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Home , Allotropes of carbon, Fullerene chemistry

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1537

Carbon – from to Functionalised Materials

Fullerene-Ferrocene Oligomers, Modification and Deposition

MICHAEL NORDLUND

ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-0019-1 UPPSALA urn:nbn:se:uu:diva-327189 2017 Dissertation presented at Uppsala University to be publicly examined in A1:107a, BMC, Husargatan 3, Uppsala, Friday, 22 September 2017 at 09:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Mogens Brøndsted Nielsen (Copenhagen University, Department of ).

Abstract Nordlund, M. 2017. Nanostructures – from Molecules to Functionalised Materials. -Ferrocene Oligomers, Graphene Modification and Deposition. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1537. 64 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0019-1.

The work described in this thesis concerns development, synthesis and characterisation of new molecular compounds and materials based on the carbon allotropes fullerene (C60) and graphene. A stepwise strategy to a symmetric ferrocene-linked dumbbell of fulleropyrrolidines was developed. The versatility of this approach was demonstrated in the synthesis of a non- symmetric fulleropyrrolidine-ferrocene-tryptophan triad. A new tethered bis-aldehyde, capable of regiospecific bis-pyrrolidination of a C60-fullerene in predominantly trans , was designed, synthesised and reacted with glycine and C60 to the desired N-unfunctionalised bis(pyrrolidine)fullerene. A catenane dimer composed of two bis(pyrrolidine) was obtained as a minor co-product. From the synthesis of the N-methyl analogue, the catenane dimer could be separated from the monomeric main product and fully characterised by NMR . Working towards organometallic fullerene-based molecular wires, the N-unfunctionalised bis(pyrrolidine)fullerene was coupled to an activated carboxyferrocene- fullerene fragment by amide links to yield a ferrocene-linked fullerene trimer, as indicated by from reactions carried out at small scale A small library of conjugated diarylacetylene linkers, to be coupled to C60 via metal-mediated hydroarylation, was developed. Selected linker precursors were prepared and characterised, and the hydroarylation has been adapted using simple arylboronic acids. Few-layer graphene was prepared and dip-deposited from suspension onto a piezoelectric polymer substrate. Spontaneous side-selective deposition was observed and, from the perspective of non-covalent interaction, rationalised as being driven by the inbuilt polarization of the polymer. Aiming for selectively edge-oxidized graphene, a number of graphitic materials were treated with a combination of ozone and under sonication. This mild, metal-free procedure led to edge-oxidation and exfoliation with very simple isolation of clean materials indicated by microscopy, spectroscopy, and thermogravimetric analysis.

Keywords: Fullerenes, Graphene, Deposition, Functionalization, Organometallic complexes

Michael Nordlund, Department of Chemistry - BMC, , Box 576, Uppsala University, SE-75123 Uppsala, Sweden.

© Michael Nordlund 2017

ISSN 1651-6214 ISBN 978-91-513-0019-1 urn:nbn:se:uu:diva-327189 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-327189)

In this house, we obey the laws of thermodynamics!

– The Simpsons

To my family.

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals:

I Nordlund, M., Andersson, C.-H., Grennberg, H. Mono- and diamides of 1,1’-dicarboxyferrocene: a stepwise approach to ferrocene-interlinked fullerene oligomers. Manuscript II Nordlund, M., Günther, T., Andersson, C.-H., Grennberg, H. Catenane dimer formation in tether-assisted trans-bis- pyrrolidination of [60]fullerene. Manuscript III Nordlund, M., Andersson, C.-H., Grennberg, H. Towards fullerene-based molecular wires: amide-interlinked ferro- cene-fullerene oligomers. Manuscript IV Nordlund, M., Kazen Orrefur, J., Grennberg, H. Diarylacety- lene-linked fullerene dumbbells via metal-catalysed cou- plings. Preliminary manuscript V Nordlund, M., Bhandary, S., Sanyal, B., Löfqvist, T., Grenn- berg, H. Side-selective self-assembly of graphene and FLG on piezoelectric PVDF from suspension. Journal of Physics D: Applied Physics, 2016, 49(7):07LT01 VI Lundstedt, A., Nordlund, M., Ahlberg, P., Grennberg, H. Edge oxidation of using a combined hydrogen peroxide - ozone treatment under sonication conditions. Manuscript Reprints were made with permission from the respective publishers.

Contribution Report

The author wishes to clarify his contribution to the research presented in papers I-VI in the thesis.

I Contributed to formulating a research problem. Conducted and optimised parts of the synthesis and characterisation. Wrote the first draft and contributed to the writing of the final manu- script. II Contributed to formulating a research problem. Designed the synthesis of the improved tether. Planned and performed a significant part of the experimental work. Supervised part of the synthesis. Contributed to the writing of the manuscript. III Contributed to the design of the target compound. Performed parts of the experimental work and characterisation. Signifi- cantly contributed to the writing of the manuscript. IV Formulated the research problem and planned the project. Per- formed a significant part of the experimental work. Supervised part of the synthesis. Wrote the first draft and contributed to the writing of the final manuscript. V Performed all of the experiments, Raman, IR, UV/vis and LOM characterisation, and contact angle measurements. Sig- nificantly contributed to the writing of the manuscript. VI Contributed to formulating a research problem and planning. Co-performed all of the experiments. Performed all LOM, UV/vis, and Raman characterisation. Significantly contributed to the writing of the manuscript. The author also wishes to point out that parts of this thesis are based on a previous thesis, which was submitted for his licentiate degree in 2015. Up- dated content has been largely rewritten, expanded with current results, and adapted to the format of a doctoral thesis. Nordlund, M., Carbon Nanostructures and : Graphene on Polymer Substrates and Organometallic Fullerene Oligomers. Fil. Lic. thesis, Acta Universitatis Upsaliensis, 2015

Contents

1 Introduction ...... 11 1.1 Carbon allotropes ...... 12 1.2 Fullerene background ...... 14 1.3 Functionalisation of C60-fullerenes ...... 15 1.4 Graphite and graphene ...... 18 1.4.1 Production of graphene ...... 19 1.4.2 Deposition of graphene from suspension ...... 22 1.5 Characterisation methods ...... 23 1.5.1 Molecules ...... 23 1.5.2 Materials ...... 24 2 Molecules to materials – a fullerene story (Papers I-IV) ...... 26 2.1 Introduction ...... 26 2.2 Stepwise approach to C60-dumbbells ...... 28 2.3 Towards longer oligomers ...... 30 2.4 Novel fullerene-based dumbbells ...... 38 2.4.1 Metal-catalysed hydroarylation of fullerenes ...... 39 2.4.2 Fullerene dumbbells via metal-catalysed hydroarylation ...... 40 2.5 Conclusion ...... 42 3 Graphene story (Papers V-VI) ...... 44 3.1 Introduction ...... 44 3.2 Manufacture and deposition of FLG ...... 44 3.3 Mild oxidation of graphitic materials ...... 48 3.4 Conclusion ...... 51 4 Concluding remarks and outlook ...... 52 5 Acknowledgments ...... 53 6 Populärvetenskaplig sammanfattning på svenska ...... 55 7 References ...... 57

Abbreviations

AIBN Azobisisobutyronitrile γ-CD γ- CNT cod 1,5-Cyclooctadiene COSY Correlation spectroscopy CV Cyclic voltammetry CVD Chemical vapor deposition DCE 1,2-Dichloroethane DCM Dichloromethane DEA Diethylamine DFT Density functional theory DMF Dimethylformamide DOSY Diffusion-ordered spectroscopy FLG Few-layer graphene GO Graphene oxide GPC Gel permeation HOPG Highly oriented pyrolytic graphite HPLC High performance liquid chromatography IR LOM Light/optical microscopy MALDI-TOF Matrix-assisted laser desorption/ionisation – time of flight m/z Mass-over-charge ratio MiW Microwave MS Mass spectrometry MWNT Multi-walled carbon nanotube NBS N-Bromosuccinimide NMR Nuclear magnetic resonance ON Overnight PAH Polycyclic aromatic hydrocarbon PE Polyethylene PEG Polyethylene glycol PET Polyethylene terephthalate

PTFE Polytetrafluoroethylene PVDF Polyvinylidene fluoride rGO Reduced graphene oxide ROESY Rotating frame Overhauser effect spectroscopy RT Room temperature SEM Scanning microscopy SWNT Single-walled carbon nanotube TEA Triethylamine TGA Thermogravimetric analysis THF Tetrahydrofuran TMEDA Tetramethylethylenediamine TOCSY Total correlation spectroscopy UV/vis Ultraviolet/visible

1 Introduction

The research described in this thesis deals with carbon allotropes and con- nects to both organic and materials chemistry. The concept of “a material” is often intuitive and is typically learnt early in life: wood and clay are materi- als; one is used to make furniture, the other to produce bricks. The terms “compound” and “”, often used interchangeably, are encountered later and require more consideration. But what does really constitute a mate- rial? How do we determine whether something is a compound or a material, and is the difference between the two important? There is no sharp boundary between them. The key to understanding these concepts lies in the descrip- tion presented by Peter Day et al.: “A material is something with properties that give it the potential for a particular application, either structural, as with a building material; functional, as with materials used to make devices (elec- tronic, optical, or magnetic); or biological, with biomedical applications.”1,2 In other words, in order to refer to something as a material, one has to reflect on functionality and potential applications. Consider the following chemical compounds: cellulose, starch and dex- tran. Their physical and chemical properties are very different from one an- other. These properties dictate the diverse applications of the materials, composed of these compounds. Cellulose is mainly used in the textile and paper industries, starch is a common food component, and dextran is used for medicinal applications, but also makes up the separation medium Se- phadex3 (Figure 1a-c). These materials can be used without knowing their structures, as long as one is able to assess the properties. Paper and starch were already in use long before their chemical structures were determined, whereas the development of dextran materials was based on understanding their structural properties. A common denominator of these three materials is their molecular composition, as they are all built up from D-glucose mole- cules. The difference in properties of these molecular materials originates from the way in which the glucose molecules are connected through glyosid- ic bonds. Unlike a molecule, a material is characterised by its macroscopic proper- ties and is homogenous on a macroscopic level. These properties differ from those of small molecules. A material does not necessarily have a well- defined structure or molecular weight. One can, for example, imagine the general structure of a branched polymer, but determining an exact structure is neither possible nor necessary. Organic chemistry provides us with an

11 extensive toolbox of reactions, which we can use to modify the existing and introduce new properties to individual compounds and materials. In this thesis, an approach to the development of carbon allotropes from molecules into functionalised materials is presented from the perspective of organic chemistry. An example of a commercial product, based on an allotrope of carbon is shown in Figure 1d. The work described herein lies at the bounda- ry between organic chemistry and .

Figure 1. Examples of D-glucose-based commercial products: (a) a nutritional sup- plement mainly consisting of molecular D-glucose (“dextrose”); (b) cellulose used in the paper to print this thesis; and c) Sephadex, a dextran-based gel- and chromatographic medium, developed by Pharmacia;3 (d) a tennis racket made of graphite composite material.

This thesis is structured according to whether the subject was primarily viewed as a molecule or a material. The first section introduces the carbon allotropes fullerene and graphene, as well as their properties. Special atten- tion is given to their manufacture, reactivity, and analysis. In the second section, the development of organometallic fullerene-based oligomers and their potential application as molecular wires is presented. Finally, the third section considers certain aspects of graphene chemistry, more specifically, non-covalent interactions and covalent functionalisation.

1.1 Carbon allotropes Most organic molecules are composed primarily of carbon and hydrogen , as well as a lower proportion of heteroatoms, mainly , nitro- gen, and sulphur. The properties of carbon atoms are responsible for the huge diversity of organic molecules that can be constructed from a seem- ingly low number of atoms. The primary reason for this diversity is the pos- sibility to connect carbon atoms with one, two or three covalent bonds, also resulting in different geometric arrangements between the atoms.4 Special cases are molecules or materials composed entirely of carbon atoms; they are called , some of which are presented in Figure 2.

12

Figure 2. Some allotropes of carbon: (a) graphite; (b) single-walled carbon nanotube (SWNT); (c) graphene; (d) C60-fullerene.

Graphite is a layered material composed of sheets of hexagonally ar- ranged sp2-hybridised carbon atoms (Figure 2a). Each carbon is cova- lently bonded to three others in a planar geometry; the sheets are held to- gether by weak non-covalent interactions. These weak interactions allow layers of graphite to slide off by mechanical force, as seen when using a pencil. The term graphene is used to describe an individual layer of carbon atoms found in graphite (Figure 2c). It can be viewed as a material with a two-dimensional molecular structure. It possesses a certain length and width while being uniformly one atom thick. Despite a seemingly simple structure, graphene is among the most recently isolated allotropes of carbon.5 Single layer graphene possesses many useful properties, such as high electric conductivity, mechanical strength, and heat conduction.6 Most of the properties that make graphene an attractive material are lost or significantly suppressed in graphite. It is no longer transparent or flexible, and the con- ductivity is significantly reduced. When only a few layers of graphene are stacked together (typically up to 9),7 the term few-layer graphene (FLG) is commonly used. Carbon nanotubes (CNT) can be viewed as straws which, like graphene, are composed of hexagonally arranged carbon atoms (Figure 2b). CNTs vary in length and diameter, often existing as multi-walled structures (MWNT), and can be considered a one-dimensional material. The ends of CNTs can be capped or open. Depending on the structure, CNTs can be conductive or semi-conductive.8 Fullerenes are spherical allotropes of carbon and can be viewed as the shortest possible CNTs, composed of only the end-caps they can be viewed as zero-dimensional objects (Figure 2d). Unlike graphene, graphite, and CNTs, the carbon atoms in fullerenes are not only arranged in a hexagonal lattice but also contain . This composition gives a positive curva- ture to the structure, thus forming a perfect or distorted sphere. Several types

13 of fullerenes exist, but the one composed of sixty carbon atoms is by far the most common. As fullerenes are generally much smaller and more well- defined than the other carbon allotropes, they are normally viewed as mole- cules and not materials. Among other allotropes of carbon can be mentioned , , and . The interesting and useful properties of carbon allotropes have made them attractive candidates in several fields, in particular in the field of nanotech- nology. Fullerenes and CNTs have made an especially noticeable contribu- tion to the applications in the field. CNT-based transistors9 and conductive circuit elements10 as well as polymer-fullerene bulk heterojunction solar cells11 are prominent examples. In order to efficiently use such materials in applications, we need to better understand their properties, reactivity, and functionality, as well as be able to control them. This includes manufacture, purification, and deposition, as well as chemical modification.

1.2 Fullerene background Spherical allotropes of carbon were predicted in the early 1970-s by several 12–14 independent groups. It was, however, not until the discovery of C60- fullerene by Kroto et al. in 198515 that the concept of fullerenes was em- braced by the scientific community. Consisting entirely of sp2-hybridised carbon atoms, the fullerenes were originally presumed to be super aromatic, i.e. act like an aromatic macrocycle with a π-orbital system delocalised over the entire molecule. However, this is not the case, as the curvature of the molecule prevents efficient overlap of the π-orbitals. Instead, there are two distinctly different types of carbon-carbon bonds: those at the boundary of two are shorter (1.38 Å) than the ones between hexagons and pen- tagons (1.45 Å).16,17 C60-fullerene is the most stable and therefore most studied member of the fullerene family. It can withstand high temperatures and pressures, and it has a long shelf life at ambient conditions in air. The smallest isolated fullerene is C20, while among the higher fullerenes, C150 is the largest one that has been 18 isolated to date, even though it is theoretically possible to obtain C3996. In terms of electronic properties, a fullerene is an insulator and is relative- ly uninteresting. On the other hand, fullerene-based materials exhibit re- markably high conductivity as well as high ionic mobility.19,20 Additionally, a single C60-fullerene can hold up to six due to resonance stabilisa- tion within the molecule.21 This property originates from the low-lying triply 22 degenerate lowest unoccupied of the C60-fullerene. Therefore, they have been used as acceptor moieties in donor-acceptor sys- tems, for example, in artificial photosynthesis systems.23 Fullerenes and their derivatives have been used in multiple other applica- tions, such as lubricants,24 conductive and semi-conductive elements in mo-

14 lecular ,25,26 catalytic materials,27,28 solar cells,29–31 contrast agents for magnetic resonance imaging,32 and even drug delivery models.33 In this thesis, C60-fullerenes were studied with their potential applications in the field of molecular electronics in mind.

1.3 Functionalisation of C60-fullerenes

The reactivity of the shorter C-C bonds in a C60-fullerene is similar to that of the isolated double bonds; the longer ones are comparable to a C-C single bond between two sp2-hybridised carbon atoms. This structural property reflects the reactivity of the C60-fullerene, as it behaves like an electron-poor , capable of undergoing many reactions typical for olefins. Fullerenes can be covalently functionalised by several methods. Some common func- tionalisation reactions are presented in Figure 3. reactions of C60-fullerenes are common (Figure 3c), and both [4+2] (Diels-Alder reac- tions)34 and [2+2] are known, as well as the [3+2] 1,3-dipolar cycloaddition. Performing 1,3-dipolar cycloaddition with an in situ generat- ed azomethine ylide yields fulleropyrrolidines. It is better known as the Prato reaction35 and is currently one of the most widely used reaction for the func- tionalisation of fullerenes.

15

Figure 3. Examples of well-established C60 functionalisation protocols: (a) Rh or Pd catalysed hydroarylation as an example of organometallic additions; (b) amine additions; (c) cycloadditions, for example, the Prato and Bingel reactions;35,36 (d) nucleophilic additions.

Radical as well as nucleophilic additions to fullerene double bonds are known (Figure 3b and d respectively). Nucleophilic additions can be per- formed with either organolithium or Grignard reagents. Amines (primary and secondary) undergo both nucleophilic and radical addition. These reac- tions tend to be non-selective and commonly lead to poly-adducts. Metal-catalysed additions are known but relatively unstudied (Figure 3a). Additions catalysed by the transition metals , palladium, , and cobalt have been reported.37–40 One of the most obvious metal-catalysed functionalisation reactions of is the Heck coupling reaction,41 which is not applicable to fullerenes since it cannot be performed on fully substitut- ed olefins lacking C-H bonds. In the studies described in this thesis, 1,3- dipolar cycloadditions to C60-fullerenes () and organometallic additions have been used. The covalent modification of fullerenes is complicated in terms of regi- oselectivity, as a non-functionalised C60-fullerene possesses 30 equivalent double bonds. Any covalent modification of a fullerene after the initial func- tionalisation will encounter regioselectivity issues. The reactivity of neigh- bouring positions will change due to steric and electronic factors, but there

16 are many more available sites of essentially equal reactivity. Substitution pattern is crucial in many applications. In order to selectively perform bis-functionalisation reactions, tether ap- proaches are common strategies (Figure 4).42–46 The reactive entities are separated by a rigid tether of desired shape and length, thus directing the second reaction on the fullerene. This approach often requires additional synthetic steps, ultimately leading to lower yield. On the other hand, it sig- nificantly improves control of selectivity in bis-functionalisation reactions, thus resulting in higher overall yields. Zhou et al. showed that the tether approach did lead to bis-substituted fullerene adducts, and the substitution pattern was highly dependent on the length, shape, and rigidity of the tether.47

Figure 4. A schematic illustration of the tether approach to the regioselective bis- functionalisation of fullerene C60.

An issue often encountered in is the low and adducts thereof. Being a relatively large, non-polar molecule, C60-fullerene has low solubility in polar solvents (alcohols, acetone), (pentane, cyclohexane), and haloalkanes (dichloromethane, chloroform). Solubility of fullerene in aromatic hydrocarbons (, ) is four orders of magnitude higher, even more in polycyclic aromatic hydrocarbons (PAH), such as substituted naphthalenes, although they are usually not con- sidered solvents. The solubility is also high in CS2, which is often used as co-solvent with deuterated chloroform in nuclear magnetic resonance (NMR) spectroscopy.48 Solubility of derivatives strongly depends on the character of introduced functional groups. Non-covalent interactions of fullerenes with other molecules are also of significance. For example, γ-cyclodextrin (γ-CD) is known to accommodate 49 a C60-fullerene in a host-guest type of system. Fullerenes-γ-CD complexes are soluble in a significantly wider range of solvents than the fullerenes themselves, and the γ-CDs can be easily removed when no longer needed. C70 has a significantly lower affinity towards γ-CD as compared to C60; therefore, γ-CD and calixarenes have been used as a cheap and efficient way 50,51 of purifying C60-fullerenes.

17 1.4 Graphite and graphene The term graphite (Figure 2a) in chemistry commonly refers to the - line flake graphite: graphite particles of flat and flaky morphology. It is the most common geological type of graphite and is therefore sometimes re- ferred to as natural graphite. Another type of graphite is the highly oriented pyrolytic graphite (HOPG). It is of synthetic origin and is the most pure form of graphite commonly available. This material consists of well-oriented (an- gular spread less than 1°) sheets of graphene, to some extent bonded cova- lently due to imperfections in the process. Both natural and synthetic graphite have been employed in a vast number of applications, ranging from pencils and lubricants to battery electrodes and as composite materials in the .52 HOPG has been primarily used for research purposes, for instance, in microscopy as a calibration material.53 The properties of a material composed of several stacked layers of gra- phene differ from those of a graphene monolayer material (Figure 2c). Un- surprisingly, the transparency decreases with an increased number of layers. Thermal and electrical conductivity is reduced due to interactions between the layers. The flexibility of graphite flakes is significantly lower than that of the corresponding graphene, as natural graphite is a brittle material. The graphene lattice forms an aromatic system of enormous area (on the molecular scale). “Sheet” and “flake” terminology is often used when dis- cussing graphene.54 “Graphene sheet” often refers to material of large size grown by chemical vapor deposition (CVD), whereas “flake” is used to de- scribe smaller, irregularly shaped pieces of graphene. The size of the flakes is dictated by the grain size of the source material.55,56 Graphene possesses some interesting and useful properties. For such a light material, it has ex- tremely high electrical conductivity, comparable with that of metals. It is highly transparent, mechanically strong, flexible, and conducts heat.6 These properties caused an explosive growth of research efforts regarding gra- phene. To understand these properties, the molecular structure became a point of interest. The edges of graphene sheets appear in either a zigzag or armchair structure, which follows from the hexagonal lattice of graphene.57 Different combinations of such edge configurations create specific patterns or regions at the edge of a graphene flake. The reactivity of edges towards oxidation and substitution reactions is expected to depend on the local struc- ture.58,59 Being a very young member of the carbon allotrope family, gra- phene has so far only found limited applications, mainly in state-of-the art devices and research.60–63 Graphite flakes consisting of roughly 2-9 stacked layers of graphene are commonly referred to as few-layer graphene.7 Properties of such material resemble those of graphene,64 whereas in flakes of twelve or more layers are virtually indistinguishable from those of graphite. Perhaps the most obvious application of graphene in an electronic device would be the replacement of

18 conductive, metal-containing, thin coatings, such as indium tin oxide. The surface that needs to be coated is significantly larger than that of a typical FLG flake. Therefore, a conductive layer of FLG flakes needs to be pro- duced, which means that the flakes need to be in contact with one another or even overlap like the scales of a fish. A film composed of overlapping FLG flakes would be able to be bent without the loss of conductivity, as the flakes or individual graphene layers in the flakes would slide on top of one another when subjected to mechanical bending.

1.4.1 Production of graphene In order for graphene to appear in commercial devices, it has to be produced in sufficient quantity and quality at a low cost. Quality comprises low defect density, homogeneity (monolayer or FLG), and purity (absence of additives). The starting material should be affordable and the manufacturing process simple and reproducible. It is also important to strive for a low environmen- tal impact in terms of energy consumption and product to waste ratio. Ob- taining sufficient quantities of high quality graphene is one of the challenges of graphene research. Assembling graphene or nanographene from simpler building blocks is commonly referred to as the bottom-up approach. Exfolia- tion of graphitic materials is the principle of the so-called top-down approach to graphene. An overview of both strategies is given in Figure 5.

Figure 5. Overview of the common methods for production of graphene and related materials, such as nanographenes, graphene oxide (GO), and reduced graphene ox- ide (rGO) from graphite (top-down) and from smaller molecules (bottom-up).

The most prominent method in the bottom-up category is CVD, where large sheets of high quality graphene can be obtained from most sources of carbon.65 Graphene is grown on the surface of a metal, which is later etched away, and the graphene film is transferred to a substrate. The method is easi-

19 ly scalable, as the size of the graphene sheet is limited only by the size of the metal substrate and that of the equipment.66 Due to the relatively high manu- facturing cost of CVD graphene, it is not the most cost-efficient method at the moment. Choice of the growth substrate, high temperatures, and the sen- sitive transfer process are common issues. of nanographenes is a method of obtaining materials similar to graphene in both structure and properties. Müllen et al.67 as well as many other researchers68 have reported syntheses of large molecules com- posed mainly of sp2-hybridised carbon atoms. Despite the elegant syntheses, nanographene obtained by this method is not likely to find its way into ap- plications. This is mainly due to the small area and high manufacturing cost, as compared to other methods. That said, nanographenes can be used as gra- phene models when studying the reactivity of graphene, as recently demon- strated with smaller PAHs.59 Characterisation of individual flakes of graphene was first reported in 2004 by Novoselov and Geim et al.5 The graphene flakes were obtained by a top-down method of mechanical peeling of layers from graphite using adhe- sive tape (Scotch tape method). The flakes yielded by this technique were of varying thickness, some of which could be identified as a single layer using optical microscopy. This exfoliation method, however simple, cannot yield a sufficient quantity of graphene for large scale applications. A fast and high-yielding process is exfoliation of natural graphite from powder or foil. It is commonly performed by ultrasonication in a suitable solvent69 and yields graphene flakes in a suspension; however, the size of the flakes is relatively small as compared to CVD. Long ultrasonication times are often needed to obtain satisfactory quantities of FLG flakes, as the inter- actions between the individual sheets in graphite are relatively strong. The downside is the risk of introducing defects into the graphene lattice, as well as the rolling and folding of the flakes.70 Intercalation facilitated exfoliation of graphite is a widely used method to reduce the ultrasonication times. Ex- foliation of graphite occurs more easily if the interactions between the sheets of graphene are weakened beforehand.56,70 Binding interactions between the individual graphene sheets in the graphitic material are weakened prior to ultrasonication by the insertion of small molecules (typically halogens, sol- vents, or salts). FLG acquired by these methods typically exhibit a distribu- tion between 1-7 layers and a low degree of defects. Addition of surfactants in the sonication step further stabilises the suspension of FLG, leading to a more efficient exfoliation, but removal of the surfactants is complicated. The most established large-scale exfoliation method involves altering the intermolecular interactions between the layers by oxidising the graphite, producing graphene oxide (GO).71,72 Reducing GO can, in turn, yield rGO flakes.73 The low cost and scalability of this method are its main advantages. The main downside is the introduction of defects to the graphene lattice, significantly affecting its properties.74 From an environmental and safety

20 perspective, this procedure is dubious, as it involves dangerous, corrosive, and toxic chemicals, while also generating large quantities of waste. An overview of the various aspects of the common methods of graphene produc- tion is given in Figure 6 and relates to a review, “A roadmap for graphene,” by Novoselov et al.75

Figure 6. Factors such as quality of the material, scalability, efficiency, and envi- ronmental impact relate to the application and affect the choice of graphene produc- tion method.75

In the studies described within this thesis, we focused on using suspen- sions of FLG. This type of graphene material was chosen for its simplicity of manufacture, reproducibility, and relatively high quality (Figure 6). A simple and reliable method of obtaining FLG-suspensions using intercalation with was previously developed and studied in our group.56 Low-cost graphite foil can be used as a starting material, and the process is carried out at room temperature with low energy consumption. This procedure elimi- nates the need for harsh conditions and produces minimal waste, which is easily converted to non-toxic salts by the neutralisation of bromine with sodium thiosulphate. This method reliably yields suspensions of FLG, which can be deposited on the desired substrate by simple techniques described further below. Volatile solvents are preferred to simplify the deposition step. The size of the graphene flakes obtained by this method depends on the grain size of the starting material and generally reaches 5-30 µm in size.

21 1.4.2 Deposition of graphene from suspension Different types of graphene (Chapter 1.4.1) require different processes for deposition onto substrates, as neither the graphene nor the substrate proper- ties are identical for all possible combinations. Similarly, the substrate limits the applicable deposition processes (Figure 7). It is essential to carefully select parameters in order to achieve the desired properties of the resulting assembly. We chose to focus on deposition of FLG from suspension, as this is related to the performed study. Transfer of CVD graphene, for example, would require different methods.

Figure 7. The substrate of choice, type of graphene, and the deposition method all need to be considered for a successful deposition.

FLG in suspension can be deposited onto a solid substrate in several ways. In the simplest case of drop deposition, a droplet of FLG-suspension is applied on a horizontal substrate surface, and the solvent is allowed to evaporate. A disadvantage of this method is a pronounced “coffee ring” ef- fect: upon evaporation of the suspension medium, the flakes tend to move towards the edges of the droplet, which leads to uneven distribution and aggregation. In dip deposition, a substrate is vertically lowered into an FLG- suspension and, after a certain time, pulled out and allowed to dry. Com- pared to drop deposition, this method gives less deposited material. This is because the process is driven by self-assembly through weak non-covalent interactions between the flakes and the substrate, as opposed to evaporation of suspension medium. Assuming that the flakes adhere sufficiently well to not be washed away by another immersion, the degree of deposition can be increased by repeating the process.69 Analysis and application of modified or non-modified graphene often re- quires it to be placed on a suitable substrate. Over the years, many substrates have been used for graphene deposition, with Si/SiO2 probably being the most common. This is largely because it simplifies identification and analy- sis of graphene using simple light/optical microscopy (LOM) observation.5 Monolayer graphene on a Si/SiO2 surface can be distinguished from bi- and

22 multilayer graphene by colour and contrast. However, Si/SiO2 does not take full advantage of the flexibility and transparency of graphene as it is neither. Flexible polymeric substrates therefore present high potential. Polyeth- ylene terephthalate (PET) and polyethylene (PE) are examples of flexible, transparent polymers that have been used as substrates for graphene.62 Like most polymers, they are insulators and, as such, have limited applications in electronics. A separate class of polymer materials is the piezoelectric one. The electri- cal properties of this class depend on the mechanical stress to which the ma- terial is subjected. The most common piezoelectric polymer is polyvinyli- dene fluoride (PVDF). It is an organic polymer consisting of aliphatic carbon chains with two hydrogen and two fluorine atoms on alternating . The material is chemically inert, with properties similar to polytetrafluoro- ethylene (PTFE). Upon stretching in an electric field (poling), the material obtains piezoelectric properties, which are permanently lost upon heating above certain temperatures, typically 70 °C.76,77 The material and the process of poling are described in more detail in Chapter 3.2. PVDF is often used in such components as stress and sound sensors, actuators, spark ignitors, etc.78

1.5 Characterisation methods

1.5.1 Molecules Nuclear magnetic resonance spectroscopy is probably the most common technique for solution-phase structure determination of organic molecules. A variety of studies can be performed, and information about the structure and properties of organic molecules can be obtained using this technique. Insolu- ble materials, such as polymers or suspensions, are less suitable for NMR analyses. Solid-phase NMR techniques are still less common than NMR spectroscopy in solution. Overall, NMR spectroscopy is typically used for characterisation of smaller molecules. Analysing large fullerene assemblies with NMR is unpractical for several reasons. The presence of multiple regioisomers and a large number of proton signals lead to complicated spectra with overlapping peaks. Solubility of such compounds is often limited. Therefore, the primary tool for the identifi- cation and characterisation of fullerene oligomers is mass spectrometry (MS), more specifically the matrix-assisted laser desorption/ionisation time- of-flight technique (MALDI-TOF). Using this mild ionisation method, heavy fullerene oligomers and fragments thereof can be identified by their mass- over-charge ratios (m/z). MALDI-TOF relies heavily on the ionisation ma- trix, which has to be carefully chosen to fit the analyte. The structural infor- mation obtained by MS is limited, as compared to e.g. NMR.

23 Among the spectroscopic methods, infrared spectroscopy (IR) is useful for determining the presence of specific functional groups. In specific cases, IR spectroscopy can also be used for quantification purposes.79,80 This is of significance for confirming formation of the fullerene adducts. Ultraviolet- visible spectroscopy (UV/vis) is used as a complementary characterisation technique.

1.5.2 Materials Characterisation of materials in general, and more specifically graphene, is often done using a combination of techniques focusing on macroscopic and molecular properties, respectively, such as microscopy and spectroscopy. An easy way to visually observe large flakes of graphene and FLG is through LOM. Simple image analysis software enables the estimation of surface coverage, flake size determination, and distribution of FLG flakes. Scanning electron microscopy (SEM) is a more advanced technique, which provides a significantly better resolution as compared to LOM. It provides detailed information on the quality of graphene (number of layers, edge shape, fold- ing, and a more precise size determination). Finally, atomic force microsco- py (AFM) is a technique that allows for an even more precise topology de- termination, including measuring height of individual atomic layers. Spectroscopic techniques are another common approach to material anal- ysis. Vibrational spectroscopy is used to detect presence of specific atomic configurations, such as functional groups. IR spectroscopy can be used for characterisation of polymers and also to distinguish graphene from GO. A very common material analysis technique, and by far the most common technique for studying graphene, is . It can provide in- sight into the quality of graphene in terms of defects, presence of contami- nants, and average flake thickness. The main complication when performing Raman measurements is the background fluorescence of the substrate. When the substrate is Si/SiO2, this problem is non-existent, as the substrate peaks lie outside the characteristic bands of graphene. Care must be taken when the substrates are organic polymers, as the background fluorescence or substrate bands can seriously disturb the measurements. High laser intensity also risks damaging the substrate. Typical Raman spectra (532 nm) of a graphite flake and CVD graphene sheet are shown in Figure 8. The most prominent characteristic bands are the G peak (1580 cm-1) and the 2D peak (2700 cm-1). The G peak originates from the in-plane stretching of the C-C bonds in the graphene lattice.81 The 2D peak is the second-order overtone of the D peak (1350 cm-1), another in- plane vibration mode. This mode, also known as “the breathing mode,” is normally forbidden, and the D peak is missing in a perfect graphene sheet. It appears due to imperfections, , and defects in the lattice, and thus pro- 81–83 vides useful information about the quality of the material. The ID/IG ratio

24 is commonly used to compare the graphene quality with respect to the de- fects between different samples.84

Figure 8. Typical Raman spectra of CVD graphene used in Paper VI (top) and a typical graphite flake used in Paper V (bottom).

The main difference between the Raman spectra of graphene and graphite lies in the ratios of the G and 2D peaks. The I2D/IG ratio close to two is char- acteristic for graphene and indicates monolayer. As the number of layers increases, the ratio of the G and the 2D bands is inverted until a signature Raman spectrum of graphite is obtained. Additionally, in graphene, the 2D peak consists of a single component, whereas in bulk graphite, the same peak contains multiple components. This is reflected in the width, shape, and relative position of the 2D peak.82 The 2D band thus provides a mean of quick identification of graphene, as well as some information about the thickness of the graphite flakes. Thermogravimetric analysis (TGA) is a useful tool of quantification of oxidation and other defects in graphitic materials. The sample is continuous- ly weighed on a precision balance while subjected to heating. Various com- ponents of sample decompose at different temperatures, and the resulting change in mass is detected. The defects in graphene are often related to oxi- dation; oxygen functional groups, such as carboxylic acids, hydroxyls, and epoxy have been reported to decompose at 150-200 °C.85–88 Care must be taken to maintain an inert atmosphere in order to avoid oxidation of the ma- terial during the measurement.

25 2 Molecules to materials – a fullerene story (Papers I-IV)

2.1 Introduction Development of new fullerene-based materials requires an understanding of functionalisation reactions, reaction conditions, and characterisation tech- niques. Starting with individual fullerene molecules, the materials can be obtained through different approaches: covalently bonded fullerene- containing polymers,89,90 non-covalent assemblies,91 and composites,92 to name a few. One of the most studied and promising areas of application for such materials is the field of molecular electronics. Such assemblies can play part of a molecular wire, a molecular chain that can take part in an electric circuit. Molecular wires are typically thought of as conductive elements; however, many organic polymers have semi-conducting rather than metallic conductive properties. A fullerene-based wire could therefore also function as other elements of a circuit, such as a , a modulator, or a resistor. The properties can be tuned by introducing suitable functional groups to the wire. It was shown that photoirradiation of Zn-porphyrine-C60 assemblies significantly improves the conductivity of that system,26 which suggested potential application of such systems as switches. For an organic , reactivity and covalent modification of molecules is of special interest, and incorporating fullerenes into oligomers93 is the main topic of the research discussed in this chapter. Synthesis of a C60-ferrocene-C60 dumbbell and studies of its electrochem- ical properties, previously performed by our research group, revealed energy or electron transfer between the individual components.94 A schematic over- view of the dumbbell assembly is depicted in Scheme 1a. Based on this promising result, we envisioned a versatile strategy of assembling fullerenes and other molecules into dimers, trimers, and longer oligomers. An overview of the process is depicted in Scheme 1b. Unlike our first reported dumbbell synthesis,94 the approach here is stepwise. A precursor is formed by coupling a fullerene C60 adduct with a ferrocene linker. This precursor is then activat- ed and coupled to a new component, for example, to a differently functional- ised C60 fullerene or other suitable molecules. The process can then be con- tinued, leading to formation of well-defined oligomers. The results of this study are presented in Chapters 2.2 and 2.3 (Papers I-III).

26

Scheme 1. Schematic illustration of different strategies towards ferrocene-linked fulleropyrrolidine dumbbells and oligomers, topic of Chapters 2.2 and 2.3.

We also investigated another method of fullerene functionalisation, the Rh-catalysed hydroarylation. This strategy is schematically illustrated in Scheme 2. The method utilises novel bis- functionalised linkers, as opposed to the ferrocene-based ones mentioned earlier. An advantage of functionalising the fullerene cage through hydroarylation is stability of the product, as opposed to the fulleropyrrolidines which are prone to degradation through oxidation and the retro-Prato reaction. Another attractive advantage is the ease with which a variety of functionalities can be introduced to the fullerene derivatives. A vast number of organoboron compounds are com- mercially available, and the synthesis of boronic acids and esters is well studied. This opens up substantial possibilities to tune the properties of the fullerene derivatives, for example, by introducing pH-sensitive or light ab- sorbing functional groups. Results of the investigation of this uncommon functionalisation strategy towards the main-chain fullerene dumbbells are given in Chapter 2.4 (Paper IV).

Scheme 2. Schematic illustration of the strategy towards fullerene dumbbells via rhodium-catalysed hydroarylation.

27 2.2 Stepwise approach to C60-dumbbells The main limitation of the previously developed synthesis of the ferrocene- linked fulleropyrrolidine dumbbell94 (Scheme 1a) is the low versatility and control over the reaction that it offers. The chosen approach is convergent, i.e. the two amide bonds between the fulleropyrrolidine 1 and the ferrocene linker 2 are formed in the same reaction step (Scheme 3). Only symmetrical assemblies can be obtained reliably by this method, as any attempts to use two different fulleropyrrolidines A and B would result in mixture of assem- blies AA, AB, and BB. Such mixtures are likely to prove difficult to sepa- rate. The complexity of anticipated product mixtures increases further if one would apply the convergent strategy to couple mono- and bis-functionalised fulleropyrrolidines, as would be required to obtain trimers and oligomers. In order to overcome this limitation, a stepwise approach was studied.

Scheme 3. Assembly of a ferrocene-linked fulleropyrrolidine dumbbell.94

The key to stepwise assembly lies in the reactivity of the carboxyferro- cene linker and our ability to activate and deactivate it. The synthesis of the linker was altered to offer formation of acyl chloride at only one of the car- boxylic groups. This synthesis is shown in Scheme 4. The functionalities in 5 were protected by esterification, selectively hydrolysed to 7 and converted to acyl chloride 8. Selective hydrolysis proved challenging, as the two ester groups of 6 are identical, offering no selectivity. In a bipha- sic system of dichloromethane (DCM) and NaOH (aq.),95 even minor chang- es in reaction time, temperature, and NaOH concentration significantly af- fected the outcome of the hydrolysis. The mono-protected carboxyferrocene 7 was at best obtained in 44% yield, and the crude always contained a mix- ture of the unprotected, mono- and bis-protected carboxyferrocene, compli- cating the purification. The hydrolysis was significantly improved by per- forming the same reaction in a one-phase system of acetone and NaOH in methanol.96 Upon hydrolysis of one of the esters, 7 precipitated out as a so- dium salt and was, after workup and purification by flash chromatography, isolated in 75% yield.

28

Scheme 4. Conditions: i) TMEDA, n-Butyl lithium, CO2 (g), yield: 52%; ii) a) DCM, DMF, (COCl)2 b) methanol, yield: 79%. iii) NaOH (5% in methanol), ace- tone, yield: 75%; iv) DCM, (COCl)2, 4 h.

Fulleropyrrolidine 194 was reacted with the acyl chloride 8, forming an amide bond. This synthesis is presented in Scheme 5. The ester of the ob- tained ferrocene-fulleropyrrolidine dyad 9 was hydrolysed under alkaline conditions to a carboxylic acid derivative 10. Upon conversion of 10 to an acyl chloride, another equivalent of 1 was added to form the dumbbell 3, completing the stepwise synthesis (Scheme 6).

Scheme 5. Conditions: i) DCM, (COCl)2, pyridine, yield: 65%; ii) LiOH, 1,2- dimethoxyethane, H2O, 115°C, 5 h, yield: 97%.

Scheme 6. Conditions: i) a) CHCl3, SOCl2, TEA, RT, stirring; b) CHCl3, TEA, stir- ring, ON.

The target molecule 3 was isolated by on silica. Its presence was confirmed by mass spectrometry and UV/vis spectroscopy, which were found to be in agreement with the reference data.94 Development and successful application of this controlled stepwise synthesis strategy opened a path towards more complex ferrocene-linked fulleropyrrolidine assemblies, such as asymmetrical dumbbells, trimers, and longer oligomers (Scheme 1b). To demonstrate the versatility of the stepwise method, we also successful- ly coupled 10 with an amino acid derivative. This can be viewed as a first step in the synthesis of fullerene and ferrocene containing peptides. Similar assemblies have been reported to possess enzyme inhibitory activity97–99 and found application in e.g. biosensors.100,101 The methyl ester of L-tryptophan was chosen for its good solubility in the reaction media and its availability. It

29 was used as a hydrochloride salt without additional purification, and there- fore, an excess of triethylamine was required to generate its free base. The reaction is shown in Scheme 7.

Scheme 7. Conditions: i) a) CHCl3, SOCl2, TEA, RT, stirring; b) CHCl3, TEA, stir- ring, 4 h, estimated yield: 82%.

As compared to the formation of dumbbell 3, this reaction proceeded more efficiently and required shorter time. This difference is likely arising from higher nucleophilicity of the primary amine of the amino acid 11 as compared to the fulleropyrrolidine 1.102,103 The product 12 was purified by column chromatography (silica) and confirmed by MALDI-TOF MS; the only side products were traced back to insufficient purification of the starting material 10.

2.3 Towards longer oligomers Having demonstrated stepwise synthesis of ferrocene-linked bis- fulleropyrrolidine 3 and fulleropyrrolidine-amino acid dumbbell 12, we moved on to longer ferrocene-linked fullerene trimers. A key element of such molecules is a bis-functionalised fulleropyrrolidine. Any attempt at uncontrolled bis-functionalisation of a C60-fullerene cage will result in a complex mixture of mono-, bis- and multi-functionalised fullerenes, as well as some polymerisation adducts. In the case of bis- and multi- functionalisation, a variety of regioisomers and conformers is expected, which will further complicate purification and characterisation. To overcome these challenges, the tether-directed approach was chosen, where a rigid tether is used to guide the functionalisation and reduce the number of possi- ble products. A carefully selected tether is expected to direct the bis- functionalisation to the opposing poles of the fullerene cage, which in turn will increase the symmetry and allow the target trimer to form a linear struc- ture, as exemplified in Figure 9.

30

Figure 9. Schematic representation of a ferrocene-linked, fullerene-based molecular wire, showing the tethered fullerene unit.

In his doctoral thesis emanating from our group, C.-H. Andersson report- ed the use of a simple biphenyl-based tether,104 its choice being based on a previously published study by Zhou et al.47 It was successfully used in a dual Prato reaction, yielding fullero-bis(pyrrolidine). This approach, although successful, met with solubility and purification issues.104 In order to improve these results, we redesigned the tether to include long aliphatic chains at 3 and 3’ positions of the biphenyl backbone. These were chosen based on the structure of the dumbbell,94 which appeared to obtain sufficient solubility when two octadecyloxy chains were introduced at every fullerene unit. Semi-empirical PM3 calculations confirmed that the introduction of such side-chains did not present significant strain or unfavourable interactions as compared to the unmodified biphenyl tether.104 The target fulleropyrrolidine trimer 16 is presented in Scheme 8 together with an overview of the synthe- sis of the tether by design 13 (route a)104 and the alkyl-functionalised tether 14 (route b).

Scheme 8. Routes to tethers 13104 and 14 and the target trimers 15104 and 16.

The synthesis route to the tether was revised, as the biphenyl backbone with reactive functional groups at the desired positions was not commercial- ly available. The backbone had to be assembled from smaller molecules. A potential benefit of assembling the tether backbone from smaller molecules

31 is that it enables introduction of additional functional groups. As an example, one could imagine functionalising the tether backbone with pH- or light- responsive functionalities, thus providing the oligomer with new properties. One could also imagine affecting the solubility of such oligomers by intro- ducing, for example, hydrophilic groups (PEG, polysaccharides, thiols etc.). Cross-coupling of 5-bromo-methylphenol derivatives was chosen due to its apparent simplicity. The overall synthesis of the fullero-bis(pyrrolidine) 21, functionalised with aliphatic side chains is presented in Scheme 9. A number of approaches to such couplings were evaluated; a summary is given in Table 1.

Scheme 9. Conditions: i) K2CO3, acetone, C18H37Br, yield: 79%; ii) THF, I2, Mg, FeCl3, yield: 68%; iii) NBS, AIBN, CCl4; iv) 4-hydroxy benzaldehyde, DMF, K2CO3, yield: 60% (from 19 to 14); v) C60, glycine, ODCB, 180 °C, 5 h, yield: 69%. For simplicity, only one of the possible regioisomers of compound 21 is shown.

Table 1. Evaluated reaction conditions for cross-coupling of aryl halides in the syn- thesis of the tether backbone. Entry Conditions Solvent Yield 1 Cu, MiW-assisted heating, 200 °C, 2 h DMF No reactiona,b 2 Pre-activated Cu, MiW-assisted heating, 220 °C, 2 h DMF No reactiona,b c,d 3 Na, reflux, 19 h dry Et2O 10 % b 4 1. Mg, I2, reflux, 1 h dry Et2O No reaction 2. cat. FeCl3, reflux, 18 h DCE 5 1. Mg, I2, MiW-assisted heating, 100 °C, 1 h dry THF 1-19 % 2. cat. ZnBr2, MiW-assisted heating, 100 °C, 1 h

6 1. Mg, I2, MiW-assisted heating, 110 °C, 1 h dry THF 40-45 % 2. cat. FeCl3, RT, O2 (g), 1h e 7 1. Mg, I2, MiW-assisted heating, 110 °C, 3 h dry THF 68-78 % 2. cat. FeCl3, 120°C, 3 h; 80°C, 64-108 h a: Reaction performed on 2-bromonaphthalene; b: starting material recovered; c: reaction performed on 3-bromotoluene; d: by 1H NMR; e: synthesis by T. Günther.105

32 The tested methods included such reactions as -mediated Ullman coupling106,107 (Table 1, entries 1 and 2), Wurtz-Fittig reaction108,109 (Table 1, entry 3), and Grignard cross-coupling (Table 1, entries 4-7). Satisfactory yields of 40-45% were obtained when the using AlCl3 as a Lewis acid cata- 110 lyst while introducing O2(g). In a separate study by T. Günther, conducted in our group, the yields were further improved to 78% (Table 1, entry 7).105 The fullero-bis(pyrrolidine) 21 was successfully synthesised and partially purified by column chromatography. Characterisation by MALDI-TOF MS revealed the molecular peak matching the expected mass of the target molecule 21 (m/z=1705). The 1H NMR spectrum shown in Figure 10 was analogous to the one obtained previously for fullero-bis(pyrrolidine) synthe- sized using tether 13.104 At least six singlets were observed between 5.5 and 6.3 ppm, which are indicative of methine protons of the fulleropyrrolidine rings.47 This indicates the different environments corresponding to various isomers.

Figure 10. Expansion of the 1H NMR spectrum of 21 showing the singlets corre- sponding to the fulleropyrrolidine methine protons indicated by circles (400 MHz, CDCl3:CS2 1:1, 25 °C).

Several minor products of the dual Prato reaction (Scheme 9, step v) shown in Figure 11 were observed with MALDI-TOF MS. Formation of the mono-functionalised fulleropyrrolidine 22 (m/z=1692) and the dimer adduct 23 (m/z=2425) was expected. An uncertainty remained regarding the peak with m/z=3410. It could correspond to the adduct 24 (mixture of isomers) or to compound 25, a dimer of the target fullero-bis(pyrrolidine) 21. The latter case is a catenane, a molecular assembly consisting of mechanically inter- locked macrocycles. Such macromolecular assemblies are in themselves interesting and have over the years received significant attention.111

33

Figure 11. Side products of the dual Prato reaction described in Scheme 9, step v, indicated by MALDI-TOF MS. Compounds 24 and 25 are two possible assemblies corresponding to the same peak in the mass spectrum.

Unfortunately, we were unable to separate the suspected catenane 25 (mi- nor component) from the major product 21 by available chromatographic methods (column chromatography on silica or aluminium oxide, C18- reversed phase silica, or preparative TLC). We therefore repeated the dual Prato reaction above using sarcosine instead of glycine (Scheme 10) to ease the purification by avoiding secondary amines in the structures. This yielded 26a, the N-methylated derivative of the tethered fullero-bis(pyrrolidine) 21 as the major product.105 Again, a molecular ion peak with m/z corresponding to twice the m/z of 26a was observed with MALDI-TOF MS.

Scheme 10. Conditions: i) C60, sarcosine, ODCB, 180 °C, 5 h, yield: 30% (26a, trans-1 and trans-2), 10% (26b).

Purification by column chromatography was successful, and the suspected catenane 26b was characterised by 1H NMR, COSY, TOCSY and ROESY.

34 A full proton signal assignment strongly supported the catenane model.105 The molecule appeared to be highly symmetrical and consisted of two iso- mers of 26a in a 1:1 ratio; one identified as trans-1, and the other is likely to be a trans-2, based on previously reported data.47 Additional support was provided by DOSY experiments. Assuming spherical molecules, the diffu- sion coefficient is inversely proportional to the hydrodynamic radius by the Stokes-Einstein equation:112 6 where k is the Boltzmann constant, T is the absolute temperature, η is the dynamic viscosity, and rs is the hydrodynamic radius of the molecule. The hydrodynamic radii of 26a and the suspected catenane 26b were found to differ by a factor close to two, which is consistent with the expected differ- ence in size between the molecules, assuming an approximate linear rela- tionship between molecular weight and hydrodynamic radius (Figure 12).

Figure 12. Overlaid DOSY spectra of 26a (top trace) and 26b (bottom trace) (300 MHz, solvent CDCl3:CS2 1:1, 25 °C). Note a two-fold difference in diffusion coeffi- cient indicative of the difference of the hydrodynamic radii.

Formation of a catenane typically requires the pre-organisation of its components. Preliminary computations using the semi-empirical PM3 meth- od and shorter -OC5H11 side chains were performed. The energies and dis- tances between the two tethers were compared for different initial orienta- tions of the side chains. The orientations and the results are given in Table 2.

35 Upon relaxation, the distance between the tether backbones and the total energy of the system remains essentially unchanged when the side chains of each tether are directed towards those of the other (Table 2, entry 1). When interaction between the chains is minimized by directing the chains away from each other, the tethers move away from one another when allowed to relax (Table 2, entry 2). For tether 13 (R=H), the distance between the teth- ers was larger than for the other two cases (Table 2, entry 3). This suggests a local energy minimum when the side chains of two tether molecules 14 are oriented in a way that benefits formation of the catenane. The pre- organization appears to be driven by the hydrophobic interactions between the aliphatic side chains and by π-interactions between the tether backbones.

Table 2. Relative orientations of the chains and results of the computational PM3 study of the interactions between two tethers 14 with short side chains (a and b), and between two tethers 13 (c). R=C5H11; R´=OC6H4CHO.

Energy (kcal/mol) Distance (Å) Entry Structure Initial Final Difference Initial Final Difference 1 (a) -325.549 -325.550 -0.001 3.934 3.935 0.001 2 (b) -319.000 -331.272 -12.282 4.458 4.724 0.266 3 (c) -87.498 -87.496 0.007 5.050 5.008 -0.042

36

Scheme 11. Conditions: i) a) CHCl3, SOCl2; b) CHCl3, TEA. To simplify the scheme, only one of the trans-1 regioisomers of compound 16 is drawn.

Having studied the exciting catenane co-products 25 and 26b, attention was again turned to the fullerene oligomers. Synthesis of the target trimer 16 was attempted through the stepwise strategy developed in Chapter 2.2. The synthetic procedure is shown in Scheme 11. After purification by column chromatography (silica), MALDI-TOF MS suggested the presence of the target trimer 16 (minor peak). Multiple other peaks were also observed; sug- gested structures are shown in Figure 13. A dumbbell 27 was observed as the main component.

Figure 13. Side products of the reaction described in Scheme 11 as indicated by MALDI-TOF MS.

Formation of the dumbbell 27 indicates that the stepwise approach de- scribed in Chapter 2.2 has the potential to yield the desired products. How- ever, harsher reaction conditions (temperature, time) may be required to improve the outcome. The sensitivity of the acyl chlorides participating in the amide bond formation is a possible cause of the low yield of 16. Alt-

37 hough all of the used solvents were dried over molecular sieves (3 Å) prior to use, considering the small reaction scale, the presence of even minor quantities of water may have affected the outcome. Purification remains an issue, as even with the solubilising side-chains, the trimer could not be sepa- rated from the side products using the available methods. More appropriate chromatographic methods, such as high-performance liquid chromatography (HPLC) or gel permeation chromatography (GPC), may be required. Characterisation of the fulleropyrrolidines revealed differences in choice of the MALDI matrix. Compared to 9-nitroanthracene and dithranol, trans- 2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was found to be superior. It produced the cleanest spectra and required a lower laser intensity, thus reducing the fragmentation. This is in line with previously reported data.113 Regardless of the matrix, we always observed a molecular ion corresponding to C60, even though it was chromatographically separated and did not appear using other techniques. This was more promi- nent when higher ionisation energies were used. We therefore speculate that C60 is formed in the spectrometer in a retro-Prato reaction upon irradiation. Such retro-cycloadditions have been reported to proceed thermally with or without a catalyst.114,115 The dependence of MALDI-TOF MS on the matrix and ionisation energy, as well as the fact that the molecules are differently prone to ionisation, means that the MALDI-TOF MS results need to be in- terpreted with care. Although a convenient technique to quickly indicate the possible presence of compounds, conclusions regarding identity and in par- ticular quantification have to be treated with utter caution. The bis-functionalised fulleropyrrolidine unit 21 was successfully synthe- sised and characterised. The tether-directed functionalisation approach re- duced the number of formed isomers while keeping the yield in-line with similar reactions.47 A host-guest study of the molecule within a bis- porphyrin tweezer system116 could provide further identification and assign- ment of the isomers. Such a study was recently initiated in the group.105 The synthesis of the trimer 16 via the selected strategy was challenging. Even though an ion peak with m/z corresponding to 16 was observed, an optimisa- tion of the reaction is required to isolate any significant quantities. Should these issues be surpassed and trimer 16 be isolated in a sufficient quantity and purity, a study of its electrochemical properties by means of cyclic volt- ammetry (CV) would be interesting to conduct. It could yield insights into properties of the oligomer and give an indication of the potential of longer wires of this kind as functional materials.

2.4 Novel fullerene-based dumbbells The Prato cycloaddition reaction employed in Chapters 2.2 and 2.3 is one of the most commonly used methods of covalent functionalisation of fullerenes.

38 The work presented in this chapter has been focused on a relatively unstud- ied reaction of fullerenes, the hydroarylation reaction, as means of linking the fullerene molecules into longer assemblies. Considering the relatively unstudied nature of the metal-catalysed hydroarylation reaction, the initial goal was to learn more of the conditions and factors affecting the outcome of this functionalisation approach. In continuation, we wanted to develop a series of fullerene dumbbells, analogous to the ferrocene-based 13 described earlier, schematically shown in Scheme 12. A fluorene-linked fullerene dumbbell is the only molecule of this kind for which a synthesis using rhodi- um metal-catalysed hydroarylation has been reported to date.117 The authors indicated difficulties arising from poor solubility of both the target and the intermediate.118

Scheme 12. General representation of the synthesis of the fullerene dumbbells via rhodium-catalysed hydroarylation.

2.4.1 Metal-catalysed hydroarylation of fullerenes Based on an extensive screening of the catalysts, rhodium complex [Rh(cod)2]BF4 was chosen due to the reported high yields and selectivities.37,38,117 The initial hydroarylation attempts with simple para- substituted arylboronic acids are summarised in Table 3. All conversions were calculated by quantitative 1H NMR, using dimethyl terephthalate as an internal standard.

Table 3. Results of rhodium-catalysed hydroarylation of fullerene C60 with para- substituted arylboronic acids. Conditions affecting the outcome are outlined.

Entry Conditions Degasification Conversion,a R = –OCH3 –CH3 –CN 1 60°C, 6 hb Sonication with bubbling n/a No reaction n/a of N2 (g) 2 60°C, 20 hc Sonication with bubbling n/a 7% n/a of N2 (g) c 3 70°C, 20h Freeze-pump-thaw (N2) 30% 37% 12% c 4 MiW, 110 °C, 1 h Freeze-pump-thaw (N2) 39% 34% 19% a: By 1H NMR using dimethyl terephthalate as internal standard; single experiments; esti- mated procedure error ±5%; b: oval-shaped magnetic stirrer; c: cross-shaped magnetic stirrer.

39 Efficient mixing of the aqueous and organic phases was found to be cru- cial, as no reaction was observed when the stirring was insufficient (Table 3, entry 1).119 This is in line with a recent computational DFT study, where the role of water is outlined as the source of hydrogen in the hydroarylation.120 Careful degasification is essential for the reaction to proceed catalytically. In this study, degasification by prolonged bubbling of the solvent with nitro- gen gas at best resulted in a 7% yield at 10% catalytic loading (Table 3, en- try 2), which indicates catalyst poisoning. Degasification by five freeze- pump-thaw cycles was required for the catalyst to perform in line with the previously reported results37,38,117 (Table 3, entries 3-4). A limited screening of a variety of para-substituted phenylboronic acids showed the effect of directing groups on the yield of the reaction. It was found that both the strongly and the weakly electron-donating groups (meth- oxy and methyl) resulted in moderate conversion. A strongly electron with- drawing group (nitrile) had a deactivating effect, and the observed conver- sions were significantly lower (Table 3, entries 3-4). The reaction time could be significantly reduced while maintaining or even increasing the conver- sions by performing the reaction at a higher temperature using microwave- assisted heating (Table 3, entry 4).

2.4.2 Fullerene dumbbells via metal-catalysed hydroarylation Having gained some insight into the reaction, attention was turned to devel- oping fullerene dumbbells through Rh-catalysed hydroarylation (Scheme 12). Attention was given to several important parameters, related to the de- sired properties of the target dumbbells.

Figure 14. A library of rigid conjugated bis-boronic acid linkers, designed for syn- thesis of fullerene dumbbells through Rh-catalysed hydroarylation.

A library of suitable linkers was designed (Figure 14).121 Firstly, to com- bat the solubility and purification issues, long aliphatic side-chains were introduced to the linker backbones. Octadecyloxy side-chains were chosen, as they had successfully been used in the fullerene dumbbell described in Chapters 2.2-2.3. It was also considered important for the linkers to be rigid, separating the fullerenes over a certain distance. Too short linkers would run the risk of sterically interfering with the hydroarylation reaction. Finally, potential electron and energy transfer across the target fullerene dumbbells is a desirable property, as the dumbbells were to serve as a model of a main- chain fullerene-based molecular wire. It was therefore important to design

40 the linkers to promote semi-conducting behaviour, which in case of conduct- ing polymers is gained through conjugation. Delocalisation of electrons throughout a π-system reduces the band gap and increases the conductive properties of the fullerene dumbbell and the possible oligomers. Both a symmetrical and an unsymmetrical linker were considered (29 and 30 re- spectively). Ethynyl linkers were used to provide conjugation, length, and rigidity to the molecules. Additional functionalisation to the linker backbone might be possible, for example, by introducing pH-sensitive groups (31). The overall synthesis is shown in Scheme 13. The synthesis relies on So- nogashira coupling reactions,122 which can be performed very efficiently and in high yields using microwave irradiation.123 Conversion of aryl halides to boronic acids can be done through the formation of a or through lithiation. The lithiation approach is generally known to provide a cleaner and easier workup and is therefore preferable.124 Purification of bo- ronic acids is difficult, as typically only small boronic acids can efficiently be recrystallised,119 while most boronic acids degrade upon column purifica- tion. It is therefore important to synthesise boronic acids with as little by- and side products as possible, and to use them without further purification.

Scheme 13. Conditions: i) NaH, THF, BrC18H37, reflux, yield: 54%; ii) CuI, (PPh3)2PdCl2, DEA, trimethylsilylacetylene, MiW 120 °C, 45 min., yield: 81%; iii) MeOH, Et2O, K2CO3, RT, 4 h, yield: 99%; iv) CuI, PPh3, (PPh3)2PdCl2, DEA, MiW 120 °C, 45 min., yield: 75%; v) CuI, (PPh3)2PdCl2, DEA, MiW 120 °C, 45 min., yield: 68%; vi) Sonogashira coupling; vii) a) n-BuLi, -78°C; b) B(OMe)3, THF, - 78°C. Dashed arrows indicate not yet performed steps.

41 The key intermediate 35 was obtained from commercially available 32 in three steps in an overall yield of 43%. The performed Sonogashira coupling reactions showed moderate to high yields of 68% (39) and 75% (38). Linker precursor 41 was also isolated during synthesis of 39 as a minor side prod- uct, presumably as a result of a Glaser coupling (Scheme 14).121,125

Scheme 14. Formation of 39 (top, Sonogashira coupling) and 41 (bottom, Glaser coupling) in the same reaction mixture. Conditions: CuI, (PPh3)2PdCl2, DEA, MiW 120 °C, 45 min., yield 68% (39).

Multiple steps in the synthesis of the linker library have to date been completed. An additional symmetrical linker precursor 41 was obtained. Conversion of the aryl bromide linker precursors to boronic acids and hy- droarylation of fullerenes remains to be performed. Should this prove suc- cessful and the purification of target dumbbells possible, the target mole- cules need to be characterised. Spectroscopic methods (UV/vis, IR), 1H and 13C NMR, and MS are essential for confirming the structure of the mole- cules. The electrochemical characterisation (CV) is expected to provide in- formation regarding whether the dumbbells are of potential interest for the concept of molecular wires.

2.5 Conclusion The synthesis of ferrocene-linked fullerene oligomers outlined in Scheme 1 requires a high degree of control, both in order to control the regioselectivity and the assembly. The previous strategy94 was improved and expanded. In Paper I, we developed and successfully applied a stepwise synthesis of a fulleropyrrolidine-ferrocene dumbbell 3 as well as its fulleropyrrolidine- tryptophan analogue 12. In Paper III, we extended this strategy to a trimer. This was done by introducing a bis-functionalised fulleropyrrolidine 21, designed and obtained with high regioselectivity through a tether-directed approach. The target trimer and its fragments were observed with MS, even though optimisations are still required to isolate the trimer in any significant quantity. An unexpected co-product in the synthesis of this molecule was a

42 mechanically-interlocked dimer 25, a catenane. Investigation of its formation is the subject of Paper II. En route towards fullerene-based molecular wires of main-chain charac- ter, we studied a more exotic rhodium-catalysed functionalisation of fuller- enes through hydroarylation.37 Factors affecting the reaction, such as degasi- fication, the two-phase solvent system, and the role of substrate substituents were investigated. The obtained yields were in line with the ones reported for similar reactions when extensive degassing conditions were used. In Pa- per IV, a library of conjugated linkers was designed and several precursors synthesised. Significant work remains to be done, both in terms of synthesis of the target novel dumbbells and the characterisation thereof. Overall, a fullerene-based main-chain molecular wire presents both an in- teresting and challenging concept. The complicated syntheses and purifica- tions revealed the complexity of fullerene and related chemistry, providing an insight into the field. During the course of these projects, several synthet- ic challenges related to the reactivity, purification, and analysis of fullerenes, ferrocenes, and derivatives thereof have been encountered and solved. The experience gained in the process is useful for any organic chemist venturing into the world of macromolecules and materials chemistry.

43 3 Graphene story (Papers V-VI)

3.1 Introduction Unlike fullerenes, which are molecules of determined structure, size and properties, graphene is a flat molecular material of large, but varying size and shape. The size is determined by the manufacturing process; it may have defects and requires different characterisation methods, as compared to the individual molecules. In order for graphene to find its way into applications, it typically has to be placed on a substrate. We were interested in depositing FLG graphene flakes on a piezoelectric polymer substrate. The results of this study are presented in Chapter 3.2 (Paper V). Graphene can be chemically modified for different purposes, such as in- creased solubility. Modifications can involve covalent (typically oxidation) or non-covalent (by surfactants, for example) chemical modifications of molecular structure. Covalent functionalisation of graphene through oxida- tion commonly requires harsh conditions and a strong oxidising agent, and thus has a low degree of control and selectivity. For example, the Hummers’ method involves the use of , permanganate, and sodi- um nitrate,71 whereas the Staudenmaier method uses concentrated sulfuric and together with potassium chlorate.72 The study of ozonolysis of PAHs as graphene models was recently conducted in our group59 and indi- cated a preference towards certain edge patterns and defects in the graphene lattice. In Chapter 3.3, the results concerning the mild oxidation of various graphitic materials are presented (Paper VI).

3.2 Manufacture and deposition of FLG The goal of this study was to deposit FLG flakes from suspension on a pie- zoelectric polymer substrate. The formed graphene layer was expected to maintain conductivity even when the substrate was bent. The long-term ob- jective was a sensor device, in which graphene acts as a transparent conduc- tive layer. The selected polymer, which has the required piezoelectric prop- erties, was poled PVDF.76,77 It is commercially available; however, the pie- zoelectric properties of PVDF are lost above 70 °C, which set temperature limitations for this study. The deposition of unmodified allotropes of carbon on the surface of poled PVDF has not been studied to any significant

44 extent,61 but the composites of PVDF and graphene or carbon nanotubes have been reported.126–128 Suspensions of FLG flakes in toluene were prepared by a previously es- tablished method.56 Drop deposition was attempted and expectedly resulted in spots of unevenly distributed FLG flakes (“coffee-ring” effect). Dip depo- sition appeared more promising;70 the FLG flakes were distributed evenly over the substrate surface, and little aggregation was found. The deposition process is schematically presented in Figure 15.

Figure 15. Schematic illustration of the process of deposition of FLG flakes from suspension onto polymer substrates via dip deposition.

Inspection of the LOM images in more detail revealed deposition of flakes on only one face of the substrate plate: the one designated by the manufacturer as “-ve”. The FLG flakes neither aggregated nor formed a monolayer, nor overlapped to any significant extent. Instead, the spacing between the flakes decreased as new ones were deposited on the unoccupied areas as seen in the SEM images in Figure 16.

45

Figure 16. SEM images of FLG flakes deposited on the “-ve” face of poled PVDF. These images were chosen to illustrate: a) surface coverage; b) a low degree of over- lap between flakes; c) a high-resolution view of thin flakes; d) a close-up of the previous image.

Apart from the poled PVDF, deposition on several other commercially available polymer substrates was also studied. These were polyethylene (PE), polytetrafluoroethylene (PTFE), and non-poled PVDF. Table 4 shows the degree of coverage achieved for different polymer substrates and after multiple immersions. The degree of coverage of the “-ve” face of the poled PVDF after one immersion (Table 4, entry 5) was estimated to be 20%, based on image analysis. This is in contrast with the low coverage of the other substrates (Table 4, entries 1-4) and the “+ve” face (Table 4, entry 4). The maximum degree of coverage that has been achieved was estimated to be 77% (Table 4, entry 13). The flakes showed very limited overlapping or contact, even at this surface coverage.

46 Table 4. Degree of surface coverage observed for different polymeric substrates upon immersions in a suspension of FLG in toluene (10 seconds). Coverage is achieved through dip deposition (single/multiple) and calculated from LOM images. Entry Substrate Number of immersions Estimated surface coverage a 1 PE 1 2% 2 PTFEa 1<1% 3 Non-poled PVDFa 13% 4 Poled PVDF, ”+ve” 1<1% 5 Poled PVDF, “-ve” 1 20% 6 Poled PVDF, ”+ve” 3<1% 7 Poled PVDF, “-ve” 3 30% 8 Poled PVDF, ”+ve” 5<1% 9 Poled PVDF, “-ve” 5 40% 10 Poled PVDF, ”+ve” 10 <1% 11 Poled PVDF, “-ve” 10 68% 12 Poled PVDF, ”+ve” 15 <1% 13 Poled PVDF, “-ve” 15 77% a: Without net dipole moment.

To explain the observed effect, one has to consider the structure of the substrates. PVDF chains can adopt several phases with alpha and beta domi- nating, where the beta possesses the dipole moment responsible for the pie- zoelectric properties of the material.129 The phases are conformers of each other, the difference lying in the relative orientation of the substituents. In non-poled PVDF, the phases are distributed randomly, giving rise to no or very localised net dipole moment. Similarly, PE and PTFE possess no net dipole moment. Stretching of the heated PVDF in an electric field (poling) largely converts alpha phase to beta.76,77 The polymer chains orient them- selves in the strong electric field with the fluorine atoms facing the positive anode, creating the “+ve” face. Similarly, the hydrogen atoms produce the “-ve” face. This treatment introduces a permanent net dipole moment; the “+ve” face carries a negative net charge, while the “-ve” face carries a posi- tive net charge. In graphene, an extensive system of π-electrons is delocalised over the flake. The flakes are attracted to the positive charge of the permanent dipole, i.e. the “-ve” face of the PVDF, and repelled by the negative charge of the “-ve” face. This behaviour is primarily dictated by polar-π interactions.130 In the absence of dipole moment in the sheets of PE and PTFE, the absorption of FLG on these substrates is guided by much weaker interactions and is therefore inefficient. In non-poled PVDF, regions of alpha and beta phase are evenly distributed. As the area of an average flake is relatively large, the attraction and repulsion to the substrate cancel out, making self-assembly on either face unlikely. In our system, the flakes show a very strong tendency towards the “-ve” face of the piezoelectric PVDF. Proportions of the beta phase in the PVDF substrates were estimated using IR spectroscopy to 34%

47 (non-poled) and 86-88% (poled); these values are in accordance with the literature.79,131 Once an FLG flake has adsorbed on the “-ve” face, the positively charged surface under the flake is blocked. The surface facing the suspension at this particular area will be dominated by the π-electrons of the adsorbed FLG flake. As a result, any additional flakes would adsorb on the unoccupied surface of the H-enriched face of the substrate rather than on top of already deposited flakes. Because the surface area of the substrate is limited and the mobility of the adsorbed flakes is low, flakes with a large area are primarily deposited in the initial immersions. The observations made in this study were rationalised. They provided an insight into the interactions and adsorptive properties of graphene with re- gard to an important piezoelectric polymer material.

3.3 Mild oxidation of graphitic materials Among the different methods for manufacturing graphene (Figure 5) the one currently used to the largest extent is the oxidation of graphitic precursors to GO and its subsequent reduction to rGO.132 This easily scalable, efficient, and inexpensive process produces flakes of rGO; the downside, however, is the formation of defects that reduce the valuable properties of graphene, such as conductivity. The process typically involves metals, acids, and harm- ful reagents, which generates large volumes of waste and is not environmen- tally sustainable.71,133 Milder methods have been applied in oxidative de- struction of PAH in environmental systems.134,135 In this project, we aimed to apply such methods to graphitic materials to achieve oxidation in an envi- ronmentally sustainable fashion. Graphitic materials chosen for the study were graphite foil, natural graphite, and HOPG. CVD graphene was used as a model. The materials were treated with combinations of aq. hydrogen peroxide (30%) and ozone, with or without bath sonication. Upon isolation by filtra- tion or freeze-drying, the samples were analysed by LOM and Raman spec- troscopy; for graphite flakes, TGA was performed. For detailed summary of the reaction conditions, see Table 1 in Paper VI. The samples subjected to sonication all demonstrated increased roughness of the surface, which can be attributed to mechanical damage. Two-fold swelling was observed for the graphite foil treated with hydrogen peroxide. Further swelling and deformation occurred upon sonication. Combination of H2O2, O3, and sonication led to a significant 40-fold swelling (Figure 17), followed by sample dissociation with increased treatment time. Similar be- haviour was observed for HOPG, which also showed swelling and dissocia- tion into sheets. The swelling of graphite flakes was found to be insignificant as compared to the results obtained for the graphite foil.

48

Figure 17. Photographs of the untreated graphite foil (1), foil treated with H2O2 (5) and treated with H2O2 and O3 on bath sonication for 6 h (8). The numbering reflects the entries in Table 1 in Paper VI. The pieces of graphite foil measure ca 5×5 mm.

From the Raman spectra, the ID/IG ratios were calculated to estimate the amount of defects in the materials (Chapter 1.5.2), for summary see Table 1 in Paper VI. The ID/IG ratios indicated introduction of increased quantities of defects to the materials upon oxidative treatment. The combined effect of O3 and H2O2 was typically found to be larger than for the individual treatment, indicating cooperative effect; sonication further increased the degree of de- fects. Time of treatment was found to be important, and long reaction times were required to achieve a degree of defects detectable by other methods (TGA). The ID/IG ratios did not indicate formation of any detectable defects at the surface of HOPG; however, at the edges, the increase in defects was apparent. The results should be interpreted with caution, as the variations in ID/IG ratios were found to be high when the measurements were repeated for the graphite flakes. TGA indicated the loss of ca 1.5 - 2 % of the sample weight between 150- 200 °C (Figure 18, left), which is the range where hydroxyl and epoxy func- tionalities are known to decompose.85–88 This value is close to the ones re- ported for edge-oxidised graphene flakes,136 and also significantly lower than what is commonly found in rGO.85–88

49

Figure 18. Left: TGA analysis of the graphite flakes, subjected to oxidative treat- ment. Right: estimated concentrations of graphene and FLG flakes in dispersions of EtOH (90% aq.), prepared from graphite flakes subjected to oxidative treatment according to Table 1 in Paper VI. The concentrations are given as average values of duplicate dispersions. The numbering reflects the entries in Table 1 in Paper VI: untreated graphite flakes (1), flakes treated with H2O2 and O3 on bath sonication for 12 (9) and 72 hours (11).

In order to see how the treatment affected physical properties, dispersions of the treated flakes in (90% in water) were prepared. The concentra- tion of flakes was estimated with UV/vis absorption using previously report- ed methods.137,138 The results were in line with the values commonly ob- served for graphene flakes and rGO.139,140 A trend in line with the one found for the ID/IG measurements was observed, as dispersibility of the treated flakes reduced with increased reaction times (Figure 18, right). The results obtained to date allow for certain conclusions. Defects have been introduced to the graphitic materials upon treatment with O3 and H2O2. The combination of the two oxidising agents was found to be more efficient, especially upon prolonged sonication, as shown by Raman spectroscopy. The defects are likely to be oxidation, as indicated by TGA. The amount of defects is low but sufficient to change the morphology (swelling, dissocia- tion) and the physical properties (dispersibility). Examples were found in the literature where edge oxidation of graphite was achieved by a variety of methods.136,141–143 Analysis of HOPG supports this pathway, as does the SEM imaging of CVD graphene treated with O3 and H2O2 (Figure 3 in Paper VI). The graphene sheet is gradually decomposed upon treatment, with de- fects and edges primarily being affected. The method produced mildly oxidised flakes of graphene and FLG, with- out the use of metals, acids, or organic reagents, while at low temperatures. Purification is a simple filtration or freeze-drying, and the reagents are wa- ter-soluble or volatile. Further studies are required to optimise the reaction conditions and to establish the degree of functionalisation and properties of the obtained materials.

50 3.4 Conclusion A study of the deposition of FLG flakes on piezoelectric polymer (PVDF) was conducted. The distinctly different faces of piezoelectric PVDF sub- strate showed opposite tendencies towards of FLG. The “+ve” (F-enriched) face repelled, while the “-ve” (H-enriched) face attracted, the FLG flakes. The maximal degree of surface coverage of the “-ve” face was estimated to be 77%. The flakes were neither overlapping nor in contact with each other to any significant extent. These results are presented and dis- cussed in Paper V. Covalent modification of graphene is the topic of Paper VI, where mild and environmentally sustainable oxidising agents, hydrogen peroxide and ozone, were used to achieve the oxidation of graphene and FLG. A wide scope of graphitic materials was subjected to the treatment, and the results were evaluated and compared. The results obtained to date indicate mild oxidation sufficient to change the physical properties of the material, such as dispersibility and morphology. SEM imaging and TGA analysis indicate the edge oxidation pathway. When conducting research at the boundary of organic chemistry and ma- terials science, one has to be aware of the differences in methods and tech- niques within the two disciplines. A variety of analysis and visualisation methods have been used in projects related to graphene research, such as LOM, SEM, and Raman spectroscopy. Due to the complexity of graphene and related graphitic materials (GO, rGO, HOPG), the interpretation of the results of various analyses presents a challenge for any researcher. The pro- jects provided a valuable insight into the field of materials science in general and graphene research in particular.

51 4 Concluding remarks and outlook

The work presented in this thesis has been dedicated to developing materials, based on or composed of the carbon allotropes C60-fullerene and graphene, and to studying their properties. The first half of this thesis is devoted to the synthesis of fullerene-based materials, with a long-term goal of application in molecular wires (Figure 9). In Paper I, we demonstrated a stepwise approach to an organometallic ful- leropyrrolidine dumbbell. It provided a useful tool to the controlled synthesis of more complex assemblies, such as a ferrocene-linked fulleropyrrolidine- amino acid dumbbell 12. It can serve as an entry point to incorporating or- ganometallic fullerene derivatives into peptides for biological applications. This stepwise strategy was applied in the synthesis of fullerene-based oli- gomers. In Paper III, a tethered fullero-bis(pyrrolidine) 21 was used in as- sembling a main-chain fullerene-based trimer 16. The target trimer was formed as a minor product, and substantial optimisation of the synthesis is therefore required. In addition, a dimer of 21 was observed and, by an inves- tigation of an N-protected analogue 26a, revealed to likely be a catenane (Paper II). Formation of fullero-bis(pyrrolidine)-based catenanes has, to the best of our knowledge, not been reported previously. In Paper IV, we evaluated and optimised an unusual metal-catalysed addi- tion to C60. We also initiated the synthesis of diarylacetylene-based linkers for use in a synthesis of conjugated fullerene dumbbells. The initial results appear promising, but even so, a significant amount of work remains before the dumbbells can be isolated and characterised. The second half of this thesis deals with graphene chemistry. Exfoliation of graphite was used due to the method’s scalability, cost-efficiency, and relatively high product quality. Deposition of FLG on a piezoelectric poly- mer (PVDF) demonstrated selective self-assembly of flakes. The results were discussed and rationalised in Paper V. Other deposition methods, such as the Langmuir-Blodgett approach, could also be investigated. Finally, in Paper VI, we evaluated the oxidation of a number of graphitic materials using environmentally sustainable oxidation conditions as an alter- native to the conventional oxidation of graphene by the Hummers’ method.71 The results indicated mild and likely edge-selective oxidation with the bene- fit of being metal and acid-free. Properties of the obtained materials remain to be evaluated. The developed method can potentially complement or re- place the methods in industrial use today.

52 5 Acknowledgments

First and foremost, I would like to express my sincere gratitude to my super- visor, Professor Helena Grennberg, for introducing me to the exciting world of organic chemistry and research. Thank you for accepting me as your graduate student, for all the support and guidance during these years, and for your open-door policy which I often abused. I admire your ability to see positive sides of every experiment or event, even when I was ready to inter- pret them as utter failures. I also wish to thank my co-supervisor, Professor Adolf Gogoll, for all the feedback and advice he has given me over the years and for his support with the NMR experiments. Champagne tasting and sur- strömming evaluation have also been most memorable!

Thank you to all my colleagues in the AGHG group (past and present) for the relaxed atmosphere, fruitful discussions, and valuable advice: Dr. Magnus Blom, Dr. Sara Norrehed, Dr. Claes-Henrik Andersson, Dr. Matthew J. Webb, and Dr. Leili Tahershamsi.

Special thanks to: Sandra Olsson for being a great companion at confer- ences. I will not soon forget our hiking and whale watching in Sydney. Dr. Hao Huang for all the fun we had both in and out of the laboratory, and of course for all the foodie experiences. Anna Lundstedt for being one of the few who actually understood what I was doing. Remember “badtunnaaa”? Xiao Huang for sharing positive energy and your strong spirit. My former students Sofie Ye, Johannes Kazen Orrefur, and Tyran Günther for being very valuable “extra hands” in the laboratory.

I would also like to thank all the people at both departments of chemistry at Uppsala University for making everyday life better, especially: Dr. Maxim, Dr. Johan Verendel, and Dr. Alexander (former lab-mates from the PGA group); Carina, Fredrik, and Dr. Karthik of the Pilarski group; Dr. Rikard, Rabia, Kjell and Aleksandra D. of the Ottosson group; Dr. Christian, Dr. Emil, Aleksandra B., Jie, Lina, Jiajie, Susanna, Dr. Jia- Fei, Keyhan, and Postprof. Thomas.

53 I would also like to acknowledge the technical staff of the department: Bosse, Gunnar, Johanna, and Tomas. Your solid efforts have frequently made my work significantly easier.

A big thank you to my dear friends from outside of the world of organic chemistry: Ilja, Malin, Oleg and Kate S., Hilde, Mathias L., Petter, Andreas, Lina N., Dr. Kate and Joe. You probably do not realise how much you have supported me over these years through game nights, an occasional beer or two, tennis, photography, or a simple conversation. Special thanks to Dr. Christopher “spionen” Bishop, who apart from the actions above, also proofread this thesis.

Finally, my warmest thanks and gratitude go to my family: my parents for their endless support, even when the aims often were incomprehensible; and my dear Randi for all her love and care, and for everything she has done and continues to do for me. It would not have been worth it without you.

The work presented in this thesis was supported by generous financing from the Swedish Research Council (Vetenskapsrådet), the Uppsala Univer- sity KoF priority programs on molecular electronics and graphene, and the Knut and Alice Wallenberg Foundation (KAW Graphene).

Personal grants were gratefully received from the Swedish Chemical So- ciety, C. F. Liljewalch’s Foundation and Anna Maria Lundin’s Fund.

54 6 Populärvetenskaplig sammanfattning på svenska

Kolnanostrukturer – från molekyler till funktionaliserade material Oligomerer av fulleren-ferrocen, modifiering och deponering av grafen Den brittiske kemisten John Dalton formulerade sin atomteori år 1803.144 Teorin beskriver hur all materia är uppbyggd av atomer, att föreningar (mo- lekyler) är kombinationer av två eller flera atomer, samt att kemiska reakt- ioner går ut på omlagring av atomer. Forskningen har utvecklats enormt sedan dess, men Daltons teori utgör fortfarande grunden för vår världsbild. Kol är ett av våra vanligaste grundämnen och har varit i fokus i den här avhandlingen. Grundämnet kol förekommer i flera former, så kallade al- lotroper (Figur 1). Ett känt exempel är diamant, som byggs upp av kolatomer i ett ordnat tredimensionellt nätverk. När kolatomer istället arragerar sig i ett tvådimensionellt nätverk, likt hexagoner i en vaxkaka, skapas kolallotropen grafen. Flera tätpackade grafenlager bildar grafit. Slutligen, kolatomer som formar en sfär bildar allotropen fulleren, där atomerna är ordnade i hexa- och pentagoner.

1 Figur 1. Kolallotroperna diamant, grafen och fulleren (C60).

Egenskaper och därmed tillämpningsområden hos dessa allotroper skiljer sig åt. Diamant är känd för sin hårdhet och optiska egenskaper, grafen har en utmärkt ledningsförmåga, betydligt högre än den hos grafit, medan fullerener normalt inte leder ström alls. När man talar om funktionalitet och tillämp- ningar av de olika ämnena brukar man kalla dem ”material”. Materialet gra- fit hittas bland annat i blyertspennor, fullerener används i vissa solceller och

1 Illustration av Josef Sivek, under licensen CC BY-SA 4.0,145 omarbetad av författaren.

55 grafen har nyligen börjat göra intåg inom elektroniken. Organisk kemi, läran om kolföreningarnas egenskaper och reaktioner, ger oss verktyg att förändra föreningar, inklusive kolallotroper, med kemiska reaktioner. Genom att på ett kontrollerat sätt modifiera och koppla ihop föreningarna kan vi utveckla nya material och förse dessa med önskade egenskaper. I den första delen av den här avhandlingen diskuteras olika strategier för att länka ihop fullerener med andra organiska föreningar till längre trådlik- nande strukturer (Figur 2). Dessa har potentiell användning som ledande molekylära material med tillämpningar inom molekylärelektonik. Arbetet bedrevs med organisk-kemiska metoder då fullerener på grund av sin storlek och egenskaper är att betrakta som molekyler. Vi har: • Utvecklat och optimerat en strategi för att koppla ihop fullerener med en organometallisk länk, och tillämpat denna för att bygga upp längre ked- jor, bestående av två respektive tre sammanlänkade fullerener. • I ovan nämnda studie formades en intressant sidoprodukt: en förening där två fullerenderivat satt ihop likt två OS-ringar. • Vi har också designat ett bibliotek av molekyler, lämpliga för samman- länkning av fullerener med en relativt ostuderad additionsreaktion, och testat denna med modellföreningar. Den andra delen av avhandlingen behandlar studier av grafen (Figur 2). Grafen bör utifrån sina egenskaper och tillämpningar betraktas som material, vilket motiverade val av arbetsmetoder. Vi har: • Deponerat grafen på ett elektriskt polariserat substrat från suspensioner och observerat att grafenbitar selektivt adsorberade enbart på den ena si- dan av substratet. • Funktionaliserat grafit genom en mild och miljömässigt skonsam metod och visat att reaktionen sker främst i kanterna av grafenlagren i grafiten. Metoden är lovande, främst då den är kostnadseffektiv och skonsam, men kräver ytterligare forskning för att leda till en praktisk tillämpning inom grafenindustrin.

Figur 2. Schematisk illustration av arbete beskrivet i denna avhandling.

Sammanfattningsvis har avhandlingen tagit upp studier av olika aspekter av kolallotropernas kemi i syfte att utveckla nya potentiellt användbara avancerade funktionella material.

56 7 References

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A doctoral dissertation from the Faculty of Science and Technology, Uppsala University, is usually a summary of a number of papers. A few copies of the complete dissertation are kept at major Swedish research libraries, while the summary alone is distributed internationally through the series Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology. (Prior to January, 2005, the series was published under the title “Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology”.)

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