SYNTHESIS, CHARACTERIZATION AND THERMAL ANALYSIS OF
TETRAHEDRAL AND CYANO-SUBSTITUTED
PERYLENE-BASED DERIVATIVES
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Ding Tian
August, 2014
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SYNTHESIS, CHARACTERIZATION AND THERMAL ANALYSIS OF
TETRAHEDRAL AND CYANO-SUBSTITUTED
PERYLENE-BASED DERIVATIVES
Ding Tian
Thesis
Approved: Accepted:
______Advisor Dean of the College Dr. Stephen Z. D. Cheng Dr. Stephen Z. D. Cheng
______Faculty Reader Dean of the Graduate School Dr. Mesfin Tsige Dr. George R. Newkome
______Department Chair Date Dr. Coleen Pugh
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ABSTRACT
Perylene diimides (PDIs) have attracted the great interests of both academic and industrial people over decades of years because of their chemical stability, thermal stability, fluorescence, and photoactive property. They have been widely used as dyes, pigments, n-type organic semiconductors and organic field effect transistors (OFETs) and so on. However, in addition to molecular chemical nature, the self-assembly structure also deeply affected material’s macroscopic property and practical application. In order to get highly ordered assembly structure, we investigated the use of PDI-based derivatives as building blocks of supramolecular self-assembly and liquid crystals via modifying PDI’s imide position by alkyl chains and functional groups. Three tetrahedral PDI-based molecules tethering with different length of alkyl tails (decyl, dodecyl, tetradodecyl) and four cyano-substituted PDIs were successfully synthesized via a systematic and convenient method, getting rid of the low solubility of perylene. The chemical structures of products were fully characterized by proton nuclear magnetic resonance (1H NMR) spectroscopy and matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy. Their thermal
stability could be maintained until 320 ℃ according to the results of thermogravimetric analysis (TGA). Differential scanning calorimetry (DSC) showed that the modified PDIs had the potential to form ordered structure by thermal annealing.
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ACKNOWLEDGEMENTS
I would like to thank my advisor, Dr. Stephen Z. D. Cheng, to offer me this precious opportunity to learn and work in his excellent group. He always gave me great inspiration, guidance and encouragement for my research, and also valuable advice for my life.
I would like to thank my faculty reader, Dr. Mesfin Tsige, who gave me lots of useful help on my thesis and presentation.
I would like to thank my mentor Dr. Chih-Hao Hsu. He taught me how to do research from the beginning, and gave me a lot of help, guidance and inspiring talk throughout my research in the past two years. I would meet great difficulty without his support.
I also would like to thank Dr. Kwang-Un Jeong, Mr. Mingjun Huang and all the other Dr. Cheng’s group members for their help on this project.
Finally, I would like to thank my parents, family, and friends in China and in U.S. for their great support and love.
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TABLE OF CONTENTS Page
LIST OF FIGURES ...... vii
LIST OF SCHEMES...... ix
CHAPTER
I. INTRODUCTION ...... 1
II. BACKGROUND ...... 7
2.1 Perylene and Perylene Diimide (PDI)...... 7
2.2 Introduction of Liquid Crystal (LCs) ...... 11
2.3 Introduction of Self-assembly ...... 14
III. EXPERIMENTAL ...... 16
3.1 Chemicals and Solvents ...... 16
3.2 Molecular Characterizations ...... 17
3.3 Synthetic Route of Modification Subunits ...... 20 3.4 Synthetic Route of Intermediate Molecule: Perylene-3,4-Anhydride-9,10- di-Decyloxycarbonyl...... 26 3.5 Synthetic Route of Tetrahedral Perylene-based Derivatives ...... 28
3.6 Synthetic Route of Cyano-substituted Perylene-based Derivatives .. 31
IV. RESULTS AND DISCUSSION ...... 34
4.1 Synthetic Route of Modification Units ...... 34 4.2 Synthetic Route of Intermediate Molecule: Perylene-3,4-Anhydride-9,10- di-Decyloxycarbonyl ...... 40
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4.3 Synthetic Route of Tetrahedral Perylene-based Derivatives ...... 43
4.4 Synthetic Route of Cyano-substituted Perylene-based Derivatives .. 48
4.5 Thermal Analysis ...... 52
V. CONCLUSIONS ...... 60
REFERENCES ...... 61
vi
LIST OF FIGURES
Figure Page
1.1 The molecular model of tetrahedral PDIs ...... 6
1.2 The molecular model of cyano-substituted PDIs ...... 6
2.1 (a) Chemical structure of perylene. (b) Exploded view of perylene ...... 7
2.2 Chemical structure of PTCDA ...... 8
2.3 Macroscopic appearance of PTCDA ...... 8
2.4 Electron distribution of CDA...... 9
2.5 Chemical structures of industrial PDI pigments ...... 9
2.6 Chemical structure of perylene diimide (PDI) ...... 10
2.7 Molecule packing models of solid, liquid crystal and liquid ...... 13 4.1 1H NMR spectra of (a) methyl 3,4,5-tridodecyloxybenzoate, (b) 3,4,5-tridodecyloxybenzylic alcohol, (c) 3,4,5-tridodecyloxybenzylbromide, (d) 3,4,5-tridodecyloxybenzylazide, (e) 3,4,5-tridodecyloxybenzylamine ..... 39
4.2 1H NMR spectra of (a) 3,4,5-tritetradecyloxybenzylamine， (b) 3,4,5-tridecyloxybenzylamine ...... 40
4.3 1H NMR spectrum of PADE ...... 42 4.4 1H NMR spectra of (a) di-(decyloxycarbonyl)-perylene-1-(3,4,5)G12, (b) anhydride-perylene-1-(3,4,5)G12, (c) tetra-PDI-1-(3,4,5)G12 ...... 45
4.5 1H NMR spectra of (a) tetra-PDI-1-(3,4,5)G10, (b) tetra-PDI-1-(3,4,5)G14 ...... 46
4.6 MOLDI-TOF mass spectra of (a) tetra-PDI-1-(3,4,5)G12, (b) tetra-PDI-1-(3,4,5)G10, (c) tetra-PDI-1-(3,4,5)G14 ...... 47
4.7 1H NMR spectra of (a) cyanobiphenyl-perylene-1-(3,4,5)G12,
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(b) cyanobiphenyl-perylene-1-(3,4,5)G14, (c) cyanophenyl-perylene-1-(3,4,5)G12, (d) cyanophenyl-perylene-1-(3,4,5)G14 ...... 49
4.8 MOLDI-TOF mass spectra of (a) cyanobiphenyl-perylene-1-(3,4,5)G12, (b) cyanobiphenyl-perylene-1-(3,4,5)G14, (c) cyanophenyl-perylene-1-(3,4,5)G12, (d) cyanophenyl-perylene-1-(3,4,5)G14 ...... 51
4.9 TGA curves of (a) tetra-PDI-1-(3,4,5)G12, (b) tetra-PDI-1-(3,4,5)G10, (c) tetra-PDI-1-(3,4,5)G14, (d) cyanobiphenyl-perylene-1-(3,4,5)G12, (e) cyanobiphenyl-perylene-1-(3,4,5)G14, (f) cyanophenyl-perylene-1-(3,4,5)G12, (g) cyanophenyl-perylene-1-(3,4,5)G14 ...... 55
4.10 DSC curves of (a) cyanobiphenyl-perylene-1-(3,4,5)G12, (b) cyanobiphenyl-perylene-1-(3,4,5)G14, (c) cyanophenyl-perylene-1-(3,4,5)G12, (d) cyanophenyl-perylene-1-(3,4,5)G14 ...... 59
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LIST OF SCHEMES
Scheme Page
4.1 Synthetic route of 3,4,5-trialkoxybenzylamine ...... 35
4.2 Synthetic route of PADE ...... 41
4.3 Synthetic route of tetrahedral PDIs ...... 43
4.4 Synthetic route of cyano-substituted PDIs ...... 48
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CHAPTER I
INTRODUCTION
Perylene is inexpensive, readily available, and robust compound.1 Many of its
derivatives, such as perylene diimides (PDIs), have excellent electroactive properties
and photoactive properties, and wonderful thermal, chemical and photochemical
stability.2-5 PDIs have attracted great interest of scientists, and been under thoroughly
study for lots of years. Not only because are they in an important class of n-type organic semiconductor with promising practical application, but also they can serve
as intriguing building blocks for supramolecular self-assembly6 and liquid crystals.7
The word “n-type” here means that electrons are the majority charge carrier in this kind of semiconductor. There is also a “p-type” semiconductor in which the holes are the majority charge carrier. PDIs exhibit large optical absorption in the visible to near-infrared spectral region, irradiate fluorescence with quantum yields near unity, and show significant charge transport properties,8 so they have been widely used as industrial dyes, pigments, displays, xerographic photoreceptors, organic field effect transistors (OFETs), light emitting diodes, solar cells, fluorescent labels and sensors in the fields of life science, biology, industry, and as artificial photosynthetic systems.9 In addition, many previous studies on aromatic molecules have shown that
π-π interactions between conjugated benzene rings can lead the formation of
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nanostructures via “bottom-up” approach (self-assembly) in nanoscale.10,11
Compared to its larger counterpart, the nanoscale organic crystals could offer even better practical advantages at the meantime avoid growing large organic single crystal, which will let researchers save materials and time and become more efficient.12
The studies of practical device application and self-assembly structure should
not be regarded as separate or unrelated parts, because, in addition to the natural
properties of PDIs, their aggregate structures also have deep influence on the
macroscopic properties. Highly and perfectly ordered supramolecular self-assembly structure of PDI-based derivatives could be a favor element for charge transport, stabilizing charge-separated states and lowering the charge combination,20 which are
critical for organic electronic devices such as OFETs and n-type organic semiconductors.12
As aromatic molecules, PDIs have four conjugated benzene rings in the core which could provide strong π-π interactions acting as the primary force to drive the self-assembly structure. Usually the coherent π-π stacking is the basic packing structure in the assembled nanostructures of PDIs.1 In order to further tune the final
self-assembly structure, the two imide and four bay positions can be functionalized
together or separately, thus providing extra secondary forces such as static
interaction, dipole moment, hydrophilic/hydrophobic interaction and so on to
intervene or modulate the coherent π-π interactions in core position. After the subtle
functionality, diverse morphologies are imparted such as nanofibers,13
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nanoparticles,14 hollow vesicles,15 and nanotubes,16 and also some more complex
structure. Percec et al 9 reported that a 2D-hexagonal columnar phase with
intra-columnar order was formed from one PDI-based derivative which was
functionalized at the imide position with first generation dendrons at high
temperature. Dehm et al 16 reported that a helical stacking conformation with
preferential M or P types can be formed when a chiral unit was introduced in the
PDI’s side chain, which showed the potential application in biotechnology. In
addition to the substituent group, the solvent also plays an important role in
self-assembly process of PDIs. Zhang et al 1 discovered that the self-assembly
structure of perylene diimide with two hydrophilic pyridyloxyl groups on the
1,7-position and alkyl tails on the imide position could be finely tuned by varying
the water fraction Rw and hydrochloric acid concentration, resulting in five different
self-assembled nanostructures as nanotapes, nanoparticles (and their 1-D assemblies),
rigid nanoplate, soft nanoplate and hollow nano-spheres (and their 1-D assemblies).
The conversion of the nanostructures was due to the different kinds of secondary
force induced by different surrounding conditions. For example, under high water
fraction and low acid concentration, the π-π stacking was dominant secondary force
driving the self-assembly. However, under high concentration of hydrochloric acid,
the nitrogen (N) on side group was protonated and charge-charge interaction became
the dominant force.
Although the solvent is crucial for PDI-based derivatives to self-assemble to desired structures as mentioned above, the main challenge, however, for this kind of
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materials is their poor solubility, which results in much trouble in synthesizing,
purifying, characterizing, and further self-assembling process. In recent years, various PDI-based derivatives have been designed and synthesized to increase its solubility in common solvents, and during which they can still remain or improve their original thermal, chemical stability and photochemical properties. The most general method is to introduce long alkyl chains to the two imide positions, which can increase the solubility in some common solvents like chloroform, dichloromethane and tetrahydrofuran. But for self-assembly only using this method just gives one secondary force—hydrophilic/hydrophobic interaction—to the system, in order to further tune the self-assembly process, also to make PDI-based materials environmental-friendly water-soluble, some other functional groups were introduced to incorporate with alkyl chain tails as modification units. Jagadeesh et al 17 designed
a new molecule with alternating flexible hexa(ethylene glycol) spacers and rigid
perylene cores ,which showed remarkable solubility increase in some polar solvents
due to the polar oxygen-carbon bond. Huang et al12 reported that the strong electron-withdrawing fluorine (F) atoms introduced into the π-conjugated core could not only increase the solubility dramatically, but also improve the ability to grow single crystal due to increased polarity induced by the strong electro-withdrawing F atoms. A highly water-soluble polyglycerol dendronized PDIs with a single clickable reaction group was synthesized by Yang18 and his co-workers, which was able to
serve as highly specific protein labels on the surfaces of living bacteria cells.
Liquid crystal is another attractive phase of matter that has the ability to
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self-organize to highly ordered structure. Struijk et al 19 studied the phase behavior
of N-alkyl-substituted perylene diimide derivatives, finding out that the liquid
crystalline phases displayed smectic layers with an additional ordering in columns
observed within them. This so called hierarchical structure (means structure in
structure) was always used as a bridge to collect, transfer, and amplify microscopic
molecular function to macroscopic material properties. Cyano is an important
functional group that can lead PDIs to liquid crystalline phase via self-assembly, which would also improve the practical application. Gao et al 21 demonstrated that
by incorporating cyano groups to perylene core, the LUMO energy level could be effectively lowered, and the extent of decrease showed an excellent linear relationship with the number of cyano group introduced.
Although many works have been done, and considerable achievements were
obtained in the modification of PDIs research, most of them are mainly driven from the application and industrial point of view. But, as the aforementioned statements, the application or property of material is tightly related to its chemical and supramolecular self-assembly structure. However, the studies of the PDI’s relationship between their microscopic molecules and macroscopic properties, the controlling factors of self-organization process and the favorable crystallization conditions are relatively less. In order to get versatile self-assembly structures from
PDIs and to precisely control them, it is important to find out how to integrate PDIs to more structurally intricate self-assembly system. So a systematical and efficient synthetic route working for various PDI-based derivatives is necessary. And unlike
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some aforementioned works concentrating on simple symmetric PDI-based
derivatives, this thesis challenge asymmetric PDIs, providing an efficient way to
synthesize three tetrahedral PDIs tethering alkyl tails with different length (Figure
1.1), and four cyano-substituted PDI-based molecules (Figure 1.2). All of them have the potential to self-assemble to ordered crystalline or liquid crystalline structure according to the preliminary physical results. In this thesis, the background information was introduced in Chapter II, the synthesis, characterization and thermal analysis part’s works were presented and discussed in Chapter III and Chapter IV, and the conclusion and future work were summarized in Chapter V.
Figure 1.1. The molecular Figure 1.2. The molecular model model of tetrahedral PDIs of cyano-substituted PDIs
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CHAPTER II
BACKGROUND
2.1 Perylene and Perylene Diimide (PDI)
Perylene is one of the organic compounds which are classified as polycyclic
aromatic hydrocarbons (PAH). The chemical formula for perylene is C20H12. It has
four conjugated benzene rings at four corners with one six-membered, nonaromatic ring in the center (Figure 2.1a), or it can be regarded as two naphthalene subunits bridged at their respective 1 and 8 carbon (Figure 2.1b). All the carbons in perylene are sp2 hybridized, so the five fused phenyl rings are co-planar. The p-orbitals of
each carbon atom are perpendicular to the molecule plane, and overlap “shoulder to
shoulder” to form a large delocalized-electron system which is termed as π bond.
There are π electrons delocalized at the two sides of molecule plane (up and down),
so perylene and its derivatives have the tendency to form π-π interactions which play
an important role in their self-assembly process and deeply influence their practical
application.
Figure 2.1. (a) Chemical structure of perylene. (b) Exploded view of perylene
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The perylene molecule’s framework of perylene is rigid due to the strong π-π
interaction, and its solubility is pretty poor. So the synthesis of perylene derivatives
is very hard until the discovery of perylene-3,4,9,10-tetracarboxylic acid dianhydride
(PTCDA) (Figure 2.2, 2.3) in the early 1910s.22,23 Since then PTCDA has been one
of the most widely used perylene derivatives. Containing two acid anhydride
functional groups at the two sides of perylene core, PTCDA always acts as a
precursor for many other perylene-based derivatives because it can be readily
modified at anhydride groups and 1,6,7,12 carbons (bay position) on the conjugated
benzene rings.
Figure 2.2. Chemical structure of Figure 2.3. Macroscopic appearance of PTCDA PTCDA
The high reactivity at these two positions is caused by electron density
distribution in PTCDA. Due to the strong electron affinity of oxygen atom in
anhydride group, the electron density is pulled toward opposite ends of the molecule,
making the bay position relatively electron deficient, which favors the nucleophilic
aromatic substitution (Figure 2.4). And the two anhydride groups in PTCDA can
undergo imidization with primary amino group, or be converted to four carbonic
anions in aqueous sodium hydroxide solution at high temperature.
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Figure 2.4. Electron distribution of CDA. The red indicates electron-efficient area, while blue indicates electron-deficient area.
When the PTCDA was imidized with primary amine derivatives, a new kind of perylene derivative was formed and termed as perylene diimides (PDIs). PDIs were firstly used as an industrial pigment called Pigment Red 179 thanks to the ground-breaking work of Harmon Colors.22 And in late 1950s, several other PDI
derivatives used as high-grade paint in automobile and fiber industry achieved the
industrial-scale production (Figure 2.5), showing favorable combination of their
thermal stability, migrational stability, weather fastness and chemical inertness as
well as high tinctorial strength with hues ranging from red to violet, and even black
shades.22, 24
Figure 2.5. Chemical structures of industrial PDI pigments
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After the high fluorescent nature was discovered in 1959,25 PDIs draw further more attention in both academic and industrial fields, and was started using as organic semiconductors, solar cells, organic field effect transistors (OFETs), photoconductors and photovoltaic devices. In recent years, the application of PDIs as building blocks for supramolecular self-assembly and self-organization has been showing interesting and promising potential, since their molecular packing behavior would deeply affect their macroscopic properties such as charge transfer efficiency.
In order to precisely tune the self-assembly process, the synthesis of more complicated, soluble and complex PDI-based derivatives are desired. Similar to the
PTCDA, the main reaction sites of PDIs are the four bay positions in center and the two imide positions at two sides (Figure 2.6).
Figure 2.6. Chemical structure of perylene diimide (PDI). The R groups represent modification units
Various functional groups and alkyl chains can be substituted at bay and imide positions, giving the PDIs desired properties. Some representative works were discussed in the Chapter I. In this thesis, we design a series of tetrahedral PDI-based derivatives and cyano-substituted PDI-based derivatives. In order to make the synthetic work feasible and efficient, a “reactive” PDI-based intermediate is required.
The word “reactive” here mainly means that this intermediate molecule should be
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soluble in common solvents, and can readily react with various modification groups to obtain versatile PDI-based derivatives. Perylene-3,4-anhydride-9,10- di-decyloxycarbonyl (PADE) is a satisfying intermediate as Xue et al 26 reported, since the anhydride group can react with primary amino group, and long alkyl chain tails provide necessary solubility. From PADE the aimed molecules in this thesis can be obtained.
2.2 Introduction of Liquid Crystal (LC)
There are four basic states of matter—solid, liquid, gas and plasma—existing in the universe. This classification is based on the difference of the form the matter takes on. The solid has a definite and fixed volume and shape because of strong interaction force between the so closed packing particles. If the particles in solid pack in a regular long-order pattern along three spatial dimensions, a crystalline solid is formed. The liquid has an inter-particle distance a little bit larger than that in solid, and totally lose its packing order. So the liquid molecules can freely move with respect to each other, but their total volume is almost fixed. Gas has nearly zero intermolecular force due to the extremely large particle-particle distance, resulting in flexible shape and volume. Plasma, which is composed of charged particle, also has no fixed volume and shape, and can respond to electromagnetic field. It is the most common matter of state in the universe.
In addition to the aforementioned four typical states of matter, there are many phases showing excellent properties and being widely used in science, technology
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and everyday life. Liquid crystal (LC) is one famous representative. Most of
cellphone and computer displays, even some proteins and cell membrane are made
of liquid crystal. Liquid crystal was first studied by an Austrian botanist named
Friedrich Reinitzer in 1888.27 He found that cholesteryl benzoate had two distinct
melting points when he increased the temperature. At first the solid sample crystal
changed into a hazy liquid, but if he increased temperature further, the hazy liquid
changed back into a clear, transparent liquid. The “hazy liquid” is now known as
liquid crystal. Just like its name, liquid crystals combine the properties of typical
crystal and typical liquid. It can freely flow just as the liquid does, but it still remains
some ordered arrangement in particle packing which is the property of a crystal.
The molecules (mesogens) forming liquid crystal have the tendency to orient
along a common axis, which is distinctively different from the molecules in typical liquid with no intrinsic order. The common axis in LC is called director. However, the degree of molecular order in LC is not as high as that in a solid (Figure 2.7).
Liquid crystal mesogens are always rod-like or disk-like with rigid molecular framework. It is better for the molecules to be at least 1.3 nm long with some alkyl chains around it, since flexible alkyl chains can give the molecules necessary mobility to align with respect to each other. But the structure should not be branched or angled, which would let molecules difficult to pack in order. The most common liquid crystal building blocks are aromatic molecules and their derivatives.
Obviously, PDIs with proper modification could be suitable to generate liquid crystal self-assembly material.
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Figure 2.7. Molecule packing models of solid, liquid crystal and liquid
If a liquid crystal is induced by heating or annealing, it is called thermotropic liquid crystal. Thermotropic liquid crystals usually form when temperature is increased to a certain temperature range, below which the material is conventional crystalline solid or amorphous matter. And if the temperature is increased further and exceeded upper level of that range, the liquid crystal phase would disappear and be replaced by a conventional disordered liquid phase. What is interesting is that even though the temperature range is continuous, it may contain many sub-ranges in which different liquid crystal phases may be formed. This behavior can be tested by differential scanning calorimetry. The thermotropic LCs may take on nematic, sematic, chiral, blue and discotic phase in different temperature sub-ranges with distinct mesogen packing order.
Not only is the temperature able to induce liquid crystal phases, a suitable solvent can also favor the liquid crystal formation. This kind of liquid crystals is called lyotropic LCs. Lyotropic LCs usually have two immiscible hydrophilic and hydrophobic parts, the LC phase is formed due to the microscopic phase separation of the two immiscible parts on a nanometer scale under certain concentration.
Different morphologies such as micelle and vesicle, and various LC phases such as
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hexagonal, lamellar, bicontinuous cubic, reverse hexagonal and inverse cubic phases could be formed under specific solution concentration and volume fraction of hydrophilic/hydrophobic part.
The cyano-substituted samples introduced in this thesis have the potential to form both thermotropic and lyotropic LCs. The synthetic route and thermal analysis data were discussed in Chapter III and IV. The LC behavior study is still in progress.
2.3 Introduction of Self-assembly
Self-assembly is the method based on “bottom-up” approach that adopts to obtain the final supramolecular structure. Compared to “top-down” approach which is a common technique used to manufacture bulk materials in nanometer scale via casting, X-ray or electron beam lithography, A new ‘‘bottom-up’’ approach opens the door of self-assembly, which provides unique opportunities to organize giant molecules into ordered arrangement via interactions between themselves to make the resulting packing structures predictable and controllable, thus getting rid of the lack of thickness shape control and surface modification mobility and sometimes the thermodynamically unstable final structures coming from “top-down” approach.29
What is more, many useful structures with certain properties are organized matter like hierarchical structure and some biological tissues, which are larger than molecule-scale and cannot be synthesized just by forming chemical bonds. However, by self-assembly, they are able to form ordered supramolecular self-assembly structures that have the complex properties far beyond the single molecule itself. But,
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in most of cases, the molecule may not spontaneously assemble to the desired structure if there is no sufficient mobility and proper driving force in the system. In other word, suitable modification on the molecule surface is required. Molecular mobility will be increased if long alkyl chains are connected to the molecule.
Introducing various functional groups may induced the individual molecule to adjust their position in an ordered arrangement via intermolecular physical interactions such as hydrogen-bonding, electrostatic (ion–ion, ion– dipole, and dipole–dipole) interactions, van der Waals interactions, hydrophobic–hydrophilic phase separations,
π-π interactions and others.
The work of the structure determination of self-assembly aggregate is still in progress and the details were not presented in this thesis. Some preliminary results showed that tetrahedral PDIs had the potential to crystallize in THF and chloroform, and cyano-substituted PDIs formed several different LC phases at a wide temperature range (about 50℃-220℃).
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CHAPTER III
EXPERIMENTAL
3.1 Chemicals and solvents
Most of the chemicals and solvents were ACS certified that could be used
directly as received. However, some of them were purified before adding to the
reaction in order to eliminate potential side reactions when necessary.
The following chemicals and solvents were used as received: methyl
3,4,5-trihydroxybenzoate (98%, Sigma-Aldrich), 1-bromododecane (97%,
Sigma-Aldrich), potassium carbonate (≥99.0%, Sigma-Aldrich),
N,N-dimethylformamide (DMF, 99.8%, anhydrous, Sigma-Aldrich), potassium
hydroxide (≥85%, ACS reagent, pellets, Sigma-Aldrich), ethanol (≥99.5%, ACS reagent, Sigma-Aldrich), deionized water, lithium aluminum hydride solution
(LiAlH4, 1.0M in THF, Sigma-Aldrich), triphenylphosphine (≥95.0%,
Sigma-Aldrich), tetrabromomethane (99.0%, Sigma-Aldrich), benzene (99.8%, anhydrous, Sigma-Aldrich), diethyl ether (≥99.0%, anhydrous, ACS reagent, contains BHT as inhibitor, Sigma-Aldrich), sodium azide (NaN3, ≥99.5%,
Sigma-Aldrich), 1-bromodecane (98%, Sigma-Aldrich), perylene-3,4,9,10-
tetracarboxylic dianhydride (PTCDA, 97%, Sigma-Aldrich), Aliquat 336
(Sigma-Aldrich), potassium iodide (≥99.5%, Sigma-Aldrich), n-dodecane (≥99.5%,
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anhydrous, Sigma-Aldrich), p-toluenesulfonic acid monohydrate (≥98.5%, ACS
reagent, Sigma-Aldrich), imidazole (≥99%, ACS reagent, Sigma-Aldrich),
1,2-dichlorobenzene (ODCB, 99%, anhydrous, Sigma-Aldrich),
4-dimethylaminopyridine (DMAP, 99%, Sigma-Aldrich), tetra(4-aminophenyl)
methane (Synthesized by Mr. Mingjun Huang), 1-bromotetradecane (97%,
Sigma-Aldrich), 4-aminobenzonitrile (98%, Sigma-Aldrich), 4-(4-aminophenyl) benzonitrile (95%, Sigma-Aldrich) methanol (Fisher Scientific, reagent grade), ethyl acetate (Fisher Scientific), dichloromethane (Certified ACS), chloroform (Certified
ACS), hexane (Certified ACS)
While, tetrahydrofuran (THF, Certified ACS, EM Science), toluene (Certified
ACS) were used after distillation to make them as extra-anhydrous solvents.
All the glasswares were steeped in the alkaline solution (potassium hydroxide dissolved in water and ethanol with 1:1 in volume) for 24 hours. Then they were under careful wash, and were put into oven at 120℃ for more than 2 hours to remove the water. The magnetic stirring bars were stored in THF when not being used.
3.2 Molecular Characterizations
The synthetic molecules were characterized by thin-layer chromatographic analyses (TLC), proton nuclear magnetic resonance (NMR) spectra and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra to prove their chemical structure, and were characterized by thermal gravity
17
analysis (TGA) and differential scanning calorimetry (DSC) to study their thermal
behavior.
3.2.1 1H Nuclear Magnetic Resonance (NMR) Spectra
1 All the H NMR and spectra were acquired in CDCl3 (Aldrich, 99.8 % D) utilizing a Varian Mercury 300 NMR spectrometer. The 1H NMR spectra were
referenced to the residual proton signals in the CDCl3 at δ 7.27 ppm
3.2.2 Thermal Gravity Analysis (TGA)
The non-isothermal decomposition experiments were carried out in a thermal
gravimetric analysis instrument (Model Q500, TA Instrument) under nitrogen
atmosphere for the temperature range from 30 ℃ to 500 ℃ with a heating rate of
10 ℃/min. Initial mass of the sample was 2.0 mg for each run of the experiments. A platinum crucible was used as the sample holder.
3.2.3 Thin-layer Chromatographic Analyses (TLC)
Thin-layer chromatographic analyses (TLC) of the modified PDI-based molecules were carried out on by spotting and developing samples on flexible silica gel plates (Selecto Scientific, Silica Gel 60, F-254 with fluorescent indicator) using mixture solvent of chloroform and acetone as eluent.
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3.2.4 Differential Scanning Calorimetry (DSC)
The thermal characteristics of the bulk samples were obtained using a
Perkin-Elmer PYRIS Diamond DSC coupled with an Intracooler 2P apparatus. A
typical sample weight was 2.0 mg for bulk samples was used. The difference in pan
weights between the reference and sample was kept less than 0.005 mg. The heating
and cooling rates were 10℃/min, 5℃/min, and 20℃/min.
3.2.5 Matrix-assisted Laser Desorption/ionization Time-of-Flight (MALDI-TOF)
Mass Spectra
Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass
spectra were acquired on a Bruker Ultraflex-III TOF/TOF mass spectrometer
(Bruker Daltonics, Inc., Billerica, MA) equipped with a Nd:YAG laser (355 nm).
The spectra were measured in positive linear mode. The instrument was calibrated
prior to each measurement with external PMMA or PS standards at the molecular
weight under consideration. The compound trans-2-[3-(4-tert-butylphenyl)-
2-methyl-2-propenylidene]-malononitrile (DCTB, Aldrich, > 98 %) served as matrix and was prepared in CHCl3 at a concentration of 20 mg/mL. The cationizing agent
sodium trifluoroacetate or silver trifluoroacetate was prepared in MeOH/CHCl3 (v/v
= 1/3) at a concentration of 5 mg/mL or 10 mg/mL. The matrix and cationizing salt
solutions were mixed in a ratio of 10/1 (v/v). All samples were dissolved in THF at a
concentration of 10 mg/mL. The sample preparation followed the procedure of
depositing 0.5 μL of matrix and salt mixture on the wells of a 384-well ground-steel
19
plate, allowing the spots to dry, depositing 0.5 μL of each sample on a spot of dry
matrix/salt, and adding another 0.5 μL of matrix and salt mixture on top of the dry
sample (sandwich method). After solvent evaporation, the plate was inserted into the
MALDI mass spectrometer. The attenuation of the Nd:YAG laser was adjusted to
minimize undesired fragmentation and to maximize the sensitivity. Data analyses
were conducted with the Bruker’s flexAnalysis software.
3.3 Synthesis of Modification Subunits
There are three kinds of small aromatic amine molecules with different length of
alkyl tails being synthesized: (1) 3,4,5-tridecyloxybenzylamine, (2) 3,4,5-
tridodecyloxybenzylamine, (3) 3,4,5-tritetradecyloxybenzylamine, which have three
alkyl tails with ten, twelve and fourteen carbons, respectively. All of them will be
subsequently connected to perylene core via imidization reaction as modification subunits to increase the mobility and solubility of the products. The synthetic route and reaction condition of these three molecules were almost same. The 3,4,5- tridodecyloxybenzylamine was chosen as a representative to show the detailed synthetic steps. And the procedure of every step can be directly used to synthesize
3,4,5-tridecyloxybenzylamine and 3,4,5-tritetradecyloxybenzylamine, only by
changing the starting reactants. The characterization results of all the three molecules
were presented in Chapter IV.
20
3.3.1 Synthesis of Methyl 3,4,5-Tridodecyloxybenzoate (Compound 1)
To a round bottom (50 ml) equipped with a magnetic stirring bar were added
3,4,5-trihydroxybenzoate (3250 mg, 17.66 mmol), 1-bromododecane (12.980 g,
64.12 mmol) and potassium carbonate (10.935 g, 79.48 mmol). Then 30 ml
N,N-dimethylformamide (DMF) was added to the flask to dissolve the mixture. The potassium hydroxide cannot totally dissolve in DMF because it is in large excess.
The mixture was stirred and refluxed in oil bath at 80℃ under N2 atmosphere for 12 hours. During the reaction, the color of solution turned to grey. After the reaction, pour all the solution into a flask with deionized water (400 ml). Cover the flask and put it into fridge at 4 ℃ for two hours. Take the flask out, and the crude product was separated by suction filtration. Collect the grey solid on the filter paper and transfer it into acetone (400 ml) to do recrystallization. The acetone was heated to boil and the crude product would totally be dissolved in the hot acetone. The acetone solution was quickly filtrated when it was still hot. Collect the filtrate and put it into the fridge for four hours. The pure product was precipitated from cold acetone and filtered to afford pure product as white solid. The product was dried in vacuum oven
1 over night at room temperature. Yield: 83.2%. H NMR (300Hz, CDCl3, ppm, δ):
0.89 (t, 9H, 3CH3), 1.28 (t, 48H, 3CH3(CH2)8), 1.49 (m, 6H, 3AlkCH2CH2CH2OAr),
1.83 (m, 6H, 3AlkCH2CH2CH2OAr), 3.90 (m, 3H, CH3OOCAr), 4.05 (m, 6H,
3AlkCH2CH2CH2OAr).
21
3.3.2 Synthesis of 3,4,5-Tridodecyloxybenzylic Alcohol (Compound 2)
To a round bottom flask (100 ml) equipped with a magnetic stirring bar were added N2 to remove air for fifteen minutes. Then the ice-water bath was given to the
flask for about five minutes until the flask was totally cooled by the ice water. A solution of Compound 1 (2994 mg, 4.34 mmol) in 18 ml freshly distilled
tetrahydrofuran (THF) was added in the round bottom flask using a syringe. The
solution was stirred vigorously for about five minutes to let the added solution be
completely cooled down, then 6.52 μl lithium aluminum hydride solution (LiAlH4,
1.0M in THF) was added slowly and dropwise by a small syringe under vigorous
stirring. Subsequently, another 15 ml THF was added. The reaction mixture was
stirred under N2 atmosphere and in ice water bath for 30 minutes, and then the ice
water bath was removed and the reaction mixture was kept stirring for another 2.5
hours at room temperature. Then the reaction was quenched by dropwise adding
deionized water (2 ml), aqueous sodium hydroxide solution (5 ml, 1M) and
deionized water (10 ml) under ice water bath and vigorous stirring. The mixture was
continuously stirred for fifteen minutes, and the granular solid was filtered off. Wash
the filter residue by enough THF and combine the washed THF with original filtrate.
The filtrate was condensed by rota-evaporation under reduced pressure. The crude
product was extracted by dichloromethane (DCM)/water. The organic layer was
collected and the solvent was removed under reduced pressure. The pure product
was obtained by column chromatography using a mixture of hexane and acetone
(97:3, v/v) as eluent. The eluent was removed by rota-evaporation and the pure
22
product showed white powder after being dried in vacuum oven at room temperature
1 overnight. Yield: 85.65%. H NMR (300Hz, CDCl3, ppm, δ): 0.89 (t, 9H, 3CH3),
1.28 (t, 48H, 3CH3(CH2)8), 1.49 (m, 6H, 3AlkCH2CH2CH2OAr), 1.80 (m, 6H,
3AlkCH2CH2CH2OAr), 3.99 (m, 6H, 3AlkCH2CH2CH2OAr), 4.61 (m, 2H,
ArCH2OH), 6.57 (m, 2H, ArH ortho to CH2OH).
3.3.3 Synthesis of 3,4,5-Tridodecyloxybenzylbromide (Compound 3)
To a round bottom flask (100 ml) equipped with a magnetic stirring bar were added a solution of Compound 2 (1600mg, 2.42 mmol), triphenylphosphine (1905 mg, 7.27 mmol), and tetrabromomethane (2378.18 mg, 7.27 mmol) in 33 ml benzene.
The reaction mixture was stirred at room temperature for 12 hours under N2 atmosphere. The color of solution became yellow. Then, 33 ml hexane was added into the flask to dilute the reaction system. The crude product was filtered out as sticky solid, and it was added into acetone (200 ml) to do recrystallization. The pure product was filtered and dried in the vacuum oven as white powder. Yield: 95%. 1H
NMR (300Hz, CDCl3, ppm, δ): 0.89 (t, 9H, 3CH3), 1.27 (t, 48H, 3CH3(CH2)8), 1.48
(m, 6H, 3AlkCH2CH2CH2OAr), 1.81 (m, 6H, 3AlkCH2CH2CH2OAr), 3.98 (m, 6H,
3AlkCH2CH2CH2OAr), 4.25 (m, 2H, ArCH2Br), 6.49 (m, 2H, ArH ortho to CH2Br).
3.3.4 Synthesis of 3,4,5-Tridodecyloxybenzylazide (Compound 4)
To a round bottom flask (100 ml) equipped with a magnetic stirring bar were added a solution of Compound 3 (1017 mg, 1.41 mmol), sodium azide (914 mg, 14.1
23
mmol) in a mixture of THF (10 ml) and DMF (35 ml). The reaction mixture was
stirred under N2 atmosphere at room temperature for 12 hours. The crude product
was extracted by DCM/water (1:1, v/v). The organic layer was collected and
condensed by rota-evaporation under reduced pressure, then was added to acetone
(150 ml) to do recrystallization using the same method as described in the synthetic
procedure of Compound 1. The pure product was filtered and dried in the vacuum
1 oven as white crystal. Yield: 97%. H NMR (300Hz, CDCl3, ppm, δ): 0.89 (t, 9H,
3CH3), 1.28 (t, 48H, 3CH3(CH2)8), 1.47 (m, 6H, 3AlkCH2CH2CH2OAr), 1.82 (m, 6H,
3AlkCH2CH2CH2OAr), 3.97 (m, 6H, 3AlkCH2CH2CH2OAr), 4.44 (m, 2H,
ArCH2N3), 6.58 (m, 2H, ArH ortho to CH2N3).
3.3.5 Synthesis of 3,4,5-Tridodecyloxybenzylamine (Compound 5)
To a round bottom flask (100 ml) equipped with a magnetic stirring bar were added N2 to remove air for fifteen minutes. Then the ice-water bath was set up. After
about five minutes when the flask was totally cooled by the ice water, a solution of
Compound 4 (993mg, 1.48 mmol) in 6.22 ml freshly distilled tetrahydrofuran (THF)
was added in the round bottom flask using a syringe. The solution was stirred
vigorously. After five minutes when the added solution was completely cooled down,
2.04μl lithium aluminum hydride solution (LiAlH4, 1.0M in THF) was added slowly
and dropwise by a small syringe under vigorous stirring. Subsequently, another 3.8
ml THF was added. The reaction mixture was stirred under N2 atmosphere in ice
water bath for 30 minutes, and then the ice water bath was removed and the reaction
24
mixture was kept stirring for another 2.5 hours at room temperature. Then the
reaction was quenched by dropwise adding deionized water (0.8 ml), aqueous
sodium hydroxide solution (1.6 ml, 1M) and deionized water (2.4 ml) under ice water bath and vigorous stirring. The mixture was continuously stirred for fifteen minutes, and the granular solid was filtered off. Wash the filter residue by enough
THF, and combine the washed THF with original filtrate. The filtrate was condensed
by rota-evaporation under reduced pressure. The crude product was extracted by
dichloromethane (DCM)/water. The organic layer was collected and the solvent was
removed under reduced pressure. After the recrystallization from acetone, the pure
product was filter and dried in vacuum oven as a white crystal. Yield: 65%. 1H NMR
(300Hz, CDCl3, ppm, δ): 0.89 (t, 9H, 3CH3), 1.28 (t, 48H, 3CH3(CH2)8), 1.44 (m, 6H,
3AlkCH2CH2CH2OAr), 1.77 (m, 6H, 3AlkCH2CH2CH2OAr), 3.90-3.97 (overlapped, m, 6H+2H, 3AlkCH2CH2CH2OAr, ArCH2NH2), 6.68 (m, 2H, ArH ortho to
CH2NH2).
3.3.6 Synthesis of 3,4,5-Tritdecyloxybenzylamine (Compound 6)
1-bromododecane was replaced by 1-bromodecane, following the same
synthetic method from compound 1 to compound 5. Yield: 58%. 1H NMR (300Hz,
CDCl3, ppm, δ): 0.89 (t, 9H, 3CH3), 1.28 (t, 48H, 3CH3(CH2)8), 1.44 (m, 6H,
3AlkCH2CH2CH2OAr), 1.77 (m, 6H, 3AlkCH2CH2CH2OAr), 3.90-3.97 (overlapped,
m, 6H+2H, 3AlkCH2CH2CH2OAr, ArCH2NH2), 6.68 (m, 2H, ArH ortho to
CH2NH2).
25
3.3.7 Synthesis of 3,4,5-Tritetradecyloxybenzylamine (Compound 7)
1-bromododecane was replaced by 1-bromotetradecane, following the same
synthetic method from compound 1 to compound 5. Yield: 69%. 1H NMR (300Hz,
CDCl3, ppm, δ): 0.89 (t, 9H, 3CH3), 1.28 (t, 48H, 3CH3(CH2)8), 1.44 (m, 6H,
3AlkCH2CH2CH2OAr), 1.77 (m, 6H, 3AlkCH2CH2CH2OAr), 3.90-3.97 (overlapped,
m, 6H+2H, 3AlkCH2CH2CH2OAr, ArCH2NH2), 6.68 (m, 2H, ArH ortho to
CH2NH2).
3.4 Synthetic Route of Intermediate Molecule: Perylene-3,4-Anhydride-9,10-
di-Decyloxycarbonyl
In order to get versatile PDI-based derivatives form a systematical method, an excellent intermediate molecule with excellent solubility and reactivity is necessary.
3.4.1 Synthesis of Perylene-3,4,9,10-Tetradecyloxylcarbonyl (PTE) (Compound 8)
To a round bottom flask (250 ml) equipped with a magnetic stirring bar were added perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA, 7840 mg, 20 mmol), potassium hydroxide (6000 mg, 107.14 mmol) and deionized water (100 ml). The reaction mixture was stirred at 70 ℃ for 30 minutes. Then the solution was filtered and the filtrate was collected. The pH value was adjusted to 8-9 by adding hydrochloric acid (HCl, 1M) dropwise. Potassium iodide (5000 mg, 4 mmol) and
Aliquat 336 (2.7 g, 4mmol) was added to solution. After the solution was stirred vigorously for 10 minutes, 1-bromodecane (35.4 g, 160 mmol) was added to the
26
solution. Then the solution was refluxed with vigorous stirring for 2 hours. The
yellow product was extracted by chloroform (1 L), followed by washing with
aqueous sodium hydroxide (3*150 ml, 15% in weight) three times. The organic
phase was dropwise added to acetone (1 L) under vigorous stirring. The precipitated
product was filtered and dried in vacuum oven. The product showed yellow solid.
3.4.2 Synthesis of Perylene-3,4-Anhydride-9,10-di-Decyloxycarbonyl (PADE)
(Compound 9)
To a round bottom flask (10 ml) equipped with a magnetic stirring bar were
added compound 8 (4941 mg, 5mmol), n-dodecane (6.75 ml) and toluene (1.35 ml).
The reaction mixture was heated to 95 ℃. When the compound 8 was dissolved,
p-toluenesulfonic acid monohydrate (948 mg, 5 mmol) was added. The solution was
stirred at 95 ℃ for 5 hours. The dark red sticky mixture was dissolved in
chloroform (90 ml). The crude product was purified by column chromatography
using a mixture of chloroform and acetone (20:1, v/v) as eluent. The solvent was
removed by rota-evaporation under reduced pressure. After being dried in vacuum
oven, the pure product was afforded as red solid. Yield: 71%. 1H NMR (300Hz,
CDCl3, ppm, δ): 0.89 (t, 6H, 2CH3), 1.28 (t, 28H, 2CH3(CH2)7), 1.83 (m, 4H,
AlkCH2CH2OOCAr), 4.34 (m, 4H, AlkCH2CH2OOCAr), 7.92-8.51 (s, 8H, ArH)
27
3.5 Synthetic Route of Tetrahedral Perylene-based Derivatives
The aforementioned molecules: a) 3,4,5-tridecyloxybenzylamine, b) 3,4,5- tridodecyloxybenzylamine, c) 3,4,5-tritetradecyloxybenzylamine are all used as modification subunits to react with anhydride group with PDIs, obtaining three kinds of tetrahedral perylene-based molecules with three different length alkyl tails: C10,
C12 and C14, respectively. In this section, the molecule having alkyl tails of twelve carbons, named tetra-PDI-1-(3,4,5)G12, is chosen as a representative to introduce its synthetic method. The other two molecules having alkyl tails of twelve carbons and
fourteen carbons respectively can be obtained from the same reaction condition and
purifying method. The characterization results of all the three molecules are
presented in Chapter IV.
3.5.1 Synthesis of di-Decyloxycarbonyl-Perylene-1-(3,4,5)G12 (Compound 10)
To a Schlenk flask (10 ml) equipped with a small magnetic stirring bar were
added compound 9 (483.05 mg, 0.7 mmol), compound 5 (600 mg, 0.91 mmol) and
imidazole (3.3 g). N2 was added into Schlenk flask smoothly for 15 minutes to
remove the air. Then 1,2-dichlorobenzene (ODCB, 2.2ml) was added. The Schlenk flask was sealed tightly, and the reaction mixture was stirred at 120 ℃ for 5 hours.
Methanol (8 ml) was added in to Schlenk flask after the reaction. The mixture was
filtered, and the red solid was washed by enough methanol and collected. The crude
product was purified by silica gel column chromatography using chloroform as
eluent. The solvent was removed by rota-evaporation under reduced pressure. After
28
being dried in the vacuum oven, the pure product was afforded as dark red crystal.
1 Yield: 75%. H NMR (300Hz, CDCl3, ppm, δ): 0.89 (t, 15H, 5CH3), 1.28-1.51
(overlapped, t, 28H+54H, 2CH3(CH2)7CH2CH2OOCAr, 3CH3(CH2)9CH2CH2OAr),
1.86 (overlapped, m, 4H+6H, 2CH3(CH2)7CH2CH2OOCAr,
3CH3(CH2)9CH2CH2OAr), 3.94 (s, 2H, AlkCH2CH2OAr), 4.07 (m, 4H,
2AlkCH2CH2OAr), 4.38 (m, 4H, AlkCH2CH2OOCAr), 7.56-7.93 (s, 8H, ArH)
3.5.2 Synthesis of Anhydride-Perylene-1-(3,4,5)G12 (Compound 11)
To a round bottom flask (50 ml) equipped with a magnetic stirring bar were added compound 10 (388 mg, 0.29 mmol), p-toluenesulfonic acid monohydrate (333 mg, 1.75 mmol) and toluene (25 ml). The reaction mixture was stirred at 100 ℃ for
2 hours. The solvent was removed by rota-evaporation under reduced pressure. The
solid residue was washed by methanol and filtered. After being dried in vacuum
oven, the pure product was afforded as dark red solid. Yield: 97%. 1H NMR (300Hz,
CDCl3, ppm, δ): 0.89 (t, 9H, 3CH3), 1.26 (m, 48H, 3CH3(CH2)8), 1.45 (m, 6H,
3AlkCH2CH2CH2OAr), 1.78 (s, 6H, 3AlkCH2CH2CH2OAr), 3.91 (s, 2H,
AlkCH2CH2CH2OAr), 4.00 (s, 4H, 2AlkCH2CH2CH2OAr), 5.30 (s, 2H, CH2Ar),
6.84 (s, 2H, ArH ortho to 3 OAlk), 8.73 (s, 8H, ArH).
3.5.3 Synthesis of Tetra-PDI-1-(3,4,5)G12 (Compound 12)
To a Schlenk flask (10 ml) equipped with a small magnetic stirring bar were
added compound 11 (170 mg, 0.16 mmol), tetra(4-aminophenyl)methane (6.54 mg,
29
0.036 mmol), 4-dimethylaminopyridine (DMAP, 6 mg), and imidazole (3 g). N2 was
added into Schlenk flask smoothly for 15 minutes to remove the air. Then
1,2-dichlorobenzene (ODCB, 2ml) was added. The Schlenk flask was sealed tightly,
and the reaction mixture was stirred at 130 ℃ for 5 hours. Methanol (8 ml) was
added in to Schlenk flask after the reaction. The mixture was filtered, and the red
solid was washed by enough methanol and collected. The crude product was
obtained by alumina column chromatography using chloroform as eluent. After that,
the crude product was purified by silica gel column chromatography using a mixture
of chloroform and acetone (40:1, v/v) as eluent. The solvent was removed by
rota-evaporation under reduced pressure. After being dried in the vacuum oven, the
1 pure product was afforded as red crystal. Yield: 53%. H NMR (300Hz, CDCl3, ppm,
δ): 0.88 (m, 36H, 12CH3), 1.23 (t, 192H, 12CH3(CH2)8), 1.45 (m, 24H,
12AlkCH2CH2CH2OAr), 1.80 (s, 24H, 12AlkCH2CH2CH2OAr), 3.92-4.02 (s, 24H,
12AlkCH2CH2CH2OAr), 5.35 (s, 8H, 4CH2Ar), 6.86 (s, 8H, ArH ortho to 12OAlk),
7.53-7.64 (s, 32H, 4C-ArH), 8.07-8.40 (s, 32H, 4ArH). MS (MOLDI-TOF): Calc:
4442.73, Exp: 4442.33 [M+Na]+
3.5.4 Synthesis of Tetra-PDI-1-(3,4,5)G10 (Compound 13)
Compound 5 was replaced by compound 6, following the same synthetic
1 method from compound 10 to compound 12. Yield: 47.5%. H NMR (300Hz, CDCl3, ppm, δ): 0.86 (m, 36H, 12CH3), 1.24 (t, 144H, 12CH3(CH2)6), 1.44 (m, 24H,
12AlkCH2CH2CH2OAr), 1.78 (s, 24H, 12AlkCH2CH2CH2OAr), 3.91-4.00 (s, 24H,
30
12AlkCH2CH2CH2OAr), 5.36 (s, 8H, 4CH2Ar), 6.83 (s, 8H, ArH ortho to 12OAlk),
7.51-7.61 (s, 32H, 4C-ArH), 8.18-8.44 (s, 32H, 4ArH). MS (MOLDI-TOF): Calc:
4106.35, Exp: 4106.34 [M+Na]+
3.5.6 Synthesis of Tetra-PDI-1-(3,4,5)G14 (Compound 14)
Compound 5 was replaced by compound 7, following the same synthetic
1 method from compound 10 to compound 12. Yield : 61%. H NMR (300Hz, CDCl3, ppm, δ): 0.87 (m, 36H, 12CH3), 1.24 (t, 240H, 12CH3(CH2)10), 1.44 (m, 24H,
12AlkCH2CH2CH2OAr), 1.78 (s, 24H, 12AlkCH2CH2CH2OAr), 3.90-3.99 (s, 24H,
12AlkCH2CH2CH2OAr), 5.36 (s, 8H, 4CH2Ar), 6.83 (s, 8H, ArH ortho to 12OAlk),
7.52-7.61 (s, 32H, 4C-ArH), 8.22-8.54 (s, 21H, 4ArH). MS (MOLDI-TOF): Calc:
4779.04, Exp: 4779.11 [M+Na]+
3.6 Synthetic Route of Cyano-substituted Perylene-based Derivatives
The synthetic works of cyano-substituted perylene-based molecules were done
through the similar chemical reaction as the synthesis of tetrahedral perylene-based
derivatives, using compound 11 as one reactant.
3.6.1 Synthesis of Cyanobiphenyl-Perylene-1-(3,4,5)G12 (Compound 15)
To a Schlenk flask (10 ml) equipped with a small magnetic stirring bar were
added compound 11 (150 mg, 0.15 mmol), 4-(4-aminophenyl)benzonitrile (37.6 mg,
0.19 mmol), 4-dimethylaminopyridine (DMAP, 10 mg), and imidazole (2 g). N2 was
31
added into Schlenk flask smoothly for 15 minutes to remove the air. Then
1,2-dichlorobenzene (ODCB, 1ml) was added. The Schlenk flask was sealed tightly, and the reaction mixture was stirred at 120 ℃ for 5 hours. Methanol (8 ml) was added in to Schlenk flask after the reaction. The mixture was filtered, and the red solid was washed by enough methanol and collected. The crude product was purified by silica gel column chromatography using a mixture of dichloromethane and acetone (150:1, v/v) as eluent. The solvent was removed by rota-evaporation under reduced pressure. After being dried in the vacuum oven, the pure product was
1 afforded as red crystal. Yield: H NMR (300Hz, CDCl3, ppm, δ): 0.89 (m, 9H, 3CH3),
1.26 (m, 48H, 3CH3(CH2)8), 1.47 (m, 6H, 3AlkCH2CH2CH2OAr), 1.79 (s, 6H,
3AlkCH2CH2CH2OAr), 3.91 (s, 2H, AlkCH2CH2CH2OAr), 4.03 (s, 4H,
2AlkCH2CH2CH2OAr), 5.30 (s, 2H, CH2Ar), 6.84 (s, 2H, ArH ortho to 3 OAlk),
7.55&7.78 (s, 8H, ArH-CN), 8.48-8.74 (s, 8H, ArH). MS (MOLDI-TOF):
Calc:1210.65, Exp: 1210.18
3.6.2 Synthesis of Cyanobiphenyl-Perylene-1-(3,4,5)G14 (Compound 16)
Compound 11 was replaced by 4-aminobenzonitrile and anhydride-perylene-1
-(3,4,5)G14, following the same synthetic method of compound 15.Yield: 1H NMR
(300Hz, CDCl3, ppm, δ): 0.89 (m, 9H, 3CH3), 1.26 (m, 60H, 3CH3(CH2)10), 1.46 (m,
6H, 3AlkCH2CH2CH2OAr), 1.79 (s, 6H, 3AlkCH2CH2CH2OAr), 3.91 (s, 2H,
AlkCH2CH2CH2OAr), 4.00 (s, 4H, 2AlkCH2CH2CH2OAr), 5.31 (s, 2H, CH2Ar),
32
6.84 (s, 2H, ArH ortho to 3 OAlk), 7.53&7.79 (s, 8H, ArH-CN), 8.61-8.78 (s, 8H,
ArH). MS (MOLDI-TOF): Calc: 1293.81, Exp: 1294.41
3.6.3 Synthesis of Cyanophenyl-Perylene-1-(3,4,5)G12 (Compound 17)
4-(4-aminophenyl)benzonitrile was replaced by 4-aminobenzonitrile, following
1 the same synthetic method of compound 15.Yield: H NMR (300Hz, CDCl3, ppm, δ):
0.88 (m, 9H, 3CH3), 1.26 (t, 48H, 3CH3(CH2)8), 1.46 (s, 6H, 3AlkCH2CH2CH2OAr),
1.79 (s, 6H, 3AlkCH2CH2CH2OAr), 3.91 (s, 2H, AlkCH2CH2CH2OAr), 4.00 (s, 4H,
2AlkCH2CH2CH2OAr), 5.31 (m, 2H, CH2Ar), 6.83 (s, 2H, ArH ortho to 3 OAlk),
7.56&7.88 (s, 4H, ArH-CN), 8.59-8.75 (s, 8H, ArH). MS (MOLDI-TOF): Calc:
1133.69, Exp: 1133.89
3.6.4 Synthesis of Cyanophenyl-Perylene-1-(3,4,5)G14 (Compound 18)
4-(4-aminophenyl)benzonitrile and compound 11 was replaced by
4-aminobenzonitrile and anhydride-perylene-1-(3,4,5)G12, following the same
1 synthetic method of compound 15.Yield: H NMR (300Hz, CDCl3, ppm, δ): 0.88 (m,
9H, 3CH3), 1.26 (m, 60H, 3CH3(CH2)10), 1.44 (m, 6H, 3AlkCH2CH2CH2OAr), 1.79
(s, 6H, 3AlkCH2CH2CH2OAr), 3.91 (s, 2H, AlkCH2CH2CH2OAr), 4.00 (s, 4H,
2AlkCH2CH2CH2OAr), 5.31 (s, 2H, CH2Ar), 6.84 (s, 2H, ArH ortho to 3 OAlk),
7.55&7.88 (s, 4H, ArH-CN), 8.65-8.76 (s, 8H, ArH). Calc: 1218.72, Exp: 1218.23
33
CHAPTER IV
RESULTS AND DISCUSSION
4.1 Synthesis of Modification Units
Three molecules: (1) 3,4,5-tridecyloxybenzylamine, (2) 3,4,5- tridodecyloxybenzylamine, (3) 3,4,5-tritetradecyloxybenzylamine were designed and
synthesize as modification units to introduce both mobility and solubility to the final
perylene-based derivatives for self-assembly.
The synthesis of modification units started from a widely-used and cheap
chemical: 3,4,5-trihydroxybenzoate. The three hydroxyl groups at 3,4,5 positions of
the benzene rings were nucleophilic substituted by 1-bromododecane,
1-bromodocane, 1-bromotetradecane, respectively, to get methyl 3,4,5-
tridodecyloxybenzoate, methyl 3,4,5-tridecyloxybenzoate and methyl 3,4,5-
tritetradecyloxybenzoate which have different length of its alkyl tails of twelve, ten
and fourteen carbons. The following discussion was taking methyl 3,4,5-
tridodecyloxybenzoate as an example. The methyl 3,4,5-tridodecyloxybenzoate was
firstly reduced to 3,4,5-tridodecyloxybenzylic alcohol by reducing agent lithium
aluminum. The hydroxide group in 3,4,5-tridodecyloxybenzylic alcohol was
subsequently replaced by bromine via “Appel reaction”, and then the bromine was
followed by the nuleophilic substitution by azide group. Finally the azide group was
34
reduced to primary amino group using lithium aluminum, getting
3,4,5-tridodecyloxybenzylamine which was able to react with the anhydride group in
PDIs. This series of reactions was summarized in Scheme 1.
R1=C12H25; R2=C10H21; R3=C14H29
Scheme 4.1. Synthetic route of 3,4,5-trialkoxybenzylamine. (1) DMF,
BrR, K2CO3; (2) THF (anhydrous), H4AlLi; (3) PhP3, C6H6, BrC4; (4)
NaN 3, DMF, K2CO3; (5) THF (anhydrous), H4AlLi
The step 1 was a nucleophilic substitution reaction, using potassium hydroxide as catalyst to provide an alkaline condition. The potassium hydroxide was in large excess and was not totally dissolved in DMF. After the reaction, just pour the mixture into 400 mL water. The undissolved potassium hydroxide and solvent DMF would completely dissolve in water because of their larger polarity, and the products would precipitate out from water because of the hydrophobicity of long alkyl chains.
The crude product was able to separate from potassium hydroxide and DMF easily by filtering out from water. Acetone had weaker polarity than water, so the product
35
was soluble in hot acetone but still remained low solubility in cold acetone. However,
the impurities (most of them are unreacted 3,4,5-trihydroxybenzoate) were insoluble
in the hot acetone. So acetone was a perfect solvent to do recrystallization, by which
we could avoid the time-consuming column chromatography. The critical step of
recrystallization was to filter the solution when the acetone was still hot. Otherwise,
some product would crystallize in advance, resulting in low productivity. After the
filtrate was cooled down and filtered, it was recommended to dry the pure product in
vacuum oven at room temperature for long time. Although elevated temperature
could accelerate the process, it would cause the product to dissolve in the residue
acetone at the beginning. When the product was precipitated from concentrated
solution again, an agglomerate sample was given, which was harder to measure precisely than powder one. And some residue acetone may be buried in the product, which would cause solvent peaks in 1H NMR spectrum.
Step 2 was a reducing reaction. Using lithium aluminum as reducing agent can make the reaction finish in four hours. It was efficient, but the lithium aluminum was a dangerous chemical which may be flammable in the air. Also, lithium aluminum was highly susceptible to water, even the moisture air could effectively decrease its efficiency. So the critical point in this step was to create an ultra-dry condition for
the whole reaction system. It could be done by replacing the air in the flask with pure
nitrogen before the reaction started. And the solvent THF should be freshly distilled.
Also, it was better to use lithium aluminum hydride solution than lithium aluminum
powder, because the powder sample may absorb water from air while measuring. In
36
order to safety, ice water bath and vigorous stirring were required when injecting the
reactants, since a large amount of heat was released immediately after the reaction
started, and it needed being removed promptly. The reaction was quenched by water
and aqueous sodium hydroxide under ice-water bath. The unreacted lithium
aluminum was converted to hydrogen and alumina as a result of the reaction with
added water. Because large amount of H2 was released while quenching, adding water must be slow and dropwise to prevent undesired explosion. Step 3 was called
“Appel reaction”, which was an organic reaction used to convert an alcohol to an
alkyl halide using tetrahalomethane and triphenylphosphine as reactants. Beginning
with the bromination of triphenyl-phosphine, the alcohol was converted to alkoxide.
And the alkoxide immediately attacked the phosphorous, releasing the bromide
leaving group. As a nucleophilic substitution reaction (SN2), the bromine attacked the carbon stereocenter resulting in the final alkyl bromide product with inverted stereochemistry.29 Then the bromine group was further substituted by azide group in step 4. The mixture solvent of DMF and THF was used because THF can promote the solubility of reactants. Finally, the 3,4,5-tridodecyloxybenzylazide was reduced to 3,4,5-tridodecyloxybenzylamine to get the desired modification unit for PDIs via the similar mechanism in step 2. It was worthy to pay attention that the final modification unit 3,4,5-tridodecyloxybenzylamine may generate some impurities if it was exposed in the air for long time (more than 24 hours), as a result of the high reactivity of amino group. So it was recommended to characterize the product immediately after it was dried, and to start the imidization reaction with PDIs
37
promptly. The 1H NMR spectra in Figure 4.1 (a-e) indicated the correct synthesis of
Compound 1, 2, 3, 4 and 5 in chapter III. Every peak was assigned to corresponding protons. The integrated area of each peak was in excellent proportional to the number of its related protons. It can be seen that the peaks of protons in alkyl tails and benzene rings were in almost the same position (ppm) of this series of molecules.
However, the protons at “e” position (the two protons connected to the carbon next to benzene ring) shifted in different molecules. That is caused by the difference of electronegativity of different functional groups next to them. For high electronegative group like –OH, the electron density of the “e” position was less and the peak shifted to position with high ppm value. The 1H NMR spectra of Compound
6 and 7 were shown in Figure 4.2.
38
(a)
(b)
(c)
(d)
(e)
Figure 4.1. 1H NMR spectra of (a) methyl 3,4,5-tridodecyloxybenzoate, (b) 3,4,5-tridodecyloxybenzylic alcohol, (c) 3,4,5-tridodecyloxybenzylbromide, (d) 3,4,5-tridodecyloxybenzylazide, (e) 3,4,5-tridodecyloxybenzylamine
39
(a)
(b)
1 Figure 4.2. H NMR spectra of (a) 3,4,5-tritetradecyloxybenzylamine， (b) 3,4,5-tridecyloxybenzylamine
4.2 Synthetic Route of Intermediate Molecule: Perylene-3,4-Anhydride-9,10-
di-Decyloxycarbonyl
As mentioned above, the solubility plays an important role in the synthetic work.
Even if the reaction indeed produces the correct product, it is impossible to separate
the product from its by-products if the solubility is poor. That is the reason why we
cannot directly use perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) to
synthesize asymmetric PDI-based derivatives. However, perylene-3,4-anhydride-
9,10-di-decyloxycarbonyl (PADE) worked well as an intermediate. The synthetic route was summarized in Scheme 2.
40
Scheme 4.2. Synthetic route of PADE (1) KOH, H2O, 0.5h, 70℃;
(2) BrC10H21, Aliquat 336, KI; (3) PTSA, n-dodecane, toluene, 95 ℃
The synthesis of PADE started from coveting PTCDA to
perylene-3,4,9,10-tetradecyloxylcarbonyl (PTE) via a phase transfer catalysis method (step 1). The phase transfer catalysis used was tetra-octyl ammonium bromide (Aliquat 336). Firstly, the two anhydride groups in PTCDA were
hydrolyzed in aqueous potassium hydroxide solution under heating and stirring,
generating the perylene-tetracarboxylic acid. Both the tetrapotassium salt of
perylene-tetracarboxylic acid and the tetra-octyl ammonium dissociated from Aliquat
336 were dissolved in water, between which anion-cation pairs were formed. The
hydrophobic anion-cation pairs tended to transfer into the organic phase composed
of 1-bromodecane. The nuclear displacement reaction happened between
perylene-3,4,9,10-tetracarboxylic anions and 1-bromodecane, generating the product
PTE. PTE is easy to extract by chloroform, and then it was recrystallized in
methanol. No further column chromatography was needed.
In step 2, the PTE was dehydrated on one side to give PADE via an
acid-catalyzed reaction at a relative high temperature (~95 ℃). A unimolecular
41
cyclization reaction happened rapidly, giving a six-membered cyclic anhydride once
two carboxyl groups formed from a PTE molecule if they were at “right” positions.
Even if the initial ester bond cleavages did not occur at “right” positions, meaning
that two ester bond cleaved at opposite side, they could be “corrected” through the
esterification-hydrolysis equilibrium because six-member rings had excellent
stability, making them thermodynamically favored in the reaction. But once the
PADE was formed, the further dehydration of the other side must be avoided.
Otherwise, it would give the very beginning reactant PDCTA. It was a good method
to push PADE out of the reaction system immediately after its formation. A mixture
of n-dodecane and toluene was used as a perfect solvent to do that, because the
reactant PTE dissolved in this mixture solvent well but the product PADE would
precipitate out. So the chemical equilibrium was always pushed forward to give high
yield. The 1H NMR in Figure 4.3 indicated the correct synthesis. Every peak was
assigned to corresponding protons. The integrated area of each peak was in excellent
proportional to the number of related protons.
Figure 4.3. 1H NMR spectrum of PADE
42
4.3 Synthetic Route of Tetrahedral Perylene-based Derivatives
The synthetic route was summarized in Scheme 3.
R1=C12H25; R2=C10H21; R3=C14H29
Scheme 4.3. Synthetic route of tetrahedral PDIs (1) Imidazole, ODCB, 120℃; (2) PTSA, 100℃; (3) Imidazole, DMAP, ODCB, 127℃
Step 1 was an imidization reaction between Compound 11 (PADE) and
modification unit Compound 5 (3,4,5-tridodecyloxybenzylamine). Imidazole and
ODCB were favorable solvent for this kind of reaction because of their special
dissolving capacity for PDIs. And they can be readily washed away by methanol after reaction. Step 2 was the acid-catalyzed dehydration reaction. The reaction was similar as the reaction of the step 2 in scheme 2. It should make sure that the product of step 1 (also the reactant of step 2) was high degree of purity after being separated by silica gel column chromatography, since the solubility of product of step 2 decreased dramatically, being too low to purify via column chromatography or recrystallization. But pure product could be afforded just by filtering and washing with enough methanol if the reactant was high purity. The 1H NMR in Figure 4.4
43
indicated the correct synthesis of these two molecules. Every peak was assigned to
corresponding protons. It was obvious that all the peaks looked the same except the
disappearance of the peak at around 4.38 ppm in Figure 4.4-b, which means that the
two decyl groups in the Compound 10 were cleaved and left, forming anhydride
group when Compound 11 was given. Step 3 was another imidization reaction
between Compound 11 and the tetrahedral core (tetra(4-aminophenyl)methane).
Since the mobility of four amino groups in the tetrahedral core was rigid compared
to the compound 5, a higher temperature (130 ℃) and a catalyst DMAP were
needed for the reaction. After the reaction, alumina column chromatography was
used to afford the crude product firstly, because there was much unreacted insoluble
Compound 11 which would block the column if the smaller size silica gel was
directly used as matrix. However, the crude product of this step had good solubility,
so it can be purified via a subsequent silica gel column chromatography using a
mixture of chloroform and acetone (40:1, v/v) as eluent to afford pure product. The
1H NMR spectrum in Figure 4.4-c indicated the correct product (Compound 12).
Every peak was assigned to corresponding protons. The integrated area of each peak
was in excellent proportional to the number of its related protons. The broad and
overlapped peaks between 7.50-8.40 ppm represented the protons on conjugated
benzene rings in the center, as a result of the delocalization of the electrons in that
conjugated system. The 1H NMR spectra of Compound 13 and 14 were shown in
Figure 4.5. The more direct and convincing evidence of the successful synthesis of tetrahedral PDIs were given by the MOLDI-TOF spectra shown in Figure 4.6, it can
44
be seen that the experimental mass were almost exactly same as the calculated mass for all these three molecules Compound 12, 13 and 14.
(a)
(b)
(c)
Figure 4.4. 1H NMR spectra of (a) di-(decyloxycarbonyl)-perylene-1-(3,4,5)G12, (b) anhydride-perylene-1-(3,4,5)G12, (c) tetra-PDI-1-(3,4,5)G12
45
(a)
(b)
Figure 4.5. 1H NMR spectra of (a) tetra-PDI -1-(3,4,5)G10, (b) tetra-PDI-1-(3,4,5)G14
(a)
46
(b)
(c)
Figure 4.6. MOLDI-TOF mass spectra of (a) tetra-PDI-1-(3,4,5)G12, (b) tetra-PDI-1-(3,4,5)G10, (c) tetra-PDI-1-(3,4,5)G14
47
4.4 Synthetic Route of Cyano-substituted Perylene-based Derivatives
The synthetic mechanism of Compound 15,16,17,18 was similar as the step 3 in
the scheme 3. The correct synthesis of these four molecules was approved by the 1H
NMR spectra in Figure 4.7. The large peak at around 7.785 ppm in Figure 4.7-a and
Figure 4.7-b came from the four protons in the middle positions of the cyano-biphenyl group, and it became as the same size as the peak at 7.55 ppm in cyano-phenyl derivatives as shown in Figure 4.7-c and Figure 4.7-d. This change obviously indicated the difference number of benzene rings in these two kinds of molecules. The more direct and convincing evidence of the successful synthesis of cyano-substituted PDIs were given by the MOLDI-TOF spectra shown in Figure 4.8,
it can be seen that the experimental mass were almost exactly same as the calculated
mass for all these four molecules Compound 15, 16, 17 and 18.
Compound 15, R=C12H25
Compound 16, R=C14H29
Compound 17, R=C12H25
Compound 18, R=C14H29
Scheme 4.4. Synthetic route of cyano-substituted PDIs (1)&(2) Imidazole,
DMAP, ODCB, 127℃
48
(a)
(b)
(c)
(d)
1 Figure 4.7. H NMR spectra of (a) cyanobiphenyl-perylene-1-(3,4,5)G12, (b) cyanobiphenyl-perylene-1-(3,4,5)G14, (c) cyanophenyl-perylene-1-(3,4,5)G12, (d) cyanophenyl-perylene-1-(3,4,5)G14
49
(a)
(b)
(c)
50
(d)
Figure 4.8. MOLDI-TOF mass spectra of (a) cyanobiphenyl-perylene-1-(3,4,5)G12, (b) cyanobiphenyl-perylene-1-(3,4,5)G14, (c) cyanophenyl-perylene-1-(3,4,5)G12, (d) cyanophenyl-perylene-1-(3,4,5)G14
51
4.5 Thermal Analysis
The synthetic products were tested their thermal stability by thermogravimetric
analysis (TGA), and were analyzed their phase behavior by differential scanning
calorimetry (DSC) analysis
4.5.1Thermal Gravimetric Analysis (TGA)
The thermal gravimetric analysis measured the change of sample’s weight with
respect to the change of temperature at a fixed rate, from which the information of
thermal stability can be obtained. When the weight loss was greater than 5%, the
sample could be thought that it had already degraded.
All the molecules synthesized (Compound 12-18) were tested by TGA at a fixed
increasing temperature as 10 ℃/min from 30℃ to 500℃. Although they had
different modification units, all of them gave similar TGA curves as shown from
Figure 4.9 (a to g). When the temperature was below 320℃, the curve was almost plateau, indicating no degradation occurred. And the 5% weight loss temperature for
these seven molecules all happed at about 355℃. It meant that the molecules had
good thermal stability under the 320℃,and they would degrade at above 355 ℃.
The 355 ℃ can be regarded as the thermal decomposition temperature of the
perylene core, since no matter what kind of modification units was connected to
perylene, they all degrade in a narrow temperature range around it. And the upper
level scanning temperature of DSC should never exceed 320℃.
52
(a)
(b)
(c)
53
(d)
(e)
(f)
54
(g)
Figure 4.9. TGA curves of (a) tetra-PDI-1-(3,4,5)G12, (b) tetra-PDI-1-(3,4,5)G10,
(c) tetra-PDI-1-(3,4,5)G14, (d) cyanobiphenyl-perylene-1-(3,4,5)G12, (e) cyanobiphenyl-perylene-1-(3,4,5)G14, (f) cyanophenyl-perylene-1-(3,4,5)G12, (g) cyanophenyl-perylene-1-(3,4,5)G14
55
4.5.2 Differential Scanning Calorimetry (DSC) Analysis
Differential scanning calorimetry (DSC) measures the difference of the heat
required to maintain the same temperature between the sample and reference as a
function of temperature, from which the phase transition behavior can be viewed.
The tetrahedral PDIs sample (Compound 12-14) showed no significant peak on
the DSC curve, even no melting peak, indicating that the aggregate structure did not
have a remarkable change within the temperature range (-20℃ to 315℃). This may
be caused by restriction of the molecule motion as a result of the strong π-π
interactions in the system, since there were four PDIs in one molecule. The sample
would degrade before it melted, so there was no melting peak, which also implied
the strong intra- and intermolecular interactions in the system. It made it hard for
molecules to pack into order in bulk only with annealing, solvent-annealing could be
another choice to induce the ordered self-assembly structure.
The four cyano-substituted PDIs (Compound 15-18) showed phase transition
behaviorwhile the temperature increased, which was indicated by three sharp peaks
(Figure 4.10 a to d). The onset temperature and change of enthalpy of each peak was marked next to each peak. According to the DSC curves, the liquid crystal phase was possible to be induced by thermal annealing.
The three peaks occurred in different range, which meant the bulk molecules lost their order step by step. The peak in the low temperature range (25℃-70℃) was caused by the melting of alkyl tails, but the aromatic moiety was still frozen. The peak in the medium temperature range (120 ℃ -250 ℃ ) represented that the
56
cyanophenyl or cyanobiphenyl group lost their order, while the perylene core did not.
The peak at high temperature (> 280℃) was given by losing order of perylene core,
now the whole molecule became disordered. In other words, it melted. So this was
the melting peak of the molecule.
All of them had relative high melting temperature (> 280 ℃), this also
indicated the strong π-π interactions in PDIs. But the long alkyl chains indeed increased the molecular mobility. By comparing Figure 4.10-a with Figure 4.10-b (or
Figure 4.10-c with Figure 4.10-d), the molecules with tetradecyl alkyl tail (b and d) had lower melting temperature than those with dodecyl alkyl tail (a and c), meaning that longer alkyl chains provided better mobility for molecules to move with respect to each other and melt. Also, it can be seen that the cyanobiphenyl PDIs had lower melting temperature than cyanophenyl PDIs. The cyanobiphenyl PDIs had two possible stacking: a) only one benzene ring overlapped with adjacent molecule and b) two benzene rings overlapped with adjacent molecule. But cyanophenyl PDIs only had one possible stacking model. So the degree of order in cyanophenyl PDIs aggregate was better than that in the cyanobiphenyl PDIs, resulting in a stronger intermolecular interaction and higher melting temperature.
57
(a)
(b)
58
(c)
(d)
Figure 4.10. DSC curves of (a) cyanobiphenyl-perylene-1-(3,4,5)G12, (b) cyanobiphenyl-perylene-1-(3,4,5)G14, (c) cyanophenyl-perylene-1-(3,4,5)G12, (d) cyanophenyl-perylene-1-(3,4,5)G14
59
CHAPTER V
CONCLUSIONS
In conclusion, three tetrahedral perylenediimide-based molecules tethering with
different length of alkyl tails (decyl, dodecyl, tetradodecyl) and four
cyano-substituted PDIs were successfully synthesized and fully characterized. These
seven perylene-based self-assembly building blocks were synthesized via a
systematical and efficient method to get rid of the low solubility of perylene,
obtaining versatile asymmetric perylene-based derivatives. Their self-assembly
behavior could be tuned via the secondary intermolecular force introduced by alkyl
chain and functional groups. The chemical structures were approved by 1H NMR and
MOLDI-TOF mass spectrum. Their thermal behaviors were studied by TGA and
DSC. The TGA curves showed a general 355 ℃ degradation temperature of PDIs.
DSC results indicated that cyano-substituted PDIs had potential to form ordered
structure by thermal annealing and their aggregate lost order step by step with
increasing temperature. The determination of self-assembly structures is in progress.
The precise parameter of crystal unit cell and molecular packing model will be
defined according to the combined results of TEM, SAXS, WAXS and computer
simulation.
60
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