Synthesis, physical properties and biradical characters of zethrene based polycylic hydrocarbons 1

Table of contents Thesis Declaration ii Acknowledgement iii Table of contents iv Summary vii List of Tables ix List of Figures x List of Schemes xiv List of Abbreviations xvi Lis

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SYNTHESIS, PHYSICAL PROPERTIES AND BIRADICAL

CHARACTERS OF ZETHRENE-BASED POLYCYLIC

HYDROCARBONS

SUN ZHE

(B.Sc., Sichuan University)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY

DEPARTMENT OF CHEMISTRY

NATIONAL UNIVERSITY OF SINGAPORE

2013

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Thesis Declaration

I hereby declare that this thesis is my original work and it has been written by me in its entirety, under the supervision of A/P Wu Jishan, (in the laboratory Organic & Supramolecular Chemistry), Chemistry Department, National University of Singapore, between August 2009 and July 2013

I have duly acknowledged all the sources of information which have been used in the thesis

This thesis has also not been submitted for any degree in any university previously

The content of the thesis has been published in:

1) Sun, Z.; Huang, K.-W.; Wu, J Org Lett 2010, 12, 4690–4693

2) Sun, Z.; Huang, K.-W.; Wu, J J Am Chem Soc 2011, 133, 11896−11899

3) Sun, Z.; Wu, J J Org Chem., 2013, 78, 9032–9040

4) Sun, Z.; Lee, S.; Park, K H.; Zhu, X.; Zhang, W.; Zheng, B.; Hu, P.; Zeng, Z.; Das, S.; Li, Y.;

Chi, C.; Li, R W.; Huang, K W.; Ding, J.;Kim, D.; Wu, J J Am Chem Soc 2013,

SUN ZHE

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Acknowledgement

The completion of this thesis depends largely on the efforts and the encouragements of many people, it would be a great pleasure for me the take this opportunity to express my sincerest gratitude to them

Foremost, I wish to give my deepest appreciation to my supervisor, Prof Wu Jishan, for his unreserved professional guidance and continuous inspiration Apart from the academic guidance, he is also a valuable mentor in my life I still remembered the words he said to me when I was frustrated by my first project: “It’s tough to do something new, don’t give up and I believe you can make it”, and the encouragement he gave when I tried to present my own ideas: “I like your idea very much, let’s make it happen” I could not have listed enough of the inspirations and supports from Prof Wu, and I could not have imagined having a better supervisor and mentor for my PhD study A sincere thanks also goes to Prof Chi Chunyan for her enlightening suggestions and kind help over the years during my PhD study

Next, I am truly grateful to all the past and present members in Prof Wu and Prof Chi’s group, Dr Zhang Xiaojie, Dr Yin jun, Dr Cui Weibin, Dr Zhao Baomin, Dr Zhang Kai, Dr Luo Jing, Dr Xiang Hongfa, Dr Li Yuan, Dr Zeng Lintao, Dr Luo jie, Dr Huang Xiaobo, Dr Cao Jing, Dr Soumyajit Das, Dr Gao Fei, Dr Jiao Chongjun, Dr Li Jinling, Dr Suvankar Dasgupta, Dr Zeng Zebing, Dr Qu Hemi, Dr Tong Chenghua, Dr Shao Jinjun, Mao Lu, Ye Qun, Ni Yong, Zeng Wangdong, Dai Gaole, Chang Jingjing, Kam Zhiming, Shi Xueliang, Lim Zhenglong, Hu Pan and Qi Qingbiao I not only gain precious research experiences working with them, but also harvest friendships for a lifetime

Furthermore, I would also thank all our collaborators including Prof Huang Kuo-Wei, Prof Juan Casado, Prof Ding Jun, Prof Dongho Kim, Dr Zhang Wenhua and so on For without the efforts of them, the accomplishment of this thesis would not have been possible

In addition, I would like to thank the National University of Singapore for providing me the research scholarship Moreover, I have greatly benefited from many staffs from chemistry department administrative office, the NMR laboratory and the Mass laboratory

Last but not least, I would like to thank my loved ones, my parents Sun Gaoyuan and Wei Hongzhen, and my wife Yang Cong Their unconditional love and support are the motive force for me throughout the entire process I will be grateful forever for their love

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Table of contents

Thesis Declaration ii

Acknowledgement iii

Table of contents iv

Summary vii

List of Tables ix

List of Figures x

List of Schemes xiv

List of Abbreviations xvi

List of Publications xv

Chapter 1: Introduction 1.1 Low band gap polycyclic hydrocarbons with either a closed-shell or an open-shell singlet biradical ground state 1

1.1.1 PHs with a closed-shell ground state 3

1.1.2 PHs with an open-shell ground state 9

1.2 Overview on zethrene-based PHs 14

1.2.1 Synthesis and reactivity for zethrene-based PHs 15

1.2.2 Applications for zethrene-based PHs 18

1.3 Objectives 20

1.4 References 21

Chapter 2: Zethrene bis(dicarboximide) and its unexpected oxidation 2.1 Introduction 26

2.2 Results and discussion 27

2.2.1 Synthesis and mechanism study 27

2.2.2 Theoretical calculations 31

2.2.3 Photophysical and electrochemical properties 32

2.3 Conclusions 35

2.4 Experimental section 36

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2.4.1 General experimental methods 36

2.4.2 Characterization data 36

2.5 References 39

Chapter 3: 7,14-Diaryl-substituted zethrene diimides as stable far-red dyes with tunable photophysical properties 3.1 Introduction 41

3.2.1 Synthesis 44

3.2.2 Photophysical properties 46

3.2.3 Electrochemical properties 48

3.2.4 Photo-stability test 50

3.3 Conclusions 52

3.5 References 58

Chapter 4: Heptazethrene bis(dicarboximide)s with a singlet biradical ground state 4.1 Introduction 61

4.2.1 Synthesis 62

4.2.2 Variable-temperature 1H NMR spectra 64

4.2.3 Theoretical calculations 65

4.2.5 Raman spectroscopic measurements 68

4.2.7 Photostability measurements 72

4.3 Conclusions 73

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4.5 References 79

Chapter 5: Dibenzoheptazethrene isomers with different biradical characters: an exercise of Clar’s aromatic sextet rule in singlet biradicaloids 5.1 Introduction 81

5.2.1 Synthesis 82

5.2.2 Variable-temperature 1H NMR, ESR and SQUID measurements 83

5.2.3 X-ray single crystal analysis and theoretical calculations 86

5.3 Conclusions 96

5.5 References 105

Appendix 1 108

Appendix 2 114

Appendix 3 127

Appendix 4 129

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Summary

This thesis describes the synthesis, physical properties and potential applications of a series

of zethrene-based molecules, including zethrene bis(dicarboximde)s, heptazethrene bis(dicarboximide)s and dibenzoheptazethrene derivatives Due to the presence of both Kekulé and biradical resonance forms, the ground state of these molecules can be either closed-shell or open-shell singlet biradical The moderate biradical character renders attractive electronic, optical and magnetic properties which allows a diversity of applications Chapter 1 firstly presents an overview of recent advances in low band gap polycyclic hydrocarbons The ground state of these molecules is either closed-shell or open-shell due to the structural difference, and they are actively participated in the materials science In the second part of this chapter, a review of zethrene-based compounds is given The theoretical calculations, synthesis and primary applications of this interesting class of polycyclic hydrocarbons are discussed

In chapter 2, the synthesis and properties of a novel zethrene bis(dicarboximde) compound are presented This molecule exhibits good stability and solubility compared to parent zethrene, and represents good candidate for far-red dyes Moreover, the unexpected oxidation reaction of this molecule is also discussed

In chapter 3, the synthesis and properties of a series of 7,14-diaryl-substituted zethrene diimides are described This study is an extension of zethrene diimide compounds and the functionalizations are allowed on both imide sites and bay region because of a novel synthetic method The possibilities of these compounds as novel dyes are also discussed

In chapter 4, the preparation of soluble and stable heptazethrene bis(dicarboximde)s is presented which is the first isolation of heptazethrene derivatives The ground state of these molecules are determined as open-shell singlet biradical, the properties are studied from both theoretical and experimental perspectives

In chapter 5, two dibenzoheptazethrene isomers are synthesized following two facile synthetic sequences Both compounds are singlet biradical in the ground state, but the biradical characters are found dependant on the number of Clar’s Sextet rings in the biradical form The physical properties are investigated with a combination of theoretical and experimental methods

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Keywords: polycyclic hydrocarbons, zethrene, biradicaloid, dye, Clar’s sextet rule

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ix

List of Tables Table 2.1 Photophysical and electrochemical properties of compounds 2-3, 2-8

Table 3.1 Photophysical data of ZDI compounds recorded in DCM

Table 3.2 Electrochemical data of ZDI compounds

Table 4.1 Photophysical and electrochemical properties of compounds 2-3 and 4-3

Table 5.1 Photophysical and electrochemical properties of compounds 5-1 and 5-2

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List of Figures Figure 1.1 Examples of low band gap PHs

Figure 1.2 Functionalized high order acenes

Figure 1.3 High order rylenes and their diimide derivatives

Figure 1.4 Structures of N-annulated rylenes

Figure 1.5 Stable bisanthene derivatives

Figure 1.6 Indenofluorene derivatives

Figure 1.7 Teranthene/quanteranthene derivatives with singlet biradical ground states

Figure 1.8 Bis(phenalenyl)s with singlet biradical ground states

Figure 1.9 Bis(phenalenyl)s with different aromatic linkers

Figure 1.10 Indenofluorene with a singlet biradical ground state

Figure 1.11 Resonance structures for zethrene and higher order zethrenes

Figure 1.12 Drain current (IDS) versus gate voltage (VG) with drain voltage(VDS) at -50 V

for the best-performing OTFT of zethrene with the active channel of W = 1 mm and L = 150

Tm as measured in air

Figure 2.1 Structures of zethrene 2-1, 7,14-disubstituted zethrene 2-2 and zethrene bis(dicarboximide) 2-3

Figure 2.2 (a) MALDI-TOF Mass spectrum of 2-8, (b) FT-IR spectrum of 2-3, (c) FT-IR spectrum of 2-8

Figure 2.3 Optimized molecular structures and frontier molecular orbital profiles of 2-3 and 2-8 Some bond lengths are indicated by arrows (Å)

10-5 M) of 2-3 and 2-8 in chloroform

Figure 2.5 Photostability measurements for 2-3 UV spectra change under irradiation of (a)

UV lamp, (b) white bulb and (c) ambient condition (d) Change of optical density of 2-3 at the

absorption maximum wavelength with the irradiation time

Figure 2.6 Cyclic voltammograms of compounds 2-3, 2-8 in dichloromethane with 0.1 M

Bu4NPF6 as supporting electrolyte, Ag/AgCl as reference electrode, Au disk as working electrode, Pt wire as counter electrode, and scan rate at 50 mV/s

Figure 3.1 Structures of zethrenes, perylene diimide (PDI), terrylene diimide (TDI) and

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zethrene diimide (ZDI)

Figure 3.2 Structures of ZDI derivatives

Figure 3.3 UV-Vis absorption and fluorescence spectra recorded in DCM solutions: (a) absorption spectra of 2-3, 3-1–3-3, (b) normalized fluorescence spectra of 2-3, 3-1–3-3, (c) absorption spectra of 3-4 and 3-5, and (d) normalized fluorescence spectra of 3-4 and 3-5 Figure 3.4 Concentration dependant fluorescence for 3-1 (a), (b); 3-2 (c), (d) and 3-3 (e), (f) Figure 3.5 Cyclic voltammograms of (a) 3-1, (b) 3-2, (c) 3-3, (d) 3-4 and (e) 3-5 in DCM (for

anodic scan) and THF (for cathodic scan) with 0.1M Bu4NPF6 as supporting electrolyte, AgCl/Ag as reference electrode, Au disk as working electrode, Pt wire as counter electrode, and a scan rate of 50 mVs-1

Figure 3.6 Photo-stability test of 2-3 and 3-4 in CHCl3 upon irradiation with (a) UV lamp (254 nm, 4W), (b) white light bulb (100 W) and (c) ambient light

Figure 4.1 Resonance structures of zethrene and heptazethrene

Figure 4.2 Molecular structures of higher order zethrenes and imide derivatives

Figure 4.3 Variable-temperature 1H NMR spectra of 4-3 in CD2Cl2 in aromatic region and assignment of aromatic protons The resonance assignment referred to the structure shown in Figure 4.2

Figure 4.4 (a) HOMO, (b) LUMO and (c) spin densities of 4-3 The calculations are

performed at CAM-B3LYP level of theory Blue and green surfaces represent α and β spin densities, respectively

Figure 4.5 Calculated structures for the closed-shell (a), open-shell singlet biradical (b) and

open-shell triplet (c) states of 4-3 All the structures have a C2 symmetry with a C2 axis along

the central six-membered ring The bond lengths are labeled for the central rings in Å

Figure 4.6 (a) Absorption spectra of 2-3 and 4-3 in chloroform, (b) UV-vis-NIR absorption spectra of 4-3 in DCM at different temperatures

Figure 4.7 Left: Raman spectra of 4-4 with different excitation wavelength, a) 532 nm, b) 633

nm, c) 785 nm and d) 1064 nm Right: electronic absorption spectra of 4-4 in CH2Cl2 solution (broken line) and in solid state (solid line)

Figure 4.8 1064 nm FT-Raman spectra in solid state of: a) 4-4, b) 4-1, and c) 4-2

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as supporting electrolyte, Ag/AgCl as reference electrode, Au disk as working electrode, Pt wire as counter electrode, and scan rate at 50 mV/s Inset: Differential pulse voltammograms

of 4-3

Figure 4.10 (a) Absorption spectral changes of 4-3 under ambient light irradiation, (b) absorption spectral changes under white light irradiation (100 W white bulb), (c) change of optical density of 2-3 and 4-3 as a function of irradiation time

Figure 5.1 Resonance structures of two DBHZ isomers and chemical structures of heptazethrene, 5-1 and 5-2

Figure 5.2 Variable temperature 1H NMR spectra (aromatic region) of 5-2 in THF-d8 and assignments of aromatic protons, the assignments referred to structure shown in Figure 5.1 (peak labelled as * is from the impurity in THF-d8)

Figure 5.3 ESR spectrum of 5-2 in toluene recorded at 298K

Figure 5.4 ΧT-T plot for the solid 5-2 The measured data was plotted as open circles, and the

fitting curve was drawn using the Bleaney-Bowers equation with g = 2.00

Figure 5.5 (a) ORTEP drawing of 5-1 and 5-2 measured at 123 K The hydrogen atoms are

omitted for clarity (b) Mean values of bond lengths and calculated NICS(1) values in the

DBHZ core for 5-1 and 5-2 (c) Calculated (UCAM-B3LYP) spin density distribution of 5-1

and 5-2; the blue and green surfaces represent α and β spin densities, respectively

Figure 5.6 Calculated SOMOs for (a) 5-1 and (b) 5-2 Left: SOMO-α, right: SOMO-β

Figure 5.7 The optimized structure of (a) 5-1 (singlet biradical), (b) 5-2 (singlet biradical), and calculated bond lengths of (c) 5-1 (singlet biradical), (d) 5-2 (singlet biradical)

Figure 5.8 OPA spectra (solid line and left vertical axis) and TPA spectra (blue symbols and

right vertical axis) of (a) 5-1 and (b) 5-2 TPA spectra are plotted at λex/2 Insert are the photographs of the solutions in chloroform

Figure 5.9 Femtosecond transient absorption spectra (left) decay-associated spectra (right) of 5-1 (top) and 5-2 (bottom) recorded in toluene

Figure 5.10 Z-scan curves of (a) 5-1 and (b) 5-2 by photoexcitation in the range from 1200 to

1700 nm

Figure 5.11 Absorption spectral changes of (a) 5-1 under white light irradiation, (b) 5-1 under ambient light, (d) 5-2 under white light irradiation, (e) 5-2 under ambient light irradiation, (c)

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