 6.3: Extended Zethrenes-Based Diradicaloids 6.3.1: Vertically Extended Zethrenes 6.3.2: Laterally Extended Zethrenes 6.4: Zethrene Isomers and Analogs 6.5: Conclusion Chapter 7: Extended para-Quinodimethanes and Quinones 7.1: Extended Para-Quinodimethanes 7.2: Extended Quinones 7.3: Quinoidal Oligothiophenes 7.4: Conclusion Chapter 8: Fused Heteropolycyclic Compounds-Based Diradicaloids 8.1: Introduction 8.2: Quinoidal Acene and Heteroacene Analogs 8.2.1: General Synthetic Strategies 8.2.2: Extended Quinoidal Acene Analogs 8.2.3: Extended Quinoidal Heteroacene Analogs 8.3: Extended Aza-Acenes and Aza-Quinodimethanes 8.4: Non-Classical Acenes Capped with Thiophenes or Thiadiazoles 8.5: Summary Chapter 9: Non-Benzenoid Polycyclic Hydrocarbon-Based Diradicaloids 9.1: Introduction 9.2: Five-Membered Ring-Containing Diradicaloids 9.2.1: Pentalene-Based Diradicaloids 9.2.2: Indacene-Based Diradicaloids 9.2.2.1: Indenofluorene and its π-extended homologues 9.2.2.2: Biphenalenyls 9.2.3: Curved Diradicaloids with Pentagons 9.2.4: Other Diradicaloids with Pentagons 9.3: Seven-Membered Ring-Containing Diradicaloids 9.4: Four- or Eight-Membered Ring-Containing Diradicaloids 9.5: Summary Chapter 10: Photo-Responsive Diradicaloids 10.1: Introduction 10.2: Pentaarylbiimidazole 10.3: Phenoxyl-Imidazolyl Radical Complex 10.4: Bis(Phenoxyl-Imidazolyl Radical Complex) 10.5: Conclusion Chapter 11: Porphyrinoid-Based Diradicaloids 11.1: Introduction—Porphyrinoid-Based Mono-Radicals 11.1.1: Electronic Flexibility and Radical-Stabilizing Ability of Porphyrinoids 11.1.2: Examples of Stable Porphyrinoid-Based Radicals 11.1.3: Spin Density Distribution Depending on Incorporated Radical Units 11.2: Classification of Porphyrinoid-Based Diradicaloids 11.3: Porphyrinoids Bearing Two Radical Units at Their Periphery (Type I) 11.3.1: Quinoidal Porphyrinoids 11.3.2: Diradicaloids Based on Porphyrin Dimers 11.3.3: Non-Kekulé Diradicals Based on Porphyrinoids 11.4: Porphyrinoid-Fused Diradicals and Diradicaloids (Type II) 11.4.1: Kekulé-Type Singlet Diradicaloids 11.4.2: Non-Kekulé Diradicals with High-Spin Ground States 11.5: Diradicaloids Based on Intrinsically Radical Porphyrinoids (Type III) 11.5.1: Corrole-Based Diradicaloids 11.5.2: Norcorroles: Strong Antiaromaticity and Singlet Diradical Characters 11.5.3: Diradicaloids Based on Core-Modified Expanded Porphyrins 11.6: Summary Chapter 12: Heteroatom (N, P, B, S, etc.) Centered Monoradicals and Diradicals 12.1: Introduction 12.2: Group 13 Element-Centered Radicals 12.2.1: Boron-Centered Radical Anions 12.2.2: Gallium-Centered Radical Cation 12.3: Group 15 Element-Based Radicals 12.3.1: Nitrogen-Based Diradicals and Dications 12.3.1.1: Nitrogen analogs of Thiele’s hydrocarbon 12.3.1.2: Nitrogen analogs of Chichibabin’s hydrocarbon 12.3.1.3: Nitrogen analogs of Müller’s hydrocarbon 12.3.1.4: Other amine-based diradical dications 12.3.1.5: Nitrogen-based radical anions 12.3.2: Heavy Pnictogen-Centered Radical Ions 12.3.2.1: Heavy pnictogen-centered radical cations 12.3.2.2: Heavy pnictogen-centered radical anions 12.4: Group 16 Element-Based Radicals 12.5: Conclusion Chapter 13: Polyradicaloids and 2D/3D Global Aromaticity 13.1: Introduction 13.2: Linear Polyradicaloids 13.3: 2D Macrocyclic Diradicaloids and Polyradicaloids 13.3.1 Expanded Porphyrinoids with Radical Character 13.3.2 Polycyclic Hydrocarbon-Based Macrocyclic Polyradicaloids Showing Hückel (Anti)Aromaticity 13.3.3 Macrocyclic Diradicaloids/Polyradicaloids Showing Baird Aromaticity 13.3.4 Global Antiaromaticity in Transition State of Macrocyclic Polyradicaloid 13.4: 3D Fully Conjugated Diradicaloid Cages and 3D Global Aromaticity 13.5: 2D CORFs 13.6: Conclusion Index