Carboxy Fantrip Monomer Was Used in the Langmuir-Blodgett Synthesis of Poly(Carboxyfantrip)

Home , Thioanisole

University of Nevada, Reno

The Synthesis and Fabrication of Two-Dimensional

Polymers

A dissertation submitted in partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Chemistry

William Bailey Thompson

Dr. Benjamin T. King/Dissertation Advisor

May 2019

THE GRADUATE SCHOOL

We recommend that the dissertation prepared under our supervision by

William Bailey Thompson

Entitled

The Synthesis and Fabrication of Two-Dimensional Polymers

be accepted in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Benjamin T. King , Advisor

Christopher S. Jeffrey , Committee Member

Robert S. Sheridan , Committee Member Lora Robinson , Committee Member

Jonathan Weinstein , Graduate School Representative

David W. Zeh, Ph.D., Dean, Graduate School

May-2019 i

Abstract

The Synthesis and Fabrication of Two-Dimensional Polymers

William Bailey Thompson

Ph.D Advisor: Professor Benjamin T. King

With the isolation of graphene, there has been a proverbial gold rush in the field of two- dimensional (2D) materials because their unique properties promise numerous applications. The synthesis of 2D materials is currently a hot field with many branches, one of them being synthetic two-dimensional polymers (2DPs). 2DPs share many similarities with other 2D materials but promise tunable properties for various applications. This dissertation focuses on the synthesis and use of monomers that can be fabricated into 2DPs by crystallization or Langmuir-Blodgett approach.

Chapter One will explore how 2DPs are classified and showcase some of the current methods for the synthesis of 2DPs. The pros and cons of each method will be highlighted and potential applications of 2DPs will be discussed.

Chapter Two describes the synthesis of the monomers fantrip and carboxyfantrip. The fantrip monomer was used in the crystalline-state synthesis of poly(fantrip) and the carboxy fantrip monomer was used in the Langmuir-Blodgett synthesis of poly(carboxyfantrip). The chapter also describes optimization of their syntheses.

Chapter Three focuses on the fabrication of poly(carboxy fantrip) by Langmuir-Blodgett techniques. The chapter contains a brief theory behind Langmuir-Blodgett and characterization ii techniques used on 2DP films. Past and current challenges of using poly(carboxy fantrip) are also present in the chapter.

Chapter 4 contains different gas flow measurement through poly (carboxy fantrip). The chapter focuses on using fabricated film for gas flow studies and explains challenges occurred in obtain gas flow data for poly(carboxy fantrip).

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This thesis is dedicated:

To my friends and family for their love and support.

Acknowledgements

I would like to thank my parents and family for their support throughout the years. They have

done so much for me over the years that I might not ever be able to repay their kindness, but I

will try my hardest to find a way.

I would like to thank my advisor Benjamin T. King for the opportunity to work on a variety of

novel projects and introducing me to the wider world of chemistry.

I would like to thank the entire chemistry department at UNR, faculty and students for all the

good memories and the camaraderie. There are just too many people to name on here.

I would like to thank the entire King group, past and present. There may never be a better group of people to work with ever again. Special thanks to Dustin, Daniel, Jonathan, Carey, Devin,

Manuel, Drew. The memories from this lab will last a lifetime.

A special shout out to Devin Grainer who helped me improve this dissertation with his

extensive chemistry knowledge and better grammar.

A special shout out to Dr. Patterson (Dustin) for all the time talking about chemistry and life.

Also, for helping me to improve my golf game.

Table of Contents

Chapter 1……………………………………………………………………………….………...1

1.1. Two-Dimensional Polymers and Fantrip…………………………………………………….1

1.2. Two-Dimensional Polymers…………………………………………………………………5

1.3. Possible Applications of 2D-Polymers…………………………………………………..…..9

1.4. Synthesizing 2DPs……………………………………………………………………….....12

1.5. Solid Surface Approach to 2DPs ……………………………………………………..……16

1.6. Solution Phase Approach…………………………………………………………………...17

1.7. Air-Water Interface Approach………………………………………………………….…..20

1.8. Precursor to Fantrip Monomer Synthesis……………………………………………….…..22

1.9. References………………………………………………………………………………….28

Chapter 2: Synthesis of Carboxy Fantrip…………………………………………………....…..31

2.1 Past Monomers for 2DP………………………………………………………………..…….31

2.2 Synthetic Route Towards Fantrip……………………………………………….……………33

2.3 Starting Synthesis of Carboxy Fantrip………………………………………………………..38

2.4 Different Methods for Cheletropic Eliminations………………………………………….….50

2.5 Lithium Reagents for Benzyne Formation……………………………………………….…..56

2.6 Carboxylic Acid Protection…………………………………………………………..………60

2.7 Rearrangement of Triptycene Core………………………………………………….……….65

2.8 Reactivity of the Monomers…………………………………………………………….……67 vi

2.9 Conclusion……………………………………………………………………………………70

2.10 Experimental Section………………………………………………………..………..…….71

2.11 References………………………………………………………………………..…….….107

Chapter 3 Fabrication of Poly (carboxy fantrip) Films…………………………………..……..110

3.1 Langmuir Films…………………………………………………………………….…….…110

3.2.1 Parts of the Langmuir-Blodgett Trough…………………………………...……….……..114

3.2.2 Solubility of Compounds……………………….…………………………………..…….116

3.3 Causes of Defects in Film…………………………………………………………………..118

3.4.1 Transferring Langmuir Films……………………………………………………....……..119

3.4.2 Horizontal Deposition……………………………….………………………………..…..121

3.5 Langmuir-Shäfer (Horizontal Transfer)………………………………………………...…..122

3.5 Substrate Functionalization……………………………………………..…………….…….122

3.6 Brewster Angle Microscopy (BAM)……..……………………………………………..….125

3.7 Optical Microscopy………………..………………………………….………………….…127

3.8 Scanning Electron Microscope (SEM)………………………..……………………………129

3.9 Poly(carboxy fantrip) Films………………………………………………………..……….129

3.10 Defects in poly (carboxy fantrip)………………………………………………………….138

3.12 Stability of Poly (carboxy fantrip) Film…………………………………………………...145 vii

3.13 Roughness of Substrates…………………………………………………………………...152

3.14 Conclusion………………………………………………………………………………....155

3.15 Experimental…………………………………………………………………………...….156

3.16 References……………………………………………………………………………...….160

Chapter 4: Poly (carboxy fantrip) Gas Flow……………………………………………....…...163

4.1 Gas Flow Through Membranes…………………………………………………………..…163

4.2 Gas Flow Through poly (carboxy fantrip)……………………………………………….…165

4.3 Conclusion…………………………………………………………………………………..169

4.4 Experimental………………………………………………………………………………..170

4.5 References……………………………………………………………………………..……172

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Chapter 1 List of Figures

Figure 1.1 Three different 1-dimensional polymer architectures……………………….…………2 Figure 1.2 Different topologies of polymers types………………………………………….……..3 Figure 1.3 A section of molecular structure of graphene, the archetypal 2D polymer…………....4 Figure 1.4 Example of poly(styrene) and poly(styrene-butadiene-styrene)……………….………6 Figure 1.5 A tiling set where a single tile being used to cover a plane…………………….……...7 Figure 1.6 In tiling there is three sets of tiles that can be described……………………….………8 Figure 1.7 A couple of possible monomer configuration………………………………….………8 Figure 1.8 Müller and co-workers monomer used to make their 2DP……………….…….……..10 Figure 1.9 An illustration of a monolayer membrane…………………………………….………11 Figure 1.10 Monomer synthesized to make the 2DP after crystallization………………….…….14 Figure 1.11 Monomer synthesized and crystalized and polymerize to form the crystals…….…..15 Figure 1.12 Structure of antrip monomer………………………………………………….……..16 Figure 1.13 Using Ullman reactions on the surface of copper produced sheets…………….……17 Figure 1.14 Triptycene core monomer was used in the synthesis of their 2DP in solution….…..19 Figure 1.15 Bai and co-workers synthesis a set of monomers when put into solution…….……..20 Figure 1.16 A terpyridine monomer unit placed on the air/water interface………………….…...22 Figure 1.17 Resonance structures of anthracene…………………………………………….……22 Figure 1.18 [4+4] cycloaddition of anthracene……………………………………………….…..22 Figure 1.19 The reactive anthracene blades attached to the antrip molecule……………….……23 Figure 1.20 The free bond rotation of Fridel Craft reaction………………………………….…..24 Figure 1.21 Antrip’s example of a regioisomer with bond rotation………………………….…..25 Figure 1.22 The less stable isoindole moiety and the fluoro substituted isoindole……….……...26 Figure 1.23 The different stackings of molecules based on their electronics……………….……27

Chapter 2 List of Figures

Figure 2.1 Antrip monomer and fantrip monomer………………………………………………..31 Figure 2.2 The monomer antrip-DEG and hydroxy fantrip……………………………………….32 Figure 2.3 HRMS of fantrip trifold with multiple stages of chelotropic elimination…………….37 Figure 2.5 TOF-HRMS of an impure sample of hexabromo triptycene carboxylic acid…………43 Figure 2.5 Crystal structure of carboxy HBT grown in THF……………….……………………43 Figure 2.6 DSC of carboxy HBT and TGA of carboxy HBT…………………………………….44 Figure 2.7 Possible bi-products of the three-fold benzyne reaction………………………………46 Figure 2.8 Carboxy trifold different byproducts after column chromatography………………….47 Figure 2.9 Cheletropic elimination into carboxyfantrip using hydrogen peroxide……………….48 Figure 2.10 Crude after cheletropic elimination with hydrogen peroxide…….………………….49 Figure 2.11 Crystal structure of carboxy fantrip using 1,4-dioxane as a cocrystal………………50 Figure 2.12 Cheletropic elimination of the N-methyl bridge by Gribble…………………………51 Figure 2.13 Carboxy fantrip precursor before pyrolysis and after pyrolysis……………………..53 Figure 2.14 Setup and synthesis of dimethyldioxirane (DMDO)…………………………………54 Figure 2.15 GCMS of DMDO reagent with thioanisole and the sulfoxide synthesized…………55 Figure 2.16 Nucleophilic aromatic substitution of isoindole……………………………………..56 Figure 2.17 A general reaction of nucleophilic aromatic substitution…………………………….57 Figure 2.18 The bromine-lithium exchange with tert-butyllithium………………………………57 Figure 2.19 Crude trifold adduct using n-BuLi and using t-BuLi ………………………………..58 Figure 2.20 Gribble and co-workers generation of Diels-Alder cycloaddition…………………..59 Figure 2.21 List of carboxylic acid protecting groups attempted…………………………………61 Figure 2.22 Triptoic acid reaction with thinoyl chloride…………………………………………62 Figure 2.23 Protection of carboxy hexabromotriptycene…………………………………………63 Figure 2.24 Synthesis of benzyl protected trifold adduct…………………………………………63 Figure 2.25 Cheletropic elimination of the benzyl ester trifold to benzyl ester fantrip………….64 Figure 2.26 Deprotection of benzyl ester fantrip into carboxy fantrip……………………………65 Figure 2.27 Demjanov rearrangement of a triptycene core from Penelle et al…………………..66 Figure 2.28 Ring opening of triptycene core……………………………………………………..66 Figure 2.29 Carboxy fantrip dried on vacuum pump right after HPLC. contains ~ 5 mg…………67 Figure 2.30 The reversibility of carboxy fantrip monomer in test tube…………………………..68 Figure 2.31 Hexabromotriptycene 1H NMR……………………………………………………..98 Figure 2.32 Optical microscopy images of partially exfoliated poly(fantrip) crystals………….105 Figure 2.33 IR spectra of fantrip monomer before irradiation and the IR of poly(fantrip)……..106

Chapter 3 List of Figures

Figure 3.1 Example of molecules being in a phases of on Langmuir-Blodgett trough...... 112 Figure 3.2: Polar head-group the amphiphile interacts with the liquid...... 113 Figure 3.3 A general setup for a Langmuir-Blodgett trough...... 114 Figure 3.4 A basic Langmuir-Blodgett trough with two movable barriers...... 115 Figure 3.5 Example of a Wilhelmy plate...... 116 Figure 3.6 Langmuir-films deposited on subphase...... 118 Figure 3.7 A vertical deposition...... 121 Figure 3.8 Examples of Shäfer’s method...... 122 Figure 3.9 Etching of silicon by HF...... 123 Figure 3.10 The cleaning of a silicon oxide layer with piranha solution ...... 124 Figure 3.11 Modification with GOPES and the immobilization of MACF antibody on a silicon nitride surface...... 125 Figure 3.12 Brewster angle microscopy of the monomer antrip-DEG...... 126 Figure 3.13 Mammalian cell where the top uses bright-field microscopy...... 128 Figure 3.14 structure of the monomer carboxyfantrip and dimer of carboxy fantrip…………. 130 Figure 3.15 Surface pressure and mean molecular area of carboxy fantrip monomer isotherm at the air/water interface……………………………………………………………………...131 Figure 3.16 Brewster angle microscopy of carboxy fantrip at the air/water interface with increasing amounts of surface pressure……………………………………………………132 Figure 3.17 SEM image of poly(carboxy fantrip) on the surface of a copper substrate………..132 Figure 3.18 Polymerization at the air/water interface ………………………………………….133 Figure 3.19 Copper (111) substrate with a ~50 µm hole ………………………………………134 Figure 3.20 Langmuir trough with copper substrate on top…………………………………….135 Figure 3.21 Poly (carboxy fantrip) covering the 50 µm hole with different coverage levels…..136 Figure 3.22 Optical microscopy of poly (carboxy fantrip) and SEM image of the same grid…137 Figure 3.23 Poly (carboxy fantrip) transferred onto SiO2 from the bottom up…………………138 Figure 3.24 Concentration of carboxy fantrip spread at the air/water interface………………..139 Figure 3.25 Optical microscopy of disclinations in poly (carboxy fantrip) film and SEM…….139 Figure 3.26 SEM of different defects that can occur during film fabrication…………………..140 Figure 3.27 Carboxy fantrip with grease impurities……………………………………………141 Figure 3.28 Optical microscopy of poly (carboxy fantrip) film on SiN………………………..142 Figure 3.29 Poly (carboxy fantrip) over SiN. ………………………………………………….143 Figure 3.30 Blank SiN surface, showing charging effects on the etched surface. ……………..144 Figure 3.31 SiN with film covering the pore…………………………………………………...145 Figure 3.32 Examples of damage to a film spanning a hole……………………………………147 Figure 3.33 Damage to poly (carboxy fantrip) film occurred from water……………………...148 Figure 3.34 Poly (carboxy fantrip) film spans over a copper grid and damage………………..148 Figure 3.35 Examples of poly (carboxy fantrip) film being pushed down into the substrate….149 Figure 3.36 Poly (carboxy fantrip) on an SiN window. …………………………………….…150 xi

Figure 3.37 SEM images of poly (carboxy fantrip) …………………………………………....151 Figure 3.38 Examples of longer exposure of electron charge and vacuum in SEM……………151 Figure 3.39 Poly (carboxy fantrip) with film covering around 100 µm holes………………….152 Figure 3.40 Steel substrates with 50 µm holes. …………………………………………..……153 Figure 3.41 Steel substrate showing its roughness. ……………………………………………153 Figure 3.42 Poly (carboxy fantrip) that has been damaged on a SiN surface. …………………154 Figure 3.43 Top down view of the Langmuir-Blodgett trough………………………………...157

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Chapter 4 List of Figures

Figure 4.1 Cartoon representation of poly (carboxy fantrip) over the TEM hole………………166 Figure 4.2 The added fluoroelastomer o-ring added onto the sample holder……………...…...167 Figure 4.3 Graph of measured permeance of gases compared to their molecular weight……...168 Figure 4.4 Setup for gas flow studies…………………………………………………………...170 Figure 4.5 Micropipettes used as a bubble flowmeter…………………………………….……171

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List of Abbreviations

1D One dimensional

2D Two dimensional

3D Three dimensional

2DP Two-dimensional polymer

HRMS High resolution mass spectrometry

TOF Time of flight

UV-VIS Ultraviolet-visible spectrophotometry

NMR Nuclear magnetic resonance

GCMS Gas chromatography mass spectrometry

DSC Differential scanning calorimetry

TGA Thermogravimetric analysis

AFM Atomic force microscopy

BAM Brewster’s angle microscopy

MMA Mean molecular area

SEM Scanning electron microscopy

TEM Transmission electron microscopy

SP Surface pressure

XRD X-ray diffraction 1

Chapter 1: 2-Dimensional Polymers and Fantrip

Anyone born after the 1950’s may not be able to remember a time in their lives without synthetic polymers. Due to their cost-effective, widespread use, and ease of mass production, polymers can be found everywhere. The world’s obsession with synthetic polymers did not start until the first commercially successful synthetic polymer, Bakelite, was developed by Leo

Hendrick in 1907.1 As of 2017, an estimated 8.3 billion metric tons of polymers have been produced since the early 1950’s, making polymers one of the most abundant materials ever produced by humanity.2 Just as the discovery and implementation of bronze and iron would define that age, polymers and their massive influence over our lives have set the recent history as the polymer age.

Naturally occurring polymers have been around since before recorded history and are found in many living animals and inorganic materials. However, it has only been relatively recently since chemists have learned how to produce and tune the properties of polymers to make a wide selection of materials, from red plastic cups to intricate medical devices. These products now account for over 300 million tons of polymer materials produced each year, and the trends shows that polymer demand will increase.3

Modern polymers are defined as long chains of small molecular repeat units that are linked together by chemical bonds.4 The first usage of the term polymers was in 18335 and used further to define other notable materials like polystyrene,6 poly(ethylene glycol),7 and poly(ethylene succinate.)8 The modern understanding of polymers did not come until the 1920’s where Hermann

Staudinger aimed to provide a molecular definition of polymers. At the time, some scientists,

2 including Emil Fischer, winner of the 1902 Nobel Prize in chemistry, thought polymers were held together by self-assembly of small molecules through weak intermolecular forces.9 It was challenging to convince the scientific community otherwise since polymers were poorly characterized and understood at the time. Hermann Staudinger was able to characterize and define polymers leading to our modern understanding, for which he was awarded the Nobel Prize in

Chemistry (1953).

Staudinger’s definition of polymers has stood the test of time and now many subclasses of polymers are used today, including, linear, branched, and networked (Fig. 1.1). Linear polymers are when small repeat units combine, almost like a paper clip daisy chain, to form a single large molecule that has no branching. A branched polymer has additional units along a polymer chain that will spread out almost like a tree branch. A networked polymer, also referred to as crosslinked polymer, forms when many branched or linear polymers chains are linked by bonds.

Linear Branched Network

Figure 1.1 Three different 1-dimensional polymer architectures which have linear growth regimes.

There is also the growth of polymers that can occur in three-dimension (3D). The overall strength occurs from these multiple strong covalent bonds repeating in all directions. Diamond is a 3D-polymer with its connecting carbon covalent bonds and is well known to be the hardest known material. MOFs (metal-organic frameworks) and COFs (covalent organic frameworks) are

Figure 1.2 Different topologies of polymers types. (a) Polymerization in one dimension, 1DP.

(b) Polymerization in a plane, 2DP. (c) Polymerize in three dimensions, 3DP. other examples of 3D-polymers.10

A two-dimensional polymer (2DP) is defined as a polymer that is one molecule thick and extends in two-dimensions. Graphene is an example of a two-dimensional polymer, it is made up of a single layer of carbon atoms bonded together in an sp2-hybridized hexagonal lattice in a plane shown in Figure 1.3.

In 2004 scientists were able to exfoliate single sheets of graphene.10,11 Single graphene sheet are a million times thinner than common printing paper and the most desirable properties of graphene are observed in the single sheets but not in bulk material. Some of the interesting properties of graphene sheets are large tensile strength,12 high electrical conductivity, high optical transparency,13 and high thermal conductivity.14

The properties of graphene have caused an interest into other types of natural 2D materials that have structures analogous to graphene, such as MoS2, TaS2, MoSe2, and hexagonal boron nitride (h-BN). 15 All these materials give vastly different properties than what has been seen in

1D materials or even graphene.15

Repeat unit of sp2- hybridized carbon atom

Figure 1.3 A section of molecular structure of graphene, the archetypal 2D polymer.

The synthesis of graphene sheets and graphene-like materials has only been in the literature for about a decade, leaving many different possibilities unexplored. Graphene sheets can be grown onto copper foil by using harsh conditions with temperatures in excess of 1000 °C.16

The desirable properties of graphene have been the driving force behind the explosion of publications synthesizing graphene, but also tunable 2D materials. The next sections will present

5 different paths taken in synthesizing 2D polymer materials and show the unique approaches that people have employed and their shortcomings in this relatively new field of chemistry.

1.2 Two-Dimensional Polymers

The field of two-dimensional polymers (2DP) is relatively new and currently there is no set definition for a 2DP. Current definitions are based on a set of criteria that are too broad or ambiguous. A definition is needed to classify 2DP materials and bring a focus to a specific field, at the same time should not exclude many materials that this author considered should be in the classification of 2DPs.

One of the pioneering researchers in 2DP, Schlüter, proposes a definition where certain criteria must be fulfilled to be a 2DP.17 The criteria includes that a 2DP must be a topologically planar molecular sheet that is composed of planar monomer units with three, four, or six reactive sites. Periodical order and crystallinity must be observable in one of its conformations of the polymer and the thickness of the 2DP must be the same thickness of its monomers and repeat units of the 2DP that are all covalently bound. The bonds of the repeat units must be preserved to allow for the 2DP to support its weight and allow for manipulation of the material. This definition excludes many materials. A more general definition of 2DP would be beneficial, which then classifications of 2DP types can be arranged similar to 1D polymers are classified.

To build a more general definition of 2DPs there should be a classification system similar to 1D polymers. One-dimensional polymers (1DPs), also known as linear polymers, are defined as a molecules that have a relatively high molecular mass that is made up of multiple repeat units of relatively low molecular mass. These linear polymers are made up of repeating units derived from

6 small molecules called monomers, e.g. styrene. Polymerizing monomers generates a linear polymer that is periodic in one dimensional chain.

Comparing poly(styrene) to a random copolymer, like stat-poly(styrene-butadiene) which is made up of a random distributed polystyrene and polybutadiene units in linear chains, the arrangement of this polymer is random, not periodic. By analogy, two-dimensional materials do not need to be periodic. While proving structure of a periodic 2DPs should be easier than a non- periodic 2DP, it should not be a reason to exclude them from being considered a 2DP.

Figure 1.4 Example of poly(styrene) and poly(styrene-butadiene-styrene) linear polymers.

(SBS). Both are considered linear polymers even though SBS can be non-periodic in its overall

structure.

Another type of 1D polymer is the linear coordination polymers, which are polymers with repeating coordination moieties that are held together by coordination, not covalent bonds. These polymers find use in many material applications such as electronics, luminescence, and magnetism.18 The main requirement for 2DPs is that they are robust enough to hold the structure together allowing for manipulation and transfer, not the type of bonds formed. If similar types of bonds were excluded from 2DPs because they are not covalently bond would unnecessarily narrow the field, losing some of the field’s potential.

Forming a 2DP definition encompassing as traditional linear polymers would make the

primary requirement of 2DPs to make planar molecular sheets. The definition would employ

topology of the molecules and use tiling theory to explain the requirements of defining something

as a 2DP.

Grünbaum and Shepard have written the definitive book on the theory of tilings and

tessellation theory; that it defines a plane tiling as a set of tiles covering a plane without gaps or

overlaps.19 Tiles comprise a set of connected vertices and edges that form a closed polygon. If a

set of tiles cover a plane without gaps or overlaps, it is a tiling.

Figure 1.5 A tiling set where a single tile being used to cover a plane with no gaps or overlaps to form a regular tessellation. Vertices are represented by red dots and the blue lines being the vertices.

Tiling is not limited to uniform tiling, it can include regular tiling, random tiling, and

convex uniform. The most common tilings are regular and semi-regular tilings. Regular tiling is

composed of a single uniform polygon. A semi-regular tiling contains more than one regular

polygon to make a repeating pattern. Random tilings can contain many polygons and a lack

periodic structure, with no discernable repeating patterns in their structure. (Fig 1.6)

Figure 1.6 In tiling there is three sets of tiles that can be described (a.) Regular tiling set. (b.) semi-regular tiling set. (c.) A random tiling set

Using tiling theory, as applied to the definition of 2DPs, would simply defining 2DPs as a

tiling, where repeat units are vertices and bonds between monomers connecting the repeat units

are the edges of the tiles. The edges are the outline of a tile that can be used to cover a plane without

overlaps or gaps. For example, this definition describes graphene as tiles where the carbon-carbon

bonds are edges and the vertices are the carbon atoms. The graphene carbon atom edges outline

hexagonal tiles that cover a plane with no overlaps or gaps.

Figure 1.7 A couple of possible monomer configurations. Red spheres represent reactive binding sites

and blue lines represent the unreactive parts of the monomer.

2DP can be defined as having 2D-planar connected structures supported by tiling theory.

Future subclasses of 2DP can be formed to narrow focus, but that narrowing should not be at expense of the field’s potential. Defining 2DP by tiling theory will make the field inclusive enough for many 2D materials and allow for a growth spurt of a relativity new field.

1.3 Possible Applications of 2DP

2DPs can often described as molecular fisherman’s nets. These polymers typically have a periodic repeat unit distribution and unchanging pore size.20

Synthetic 2DPs are often compared to graphene, a natural material used as a reference point when designing 2DPs. Graphene is a naturally occurring 2DP with a hexagonal carbon scaffold that many synthetic 2DPs emulate. However, graphene’s dense array of hexagonal rings does not contain pores, making it more like an ultrathin homogeneous film rather than a molecular fisherman’s net. Graphene possess unique electronic properties attributed to its fully conjugated hexagonal structure, which other current 2DPs do not have.21 Current 2DPs found in the literature cannot match or surpass the electronic or mechanical properties of graphene.22 Despite this, there is still much to be excited over. Recent synthetic 2DP materials do show considerable strength and have the potential for use in applications such as membranes and tunable electronics.

While graphene has unprecedented electronic features, 2DPs can be manipulated for different uses. The potential for tailoring 2DP pore sizes to a specific guest molecule has several possible applications in the chemical and biological fields. An application of synthetic 2DPs can potentially be used as dielectrics, due to their low conductivity and planar extension. Their properties may extend over an entire area of a wafer with a monolayer of insulator 2DP sheet that

could achieve coating over an entire surface. However, it should be noted that coating an entire

surface using a 2DP is extremely challenging.

Müller et. al. has shown that upon polymerization of an anthracene-based monomer,

fluorescence response changes.23 Using this property on their 2DP monolayers shows use as

rewritable molecular paper. This process has been observed using anthracene dimerization where

(a.) (b.)

(c.)

Figure 1.8 Reprinted with permission from (22). Copyright 2017 John Wiley and Sons. (a.) Müller and co-worker’s monomer used to make their 2DP. (b.) The reactive anthracene blades of the monomer undergo a [4+4]-cycloaddition but can be reversed by different wavelengths or under sufficient heat. (c.)

Example of the polymer on a silicon surface, where the hole represents bleaching occurring with a 405 nm laser, which then can be reserved by apply heat.

11 a light-induced dimerization undergoes a thermally reversible process to the original material. The disappearance of fluorescence is observed after polymerization and doing thermal depolymerization will reverse and bring fluorescence back. An example of this published by

Müller and co-workers outlines how they used a laser to mark parts of their polymer and were able to reverse the markings by using the polymers thermal reversibility (Fig 1.8).23

A potential use of some synthetic 2DPs is for separations of gases and liquids, in which the uniform pore size of the 2DP make them ideal for separations. In some cases the pore sizes of

2DPs might be tuned by synthesis, making it possible to target specific separations of gases or liquids. The most exciting property of 2DP membranes is their thinness, at the molecular limit.

The thinness of 2DPs could lead to unprecedented separations of gases and liquids.

Figure 1.9 An illustration of a monolayer membrane (right) compared to membrane that is made from bulk material (left). Larger the membrane, the more interactions of the gases/liquids will have with the pores, slowing down the flow.

2DPs may find utility with optoelectronics devices due to their optical properties. One suggestion includes filling 2DP pores with guest molecules and using the nonlinear-optical effects

24 of the guest molecules. Some 2DP single crystals have enantiomorphic space group P31 which are expected to produce nonlinear-optic effects.25 By placing active components into the pores, these 2DPs could be used to modulate optical wavelength in some devices. However, co- crystallizing guest molecules into 2DP does not guarantee all the pores would be filled and could change the 2DP entire crystal packing.24 This process would be challenging since small changes to the crystal structure may lose the nonlinear-optic effect desired.

2DPs could be potentially used as molecular scaffolds and are being developed by the King group. Further details of this attempt are in chapter 4 of this dissertation. Using the 2DP as a scaffold to place modifications onto the surface of the polymer may find application as a sensor in electronics or biological fields. The 2DP’s regularity and predictive structure makes them attractive for grafting other functional groups onto the surface and turning them into a sensor. The number of active sites on the 2DP can be reliably calculated over a specific area. The modifications could be done post-polymerization and could tune the properties of the polymer to fit a desired application. The main difficulty in this process would be in characterizing and proving structural change after polymerization, while not damaging a one-molecule thick surface.

1.4 Synthesizing 2DPs

Currently, the main method for synthesizing 2DPs is the single crystal approach. This approach performs topochemical reactions on lamellar crystals. Topochemical reactions occur at the boundary of the solid phase and rely on orientation of the molecules within a crystal. Much of the topochemistry work that has been used in 2DPs synthesis has roots in work by Schmidt and

Cohen.26 A common method to the synthesis of 2DPs via the single crystal approach uses a photochemically reactive monomer that is crystallized to orientate reactive units near each other.

Upon irradiation, covalent bonds are formed from the reactive units of the monomer and a lamellar crystal composed of 2DP sheet is formed. This order can be exploited and produces 2DPs by exfoliating the crystal with different solvents, heat, or sonication to make monolayer 2DP sheets.

A single crystals 2DP synthesis approach can give excellent insight into the structure and shape of the likely 2DP via x-ray diffraction. A significant downside to this single crystal approach is from the limited sizes of crystal growth. Some synthesis of 2DPs can produce large amounts of material that can be crystalized but are limited based on size of a single crystal. Another challenge in the single crystal approach is in exfoliation of the crystal because exfoliation may not always lead to a monolayer or an undamaged sheet. Testing for a monolayer after exfoliation is typically accomplished by using AFM (atomic force microscopy). While the single crystal approach to 2DPs is not optimal for application purposes, it is unrivaled in terms of characterization.

One of the first reported organic 2DP was in 2012 by Kissel et. al., where the photochemical reactive monomer had a 25-step synthetic sequence.27 The reactive monomer, a cup-shaped macromolecule, was crystalized to form a hexagonal lamellar single crystal (fig 1.10).

Once crystalized, the molecule was irradiated with a 300 mW, 470 nm LED to form the polymerized lamellar crystal by a [4+2] cycloaddition. It was hypothesized this was allowed because the reactive ends were topochemically oriented, and were forced to react, unlike if they

14 were in a solution. Afterward, the crystal was exfoliated by heating in NMP for three days to produce 2DP sheets on the order of 10 nm which were isolated for characterization.

Figure 1.10 Reprinted with permission from 26 Copyright 2012 American Chemical Society. On the left is the monomer synthesized to make the 2DP after crystallization and polymerization which the product can be seen on the right (scale = 10 nm).

Another single crystal approach reported in 2017 by Lange et al., was achieved by using a

[2+2] cycloaddition reaction.28 The monomer synthesis was shorter than that of Kissel et. al, comprising of only four synthetic steps. It was scaled up to 35 grams and produced larger sheets up to around 500 µm in size.28 Their idea to polymerize the crystal using [2+2] cycloaddition came from Hesse and Hünig who used 2,6-di(tert-butyl) styryl pyrylium salts, which reliably polymerized when irradiated at 530 nm LED at 4 °C. Their proof of polymerization was the disappearance of the olefin resonance at 1585 cm -1 via FTIR. Exfoliation of the crystal used various solvents DMSO, DMF, acetonitrile or γ-butyrolactone (GBL). The difficulty with this process was forming the monomer crystal which was said to disintegrate if improperly handled and had to remain in the mother liquor to avoid cracking. The authors acknowledge that exfoliation

15 of the crystals to obtain single sheets was difficult, where one crystal was exfoliated for 45 days in GBL at 80 °C and still had multilayers.

Left is the monomer synthesized and crystalized and polymerize to form the crystals seen on the right.

Another example of single crystal approach to 2DPs was published in 2013 by King’s group at UNR where Dr. Rhada Bhola was able to synthesize a threefold-symmetrical anthracene monomer, e.g. antrip (fig 1.12).29 This molecule was based on a triptycene core and was designed to exploit the π-π interactions between anthracene blades. This was intended to promote a hexagonal packing during crystallization. The intermolecular π-π interactions of the anthracene blades promote overlap between the medial positions which can undergo [4+4] cycloaddition when irradiated. The antrip monomer packed in quasi hexagonal units that were in a pseudo-lamellar structure. Polymerization was induced using a 365 nm light and dimerization occurred between the anthracene blades. Due to the pseudo-lamellar structure of the antrip monomer crystal, rearrangement during irridation occurred after polymerize. This molecular motion resulted in the loss of crystal integrity which made analysis by single-crystal XRD not possible. However,

formation of the polymer was indirectly confirmed by using IR spectroscopy, solid state NMR,

and AFM.

medial positions

Structure of antrip monomer (left). The single crystal structure showing the packing of the molecule in almost lamellar arrangement.

1.5 Solid Surface Approach to 2DPs

Another synthetic method for 2DPs can be accomplished by depositing shape-persistent

monomers onto a heated metal surface.29 Most commonly this approach uses a heated coinage

metal surface that triggers the polymerization of the monomers forming the 2DP. The 2DP is often

characterized by high-resolution STM on the same coinage metal surface. This reveals the overall

structure of the polymer, the domain sizes, and possible defects. This method for producing 2DPs

lends itself very well for characterization but is somewhat less common. This may be due to the

difficulty in manipulating the 2DP post-polymerization, making it impracticable for application

purposes.

An example of the solid state approach from Bieri et al., who synthesized an hexapod-

substituted cyclohexane-m-phenylene monomer. Bieri et al. deposited the monomers on a Cu

(111) surface under UHV conditions, followed by the monomers undergoing an Ullman coupling

(fig 1.13).30 The monomers were deposited onto a copper surface at room temperature, then the

temperature was raised to 525 K with a pressure around 2 x 10-10 mbar, forming the 2DP on the

surface. While the reaction was successful and easily characterized, this synthetic approach lacks

transferability, suffers from small domains and defects as shown in Figure 1.13.

Ullman reactions on the surface of copper produced sheets. 1.6 Solution Phase Approach to 2DPs

There are multiple reports of making 2DP in the solution phase. Making 2DPs in solution

requires a monomer designed to react in a lateral manner while in solution. After the monomer

reacts, it forms polymer sheets that stack to yield lamellar solids. Upon closer inspection of the in-

solution growth of 2DPs, the challenge lies in monomers ability to laterally react and form large

sheets before precipitating. Additionally, growth in the solution phase can permit out-of-plane

additions. These out-of-plane additions are due to the monomers unconfined bond rotation during

polymerization. Without confinement of the monomers, defects and small domain sizes are typical

18 for the solution phase synthesis of 2DPs. The idea of using in-solution 2DP synthesis is attractive for its potential simplicity in forming 2DPs. However, the method has challenges such as isolation of single sheets and obtaining large domain sizes, all of which need to be addressed before being used in larger scale synthesis of usable films.

Zhao et al. were able to synthesize a 2DP monolayer material by using 9,10-dimethyl-

2,3,6,7,12,13-hexahydroxtriptycene in a condensation reaction with a 1,4-benzenediboronic acid

(Fig 1.14). Both monomer units were reacted under argon for days at 85 °C in a mesitylene-dioxane solution.31 Subsequently, the solution produced a floating film, and the solvent was evaporation under reduced pressure, and the film collected by filtration. Evidence that the polymerization reaction took place was determined by FTIR where the disappearance of visible hydroxyl bands was observed and the appearance of boronate ester bands at 1351 and 1272 cm-1 were detected.

Though the simplicity of their solution phase approach to 2DPs seems attractive, it entirely rests on a suitable solvent to allow for lateral reactions and for the film to crash out of solution. Zhao et al. can be lauded in finding the ideal conditions to make a 2DP using a solution phase approach.

However, criticism can still be applied to the solution phase approach due to the lack of confinement of the monomers which have the ability for out-of-plane additions.

Their triptycene core monomer was used in the synthesis of their 2DP in solution using well known reaction conditions. Another solution phase preparation of 2DP monolayers was recently published by Bai et al., who were able to synthesize a monolayer in water by free radical polymerization (Fig 1.15).32

This was accomplished by using a amphiphilic monomer that had two maleic acid groups that would self-assemble in the solution after addition of tetramethylguanidine. Afterward, sodium p- styrenesulfonate (SSS) was added with potassium peroxydisulfate to make the polymer sheets by free radical polymerization. The self-assembly of the supramolecular 2DP sheets are assumed to be driven by multiple interactions. The monomers contain biphenyls that had π-π interactions, hydrogen bonding between amide and carbonyl groups, and contained hydrophilic and hydrophobic components. While there was large domain growth in the solution phase, the weak

π-π interactions between the sheets still required exfoliation. Fortunately, the sheets retained their integrity and could be characterized.

Society. Bai and co-workers were able to synthesis a set of monomers when put into solution would produce sheets of film as seem on the image on the right.

1.7 Air-Liquid Interface Approach to 2DPs

Another approach to synthesizing 2DPs is preorganization of monomer units at the air-

liquid interface followed by polymerization. This method uses Langmuir-Blodgett techniques (LB

trough), a well-known method that can confine monomers to a two-dimensional interface.17 The

monomers used on the air-water interface must be amphiphilic, where the monomer contains a

hydrophilic and a hydrophobic segment of the molecule. The amphiphlic monomers can organize

into the appropriate air and water interface which confines them in two dimensions. A great

advantage to using the LB trough is that films created by this method are only limited to the size

of the LB trough, and transfer onto substrates for further studies is common practice. Of all the

mentioned methods of synthesizing 2DPs, this approach appears to be the most promising for real-

world applications due to the possibility of generating large sheets.

An example of a monomer unit that floats on the LB trough is the antrip-DEG monomer

from the King group. This consisted of a hydrophobic portion (antrip body) that would float on

21 the surface of the water, and a hydrophilic portion (DEG chain) that would reside in the water.17

After depositing the antrip-DEG monomers onto the surface of the water the LB trough would compress the monomers, forcing the anthracene blades of the monomers near each other and giving a hexagonal packing. Once full compression of the monomers occurred, the monomer film was irradiated with a 365 nm light to yield a 2DP on the surface of the water. This polymer was transferred to other substrates for further characterization and structure elucidation.

Another synthesis of a 2DP at the air-water interface was published by Schlüter and co- workers employing a hexafunctional terpyridine monomer (Fig 1.16).33 They were able to deposit the monomer on an air-water interface. A divalent cation in the water phase induced polymerization of the monomer upon complexation. Langmuir trough surface pressure/mean molecular area (MMA) measurments, gave an isotherm with 520 Å/molecule, demonstrating that the monomer was laying flat and organized on the interface. Upon transferring the monolayer, analysis showed its thickness was ~0.8 nm. Molecular resolution of overall structure of the polymer sheet was not possible through using AFM, FTIR, and UV-vis they were able to deduce the existence of a 2DP.

Figure 1.16 A terpyridine monomer unit placed on the air/water interface, afterwards complexes with metal cations, resulting in a 2DP.

1.8 Precursor to Fantrip Monomer Synthesis

In the King group, our first experience with 2DPs was with the growth of antrip crystals

that produced 2D-sheets when exfoliated.28 Antrip was the first monomer choosen due to its rigid

triptycene core and three bladed construction. These three blades were made up of anthraceno

units, which allowed for its organization through pi-stacking and selective polymerization. The

rigid triptycene core was intended to maintain structure and orientation through polymerization

conditions. An anthraceno group’s ability to pi-pi organize as well as undergo polymerization

made them the ideal functional group to be used in a monomer for 2DP synthesis.

Anthrance has three linearly fused benzene rings and 14 π-electrons. These π-electrons are

arranged where it is only possible to have one fully aromatic ring, “Clar sextet”. Using circles to

represent sextets there are three resonance structures as shown below (Fig 1.18).

Figure 1.17 Resonance structures of anthracene. Most reactions of anthracene involve the 9- and 10-positions as can be seen in the [4+4]

cycloaddition of anthracene shown in figure 1.18. Contributing factors for the reactivity of the 9,10

positions are attributed to the greater aromatic stabilization resulting from the increased sextets

formed after cycloaddition.

Figure 1.18 [4+4] cycloaddition of anthracene takes place at the 9,10 position and forms into anthracene dimer.

Anthracene’s photodimerzation is well known and reliable, which is why the anthraceno group was choosen to be used in our antrip monomer (Figure 1.19). However, attaching anthraceno blades onto a triptycene core to make a threefold monomer proved to be synthetically tricky. The synthesis of antrip was originally reported by Swager and Long in 2001, and the synthesis was modified and improved by a prior King group member, Dr. Rhada Bhola.34,35

Figure 1.19 The reactive anthracene blades attached to the antrip molecule (left). When anthracene blade react with each other once forms a dimer (right)

Swager’s approach to constructing antrip was to build the anthraceno blades of the monomer using a threefold Friedel-Crafts reaction.17 Their synthesis started from triptycene, which reacted with phthalic anhydride giving a tris-ketoacid. This ketoacid was then cyclized using sulfuric acid at 100 °C to get a quinone intermediate that was then followed up by a reduction using mercury-aluminum (Al-HgCl2) to give the final product, antrip, with an overall yield of

1.8%. Swager’s approach required harsh conditions and was not reproducible in our group, which lead to the King group trying a different method to synthesize antrip. With modifications to the

Swager synthesis, Dr. Bhola was able to obtain the product, but overall yields were still low. The

24 reasoning for the low yields reported by Dr. Bhola and Dr. Patterson was the formation of unwanted regioisomers during the Friedel-Crafts steps.36 The regioisomeric problem of the route began with the initial Friedel-Crafts reaction where the intermediate has free bond rotation, as illustrated in figure 1.20. The bond rotation allows for different sites (marked in red) that can react during the cyclization step with sulfuric acid.

Figure 1.20 The free bond rotation shown in blue of the intermediate caused side products to form and significantly lower the yield of the desired product. The cyclization completes the carbon framework of the monomer, but when bond rotation occurs, it can lead to different regioisomeric products. The bond rotation can lead to acene homologation on the proximal sites of the triptycene core rather than the desired distal sites, as can be seen in figure 1.21. Being a threefold reaction, there was a possibility of producing six different regioisomers. These regioisomers have the same molecular mass and very similar Rf values which increased the difficulty in purification and characterization. The yields of antrip were slightly improved by Dr. Bhola and Dr. Patterson, but these regioisomers were too problematic and

25 produced lower than desired yields. Avoiding the regioisomers inherent to Friedel-Crafts approach was a priority to increase yields. With the problematic regioselectivity of the previous route to antrip, it was decided to find a method that would lock in the regio chemistry. The new approach tried used a one-step Diels-Alder route to achieve acene homologation of the triptycene core.

Figure 1.21 (left) the desired product, antrip. (right) an example of a regioisomer with bond rotation producing a defective product after cyclization.

Gribble in 1985 reported using isoindole moieties with a benzyne intermediate, proceeding through a Diels-Alder reaction to obtain acene homologation. 37 Gribble’s benzyne intermediate was generated from a ortho-dihalogenated benzene using a lithium-halogen exchange reaction.

The benzyne intermediate would react in a Diels-Alder reaction with the isoindole species, creating

Scheme 1.1 Acene elongation using a Diels-Alder and cheletropic elimination approach.

26 the carbon framework of the desired anthrecno unit. The resulting N-methyl bridge was cheletropically eliminated using mCPBA to obtain the final product.

The Diels-Alder route to synthesizing antrip was initially developed by professor King while on sabbatical at ETH Zurich. His original route used N-methyl-isoindole. Unfortunately, this molecule has poor thermal stability and its diene character is destroyed within hours at room temperature. This degradation even happens while being kept under inert and cold conditions. The instability of the N-methyl-isoindole prevented Dr. King from making a usable amount of antrip.

Therefore, a more stable N-methyl-isoindole moiety was needed for the initial Diels-Alder reaction toward the monomer. A new N-methyl-isoindole moiety was used which possessed fluorene substituents to promote better stability. This increased stability made the synthesis of usable amounts of monomer using the Diels-Alder route possible. (Fig 1.22).39

Figure 1.22 (left) The less stable isoindole moiety. (right) The fluoro substituted isoindole. An added benefit to using fluorinated isoindole in the monomer synthesis was immediately noticed. The fluorinated anthrceno blades had an important impact on intermolecular stacking with adjacent blades due to dipole interactions. Using fluorinated isoindole not only helped us realize the Diels-Alder approach, but also improved the monomers ability to self-organize.

Fluorinated acenes have different intermolecular stacking pattern than their non- fluorinated counterparts. For example, anthraceno blades stack preferably with themselves in an off-center or edge-to-face arrangement. However, a face-center stacking is unfavorable because of

27 quadropolar electronics. Having the fluorenes on one side of the anthraceno blade reduces repulsion forces created during face-centered stacking. This makes face-centered stacking the preferred arrangement for fluorinated monomer blades.38

This is due to fluorenes ability draw electron density to one side of the blade while leaving the other side electron deficient. This makes a dipole within the blade that favors face-centered stacking (Figure 1.23)

Figure 1.23 The different stackings of molecules based on their electronics.

1.11 References.

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2 Finch, C. A. Polymer International 1991, 24 (3), 192–192.

3 Peplow, M. Nature 2016, 536 (7616), 266–268.

4 Staudinger, H. Ber. Dtsch. Chem. Ges. 1920, 53, 1073–1085.

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6 E. Simon, Liebigs Ann. Chem., 1839, 31, 265.

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8 A- V. Lourenco, Ann. Chim. Phys., 1863, 3, 67.

9 Hudson, C. S. J. Chem. Ed. 1941, 18 (8), 353.

10 Boehm, H. P.; Clauss, A.; Fischer, G. O.; Hofmann, U. Zeitschrift fur anorganische und allgemeine Chemie 1962, 316 (3–4), 119–127.

11 Novoselov, K. S. Science 2004, 306 (5696), 666–669.

12 Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321 (5887), 385–388.

13Kuzmenko, A. B.; van Heumen, E.; Carbone, F.; van der Marel, D. Phys. Rev. Letters 2008, 100 (11).

14 Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano Letters 2008, 8 (3), 902–907.

15 Miró, P.; Audiffred, M.; Heine, T. Chem. Soc. Rev. 2014, 43 (18), 6537–6554.

16 Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, Science 2009, 324 (5932), 1312–1314.

17 Sakamoto, J.; van Heijst, J.; Lukin, O.; Schlüter, A. D. Angew. Chem. International Ed. 2009, 48 (6), 1030.

18 Lopez, S.; Keller, S. W. Inorganic Chem. 1999, 38 (8), 1883–1888.

19 Grünbaum, Branko; Shephard, G. C. Tilings with congruent tiles. Bull. Amer. Math. Soc. 1980, no. 3, 951--973.

20 Kissel, P.; van Heijst, J.; Enning, R.; Stemmer, A.; Schlüter, A. D.; Sakamoto, J. Org. Letters 2010, 12 (12), 2778–2781.

21 Wang, L.; Gao, Y.; Wen, B.; Han, Z.; Taniguchi, T.; Watanabe, K.; Koshino, M.; Hone, J.; Dean, C. R. Science 2015, 350 (6265), 1231–1234.

22 Kory, M. J.; Wörle, M.; Weber, T.; Payamyar, P.; van de Poll, S. W.; Dshemuchadse, J.; Trapp, N.; Schlüter, A. D. Nature Chem. 2014, 6 (9), 779–784.

23 Müller, V.; Hungerland, T.; Baljozovic, M.; Jung, T.; Spencer, N. D.; Eghlidi, H.; Payamyar, P.; Schlüter, A. D. Adv. Materials 2017, 29 (27), 1701220.

24 Servalli, M.; Schlüter, A. D. Annual Rev. Materials Research 2017, 47 (1), 361–389.

25 Kory, M. J.; Wörle, M.; Weber, T.; Payamyar, P.; van de Poll, S. W.; Dshemuchadse, J.; Trapp, N.; Schlüter, A. D. Nature Chem. 2014, 6 (9), 779–784.

26 Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87 (2), 433–481.

27 Kissel, P.; Erni, R.; Schweizer, W. B.; Rossell, M. D.; King, B. T.; Bauer, T.; Götzinger, S.; Schlüter, A. D.; Sakamoto, J. Nature Chemistry 2012, 4 (4), 287.

28 Lange, R. Z.; Hofer, G.; Weber, T.; Schlüter, A. D. J. American Chem. Soc. 2017, 139 (5), 2053.

29 Bhola, R.; Payamyar, P.; Murray, D. J.; Kumar, B.; Teator, A. J.; Schmidt, M. U.; Hammer, S. M.; Saha, A.; Sakamoto, J.; Schlüter, A. D.; et al. J. American Chem. Soc. 2013, 135 (38), 14134– 14141.

30 Bieri, M.; Nguyen, M.-T.; Gröning, O.; Cai, J.; Treier, M.; Aït-Mansour, K.; Ruffieux, P.; Pignedoli, C. A.; Passerone, D.; Kastler, M.; Müllen, K.; Fasel, R. J. American Chem. Soc. 2010, 132 (46), 16669.

31 Zhou, T.-Y.; Lin, F.; Li, Z.-T.; Zhao, X. Macromolecules 2013, 46 (19), 7745.

32 Li, Z.; Tang, M.; Dai, J.; Wang, T.; Wang, Z.; Bai, W.; Bai, R. Macromolecules 2017, 50 (11), 4292.

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37 Gribble, G. W.; LeHoullier, C. S.; Sibi, M. P.; Allen, R. W. J. of Org. Chem. 1985, 50 (10), 1611–1616.

38 Martinez, C. R.; Iverson, B. L. Chemical Science 2012, 3 (7), 2191.

Chapter 2: Synthesis of Carboxy Fantrip

Chapter 2.1: Past Monomers for 2DP

While effective, the synthesis of the monomers fantrip and antrip and their subsequent 2DP crystal polymerization worked the approach lacked the scalability to make large 2DP. sheets1,2

The 2PD sheets could only be as large as the crystal itself, which is around 0.1-0.2 mm in 2- dimensions and a fraction of that in the third dimension. Our main goals were to increase the area of the 2DP that could be fabricated after polymerization, and this could be accomplished by synthesizing a monomer that could be used at the air-water interface. The synthesis of a polymer at air-water interface would only be limited to the size of workable equipment rather than the size of grown crystals. Also, adding the monomer onto the air-water interface would greatly decrease the amount of monomer required to synthesize 2DPs, making low yield reactions bearable for further studies.

Figure 2.1 (left) Antrip monomer. (right) Fantrip monomer The first 2DP used at the air-water interface was developed by the King group and used antrip-DEG monomer which could be immobilized on the air/water interface and polymerized into a 2DP sheet.3 The thin sheets that were made from antrip-DEG monomer had periodic and ultrahigh pore density but were difficult to manipulate after formation. The antrip-DEG monomer

32 was also difficult to synthesize with low yields and was difficult to form the desired 2DP sheets without expressive defects.

Antrip-DEG monomer may have worked at the air-water interface to make 2DPs, but we wanted a better synthesis into the monomer as well a better finished 2DP. To make a better 2DP we thought to synthesize a fantrip monomer with a solubilizing group to use at the air-water interface. The fantrip synthesis as described in chapter 1, had been improved to garner larger yields of the monomer and with the addition of fluorines of fantrip, we hoped would make work at the air-water interface easier. While the DEG (diethylene glycol) chain on antrip-DEG might have been sufficient to make the fantrip monomer usable at the air-water interface, it was not the functional group we initally targeted.

Figure 2.2 (left) The monomer antrip-DEG. (right) Hydroxy Fantrip. Our target goal then changed to place a carboxylic acid at the bridgehead. This had two advantages. One, it enabled to use the monomer at the air-water interface and two, post- polymerization of the functionalized 2D film was possible. Having a reactive carboxylic acid functional group at the bridgehead would give options for the synthesis of multiple products to be made from a single monomer unit post-polymerization.

2.2 Synthetic Route Towards Fantrip

Preliminary research work by Dr. Bhola and Dr. Kumar, both former King group members,

laid the groundwork for the synthesis of fantrip.1 A former post-doc, Dr. Kissel, lab improved upon

their work and published the synthesis of fantrip in 2014, which used the Diels-Alder approach to

acene enlogation instead of Fridel Craft approach.4 Dr. Kissel’s published work was primarily on

the single crystal polymerization to 2DPs approach. While this work was groundbreaking, the

synthesis of fantrip was not optimized to produce large quantities. Dr. Kissel’s best overall yields

for fantrip, in a three-step synthesis, was around 0.9% yield for a total of 11 mg. This amount of

monomer obtain by his synthesis was not sufficient enough to supply collaborators who wanted

the monomer to run a vast array of experiments on the polymer. My contribution to this project

was not in the synthesis of the monomer, but to build upon his old work and improve the synthesis

of the monomer from an overall 0.9% yield (11mg) to a 16% overall yield with 1.13g of fantrip

monomer. The improvement in the fantrip synthesis happened in every step in the synthesis, by

improving yields and simplifying workup of the synthesis. Detailed descriptions of the procedures

Scheme 2.1 Complete synthesis of fantrip.

34 found in the experimental section, but key points are highlighted in this next section. The synthetic scheme of fantrip is shown below in Scheme 2.2.

Simple improvements were added on the workup of the isoindole precursor (5) and isoindole (4) to improve yields. A more detailed listing of the improvements is written in experimental section of this chapter 2, but the main improvement of purification of these molecules involves replacing the column chromatography step and recrystallization with the use of sublimation. Yields and consistency of product (4) and (5) were improved by using sublimation, especially in the case reactive isoindole (4). The fluorines on the isoindole (4) may help stabilize the product, but if the pure yellow-orange product when put into a solution for recrystallization, there commonly was a 30-40% loss of material. The material is still reactive even with the fluorines and it is most likely that the material dimerizes with itself, which accounts for the loss of material when recrystallized or subject to chromatography.3

Large improvements were made on the synthesis of hexabromotriptycene (3), which involve changing some key features on the synthesis. Product (3) was a key component in the overall synthesis of fantrip and was the limiting reagent in the synthesis of the targeted molecule; the best yields of the (3) were around 29% yield before optimization.

Scheme 2.2 Reaction of triptycene to desired hexabromotriptycene (3). The first problem identified when following the previous reaction procedure was the amount of solvent used in the reaction. The amount of solvent used in the previous procedure was

35 enough to dissolve the initial starting material, triptycene, but brominated products created are less soluble than the starting material causing them to crash out before completion of the reaction. The other products synthesized would be under brominated material, where there would only be only four or five bromines on the triptycene molecule seen by HRMS, resulting in a dark brown semi oil-solid product that was not possible to separate out the under brominated side products. Solving the under-bromination problem was achieved by increasing the solvent volume used that is needed for the product (3) to be completely soluble, thus the solvent resulting in less dark-brown product and higher yields. Another improvement performed was properly drying the solvent before use in brominating. Removing the excess water from chloroform was accomplished by two different methods and produced the same result. One was drying the solvent, chloroform, by distillation over calcium chloride before use and the other much simpler method was adding activated 4 Å molecular sieves into the solvent for at least 24 hours to obtain dry chloroform.5 Removing water from the reaction stops the iron catalyst from being consumed, which may have been another reason for the under the brominated material. These simple improvements to the reaction increased the yield average to around 78% and the resulting workup was much simpler due to the decrease in the oily under brominated material.

The most difficult step in the synthesis of fantrip is the making of the trifold product (2).

This reaction involves a three-fold benzyne reaction with the fluorinated isoindole intermediate

(4). Benzyne is a very reactive species that can make a variety of unwanted side products and having three of them in a single reaction can cause some unexpected products.

The first approach to improving the reaction was by varying the quantity of isoindole (4) starting material added to the reaction. At minimum, three equivalents of (4) isoindole are needed to produce the desired product, but adding more may reduce side products produced by the benzyne

36 reaction. Adding 3.3, 4, and 6 equivalences of (4) were tested and showed no significant difference between each set of additions and it was discovered that unreacted (4) elutes as the first band from a silica plug using dichloromethane, but still had problems of low yields.

Scheme 2.3 Trifold reaction of triptycene with isoindole to make fantrip trifold adduct (2). A small improvement to this trifold reaction came from the previous improvement of starting material (4). When doing sublimation of (4) instead of recrystallization to purify the material, which consistently improved yields. We also investigated the hexabromotriptycene (3) starting material, which when recrystallized with acetone had acetone trapped inside their crystals even with high heat and under vacuum. To counter this, we dissolved the hexabromotriptycene (3) in tetrahydrofuran (THF) and then evaporated under reduced pressure to remove any acetone trapped inside the crystals. Repeating this procedure a couple of times significantly reduced the amount of acetone found in the material and our overall yields improved slightly. Another thing that may have helped improve the overall yields of the reaction to make fantrip into the gram quantity was the reducing the amount of purification of the fantrip trifold (2) before moving to the next step. Removal of the excess isoindole (4) and lithium salts was done by using dichloromethane with a silica plug, but getting a pure product was still challenging. The N-methyl bridges of the fantrip trifold (2) make a variety of isomers both in the anti and syn- confirmations. These isomers make characterization a difficult problem since their Rf values are extremely close to each other.

Additionally, the N-methyl bridges would spontaneously eliminate when exposed to the

atmosphere, which started to create a variety of side products as seen in the high-resolution mass

spectrum data Figure 2.3.

Figure 2.3 HRMS of fantrip trifold (2) with multiple stages of chelotropic elimination. Each blade that lost one of the N-methyl bridges would create a new isomeric product

mixture in the solution with a similar Rf value, making purification difficult. Purification of this

material lead to a poor yield of 25% yield of mostly pure material, but the low yield may be more

reflective the extreme conditions used to purify the material. Using the crude material in the next

reaction is my recommendation to achieve higher overall yields.

The last synthetic step in the synthesis of fantrip (1) is the elimination of the N-methyl

bridges by the cheletropic elimination (Scheme 2.4). Previously reported attempts used

benzyltriethylammonium chloride and a sodium hydroxide solution for the elimination to form the

fantrip monomer (1) but does not produce good yields of the product even with clean fantrip trifold

(2). The first attempt at improving the yield was to use mCPBA (meta-chloroperoxybenzoic acid)

to procedure elimination, which by HRMS showed to perform the elimination effectively. The

downside was that the monomer and mCPBA we were not able to be separated by various

purification techniques and was abandoned after a couple of attempts. Using hydrogen peroxide

(H2O2) as O-atom transfer reagent to induce cheletropic elimination was saved as a last resort since

there was concern about over-oxidizing the acene blades of fantrip (1). But when employing this

reagent, there was no evidence of over-oxidation by APPI-TOFMS or by 1H NMR. The yields of

fantrip (1) significantly increased when using semi-pure fantrip trifold (2) compared to the

previous method: ~84% yield by the improved synthesis, compared to the old method of ~9%).

Scheme 2.4 Cheletropic elimination step for synthesizing fantrip (1).

Chapter 2.3: Starting Synthesis of Carboxy Fantrip

We still wanted to be able to change the functionalization of the monomer at the bridgehead

after polymerization. The carboxylic acid functional group seemed quite promising in both

synthesis and floating at the air-water interface. The triptycene core that we used in previous

monomers had a carboxylic acid version known in the literature (9,10[1',2']-Benzenoanthracene-

9(10H)-carboxylic acid), and there were other carboxylic acids already used at the air-water interface.6

Scheme 2.5 Basic retrosynthetic pathway to carboxy fantrip (1), using a Diels-Alder approach

with isoindole (2) and our carboxy hexabromotriptycene (3).

The synthesis of carboxy fantrip (1) as seen in scheme 2.1, requires two pieces to synthesize the target monomer, isoindole (2) and carboxy hexabromo-triptycene (3). The carboxy hexabromotriptycene (3) required the bromination of known carboxytriptycene.5

Scheme 6.2 Synthesis of carboxytriptycene (9,10[1',2']-Benzenoanthracene-9(10H)-carboxylic acid) (4) Synthesizing carboxy hexabromo-triptycene (3) required a few changes to the literature preparation for safer reactions and increase yields. A safer route for synthesizing the triptycene core (6) used in only dichloromethane (DCM) in the benzyne reaction as a solvent. Using DCM as solvent in small scale reactions was sufficient, but not for scaled up reactions. Scaling the reaction posed the danger of larger amounts of gas and heat evolve from the reaction and DCM does not absorb enough heat to do this reaction safely. The addition of dimethoxyethane (DME) as a co-solvent allows for increased heat absorption which allowed for larger scale reactions, increasing from 1 gram reactions to 10 grams safely. The yields were comparably better than the literature and did not require column chromatography to obtain pure triptycene acetal (6).

Another change from the literature was from the synthesis of (4) via Jones Oxidation which the amount of chromium trioxide was reduced to two equivalents from three to four equivalents as reported.6 Any unreacted material can be easily be separated. When an excess amounts of chromium trioxide was added it caused degradation of the product, leading to low to no yields.7

Our protocol for the bromination of previous triptycene worked poorly for carboxy

triptycene. The six-fold bromination of the triptycene carboxylic acid was not straight forward and

yields were low with difficult purifications. The carboxy hexabromotriptycene (3) could not be

obtain by a flash chromatography or recrystallization, but the reaction was inconsistent day to day.

The first assumption was that there may be regiochemistry problems with brominations occurring

at the proximal sites of the triptycene molecule rather than the desired distal positions.

Scheme 2.7 Six brominations of the triptycene core of both regular triptycene (top) and carboxy triptycene (3) (bottom).

Time of flight high resolution mass spectrometry (TOF-HRMS) was used to elucidate what

was happening. The appearance of regular hexabromotriptycene, peak 726.56 m/z, demonstrates

decarboxylation during this reaction. The next noticeable problem of this reaction was with the

under brominated carboxylic acid product at 690.64 m/z and under brominated non-carboxylic

acid product at 646.65 m/z. Adding additional bromine to under brominated material did yield

carboxy hexabromotriptycene (3), but some would react further resulting in over brominated

material as seen at peak 848.46 m/z. The over brominated material was difficult to remove from

42 the hexabrominated product, (3) and accounted for lower yields. The low solubility of the desired product (3) and similar polarity of the over brominated material made column chromatography unfeasible. Triturations with hot acetone or chloroform attempts were successful to a degree but was time consuming and required multiple attempts and yields did not improve. In my opinion it was less challenging to contend with under brominated products than the over brominated material.

The under brominated triptycenes that would be seen in the bromination reaction of (3) are around

~ 692, 613, 535, and 456 m/z and all could be removed these impurities was to triturate the crude product with cold acetone until a large portion of the black oil was removed and a gray solid was obtained. Afterwards recrystallizing crude (3) with hot acetone was suitable, to obtain white crystals without impurities. The product is poorly soluble in acetone, but the under brominated material is more soluble and will wash away upon filtration. If solubility becomes a problem during recrystallization, add a few drops of tetrahydrofuran or methanol to increase the solubility.

High purity crystals of (3) can be obtained by recrystallization from THF using slow evaporation (room temperature) to get X-ray quality crystals as shown in Fig 2.5. Other solvents like acetone and methanol would provide microcrystals or powder, of high purity, but not suitable for X-ray diffraction to get data.

Figure 2.5 Crystal structure of carboxy HBT (3) grown in THF.

Figure 2.5 TOF-HRMS of an impure sample of hexabromo triptycene carboxylic acid.

The carboxy hexabromotriptycene (3) purified by recrystallization would have solvent left

inside the crystal structure. DSC revealed loss of solvent at ~60 °C and ~150 °C. The amount of

solvent in carboxy hexabromotriptycene (3) crystals determined by TGA was water with around

~9.6% weight content as shown in the figures 2.5. Performing differential scanning calorimetry

(DSC) and thermogravimetric analysis (TGA) on carboxy hexabromotriptycene (3) showed that

there were multiple solvents and a significant amount of solvent left in the product. Both samples

were both heated to ~70 °C under reduced pressure to remove any excess solvent in the sample,

but by DSC and TGA it seemed that the carboxylic acid and the structure of the (3) held onto

solvent more tightly than previously believed. Decomposition of (3) occurred at ~375 ° C which

signified that drying of the product could be taken up to higher temperatures around 130 °C under

reduce pressure to remove any excess water without worry of decomposition. Reducing the amount

of water in the reactants increased the yield and reproducibility of the product carboxy

hexabromotriptycene (3), but still problems of under brominated material occasional occurring if

the solvent and reactants are not properly dried before bromination.

Figure 2.6 DSC of carboxy HBT (top), TGA of carboxy HBT (bottom).

The next step of the synthesis towards making carboxy fantrip involved a triple benzyne reaction using the carboxy hexabromotriptycene (3) and tetrafluorisoindole to make carboxy fantrip precursor (1a). Overall, the reaction is similar via the reaction of ortho-dihalogens of carboxy hexabromotriptycene (3) used n-BuLi to generate an aryne intermediate that undergoes a regiospecific Diels-Alder reaction with the tetrafluoroisoindole to make carboxy fantrip precursor

(1a).

(3) variable yields (1a)

Scheme 2.8 Carboxyfantrip three-fold benzyne reaction. The benzyne reaction used to synthesize carboxy fantrip precursor (1a) is very reactive and yielded a variety of byproducts along with our intended product carboxy fantrip precursor (1a).

Using HR-TOFMS analysis as shown in the figure 2.6, it can be demonstrated that a range of reduced materials byproducts were obtained. The main types of byproducts seen by mass spectrum were benzynes that abstracted a proton from the reaction mixture, and unreacted ortho-dibromides that did not react. Another type of bi-product generated, is believed to come from the benzyne reacting with butyl bromide, which was generated in the reaction mixture and seen as the peak at

~837 m/z.

Figure 2.7 Possible bi-products of the three-fold benzyne reaction. HR-TOFMS analysis of regular fantrip trifold and the carboxy fantrip precursor (1a) showed that a wider range of byproducts formed during the benzyne reaction of the carboxy fantrip precursor (1a). Attempts were made trying to purify the crude reaction mixture by column chromatography and material was separate the into parts as shown in figure 2.7 but unable to provid pure carboxy fantrip precursor (1a). Attempts to further purify the fractions that contained the carboxy fantrip precursor (1a) failed due to Rf values being too close or the trifold adduct would be exposed to oxygen too long and one or two of the blades would start the cheletropic elimination of the amine creating another product. Best estimations for yield of this reaction after column chromatography would be ~27% of material when measuring mass of fractions containing carboxy fantrip precursor (1a). Finding a yield or complete characterization for this reaction was

47 not achieved due to the challenging nature of separating the byproducts and large number of compounds generated in the synthesis.

Figure 2.8 Carboxy trifold different byproducts after column chromatography. The desired (1a) eluted early on the reverse phase HPLC. (2:98 ethyl acetate:methanol).

The crude carboxy fantrip precursor (1a) was used in the next reaction after a short column to synthesize the monomer of carboxy fantrip (1) in a three-fold cheletropic elimination of the methyl amines bridges on the anthraceno blades. Elimination of the methyl amines was facilitated by using oxidative reagents which would form two lower energy molecules of methyl nitrosium and aromatization of anthracene. The driving force of the elimination comes from the extension of conjugation on formation of aromaticity into the anthraceno blade and the generation of the methyl nitrosium byproduct. Hydrogen peroxide was used as a reagent to facilitate the cheletropic elimination from carboxy fantrip precursor (1a) into the monomer carboxy fantrip (1) synthesis but using this reagent for the elimination presented a few challenges.

A challenge of using hydrogen peroxide for the cheletropic elimination occurred during workup, where the separation of reagents from the monomer, carboxy fantrip (1), was challenging.

1-2 % yield

Figure 2.9 Cheletropic elimination of (1a) into carboxyfantrip (1) using hydrogen peroxide.

In previous work on parent fantrip, workup used hydrogen peroxide for cheletropic elimination of

fantrip precursor involved adding water to precipated the product, which allowed for the removal

of hydrogen peroxide from the desired product. However, working up carboxy fantrip (1) after

being mixed in with hydrogen peroxide and water, the product would stay in the water. Aqueous

workups would not remove hydrogen peroxide from carboxy fantrip (1) and most of the product

could not be fully extracted using multiple organic solvents.

The byproducts of the cheletropic elimination of carboxy fantrip precursor (1a) into

carboxy fantrip (1) are shown in figure 2.5 and figure 2.8, where there are many of the same

products from the reaction that would be formed after cheletropic elimination. It can be observed

too that some of the anthraceno blades did not fully undergo cheletropic elimination, leading to

new byproducts making purification of carboxy fantrip (1) challenging. Initial reactions of this

final step would only yield around 1-2% of (1) after a long silica column followed by a reverse

phase HPLC column with 5:95 ethyl acetate:methanol with 0.1% of TFA. Purification often

required multiple passes through the HPLC column was often required to obtain pure carboxy

fantrip (1).

Figure 2.10 Crude (1) after cheletropic elimination with hydrogen peroxide.

Purification by trituration and recrystallization of carboxy fantrip (1) was attempted many

times but was unsuccessful. Crystal growth of carboxy fantrip (1) was only successful using a

cocrystal material (1,4-dioxane) in ethyl acetate by slow evaporation (room temperature) as shown

in figure 2.9. We thought that the carboxylic acid groups of the monomers would hydrogen bond

with each other and generating pairwise packing. Conversely, the oxygen of the 1,4-dioxane

molecules coordinate to one carboxy fantrip monomer. Many different solvents without dioxane

were attempted but none with succeed in growing a crystal; The material would powder out of

solution, and no X-ray quality crystals were formed. Though attempts at growing crystals used a

total of ~120 mg of carboxy fantrip, further attempts were not possible due to the lack of available

carboxy fantrip monomer (1). Also, when growing crystals, the pure product would react with

50 itself if the crystals were exposed to light when evaluating them, leaving most of the material used to grow crystals useless for further use. If the synthesis was improved to synthesize gram scale of carboxy fantrip (1), further crystallization experiments should be attempted.

Chapter 2.4: Different Methods for Cheletropic Eliminations

Figure 2.11 Crystal structure of carboxy fantrip using 1,4-dioxane as a cocrystal.

With the lower than expected yields of carboxy fantrip (1) compared to regular fantrip, we thought that the hydrogen peroxide may have over oxidized the aceno blades of the carboxy fantrip

(1). To test if the carboxy fantrip (1) was over oxidizing we set up a small experiment where ~5 mg of pure carboxy fantrip was mixed with 20 eq of hydrogen peroxide and dissolved in tert- butanol set at reflux for 12 hours. After this reaction we were able to recover around 94% of the carboxy fantrip and were unable to find any over oxidized material by proton NMR or HR-

TOFMS. The loss of material was most likely due to carboxy fantrip (1) not coming out of an aqueous solution during workup and transfer losses. Though oxidation could have occurred at an intermediate stage, it seemed that the hydrogen peroxide was not able to damage the carboxy fantrip (1).

We were not fully convinced that hydrogen peroxide was the best reagent for the synthesis of carboxy fantrip (1); this was primarily due to the persistent low yields and challenging workup.

We investigated into gentler methods for cheletropic elimination of our carboxy fantrip precursor

(1a), which we hoped would increase the yields of carboxy fantrip monomer (1).

Figure 2.12 Cheletropic elimination of the N-methyl bridge by Gribble. A gentler cheletropic elmination method used by Gribble in 1985 used the reagent meta- chloroperoxybenzoic acid (mCPBA) for their cheletropic elimination reaction as shown in figure

2.10.8 Using mCPBA for carboxy fantrip precursor (1a) we were able to see that we made our monomer, carboxy fantrip (1) by proton NMR and HR-TOFMS. However, it was challenging to remove excess mCPBA from the monomer (1). Trituration, crystallizations, and column chromatography all were attempted to fully purify the monomer with no success. Large amounts of the mCPBA could be removed by these methods, but there always was trace amounts left behind. We abandoned this method due to the extensive amount of time spent on trying remove excess mCPBA and the yields of the impure carboxy fantrip (1) were not meaningfully improved.

Scheme 2.9 Hart deamination by pyrolysis.

Another cheletropic elimination method was attempted to removing the N-methyl bridge from carboxy fantrip precursor (1a), using Hart’s method via pyrolysis as shown in Scheme 2.5.9,10

DSC studies on pure carboxy fantrip (1) showed that carboxy fantrip can be heated to around 200

°C before decomposition. The yields reported in the literature were very high and the procedure cited often, which lead us to attempt this procedure. Unfortunately, when we exposed our trifold adduct (1a) to high heat (190 °C for ~8 hours) under reduced pressure we created black powder, as seen in figure 2.11. Trying to characterize the powder was note feasible but filtering the black material with ethyl acetate and methanol obtained a pale-yellow powder which contained our monomer, carboxy fantrip (1). Purification was required but yields of the first attempt were around

~9% yield which was about four times greater than using hydrogen peroxide. This method did seem promising at first, but further attempts failed to produce consistent results. Attempts were made to increase or reduce reaction time, vacuum pressure, and temperature, but nothing seem to give consistency. Sometimes yields would be ~5-6% and other times nothing was able to be salvage from the high heat.

Figure 2.13 (left) Carboxy fantrip precursor (1a) before pyrolysis. (right) After pyrolysis. Explaining the low yields from the pyrolysis of carboxy fantrip precursor (1a) we hypothesis that a free nitrosonium cations byproduct was being generated and was reacting with the other molecules of carboxy fantrip precursor (1a). To reduce the chances of the liberate nitrene cation from reacting with our product we tried doing the pyrolysis in solvent, where the nitrosonium cation would react with the solvent before the carboxy fantrip precursor (1a). From literature examples, we assume that the deamination may occur slightly over 180 °C and used a higher boiling solvent, 1,2,4-trichlorobenzene for our experiment to see if we could have the nitrosonium cation react with the solvent instead of the product. The solvent was heated to 190 °C over 12 hours and found that deamination did occur. Getting an accurate yields for the reaction was challenging due to small amounts of 1,2,4-trichlorobenzene solvent was still in the solid and not being able to remove the solvent by heating and reduced pressure, or trituration. The inconsistency of the reaction and the high boiling solvent was too challenging of a method and we had to abandon it to find a better way to eliminated the N-methyl bridge on our trifold adduct (1a).

Cheletropic elimination of carboxy fantrip precursor after (1a) was using the reagent

DMDO (dimethyldioxirane), also known as Murray’s reagent. This reagent is known to be a mild oxidant that can be used in large quantities and has a low boiling point. The downside to the reagent was that it had to be made fresh and tested for concentration each run and the setup to make the reagent was a tedious process as shown in Figure 2.12.11,12

Figure 2.14 Setup and synthesis of dimethyldioxirane (DMDO).

The challenge to this synthesis of DMDO was collecting enough DMDO reagent to attempt the cheletropic elimination of carboxy fantrip precursor (1a). The literature states they were able to collect concentrations at 100 mM at a time, but the concentrations were ~ 35-50 mM and was never consistent from batch to batch.11 However, this was not a big problem since we only need a small amount of this reagent to deaminate our carboxy fantrip precursor (1a) and we could quickly check the concentration of our reagent easily. The concentration of the reagent was checked by adding a known amount of thioanisole to some of the DMDO reagent and measuring the amount of the corresponding sulfoxide that was synthesized. Tests could be performed using GCMS to

measure the concentration as shown in Figure 2.13. After the reagent was synthesized and added

to our trifold adduct (1a) we obtain consistent yields of ~5-7%. The reaction was simple in that

the trifold adduct could be dissolved in acetone and the reagent could be added directly to the

solution and stirred for 2 hours to obtain carboxy fantrip (1). Workup involved only evaporating

the excess reagent and acetone from the reaction mixture under reduced pressure and did not

require any additional steps to remove high boiling solvents or charred material. The DMDO

reagent is effective enough to be used for the cheletropic elimination in our trifold adduct and was

used for future oxidations. However, even with this mild oxidation method, it did not dramatically

improve yields of our synthesis, and we started to look at other areas in the synthesis to improve.

Figure 2.15 GCMS of DMDO reagent with thioanisole and the sulfoxide synthesized.

Chapter 2.5 Lithium Reagents for Benzyne Formation

To improve the synthesis and yields of carboxy fantrip (1), we turned our focus to the

most uncertain step of our synthesis; the formation of the carboxy fantrip precursor (1a). This step

has a three-fold benzyne reaction that forms the trifold adduct seen in Scheme 2.4. The area of

improvement we seen as easiest was by changing the organolithium reagent and conditions we

applied to form the benzyne.

The organolithium reagent n-BuLi worked well in our earlier synthesis of fantrip

mentioned in chapter 1, but there was a noticeable difference in byproducts between fantrip and

carboxy fantrip (1). Our investigation began by trying to elucidate possible byproducts in our HR-

TOFMS carboxy fantrip precursor (1a) samples, in which we found a mass difference of 38 amu

between each peak. The byproducts were not immediately apparent, but we determined what may

have been occurring was the replacement of a fluorine by a butyl group. We tested this nucleophilic

aromatic substitution with pure tetrafluoro isoindole and added butyl lithium at −78 °C and found

that this substitution was occurring as shown in figure 2.14. This substitution occurring at any

fluorine on our carboxy fantrip precursor (1a) would lower our yield and could have been reacting

with anyone of the 12 fluorines on our carboxy fantrip precursor (1a).

Evident by GC-MS

Figure 2.16 Nucleophilic aromatic substitution of isoindole. Compounds with fluorines under similar conditions in the literature did not specifically

mention the nucleophilic substitution issue, but fluorine specific chemistry books did mention

nucleophilic aromatic substitution occurring as shown in figure 2.15.13 The substitution of

aromatic fluorines with nucleophilic bases are well known and with that finding we determined

that we need to change our organolithium reagent to something less nucleophilic.

Figure 2.17 A general reaction of nucleophilic aromatic substitution. Sec-butyllithium was the first organolithium reagent we used instead of n-BuLi to generate

our benzynes to make carboxy fantrip precursor (1a). At −78 °C the difference was not discernable

compared to n-BuLi and we obtained similar results such as trifold being oily and evidence of

nucleophilic aromatic substitution. With this data, we moved on to using a bulkier organolithium

reagent, tert-butyllithium.

Using tert-butyllithium (t-BuLi) for the trifold reaction yielded improved results of crude

material, where n-BuLi produced an oily substance, the t-BuLi produced a similar texture to our

starting reactants where it was flakey. However, the cheletropic elimination of the trifold yielded

much lower amounts of product (~1%) than using n-BuLi and some of the starting material was

still observed. It was Dr. Lawrence Scott in our group meeting who pointed out that when using t-

BuLi that we needed to add two equivalents, since the reagent will react with itself to form

isobutene and isobutane as shown in figure 2.17.14

Figure 2.18 The bromine-lithium exchange with 2 equivalent addition of tert-butyllithium.

Adding two equivalents of t-BuLi per intended benzyene and deprotonation used 8.2 equiv improved the synthesis of carboxy fantrip trifold precursor (1a) eliminating unreacted starting material. The material now was a brown flakey solid with no oily substance left behind, the difference between the organolithium reagents is shown in figure 2.17. Purification by column chromatography was improved by this new method, with defined bands to separate out byproducts from our carboxy fantrip precursor (1a), but we were still unable to obtain pure trifold adduct. The overall yield did not improve from n-BuLi, to t-BuLi even though there was a change in the crude material appearance.

(a.) (b.)

(c.) (d.) Figure 2.19 Crude trifold adduct (1a) (a.) using n-BuLi. (b.) using t-BuLi (c.) column

using material a. (d.) column using material b.

We attempted to use another organolithium reagent, phenyllithium, when we found a literature method used by Gribble, for a triple benzyne reaction using phenyllithium as shown in figure 2.18.15 When we attempted to use this method to synthesize the carboxy fantrip precursor

(1a), there was no reaction, and we recovered most of our starting material (~94%). The phenyllithium was purchased, tested immediately before use, and found to be active. The solvent was changed to allow for higher reflux temperatures, but no reaction was observed.

Figure 2.20 Gribble and co-workers generation of Diels-Alder cycloaddition of 1,3,6- naphthotriyne by benzyne.

From these studies, we concluded that the tert-butyllithium was the preferred organolithium reagent to synthesize our carboxy fantrip precursor (1a), not for increased yields, but ease of purification. However, these studies did not increase the yield of our product significantly and we needed to improve other features of our synthesis towards carboxy fantrip (1).

Fantrip yields were good while the carboxy fantrip (1) yields were significantly lower, and the presence of a carboxylic acid functional group at the bridgehead position is likely the cause of this disparity. The carboxylic acid might be interfering with the benzyne chemistry either by coordination or water retention in the starting material leading would consumption of the organolithium reagent. Initially, we tried adding reagents that would react with water and not interfere with the benzyene reaction, but there was no change in the yields of the reaction. From there we explored protecting groups for carboxylic acids prior to lithiation.

Chapter 2.6 Carboxylic Acid Protection

There are a variety of carboxylic acid protecting groups described in the scientific literature that could protect the functional group and be easily deprotected. The challenges protecting the carboxylic acid at the hexabromo triptycene was having the resistance of reaction conditions, easy of installing the protecting group and the ability to remove the group. There was a fine line where we needed a protecting group that could survive strong alkyl lithium conditions and could react at a very hinder position. Removal of protecting group was important too, because afterwards we needed deprotecting conditions that would not damage the molecules.

The first methods we attempted are shown in figure 2.19, all were unsuccessful. Reaction

(a.) was a microwave reaction to make methyl esters using dimethyl carbonate (DMC) with DBU as a catalyst with literature obtained yields ~99% and shorter times ~20-40 minutes.16 This reaction was attempted using a microwave reactor and by standard methods, of which both yielded only starting material even when we increased the reaction time to 24 hours. To synthesize a tert-butyl ester we followed procedure (b.) protect the carboxylic acid with a tert-butyl ester by making isobutylene in situ.17 This reaction was not successful even when we increase the concentration of isobutylene and pressure of the reaction flask. The literature did not use sterically hindered carboxylic acids in their literature and it seems reasonable that the steric hindrance of our carboxylic acid was preventing protection by common methods. We then attempted method (c) which uses catalytic scandium triflate and DMAP and was used on a few more hindered acids in literature studies.18 The reaction was not successful with carboxy hexabromotriptycene (3), but to make certain we were doing the reaction correctly, we repeated one literature studies of (mesitoic acid), and we were able to reproduce the yields in the literature report. Another method suggested in a group meeting was to attempt method (d.) which employed an oxazoline protecting group

using DCC coupling.19 Even though the method was cited over 200 times and used in many other

substrates, we observed no reaction and recovered carboxy hexabromotriptycene (3). We found

another literature procedure that had a carboxylic acid at the bridgehead position of a triptycene

and conversion to a methyl ester by method (e.) was possible using sulfuric acid and methanol.20

This reaction did yield a small amount of the methyl ester but increasing the reaction time to 72

hours it did not seem to increase the yield. In the supporting information, the authors acknowledge

the same issue and suggested an alternative method for forming the methyl ester via reaction with

thinoyl chloride.

Figure 2.21 List of carboxylic acid protecting precedent that failed on carboxy hexabromotriptycene (3).

The protection of our carboxylic acid started by synthesizing an acyl chloride adduct of

triptoic acid then adding a nucleophile, methanol, which would form the methyl ester on the

bridgehead position of triptycene as shown on figure 2.20. We were able to reproduce the results

from literature sources with lower than reported yields, (~65% compared to 91% yield), but the

success was encouraging after many failures.16 We did not move further with the triptycene methyl

ester due to an inability to deprotect it with boron tribromide and many other methods deprotect

this group using bases that may substitute with our fluorines as discussed in chapter 2.4.21

~55 % yield Figure 2.22 Triptoic acid reaction with thinoyl chloride and methanol to make the methyl ester. The next protecting group we decided to use was a benzyl ester on carboxy

hexabromotriptycene (3) by route of an acyl chloride as shown in figure 2.21. Using the acyl

chloride route to get to the benzyl ester yielded a reasonable amount of product with average yields

of ~58%, but the reaction was not that efficient considering the destruction of excess starting

material. The acyl chloride formation was slow compared to other substrates which often occur in

less than an hour, since shorter reaction times and removal of the excess thionyl chloride recovered

starting material.22 Reaction times with thionyl chloride decreased the yield, forming black sticky

oil, that contained unidentifiable impurities. Optimization studies found that letting the reaction

proceed 4-6 hours before removing excess thionyl chloride yielded the best results, where starting

material could be recovered after addition of the benzyl alcohol. Shorter reaction times needed a

short silica column, 6-hour reaction times formed would have some black oil that could be easily

removed by filtration with cold DCM. The benzyl alcohol added to the reaction action oily acyl

chloride, as the solvent and reagent and would be completed at room temperature in ~36 hours at

room temperature. After the addition of the benzyl alcohol the reaction can be set to reflux to

shorten the reaction time to 12 hours, but this reaction is slow compared to other literature acyl

chlorides and needs additional time to go to completion.18 The benzyl ester triptycene (4a), when

63 tested by TGA for water content, was found to have very low water content, <1% by weight, which was enough for us to confidently go forward with our lithation to make the trifold adduct.

~58% yield

Figure 2.23 Protection of carboxy hexabromotriptycene (4) to a benzyl ester (4a). We moved forward with our synthesis using the benzyl-protected hexabromo triptycene

(4a), Figure 2.23. The difference was the increase in solubility of (4a) compared to (4) in THF, but we tried to keep the conditions the same for reproducibility purposes. The benzyl protected trifold (4b) byproducts seen by HR-TOFMS were almost identical to the unprotected carboxylic acid trifold (1a) with the addition of a benzyl ester. With the increased solubility, we attempted to perform column chromatography to see if purification would be improved, but we ran into similar challenges of overlapping Rf values, making purification at the trifold stage extremely challenging.

~72% crude yield

Figure 2.24 Synthesis of benzyl protected trifold adduct (4b).

We moved forward to synthesize the benzyl ester fantrip (4c) after the cheletropic elimination of the benzyl ester trifold (4b). Using DMDO as the reagent to perform the cheletropic elimination, induced oxidation and subsequent elimination. Similar results were obtained by using hydrogen peroxide since the benzyl ester fantrip (4c) is not hydrophilic and aqueous workup could be used. The purification of the benzyl ester fantrip (4c) was still difficult, but could be accomplished with a column of 40:60 DCM:hexanes followed by a recrystallization in benzene.

Crystal growth experiments were attempted in various solvents and conditions but did not produce x-ray quality crystals. Yields of both methods did improve slightly, but only to ~8%, which is not enough to justify going through extra steps.

~8-10% yield

Figure 2.25 Cheletropic elimination of the benzyl ester trifold (4b) to benzyl ester fantrip (4c) The final step taken was to deprotect the benzyl ester fantrip (4c) to make carboxy fantrip

(1). Testing the deprotection using Pd/C with hydrogen gas worked sufficiently on the hexabromo benzyl ester (4a), and it was a good candidate for deprotection of the benzyl ester fantrip (4c).23

However, this reaction reduced the anthracene blades of the benzyl ester fantrip (4c) to a mixture of byproducts and resulted in no product being formed. Luckily, we found another procedure that worked well, using SnCl4 to cleave the benzyl ester and forming the carboxylic acid as shown in

Figure 2.26.24 The yields were ~82%, but non-reactive starting material could be recovered and reacted again to obtain the carboxy fantrip (1) as the product.

~82% yield

Figure 2.26 Deprotection of benzyl ester fantrip (4c) into carboxy fantrip (1).

Chapter 2.7 Rearrangement and Ring Opening of Triptycene Core

With only small improvements in our yields using protecting group, we had to contemplate other potential causes of the carboxy fantrip monomer lower yields relative to the unsubstituted fantrip. The main suspect for the lower yields was the carboxylic acid functional group at the bridgehead of the triptycene. Searching the literature, we found reports about rearrangement of a triptycene core with a functional group on the bridgehead.25 One study by Penelle et al. performed diazotization of 9-methylene-amine-triptycene.26 Penelle et al. hypothesized that upon diazotization, the neighboring benzene ring of the triptycene skeleton rearranged via an ring- expansion involving the an electrophilic C on the bridgehead, forming product (B) as an intermediate as shown in Figure 2.27. This result was also confirmed by Minoura et al. in 2006, demonstrating that the triptycene ring can expand if there is partial positive carbon next to the bridgehead.27

Figure 2.27 Demjanov rearrangement of a triptycene core from Penelle et al.

Another possible side product we theorize is reductive ring-opening cleavage of the

triptycene core. Searching the literature, Ross and Walsh published on the reductive cleavage of

triptycene, showing a ring opening of the core.28 They used both potassium and sodium metal to

generate the radical anion shown in Figure 2.28. Afterwards they added methanol followed by

sulfur aromatization to produce their final product.

Figure 2.28 Ring opening of triptycene core

Some of our reaction conditions use organolithium reagents and could be having a similar

ring opening reaction with our 9-carboxylic acid triptycene. The carboxylic acid at the 9 position

of the triptycene could stabilize an anion at the 9 position of triptycene as the triptycene core opens,

leaving a phenyl radical that would then abstract a proton from the solvent or other source.

The presence of the carbonyl may facilitate ring expansion or ring opening, providing

undesired decomposition pathways. Benzyl protection would not suppress these pathways. This

67 hypothesis could explain why our yields did not dramatically improve with the protecting of the carboxylic acid. To avoid this ring expansion or opening, we may have to find a way to make the carbon on the bridgehead of the triptycene less electrophilic and/or avoid reductive conditions.

Introducing the carboxylic acid functional group later in the synthesis may reduce or eliminate these types of problems.

Chapter 2.8 Reactivity of the Monomers – Unwanted Polymerization

The carboxy fantrip monomer (1) may be obtained from two different pathways described above, though both suffered from lower than desired yields. Reactions can be scaled up, but the yields decreased slightly. The reason for this may be tied to the reactivity of the monomer when it is in close proximity to itself, even in dark, cold conditions. As shown in Figure 2.29, when there are small quantities of monomer in a vial it is a white powder, but increased amounts of monomer

(~ 10 mg), start to turn orange. 1H NMR primarily contains the peaks for the carboxy fantrip monomer (1) (> 95%), but there are other smaller peaks also present. Both of the vials from the

Figure 2.29 come from the same HPLC sample and dried by the same method, but one sample was more concentrated when collected, thus it is unlikely that outside impurities could be interfering with this result.

(a.) (b.) Figure 2.29 Carboxy fantrip (1) dried on vacuum pump right after HPLC. (a.) vial has ~ 1 mg (b.) contains ~ 5 mg.

To test this further, we set up an NMR tube reaction putting pure carboxy fantrip monomer

(1) dissolved in DMSO and exposed the test tube to 365 nm light for a few hours as shown in

Figure 2.26. After a few hours solids precipitated, which is likely the formation of the dimers, trimers, ect. By HR-TOFMS we can tell that the dimer is forming, but trimers and larger were not observed. DSC traces of fantrip crystals show that the dimerization is reversible at 178 °C, and with a similar dimer we used a temperature of 180 °C to heat our NMR tube and break the carboxy fantrip dimer. After heating for a few hours then letting the NMR tube cool to room temperature, the solids that once were in the tube were dissolved once again. The dimer was still slightly visible by 1H and HR-TOFMS, but it decreased in concentration, showing some reversibility.

365 nm

185 °C

Figure 2.30 The reversibility of carboxy fantrip monomer (1) in test tube.

Heating the temperature beyond 180 °C was not possible in the experiment because DMSO decomposes rapidly at high temperatures and we wanted to avoid forming byproducts in the solution. We did attempt to heat impure solid carboxy fantrip (1) sample that had dimer as the impurity, but we did not see a change in the amount of dimer in the sample. We did not attempt to increase the temperature further because our DSC measurements showed that the carboxy fantrip monomer (1) would start decomposing at 198 °C.

With this we believe that doing larger scale reactions may produce more carboxy fantrip monomer (1), but larger quantities promote dimerization reactions, causing yields to decrease. This is something to be aware of in the future development of new pathways to carboxy fantrip (1).

Chapter 2.9 Conclusion

The synthesis of carboxy fantrip molecule is possible, but extremely challenging compared to its predecessor, fantrip. The main problem for the synthesis is not during the cheletropic elimination stage due to possible over oxidation, but rather during the generation the three-fold benzyne reaction. We think that this might be something along the lines that the triptycene core is rearranging and causing for many byproducts.11 The benzyne reaction is quite reactive and could be reactive with the carboxylic acid on the carboxy fantrip molecule in some manner that we do not know about. However, this is most unlikely due to the benzyl ester fantrip had extremely low yields as well. The functional group on the bridgehead is the most likely culprit that causes the lower yields. Similar molecules like antrip-DEG or Fantrip-hydroxide synthesized by other group members had low yields, which they may have experienced the same problem as carboxy fantrip.

The main way to deal with this problem would be to introduce the carboxylic acid on the bridgehead after the triple-benzyne reaction. This should improve the yields greatly even if it means adding a few more steps to the synthesis.

Chapter 2.10 Experimental Section

Unless otherwise stated, all reactions were performed under nitrogen using standard

Schlenk techniques. Tetrahydrofuran and diethyl ether were distilled over sodium/benzophenone.

N-Methylpyrrole was distilled over calcium sulfate prior to use. Chemicals were purchased from

Acros Organics, Fisher Scientific, Oakwood Products Inc., TCI, and Sigma-Aldrich and used without further purification.

1H, 19F and 13C NMR spectra were recorded on Varian 500 MHz or Varian 400 MHz NMR

Systems Spectrometers. Spectra were recorded in deuterated chloroform (CDCl3) or deuterated methanol (MeOD). Tetramethylsilane (TMS, set to 0 ppm) was used as an internal standard for chemical shifts, with exception of monomer 1, where chloroform was referenced instead.

Purification of monomer 1 was perfomed on Water 600 controller, HPLC and UV-Vis detector, using a XTerra MS C18 5 μm 4.6 x 100mm column. Mass spectra were recorded using an Agilent

6230 HRMS-TOF. The instrument was operated with an electrospray ionization negative (ESI neg) on a time of flight (TOF) instrument in the negative mode. Ammonia formate was added to samples to promote ionization. IR spectra were recorded by FTIR in the range of 4000-600 cm-1.

2-(antracene-9-yl)-1,3-dioxalane: (7)

Anthracene-9-carboxaldehyde (10.43g, 50.57 mmol) was placed into a 500 mL round bottom flask with ethylene glycol (6 g, 97.31 mmol) and p-toluene sulfonic acid (0.021g, 0.011 mmol) was mixed together in 200 mL of toluene. A Dean Stark trap was attached along with a reflux condenser and the solution was heated to reflux for 48 hours. The reaction was then cooled to room temperature which formed bright yellow precipitate, which was filtered and washed with ice cold water to yield 10.53 g for 83% yield.

1 H NMR (400 MHz, CDCl3) δ: 4.25-4.29 (m, 2H), 4.49-4.55 (m, 2H), 7.09 (s, 1H), 7.42-7.51 (m,

4H), 7.98-8.01 (m, 2H), 8.50-8.54 (m, 3H)

1H NMR data match literature procedure.29

9,10[1',2']-Benzenoanthracene-9(10H)-1,3-dioxalane: (6)

5.2g (20.77 mmol) of 2-(antracene-9-yl)-1,3-dioxalane was added into a three-neck 500 mL round bottom with 150 mL of dichloromethane and 4.68 mL (70.47 mmol) of isoamyl nitrite where the solution was heated to reflux. 5.7 g (41.56 mmol) of anthranilic acid was dissolved in

40 mL of dimethoxyethane and then gently heated to induce anthranilic acid to completely dissolve. The anthranilic acid solution was slowly added by syringe pump over a period of an hour.

The reaction was then cooled to room temperature and then another 4.68 mL (70.47 mmol) of isoamyl nitrite was added to the reaction then reset the heat to reflux the solution. Another 5.7g

(41.56 mmol) of anthranilic acid was dissolved in 40 mL of dimethoxyethane and then slowly added to the refluxing solution over a period of an hour. After addition the reaction was left at reflux for an additional hour to ensure the reaction was complete. The reaction was then cooled to room temperature and the solvent was removed by reduced pressure. The remaining gray solid was washed with copious amount of ice cold methanol (around 300 mL) to produce a light gray powder to yield 4.34g (64%) of product.

1 H NMR (400 MHz, CDCl3) δ: 4.31-4.40 (m, 2H), 4.45-4.54 (m, 2H), 6.33 (s, 1H), 6.94-7.01 (m,

4H), 7.34-7.36 (m, 2H),

1H NMR data match literature procedure.30

9,10[1',2']-Benzenoanthracene-9(10H)-carboxaldehyde: (5)

6.3 g (19.30 mmol) of 9,10[1',2']-Benzenoanthracene-9(10H)-1,3-dioxalane was added into a 500 mL round bottom with 150 mL of acetic acid, and 2 mL of hydrochloric acid and then reaction was set to reflux for 12 hours. Afterwards the reaction was cooled to room temperature and then poured into 250 mL of ice cold water to form an off-white precipitate that was filtered and washed with an additional 200 mL of ice cold water to yield 4.85 g of product (89%).

1 H NMR (400 MHz, CDCl3) δ: 5.39 (s, 1H), 7.00-7.06 (m, 6H ), 7.40-7.44 (m, 3H ), 7.59-7.63 (m,

3H ), 11.21 (s, 1H)

1H NMR data match literature procedure 31

9,10[1',2']-Benzenoanthracene-9(10H)-carboxylic acid: (4)

3.1 g (10.98 mmol) of 9,10[1',2']-Benzenoanthracene-9(10H)-carboxaldehyde was added into a 500 mL round bottom with 100 mL of acetone, 25 mL of water and 2 mL of sulfuric acid.

The round bottom was fitted with a reflux condenser and then 1.5 g (15.10 mmol) of chromium trioxide was added into the reaction mixture and then heated to reflux for 2 hour. The reaction was cooled to room temperature and 200 mL of ice cold water was added to the solution to precipitate out a white solid which was filtered and washed with an additional 300 mL of water to yield 3.05 g for a 97 % yield.

1 H NMR (400 MHz, CDCl3) δ: 5.39 (s,1H), 7.03-7.08 (m, 6H ), 7.38-7.43 (m, 3H ), 7.85-7.90

(m, 3H )

1H NMR data match literature procedure 32

Carboxy hexabromo-triptycene: (3)

9,10[1',2']-Benzenoanthracene-9(10H)-carboxylic acid (2 g, 6.70 mmol) was added into chloroform ~350 mL then iron powder was added (0.2, g, 3.59 mmol) at room temperature under stirring. The mixture was stirred for 30 minutes then bromine (2.07 mL, 6.42 g, 40.20 mmol) in chloroform (~20 mL) was added over ~20 minutes by addition funnel. The reaction was then stirred under reflux for eight hours, then cooled to room temperature and the solvent was removed under pressure. The resulting reddish-brown crude oily material was dissolved in ethyl acetate

(~150 mL) and filtered through a pad of silica gel using with additional (~300 mL) ethyl acetate.

The solvent was then removed under reduced pressure and the resulting brownish solid was

77 dissolved in hot tetrahydrofuran (~200 mL) and cooled to 0°C to give pure 3 as white crystals. The mother liquor was concentrated to (~100 mL) and cooled again to give additional 3 for a total of

(3.22 g, 4.17 mmol, 62 %)

1 13 H NMR (400 MHz, CDCl3) δ: 8.09 (s, 3H), 7.64 (s, 3H), 5.19 (s, 1H) C NMR (500 MHz, CDCl3)

δ: 170.33, 146.05, 143.87, 130.58, 129.71, 122.91, 122.33, 61.34, 51.81.

HRMS (ESI Neg): m/z calcd for [C21H8Br6O2] 764.5546; found 764.5581 Mp: > 300 °C

For best results on this reaction, the starting material 9,10[1',2']-Benzenoanthracene-

9(10H)-carboxylic acid should be dried under reduced pressure at ~120 °C for a 2-3 hours before use to help remove excess water. Check aliquots by mass spectra to see if there is still under brominated material left, if so, add in small portions of bromine (~10% of whatever you originally added) to push the reaction to completion. Over brominated material is possible and should be aware not to add too much excess bromine in at a time.

Carboxy fantrip trifold: (1a)

A solution of carboxy hexabromotriptycene (1g, 1.29 mmol) and 2-methyl-4,5,6,7- tetrafluoroisoindole (0.82 g, 4.01 mmol) in tetrahydrofuran (300 mL) was cooled to −78 °C.

Then tert-BuLi (1.6 M in hexanes, 4.91 mL, 7.87 mmol) was added slowly, dropwise, over a period of 20 minutes. The reaction mixture was slowly warm up to room temperature after 1 hour. Afterwards the solvent was removed under reduced pressure and the orange-yellow crude reaction mixture was dissolved in DCM (~200 mL) and filtered through a pad of silica gel with additional DCM (~ 400 mL). The front fractions were heated to 70 °C at 250 mTorr for 1 hour to sublime any unreacted tetrafluoroisoindole. 1.03g of crude trifold was left over and was used in next step without any further purfication. Full purification of compound 1a was not possible due to the large number of isomers as well as the limited stability of the compound on silica gel and was used without further purification.

HRMS-TOF (ESI Neg): m/z calcd for [C48H23F12N3O2] 900.154; found 900.153

Carboxy fantrip, monomer (1)

A solution of 1a (1.03 g, 1.14 mmol) and dimethyldioxirane (0.098 M, 37.30 mL, 3.65 mmol) in acetone (100 mL) was stirred at room temperature for 14 hours. The solution was reduced under pressure to give 902 mg of impure crude product. The crude product was passed through of plug a silica with 300 mL of DCM then 200 mL of ethyl acetate, and the solution was evaporated under reduced pressure. Further purification was accomplished by reverse phase

HPLC column at 0.75 mL/min with 5:95 ethyl acetate:methanol mixture to give pure monomer 1

(46.06 mg, 0.046 mmol, 4.94%)

[Note: Yields of monomer 1 can be inconsistent and we found that yields can range from 1-10%]

1 H NMR (400 MHz, CDCl3) δ: 8.64 (s, 2H), 8.61 (s, 2H), 8.54 (s, 2H), 8.17 (s, 2H), 5.91 (s, 1H)

13 C NMR (500 MHz, CDCl3) δ: 174.92, 146.54, 144.58, 142.74, 141.72, 139.73, 134.71, 127.93,

19 125.90, 124.06, 122.53, 64.39, 57.24 F NMR (CDCl3, 400 MHz) δ: -150.19 (t, 16 Hz, 3F), -

150.47 (t, 3F), -158.55 (t, 16Hz, 3F), -158.91 (t, 16 Hz, 3F)

HRMS (ESI Neg): m/z calcd for [C45H14F12O2] 814.0801; found Mp: > 300 °C

Wavelength @358

Dimethyldioxirane (DMDO):

To a 1-L, three-necked, round-bottomed 80 mL of water, acetone (25 mL, 0.34 mol), and sodium bicarbonate (48 g), is equipped with a magnetic stirring bar and was cooled with an ice bath. A dry ice cold finger was attached to the top of the reaction flask which was connected to another dry ice cold finger that has a vacuum attachment and 250 mL round bottom attached at the bottom for collection. A solid addition flask containing Oxone (90 g, 0.15 mol) is added and vigorously stirred throughout the addition of reagents and was stirred for another hour. After stirring the reaction vessel was put under reduce pressure around 300 mmbar where condensation begins to occur on the second dry ice cold finger. The yellow solution was dried oversodium sulfate and the dimethyldioxirane in acetone was collected in the receiving flask and collection is completed when no more condensation can be seen on the cold finger.

The concentration of the DMDO solution was measured against thioanisole by GCMS. 33

Benzyl ester hexabromo-triptycene: (4a)

To a 500 mL round bottom flask 1 g of carboxy hexabromotriptycene (1.29 mmol) was added with a stir bar. Then 30 mL of thionyl chloride was added to the reaction then set to reflux for 5-6 hours. Afterwards, the reaction was cooled to room temperature then the excess thionyl chloride was pumped off at reduced pressure. 40 mL of benzyl alcohol was added to the reaction then set to reflux for 14 hours. Then the excess benzyl alcohol was pumped off by vacuum pump until no liquid remained and only an oily brownish solid was left behind. The solid was then dissolved in 20-30 mL and passed through a silica plug with an additional 200 mL of DCM. The

88 solution was then evaporated by reduced pressure where 0.64 g of a white solid product 4a was recovered at a 58% yield.

Note: Some excess benzyl alcohol is persistent in the product even after many hour under reduce pressure, filter the solid with ~100 mL of ice-cold methanol, then dry off the solid. Aqueous workup can achieve similar results.

1 H NMR (400 MHz, CDCl3) δ: 5.15 (s, 1H), 5.70 (s, 2H), 7.43-7.50 (m, 3H), 7.60 (m, 5H), 7.86

(s, 3H)

13 C NMR (500 MHz, CDCl3) δ: 51.30, 59.19, 68.22, 121.94, 122.47, 128.62, 129.18, 129.38,

129.57, 129.60, 134.21, 142.05, 144.07, 167.57.

HRMS (ESI Neg): m/z calcd for [C28H14Br6 O2] 861.83 found; 861.65

Benzyl ester fantrip trifold: (4b)

A solution of benzyl hexabromotriptycene (0.33g, 0.43 mmol) and 2-methyl-4,5,6,7- tetrafluoroisoindole 2 (0.27 g, 1.37 mmol) in tetrahydrofuran (100 mL) was cooled to −78°C.

Then tert-BuLi (1.6 M in hexanes, 1.60 mL, 2.62 mmol) was added slowly, dropwise, over a period of 20 minutes. The reaction mixture was allowed to slowly warm up to room temperature after 1 hour. Afterwards the solvent was removed under reduced pressure and the orange-yellow crude reaction mixture was dissolved in DCM (~100 mL) and filter through a pad of silica gel with additional DCM (~ 200 mL). The front fractions was heated to 70 °C at 250 mTorr for 1 hour to sublime any unreacted compound. 0.34 g of crude compound was left over and was used in next step without any further purification. Full purification of the trifold was never done due

92 to the large number of isomers as well as the limited stability of the compound on silica gel and was used without further purification.

HRMS (ESI Neg): m/z calcd for [C55H29F12N3O2] 991.82; found 991.19

Benzyl ester-Fantrip: (4c)

A solution of 4b (0.34 g, 0.35 mmol) and dimethyldioxirane (0.088 M, 10.1 mL, 0.88 mmol) in acetone (50 mL) was stirred at room temperature for 14 hours. The solution concentrated was reduced under pressure to give 0.29 mg of impure crude product. The crude product was passed through a plug of silica with 200 mL of DCM which then the solution was evaporated under reduced pressure. Further purification went through a silica column using

40:60 hexanes:DCM, which afterwards semi-pure material yellowish solid was obtained then recrystallized with benzene to form a pure white powder 4c. (2.44 mg, 0.0027 mmol, 7.87%)

1 H NMR (400 MHz, CDCl3) δ: 5.83 (s,1H), 6.01 (s, 2H), 7.60-7.70 (m, 4H), 7.80-7.85 (m, 2H),

8.09 (s, 3H), 8.30 (s, 3H), 8.35 (s, 3H), 8.48 (s, 3H).

13 C NMR (500 MHz, CDCl3) δ: 53.27, 59.53, 67.75, 118.58, 120.27, 121.99, 123.91, 129.25,

129.58, 130.56, 130.63, 131.06, 135.18, 138.45, 140.29, 169.12.

19 F NMR (400 MHz, CDCl3) δ: -159.18 - -159.09 (t, 6F), -158.80 - -158.71 (t, 6F).

HRMS (ESI Neg): m/z calcd for [C52H20F12O2] 904.69; found 904.17.

Deptrotection of benzyl ester fantrip to carboxy fantrip.34

The benzyl ester fantrip (4c) 0.0074 g (0.0081 mmol) was dissolved in anhydrous DCM (10 mL) under nitrogen. While stirring, SnCl4 (0.0041mmol, 0.5 equiv.) was added and the reaction was sealed and heated to 40 °C for 12 hours or overnight. Reaction can be checked by TLC after a few hours, but reaction times over 6 hours were necessary before seeing any product. Once complete the reaction mixture was quenched with HCl (1 mol/L,1 mL) and worked up by aqeous extraction. The organic layer was washed with brine and then dried sodium sulfate and the product was dried under reduced pressure to obtain 0.0051 g of a white powder, carboxy fantrip

(1) for 82% yield. HPLC was not required, but if the produced is slightly yellow, use conditions for purifying carboxy fantrip (1).

General information: All reactions were done under nitrogen atmosphere except when specified.

Starting material triptycene was purchased and used as received from AKSci. Mass spectra were recorded with an atmospheric pressure photoionization (APPI) source on a time-of-flight (TOF) instrument in the positive mode. IR spectra were recorded by FTIR in the range of 4000-600 cm-

1. All NMR measurements were taken on either a 400 or 500 MHz Varian instrument.

Hexabromotriptycene (3): Triptycene (0.5 g, 1.9 mmol) was dissolved in 250 mL of chloroform.

Iron powder (0.02 g, 0.27 mmol) was added, followed by addition of bromine (0.6 mL, 11.5 mmol) by addition funnel at reflux. 3h after addition, the reaction was cooled to room temperature and eluted through a pad of silica, removing the iron. The solute was evaporated under reduced pressure, resulting in a mixture composed primarily of the product and some partially brominated product. The product mixture was then filtered and washed with cold methanol (150 mL) which removes nearly all impurities, sometimes all impurities will be removed, and material can be used.

If impurities persist, the remaining solid is then recrystallized twice using hot acetone. The crystallization yielded the product as a white powder 1.11 g (78%). The 1H NMR and mass spectrum match to previously reported results.

1 H NMR (400 MHz, CDCl3) 훿 7.62 (s, 6H), 5.23 (s, 2H). HRMS APPI-TOF positive mode calculated 727.70 found 727.57.

Figure 1.31 Hexabromotriptycene 1H NMR.

Indole precursor (5) Using a 250 mL round bottom flaks, 4.5 mL (7.1g, 34.9 mmol) of chloropentafluorobenzene was added to 125 mL of dry diethyl ether then cooled to -78° C under nitrogen. After multiple purge cycles (three minimum), with nitrogen 24 mL of 1.6M n-BuLi (38.4 mmol) was added slowly over an hour by syringe pump while the reaction was stirring. After addition of n-BuLi the reaction was allowed to stir for an hour keeping the temperature at -78° C.

12.5 mL (11.5 g, 140.9 mmol) of N-methylpyrrole was added to the solution over an hour period by syringe pump while the solution was kept at -78° C. After the addition the solution was allowed to warm to room temperature overnight. Any unreacted n-BuLi was quenched with isopropanol then the solution was evaporated under reduced pressure to produce a black oil. Purification of

99 the crude material was completed by sublimation of the oil at 130° C under reduced pressure, to sublimate oily white crystals. These crystals were then recrystallized with hot hexanes to remove excess N-methylpyrrole to yield 3.9 g of pure product for a 48.3% yield. 1H NMR and melting

1 temperatures match previously reported results. H NMR (400 MHz, CDCl3) 훿 6.98 (s, broad, 2H),

4.86 (s, very broad, 2H), 2.14 (s, broad, 3H). Mp: 73-76.2 °C Literature value Mp: 74.5-76 °C

Isoindole (4). A 500 mL round bottom was filled with 3.9 g (17.02 mmol) of isoindole precursor and 250 mL of dichloromethane, then brought to reflux while stirring. Bis(2-pyridyl)-1,2,4,5- tetrazine was added (4.3 g, 18.01 mmol) to the stirring solution under nitrogen and was reacted for

12 hours. The solution was then cooled to room temperature and passed through silica gel with dichloromethane to remove any excess Bis(2-pyridyl)-1,2,4,5-tetrazine. Another silica gel

100 filtration may be required if excess Bis(2-pyridyl)-1,2,4,5-tetrazine remains. The remaining solution was evaporated under reduced pressure to produce an orange/yellow solid. Purification of this solid was accomplished by sublimation, in the dark (covered by aluminum foil) at 130 °C under reduced pressure to obtain 2.4 g (69.8% yield) near pure yellow powder isoindole product.

1H NMR and GCMS data match previously reported results.

1 H NMR (400 MHz, CDCl3) 훿 7.26 (s, 2H), 4.00 (s, 3H). Mp: 169.2-171.6 °C. GCMS: Calculated

203.14 found 203.00. Literature 172-174 °C.

Fantrip tri-fold adduct (2) A dry 500 mL round bottom was filled with 3.0 g (4.1 mmol) of hexabromotriptycene and 3.4 g (16.4 mmol) of isoindole which was dissolved in 250 mL of dry

THF. This solution was then cooled to -78° C, then purged and refilled with nitrogen three times.

1.6 M n-BuLi 7.9 mL (12.71 mmol) was added by syringe pump over a period of 30 minutes while the reaction mixture was stirring. After the addition, the reaction was stirred for another hour. The reaction was then quenched with isopropanol and allowed to warm to room temperature. Once at room the solvent was evaporated under reduced pressure to obtain a brown oily solid. The crude product was dissolved in dichloromethane and filtered through silica gel to remove excess salts and excess isoindole. To obtain the main product the mobile phase was changed to ethyl acetate to elute a brown solution. This solution was evaporated under reduced pressure to give a semi-pure brown solid product mixture of 3.1 g. This crude product can proceed to the next step to make fantrip. For further purification a careful column using silica gel and 80:20 ethyl acetate and hexanes can be used to elute a light brown solid which was filtered and washed with cold methanol to obtain of relatively clean trifold adduct 0.89 g (25% yield). 1H NMR and mass spectrums match

101

1 literature reports. H NMR (400 MHz, CDCl3) 훿: 7.40-7.20 (m, 6H), 5.22-4.95 (m, 8H), 2.25-2.00

(m, 9H). C47H23F12N3 APPI Pos found 857.19 calculated 857.68

Fantrip (1) A 250 mL round bottom flask is filled with fantrip adduct 1.5 g (1.75 mmol) and 95 mL of tert-butanol. The tert-butanol was brought to reflux which then the fantrip precursor was dissolved into the solution. After the solution was brought to reflux 20 eq of H2O2 was added to the solution and stirred overnight. The solution was then cooled to near room temperature and then

35 mL of cold water was added to the solution, which precipitated a pale yellow solid. This yellow solid was filtered and washed with 200 mL of additional water 1.13 g of product was collected for an 84 % yield. 1H NMR and mass spectrums match literature reports.

102

1 13 H NMR (400 MHz, CDCl3) 훿: 8.53 (s, 6H), 8.13 (s, 6H), 5.92 (s, 2H). C NMR (500 MHz,

19 CDCl3) 훿: 141.7, 140.7, 137.1, 131.2, 122.4, 119.1, 118.8, 53.0. F NMR (CDCl3 400 MHz) 훿: -

150.6 (dd, 6F), -159.1 (dd, 6F). High Res TOFMS: C44H14F12 Calculated 770.56 found 770.11

103

3,6-bis(2-pyridyl)-1,2,4,5-tetrazine: In a 1 L round bottom 2-cyanopyridine (40.16 g, 386.72 mmol) in anhydrous ethanol (400 mL) was added with hydrazine monohydrate (44 mL) and the reaction mixture was set to reflux for 22 hours. The solution was cooled to room temperature then to 0 °C for 1 hour and the resulting orange precipitate that was collected by filtration and washed with cold ethanol (50 mL). Drying the orange solid under reduced pressure gave 3,6-di(2-pyridyl)-

1,2,4,5-tetrazine (14.73 g, 60.94 mmol). This solid was then placed into a 1L Erlenmeyer flask at room temperature in a solvent of glacial acetic acid (140 mL) and ethanol (140 mL) and stirred for

10 minutes. While contining to stir, a solution of sodium nitrate (12.7 g, 184.02 mmol) in water

(20 mL) was slowly added dropwise over 10 minutes. During addition, gas formation was observed. After addition, the reaction mixture was stirred at room temperature for an hour. The

104 resulting solution was then poured onto ice and stirred until the ice melted. A purple solid was collected by filtration, and then washed with water and dried under reduced pressure to produce

3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (12.20 g, 50.62 mmol, 85%) as a deep purple crystalline solid.

1H NMR and melting points match previously reported results.

1 H NMR (400 MHz, CDCl3) 훿: 8.97 (ddd, 2H), 8.74 (dd, 2H), 8.01 (ddd, 2H), 7.58 (ddd, 2H). Mp:

226.5-228.8 °C. Literature 222-230 °C.

Fantrip crystal preparation and Exfoliation of 2DP sheets: To grow fantrip crystals for polymerization, 1 mL of warm (near boiling) chloroform for every mg of fantrip was used. After fantrip was dissolved in the solution, it was allowed to cool to room temperature and kept uncovered in a dark area to slowly allow evaporation to occur. At room temperature and after

105

around 24 hours of allowing the solvent evaporation, yellow crystals should be seen. Crystals

were collected by filtration and washing with cold chloroform and were dried under reduced

pressure.

100 mg of fantrip crystals were put inside a 20 mL vial with 10 mL of a 1:5 mixture of

chloroform: hexanes solutions under argon. The crystals did not dissolve in this mixture at room

temperature. Using a 395-405 nm LED the crystals were irradiated for 3-5 hours. Every 60 minutes

the vial was shaken around to mix the vial. Upon completion of irradiation, the yellow crystals

become orange-yellowish, poly(fantrip). After this, the crystals were sonicated in the mixture for

1-2 hours to produce to 2DP sheets of poly fantrip.

Figure 2.32 Optical microscopy images of partially exfoliated poly(fantrip) crystals.

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Figure 2.33 IR spectra of fantrip monomer before irradiation (top) and the IR of poly(fantrip)

(bottom).

107

Chapter 2.11 References

1 Kissel, P.; Murray, D. J.; Wulftange, W. J.; Catalano, V. J.; King, B. T. A Nature Chem. 2014, 6 (9), 774–778.

2 Bhola, R.; Payamyar, P.; Murray, D. J.; Kumar, B.; Teator, A. J.; Schmidt, M. U.; Hammer, S. M.; Saha, A.; Sakamoto, J.; Schlüter, A. D.; et al. J. American Chem. Soc. 2013, 135 (38), 14134–14141.

3 Murray, D. J.; Patterson, D. D.; Payamyar, P.; Bhola, R.; Song, W.; Lackinger, M.; Schlüter, A. D.; King, B. T. J. American Chem. Society 2015, 137 (10), 3450–3453.

4 Kissel, P.; Murray, D. J.; Wulftange, W. J.; Catalano, V. J.; King, B. T. Nature Chemistry 2014, 6 (9), 774–778.

5 Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory Chemicals, 4. ed., reprint.; Butterworth-Heinemann: Oxford, 2002.

6 Jadhav, S. A. Central European J. Chem. 2011, 9 (3), 369–378.

7 Skvarchenko, V. R.; Shalaev, V. K.; Klabunovskii, Rus. Chem. Rev. 1974, 43 (11), 951–966.

8 Gribble, G. W.; LeHoullier, C. S.; Sibi, M. P.; Allen, R. W. J. Org. Chem. 1985, 50 (10), 1611–1616.

9 Hart, H.; Lai, C.; Chukuemeka Nwokogu, G.; Shamouilian, S. Tetrahedron 1987, 43 (22), 5203–5224.

10 Hart, H.; Shamouilian, S. J. Org. Chem. 1981, 46 (24), 4874–4876.

11 Mikula, H.; Svatunek, D.; Lumpi, D.; Glöcklhofer, F.; Hametner, C.; Fröhlich, J. Org. Process Res. Dev. 2013, 17 (2), 313–316.

12 Murray, W., R; Org. Syntheses 1997, 74, 91.

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13 Chambers, R. D. Fluorine in Organic Chemistry; Blackwell Publishing Ltd.: Oxford, UK, 2004.

14 Waldmann, C.; Schober, O.; Haufe, G.; Kopka, K. Org. Letters 2013, 15 (12), 2954–2957.

15 Mannes, P. Z.; Onyango, E. O.; Gribble, G. W. J. Org. Chem. 2015, 80 (21), 11189–11192.

16 Shieh, W.-C.; Dell, S.; Repič, O. Tetrahedron Letters 2002, 43 (32), 5607–5609.

17 Taber, D. F.; Gerstenhaber, D. A.; Zhao, X. Tetrahedron Letters 2006, 47 (18), 3065–3066.

18 Zhao, H.; Pendri, A.; Greenwald, R. B. J. Org. Chem. 1998, 63 (21), 7559–7562.

19 Meyers, A. I.; Temple, D. L.; Haidukewych, D.; Mihelich, E. D. J. Org. Chem. 1974, 39 (18), 2787–2793.

20 Yoon, I.; Suh, S.-E.; Barros, S. A.; Chenoweth, D. M. Organic Letters 2016, 18 (5), 1096– 1099.

21 Khurana, J.; Arora, R. Synthesis 2009, 2009 (07), 1127–1130.

22 Norman, R. O. C.; Coxon, J. M. Principles of Organic Synthesis, 3. ed, repr.; Thornes: Cheltenham, 2001.

23 P. K. Mandal, J. S. McMurray, J. Org. Chem., 2007, 72, 6599-6601.

24 Baker, A. E. G.; Marchal, E.; Lund, K. A. R.; Thompson, A. Canadian J. Chem. 2014, 92 (12), 1175–1185.

25 Skvarchenko, V. R.; Shalaev, V. K.; Klabunovskii, E. I. Rus. Chem. Reviews 1974, 43 (11), 951–966.

26 Cristol, S. J.; Pennelle, D. K. J. Org. Chem. 1970, 35 (7), 2357–2361.

27 Yamamoto, G.; Koseki, A.; Sugita, J.; Mochida, H.; Minoura, M. Bull. Chem. Soc. Jpn. 2006, 79 (10), 1585–1600.

28 Walsh, T. D.; Ross, R. T. Tetrahedron Letters 1968, 9, 3123–3126.

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29 Seeleib, Y.; Nemecek, G.; Pfaff, D.; Süveges, B. D.; Podlech, J. Synthetic Comm. 2014, 44 (20), 2966–2973.

30 Kornfeld, E. C.; Barney, P.; Blankley, J.; Faul, W. J. Medicinal Chem. 1965, 8 (3), 342–347.

31 Kimura, M.; Okamoto, H.; Kashino, S. Bull. Chem. Soc. Japan 1994, 67 (8), 2203–2212.

32 Yang, N. C. C.; Yang, X. Q. J. American Chem. Society 1987, 109 (12), 3804–3805.

33 Mikula, H.; Svatunek, D.; Lumpi, D.; Glöcklhofer, F.; Hametner, C.; Fröhlich, Org. Process Res. Dev. 2013, 17 (2), 313–316.

34 Baker, A. E. G.; Marchal, E.; Lund, K. A. R.; Thompson, A. Canadian J. Chem. 2014, 92 (12), 1175–1185.

110

Chapter 3 Langmuir-Blodgett Films of Poly(carboxy fantrip) Films

Langmuir-Blodgett techniques are often touted for their ability to make ultrathin organic films with organized, controlled structures.1 Often in the literature Langmuir-Blodgett films are made to imitate single layer graphene-like structures or used to make ultrathin membranes.2 The

Langmuir-Blodgett technique’s ability to make single layer materials caught the interest of material scientists in the pursuit of making 2-dimesional polymers (2DPs).

As shown in Chapter 1, it was possible to make a 2DP material by exfoliating crystals of fantrip into sheets that are single-layer porous structures.3 The immediate problem with such a method is that the sheets are limited in size and fabrication potential. Synthesizing carboxy fantrip, was the response to finding a method for fabricating monolayer, porous graphene-like structures based on fantrip. Having the carboxylic acid act as an hydrophilic anchor for the water surface makes it possible to place the monomer onto a Langmuir trough and fabricate Langmuir-Blodgett films. These Langmuir films make it possible to transfer a porous monolayer onto different substrates and not be limited in the size of sheets that can be grown.

3.1 Langmuir Films Introduction

When a drop of an insoluble liquid is deposited onto another liquid, it will spread out across the surface over time and make a film. The first written records of this observation were by Pliny the Elder in 560 BC where he noted separation of the two liquids, oil and water.4 The first experimental recording of oil on water was by Benjamin Franklin who noted that the ship’s cooks pouring oil over the side of the ship made a glassy surface.6 Benjamin Franklin took the same oil and went around pouring small amounts of the oil over ponds and lakes and had the same observation as he noticed on the ship.5 Many years later it was Agnes Pockels who piloted the

111 study of oil base films over water to the modern era, where all the experiments she performed were in her own kitchen. After these studies she told of her results to Lord Rayleigh, which led him to study these oil-based films over water. Lord Rayleigh was able to first calculate the width of the films and deduce that the films had the thickness of one molecule.6 From the Rayleigh results, another scientist, Langmuir, started to take interest in the studies and improved the experimental setups and conditions. Langmuir’s improvement was in the development of the basic equipment used to make the oil base films uniformed and easier to prepared, known as a Langmuir trough.

Langmuir’s assistant, Katharine Blodgett, worked with Langmuir for many years, perfecting many of the transferring techniques from the trough to substrates, of which many are still used in surface science. Both of their dedicated studies of the films over the surface of water lead these films to be known as Langmuir-Blodgett films, though literature today shortens the term as Langmuir films. For the work done by Langmuir on studies of molecular films on liquid surfaces, he was awarded the 1932 Nobel Prize for chemistry. Most of the basic film science was developed from both Langmuir and Blodgett’s work over the last century and allowed for more complex Langmuir-

Blodgett films to be developed.

Langmuir-Blodgett films are formed when a sufficient amount of amphiphile is deposited onto the water surface. Amphiphiles are defined as molecules that have a polar (hydrophilic) group on one end and nonpolar (hydrophobic) group on the other. Examples of amphiphiles used as

Langmuir films include stearic acid, alkanethiols, and perfluoronated alkanethiols.

The hydrophobic end of the amphiphiles is often a long alkyl chain that is hydrogenated or perfluoronated. The hydrophilic end-groups of the amphiphiles often use alcohols, amines, amides, nitriles, aldoxime, ketoxime, and carboxylic acids. When an amphiphile is deposited onto the water surface and allowed to dissipate, the polar end is attracted to the water whereas the alkyl chain

112

(hydrophobic) floats on top of the water. The area of the amphiphile can be varied by applying pressure to the molecules with the use of a movable barrier, bringing the amphiphile closer together and making the Langmuir-Blodgett film. An example of the effects of pressure on an amphiphile is shown in figure 3.1.

Figure 3.1 (a.) Example of molecules being in a gas “phase” on the trough going towards a solid “phase” once barriers have been closed. (b.) Surface pressure [mN/m] vs. area per molecule [Å2/molecule] for stearic acid showing the three “phases” that occur when compression occurs.

113

When the amphiphile is first deposited onto the water surface, the area per molecule is large and this is known as the disordered gaseous phase. This gas phase possesses little to no short- range or long-range order and there is little interaction between the molecules. As pressure is applied, the area decreases, and a phase transition happens where the gas phase goes into a liquid- like phase and exhibits some short-range but no long-range order in the film. Compressing the film further, the area shrinks, and the film goes into a solid-like phase.

In a perfect Langmuir film, all the condensed phases will be observed, but this is not true for every amphiphile where all transitions may not be observed. Condensed phase formation often depends on the combined intermolecular interactions of film-film and film-water interactions. The amphiphile hydrophilic head-group is in the liquid layer and participates in hydrogen bonding to the liquid (usually water, but other liquids with high surface tensions, like mercury can be used).

Tail group van der Waals /electrostatic interactions

Electrostaic interactions or Surface-active hydrogen bonding head group Liquid

Figure 3.2: The polar head-group (hydrophilic) of the amphiphile interacts with the liquid through electrostatic interactions or hydrogen bonding. When barriers close and pressure is applied, the tail groups (hydrophobic) rearrange to maximize attractive interactions.

114

The hydrogen bonding between the amphiphile and the liquid is a weak interaction, around 20 kJ/mol-1, which keeps the amphiphile on the surface of the water.6

3.2.1 Parts of the Langmuir-Blodgett Trough

The design of the equipment for deposition of Langmuir-Blodgett films is crucial for making consistent films. For a standard Langmuir trough experiment, a few µg of an amphiphile are spread across a surface of hundreds mL of water, followed by compression, resulting in a

Langmuir film. Compression speeds of the amphiphile must be very slow and precise control otherwise the Langmuir film may experience defects. Avoiding defects in the Langmuir films relies on using the ideal conditions and equipment and knowing how each part affects the formation of Langmuir films.

LED lamp

dipper

Wilhemy plate

movable barriers trough with well

Figure 3.3 A general setup for a Langmuir-Blodgett trough.

For making defect free Langmuir films the first consideration is the material that the

Langmuir-Blodgett trough is made from. Material used for all parts that come in direct contact

115 with the subphase should be hydrophobic, inert, and not dissolve in organic solvents.

Polytetrafluoroethylene (Teflon) is often used to build troughs since it is a material that is easily formed into desired shapes and unreactive to most compounds.

The barriers of the trough are independent of the trough but still made of the same material to avoid contamination when applying pressure to form the monolayer ‘solid’ phase. Barriers are connected to a motor that allows for various speeds in contracting the area of the trough. If the amphiphile changes phases too quickly or slowly, it could cause defects in the creation of the monolayer solid. Troughs can have one, two, or even three barriers with different forces for a whole range of purposes, but most common troughs have two barriers, which allows for the creation of monolayers and removal of surface contaminants.

Monolayer

Barrier Barrier

Trough Well for LB trough

Figure 3.4 A basic Langmuir-Blodgett trough with two movable barriers.

Measuring the forces applied by the barriers and the amphiphiles commonly measures the surface pressure on the Langmuir-Blodgett trough by Wilhelmy plate technique. A measurement using the Wilhelmy technique measures the surface pressure by hanging a plate connected to a microbalance which is sensitive to pressures around 10-3 mN m-1 that is dipped into the subphase where the ‘well’ resides and allowing detection of changes in surface pressure.6 Forces acting

116 upon the plate are from gravity and surface tension pushing down, and buoyancy, pushing upwards; water being moved by the monolayer or evaporation causes changes in these forces. The net downward force can be solved by equation 3.1, where a rectangular plate, with dimensions l, w, and t and the material density of the plate, ρw, immersed in a depth of h in a liquid of density

ρL. ϒ is the surface tension of the liquid, θ is the contact angle on the plate and g is the gravitational constant.7 A plate that is entirely wetted is chosen (where θ = 0, typically platinum plates or chromatography paper is used) and the change in force on the stationary plate is measured.

F = ρwglwt + 2 ϒ(t + w) cos θ - ρLgtwh (Eq. 3.1)

microbalance

h θ

w t

Figure 3.5 Example of a Wilhelmy plate with the left being a front view and the right being a side view of the plate.

3.2.2 Solubility of Compounds

When formulating ideas for new possible amphiphiles to use for Langmuir films it is important to think about the solubility of the overall compound. Most amphiphiles can be simply

117 divided into two categories; those that are soluble in water and those that are not. Amphiphiles soluble in water are generally polar compounds that carry an uneven distribution of charge, such as water.

The electric dipole moment of an amphiphile is often associated with certain functional groups that are frequently used in Langmuir film fabrication as shown in Table 3.1. Molecules like stearic acid can possess both nonpolar and polar groups. In stearic acid the solubilizing unit is the carboxylic acid group (polar) of the compound, while the long hydrocarbon chain (nonpolar) portion is immiscible with the aqueous solution. It is important to use polar functional groups in the solubilizing unit that will allow for monolayer formation to take place, but if the group is too solubilizing the molecule may dissolve in the aqueous subphase and not form a monolayer film.

The balance between the hydrophobic and hydrophilic portions of the amphiphile is crucial in monolayer formation.

Chemical group Dipole moment, µ [Debye] Example

benzene ring 0 Benzene

cis alkene 0.33 cis-2-butene

amines 1.17 Propylamine

alcohols 1.68 Propanol

carboxylic acids 1.75 propranoic acid

nitro 3.66 Nitropropane

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cyano 4.02 Cyanoethane

Table 3.1 Example of various chemical groups and their dipole moments.

3.3 Causes of Defects in Films

Defects in Langmuir films have several forms. These sort of defects, called disclinations, 8

are caused by breaks in molecules when forming the monolayer.9 Disclinations are seen in fatty

acids layers, where two opposite signs form the defects in the films that resemble a hole as shown

in Figure 3.6.10

(a) (b)

Figure 3.6 Langmuir-films deposited on subphase: (a) Film that does not have any defects. (b)

Film with a disclination defect.

Other types of defects can occur in the films by means of using a low quality water

subphase. Water that is not of the highest quality will produce films that do not span properly or

contain defects that arise from impurities in the water.11 The water to use for making Langmuir

films has an organic content less than 20 ppb and a low conductivity of 18M ohmcm. Distilled

water that is provided in most laboratories is only around 1M-ohmcm and will affect film

production, especially if using fatty acids where the mineral ions will combine with the dissociated

fatty acids. Storing and transporting the water used in the formation of Langmuir films should be

119 taken into consideration due to impurities from the container that may affect experimental work.

Water should be stored only in borosilicate glass bottles and not stored longer than a couple days, after which leaching will occur. Plastic or even Teflon bottles should not be used since the surfactants can contaminate the subphase.

Defects in the Langmuir films can occur by improper spreading of the amphiphile onto the subphase.12 The amphiphile is often dissolved in a solvent that will not react with the material or the trough, like chloroform or n-hexane, and the solution is spread across the surface in a known concentration.

Defects can occur by improper cleaning of the surface of the subphase of the LB trough where clean areas are essential for obtaining high quality Langmuir films. A list of the most common contaminants that have shown up in the subphase of LB troughs includes human skin oil, silicone diffusion pump oil, silicone vacuum grease, and residues of cleaning solvents.13 The most common way to clean the subphase is to use a glass capillary tube or pipet with a suitable pump and remove the top layer of water in sweeping fashion to reduce the amount of contaminants on the surface that could affect the quality of the film.14

3.4 Transferring Langmuir Films

There are multiple ways to transfer a film onto the surface of a substrate, but it generally occurs when a solid substrate interacts with and holds up a Langmuir film. The substrate contact with the surface of the subphase leads to the formation of a meniscus which transitions from the surface of the liquid to the surface of the substrate. When the liquid becomes thinner and gives way to the solid substrate there is a transfer of the film onto the substrate. Substrates can be lowered into the subphase quickly without affecting monolayer transfers, but withdrawing the substrate must proceed slowly, otherwise transfer of the film onto the substrate may not occur. The rate of

120 removal of the substrate from the subphase should not be faster than the rate of water draining from the substrate. Slow speeds, around a few mm s-1 to 10 µm s-1, are common, and help improve film deposition.15

3.4.1 Vertical Deposition

Transferring Langmuir films can also be accomplished vertically as shown in Figure 3.7, where the surface of the liquid comes in contact to the surface of the substrate while there is film on the surface of the air-water interface. When the substrate is pulled in a vertical fashion out of the subphase and barriers are used to retain the form of the Langmuir film, then the film will be transferred to the substrate, either on one side or both sides, depending on the substrate. Vertical transfer will occur if the contact angle of the substrate differs from that of the film being transferred onto the substrate. Also, it is important that the rate of the substrate removal from the subphase is not too high, otherwise the thinning of the liquid between Langmuir film and substrate will not be enough and transfer will fail.

121

Figure 3.7 A vertical deposition where the hydrophilic tail of a Langmuir-Blodgett

film is deposited on a hydrophilic substrate.

3.4.2 Langmuir-Shäfer (Horizontal Transfer)

When a substrate approaches parallel to the surface, the substrate can be pushed onto a film parallel to the interface. Horizontal transfer may have been first used in a scientific study by Shäfer and Langmuir, but this technique had already been used for hundreds of years in Japan as an art form.16 The technique is called sumi-nagashi (‘ink flow’), which uses the method to transfer marbled patterns onto paper.

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Transferring Langmuir films onto a substrate by this method is commonly known as

horizontal lifting, or Shäfer’s method. The substrate can come from the top down to apply the

hydrophobic portion of the amphiphile onto the substrate or it can come from the bottom up

approach to apply the hydrophilic side of the amphiphile to the substrate. A gentler approach can

be made by placing the substrate at ~45° in the interface and lifting to apply the hydrophilic portion

of the amphiphile to the substrate. Examples of these transfer techniques are shown in figure 3.8.

Figure 3.8 Examples of Shäfer’s method. Left: shows a substrate being pressed down onto the

Langmuir film. Middle: shows a bottom-up approach where the hydrophilic portion of the molecule is attached to the substrate. Right: substrate is placed into the interface at a ~45° angle and lifting up.

3.5 Substrate Functionalization

When transferring a Langmuir film onto a substrate, the contact angle measurement and

the forces acting on the substrate need to be considered. Changing the contact angle of the substrate

can be accomplished by performing reactions on the substrate itself before transferring of the film.

Alterations to substrate surfaces can lead to more hydrophilic or more hydrophobic characteristics,

depending on how the surface is treated.

The most common substrates that are functionalized in surface science are silicon wafers,

which are often used in fabricating electronics and photovoltaics. The natural silicon surface reacts

with oxygen, which forms a layer of silicon dioxide on the surface. The silicon dioxide layer is

123

removed before further experimentation. Selective removal of silicon dioxide layer is done by

changing the functionalization of the substrate into various groups like hydrogen, hydroxyls, and

silanization, etc., is a process known as etching.17

The most common used method for etching the silicon wafer uses a buffer solution of

ammonium fluoride (NH4F) in a volume ratio of 20-parts NH4F to 1-part HF called a buffered

oxide etch (BOE). This buffer will etch off the silicon oxide layer and silicon nitride but will not

damage the bare silicon below the oxide layer. Removing this layer is often used to remove any

surface or subsurface contaminations that may have formed over time, leaving behind a hydrogen-

terminated surface.18 Hydrogen-terminated surfaces are often used to transfer materials onto the

surface or used to react the substrate for further alkylation experimentation.19

substrate

Figure 3.9 Etching of silicon by HF occurs when the highly polar H-F bond reacts by breaking the

Si-O bond and forming a strong Si-F bond, releasing water. HF reacts with the Si-Si bond to make a hydrogen-terminated surface.

Oxide layers are not always hydrogen terminated and are reacted to obtain a different

contact angle for transfer, but before the oxide layer can be reacted, cleaning of the substrate

surface of any organic contaminates must occur by a different method. Contaminants arise from

human skin grease, silicon grease or metals from impure water, which could affect further

experimental data by adding roughness or incomplete functionalization across the substrate.

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Cleaning the substrate with organic solvents does not get the substrate clean enough and generally

this requires a common cleaning mixture used in surface chemistry, e.g. piranha solution. The

mixture is made up of 5:1 sulfuric acid and hydrogen peroxide making a very strong oxidant

solution that removes organic contaminants and metals and which will hydroxylate surfaces

making them hydrophilic.20

Once the substrate is cleaned and hydroxylated it is possible to perform silanization upon

the surface to reduce the hydrophilicity of the substrate. A common silanization uses trimethylsilyl

chloride to make a silylated surface.21

contaminates Pirahna Silanization

solution substrate substrate substrate

Figure 3.10 The cleaning of a silicon oxide layer with piranha solution to remove any organic contaminants, which produces a clean surface of hydroxyls. After the surface is cleaned, silanization can be done to obtain a different surface than the original substrate.

More complex functionalization has been done and has been used to immobilize a surface

for further studies or to make self-assembled monolayers, whether for electronic or biological

studies. This immobilization is typically done by making strong chemisorption bonds upon the

surface of the substrate.

Dang and co-workers modified a silicon nitride (SiN) surface by silanization using 3-

(glycidyloxypropyl) dimethylethoxysilane (GOPES), which was covalently bound with

125

glutaraldehyde (GTA) for investigating the immobilization of anti-cytokeratin-FITC (MACF)

antibody. Their study required a consistent surface to measure the efficiency of the antibody

immobilization as a single monolayer which was achieved by using similar cleaning and

silanization methods as described above.

Figure 3.11 Reprinted with permission from [22] Modification with GOPES and the immobilization of MACF antibody on a silicon nitride surface.

There is a vast library of different substrates and methods for functionalization. The

methods given above are the basic approaches for doing ‘wet’ surface science, mainly due to their

well-known methods and ease of making new substrate surfaces. A large review of surface

functionalization methods and substrates has been performed by Riddiford and co-authors22 and

another by Singh.23 Both are a decent starting point for finding new ways to functionalize

substrates to suit certain experimental purposes.

3.6 Brewster Angle Microscopy (BAM)

126

Being able to optically tell if there is a monolayer on the surface of the air/water interface

or on a substrate is helpful in an area where other classical characterization techniques may not be

available. Imaging monolayers is not a trivial matter and often pushes the limits of what our current

instrumental methods can do. A popular choice for viewing monolayers on the air/water interface

is the use of Brewster angle microscopy (BAM). This technique uses both a microscope and a

polarized light source over a liquid surface to image the light reflecting from the liquid surface.

This is possible because there is no P-polarized reflection from a pure liquid, but when there is

material on the surface it will affect the liquid surface and reflect light, making an image. In most

cases images are taken over a period to show the uniformity of the monolayer formed as the

pressure of the barriers is increased. This gives the ability to observe the phase changes, domain

Brewster angle microscopy of the monomer antrip-DEG as pressure increases on the air/water interface.

127 sizes, packing and shape, which would not otherwise be possible without disturbing the monolayer as can be seen in Figure 3.12 for antrip-DEG.24 The unfortunate part of using BAM is that the imaging apparatus can be a bulky setup and only works with an air/water interface making its use limited.

3.7 Optical Microscopy

One of the most popular methods for imaging is using the optical microscope. The first microscope was first made around 1595 by Zacharias Jansen in Middleburg, Holland using a simple setup that was said to get blurry images with around 9x magnification. Modern day microscopes have improved quite a bit in terms of magnification and resolution. The basic principles of optical microscopy use lenses and visible light to view a magnified image. The simplest technique observes an image by illuminating the object from below is called bright-field microscopy. A more complex technique requires the use of prisms such as differential interference contrast.

Differential interference contrast (DIC) is a mode of light microscopy that uses dual-beam interference optics, which transform local gradients in the optical path length and form regions of contrast in the image. This gives the images a characteristic darkness, almost shadow-like, which can be seen in Figure 3.13.25 This gradient that is generated from the DIC occurs due to optical path length being a product of refractive index and thickness. The difference in the darkness of the images should be referred to as optical path differences and not refractive index or physical thickness without knowing the object’s true thickness previously by a different technique.

128

There is a limit to optical microscopy in creating a usable image and this is due to the

resolution of the instrument. Resolution is defined as the shortest distance between two points on

an object that can be distinguished by a person as distinct parts.

Figure 3.13 Reprinted with permission from [25] Mammalian cell where the top uses bright-field microscopy and the bottom uses differential interference contrast.

The resolving power of an object is set by the numerical aperture (NA), and the total

resolution of the microscope optical ability is reliant upon the numerical aperture of the substage

condenser. Typically, the higher the numerical aperture of the total system, the better the

resolution. Limits to the resolution of optical microscopy are set by physical laws that cannot be

overcome by altering objective lens or aperture design. The limitation is usually described as

diffraction limit or Abbe resolution found in Equation 3.2, the ability to tell the difference between

two points by lateral distance less than half the wavelength of light used to image the object.

휆 푑 = 2푛 sin 휃

129

Equation 3.2 Abbe resolution equation, where d is the resolvable size, 흀 is the wavelength of light, n is the refractive index of the medium being imaged, i.e. air, water, oil. θ is the half-angle of the optical objective lens. (n sin θ) can be simplified as the numerical aperture (NA).

This means that the ways to increase resolution would be to decrease the wavelength,

change the medium of the image to have larger refractive index, or increase the numerical aperture.

The most powerful objective lenses under ideal conditions are still limited to a resolution of 0.2-

0.25 µm due to transmission of glass at wavelengths under 400 nm and physical constraints of

numerical apertures.26 Achieving this level of resolution requires near ideal conditions.

3.8 Scanning Electron Microscopy (SEM)

When more resolving power is required many people turn to scanning electron microscopy.

Resolution differences between SEM and optical microscopy can be quite dramatic. The technique

focuses an electron beam over an object while under vacuum, where the beam interacts with the

sample making signals that are used to create an image. To achieve high-resolution images,

substrates or objects must be electrically conductive; otherwise a blurry image is obtained due to

charging effects.27

3.9 Poly(carboxy fantrip) Films

The synthesis of carboxy fantrip (Figure 3.14) was performed in 5 steps starting from 9-

anthracenecarboxaldehyde as reported in chapter 2. The synthesis of carboxy fantrip reported in

chapter 2 will not be expanded upon in this section, rather the focus will be on poly (carboxy

fantrip) film fabrication. Reactivity of this monomer relies on the tetrafluoroanthraceno blades,

130 which can react in a well-known [4+4] cycloaddition forming a [2.2.2] bicyclic core that is persistent, rigid, and with an internal free volume. The carboxy fantrip monomer acts as the amphiphile with the carboxylic acid at the bridgehead position of the monomer acting as the hydrophilic anchor at the air-water interface and the hydrophobic segments, the tetrafluoroanthraceno blades, lying perpendicular to the interface.

(a) (b)

Figure 3.14 (a) structure of the monomer carboxyfantrip. (b) Dimer of carboxyfantrip.

The poly (carboxy fantrip) film at the air/water interface was achieved by using a 0.1 mg/mL solution of carboxy fantrip in 1:0.1 chloroform:isopropanol spread on the water surface of the Langmuir-Blodgett trough and then compressing it. The resulting isotherm (Figure 3.15) shows a rise in surface pressure starting at ~215 Å2/molecule where the mean molecular area is calculated to be 180 Å2/molecule when fully compressed. This isotherm is closely related to the area per molecule calculated from the crystal structure of fantrip.28 This mean molecular area has been reported by our collaborators to be ~160 Å2/molecule.29 The cause of this lower mean molecular area measurement is most likely due to their use of lower surface pressures to avoid double-layer

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formation. 160-180 Å2/molecule will still produce a suitable film for transfer with little to no

visible defects.

Figure 3.15 Surface pressure (SP) and mean molecular area (MMA) carboxy fantrip monomer isotherm at the air/water interface.

Using Brewster angle microscopy, the spreading and morphology of the monomer, carboxy

fantrip, can be tracked at the interface as pressure increases. In Figure 3.16, when carboxy fantrip

initially spreads across the interface there are islands formed with sharp corners and straight

edges.30 When compression occurs, these islands start to merge and a homogeneous film forms

when 10 mN/m pressure is reached. With the initial spreading of carboxy fantrip, there are ordered

structures and domains that occur after the film has formed as seen in Figure 3.22.

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Figure 3.16 Brewster angle microscopy of carboxy fantrip at the air/water interface with

increasing amounts of surface pressure. Images taken by Daniel Murray.

Figure 3.17 SEM image of poly(carboxy fantrip) on the surface of a copper substrate. In the image one can see the domains similar to the BAM image.

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Polymerization was done at room temperature in air using a 5 W 365 nm LED to irradiate

the monolayer. When polymerization occurs, there is an observable dip in surface pressure. The

rebound of the surface pressure occurs by continually closing the barriers to the desired pressure;

the decrease in surface pressure happens because of the reduced lattice size of the polymerized

film. Measuring the MMA of both monomer and polymer shows that the polymer film is around

10-20 Å2/(molecular unit) smaller than the monomer film. Since the polymerization occurs faster

than the barriers can close, there is a dip in surface pressure until the barriers catch up with the

finished polymer film. Figure 3.18 shows a monomer monolayer being formed then polymerized

to form the polymer, poly (carboxy fantrip).

Figure 3.18 When polymerization at the air/water interface begins there is a dip in surface pressure which happens due to molecules coming closer together after polymerization.

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After irradiation is completed and the polymer monolayer has formed on the surface of the

air/water interface, the film can be transferred to a substrate for further experimentation. The main

choice of substrate used for the transfer was copper (111) gilder aperture grids, which have a near

hydrophobic surface with a water contact around 81°. Due to the semi-hydrophobic nature of the

substrate, it is best to use a top-down approach, the Langmuir-Shäfer method, where after transfer

the carboxylic acid functional groups are pointed upwards. A Teflon holder (basket) was often

used with a dipper to push the substrate through the poly (carboxy fantrip) film on top of the

air/water interface then slowly raise the film out of the subphase (0.5 mm/min) to avoid tears in

the film coverage during transfer. Small amounts of carbon conductive double-sided tape were

used to keep copper substrates on the basket.

film 50 µm hole

Figure 3.19 (left) copper (111) substrate with a ~50 µm hole used to span poly (carboxy fantrip) film over. (right) Teflon holder used to be able to do a top-down approach, the Langmuir-Shäfer method. (Scale is 10 mm)

The methods described above work well, with successful full coverage over the copper

substrate hole of around 33% of the substrates used. Most of the failures occurred due to bending

of the copper substrate while removing the tape. If the substrates are unsecured, however, they end

up floating in the Langmuir trough with no film. If there is too much tape, the thin substrates end

up bending when being removed for further studies. Previous methods created by Daniel Murray

135 used the same idea of Langmuir-Shäfer transfer, by laying the substrates on top of the film. Once the substrate was on the surface of the film, removal was possible by using a newspaper placed over the backside of the substrate then pulled up as shown in Figure 3.20. The method allows for many of substrates to be placed on the surface at once, but the major downside was that the successful transfer rate was about 10% of the substrates used. The unsuccessful transfer rate could be due to the film being more strongly bound to the water than the physisorption forces of the substrate. Most likely the low transfer rate was due to lifting the substrate out of the subphase which requires an extremely careful hand, where any twisting of the substrate could cause damage to the poly (carboxy fantrip) film.

Figure 3.20 (left) Langmuir trough with copper substrate on top of poly (carboxy fantrip) film.

(right) A copper substrate that was lifted out of the trough using a small piece of newspaper.

Imaging the poly (carboxy fantrip) film on top of the substrates is not straightforward. The easiest substrates to image were copper apertures grids with 50 µm holes where it was easy to tell if there was complete coverage or a tear by optical microscopy, as can be seen in Figure 3.28.

However, even with the hole, it can be deceiving to tell if the film spans over the entire hole without

136 seeing a tear in the substrate, since the image may seem to come out as hazy, as can be seen comparing Figure 3.21 (a) and (b). Using differential interference contrast (DIC) microscopy can enhance the images seen in torn films by giving them a shadow-like effect as can be seen in Figure

(a) (b)

Figure 3.21 Poly (carboxy fantrip) covering the 50 µm hole with different coverage levels. (a)

Complete coverage. (b) Ruptured hole. (c) Torn film using DIC microscopy. (d) Torn film using

DIC microscopy.

3.28 (c) and (d).

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Imaging copper (111) grids by bright field optical microscopy is a fast method to check if the film is on the substrate but it lacks the detailed imaging that scanning electron microscopy

(SEM) can provide as shown in Figure 3.22. It can be seen clearly by SEM that holes either have complete coverage of film or have a torn film. However, if the film has no defects in the transfer, it makes identification of film transfer more challenging and creates uncertainty.

Figure 3.22 (left) Optical microscopy of poly (carboxy fantrip) (right) SEM image of the same grid.

Using water contact angle measurements on substrates without and with the film is a widely used method to see whether changes on the surface have occurred. Water contact angle measurements may not tell how much film covers the substrate, but it can tell on which areas the film does reside and where bare substrate exists. The use of SiO2 wafers as substrates is a popular choice for water contact angle methods, because clean SiO2 has a water contact angle around 88°.

When the film is applied to the surface using the top-down Langmuir-Shäfer transfer method,

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where the carboxylic acids pointed up, it makes the surface more hydrophilic, obtaining water

contact angle measurements around 60°. Using the bottom-up approach where the film is

transferred upwards at a 45° angle, water contact measurements are around 93°, which is due to

the carboxylic acid being transferred down, making the surface slightly more hydrophobic. This

result is in line with what Kim and Nguyen observed using single layers of graphene on top of a

copper surface.34

Figure 3.23 (left) poly (carboxy fantrip) transferred onto SiO2 from the bottom up. (right) water droplet ontop of a SiO2 surface after film transfer with a water contact angle measurement around 93°. 3.10 Defects in poly (carboxy fantrip)

Imaging poly (carboxy fantrip) film that has complete coverage with no defects over the

substrate is difficult; contrast in the images is often the only way to locate positioning of the poly

(carboxy fantrip) film. There are ways to introduce defects into the fabrication of the film using a

0.3 mg/mL solution of carboxy fantrip monomer in 1:0.1 chloroform:isopropanol to make a film

that is easier to monitor when testing a new substrate. Making a film that has a mean molecular

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area of around 225-230 Å2/molecule, as shown in Figure 3.24, can result in usable films, without

the risk of collapsing the film.

Figure 3.24 Too high a concentration of carboxy fantrip spread at the air/water interface, leading to

the different layering of film. Depositing concentrated monomer solution on top of the air/water surface will result in a

higher MMA and that will form disclinations within the monomer layer. Disclinations occur as the

molecules deposited onto the water surface group up and form islands with other molecules. When

Figure 3.25 (left) optical microscopy of disclinations in poly (carboxy fantrip) film. (right) SEM image where the holes appear to still have a film-like quality.

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irradiation occurs on the trough the surface pressure drops making a film with a mean molecular

area of 160 Å2/molecule without (or with less) disclinations, which is the result of islands of

monomers reacting with other islands and forming a complete film.

Controlling disclination formation as shown in Figure 3.26 requires precise spreading of

the monomer on the water surface in the exact concentrations, otherwise defective films are

formed, as shown in Figure 3.25. However, the main purpose of introducing defects without

introducing new impurities is to help monitor film transfer onto a new substrate, and different

disclinations patterns are acceptable.

Figure 3.26 SEM of different defects that can occur during film fabrication.

Trying to introduce new defects into the film is useful for imaging. The most common

occurrence of defects formation is typically from the introduction of impurities into either the

subphase or the monomer solution itself. A common impurity is skin grease that often comes from

handling the monomer solution and aqueous phase. With small amounts of grease there will be a

significant drop in MMA as shown in Figure 3.27, which most likely occurs by changing the

packing arrangements. When this occurs, the anthracene blades are not able to pack close enough

with each other to be polymerized once irritation occurs and results in the lack of formation of a

transferable film.

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Figure 3.27 Carboxy fantrip with grease impurities. Polymerization still can occur but is

limited.

3.11 Transferring Poly (carboxy fantrip) Film onto Different Substrates

Being able to transfer Langmuir-Blodgett films onto substrates is essential to the process for making films created on the trough. This is especially true when collaborators have the need for different substrates for their own experiments on these films. One of our collaborators desired a hydrophilic silicon nitride substrate for their physical experiments. Their experimental goal was to measure the Josephson effect in superfluid helium 4. Josephson effects in superconductivity are used to fabricate SQUIDs (superconducting quantum interference device) and superconducting qubits. They were thinking that with the superconducting, Josephson effects pairs of electrons could tunnel through a resistive barrier, which is called the Josephson effect.31 Helium atoms are much more massive than electrons, and as such can't tunnel significantly through physical barriers.

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What is needed is to constrict their flow to very narrow channels, on the order of sub-nanometers.

It may be possible to do this with the poly (carboxy fantrip).

Using semi-hydrophobic substrates such as copper requires top-down transfer to have the carboxylic acid groups being upward after the transfer occurs. A substrate that is hydrophilic like etched silicon nitride (SiN) requires a bottom-up transfer approach, where the hydrophobic side of the poly (carboxy fantrip) is transferred pointed upward. A top-down transfer approach does not work using SiN substrates with poly (carboxy fantrip); the physisorption forces are not strong enough to stay on the substrate as shown in Figure 3.28.

Figure 3.28 (left) Optical microscopy of poly (carboxy fantrip) film on SiN using top-down

approach. (right) SEM of the same film.

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The difficulty with the SiN surface did not only reside in the transfer of the poly (carboxy fantrip) film but rather the confirmation of film transfer over a 2-µm hole. To be able to measure the Josephson effect, there needs to be confirmation of complete coverage. Water contact measurements were not ideal because forming water droplets that small is not a trivial matter and requires super-hydrophobic needles and extensively sensitive micro-droppers. Another problem is that trying to observe a 2-µm hole by optical microscopy is near to the theoretical limits of the technique due to the Abbe diffraction limit. The difficulty of resolving whether the film covers the

2-µm hole is not straightforward as shown in figure 3.29 where the resolution is too low.

Figure 3.29 Poly (carboxy fantrip) over SiN. The yellow portion is etched SiN with a 2 µm hole.

Even using SEM imaging did not seem to be a viable technique to be able to tell if the film was covering the 2-µm hole. Focusing the SEM beam on a smaller area with the film on the substrate generally caused the film to blow away due to the focused energy of the SEM beam. This was a common observation when using SiN, which is not a conductive surface and gives charging effects, as can be seen in Figure 3.30. However, if there is film present on top of the substrate, there are no charging effects, giving the impression that the film may be conductive.

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Figure 3.30 (left) Blank SiN surface, showing charging effects on the etched surface. (right)

Poly (carboxy fantrip) on the surface showing no charging effects.

Without the ability to determine complete film coverage over the SiN 2-µm hole, it was decided to start primary studies on larger holes of around 20 µm. Using the same SiN substrates with the 2-µm hole, it was possible to etch away the window using hot phosphoric acid and create a hydrophobic surface of water contact angles around 20° after etching. Doing the same Langmuir-

Shäfer transfer techniques going at a 45° lifting angle from the bottom up, it was possible to get film transfers across the substrates and the hole. However, an unexpected problem arose in that even by optical microscopy it was still hard to tell if there is complete film coverage. The SiN surface being an extremely smooth surface makes the poly (carboxy fantrip) film is a single layer, to take the form of the substrate which does not give any contrast to tell if the film is spanning the hole. When the hole cover with film was poked by a small needle, the film rolled up, giving contrast to the image as observed in Figure 3.31.

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Figure 3.31 (left) SiN with film covering the pore. (right) after film was punctured with a needle.

3.12 Stability of Poly (carboxy fantrip) Film

Transferring a monolayer film onto a substrate is a challenge for multiple reasons but keeping a monolayer of poly (carboxy fantrip) film intact on a surface is another challenge. The poly (carboxy fantrip) organic films on the substrates are held by physisorption forces and their stability may be limited.32 Once the film has been fabricated and placed on a surface, there is a limited lifespan of the film, where damage can occur by oxygen, light, and dust particles.

Poly (carboxy fantrip) film seems to be resistant to atmospheric oxygen sources, and no observable damage is seen by optical microscopy after a month. Keeping fabricated film out of fluorescent light is recommended, but some exposure did not seem to immediately damage the film. After a week of exposing multiple poly (carboxy fantrip) films to fluorescent light, small cracks can be seen forming in the film, which may be the result of small amounts of the film degradation.

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Another source of tears in the poly (carboxy fantrip) film occurs while handling substrates after the film has been transferred from the air/water interface. The poly (carboxy fantrip), being a single layer film, is quite easily damaged, especially since the film is kept on the substrate by physisorption.

Damage can occur after transfer from the air/water interface, where the substrate is still wet, and incorrectly drying the substrate can damage the film, because the water that lays on the substrate can roll the film or even ruffle it, as shown in Figure 3.32 (a). When drying the substrate, it is important to make certain to have it flat and let the water dry by natural evaporation. Blowing off the water with a stream of nitrogen or trying to shake off the water can cause damage to the film.

After the film has been transferred and dried, damage can occur by improper handling, especially when the film is spanning a hole, which allows for air currents to damage the film. When handling the substrate, lowering it down quickly causes a small air current to lift the film, as shown in Figure 3.31 (b). It is recommended to move the substrates with film spanning the hole horizontally as much as possible and transfer them into carrying containers to avoid air currents.

The film can also be pushed down by blowing a stream of nitrogen over the surface.

Less commonly, damage can occur to film spanning a hole when passing gases through or placing water droplets on top of the surface of the hole. If too much pressure is applied when passing gas through the film, it can push the film into the hole as shown in Figure 3.31 (c). The same is true when trying to place water droplets on top of the spanned film if the droplet is not laid on the surface of the film but rather dropped from a small distance, it can be enough force to push the film in.

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(a)

(b)

(c)

Figure 3.32 Examples of damage to a film spanning a hole. (a) water running over the film and ruffling the surface. (b) Film being pushed up by air currents. (c) Film being pushed down by gas or water droplet.

Water damage to poly (carboxy fantrip) films is more prevalent when the carboxylic acid

groups are transferred away from the substrate than when the carboxylic acids are on the substrate.

With the water over the surface of the film, the carboxylic acids will hydrogen bond to the water

and if the water is flowing down at an angle will ruffle the film as shown in figure 3.33. This

damage is not observed as often if the film is transferred by a bottom-up approach, where the

carboxylic acid groups are faced down. The hydrophobic side of the poly (carboxy fantrip) will

not interact as strongly with water as the carboxylic acid group would. Though the damage can

still be observed with the bottom-up approach film, hemi-wicking may be occurring which will

bond with the carboxylic acids and pull up the film slightly.

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Figure 3.33 Damage to poly (carboxy fantrip) film occurred from water.

The most common damage that occurs to poly (carboxy fantrip) films when spanning a hole is air-currents coming underneath them. This damage was observed by using copper grids, where films span over a large surface of the hole and it is easy to induce damage by quickly pulling down the grid through the air. The air currents rupture large portions of the grids and leave very few areas with film intact as shown in figure 3.34.

Figure 3.34 (right) Poly (carboxy fantrip) film spans over a copper grid. (right) after film was damaged by air currents.

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The least common damage to the poly (carboxy fantrip) film was when the film pushed

into the hole of the substrate. This sometimes occurs when trying to pass gas through the film or

when water gets dropped on the surface of the hole and pushes the film in, as can be seen in Figure

3.35. It is not very common to see this sort of damage since most of the time the film spanning the

hole will be torn off completely.

Figure 3.35 Examples of poly (carboxy fantrip) film being pushed down into the substrate.

Damage to the film may also occur if there is water trapped underneath the monolayer.

This can occur during the bottom-up film transfer approach by transferring a layer of water under

the poly (carboxy fantrip) film. The film has pores big enough to allow water to go through but

requires some force to remove the water from under the monolayer. Without removal of water

under the film, the heat generated from optical microscopy causes the small layer of water to

evaporate too quick which rolls up the film as shown in Figure 3.36. Proper drying of the substrate

after film transfer is a must, and a full day at room temperature is enough to avoid this sort of

damage to the poly (carboxy fantrip) film.

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Figure 3.36 Poly (carboxy fantrip) on an SiN window. The orange corners only formed after

focusing with an optical microscope onto the center.

Damage to the poly (carboxy fantrip) film can occur during film imaging, especially using

SEM where the electron beam is focused too much and can cause ruptures to form, even in the lowest energy setting. In most SEM imaging, vacuum is also applied (~1.9 x 10-4 Pa for our SEM images), which can pull the poly (carboxy fantrip) film away from the substrate as shown in Figure

3.37. In obtaining images for poly (carboxy fantrip) film on a copper substrate, the voltage should stay around 5.0 kV and below. For non-conductive surfaces like Kapton or silicon nitride using

5.0 kV and above is possible for quality images, but it will often destroy the film or displace the monolayer. When the charge increased and focused on the area of the monolayer, the film will move around almost like a caterpillar. Close inspection of the film by SEM does provide better resolution than optical microscopy but at the expense of the sample.

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Figure 3.37 SEM images of poly (carboxy fantrip) where (left) vacuum has pulled up the film

from the hole; (right) film torn and pulled up by charge and vacuum.

The durability of the poly (carboxy fantrip) film is not enough to keep its form after doing

SEM imaging. From many tests, it was observed that a film spanning over any hole has a very low chance of remaining intact, which is shown in Figure 3.38. Imaging must occur very quickly to limit the damage of the vacuum and electron beam. Damage usually starts to occur after around thirty minutes of samples being under vacuum. The reason for this damage may be from vacuum pulling up water that may be trapped under the film or held by the carboxylic acid groups.

Figure 3.38 Examples of longer exposure of electron charge and vacuum in SEM.

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A poly (carboxy fantrip) film that is fabricated and transferred from the air/water interface has a limited distance it can span over an unsupported hole. Transfers to substrates with 10-50 µm holes are enough to support the film without having many breaks in the overall film; even a 100

µm film can be transferred with reliability as seen in Figure 3.39. However, above 100 µm the amount of film that will span an unsupported hole drops dramatically.

Figure 3.39 Poly (carboxy fantrip) with film covering around 100 µm holes.

3.13 Roughness of Substrates Being able to completely transfer the film over the entire substrate without any defects is the main desirable feature in a substrate selection. Substrate roughness plays a large role in the ability to transfer and image poly (carboxy fantrip) films. With the film being a single layer, it will take the form of the substrate surface the best it can, but if the substrate is too rough, the film will not span the gap, as shown in Figure 3.40 with a steel substrate. The film can be seen covering around the hole, but it is spotty, and the transfer was not as reliable as other smoother substrates.

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Figure 3.40 Steel substrates with 50 µm holes. The roughness of the steel around the center

prevents the film from being able to span the hole.

Using smoother substrates, like copper grids, allows for the film to span across a hole as seen in previous images. However, the successful transfer rate for film completely spanned across a 50-µm hole is about 33%. The failures most likely happen due to the roughness of the copper hole, as can be seen in Figure 3.41, where little ‘teeth like’ indentations can sometimes catch the film and cause defects. With the steel substrate that has a raised indentation around the hole this is the most likely cause of the film not being able to span fully and generate defects as sometimes happens with the copper substrates.

Figure 3.41 (left) Steel substrate showing its roughness. (right) Copper substrate hole with the teeth-like indentations can be seen.

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Smoother substrates like quartz, silicon oxide, or silicon nitride have a higher transfer rate and are reliable substrates. Though reliable in transfer use, the difficulty with these substrates lies in their inability to be imaged for the presence of film over the surface. The surfaces are very smooth after cleaning, and it is nearly impossible to tell if film spans over the surface without damaging the film as shown in Figure 3.42. Kapton (polyimide) is another substrate that has a smoother surface than steel or copper and should be able to accept film transfer onto the surface.

Even with or without etching the surface of polyimide, the difficulty lies in being able to tell if the film is on the surface without a hole or some recognizable contrast. This lack of transfer of film onto polyimide may be more of a condition of the surface energies rather than surface roughness.

Figure 3.42 (left) Poly (carboxy fantrip) that has been damaged on a SiN surface.

(right) Polyimide surface after etching and attempted film transfer.

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3.14 Conclusion

Langmuir films have been well studied over the years and under certain conditions one can expect to obtain certain results. However, there are still many challenges to making new films and the reported literature is a useful guide, but not a complete tell-all. Poly (carboxy fantrip)

Langmuir films were not an exception to the rule and worked well on silicon and copper substrates, but many challenges did still arise. The two most immediate challenges for poly (carboxy fantrip) films in the future that need to be addressed is finding a new substrate that can span the film over a pore, and finding a reliable, non-damaging imaging technique. Cheaper metal substrates like copper, nickel, and steel have been attempted, but do not work particularly well. Silicon based substrates do work well but having etched holes onto the substrates of silicon do create imaging problems. Gold substrates may be pricey but could work with increased success of film fabrication due to gold’s hydrophilic nature. The imaging of poly (carboxy fantrip) on larger sheets works well with optical microscopy, but a much better technique is needed for films spanned over a hole that is < 5 micrometers. SEM can image that small, but still causes damage to the film. Non-contact

AFM would work quite well but would be extremely challenging and tedious in figuring out if the film is completely covering a spanned area.

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3.15 Experimental

Every substrate was cleaned immediately before use, to remove any impurities or oxide layers on the surface of substrates. HPLC grade or higher solvents were used for washing, and only 18.2 MΩ·cm water was used for making solutions or washing. Only new glassware and vials were used to avoid any contamination. Drying of substrates only used a nitrogen stream; compressed air from the hoods was of unacceptable quality and often resulted in contamination.

Poly (carboxy fantrip) trough procedure: To a cleaned trough water surface, 12 µL of the monomer carboxy fantrip dissolved in 1:0.1 chloroform:isopropanol to a concentration of 0.14 mg/mL is spread across the trough in four 3 µL droplets to different areas of the air/water interface of the water surface at 25 °C. Afterwards, the surface is left undisturbed for a minimum of 15 minutes to allow the monomer to spread. Once the time has passed the barriers are slowly closed at a rate of 4 mm/min until a pressure of 10 mN/m has been obtained. The monomer then is irradiated with a 365 nm LED for 2 minutes, when a large drop of pressure is observed. The barrier is then closed continually at a rate of 4 mm/min after irradiation has occurred until the pressure returns to around 10 mN/m.

A 400 nm LED can be used instead of 365 nm for irradiation to fabricate the polymer.

However this will require around 10 minutes of exposure to fabricate complete polymer film.

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Etching silicon nitride: A preliminary wash of a 5x5mm low-stress silicon nitride substrate with

(1) (2)

(3) (4)

Figure 3.43 Top down view of the Langmuir-Blodgett trough, with the two barriers on

each side and the Wilhelmy plate in the middle. The numbers represent the order of how

each of the 3 µL drops should be added for proper spreading of the monomer. a 2 µm window (purchased from Norcada) should be done using ~20 mL of chloroform, then isopropanol, then water to remove any immediate surface impurities. Using Teflon tweezers to hold the substrate in place, dip it into fresh boiling phosphoric acid to etch away a portion of the silicon nitride. The substrate should not be in the phosphoric acid for more than 10 minutes, otherwise, the SiN window will be loosened and removed, leaving ~20 µm hole in the substrate.

Copious washing with water should follow after removal from acidic solution, generally around

100 mL. Water contact angle measurements after around 5 minutes of exposure to acid should result in an angle of ~35°. After the window has been etched away, the etched surface can result in water contact angle measurements around 20° depending how much time the substrate is exposed to the acid. The color change of the substrate is normal, and depending on how long the substrate is etched it will be blue, purple, yellow, silver, or brownish red. The phosphoric acid

158 should be changed out after a couple of hours of use, to avoid contamination as well as the decreased etching effectiveness of the acid. With these substrates avoid using a sonicator at any point during washing, otherwise, the SiN window can be knocked out.

Reacting polyimide (Kapton) to polyamic acid: Preliminary washing of the polyimide using ~20 mL of chloroform, then isopropanol, then water should be done to remove any immediate surface impurities. Afterwards, the polyamide is dipped into a 1M solution of KOH for a minimum of 10 minutes to form the potassium polyamate. Around 20 mL of water should be used to wash the substrate to remove any additional KOH. Afterwards dip the substrate into a solution of 1M HCl for around 10 minutes to form the polyamic acid. This should be followed by copious washing with water and blowing off any additional water with a stream of nitrogen, and then the imide should be stored. Water contact angle measurements of clean polyimide should give angles around

73°. Converting into polyamic acid will result in water contact measurements around 35°.

Scheme 1 General procedure of converting polyimide into polyamic acid.

Etching silicon dioxide: Sonicate the substrate using ~20 mL of chloroform, then isopropanol, then water to remove any immediate surface impurities. Sonication of the substrate should be for

159 at least 5 minutes for each solvent. The substrate should be sonicated two more times again for another 5 minutes in a washing of water to remove any trace organic material. Using piranha solution in a 5:1 mixture of sulfuric acid and hydrogen peroxide, sonicate the substrate for 10 minutes. Make sure the piranha solution is near room temperature before adding it to the substrate to avoid cracking it. Afterwards, remove the substrate from the piranha solution and wash it with copious amounts of water, a minimum of 200 mL. Blow any additional water off the substrate with a stream of nitrogen, and then store the substrate in a clean, dust free environment. Piranha solution is extremely dangerous when mixed with organics; take extra precautions when using and dispose of in a separate waste container when using.

Trimethylsilane terminated silicon dioxide: After following the same procedure given above for etching silicon dioxide, the substrate was taken and placed in a 20 mL vial with a screw cap. Inside the vial, a smaller vial is placed inside with 2 mL of trimethylsilyl chloride and the 20 mL vial is screwed closed and left at room temperature overnight. Afterwards, the substrate was taken and washed with around 100 mL of water and dried with stream of nitrogen before use of the substrate.

Hydrogen-terminated silicon: The substrate should be sonicated using ~20 mL of chloroform, then isopropanol, then water to remove any immediate surface impurities. Sonicate the substrate for at least for 5 minutes for each solvent. Add 1M of ammonium fluoride solution to the cleaned substrate and sonicate for 15 minutes. Afterwards wash with a minimum of 100 mL of water. The substrate should be dried with a stream of nitrogen to remove any water droplets and stored in a clean vial until it is needed.

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3.16 References

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4 Petty, M. C. Langmuir-Blodgett Films: An Introduction; Cambridge University Press: Cambridge ; New York, 1996.

5 Tanford, C. Ben Franklin stilled the waves: an informal history of pouring oil on water with reflections on the ups and downs of scientific life in general; Oxford University Press: Oxford [England] ; New York, 2004.

6 Surface science: an introduction; Oura, K., Ed.; Advanced texts in physics; Springer: Berlin ; New York, 2003.

7 Pallas, R. N; Iwahashi, M.; Middleton, R. S. Proc. R. Soc. Lond 1984. 396, 143-154.

8 Tredgold, R. H. Order in Thin Organic Films.; Cambridge Univ Pr, 2005.

9 Bryce, M. R. Adv. Materials 1995, 7 (1), 93.

10 Petty, M. C. Molecular electronics: from principles to practice; Wiley series in materials for electronic and optoelectronic applications; John Wiley & Sons: Chichester, England ; Hoboken, NJ, 2007.

11 Barraud, A.; Vandevyver, M. Thin Solid Films 1983, 99 (1–3), 221.

12 Peterson, I. R.; Russell, G. J.; Earls, J. D.; Girling, I. R. Thin Solid Films 1988, 161, 325.

13 Petty, M. C. Langmuir-Blodgett films: an introduction; Cambridge University Press: Cambridge ; New York, 1996.

14 Pethica, B. A. Thin Solid Films 1987, 152 (1–2), 3.

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15 Malcolm, B. R. Thin Solid Films 1989, 178 (1–2), 17.

16 Hughes, S. Washi, the world of Japanese paper; Kodansha International: Tokyo; New York etc., 1982.

17 Handbook of Silicon Wafer Cleaning Technology.; Elsevier Science Ltd, 2017.

18 Deal, B. E.; Grove, A. S. J. Applied Phy. 1965, 36 (12), 3770.

19 Rivillon Amy, S.; Michalak, D. J.; Chabal, Y. J.; Wielunski, L.; Hurley, P. T.; Lewis, N. S. J. Phys. Chem. C 2007, 111 (35), 13053.

20 Sun, P.; Liu, G.; Lv, D.; Dong, X.; Wu, J.; Wang, D. RSC Adv. 2015, 5 (65), 52916.

21 Banga, R.; Yarwood, J.; Morgan, A. M. Langmuir 1995, 11 (2), 618.

22 Danielli, J. F.; Pankhurst, K. G. A.; Riddiford, A. C. Recent Progress in Surface Science: Volume 1.; Elsevier Science: Burlington, 2013.

23 Advances in surface science; Nalwa, H. S., Ed.; Experimental methods in the physical sciences; Academic Press: San Diego, 2001.

24 Murray, D. J.; Patterson, D. D.; Payamyar, P.; Bhola, R.; Song, W.; Lackinger, M.; Schlüter, A. D.; King, B. T. J. American Chem. Soc. 2015, 137 (10), 3450.

25 Pluta, M. Advanced light microscopy; PWN ; Elsevier : Distribution for the USA and Canada, Elsevier Science Publishing Co: Warszawa : Amsterdam ; New York, 1988.

26 Murphy, D. B.; Davidson, M. W. Fundamentals of light microscopy and electronic imaging, 2nd ed.; Wiley-Blackwell: Hoboken, N.J, 2013.

27 Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Lifshin, E.; Fiori, C.; Springer Science+Business Media. Scanning electron microscopy and X-ray microanalysis: a text for biologists, materials scientists and geologists; Springer Science+Business Media: New York, 2014.

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28 Kissel, P.; Murray, D. J.; Wulftange, W. J.; Catalano, V. J.; King, B. T. Nature Chem. 2014, 6 (9), 774.

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30 Murray, D. Exploring the Synthesis of Two-Dimensional Polymers: From Langmuir Films to Crystals. University of Nevada, Reno. 2016.

31 Josephson, B. D. Physics Letters 1962, 1 (7), 251–253.

32 Chi, L. F.; Eng, L. M.; Graf, K.; Fuchs, H. Langmuir 1992, 8 (9), 2255.

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Chapter 4: Poly (carboxy fantrip) Gas Flow

Chapter 4.1: Gas Flow Through Membranes

Polymeric membranes have a plethora of uses and potential functions in gas separation, reverse osmosis, catalytic reactions, etc.1 Modern polymeric membranes are built on the basis of research that was done over 150 years ago starting from Thomas Graham’s research.1 Thomas

Graham first performed experiments on the transport of gases and vapors trough a wet pig bladder that was inflated with carbon dioxide till the bursting point in 1829.2 His experiments throughout his life developed and tested permeability rate measuring devices and postulated mechanisms of the permeation processes.3 He viewed the permeation process as a solution of gas that was upstream on the surface of the membrane, which would diffusion across the membrane, and then evaporate downstream of the membrane surface. This is the basis of the solution-diffusion model which many membranes used today are based upon.4 Progress on membrane technology and science was prolific in the 19th and early 20th century with major findings from Rayleigh, Knudsen,

Poiseuille and many others.5

One major finding that came out of the 20th century was the equation of gas flow through polymeric membranes, known as the Knudsen diffusion model.1 The Knudsen diffusion model predicts that for gas passing through a porous membrane, the pressure is proportional to the number of molecules that pass through the hole(s) and is inversely proportional to the molecular mass.

This means that it is possible to have partial separation of gases if they have different molecular masses.6 Flowing dynamics of gases are characterized by the ratio of the molecular mean free path,

흀, in relation to the diameter of a cylinder through which it is flowing, which is the Knudsen number (Kn) as shown in equation 4.1.

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λ 푲풏 = (4.1) 풅

From this equation, different flow regimes can be determined, which give a good estimate for molecule collisions occurring with each other or the inner pores of the membrane (or cylinder).

There are three different flow regimes most commonly used for polymeric membranes known as free molecular flow, continuum flow, and transitional flow. Free molecular flow is when the mean free path is the same order as or greater than, the d dimension, producing a large Knudsen number, which means that the gas dynamics are dominated by collisions with the walls of the pore.

Continuum flow is when the mean free path is small compared to the d dimension, making a small

Knudsen number, meaning that intermolecular collisions are more frequent than collisions with the walls of the pore. Transitional flow occurs at the intermediate values of Knudsen number where wall collisions in the pore and gas collisions are influencing the flow characteristics. The Knudsen number can be expanded as shown in equation 4.2 to give a better picture of how different conditions can change the flow regime. In this equation, P = pressure, d = dimensions of pore, R

= universal gas law, T = absolute temperature, and M = molecular mass of gas.

ή π 푹푻 푲풏 = √ (eq 4.2) 푷풅 ퟐ 푴

The flow regime gives the basic principle of what type of gas collision will most likely occur in a membrane, but it does not tell how quickly the gas will flow theoretically. For that we use an effusion flux equation as shown in equation 4.3. In this equation, P is the pressure, Ap is the area of the pore, Np is the number of pores, M is the molecular mass of the gas, R is the universal gas constant, and T is the temperature.7

PA N Effusion flux rate = p p (eq 4.3) √2πMRT

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Equation 4.3 gives a good estimation of how much flux one should have through a porous material if there were no collisions, meaning that whatever flux is given from this equation, the experimental value will be lower. This equation has its own faults in that the flux rate is dependent on the mass of the gas being passed through a pore, while the diameter of a gas should also be taken into consideration. However, when the pore of a membrane is larger than the gas molecule, the gas will fit through at the same rate no matter the size, assuming there are no collision or other attractive or repulsive interactions between the membrane and the gas.

4.2: Gas Flow Through poly (carboxy fantrip)

A poly (carboxy fantrip) film as a porous membrane is unique compared to other polymeric membranes because it is a monolayer and the pores are identical with a minimal number of defects.

For other polymeric membranes determining how many pores, the size of the pores, or the exact thickness of the membrane used for gas separation is not trivial.8 Using the Langmuir method to make poly (carboxy fantrip) as a polymeric membrane reduces the number of unknowns, since the thickness, number of pores and the size of pores can be directly measured.

Characterization of poly (carboxy fantrip) films as transport membranes was achieved by measuring fluxes of nitrogen, hydrogen, helium, sulfur hexafluoride, and carbon dioxide through the polymer. A copper TEM grid with a single 50 µm hole was spanned with a poly (carboxy fantrip) film which allowed for consistency and ease of determining the quality of the film that spanned over the pore, shown in Figure 4.1. The hole was large enough to be placed into a flow cell and measure the flux of gas through the polymer covered aperture by use of a capillary bubble flow meter.

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(A.) (B.)

(C.) (D.)

Figure 4.1 (A.) Cartoon representation of poly (carboxy fantrip) over the TEM hole. (B.) &

(C.) poly (carboxy fantrip) spanning over the 50-micrometer hole with no defects. (D.) poly

(carboxy fantrip) spanned over the hole with some visible defects.

To model the gas flow across poly (carboxy fantrip), Knudsen’s kinetic molecular theory of gases was used. The system provides that the flow rate should follow a molecular flow regime and allows for explanation of the observed flow rate by using an effusion mechanism, making the only collision factor to worry about is the probability of a gas molecule hitting a pore. This was possible because the pores of poly (carboxy fantrip) are smaller than the mean free path of the gas molecule and the probability of intermolecular collisions within the pore are low. Thus, the flux

167 effusion Equation 4.3 was used to obtain the theoretical permeance which was compared to the measured permeance for different gases as shown in Table 4.1.

Table 4.1 The theoretical and measured permeance for poly (carboxy fantrip)

The measured permeance for each gas was lower than the theoretical permeance, which may be due to small amounts of gas molecules hitting the pore walls or kinetic collisions. This may be an indication that the pore size of poly (carboxy fantrip) may be slightly flexible and not static, which may allow the larger sulfur hexafluoride gas to pass through the pore. Adding fluoroelastomer o-rings to the sample holder seemed to fix the problem, but if care was not taken, the copper substrate could be sheared, destroying the sample as shown in Figure 4.2. Adding the

Figure 4.2 The added fluoroelastomer o-ring added onto the sample holder to measure gas flow.

168 o-ring to the sample holder and re-measuring other gases did not significantly change measured permeance gas values.

The overall measured permeance values are close to the theoretical values for each gas measurement which is shown in Figure 4.3. There were a few complications of film tearing and leaks in sample holder which made acquiring this data extremely tedious and difficult.

CO2

SF6 N2

Figure 4.3 Graph of measured permeance of gases compared to their molecular weight.

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4.3 Conclusion

Poly (carboxy fantrip) films made by Langmuir methods give a nanoporous 2DP, which can allow for gas flux through the pores in good agreement with theoretical values. The 2DP film was spanned over holes on TEM grids allowing for gas flux measurements to occur and the ~ 1 nm thickness of the films provides an ultrathin membrane that gives ballistic gas flux measurements. The future of this project should focus on selective gas permeability of two different gases to be able to maximize the point of the potential of poly (carboxy fantrip) nanoporous membranes in the field of membrane separations.

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4.4 Experimental

Gas Transport for poly (carboxy fantrip) films

For flux measurements poly (carboxy fantrip) films must first be transferred to a single hole TEM grid with a 50 µm hole (Gilder GA50). Transferring the film onto the surface of the grid was described in chapter 3 and will not be repeated here. Figure 4.4 shows the basic setup for measuring gas flux values.

Figure 4.4 Setup for gas flow studies.

Upon placing the substrate with the film into the sample holder (Figure 4.5, (B.)), the system was purged with a selected gas for > 30 minutes. The pressure was kept at 1.5 in H2O, because larger pressures should be avoided to prevent tearing of the poly (carboxy fantrip) film. A

25-µL micropipette (Figure 4.5 (A.)) was attached to act as a bubble flow meter to measure the

171 time it takes for gas to pass through the 2DP membrane. Repeated measurements were performed on the same film up to ~10 times to make sure the flow had stabilized.

(A.) (B.) Figure 4.5 Micropipettes used as a bubble flow meter, (B.) Sample holder used to hold

copper substrates.

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4.5 References

1 Drioli, E.; Giorno, L. Basic Aspects of Membrane Science and Engineering; Elsevier: Amsterdam, 2010. 2 Graham T. Roy. Inst. J., 1829.

3 Graham T. Phil. Mag., 1866, 32, 401

4 Stannett, V. J. Membrane Sci. 1978, 3 (2), 97–115. 5 Kesting, R. E.; Fritzsche, A. K. Polymeric Gas Separation Membranes; Wiley: New York, 1993. 6 W Steckelmacher. Rep. Prog. Phys. 1986, 49, 1083-1107

7 Paul, D. R., Yampolʻskii, Y. P., Eds.; Polymeric Gas Separation Membranes; CRC Press: Boca Raton, 1994.

8 Sanders, D. F.; Smith, Z. P.; Guo, R.; Robeson, L. M.; McGrath, J. E.; Paul, D. R.; Freeman, B. D. Polymer 2013, 54 (18), 4729–4761.