Part I: Synthesis and Study of Nonacene Derivatives; Part Ii: Optoelectronic Properties of Metal-Semiconductor Nanocomposites I

PART I: SYNTHESIS AND STUDY OF NONACENE DERIVATIVES;

PART II: OPTOELECTRONIC PROPERTIES OF METAL-SEMICONDUCTOR NANOCOMPOSITES IN STRONGLY COUPLED REGIME

Dmitriy Khon

A Dissertation

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

August 2011

Committee:

Douglas C. Neckers, Advisor Mikhail Zamkov, Co-advisor George S. Bullerjahn, Graduate Faculty Representative Ksenija Glusac Thomas H Kinstle

Dmitriy Khon

Douglas C. Neckers, Advisor

Mikhail Zamkov, Co-advisor

Acenes are polycyclic aromatic hydrocarbons (PAHs) consisting of linearly fused benzene rings. In the recent past, acenes have been of interest from fundamental and applied perspectives. Smaller acenes such as benzene, naphthalene, and anthracene are among the most studied organic compounds and their properties are well explored.

Pentacene has received considerable attention as the most promising active semiconductor for use in organic thin film transistors (TFT) because of its high charge- carrier mobility; however, poor environmental stability is one of the problems limiting its practical application. As the number of rings increases, the members of the acene family become increasingly reactive.

The successful synthesis of heptacene developed by Mondal et al used the Strating-

Zwanenberg photodecarbonylation reaction. The lesser stability of the tetracene moieties in the nonacene photoprecursor compared to the anthracene moieties of the heptacene process make its synthesis more challenging. The latter scheme requires 2,3- dibromoanthracene as one of the starting materials. Besides the poor solubility of 2,3- dibromoanthracene, failure was also due to insufficient formation of anthracyne upon treatment of 2,3-dibromoanthracene with n-BuLi. Although the initial idea didn’t work we used the same scheme replacing 2,3-dibromoanthracene with 7,8-dibromo-1,4- dihydroanthracene. The reaction of the latter with 5,6,7,8- tetramethylenebicyclo[2.2.2]oct-2-ene gave 1,4,7,8,9,12,15,18,19,20-octadecahydro-

III 8,19-diethenononacene albeit in low yield. Multiple attempts to dehydrogenate the nonaromatic rings using DDQ and other reagents under various conditions failed to produce the desired compound.

Recently Miller reported the synthesis of relatively stable heptacene derivatives having a combination of arylthio and o‐dialkylphenyl substituents. Miller’s scheme used

1,2,4,5-tetrakis(bromomethyl)-3,6-bis(4’-t-butylthiophenyl)benzene as the core precursor. Another synthetic approach has been undertaken that employs Miller’s

1,2,4,5‐tetrakis(bromomethyl)‐3,6‐bis(4’‐t‐butylthiophenyl)benzene in its core.

First attempts to react the latter with 1,4‐anthraquinone to produce nine linearly fused ring system were unsuccessful. Interestingly in both approaches we used, a dienophile benzyne‐type and quinone‐like with more than one fused ring were unreactive in subsequent Diels‐Alder reactions. So similarly to the prior scheme, a dienophile with terminal nonaromatic ring (6,7,8,9‐tetrahydro‐1,4‐anthraquinone) was used along with 1,2,4,5‐tetrakis(bromomethyl)‐3,6‐bis(4’‐t‐ butylthiophenyl)benzene to yield a nine‐ring backbone structure which was treated with mesityl magnesium bromide followed by reduction to yield 1,2,3,4,12,13,14,15-

Octahydro-8,19-bis(4'-t-butylphenylthio)nonacene. Unfortunately this compound wasn’t isolated or properly characterized.

Combining metal and semiconductor domains in a single nanocrystal offer a unique opportunity for the development of hybrid nanoscale composites with functionalities that extend beyond those of isolated materials. The presence of powerful carrier confinement in these nanoparticles joint with tunable geometry of the semiconductor-metal interface gives rise to novel optoelectronic properties that can potentially add up to a wide range of

IV applications. Recently, Au/CdS and Au/CdSe heterostructures containing gold domains grown onto cadmium chalcogenide semiconductor nanorods (NRs) have come forward as a model system for studying such hybrid nanomaterials.

In this work we have developed several chemical routes to CdSe/CdS core-shell nanocrystals (NCs) with each of them leading to different shape of nanocrystals. Also a simple chemical method for growing Au domains onto CdS nanorods and CdSe/CdS NCs in oleylamine was developed. The size of Au NCs can be precisely tuned by adjusting the temperature of the reaction mixture, while the shape of Au/CdS nano-composites

(matchstick or barbells) is controllable via the reaction rate. All of the aforementioned nanocomposites had semiconductor emission quenched and further study is necessary to determine whether a significantly wider band gap shell material can prevent fast transfer of excited carriers from semiconductor into metal domains.

V ACKNOWLEDGMENTS

First, I would like to thank Dr. Douglas C. Neckers for the support, guidance, and patience extended towards me as a graduate student. I am grateful for his time, effort, continuous encouragement, and trust in my abilities.

I am truly thankful to Dr. Thomas H. Kinstle, my organic chemistry teacher as well as one of my committee members, for his invaluable support and passion for teaching, which has been partially passed on to me.

I must thank Dr. Mikhail Zamkov for introducing me to the field of nanochemistry and giving me an opportunity to work in his group. I highly appreciate his professionalism, support, and optimism.

I would like to thank Dr. Ksenija Glusac and Dr. George S. Bullerjahn for serving in my committee and for their inspiration to maintain the good work.

I am truly greatful to Dr. Rajib Mondal, and Dr. Yuewei Zhao for mentoring, encouraging, criticizing and reviewing my work at the beginning of my graduate studies.

It is my pleasure to thank former and present members of Dr. Neckers’ group for stimulating and friendly environment namely Ravi, Thilini, Kelechi, Sujeewa, Zhao, Cai,

Hannah, Leandro, Alexey, Erandi, Nadeeka, Puran, and others who have been great help both inside and outside the lab for the past few years.

I would like to thank all members of Dr. Zamkov’s group especially Anna and Krishna for their priceless help in the field of nanochemistry.

Also I would like to thank Nora and Alita for their administrative help at the department and Romanowicz, Chen, and Doug for their technical support.

VI At last but not least I would like to thank my family for their enormous support and especially my sister Elena who never let me relax too much and for her invaluable help in

Dr. Zamkov’s lab.

VII Table of Contents

Chapter 1. Acenes: Structure, Reactivity, and Synthesis ...... 1 Introduction...... 1

Structure and Reactivity ...... 2

Synthesis ...... 7

References...... 15

Chapter 2. Attempted Synthesis of Parent Octacene and Nonacene ...... 18

Introduction...... 18

Synthesis ...... 19

Conclusion ...... 23

Experimental Section...... 25

References...... 30

Chapter 3. Synthesis of Heptacene/Nonacene Derivative ...... 32

Introduction...... 32

Synthesis ...... 33

Experimental Section...... 39

References...... 44

VIII Chapter 4. Exciton-Plasmon Interaction in Metal/Semiconductor

Nanocomposites...... 46

The morphology of Au/CdS(Se) colloidal nanocomposites...... 46

Exciton-plasmon interaction in Au/CdS nanocomposites...... 50

References...... 57

Chapter 5. Synthesis and Optoelectronic Properties of Au/CdS and

Au/CdSe/CdS Nanocomposites ...... 61

Au/CdS Nanocomposites...... 61

Au/CdSe/CdS Nanocomposites...... 70

Experimental Section...... 74

References...... 80

IX List of Figures

CHAPTER 1 Figure 1 Structure of Acene 2

Figure 2 Computed S0-T1 energy gaps for polyenes, acenes, and cyclacenes 3 Figure 3 Clar’s Sextet concept 4

Figure 4 Correlation of the activation (Ea) versus the reaction (ΔE) energies of acetylene addition to acene ring 5 Figure 5 Herringbone (left) and π-stacking (right) arrangements of pentacene 6

Figure 6 Persistent nonacene derivative 10 13

CHAPTER 2 Figure 2.1 Attempt to form nonacene backbone structure (2.12) 21 Figure 2.2 Formation and reactivity of “benzynes” from 1,2-Dibromobenzene to 2,3- Dibromoanthracene towards Diels-Alder reaction 21 Figure 2.3 Products of addition reaction of compound 2.6 with compound 2.10 22

CHAPTER 3 Figure 3.1 Attempt to couple compound 3.4 with 1,4-anthraquinone 35 Figure 3.2 Attempted aromatization of compound 3.10 36 Figure 3.3 MALDI-MS spectrum showing peak for compound 3.12 38

CHAPTER 4 Figure 4.1 (a,b). HAADF-STEM images of Au/CdS heterostructures showing the color contrast between gold (bright) and semiconductor (dark) domains. (c, d). TEM images of the two areas shown in (a) and (b) 48 Figure 4.2 Evaporation-induced assembly of Au/CdS nano-composites into (a) end- to-end “chains” and (b) two-dimensional superlattices 49 Figure 4.3 Depiction of Plasmon Resonance in Au nanocrystal 51

X Figure 4.4 (a) Steady-state absorption of Au/CdS heterostructures comprising 15.7- nm Au domains. A representative TEM image is shown in the insert. (b) Transient absorption spectra of 15.7-nm-Au/CdS nanocomposites resulting from the excitation at λ=400 nm with 120 fs pump pulses 53 Figure 4.5 Plasmon suppression in Au/CdS nanocomposites 54 Figure 4.6 Energy diagram showing the effect of strong inter-domains coupling on electronic energies in Au/CdS heterostructures 55

CHAPTER 5 Figure 5.1 TEM images of Au/CdS nano-composites fabricated using various synthetic conditions 63 Figure 5.2 TEM images and statistical size distributions of Au/CdS nano-composites grown at five different temperatures 65 Figure 5.3 Evolution of the Au/CdS absorption spectra during the synthesis 67 Figure 5.4 Au/CdS nano-composites synthesized from low-aspect-ratio nanocrystals 69 Figure 5.5 CdSe/CdS core-shell nanocrystals synthesized by method A 71 Figure 5.6 CdSe/CdS core-shell nanocrystals synthesized by method B 72 Figure 5.7 Au/CdSe/CdS nanocomposites with CdSe/CdS core-shell nanocrystals synthesized by method C 72 Figure 5.8 Au/CdSe/CdS nanocomposites with CdSe/CdS core-shell nanocrystals synthesized by method A 73

XI List of Schemes

CHAPTER 1 Scheme 1 Photogeneration of Heptacene 2 in PMMA matrix 7 Scheme 2 Synthesis of 7,16-silylethynylheptacenes 8 Scheme 3 Synthesis of heptacene derivatives 6a-c 9 Scheme 4 Synthesis of heptacene derivatives 7a,b and 8a,b 11 Scheme 5 Synthesis of heptacene derivative 9 12 Scheme 6 Synthesis of Octacene 11 and nonacene 12 14

CHAPTER 2 Scheme 2.1 Synthesis of 5,6,7,8-tetramethylenebicyclo[2.2.2]oct-2-ene (2.6) 19 Scheme 2.2 Synthetic scheme for nonacene photoprecursor 20 Scheme 2.3 Synthesis of 2,3-Dibromoanthracene (2.11) 20 Scheme 2.4 Modified synthetic scheme for nonacene photoprecursor 22 Scheme 2.5 Synthetic scheme for octacene photoprecursor 23 Scheme 2.6 Bettinger’s synthesis of octacene and nonacene photoprecursors 24

CHAPTER 3 Scheme 3.1 Proposed Synthetic scheme for substituted nonacene derivative 33 Scheme 3.2 Synthesis of intermediate 3.4 34 Scheme 3.3 Revised synthetic scheme for substituted nonacene 34 Scheme 3.4 Synthesis of compound 3.9 35 Scheme 3.5 Modified organometallic reaction step for Miller’s synthesis of substituted heptacene 36 Scheme 3.6 Synthetic scheme to obtain compound 3.12 37

XII List of Abbreviations

OFET Organic field-effect transistor

HOMO Highest occupied molecular orbital

LUMO Lowest unoccupied molecular orbital

DFT Density functional theory eV Electron volt

Ea Activation energy kcal Kilocalorie mol Mole nm Nanometer

PMMA Polymethylmethacrylate h Hour

NMR Nuclear magnetic resonance

NIR Near infra red

W Watt

UV/Vis Ultra violet/ visible IR Infra red n-BuLi n-Butyllithium GC Gas chromatography DDQ Dichlorodicyanoquinone NMO 4-methylmorpholin-N-oxide TLC Thin layer chromatography

XIII GCMS Gas chromatography mass spectrometry MALDI Matrix assisted laser desorption ionization ml Milliliter g Gram Å Angstrom

THF Tetrahydrofuran TsCl Tosyl chloride DIP Direct insertion probe DMSO Dimethyl sulfoxide mmol Millimole sat. Saturated aq. Aqueous

M Molar mg Milligram DMF Dimethylformamide NBS N-Bromosuccinimide AIBN Azobisisobutyronitrile RT Room temperature NR Nano rods DDAB Dodecyldimethylammonium bromide DDA Dodecylamine TEM Transmission electron microscopy STEM Scanning transmission electron microscopy SP Surface plasmons M-S Metal-semiconductor

XIV NC Nano crystal fs Femtosecond ODE 1-Octadecene OA Oleic acid TOP Tri-n-octylphosphine TOPO Tri-n-octylphosphine oxide ODPA n-Octadecylphosphonic acid ODA Octadecylamine PL Photoluminescence

Part I: Synthesis and Study of Nonacene Derivatives

Chapter 1. Acenes: Structure, Reactivity, and Synthesis

Introduction

Acenes are polycyclic aromatic hydrocarbons consisting of linearly fused benzene rings.1

The smallest acenes, benzene, naphthalene, and anthracene, are among the most studied organic molecules, while pentacene and its derivatives has received much attention as an active layer material in organic field-effect transistors (OFETs)2 due to its high charge- carrier mobility3. Interest in the synthesis of acenes larger than pentacene has been largely increased in the last decade, since increased conjugation length in acenes is expected to be beneficial for some applications in organic electronics, and significant efforts have been devoted to the development of appropriate synthetic methodology3.

However, the synthesis of larger stable acenes is a difficult and challenging task because of their very low solubility, poor stability in the presence of light and oxygen, and high reactivity towards Diels-Alder reaction and dimerization, in addition to the difficult multistep synthetic approaches required. Therefore, successful experimental studies on larger acenes are very limited. In recent years significant progress has been made in the synthesis of larger acenes, and stable and fully characterized heptacene derivatives were synthesized4,5,6,7,8. Acenes can be considered to be building blocks of carbon nanotubes and graphene, and studies on larger acenes may add to understanding of their properties.

For example, the chirality of carbon nanotubes can be described as arising from different arrangements of the acene chains that are responsible for its metallic or semiconducting electronic properties9. Although there are numerous studies on the electronic properties of larger acenes using computational techniques3, their electronic structure, aromaticity, and

1 HOMO–LUMO gaps are still not completely understood.

Structure and Reactivity

The acenes can be represented by few limiting valence bond structures as shown in

(Figure 1a, 1b and 1c)10. Electronic properties depend on the preferred structure.

Valence-bond theory suggests all three structures are energetically similar and 1a has the lowest energy 11. The stability of acene depends on a number of factors and calculations

3 favoring the stability of all three structures are reported . The cis-distorted form is more stable than the trans-form, while long range Coulomb interactions were considered. Non interacting models conclude the trans-form the most stable form while the undistorted

10 structure is preferred . According to Houk et al., larger acenes consist of two fully

10 delocalized non-alternating ribbons joined by relatively longer bonds (Figure 1b) . The energy levels are discrete in case of a particular finite acene and the HOMO-LUMO gap

(ΔE) decreases with the increase in number of rings for larger acenes.

Figure 1 Structure of Acene

Figure 2 Computed S0-T1 energy gaps for polyenes, acenes, and cyclacenes

Computational studies by Houk et al. have predicted that in polyacenes with number of rings more than eight, the triplet state of polyacene should be thermally accessible10. A singlet disjoint biradical character in the ground states of larger acenes is predicted based on calculations, where all the valence shell electrons do not participate in chemical bonding12. However, the ground states will still remain singlet states because of their disjointed biradical nature. The vertical transition energies decrease significantly from benzene to octacene (Figure 2). In addition singlet and triplet ground states become closer. The singlet-triplet gap decreases from hexacene to heptacene and octacene. Using spin-polarized DFT, Jiang and Dai predict antiferromagnetic ground states for larger acenes and polyacenes13.

3 There are at least three families of fused benzenoid compounds, including acenes, phenacenes with zig-zag type condensation, and helicenes with ortho-condensation.

Compounds of the latter family are found to be more stable in comparison with corresponding isomers in the former families14. Increasing the number of rings in acenes not only decreases the band gap, but also increases the proton and electron affinities, and reduces the ionization potential15 therefore reducing the stability of this class of compounds.

The unstable nature of larger acenes can be explained by Clar’s sextet concept which

1 approaches the matter qualitatively . In case of acenes, whether its benzene or anthracene or one of the larger analogs, there is only one π-electron sextet (Figure 3). Thus, for larger acenes one π-electron sextet is shared over a larger number of rings and the larger assemblies become increasingly less stable. This is explained through the sequential loss

16 of benzenoid character (aromaticity) according to molecular orbital theory

Figure 3 Clar’s Sextet concept

The main problem in working with the polyacenes is their high sensitivity to air and light.

As the number of rings increases, acenes become increasingly reactive, with the central ring being the most reactive19,17. Photooxidation with molecular oxygen and dimerization of the longer acenes are the major degradation pathways18.

The reactivity of acenes is illustrated first with anthracene, as reactive dienes in Diels-

4 Alder addition reactions19. The reactivity increases from the edges of the molecule to the center acene rings. The magnitude of HOMO coefficients increase inwards while the ring at the center becomes more reactive. For example, the central rings of pentacene and anthracene have activation energies (Ea) towards the Diels-Alder addition with acetylene of 24.0 and 29.4 kcal/mol, respectively while the (Ea) for the rings at the edge of pentacene is 32.7 kcal/mol19.

Figure 4 Correlation of the activation (Ea) versus the reaction (ΔE) energies of acetylene addition to acene ring

The stability of acenes towards nucleophilic and electrophilic additions as well as Diels-

Alder addition with oxygen were also addressed computationally17,18. According to these calculations the reactivity of the acenes with water and HCl increases from benzene to hexacene and then remains constant owing to the biradical character of the ground state

5 of higher acenes. These reactions are calculated to be exothermic with the activation barrier (Ea) for HCl addition being lower (~27 kcal/mol) than that for the addition of water. The addition of HCl to benzene has (Ea) of 44 kcal/mol, whereas it is only 16-18

18 kcal/mol for pentacene-nonacene . The (Ea) value for singlet oxygen addition to acenes, is in the same region as that of HCl addition. This value for benzene, anthracene, and pentacene is about 48, 29, and 20 kcal/mol, respectively with singlet oxygen18. Both triplet and singlet oxygen react with acenes and give the same products (endoperoxides).

Concerted20 as well as biradical stepwise mechanisms, starting from anthracene,17 for the addition of oxygen have been suggested.

Acenes usually adopt one of two common packing motifs: (I) the “herringbone” arrangement in which aromatic edge-to-face interactions dominate, and (II) the coplanar arrangement, wherein π-electron rich faces stack on each other to form two dimensional electronic coupling (Figure 5)3.

Figure 5 Herringbone (left) and π-stacking (right) arrangements of pentacene

6 Synthesis

Clar first claimed the synthesis of heptacene in 194221, but it was questioned in later reports in 194322 and 195523. It was finally withdrawn in 195724. Until 1986 there was no significant progress made in this area, when Fang reported the synthesis of larger acenes in his PhD dissertation, which was written under the supervision of Chapman at UCLA25.

Thermolysis of the heptacene dimer was reported to produce heptacene25. However, heptacene samples were always contaminated with heptacene dimer and dihydroheptacene and pure heptacene was not obtained. Heptacene formation was confirmed by accurate mass measurement (using mass spectrometry) and by the λmax value for the highest wavelength absorption band in the sublimed film (968 nm) and in 1- methylnaphthalene solution (at 220ºC, 752 nm)25.

In 2006, twenty years later, Neckers and co-workers, in pioneering work, obtained unsubstituted (parent) heptacene 1.2 by photodecarbonylation of a dione precursor at

395nm (Scheme 1.1) in a poly(methyl methacrylate) (PMMA) matrix4. The reported

Scheme 1 Photogeneration of Heptacene 2 in PMMA matrix

λmax value (760 nm) recorded in the PMMA matrix was in agreement with Fang’s report25 on unsubstituted heptacene. Heptacene was found to be stable for only 4 h in the

PMMA matrix4, which also confirmed the high reactivity of this compound. It

7 immediately converts into the oxygen adduct (mainly endoperoxides) in the presence of air in solution4. Heptacene that lacks protecting groups is highly unstable at room temperature26, thus to stabilize the larger acenes protecting groups are required.

In 2005 Anthony and coworkers, in groundbreaking work, synthesized 7,16-

5 silylethynylheptacenes 3-5 (Scheme 2) . The λmax value (852 nm in CH2Cl2) for 1.5 is red-shifted by about 92 nm relative to that of the unsubstituted heptacene4. Compound 5, with bulky protecting groups on the central ring, and optical and electrochemical gaps of

1.36 and 1.30 eV, respectively, was stable enough to perform chromatographic purification, spectroscopic characterization, and a single-crystal X-ray study.

Scheme 2 Synthesis of 7,16-silylethynylheptacenes

Tris(trimethylsilyl)silylethynyl groups, with diameter of about 40% of the length of heptacene, provide sufficient protection, mostly through steric effects. Heptacene 5 is very soluble and is stable in the solid state for a week under no light and air conditions; however, it decomposes within a few hours when exposed to air5. This work by

Anthony_s group5 shows that heptacene, when substituted with significantly bulky groups, is a synthetically accessible and stable molecule.

Wudl6 and Miller7 groups have made new progress in the synthesis of substituted heptacenes in the recent years. The nucleophilic addition of organometallic reagents to an

8 acene quinone, followed by dehydroxylation, is the most common synthetic route to functionalized acenes3. A different synthetic method to functionalized heptacenes has been established by Wudl et al. It utilizes double Diels–Alder cycloaddition between a

“bisanthracyne” and dienes, followed by reduction to give stable substituted heptacene derivatives (Scheme 3)6. When substituents with the alkoxy side chains were

Scheme 3 Synthesis of heptacene derivatives 6a-c

employed the solubility significantly improved but compound 6b turned out to be too reactive to be isolated or characterized completely. The aryl groups in the central ring were replaced by triisopropylsilylethynyl groups to further improve stability, forming 6c.

Compound 6c is stable for over three weeks to the atmosphere oxygen and light when coated in mineral oil and for 41 h when kept in degassed toluene solution. The bulky substituents effectively prevent dimerization and polymer formation even after recrystallization over three weeks6. Although the reactivity of heptacene derivatives may

9 be due to a singlet diradical character in the ground state which is supported by calculations12, this is not detectable spectroscopically (1H NMR signals show sharp splitting and narrow linewidths)5,6. Trialkylsilylethynyl groups red-shift the NIR absorption of 6c by approximately 20 nm (863 nm in toluene) relative to 6b. The optical

HOMO–LUMO gap of 6c (1.35 eV,) matches the electrochemical HOMO–LUMO gap

(1.38 eV) very well6. This report by the Wudl group6 greatly expands the synthetic methodologies available for the synthesis of large acenes and describes a substance that is sufficiently stable to enable potential electronic applications.

Additional significant progress was recently accomplished by Miller’s group7. In their study of a substituent effect for a large series of pentacene derivatives, they concluded that steric effects, electronic effects, and the position of the substituents are all important factors for determining photooxidative resistances and HOMO–LUMO gaps27. Miller et al. prepared persistent heptacene derivatives using 2,6-dimethylphenyl and thioaryl substituents (Scheme 4)7. Ortho-Alkyl-substituted phenyl groups are superior to phenyl substituents in enhancing photooxidative resistance: the ortho-alkyl groups lie directly over and under acene’s π system, thus providing better steric hindrance7. Thus thioalkyl and thioaryl substituents enhance photooxidative resistance more than silylethynyl substituents27. Consistent with the impressive substituent effects of arylthio substituents, heptacene derivative 7b is even longer lived than 8a. Compound 8b is as an especially persistent heptacene derivative with a small HOMO–LUMO gap (1.37 eV) comparable with those of 5 and 67. Heptacene derivative 8b is soluble in a variety of solvents and stable for weeks in solid state, for 1–2 days in solution if kept away from light and air, and for several hours in solution when directly exposed to both light and air. All solids

10 7a,b and 8a,b are dark green in color. The longest λmax for 8b is 865 nm (in CH2Cl2), which is comparable with 6c (863 nm) and 5 (852 nm)7. New protecting groups for large

Scheme 4 Synthesis of heptacene derivatives 7a,b and 8a,b

11 acenes were successfully introduced, significantly improving the possible opportunities for larger acenes7.

Chi and co-workers recently reported another stable substituted heptacene8. Although structurally it is very similar to compound 6c, and differs only in electron withdrawing trifluoromethyl substituent on phenyls, the synthetic methodology used isonaphthofuran precursors in a Diels-Alder reaction with benzoquinone followed by reduction to yield the corresponding heptacene quinone which was treated with triisopropylsilylethynyl magnesium chlride followed by another reduction to give compound 9 (Scheme 5)8.

Compound 9 has a yellow-green color and a NIR absorption maxima at 870 nm. The

Scheme 5 Synthesis of heptacene derivative 9 photostability of deoxygenated toluene solutions of 9 towards ambient light, white light, and UV light (4W) was monitored and estimated half-lives of 1950, 200, and 100 min. respectively were obtained. When a toluene solution of 9 was exposed to both ambient light and air it could still be detected after 47 h, which is more stable then 6c6, this is due to electron withdrawing groups on phenyls8. An electrochemical energy gap of 1.32 eV is obtained for 9, which is in agreement with the optical band gap. Interestingly in the

12 absence of trifluoromethyl substituents there is an upshift of about 0.1 eV for both HOMO and LUMO levels as reported in Wudl’s heptacene derivative, which has a HOMO at -4.8 eV and LUMO at -3.5 eV6.

In 2010 two very important developments in the field of larger acenes were reported, which significantly increase the synthetically accessible limit of larger acenes. Persistent substituted nonacene has been reported by Miller’s group28. Nonacene 10 (Figure 6) has a very small HOMO–LUMO gap and is stable as a solid in the dark for at least six weeks.

It was characterized by 1H NMR, 13C NMR, laser-desorption mass spectroscopy,

UV/Vis/NIR, and fluorescence techniques28, although its degree of purity cannot be

Figure 6 Persistent nonacene derivative 10

assessed. The synthetic approach to 10 was very similar to one that was employed to obtain 7a,b and 8a,b with the development of thioaryl substituted “dienophile” to ensure stability of formed nonacene. The HOMO–LUMO gap of 10 obtained from the onset of the longest-wavelength absorption is 1.12 eV, which is the smallest experimentally

13 measured HOMO–LUMO gap for any acene28. Nonacene 10 was computationally predicted to be a closed-shell species because of the special arrangements of its thioaryl substituents, which contribute greatly to its stability.

Another very important recent advance in the area comes from Bettinger’s group, who reported the synthesis and spectroscopic detection of unsubstituted (parent) octacene and nonacene in matrix isolation studies.29 The synthetic methodology use by Bettinger group

(Scheme 6)29 followed closely the procedures developed by Neckers et al4 for

Scheme 6 Synthesis of Octacene 11 and nonacene 12

synthesis of heptacene 2. The bandgap of polyacene was determined by extrapolation from the HOMO–LUMO gaps of unsubstituted acenes referring to UV/Vis/NIR detection of unsubstituted octacene and nonacene. In the same manner, a 1.2 eV bandgap was extrapolated for polyacene29.

Overall, recent experimental studies on heptacene and nonacene derivatives pave the way for investigations of the electronic properties of larger acenes and provide insight into their structure and reactivity9. Protecting groups and their selection are very important for the stabilization of acenes larger than pentacene. These materials have low HOMO–

14 LUMO gaps and it is expected that future efforts will not only increase the understanding of the electronic structure of larger acenes, but could also lead to their application in organic electronic devices9.

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14. Portella, G.; Poater, J.; Bofill, J. M.; Alemany, P.; Solá, M. J. Org. Chem., 2005, 70,

2509.

15. (a) Rienstra-Kiracofe, J. C.; Barden, C. J.; Brown, S. T.; Schaefer, H. F., J. Phys.

Chem. A. 2001, 105, 524; (b) Deleuze, M. S.; Claes, L.; Kryachko, E. S.; François, J.-P.

J. Chem. Phys. 2003, 119, 3106.

16. Suresh, C. H.; Gadre, S. R. J. Org. Chem. 1999, 64, 2505.

17. Reddy, A. R.; Bendikov, M. Chem. Commun. 2006, 1179 – 1181.

18. Reddy, A. R.; Fridman-Marueli, G.; Bendikov, M. J. Org. Chem. 2007, 72, 51 – 61.

19. Schleyer, P. v. R.; Manoharan, M.; Jiao, H.; Stahl, F. Org. Lett. 2001, 3, 3643-3646.

20. Chien, S. -H.; Cheng, M. -F.; Lau, K. -C.; Li, W, -K. J. Phys. Chem. A 2005, 109,

7509.

21. Clar, E. Ber. Dtsch. Chem. Ges. B 1942, 75, 1330 – 1338

22. Marschalk, C. Bull. Soc. Chim. 1943, 10, 511 – 512

23. Bailey, W. J.; Liao, C.-W. J. Am. Chem. Soc. 1955, 77, 992 – 993

24. Boggiano, B.; Clar, E. J. Chem. Soc. 1957, 2681 – 2689.

25. Fang, T. Heptacene, Octacene, Nonacene, Supercene and Related Polymers, Ph.D.

Dissertation, University of California, Los Angeles, CA, 1986.

26. Mondal, R.; Tönshoff, C.; Khon, D.; Neckers, D. C.; Bettinger, H. F. J. Am. Chem.

Soc. 2009, 131, 14281 – 14289.

27. Kaur, I.; Jia, W.; Kopreski, R. P.; Selvarasah, S.; Dokmeci, M. R.; Pramanik, C.;

McGruer, N. E.; Miller, G. P. J. Am. Chem. Soc. 2008, 130, 16274 – 16286.

28. Kaur, I.; Jazdzyk, M.; Stein, N. N.; Prusevich, P.; Miller, G. P. J. Am. Chem. Soc.

2010, 132, 1261 – 1263.

29. Tönshoff, C.; Bettinger H. F . Angew. Chem. Int. Ed. 2010, 49, 4125 –4128

17 Chapter 2. Attempted Synthesis of Parent Octacene and Nonacene

Introduction

Acenes are linear polycyclic aromatic hydrocarbons. The most common way to synthesize smaller acenes (up to pentacene) is by reduction of the corresponding quinones. A similar approach used for higher acenes proved to be unsuccessful since the quinones were overreduced to hydrogenated acenes123. Other synthetic methods for larger acenes also failed for almost half a century. This is because of their higher reactivity towards Diels-Alder additions, which ultimately result in formation of either dimers, or oxygen adducts (endoperoxides)4.

Clar first claimed5 the synthesis of heptacene, octacene, and nonacene, but it was questioned in later reports6,7 and then it was finally withdrawn8. Later in 1986 Fang reported synthesis of octacene and nonacene4 but the products lacked proper characterization, were always contaminated with dimers and the overall work was never completed. In 2006 Mondal, Neckers et al. reported synthesis of unsubstituted (parent) heptacene using Strating-Zwanenberg photodecarbonylation9. The Strating group, while seeking the dimer of carbon monoxide, employed this decarbonylation. First in 196910.

An impressive attribute of this reaction is that it cleanly produced an acene following the expulsion of two molecules of carbon monoxide. A semirigid polymer matrix and inert gas matrices was employed by Mondal et al9,11 to reduce the reactivity of heptacene, record its thermal decomposition, and subsequently to record its UV-vis-NIR absorption, as well as, its IR spectra.

18 After the report on synthesis of unsubstituted heptacene by Mondal, Neckers et al9 a group from Germany led by Dr. Holger Bettinger offered a collaboration to study photostability of higher acenes in solid noble gas matrix. This collaborative work yielded a publication on photochemistry and stability of pentacene, hexacene, and heptacene11.

Soon after that Bettinger group started undertaking a similar synthesis of octacene and nonacene photoprecursors that were used in Strating-Zwanenberg photodecarbonylation and matrix isolation studies12.

In this chapter attempted synthesis of octacene and nonacene photoprecursors following synthetic methodologies developed by Mondal, Neckers et al9 will be discussed.

Synthesis

All the synthetic schemes for both nonacene and octacene photoprecursors required the crucial starting material, 5,6,7,8-tetramethylenebicyclo[2.2.2]oct-2-ene. This was synthesized (Scheme 2.1) following modified reported procedures13,14. Nonacene was

Scheme 2.1 Synthesis of 5,6,7,8-tetramethylenebicyclo[2.2.2]oct-2-ene (2.6)

19 first chosen as a primary synthetic target because it is symmetric from the synthetic prospective and requires fewer steps to synthesize when compared to octacene. The synthetic approach initially developed for nonacene (Scheme 2.2), though straightforward, was also expected to be more challenging because of potential solubility problems and lesser stability of the tetracene moieties in

Scheme 2.2 Synthetic scheme for nonacene photoprecursor nonacene photoprecursor 2,3-Dibromoanthracene, one of the starting materials, was synthesized following procedures developed by Lin and Chou15(Scheme 2.3).

Scheme 2.3 Synthesis of 2,3-Dibromoanthracene (2.11)

20 The initial step of coupling 2.6 and 2.11 to afford compound 2.12 failed. No product was observed (Figure 2.1). Failure was due to poor solubility of 2,3-dibromoanthracene and insufficient formation of “anthracyne” upon treatment of 2,3-dibromoanthracene with n-

BuLi. The reactivity of 1,2-dibromobenzene;

Figure 2.1 Attempt to form nonacene backbone structure (2.12)

2,3-dibromonaphthalene; 2,3-dibromoanthracene(2.11) and 6,7-dibromo-1,4- dihydroanthracene(2.10) towards Diels-Alder reaction with furan as the diene was examined to confirm that 2.11 does not form an “anthracyne” type intermediate (Figure

2.2) 2,3-dibromoanthracene under a wide range of conditions does not react with furan.

Figure 2.2 Formation and reactivity of “benzynes” from 1,2-Dibromobenzene to 2,3- Dibromoanthracene towards Diels-Alder reaction

21 The progress of all these reactions was monitored by GC. Products were not isolated nor properly characterized since the goal of the experiments were to examine the reactivity.

Reactions were performed on microscale. These results led to development of an alternative synthetic scheme, which utilized compound 2.10 instead of unreactive 2.11 in the initial Diels-Alder step (Scheme 2.4). A subsequent aromatization of the terminal

Scheme 2.4 Modified synthetic scheme for nonacene photoprecursor rings should yield the desired 2.12.The reaction of 2.6 with 2.10 gave two products: the monoadduct 2.14 was the main one (40% yield) and double addition product 2.13

(Figure 2.3) was minor one (20% yield). Compound 2.14 is one of the crucial intermediates in the synthetic scheme that targets octacene photoprecursor (Scheme 2.5).

Figure 2.3 Products of addition reaction of compound 2.6 with compound 2.10

22 Compound 2.13 was then treated with dichlorodicyanoquinone(DDQ) to give 2.12, which was only characterized by mass spectrometry and used in subsequent oxidation with osmium tetroxide with only traces of expected diol to be detected by MALDI-MS.

Scheme 2.5 Synthetic scheme for octacene photoprecursor

Compound 2.14 in cycloaddition reaction with 2,3-dibromonaphthalene gave 2.15.

Refluxing in toluene with DDQ(dichlorodicyanoquinone) yielded 2.16 characterized only by mass spectrometry. It was used directly in the next reaction using a catalytic amount of OsO4 in the presence of 4-methylmorpholin-N-oxide (NMO) but again only traces of desired diol were detected by Mass Spectrometry.

Conclusion

The synthesis of octacene and nonacene α-diketones following the synthetic schemes above failed, presumably due to poor solubility and low stability of the precursor molecules. Compounds 2.13 and 2.15 are synthetically the most elaborated fully

23 characterized intermediates for the preparation of nonacene and octacene, respectively, while 2.12 and 2.16 are characterized by Mass Spectrometry only.

Scheme 2.6 Bettinger’s synthesis of octacene and nonacene photoprecursors

After our collaborative work on the stability and photochemistry of pentacene, hexacene, and heptacene11 octacene and nonacene were reported by Bettinger et al in 2010.12 The photochemical synthesis used a very similar approach to that reported by Mondal et al9.

24 Octacene and nonacene α-diketones (Scheme 2.6) were used, and the crucial difference was the use of two bridged α-diketones per molecule of photoprecursor. Their synthetic approach was considered at the the beginning of my project but was intentionally not developed in Bowling Green since it had already been undertaken by our collaborators in the Bettinger group.

Experimental Section

General Procedures. The starting materials 1,2,4,5-tetrabromobenzene and bicyclo[2.2.2]oct-7ene-exo2,3,5,6-tetracarboxylic-2,3,5,6-dianhydride (2.2) were purchased from Aldrich and used as received. Organic solvents were either spectroscopic grade or purified by distillation and dried before use with proper drying reagents. TLC and silica gel(230-400 mesh) were purchased from Sorbent Technologies Co. NMR spectra were recorded from a BRUKER Avance 300MHz NMR spectrometer using TMS as internal standard. Mass spectra were recorded on SHIMADZU GCMS-QP5050A gas chromatograph mass spectrometer (GCMS) or BRUKER DALTONICS OMNIFLEX matrix assisted laser desorption ionization mass spectrometer (MALDI-MS). Progress of reactions was monitored either by TLC or GCMS.

Synthesis:

(±)-Exo-2, endo-3, exo-5, endo-6-Tetrakis(ethoxycarbonyl)bicyclo[2.2.2] oct-7-ene

(2.3)13,14. Anhydride 2.2 (25 g, 0.1 mole) and 0.25 g p-toluenesulfonic acid in a mixture of 150 ml EtOH and 100 ml toluene was heated under reflux in reactor fitted with a

Soxlet extractor containing 10 g of molecular sieves (3-4 Å) for 3 days. The molecular

25 sieves were renewed every 12 hours. After cooling a suspension of sodium hydride (0.4 g, 55% in oil 0.01 mol) in 7 ml of ethanol was added. The mixture was heated at 125-

130ºC for 6 h. After completion of the reaction the mixture was cooled somewhat and additional sodium hydride in batches of 0.1g (55% in oil) was added slightly, followed by heating for 3 hours. The mixture was rotary evaporated and the liquid residue is used as

1 such in the next step. H NMR(CDCl3): 6.4(t, 2H), 4.2(m, 8H), 3.5 (m, 2H), 3.1 (m, 4H),

1.3 (m, 12H) ; MS(EI): 396(0.8), 350(15), 276(20), 177(32), 175(20), 15(100), 150(33),

149(22).

(±)-Exo-2, endo-3, exo-5, endo-6-tetrakis(hydroxymethyl)bicyclo[2.2.2] oct-7-ene

(2.4).13,14 A solution of crude 2.3 (40 g, 0.1 mol) in 150 ml of THF was added dropwise to saturated suspension of lithium aluminum hydride (7.2 g, 0.2 mol) in 300 ml of THF under argon over period of 3 hours. After heating under reflux for 2 days, and then cooling to room t, 40 ml of a saturated solution of Na2SO4 in water was added portionwise. After again heating to the boiling the mixture was filtered rapidly over SiO2

(40g) and the SiO2 + salts were extracted with hot EtOH(2×300 ml, 2 hours heating under reflux and hot filtration). The extract was evaporated to yield: 17.4 g (75%) racemic

1 mixture of both isomers of 2.4; H NMR (D2O): 6.2(m, 2H), 4.6(s, OH), 3.6(m, 2H),

3.4(br.s, 2H), 3.2(br.s, 2H), 3.13(br.s, 2H), 2.54(m, 2H), 1.41(m, 2H), 1.06(m, 2H); MS

(EI): 229(14), 212(12), 211(55), 194(89), 181(11), 175(100), 145(87).

(±)-Exo-2, endo-3, exo-5, endo-6-tetrakis(p-toluenesulfonylmethyl) bicyclo[2.2.2] oct-

7-ene (2.5) . 13,14 To a the mechanically stirred solution of tetrol 2.4 (17.4 g, 0.076 mol) in

150 ml anhydrous pyridine TsCl (72 g, 0.38 mol) was added portionwise at 0ºC. The mixture was stirred for an additional 5 hours at 0ºC and then extracted with CH2Cl2

26 (3×100 ml) and the organic phases washed with sat. NaHCO3 (2×150 ml) then with 3N

HCl(3×100 ml) and finally dried (Mg SO4). The solution was evaporated to dryness to yield 51.1 g (80%) dark yellow resin. MS (DIP): 844(0.96), 534(20), 379(31), 224(84),

1 196(100). H NMR (CDCl3): 7.8-8.0(m, 16H), 6.4(m, 2H), 3.2-3.9(m, 8H), 3.0(m, 2H),

1.4-1.8(m, 4H), 1.67(s, 12H).

5,6,7,8-Tetramethylenebicyclo[2.2.2]oct-2-ene (2.6). t-BuOK (33 g, 0.3 mol) was added portionwise to the solution of 51.1 g (0.06 mol) of tosylate 2.5 in 100 ml of DMSO while stirring mechanically at 0ºC. The mixture was stirred for an additional 2 hours at room temperature, after which reaction mixture was poured onto 100 g of ice. The mixture was extracted with hexanes (3×100 ml). The extract was then dried and evaporated. The light yellowish residue was purified on a silica gel column using hexanes as eluent to afford

1 6.3 g (67%) colorless crystals. H NMR (CDCl3): 6.4(dd, 2H), 5.15(s, 4H), 4.95(s, 4H),

3.85(dd, 2H); MS (EI): 156(88), 141(83), 128(39), 115(61), 104(100), 91(12), 78(24).

6,7-Dibromo-1,4-epoxynaphthalene (2.8). n-BuLi (2.5 M in hexane) 0.5 ml (1.25 mmol) was added dropwise to a solution of 4 g (1 mmol) 1,2,4,5-tetrabromobenzene and

1 ml (∼10 mmol) of furan in 150 ml of THF at -78 ºC. The mixture was stirred at -78 ºC for 3 h and then allowed to warm to rt. The excess n-BuLi was quenched with methanol.

Solvents were removed by rotary evaporation to yield 2.3 g (75%) of crude 2.8, which was used in the next step without further purification.

6,7-Dibromo-9,10-epoxy-1,4,4a,9,9a,10-hexahydroanthracene (2.9)12. A solution of

6,7-dibromo-9,10-epoxy-1,4,dihydronaphthalene 2.8 (6 g 19.9 mmol) and 2,5- dihydrothiophene-1,1-dioxide (sulfolene – 2.8 g 23.7 mmol) in o-xylene (25 ml) was

27 sealed in an autoclave and heated 145 ºC for 6 h. Evaporation of solvent followed by chromatography on silica gel using CH2Cl2/hexanes (1/9) as eluent affords 2.9; yield:

1 6.27 g (88%); H NMR (CDCl3): 7.45(s, 2H), 5.91(dd, 2H), 4.92(s, 2H)1.89-2.4(m 6H);

MS (EI): 356(38), 336(25), 304(18), 276(100).

6,7-Dibromo-1,4-dihydroanthracene (2.10)12. A solution of 2.9 (3 g 8.42 mmol) in toluene (150 ml) was placed in a 250 ml round-bottom flask followed by addition of

EtOH (25 ml) and 70% aq. perchloric acid (15 ml). The mixture was vigorously stirred at

80ºC for 3 h., the organic layer separated, and the aqueous layer extracted with CH2Cl2

(3×100 ml). The combined organic layer was washed with sat. aq. NaHCO3 and brine, and then dried (Mg SO4), filtered and concentrated in vacuo. The light brown residue obtained was purified by chromatography on a silica gel column with CH2Cl2/hexanes

1 (1/9) as eluent to furnish 2.10; yield: 2.56 g (90%); H NMR (CDCl3): 8.02(s, 2H),

7.48(s, 2H), 6.01(t, 2H), 3.54(d, 4H); MS (EI): 338(54), 336(41), 258(19), 207(17),

178(100), 176(76), 89(92).

2,3-Dibromoanthracene (2.11)12. To a stirred solution of 2.10 (2 g 5.96 mmol) in dry benzene (300 ml) was added DDQ (2.4 g 10.04 mmol). The mixture was refluxed for 4 h, and the resulting insoluble hydroquinone was removed by filtration, of the hot solution.

Pure compound 2.11 precipitates as flakes when the solution was slowly cooled to room temperature. The brownish orange mother liquor was evaporated under reduced pressure, and the crude product obtained was purified by chromatography on a silica gel column

1 with hexane as eluent to give 2.11; total yield: 1.5 g (85%); H NMR (CDCl3): 8.04(s,

2H), 7.81(s, 2H), 7.75(d, 2H), 7.3(d, 2H). MS (EI): 338(23), 336(43), 207(38), 176(36),

88(62), 49(100).

28 9,10-Bimethylene-1,4,7,8,11,12,-decahydro-6,13-diethenopentacene and

1,4,7,8,9,12,15,18,19,20-octadecahydro-8,19-diethenononacene (2.14 and 2.13). 2.7 mL (6.75 mmol) of nBuLi (2.5 M in hexane) was added drop-wise under argon atmosphere to a suspension of 2.6 (231 mg, 1.48 mmol) and 2.10 (1 g, 2.96 mmol) in 125 ml of dry toluene cooled to -60o C. The mixture was stirred for three hours at -60o C, then the temperature was allowed to increase very slowly to r. t. and the excess n-BuLi was quenched with methanol. The product was concentrated on a rotary evaporator and purified by a silica gel column using CH2Cl2/hexanes mixture (10% by volume) to give

1 200 mg of 2.14 (40%) and 150 mg of 2.13 (20%). H NMR for 2.14 (CDCl3): 7.51 (s,

2H), 7.48 (s, 2H), 6.51 (dd, 2H), 6.01 (s, 2H), 5.09 (s, 2H), 4.88 (s, 2H), 4.01 (t, 2H), 3.71

(s, 4H), 3.53 (s, 4H). MS (EI): 334(100), 278(41), 230(50). HRMS (EI):

1 334.1722(meas.), 334.1722(calc.). H NMR for 2.13 (CDCl3): 7.50 (s, 4H), 7.47 (s, 4H),

6.91 (dd, 2H), 6.00 (s, 4H), 4.38 (s, 2H), 3.79 (s, 8H), 3.52 (s, 8H). HRMS (EI):

512.2507(meas.), 512.2504(calc.).

1,4,7,8,9,16,17,18-Octahydro-8,17-diethenooctacene (2.15). nBuLi (2.5 M in hexane)

0.55 mL (1.4 mmol) was added dropwise under an argon atmosphere to a suspension of

2.14 (200 mg, 0.6 mmol) and 2,3-dibromonaphthalene (172 mg, 0.6 mmol) in 25 ml of dry toluene cooled to -78o C. The mixture was stirred for three hours at -78o C, then the temperature was allowed to increase very slowly to r. t. and the excess n-BuLi was quenched with methanol. The product was concentrated on a rotary evaporator and purified by a silica gel column using CH2Cl2/hexanes mixture (5% by volume) to yield 70

1 mg (25%) of 2.15 as white solid; H NMR (CDCl3): 7.71 (s, 2H), 7.60 (s, 2H), 7.50 (s,

2H), 7.47 (s, 2H), 7.35 (s, 2H), 6.91 (dd 2H), 6.00 (s, 2H), 4.40 (t, 2H), 3.81 (s, 4H), 3.79

29 (s, 4H), 3.52 (s, 4H). MS (EI): 460(100), 267(28), 228(72). HRMS (EI):

460.21861(meas.), 460.21910(calc.).

8,19-Diethenononacene (2.12). To a solution of 2.13 (150 mg, 0.29 mmol) in 25 ml of toluene was added 1 g (4.4 mmol) of DDQ under inert atmosphere. The mixture was refluxed for 2 h and then cooled down to rt. The product was concentrated on a rotary evaporator and purified by a silica gel column using CH2Cl2/hexanes mixture (10% by volume) to give 30 mg of crude 2.12, which wasn’t purified further in attempt to save enough compound for the next reaction. MS (EI): 505(63), 252(84), 149(100).

8,17-Diethenooctacene (2.16). To a solution of 2.15 (70 mg, 0.15 mmol) in 25 ml of toluene was added 1 g (4.4 mmol) of DDQ under inert atmosphere. The mixture was refluxed for 2 h and then cooled down to rt. The product was concentrated on a rotary evaporator and purified by a silica gel column using CH2Cl2/hexanes mixture (10% by volume) to give 18 mg of crude 2.16, which wasn’t purified further in attempt to save enough compound for the next reaction. MS (EI): 454(28), 265(24), 207(59), 125(56),

111(100).

References

1. Clar, E. Polycyclic Hydrocarbons; Academic Press: London and New York, 1964;

Vols. 1, 2.

2. Bailey, W. J.; Liao, 96 C. -W. J. Am. Chem. Soc. 1955, 77, 992

3. Satchell, M. P.; Stacey, B. E. J. Chem. Soc. C: Organic. 1971, 3, 468. 4. Fang, T. Heptacene, Octacene, Nonacene, Supercene and Related Polymers.

Ph.D.Thesis, University of California, Los Angeles, CA, 1986.

5. Clar, E. Ber. Dtsch. Chem. Ges. B 1942, 75, 1330 – 1338

6. Marschalk, C. Bull. Soc. Chim. 1943, 10, 511 – 512

7. Bailey, W. J.; Liao, C.-W. J. Am. Chem. Soc. 1955, 77, 992 – 993

8. Boggiano, B.; Clar, E. J. Chem. Soc. 1957, 2681 – 2689.

9. Mondal, R.; Shah, B. K.; Neckers, D. C. J. Am. Chem. Soc. 2006, 128, 9612 – 9613.

10. Strating, J.; Zwanenburg, B.; Wagenaar, A.; Udding, A. C. Tetrahedron Lett. 1969,

10, 125.

11 Mondal, R.; Tönshoff, C.; Khon, D.; Neckers, D. C.; Bettinger, H. F. J. Am. Chem. Soc.

2009, 131, 14281 – 14289.

12. Tönshoff, C.; Bettinger H. F . Angew. Chem. Int. Ed. 2010, 49, 4125 –4128

13. Gabioud, R.; Vogel, P. Tetrahedron 1980, 36, 149-154.

14. Ten Hoeve, W.; Huisman, B. Method of preparation of a precursor Oligocene; PCT

Int. Appl. 2004, 21 pp.

15. Lin, Ch.-T.; Chou, T.-Ch. Synthesis 1988, 628-630.

31 Chapter 3. Synthesis of Heptacene/Nonacene Derivative

Introduction

In recent years pentacene and its derivatives have received much attention as active layer materials in organic field-effect transistors (OFETs)1 due to their high charge-carrier mobility2. Interest in the synthesis of acenes larger than pentacene has increased in the last decade, since increased conjugation length in acenes is expected to be beneficial for some applications in organic electronics. Significant efforts have been devoted to the development of appropriate synthetic methodology2. However, the synthesis of larger stable acenes is difficult and challenging because of their very low solubility, poor stability in the presence of light and oxygen, and high reactivity towards Diels-Alder reactions and dimerization. Difficult multistep synthetic approaches are required.

Although several unsubstituted larger acenes3,4 have been reported including heptacene, higher acenes that lack protecting groups are highly unstable at room temperature.5 Thus to stabilize the larger acenes, protecting groups are required. To address these issues a number of substituted larger acenes6,7,8,9 have been synthesized over the last several years. Most notably Miller’s group synthesized the substituted nonacene10 last year.

Several methodologies are now available to prepare substituted higher acenes from organometallic reactions with corresponding quinones and subsequent reduction to using the substituted core in Diels-Alder type reactions to grow the number of rings. Miller et al. found that arylthio substituents on the central ring of larger acenes are among the best stabilizing groups11. In this chapter we describe the attempted synthesis of heptacene/nonacene derivatives closely following the reports from Miller et al.8,10

32 Synthesis

The first synthetic target of the project was substituted nonacene (Scheme 3.1) which

Scheme 3.1 Proposed Synthetic scheme for substituted nonacene derivative required crucial intermediate 3.4, which was synthesized (Scheme 3.2) following modified reported procedures8. Durene was converted to diiododurene using iodine and periodic acid dihydrate12, which was coupled with 4-tertbutylthiophenol using Cu(I) as a catalyst. The arylthiodurene was brominated with NBS using catalytic amounts of AIBN and light irradiation to form 3.4.

Scheme 3.2 Synthesis of intermediate 3.4

Unfortunately the first reaction of Scheme 3.1 didn’t give the desired nonacene diquinone (Figure 3.1), so a different synthetic approach was developed (Scheme 3.3).

This sequqnce utilizes the tetrahydroanthraquinone 3.9 instead of unreactive 1,4- anthraquinone. It was planned to aromatize the octahydrononacenediquinone 3.10 with

Scheme 3.3 Revised synthetic scheme for substituted nonacene

Figure 3.1 Attempt to couple compound 3.4 with 1,4-anthraquinone

DDQ and subsequently react with the organometallic reagent followed by reduction to obtain substituted nonacene as in Scheme 3.1. This approach required intermediate 3.9, which was synthesized (Scheme 3.4) following modified reported procedures.13

Scheme 3.4 Synthesis of compound 3.9

Compound 3.10 was successfully synthesized by coupling 3.4 with 3.9 in presence of potassium iodide in DMF. But treatment of 3.10 with DDQ didn’t give the expected nonacenediquinone (Figure 3.2).

Figure 3.2 Attempted aromatization of compound 3.10

At the same time Miller’s synthesis of arylthiosubstituted heptacene8 was reproduced

Scheme 3.5 Modified organometallic reaction step for Miller’s synthesis of substituted heptacene

36 (Scheme 3.5) using the modified organometallic addition step. The formation of aryllithium reagent from the corresponding aryl bromide and butyl lithium at -78 °C was replaced by the commercially available Grignard reagent and the reaction conditions changed to RT without significant effect on the formation of the final product 3.15. Based on these findings another synthetic scheme (Scheme 3.6) was developed targeting compound 3.12.

Scheme 3.6 Synthetic scheme to obtain compound 3.12

The treatment of 3.10 with Grignard reagent with a subsequent acidic work up didn’t furnish the expected compound 3.11. Only a trace of 3.11 was detected by MALDI-MS.

However the MALDI-MS spectrum showed a high and clean peak for desired compound

37 3.12 (Figure 3.3). When the solution was irradiated with long-wave UV light yellow- green fluorescence characteristic to arylthiosubstituted heptacenes was observed.

Unfortunately this product of the reaction couldn’t be purified and properly characterized.

The same results were obtained using organolithium reagents instead of commercially

Figure 3.3 MALDI-MS spectrum showing peak for compound 3.12 available Grignard reagent. We anticipate that terminal saturated rings facilitate the aromatization at aryl-substituted positions although no plausible mechanism is proposed.

38 Experimental Procedures

General Procedures. The starting materials 1,2,4,5-tetramethylbenzene and 1,2- cyclohexanedicarboxylic anhydride were purchased from Aldrich and used as received.

Organic solvents were either spectroscopic grade or purified by distillation and dried before use with proper drying reagents. TLC and silica gel (230-400 mesh) were purchased from Sorbent Technologies Co. NMR spectra were recorded from BRUKER

Avance 300MHz NMR spectrometer using TMS as internal standard. Mass spectra were recorded on SHIMADZU GCMS-QP5050A gas chromatograph mass spectrometer

(GCMS) or BRUKER DALTONICS OMNIFLEX matrix assisted laser desorption ionization mass spectrometer (MALDI-MS). Progress of reactions was monitored either by TLC or GCMS.

Diiododurene (3.2)12: Durene (1.34 g, 0.01 mol) was placed into round bottom flask together with periodic acid dihydrate (0.92 g, 0.0043 mol), and iodine (2.55 g, 0.01 mol).

A solution of 3 ml of concentrated sulfuric acid and 20 ml of water in 100 ml of glacial acetic acid was added to this mixture. The resulting purple solution was heated at 65–70° with stirring for approximately 1 hour until the color of iodine disappeared. The reaction mixture was diluted with approximately 250 ml of water, and the white-yellow solid that separates was collected by filtration and washed three times with 100-ml portions of water. The product was recrystallized from ethanol to yield 3.01 g (78%) of diiododurene

1 3.2 as fine colorless needles. H NMR (CDCl3): 2.49 (s, 12H). MS (EI): 386(82),

259(29), 117(100).

1,4-Bis(4'-t-butylphenylthio)-2,3,5,6-tetramethylbenzene (3.3): Copper iodide (0.07 g,

0.037 mmol) and cesium carbonate (2.45 g, 27.38 mmol) was added to a solution of 3.2

39 (1.0 g, 3.42 mmol) and 4-tbutylthiophenol (1.25 g, 7.52 mmol) in dimethylformamide

(20.0 mL). The resulting suspension was deoxygenated with bubbling argon for 15 min, sealed in a seal tube, and heated at 150ºC for 1 day. The reaction mixture was then cooled, filtered and extracted with ethyl acetate. The organic layer was separated and washed with water followed by brine and then dried to give the crude product which was further purified by column chromatography using hexanes : dichloromethane (10:1) as eluent to give white solid product (1.28 g, 81% yield). 1H NMR (CDCl3): 7.23 (d, 4H),

6.87 (d, 4H), 2.50 (s, 12H), 1.28 (s, 18H). MS (EI): 462(84), 447(42), 216(36), 117(43),

91(73), 57(100).

1,4-Bis(4'-t-butylphenylthio)-2,3,5,6-tetra(bromomethyl)benzene (3.4)8: A mixture of

3.3 (1.0 g, 2.16 mmol), N-bromosuccinimide (1.73 g, 9.72 mmol) and AIBN (30 mg) in dry CHCl3 (30 ml) was irradiated in UV-reactor (λ=354 mn) for 5 h under reflux. After cooling, succinimide precipitated as colorless crystals and was filtered off and the filtrate was washed with subsequently by sat. sodium bisulfite, sodium bicarbonate, and brine and then dried over magnesium sulfate. The evaporation of solvent gave the compound

3.4 (1.5 g, 90% crude) as a white solid. 1H NMR (CDCl3): δ 7.29 (d, 4H), 6.93 (d, 4H),

4.99 (s, 8H), 1.28 (s, 18H). MS (EI): 779(38), 701(23), 425(100), 345(91).

1,2-Bis(hydroxymethylene)-cyclohexane (3.5)13: A solution of 15.4 g (0.1 mol) of 1,2- cyclohexanedicarboxylic anhydride in 30 ml of THF was added dropwise to a suspension of 3.8 g LiAlH4 in 300 ml of THF at 0ºC. After the addition was complete stirring and heating under reflux was continued for 3 h, then 10 ml of water was added and the mixture was stirred overnight at RT. The products were filtered using a glass-fritted funnel. The filtrate was dried over magnesium sulfate and neutralized with NH4Cl and

40 the solvents evaporated yielding 10 g (69%) of crude 3.5 as light yellow viscous syrup, which was used in the next reaction without further purification. 1H NMR (CDCl3): 4.9-

4.6 (bs, 2H), 3.78-3.47 (m, 4H), 1.93-1.84 (m, 2H), 1.56-1.35 (m, 8H).

1,2-Bis(iodomethylene)-cyclohexane (3.6)13: Red phosphorus 1.7 g (55 mmol) was dissolved in 150 ml of toluene under Ar, iodine 20.6 g (164 mmol) was added to the mixture and the mixture was refluxed for 30 min. A solution of 10 g 3.5 in 30 ml of toluene then was added dropwise. The mixture was refluxed overnight. After cooling the mixture was washed sat. sodium thiosulfate solution, sat. sodium bicarbonate solution, and with brine. After drying with magnesium sulfate the solids were filtered off and the solvent evaporated to yield 23 g (92%) of crude 3.6 as light yellow liquid. MS (EI):

364(1.5), 237(20), 109(96), 67(100). 1H NMR (CDCl3): 3.14 (dd, 4H), 2.15-2.00 (m,

2H), 1.7-1.2 (m, 8H).

1,2-Bis(methylene)-cyclohexane (3.7)13: Iodide 3.6 (10 g, 28 mmol) was added to a solution of 4.6 g KOH (82 mmol) in 15 ml of dry methanol. The solution was stirred and refluxed for 3 h. After cooling the lower layer was discarded and the upper layer dried with magnesium sulfate. Filtering off the solids gave 1.8 g (60%) of 3.7 as yellow viscous liquid. MS (EI): 108(58), 93(98), 79(100). 1H NMR (CDCl3): 4.92 (s, 2H), 4.64 (s, 2H),

2.25 (s, 4H), 1.63 (s, 4H).

5,6,7,8-Tetrahydro-1,4-anthraquinone (3.9)13: A solution of 1.8 g (16.7 mmol) of diene

3.7 in 10 ml of DMSO was added to a solution of 2 g (18.5 mmol) of benzoquinone in 10 ml of dichloromethane under inert atmosphere and the mixture was stirred at RT for 1 day forming 3.8. After 1 day the mixture was exposed to atmospheric air and stirred at

RT for 2 more days then 30 ml of dichloromethane was added to the mixture and it was

41 washed with water 3 times (25 ml each), sodium bicarbonate, and brine. The solvent was removed and residue was purified by column chromatography using EtOAc/hexanes 1/10 mixture as eluent to yield 2.3 g (62%) of 3.9. MS (EI): 212(100), 197(47), 130(59). 1H

NMR (CDCl3): 7.73 (s, 2H), 6.87 (s, 2H), 2.86 (m, 4H), 1.81 (m, 4H).

7,16-Bis(4’-t-butylthiophenyl)heptacene-5,9,14,18-tetraone (3.13)8: To a clear solution of 3.4 (0.2 g, 0.260 mmol) in 8 mL of DMF was added 1,4-naphthalenedione (0.083 g,

0.52 mmol) and potassium iodide (0.425 g, 2.56 mmol). The resulting reddish brown suspension was heated and stirred at 150°C for 4h. After cooling to RT, the orange- yellow solids were filtered via vacuum filtration. The solids were washed with water,

1 acetone and dried to yield the product (0.104 g, 52%). H NMR (CDCl3): 10.12 (s, 4H),

8.42 (m, 4H), 7.86 (m, 4H), 7.19 (d, 4H), 7.14 (d, 4H), 1.18 (s, 18H). MALDI-MS:

766.22 [M+].

7,16-Bis(4’-t-butylthiophenyl)-5,9,14,18-tetrahydroxy-5,9,14,18-tetrahydroheptacene

(3.14)8: Compound 3.13 (0.1 g, 0.13 mmol) was added to a solution of 2,6- dimethylphenylmagnesium bromide (1M solution in THF, 2 ml, 2 mmol) in 25 ml of dry

THF at RT and the mixture was stirred overnight. To the reaction mixture was added 10 mL of sat. NH4Cl. The mixture was extracted with CH2Cl2 (2x20 mL), the organic layer was washed with water and dried over magnesium sulfate. The solvent was removed under vacuum until ~5 mL remained, at which point 100 mL of pentane were added resulting in the formation of a precipitate. The desired tetraol 3.14 was isolated by vacuum filtration (0.09 g, 75% crude) and used in the next reaction without further purification. MALDI-MS: 1190.05[M+].

7,16-Bis(4’-t-butylthiophenyl)-5,9,14,18-tetrakis(2’,6’-dimethylphenyl)heptacene

42 (3.15)8: To a mixture of crude 3.14 (0.02 g, 0.017 mmol) in 10 mL of 1,4-dioxane was added anhydrous SnCl2 (1.0 g, 5.26 mmol). To the suspension was added 1 mL of 10%

HCl and the resulting mixture was stirred at room temp without light for 0.5 h under Ar.

After completion of the reaction, dark precipitates were filtered under a Ar atmosphere with the complete exclusion of light. The solids were washed with 50 mL of water followed by 50 ml of methanol to yield 7,16-bis(4'-t-butylthiophenyl)-5,9,14,18-

1 tetrakis(2',6'-dimethylphenyl)heptacene 3.15 as dark green solids. H NMR (CDCl3):

9.34 (s, 4H), 7.41 (m, 8H), 7.32 (m, 4H), 7.16 (m, 4H), 7.09 (m, 4H), 7.01 (d, 4H), 6.69

(d, 4H), 1.94 (s, 24H), 1.37 (s, 18H). MALDI-MS: 1122.52 [M+].

1,2,3,4,12,13,14,15-Octahydro-8,19-bis(4'-t-butylphenylthio)nonacene-6,10,17,21- tetraone (3.10): To a clear solution of 3.4 (0.2 g, 0.256 mmol) in 8 mL of DMF was added 3.9 (0.109 g, 0.514 mmol) and potassium iodide (0.425 g, 2.56 mmol). The

resulting brown suspension was heated and stirred at 150°C for 4h. After cooling to RT, the orange solids were filtered via vacuum filtration. The solids were washed with water,

1 acetone and dried to yield the product 3.10. H NMR (CDCl3): 10.07 (s, 4H), 8.10 (s,

4H), 7.27 (d, 4H), 7.18 (d, 4H), 2.97 (s, 8H), 1.89 (s, 8H), 1.19 (s, 18H). MALDI-MS:

874.69 [M+].

1,2,3,4,12,13,14,15-Octahydro-8,19-bis(4'-t-butylphenylthio)nonacene (3.12):

Compound 3.10 (0.1 g, 0.114 mmol) was added to a solution of 2,6- dimethylphenylmagnesium bromide (1M solution in THF, 2 ml, 2 mmol) in 25 ml of dry

43 under vacuum until ~5 mL remained, at which point 100 mL of pentane were added resulting in the formation of a brown precipitate, which appeared not to be expected compound 3.11. When exposed to long-wave UV light the solution fluoresced with yellow-green light characteristic for arylthiosubstituted heptacenes. Compound wasn’t properly purified and was only characterized by MALDI-MS: 1287.90 [3.12 M+].

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Part II: Optoelectronic Properties of Metal-Semiconductor Nanocomposites in Strongly Coupled Regime Chapter 4. Exciton-Plasmon Interaction in Metal/Semiconductor Nanocomposites The morphology of Au/CdS(Se) colloidal nanocomposites

Combining metal and semiconductor domains in a single nanocrystal offer a unique opportunity for the development of hybrid nanoscale composites with functionalities that extend beyond those of isolated materials1,2,3,4,5,6,7,8,9. The presence of powerful carrier confinement in these nanoparticles joint with tunable geometry of the semiconductor-metal interface gives rise to novel optoelectronic properties that can potentially add up to a wide range of applications10,11,12.

Recently, Au/CdS and Au/CdSe heterostructures containing gold domains grown onto cadmium chalcogenide semiconductor nanorods (NRs) have come forward as a model system for studying such hybrid nanomaterials13,14,15,16,17,18. Besides being a system of choice for advancing synthetic procedures and exploring plasmon-exciton interactions, these nano-composites have also been considered for applications in areas of solution-processed solar cells19,20 and nanoscale wiring21. For instance, CdSe NRs with gold tips grown on both ends lead to a 105-fold increase in carrier conductivity in contrast to pristine CdSe nanorods placed on metal contacts21, which shows the potential of these heterostructures as nanoscale electrical interconnects. On the other hand, CdS NRs with gold tips grown on one end only can be employed as charge-separating components in photocatalytic and photovoltaic devices10.

To date, the deposition of gold domains onto CdS NRs has been shown using both thermal14 and light-assisted16 methods. The former approach was initially reported by Mokari et al.5 for the synthesis of Au/CdSe nano-composites and relied on the reduction of AuCl3 salts in a toluene

46 suspension of CdSe nanorods, dodecyldimethylammonium bromide (DDAB), and dodecylamine

(DDA). A partial reduction of Au ions in solution followed by their aggregation at lattice defects lead to the formation of small gold islands along lateral surfaces, as well as larger gold domains at both selenium (sulfur) and cadmium-rich facets of CdSe (S) NRs. The use of extended reaction times in this case, enabled a selective growth of gold domains onto one (matchsticks) or both (barbells) tips of semiconductor nanorods, with average sizes of gold domains reaching 10 nm only after 3 days of the reaction time. Such slow, tip-selective deposition was ascribed to electrochemical Ostwald ripening22, by which minor Au domains are dissolved in favor of the bigger tip. A significant improvement in the rate of Au growth onto one of the nanorod facets was recently reported by Carbone et. Al16 through a light-assisted gold deposition. It was demonstrated that ultraviolet (UV) irradiation of Au/CdS nano-composites facilitates the transfer of excited electrons from CdS NRs to Au domains, which accelerates the process of AuCl3 reduction at one of the tips. As a result, the light-assisted method can enable the growth of Au tips larger than 10 nm, which leads to efficient plasmon oscillations supported in absorption spectra of Au/CdS colloids through a typical plasmon peak.

Until recently, a limited control over the size and spatial arrangement of gold domains was reached by balancing the interchange of thermal deposition, electrochemical Ostwald ripening, and light-induced reduction of gold. Recent experiments17 proved that a careful combination of light- and thermal-assisted methods can, in principle, be used to control the structure of Au/CdS nano-composites. On the other hand, a more thorough approach producing a wider range of domain sizes and structural types of Au/CdS colloids and relying on a single synthetic variable was needed to expand Au/CdS morphologies and facilitate better reproducibility of experimental results. Zamkov group recently achieved this23 by developing a simple chemical route for

47 Figure 4.1 (a,b). HAADF-STEM images of Au/CdS heterostructures showing the color contrast between gold (bright) and semiconductor (dark) domains. (c, d). TEM images of the two areas shown in (a) and (b). growing gold domains onto CdS nanorods in oleylamine. The size of Au NCs can be fine tuned by regulating the temperature of the reaction mixture, while the shape of Au/CdS nanocomposites (matchstick or barbells) is controlled via the reaction rate (Figure 4.1)23. In contrast

48 to standard techniques for producing gold tips on semiconductor nanorods, this method does not employ DDA/DDBA reducer/surfactant combination and can furnish large-size Au tips without relying on UV-irradiation of the reaction mixture. Synthesized Au/CdS nano-composites demonstrated evaporation-induced self-assembly onto TEM grids (Figure 4.2)23 either through end-to-end or side-by-side coupling of Au domains, which should enable their incorporation into large-area optoelectronic devices.

Figure 4.2 Evaporation-induced assembly of Au/CdS nano-composites into (a) end-to-end “chains” and (b) two-dimensional superlattices

49 Exciton-plasmon interaction in Au/CdS nanocomposites

Bulk heterojunctions of metal and semiconductor materials have long been of importance to fundamental science and device engineering due to the unique relations of respective domains through the formation of the space-charge region,24 which gives rise to plentiful technological applications including Schottky barrier solar cells25, solid state lasers26, light emitting diodes27, and field effect transistors28. Of a particular significance is a fundamental interaction between semiconductor excitons and surface plasmons (SP) of metal nanoparticles (Figure 4.3)29, which results from the alteration of the exciton dipole moment because of local electromagnetic modes of SP.30,31 This interaction is facilitated by the nanoscopic nature of both material domains and has a unique effect on optoelectronic properties of a composite M-S system, which has been demonstrated to significantly change the energy flow that occurs across M-S junctions. For instance, the presence of plasmon radiative field in metals, caused by resonant oscillations of low- energy conduction electrons, can strongly influence the dynamics of excitons in semiconductor NCs via two distinct interaction mechanisms, including plasmon-exciton energy transfer, and modification of the local radiation field in S domains.

Numerous studies have explored the nature of exciton-plasmon interactions in a weak coupling regime, distinguished by the presence of a significant potential barrier at the interface of S and M components, which was accomplished experimentally by means of spacer molecules or non-epitaxial domain coupling. For these composites, the emission intensity in S domains was increased32,33,34,35,36,37,38,39 due to the plasmon-induced enhancement of S radiative rates,40,41 and was consequently studies towards improving

Figure 4.3 Depiction of Plasmon Resonance in Au nanocrystal

the process of light amplification in lasers42,43,44,45,44. Likewise, the plasmon-induced enhancement of the electric field in S domains has been shown to increase the absorption crosssection of S nanocrystals46,47, thus advancing the development of light-concentrating nanocomposites to assist the energy harvesting mechanism in photovoltaic and photocatalytic applications43. Finally a weak exciton-plasmon interaction, achieved through non-epitaxial coupling of M and S domains by means of a core/shell

51 morphology, has been utilized for controlling the spin of Au nanoparticles41, with potential application of this phenomenon in quantum information and spintronics48,49,50.

When metal and semiconductor components are joined directly, without using a spacer moiety, the potential barrier for charges residing in adjacent domains may become sufficiently diminished to allow inter-domain charge transfer, and corresponding mixing of electronic states at the M-S interface. In addition, the enhanced electromagnetic field of M plasmons in such strong coupling mode may create further alterations of carrier dynamics associated with shared oscillations of the excitation energy between S and M domains, often referred to as Rabi oscillations.51,52 In line with these expectations, several recent experimental reports have indicated that directly joined M-S nanocomposites demonstrate optoelectronic properties that are significantly different from those of isolated M and S nanoparticles, and are repeatedly overlooked by existing theoretical models, which cast doubt on the nature of exciton–plasmon interactions in epitaxial M-S nanocomposites. For example, it has been reported that a direct growth of Au tips onto

CdSe nanorods results in quenching of the fluorescence in the S domain caused by an ultrafast transfer of S electrons into Au. Nonetheless, a different explanation based on the existence of sub-band gap states at Au/CdS interface cannot be excluded. Furthermore, there is basically no theoretical report for the energy-dependent changes in the absorbance profile of the S component emerging as a result of direct coupling to M domain. Explaining these phenomena requires a deeper knowledge of basic electron processes that appear in strongly coupled M-S nanocomposites. Finding answers to these questions will help the formulation of design principles needed for the achievement of epitaxial M-S nanocomposites in future nano-electronic device architectures.

52 Recently Zamkov group and co-workers performed29 femtosecond transient absorption spectroscopy studies to investigate the nature of exciton-plasmon interactions in

Figure 4.4 (a) Steady-state absorption of Au/CdS heterostructures comprising 15.7- nm Au domains. A representative TEM image is shown in the insert. (b) Transient absorption spectra of 15.7-nm-Au/CdS nanocomposites resulting from the excitation at λ=400 nm with 120 fs pump pulses. The recovery of ΔA shows an expected plasmonic feature, which resembles the corresponding TA dynamics of isolated Au

53 epitaxially coupled Au/CdS nanocomposites. It was shown that mixing of conduction states at the interface of metal and semiconductor domains leads to the onset of unique optoelectronic properties (Figure 4.4) 29, which are not observable in non-epitaxial metal- semiconductor heterostructures. One of these properties is the complete suppression of

Figure 4.5 Plasmon suppression in Au/CdS nanocomposites

surface plasmon oscillations in Au domains, which is ascribed to the delocalization of plasma electrons into CdS fraction of the structure (Figure 4.5)29. Thorough analyses of the transient kinetics as well as steady-state absorption data uncovered that resonant plasmon oscillations are quenched in nanocomposites comprising small-diameter Au domains (< 6 nm), which differs from the plasmon dynamics in weakly coupled M-S systems. In addition to plasmon suppression, it was also found that the formation of bound electron-hole pairs (excitons) in CdS fraction of the structure is strongly hindered as a result of the presence of gold domains. This phenomenon was explained in term of

54 Figure 4.6 Energy diagram showing the effect of strong inter-domains coupling on electronic energies in Au/CdS heterostructures, which gives rise to the onset of new electronic states. First, the interface of Au and CdS materials can support the formation of trap states, which energies are located between the Fermi level of Au nanoparticles and the conduction band of CdS NCs. Photoinduced filling of these states is manifested by the spectrally-wide photoinduced absorption signal observed in TA measurements. Second, excitation bands of both Au and CdS domains are “bent” at the interface similar to those of bulk heterostructures to account for delocalization of carriers across inter-domain boundary.

the proposed mixing of conduction states at the boundary of M and S materials, which creates favorable conditions for valence electrons in CdS to be excited directly into interfacial states hence diminishing the probability of band gap transitions (Figure 4.6)29.

By using ultrafast spectroscopy it was demonstrated that it is the proposed inter-domain mixing of electronic states rather than the electron transfer process, which results in

55 fluorescence quenching in CdS domains. Further evidence of the proposed interfacial state mixing is supplied by the observation of a characteristic, long-lived photoinduced absorption (positive TA signal), which corroborates the interfacial character of excited states in Au/CdS heterostructures.

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60 Chapter 5. Synthesis and Optoelectronic Properties of Au/CdS and Au/CdSe/CdS Nanocomposites

Au/CdS Nanocomposites

Isolated gold and silver nanocrystals’ synthesis via a reduction of corresponding metal salts by oleylamine ligands has been previously demonstrated in several reports1,2,3,4,5. In these works, the reaction of AuCl3 or HAuCl4 with oleylamine was proceeded in a single phase solvent (e.g. toluene) at 60-80 °C leading to a slow reduction of Au(III) ions to Au(I) and consequently to neutral Au clusters. Careful observation of the growth kinetics during the synthesis has revealed5,6 that oleylamine molecules created complex aggregates with gold salt immediately upon mixing of these two agents. At a higher temperature (< 80 °C), these complexes disintegrated into very small particles, which ultimately recombined together into bigger and thermally stable nanocrystals with an average size varying from 5 to 10 nm. Within this method, the nanoparticle growth rate could be altered either by adjusting the concentration of oleylamine in the toluene solution or by changing the reaction temperature. Alternatively, in the present synthesis the control over nanoparticle growth was simplified by using oleylamine both as a reducing agent and the reaction solvent. This results in a set ratio of Au to oleylamine in the solution, such that the nanoparticle growth rate can be adjusted only by varying the temperature of the reaction mixture.

In a typical synthesis, CdS nanorods were initially made by growing CdS linear extensions from

(000±1) faces of small-diameter CdS seeds7, according to a procedure modified from Ref8.

Synthesized CdS NRs were cleaned several times with toluene/ethanol combination and saved for later use. For the deposition of Au, 3 ml of oleylamine were degassed in vacuum at 120°C for 1 hour and consequently cooled down to 60°C. After purification, all of the oleylamine then injected into

61 the three-neck flask containing 0.0113g of Au (III) chloride, and the temperature was increased to 60

°C. When the solution became yellow and clear (approximately after 5 min of heating), 4 mg of CdS nanorods in 0.1 ml of toluene were injected via syringe. The exact amount of CdS NRs for the synthesis of Au/CdS nano-composites was calculated using an empirical approach, whereby the nanorod absorption at 450 nm (excitonic feature) times the volume of the colloidal suspension (in mL) was set to be in the range of 18 to 25. For instance, Au/CdS nano-composites shown in Figure

5.1a9 were synthesized using 20 units of CdS NRs (corresponding to 4 mg). Upon injection of CdS

NRs into AuCl3/oleylamine mixture, the heat was brought to the flask and the solution was heated at a rate of ~ 1.5-3 °C/min. When the desired temperature was reached, the reaction was quenched by removing the heat source and injecting 5 ml of room-temperature toluene into the mixture.

The effect of the reaction temperature on the size of gold tips is demonstrated by transmission electron microscopy (TEM) images of Au/CdS nano-composites in Figure 5.19. Heating the

AuCl3/oleylamine mixture to approximately 105 °C facilitates the formation of 4-5 nm Au domains on both ends of CdS nanorods (Figure 5.1c)9. Some side-wall growth of minor gold nanocrystals is also observed at this stage and is ascribed to the reduction-induced assembly of Au clusters at defects of the CdS lattice. The two examples of Au/CdS nano-composites in Figure 5.1 (b,c)9 prove that the growth of Au NCs along defects of the CdS lattice can be curbed in nanorods showing a better lattice quality. Using high-resolution TEM analysis the defect growth kinetics was captured through monitoring a step-by-step formation of small Au nanoparticles from surface-trapped Au ions. Deposition of large-diameter gold tips onto one of the nanorod facets (Figure 5.1a,b)9 is accomplished by raising the temperature of the reaction

62 Figure 5.1 TEM images of Au/CdS nano-composites fabricated using various synthetic conditions. (a). Matchstick-shaped Au/CdS nano-composites comprising plasmonic size gold domains. (b). HR-TEM images of nano-composites shown in (a). (c). barbell-shaped Au/CdS nano-composites comprising 4.5-nm gold tips. (d). Au/CdS hetero-dimers consisting of a single gold tip grown onto a round-shaped CdS NCs (e). HR-TEM image of an Au/CdS heterojunction. (f). CdS NRs comprising multiple Au domains.

63 mixture to above 120 °C. This facilitates the dissolution of smaller Au domains and simultaneous aggregation of larger Au nanoparticles via electrochemical Ostwald ripening10. It was essential to maintain the heating rate below 3.3 °C/min to ensure the growth of large-size Au domains, while heating of the solution at a faster rate (>4 degrees/minute) leads to the formation of isolated Au NCs that use up a large portion of AuCl3 from the solution hence preventing the development of Au tips.

The crystalline nature of fabricated nano-composites is confirmed by high-resolution TEM (HR-

TEM) images in Figures 5.1b,e9. Both CdS and Au domains show characteristic lattice fringes indexed to respective spacing values of hexagonal wurtzite and face-centered cubic (fcc) crystal phases. The analysis of several samples has shown that the lattice of CdS NC’s was usually maintained during the high-temperature synthesis with the exception of some scattered lattice irregularities denoted by small Au domains developing along the defects and the opposite tip of CdS

NC’s. Such side-wall nucleation of gold displayes a set of small-size (1 nm < d < 2 nm) gold nanocrystals that form on the surface of a CdS NC’s. The structure of large-diameter gold tips grown onto one side of CdS NC’s is demonstratd in Figure 5.1e9. Judging by the spacing of (111) and

(200) atomic planes, the growth of Au tips occurs in the face-centered cubic phase with multiple twinnings of the nanoparticle shape.

The effect of the reaction temperature and the heating rate on the shape of Au domains is further investigated in Figure 5.29. The initial enhancement in the average size of gold tips further than those of defect-localized domains was observed at T ≈ 95-100 °C. The heating rate of the solution plays a critical role at these temperatures determining whether the tip grows on one or both ends of

CdS nanorods, such that a faster heating rate (≈ 3 degrees/minute) leads to the growth of barbell- shaped nano-composites (Figure 5.2b)9, while slow heating (~ 1.5-2.3 degrees/minute) facilitates

64 Figure5.2 TEM images and statistical size distributions of Au/CdS nano-composites grown at five different temperatures. Average diameters of Au domains for these samples were (a). d=3.15 nm (b). d=4.87 nm (c). d=8.7 nm (d). d= 12.6 nm (e). d= 15.67 nm.

65 the growth of Au/CdS matchsticks (Figure 5.2a)9. The average size of Au domains was measured to be 3.2 nm (matchsticks) and 4.9 nm (barbells) with theassociated dispersion of sizes of 12% and

11%, correspondingly. For the case of nano-barbells, additional raise in the temperature of the solution to 120 °C results in the dissolution of one of the tips and simultaneous selective deposition on the other, leading to an nearly exclusive development of matchstick-shaped heterostructures

(Figure 5.2c)9, regardless of the heating rate. As a consequence, the largest value of the tip diameter for nano-barbells obtained by this method was limited to 6 nm (at ~ 110 °C), due to the conversion of these structures into matchstick-shaped composites at elevated temperatures. The average size of the larger tip grew to 12.6 nm, upon additional increase of the reaction temperature to 130 °C, whereas the shape of Au domains diverged from the spherical. This tendency is evidently observed in the final sample synthesized at 140 °C (Figure 5.2e)9, where a variety of Au shapes, including triangular, pyramidal, and cylindrical can be recognized. The raise of the reaction temperature was supplemented by the growing dispersion of NC sizes, which altered from 7% for 8.0-nm tips to 10% for 15-nm domains. Further research will be needed to optimize the reported synthetic protocols in order to allow lower dispersion of shapes for Au tips with diameters larger than 15 nm. For instance, introduction of additional stabilizing ligands other than oleylamine may be helpful in reaching the desired size-focusing of Au nanoparticles.

66 Changes in optical properties of Au/CdS nano-composites during the development of Au domains are investigated in Figure 5.39. At T=90 °C, the absorption profile of Au/CdS nanoparticles shows clear changes from that of pure CdS nanorods, with the major difference occurring in the wavelength range above λ=550 nm, where the absorption of heterostructures is enhanced. This red tail is ascribed to the contribution from interfacial trap states as well as to optical transitions in small-size gold clusters growing on the surface of CdS nanorods. The diminishing of the excitonic feature in

CdS NRs at this stage is ascribed to the delocalization of carriers into small gold clusters and oleate complexes that form on the semiconductor surface. Spectral changes that are taking place during the initial heating of the mixture are accompanied by the visible change of the solution color from

Figure 5.3 Evolution of the Au/CdS absorption spectra during the synthesis. The initial spectrum (blue curve) represents the absorbance of pure CdS nanorods. The average size of gold domains for the subsequent spectra are as follows: green ≈ 2.9 nm, olive ≈ 8.1 nm, red ≈ 15.7 nm.

67 yellow to light brown, whereas additional heating of the flask to above 100 °C leads to further darkening of the solution and corresponding raise in the amplitude of the absorption tail (for λ > 500 nm). At this point of the reaction, the average size of the Au tip reaches 4 nm, giving rise to a small absorption feature at λ=550 nm, correlated to the surface plasmon resonance in Au nanoparticles11,12,13. With increasing size of Au domain this peak becomes narrower and more prominent, which is supplemented by a gradual change the solution color from dark brown to violet.

For tips greater than 9 nm in diameter, the amplitude of the plasmon peak became comparable to that of an excitonic absorption feature in CdS nanorods (Figure 5.3, red curve)9, whereas, for Au NCs with sizes of 20 nm and greater this absorption feature befall inhomogenously broadened due to large dispersions of Au shapes (prisms, rods, spheres, etc.). It is noteworthy that upon development of Au domains, the band-gap fluorescence of CdS nanorods becomes entirely quenched. Such suppression of emission in Au/CdS nano-composites has been observed previously14 and is ascribed to the fast transfer of excited carriers from CdS into Au domains.

The illustrated technique for the deposition of gold tips onto CdS is free of the nanorod aspect ratio and can be expanded to nanocrystals with other spatial geometries. For instance, heterostructures containing spherical CdS domains can be potentially used as model systems for studying non-linear optical behavior on nanoscale15,16, whereby contributing to the fundamental understanding of exciton-plasmon coupling in metal-semiconductor nano-composites. Several TEM images of

Au/CdS hetero-dimers comprising low- aspect-ratio wurtzite CdS NCs used for seeding the growth of gold domains at three different reaction temperatures are shown in Figure 5.49. Similar to nanorods, the low-temperature (T<95 °C) deposition of gold tips onto almost spherical CdS NCs appears primarily at surface defects that confine the assembly of small gold

68 Figure 5.4 Au/CdS nano-composites synthesized from low-aspect-ratio nanocrystals. The growth of nanoparticles was terminated at reaction temperatures of: (a) 145 °C, (b) 130 °C, and (c) 95 °C.

69 clusters, as shown in Figure 5.4c.9 Raising the reaction temperature to above 130 °C leads to dissolution of some smaller domains in favor of a single Au nanoparticle that can grow both on facets and lateral walls of elongated CdS NCs (Figure 5.4a,b).9 The size of these Au tips can be adjusted in the range of 5 to 15 nm, utilizing the same range of reaction temperatures as in the case of nanorods-shaped semiconductor domains. The noticeable similarity between the shape of gold domains of synthesized Au/CdS hetero-dimers and those of CdS nanorods indicates that the rate of

Au deposition onto CdS is not strongly connected with the nanorod aspect ratio.

Au/CdSe/CdS Nanocomposites

Based on procedures developed for synthesis of Au/CdS nanocomposites and the fact that Au growth onto surface of CdS does not depend on shape of nanocrystal by this methodology we pursued study of gold/semiconductor nanocomposites comprising CdSe/CdS core-shell NC’s as semiconductor domain. Since suppression of emission in Au/CdS nano-composites has been observed previously17 and is ascribed to the fast transfer of excited carriers from CdS into Au domains we wanted to investigate whether excited carriers from CdSe with a substantial shell of different band gap material

(i.e. CdS) would be sufficient enough to prevent the transfer and consequently simultaneously observe exciton emission along with plasmon resonance by proposed nanocomposites.

Three different methods for CdS shell growth onto CdSe core were developed each varying in number of shell material injections and amount of the material in every injection. In all three methods shells were grown onto seeds prepared by modified procedure from Ref. 7. In method A initial thin shell (1-2 nm) was grown onto CdSe seeds by adding seeds and sulfur precursors to Cd precursors at very high temperature (380 °C) and subsequent shell material was added with two injections at the same temperature each carrying as much Cd and sulfur precursors as for the initial

70 shell development yielding relatively large nearly spherical nanocrystals (Figure 5.5). These NC’s were later used to grow Au domains.

Figure 5.5 CdSe/CdS core-shell nanocrystals synthesized by method A

In method B different approach was pursued. Core-shell NC’s with initial thin shell grown onto

CdSe seed by adding seeds and sulfur precursors to Cd precursors at very high temperature were purified and used as seeds for subsequent similar procedure with this procedure being repeated once more. This method furnished nanocrystals with irregular shape and one of the faces growing nanorod-like CdS linear extension (Figure 5.6). These NC’s were not used for gold deposition due to their irregular shape.

Method C used approach in which core-shell NC’s with initial thin shell grown onto CdSe seed

Figure 5.6 CdSe/CdS core-shell nanocrystals synthesized by method B

by adding seeds and sulfur precursors to Cd precursors at very high temperature and consequent addition of shell material via multiple injections with increasing amount of Cd and sulfur precursors for growth of several monolayers of CdS. This yielded triangular shaped NC’s which were then used for growing Au tips.

Figure 5.7 Au/CdSe/CdS nanocomposites with CdSe/CdS core-shell nanocrystals synthesized by method C

Figure 5.8 Au/CdSe/CdS nanocomposites with CdSe/CdS core-shell nanocrystals synthesized by method A

Fabricated NC’s were washed with either hexane/ethanol or toluene/methanol combination and stored for later use. For the deposition of gold, 3 ml of oleylamine were degassed in vacuum at

120°C for 1 hour and subsequently cooled down to 60°C. After purification, all of the oleylamine then injected into the three-neck flask containing 0.0124g of gold (III) chloride, and the temperature was raised to 60 °C. When the solution became yellow and clear (approximately after 5 min of heating), 5 mg of CdSe/CdS nanocrystals in 0.1 ml of toluene were injected via syringe. The exact amount of CdS NRs for the synthesis of Au/CdSe/CdS nano-composites was calculated using an empirical approach, whereby the nanorod absorption at 450 nm (excitonic feature of CdS) times the volume of the colloidal suspension (in mL) was set to be in the range of 18 to 25. For instance,

Au/CdSe/CdS nano-composites shown in Figure 5.8 were synthesized using 25 units of CdSe/CdS

NCs (corresponding to 5 mg). Upon injection of CdSe/CdS NCs into AuCl3/oleylamine mixture, the heat was brought to the flask and the solution was heated at a rate of ~ 1.5-3 °C/min. When the

73 desired temperature was reached, removing the heating mantle from the flask and injecting 6 ml of room-temperature toluene quenched the reaction.

In conclusion, we have developed several chemical routes to CdSe/CdS core-shell NCs with each of them leading to different shape of nanocrystals. Also a simple chemical method for growing Au domains onto CdS nanorods and CdSe/CdS NCs in oleylamine was developed. The size of Au NCs can be precisely tuned by adjusting the temperature of the reaction mixture, while the shape of

Au/CdS nano-composites (matchstick or barbells) is controllable via the reaction rate. All of the aforementioned nanocomposites had quenched emission and further study is necessary to determine whether a significantly wider band gap shell material can prevent fast transfer of excited carriers from semiconductor into metal domains.

Experimental Section

Chemicals. Gold (III) chloride (99%, Aldrich), oleylamine ( tech., 70%, Aldrich), sulfur (99.999%,

Acros), 1-octadecene (ODE, tech., 90%, Aldrich), cadmium oxide (99.99%, Alrdich), oleic acid

(OA, tech., 90%, Aldrich), tri-n-octylphosphine (TOP, 97%, Strem), tri-n-octylphosphine oxide

(TOPO, 99%, Aldrich), n-octadecylphosphonic acid (ODPA, PCI Synthesis), n-hexylphosphonic acid (HPA, PCI Synthesis), octadecylamine (ODA, 90%, tech., Acros), hexane (anhydrous, 95%,

Aldrich), methanol (99.8+%, EMD), chloroform (anhydrous, 99+%, Aldrich), ethanol ( anhydrous,

95%, Aldrich) and toluene ( anhydrous, 99.8%, Aldrich) were used as purchased. All reactions were performed under argon atmosphere using the standard Schlenk technique.

Characterization. UV-vis absorption and photoluminescence (PL) spectra were recorded using

CARY 50 Scan spectrophotometer and Jobin Yvon Fluorolog FL3-11 fluorescence spectrophotometer. PL quantum yield of hetero-NCs was determined relative to known QYs of

74 several organic dyes excited at 400-440 nm. High resolution transmission electron microscopy measurements were carried out using JEOL 311UHR operated at 300 kV. Specimens were prepared by depositing a drop of nanocrystal hexane solution onto a formvar-coated copper grid and letting it dry in air.

Synthesis of CdS Nanocrystals: CdS seeds were fabricated according to the procedure reported in ref 18.

In a typical synthesis, the mixture of cadmium oxide (0.0384 g, 0.3 mmol), OA (0.9 mL), and

ODE (12.0 mL) in a 50 mL 3-neck flask was heated to 300° C until the solution turned optically clear and colorless. At this point, a sulfur precursor solution made by dissolving sulfur powder

(0.048 g, 1.5 mmol) in ODE (4.5 mL) at 200° C was quickly injected, and the temperature was stabilized at 260° C for the nanocrystal growth. The reaction was stopped after 5-9 min, and nanocrystals were isolated from the growth solution by precipitation with methanol, followed by repeated hexane/methanol extractions. The average diameter of fabricated CdS nanocrystals was in the range of 3.5-4.0 nm, depending on the growth time.

Synthesis of CdS Nanorods. The amount of CdS seeds for the synthesis of CdS nanorods was calculated using an empirical approach, whereby the product of the particle absorption at 400 nm

(excitonic feature) and the volume of the colloidal suspension (in mL) was set to be in the range of 5-10. For instance, CdS nanorods shown in Figure 1a were synthesized using 8 units of CdS seeds. Overall, it was determined that using lower amounts of CdS seeds generally yields high- aspect ratio nanorods, however, when less than 5 units of CdS is used for seeding, a small amount of tetrapods can also form during the reaction, alongside high-aspect-ratio nanorods.

In a typical synthesis of CdS nanorods, CdS (8 units) seed powder was dispersed in 1.8 mL of

TOP and subsequently introduced into the sulfur injection solution at 60° C, previously prepared

75 by dissolving sulfur (0.120 g, 3.75 mmol) in TOP (1.81 mL) at 200° C. Separately, the mixture of cadmium oxide (0.060 g, 0.47 mmol), TOPO (3.0 g), ODPA (0.290 g), and HPA (0.080 g) in a

50 mL 3-neck flask was exposed to vacuum at 150° C for ca. 30 min. Subsequently, the system was switched to Ar flow and heated to above 350° C until the solution turned optically clear and colorless. At this point, TOP (1.81 mL) was added to the flask as the Cd precursor coordinating solvent. The rod growth was initiated by a quick injection of the seed/sulfur solution at 380° C.

After the temperature recovered to 350° C the nanorods were allowed to grow for an additional

7-9 min. Purification of CdS nanorods was similar to those of CdS seeds.

Au growth on CdS Nanorods. In a typical procedure, oleylamine (3 ml) was degased at 120°C and pumped for about 30 min to remove any residual air from the system. At this step, the system was switched to Ar flow and cooled down to 90°C, before it was injected via syringe into the reaction flask which contained Gold (III) chloride (0.0113g, 0.0373 mmol) and started heating. At 60-70°C, when Gold (III) chloride was dissolved in oleylamine, 0.5 mL of toluene- suspended CdS nanorods (60 times diluted injection volume showed the excitonic absorption peak of 0.75, which corresponds to approximately 12 mg of dry nanorods) was added via syringe to the reaction flask which was slowly heated to 140°C in 0.1 mL amounts every 10 min. After

30-40 min, the reaction was stopped by cooling the flask to 50°C and adding excess toluene.

Purification of Au/CdS tipped nanocrystals. The final product was precipitated from toluene by adding ethanol (1:1) at 50°C. The subsequent cleaning was done using toluene/ethanol extraction.

Synthesis of CdSe/CdS NCs Method A. The amount of CdSe seeds for the synthesis of

CdSe/CdS NCs was calculated using an empirical approach, whereby the product of the particle

76 absorption at 586 nm (excitonic feature) and the volume of the colloidal suspension (in 1 mL) was set to be in the range of 10-15. For instance, CdSe/CdS NCs shown in Figure 5.5 were synthesized using 12 units of CdSe seeds. Overall, it was determined that using lower amounts of

CdSe seeds generally yields high-aspect ratio spherical nanocrystals, however, when less than 7 units of CdSe is used for seeding, a significant amount of nanorods is forming in the reaction.

In a typical synthesis of CdSe/CdS NCs, CdSe seed powder was dispersed in 1.8 mL of TOP and subsequently introduced into the sulfur injection solution at 60° C, previously prepared by dissolving sulfur (0.120 g, 3.75 mmol) in TOP (1.81 mL) at 200° C. Separately, the mixture of cadmium oxide (0.060 g, 0.47 mmol), TOPO (3.0 g), ODPA (0.290), and HPA (0.080 g) in a 50 mL 3-neck flask was exposed to vacuum at 150° C for ca. 30 min. Subsequently, the system was switched to Ar flow and heated to above 350° C until the solution turned optically clear and colorless. At this point, TOP (1.81 mL) was added to the flask as the Cd precursor coordinating solvent. The shell growth was initiated by a quick injection of the seed/sulfur solution at 380° C.

After the temperature recovered to 350° C the shell were allowed to grow for another 7-9 min.

Additional sulfur and Cd precursors injection solutions of the same amounts as for initial shell were added after about 9 min. and this procedure repeated once more in another 10 min.

Purification of CdSe/CdS NCs was similar to those of CdS NC’s.

Synthesis of CdSe/CdS NCs Method B. The amount of CdSe seeds for the synthesis of

CdSe/CdS NCs was calculated using an empirical approach, whereby the product of the particle absorption at 586 nm (excitonic feature) and the volume of the colloidal suspension (in mL) was set to be in the range of 10-15. For instance, CdSe/CdS NCs shown in Figure 5.6 were synthesized using 12 units of CdSe seeds. Overall, it was determined that using lower amounts of

77 CdSe seeds generally yields high-aspect ratio spherical nanocrystals, however, when less than 7 units of CdSe is used for seeding, a significant amount of nanorods is forming in the reaction.

After the temperature recovered to 350° C the shell were allowed to grow for another 7-9 min.

These nanocrystals were purified and used as seeds in subsequent procedure with the same amount of sulfur and Cd precursors as for initial shell growth and resulting NC’s were once more used as seeds after purification in procedure using same amounts of Cd and sulfur as for initial shell growth.

Synthesis of CdSe/CdS NCs Method C. The amount of CdSe seeds for the synthesis of

CdSe/CdS NCs was calculated using an empirical approach, whereby the product of the particle absorption at 586 nm (excitonic feature) and the volume of the colloidal suspension (in mL) was set to be in the range of 10-15. For instance, CdSe/CdS NCs shown in Figure 5.7 were synthesized using 12 units of CdSe seeds. Overall, it was determined that using lower amounts of

CdSe seeds generally yields high-aspect ratio spherical nanocrystals, however, when less than 7 units of CdSe is used for seeding, a significant amount of nanorods is forming in the reaction.

78 In a typical synthesis of CdSe/CdS NCs, CdSe seed powder was dispersed in 1.8 mL of TOP and subsequently introduced into the sulfur injection solution at 60° C, previously prepared by dissolving sulfur (0.120 g, 3.75 mmol) in TOP (1.81 mL) at 200° C. Separately, the mixture of cadmium oxide (0.060 g, 0.47 mmol), TOPO (3.0 g), ODPA (0.290), and HPA (0.080 g) in a 50 mL 3-neck flask was exposed to vacuum at 150° C for ca. 30 min. Subsequently, the system was switched to Ar flow and heated to above 350° C until the solution turned optically clear and colorless. At this point, TOP (1.81 mL) was added to the flask as the Cd precursor coordinating solvent. The shell growth was initiated by a quick injection of the seed/sulfur solution at 380° C.

After the temperature recovered to 350° C the shell were allowed to grow for another 7-9 min.

For additional injections solution of sulfur (0.360 g) in TOP (5.43 mL) at 200° C was made.

Separately, the mixture of cadmium oxide (0.180 g), TOPO (9.0 g), ODPA (0.870), and HPA

(0.240 g) in a 50 mL 3-neck flask was exposed to vacuum at 150° C for ca. 30 min. After initial shell growth 0.4 ml of sulfur solution at RT and 0.6 ml of Cd solution at 150° C were added simultaneously to the reaction and shell allowed to grow for 10 more min. The shell material solutions were then injected every 10 min. with each consequent injection increasing amount of solution by 0.1 ml for sulfur and 0.15 ml for Cd respectively.

Au growth on CdSe/CdS NCs. In a typical procedure, oleylamine (3 ml) was degased at 120°C and pumped for about 30 min to remove any residual air from the system. At this step, the system was switched to Ar flow and cooled down to 90°C, before it was injected via syringe into the reaction flask which contained Gold (III) chloride (0.0124g, 0.04 mmol) and started heating.

At 60-70°C, when Gold (III) chloride was dissolved in oleylamine, 0.5 mL of toluene-suspended

CdS nanorods (60 times diluted injection volume showed the excitonic absorption peak of 0.75, which corresponds to approximately 12 mg of dry nanorods) was added via syringe to the

79 reaction flask which was slowly heated to 140°C in 0.1 mL amounts every 10 min. After 30-40 min, the reaction was stopped by cooling the flask to 50°C and adding excess toluene.

Purification of Au/CdSe/CdS nanocrystals. The final product was precipitated from

hexane by adding methanol (1:1) at 50°C. The subsequent cleaning was done using hexane

/methanol extraction.

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