Self-Assembly of Facial Oligothiophene
Amphiphiles
Amphiphiles
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. ir. K. C. A. M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedingen op woensdag 05 maart 2014 om 10.00 uur door
Prof. Dr. J. H. van Esch
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. Dr. J. H. van Esch Technische Universiteit Delft, promotor Prof. Dr. H. Frauenrath École Polytechnique Fédérale de Lausanne Prof. Dr. A. del Guerzo Université Bordeaux 1
Prof. Dr. E. J. R. Sudholter Technische Universiteit Delft Prof. Dr. S. J. Picken Technische Universiteit Delft
Dr. A. H. de Vries Rijksuniversiteit Groningen
Dr. R. Eelkema Technische Universiteit Delft
Prof. Dr. ir. M. T. Kreutzer Technische Universiteit Delft, reservelid
The work described in this thesis was carried out in the Advanced Soft Matter (ASM) group at Delft University of Technology, the Faculty of Applied Sciences, the Dapartment of Chemical Engineer-ing (Chem-E) and was funded by the Netherlands Organization of Scientific Research (NWO).
© Dainius JANELIUNAS, 2014
ISBN 978-90-8891-000-5
All rights reserved. The author encourages the communication of scientific contents and explicitly allows reproduction for scientific purposes, provided the proper citation of the source. Parts of the thesis have been published in scientific journals and copyright is subject to different terms and con-ditions.
Chapter 1
1 Introduction 1
1.1 Self-assembly as a tool from nature 2
1.2 Self-assembling π-conjugated compounds in water 2
1.3 Self-assembling dynamic covalent π-conjugated polymers in water 3
1.4 Close look at self-assembly: molecular dynamics in silico 4
1.5 Scope and outline of the thesis 4
1.6 References 6
Chapter 2
2 Self-assembling π-conjugated compounds in aqueous systems: structure and applications 9
2.1 Introduction 10
2.2 Oligomeric and polymeric π-conjugated systems 10
2.2.1 General structure of π-conjugated materials 11
2.2.2 What drives the self-assembly of π-conjugated compounds? 12
2.3 Variety of π-conjugated systems in water 13
2.3.1 Making a water soluble π-conjugated systems 13
2.3.2 Water soluble π-conjugated polymers 15
2.3.3 π-Conjugated foldamers in water 17
2.3.4 Self-assembling π-conjugated amphiphiles 19
2.4 Thiophene derivatives for aqueous systems 22
2.4.1 Water soluble polythiophenes 22
2.4.2 Self-assembling thiophene low molecular weight amphiphiles 25
2.5 Conclusions 27
2.6 References 28
Chapter 3
3 Designing new symmetrical facial oligothiophene amphiphiles 31
3.1 Introduction 32
3.2 Results and discussion 33
3.2.1 Design of new thiophene facial amphiphiles 33
3.2.2 Synthesis of symmetrical thiophenes 35
3.2.3 Aggregation behaviour in water 36
3.2.4 Phase behaviour in water 37
3.2.5 Morphology of aggregates 38
3.2.6 Photo-physical properties 40
3.2.7 Backbone configuration studies 41
3.3 Conclusions 43 3.4 Experimental section 44 3.4.1 General Remarks 44 3.4.1 Synthesis 45 3.5 References 47 Appendix A 48
4.1 Introduction 54
4.2 Design of the dynamic polymers 55
4.3 Results and discussion 55
4.4 Conclusions 59 4.5 Experimental section 60 4.5.1 General remarks 60 4.5.2 Synthesis 60 4.6 References 61 Appendix B 62 Chapter 5 5 Coarse-grained Martini model for facial thiophene amphiphiles 67 5.1 Introduction 68 5.2 Model and methods 69
5.2.1 The design of new mapping scheme 69 5.2.2 Non-bonded interactions 71 5.2.3 Bonded interactions 72 5.2.4 Methods and simulation parameters 73 5.3 Results and discussion 75 5.3.1 Interaction levels and free energies of solvation 75
5.3.2 Parameterization of bonded interactions 77 5.3.3 Coarse-grained terthiophene behaviour in water 80
5.4 Limitations 81
5.5 Conclusions 82 5.6 Reference 82
Appendix C 84 Chapter 6 6 Self-assembly of facial thiophene amphiphiles in water: molecular dynamics with coarse-grained Martini model 85 6.1 Introduction 86 6.2 Model and methods 87
6.3 Results and discussion 89 6.3.1 Aggregation in water 89
6.3.2 Surface activity 91 6.3.3 Dihedral angles and curvature 93
6.3.4 Aggregate morphology in water 97
6.3.5 Incorporation into phospholipid bilayer 104
6.4 Outlook and limitations 106
6.4.1 Self-assembly of symmetrical facial thiophene amphiphiles in water 106
6.4.2 Limitations 109 6.5 Conclusions 109 6.6 References 110 Summary 113 Samenvatting 117 Acknowledgements 121
About the author 123
List of publications 123
1.1 Self-assembly as a tool from nature
Self-assembly is a process by which disordered building blocks form an ordered structure through a spontaneous and autonomous organization.1 Nature delivers a broad variety of functional
molecular nanostructures that are built by this process. Such systems play abundant roles from ca-talysis and transport to compartmentalization and hierarchical structure control in living organisms.2
Hence, scientists are inspired to investigate and mimic natural assemblies, and design and develop new functional synthetic self-assembling systems for novel application fields. There have been many breakthroughs in the field of self-assembly over the past decades. The current knowledge paved the way to understand, up to a great extent, the behaviour of this marvelous natural process and enabled the design of synthetic molecular building blocks with predictable properties and self-assembly behaviour.3-7 Control over shape and nature of these entities gives a possibility to develop and manipulate architectures like foldamers, hydrogels, micelles and vesicles.8-10 In this way, excit-ing functional systems are developed, which can find application in fields like drug delivery10-12 or regenerative medicine13, 14. The possibility to assemble building blocks via a bottom-up approach, allowing easy modification, also made self-assembly beneficial for the electronics industry.15 Here,
the major building blocks in self-assembly are solution-processable π-conjugated organic materi-als.16 This process, for instance, allows the controlled formation of structures that are smaller than
any lithographic technique can provide in the semi-conductor industry,17 as well as flexible devices
and printing of electronic components.
1.2 Self-assembling π-conjugated compounds in water
π-Conjugated molecules are successfully used in energy, information and other technologies for several decades now.18 Water-soluble or amphiphilic π-conjugated derivatives expand the
con-ventional application fields to biological systems. Such compounds are mainly used to develop chemical sensors for biological analytes.19-21 Here, self-assembly plays a major role as well. The
formation of, and control over, supramolecular structures is crucial for the construction of highly efficient electronic systems based on π-conjugated organic molecules.22-32 The formation is driven
by reversible, non-covalent interactions between individual molecules, such as ionic or hydrophobic interactions, hydrogen bonds and π-π stacking. Control over the morphology of the assembled structures is typically achieved by tuning the shape of the molecular building blocks. For instance, the introduction of specific hydrophilic-hydrophobic substitution pattern on a π-conjugated back-bone yields self-assembling amphiphilic molecules, where the structure of the resulting supramo-lecular aggregates is determined by the structure-shape concept.33 The dynamicity and ease of
building block exchange make self-assembly of π-conjugated compounds a generic and efficient tool for the development of new functional systems.
This research is focused on the self-assembly of π-conjugated facial thiophene amphiphiles. These heterocycles are highly promising π-conjugated building blocks because: (i) their photo-physical properties can be easily adjusted by substituting the different positions along the covalently conjugated backbone; and (ii) it is possible to include various functional groups in these molecules for later reactions and interactions with other objects such as surfaces, other polymers, or biomole-cules.34 Introduction of hydrophilic side chains to the α position of the hydrophobic thiophene back-bones yield head/tail amphiphiles, while the substitution of the β positions with the same groups gives facial thiophene amphiphiles. Further tailoring of these derivatives can yield ordered electron-ic systems in aqueous media by solution-processing techniques, suitable for biosensors and optoe-lectronics devices.35-41 We reported extensive studies on substituted oligothiophene facial am-phiphiles. These compounds are able to form well defined structures in water.42 As an additional
structural motif, the curvature of oligothiophene amphiphiles can be implemented by manipulating the nature and position of the side groups on the oligothiophene backbone.43 It was found that these derivatives can be successfully applied in self-assembling antenna systems44, cell membrane imag-ing and efficient charge-transport platforms in water45. A deeper understanding of their self-assembling processes and shape dynamics in water is of great importance. However, the complicat-ed synthesis of studicomplicat-ed amphiphiles hampers their research and application possibilities. This moti-vates to look for easily accessible new facial oligothiophene amphiphiles, as will be described in Chapter 3.
1.3 Self-assembling dynamic covalent π-conjugated polymers in water
Besides typical application in optoelectronic devices, conventional π-conjugated polymers in aqueous environment found use in sensors and bio-medical imaging46, pH47 and chemical48 de-tectors. Despite the variety of optical properties and efficient charge mobility in these materials,49 their features can be rather static and any tuning requires laborious synthetic procedures.50 Then,
aqueous supramolecular π-conjugated polymers are considered as candidates for organic transistors, photovoltaic cells and light-emitting devices.30, 31 In contrast to their ease of formation by
self-assembly and highly dynamic and responsive nature,51 these materials exhibit heavily inferior
ously has been used for the fabrication of reversible π-conjugated macromolecules, albeit mostly in organic solvents, not in water.55-57 This stems from the fact that imine-derived dynamic bonds, which can offer efficient π-conjugation, are usually not stable in water.58 However, we recently showed that it is possible to stabilize them through self-assembly in supramolecular aggregates.59, 60 With this knowledge we set out to develop new self-assembling dynamic covalent π-conjugated polymers in water, as will be described in Chapter 4.
1.4 Close look at self-assembly: molecular dynamics in silico
The Nobel Prize 2013 in Chemistry left no doubt that the development and application of computational models for the molecular dynamics of complex chemical systems is an important and rewarding research field. Computational simulations have been proven to be a powerful tool to in-terpret and predict research data, and to describe processes that cannot easily be studied experimen-tally.61-64 All-atom and ab initio modeling techniques are mature, but still rapidly growing research fields.65-73 However, self-assembly of molecular systems into supramolecular structures is usually a too large and too complex process for these simulation methods. Fortunately, coarse-grained mod-els74, where the systems are simplified by grouping atoms into fewer interaction sites, are powerful
enough to capture the size and time scales of large and complex self-assembly events. We are par-ticularly interested in a coarse-grained Martini modeling force field, developed by Marrink et al.75, 76 This flexible model was successfully applied to investigate molecular dynamics of spontaneous
organization in biomolecule systems, bilayers, lipids, surfactants and even precise positioning of functional elements on graphite by self-assembly.75, 77, 78 Unfortunately, application of the Martini
force field to describe the assembly of π-conjugated compounds is nearly absent from the litera-ture.79-81 This motivated us to develop a coarse-grained Martini force field for facial oligothiophene amphiphiles. In Chapter 5 we describe a robust model that is easy to use and preserves the position of side chains on the π-conjugated thiophene backbones, allowing the description of the backbone curvature in the simulation. In the last chapter 6, it is shown to be a powerful tool to observe supra-molecular organization of variously substituted oligothiophenes and their interactions with already existing Martini self-assembling systems at the molecular level.
1.5 Scope and outline of the thesis
The aim of this work is to develop and understand the self-assembling processes of oligothi-ophene amphiphiles in water. The second chapter of this thesis is a review of the most common π-conjugated platforms for aqueous systems. First the basic structural design and self-assembly
prin-ciples are discussed. Later, several examples of such systems and their application in optoelectronic devices, chemical and biological sensors are described. The chapter ends with the introduction to self-assembling thiophenes, as potential candidates to construct well-defined architectures in water for the development of smart functional materials. The third chapter is about spontaneous formation of dynamic imine bonds between amphiphilic thiophene aldehydes and various aromatic bis-amines in water. Strikingly, the generally unfavorable imine formation in aqueous media here is enhanced by the aggregation of oligothiophene building blocks. The study represents a new and easy entry into fully π-conjugated, water-soluble responsive polymers. In the fourth chapter we of-fer a straightforward method to construct conjugated thiophene amphiphiles. The developed design allows the late-stage introduction of hydrophilic groups, aiding both purification and ease of struc-ture variation and yielding new symmetrical oligothiophene facial amphiphiles. Although, with re-spect to regioregular thiophene amphiphiles, the same ratio of hydrophilic and hydrophobic substit-uents is used in the new substitution pattern, a drastic decrease of the critical aggregation concentra-tion and increase of the aggregate size is observed. This behaviour will be characterized using sev-eral techniques. The photophysical properties will also be compared with the regioregular ana-logues. The last two chapters are dedicated to the coarse-grained molecular dynamics model devel-opment and self-assembly studies of various facial oligothiophene amphiphiles in water. We will establish a coarse-grain mapping scheme to preserve the substitution pattern of the thiophene heter-ocycle rings. Then a new Martini coarse-grained force field for the facial thiophene amphiphiles will be parameterized and validated. The developed model will be proven powerful enough to cap-ture the dynamics of the structural evolution in water at the molecular level. Moreover, the mor-phologies of the formed aggregates will be reproduced quite well. We will successfully apply this model to investigate the behaviour of facial thiophene amphiphiles at the air-water interface, in bulk solution and in a synthetic bilayer.
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2 Self-assembling π-conjugated compounds in aqueous
systems: structure and applications
2.1 Introduction
During the past few decades interdisciplinary studies of π-conjugated derivatives have led to a better fundamental understanding of their electronic, optoelectronic and photonic proper-ties. It enabled their application in energy, information and other technologies.1 To name an
ex-ample, the combination of an efficient solar light absorption and good charge separation and transport in organic π-conjugated thin films is promoting the development of cheap, large-area photovoltaic devices.2-7 Development of printable and flexible thin-film electronics8 is based on
high-mobility charge transport properties in organic π-conjugated molecules as well.9-12 Efficient
exciton luminescence of these compounds found application in large-surface organic light emit-ting diodes (OLEDs), color displays and solid-state lighemit-ting.13-16 Recently, great attention is paid
to water soluble and self-assembling π-conjugated materials. Compatibility with aqueous media expands the previously mentioned, conventional application fields in electronics to biological systems. Water soluble π-conjugated semiconducting and conducting polymers are used in de-veloping chemical sensors for biological analytes, such as viruses, proteins and DNAs, as well as various chemo-sensors.17-19 It is important to note, small molecules and oligomers can be re-ferred to as electronic materials when featuring a supramolecular ordering resulting in the ability to π-π interact. Here, as for conjugated polymers, the structural order, interchain interactions, molecular order and finally the resulting packing of the molecules influence charge transport and separation, as well as optical properties.11, 20 However, small molecules and oligomers typically
have more defined chemical structure than polymers. For this reason the investigation of self-assembling behaviour of non-polymeric π-conjugated molecules in organic solvents and espe-cially water21 is of great importance.
This chapter is a review of the basic structural design and self-assembly of π-conjugated molecules in water. The most common π-conjugated platforms and basic self-assembling princi-ples are discussed first. The second part of the review is about π-conjugates in aqueous media. It provides a basic knowledge about making these platforms compatible with water, self-assembling behaviour and application. The review ends with aqueous π-conjugated thiophene based polymers and oligomers, the materials that were the main target of our research.
2.2 Oligomeric and polymeric π-conjugated systems
A conjugated system is a system where π-orbitals with delocalized electrons are connect-ed via alternating single and multiple bonds, typically lowering the overall energy of the
mole-cule. The structure of the π-conjugated backbone can be linear, cyclic (aromatic or non-aromatic) and mixed. The shape, solubility, photo-physical and electronic properties can be tuned by intro-ducing various substituents to it. Here a few conventional aromatic platforms and generic self-assembly principles will be discussed.
2.2.1 General structure of π-conjugated materials
A broad variety of molecular designs for π-conjugated small molecules, oligomers and polymers were developed for the studies focused on organic synthesis, science and application of organic semiconductors. A detailed review is beyond a scope of this study, so only few classes are presented here (Figure 2.1).22-26
Figure 2.1. Parent structures of several conjugated small molecules, oligomers and polymers. [Reproduced
from ref. 32].
The performance and application possibilities of these systems are determined by three factors: (i) chemical structure, (ii) purity and (iii) supramolecular organization. Very good syn-thetic control and tunability of optical and redox properties, and electronic band and molecular structure of π-conjugates is now available.27-31 Decrease of impurities and irregularities, that can
interrupt conjugation, cause photo-damage or act as a trap, has a positive effect on the stability of these backbones and the photophysical performance of these materials. Conductivity due to charge transfer is an important property for the application posibilities of such systems. This fea-ture strongly depends on the supramolecular organization of the conjugated backbones. For this reason control over morphology at every structural hierarchy level is very important for both small and polymeric π-conjugated systems. The ability to form “microcrystalline” phase typical-ly lead to enhanced electronical properties of an entire system. To gain control over this,
solubil-blue-shifts. In the first case highly ordered π-stacked aggregates are formed, which is impossible in the latter scenario. A major concern in current research is the effect of chemical composition of π-conjugates on their supramolecular organization.32
These findings can be illustrated with the properties of the most popular semiconducting polymer, poly(3-hexylthiophene). The regioregular head-to-tail polymers are able to form lamel-lar arrays of rods, exhibiting very high charge mobilities (0.01-0.1 cm2/V s). In contrast, head-to-head or tail-to-tail analogues do not show defined inter-chain interactions, thus the charge carrier mobilities are typically low (10-5 cm2/V s). The molecular weight has a significent effect on how
these molecules are packed. Low molecular weight regioregular thiophenes show very well de-fined rodlike crystalline structures, which contain continuous pathways for the charge carriers, giving up to four orders of magnitude higher mobilities than polymers. In contrast, the large do-mains of amorphous high molecular weight polythiophenes typically lead to decreased conduc-tivity. For this reason various approaches, which may lead to a higher ordering of these π-conjugates are investigated.32
2.2.2 What drives the self-assembly of π-conjugated compounds?
The self-assembly of π-conjugates can be roughly explained by the main principles of supramolecular chemistry. It is a chemistry based on non-covalent bonds.33 Compared to cova-lent bonds, the non-covacova-lent interactions are generally weaker.32 The most important interactions
for self-assembly of π-conjugated compounds in water are hydrogen bond and π-π stacking in-teractions. A hydrogen bond is formed between an acidic hydrogen donor and an acceptor bear-ing a lone nonbondbear-ing electron pair. These bonds are perfect secondary interactions for the con-struction of supramolecular structures because they are very selective and directional. The strength depends on the polarity of the environment, position and number of the interacting sites. π-π stacking interactions strongly depend on the solvent as well. In non-aqueous media, this in-teraction is typically weaker than in water. The inin-teractions between the solvent molecules here are weaker, so solvophobic forces play a minor role. However, in water they are driven by a hy-drophobic effect, due to higher energy of water molecules solvating the aromatic surface than bulk water. Here these interactions are typically strong.32 More details about self-assembly of π-conjugated molecules have been extensively reported by Meijer, Stupp and Aida et al.32, 34-41
2.3 Variety of π-conjugated systems in water
It is known that due to the strongly hydrophobic nature of π-conjugated segments and π-π stacking interaction of π-planes, π-conjugated compounds can be poorly soluble in water. To in-crease the solubility, these structures are augmented with hydrophilic substituents. The amount and positions of such substituents on the π-conjugated segments influence the behaviour in water of an entire molecule. The π-conjugated compounds are designated as water-soluble when hy-drophilic and hydrophobic substituents are not separated from each other in clear domains.42, 43
In contrast, amphiphilic π-conjugates have a clear separation of hydrophobic and hydrophilic domains in the structure.44 The molecular structure, self-assembling behaviour and application of
such systems will be discussed in this part of the review.
2.3.1 Making a water soluble π-conjugated systems
To introduce solubility in water to conventional conjugated compounds, a widely used approach is to attach hydrophilic side groups to a hydrophobic π-conjugated part. The most commonly used are ionic sulfonate, ammonium, carboxylate or phosphate substituents. Moreo-ver, non-ionic, high polarity, side groups may be introduced instead of ionic systems. Most re-current are hydroxyl and ethylene glycol45 or sugar-based46 substituents. Ionic systems require
careful tuning of the pH of the solution to prevent clustering and phase separation of the mole-cules. Also, non-specific electrostatic interactions can overwhelm the specificity of any other interactions important for biosensors.47 The involved ions can migrate to the active layers of
semiconducting devices and reduce their long-term stability.48 Non-ionic systems do not suffer
from these drawbacks.
Substances containing clearly distinguished hydrophilic and hydrophobic segments are referred to as amphiphiles or surfactants.49, 50 These compounds can interact with the interfaces between hydrophilic and hydrophobic domains. In an absence of such an interface, the hydro-phobic parts of amphiphiles can interact with each other and assemble in water, forming a wide variety of architectures.51 Such behaviour makes surfactants very important in everyday life
ful-filling many different functions: from being the major building blocks of cell membranes, to de-tergents, cosmetics and drug delivery platforms. To extend these common surfactants with the electronic and optical properties of conjugated systems, great attention has been paid to
π-signs of amphiphiles: head/tail amphiphiles and facial amphiphiles.51 The first one is when the
hydrophilic and hydrophobic groups are separated by the shorter axis of the molecule (Figure 2.2a). Such derivatives adopt cone or cylindrical structures in water. The self-assembly occurs when the hydrophobic parts are brought together and extends in the direction perpendicular to the normal of the surface. The morphology of the aggregates formed generaly follows the struc-ture-shape concept (Figure 2.2c).52
Figure 2.2. (a) Schematic representation of head to tail surfactants; (b) Structure of facial surfactants; (c)
Structure-shape concept. [Reproduced from ref. 52].
In facial amphiphiles, instead of a short molecular axis, the long one separates the philic and hydrophobic groups (Figure 2.2b). Such structures posses a clearly distinct hydro-philic and hydrophobic faces. The hydrophobic moities in these structures are almost fully ex-posed at one side face of the molecule. To completely cover the hydrophobic domains from the solvents, these amphiphiles also interact with each other’s hydrophobic parts. Thus self-assembly of these amphiphiles is self-complememntary.53 In general, these principles can be successfully applied in designing self-assembling head/tail or facial π-conjugated amphiphilic systems in water.
Most popular π-conjugated platforms for aqueous systems are thiophenes, fluorenes, phenylenes and phenylene vinylenes. However, BODIPY dyes, porphyrines, N-fluorenylmethoxycarbonyl diphenylalanines, perylene bisimides, and pyrenes are also actively investigated.32, 42, 54 These hydrophobic building blocks are usually substituted with hydrophilic
side groups, such as ethylene glycols, carbohydrates or various ionic phosphate or ammonium derivatives. Some water-soluble polymers and self-assembling π-conjugated examples will be
Hydrophilic Hydrophobic
a) b) c)
discussed in this part. The thiophene derivatives for aqueous systems, which are the main topic of our research, will be discussed in section 2.4.
2.3.2 Water soluble π-conjugated polymers
During the past decade water-soluble π-conjugated polymers found potential application in optoelectronic devices, chemical and biological sensors.18, 19, 55, 56 One of the first classes of water-soluble π-conjugated polymers to appear were poly(phenylene vinylenes)57, 58,
poly(p-phenylenes)59, 60 and poly(phenylene ethylenes).61-63 The most recent and intense development is
in aqueous π-conjugated polyfluorene derivatives.64-67 Wang et al. showed that a good solubility
of hydrophobic conjugated fluorene backbone in water can be achieved by introducing a high number of a charged substituents to it (Figure 2.3).68
Figure 3. (a) The structure of fluorene-based water soluble polymer; (b) Appearance (left) and emission
(right) of aqueous polymer solutions of different concentrations: a) dilute solution (1 × 10−3 mg · mL−1), b) 50 mg · mL−1, c) 100 mg · mL−1. The irradiation was 365 nm light at room temperature. [Reproduced from ref. 68].
Cationic tetraalkylammonium residues were used as water-soluble groups here. The re-sulting polymer was highly soluble in water and exhibited high fluorescence quantum yield. The performance of light-emitting diodes and biosensors can be improved with this compound due to an excellent solubility, supramolecular organization and electron injection ability.
A highly selective and sensitive sensor for cytochrome c, an important compound in the mitochondrial respiratory chain, was introduced by Wang et al.69 It is based on a
fluorene-phenylene conjugated backbone with anionic water-soluble phosphate side chains (Figure 2.4).
a) b)
Figure 2.4. (a) Chemical structure of water-soluble fluorene-phenylene polymers; (b) Schematic
representa-tion of the carepresenta-tionic analytes sensor based on fluorescence quenching of anionic PFHPNa and PFPPNa. [Re-produced from ref. 69].
It is important to note, that here, as well as for other charged water-soluble conjugated polymers (polyelectrolytes), biosensing can occur upon binding to an oppositely charged biolo-gocal analyte molecule.
Xue et al.70 designed and synthesized a polyfluorene derivative, where instead of charged
groups, ethylene oxide and carbohydrate residues are used to enhance the compatibility with aqueous media and live cells (Figure 2.5).
Figure 2.5. (a) Polymer structures. (b) Fluorescent microscopy images of fluorescent glycopolymer-stained E.
coli bacteria clusters with 109 and 106 bacterial cells (top and middle), respectively, and fluorescent glyco-polymer-stained E. coli bacterial cells of the ORN178 strain (bottom). The scale bar is 10 µm. [Reproduced from ref. 70].
Here a post-polymerization functionalization approach was used to introduce a biocom-patible glycoside-tethered group to a water-soluble ethylene glycol spacer. The specific interac-tions of this neutral polymer led to a successful application for the detection of an Escherichia Coli bacteria.
Water-soluble polymers are attractive candidates to improve the efficiency of organic electronic devices. These can now be constructed using multilayer complex films, by alternating
a) b)
a) b)
a) b)
conventional oil-soluble polymers with the water-soluble ones.71-73 Such films are used as active
layers of the devices, which can be cast from environmentally friendly solvents like water or al-cohols. An example could be a highly efficient polymeric light-emitting diode, which was made using alcohol or water soluble conjugated polymers as emitting and/or electron injecting layers.73
2.3.3 π-Conjugated foldamers in water
Folding is usually utilized by nature to organize molecules into ordered structures. The best example is the folding of one or more linear polypeptide chains, leading to the formation of a functional protein. Natural π-conjugated light harvesting complexes, such as chlorophyll dyes depend on rigidifying protein frameworks as well. The foldamer concept is one of the most promising strategies to mimic this natural phenomenon.74 For this reason this class of
com-pounds is reviewed separately here.
An oligomer, that is able to fold into a conformationally ordered state in solution, is called a foldamer. This secondary structure is stabilized by non-covalent interactions between non-adjecent repeating units: π-stacking between π-planes, H-bonding, solvophobic forces or metal coordination. Moore et al. provided a detailed in-depth study about this fascinating class of molecules.75 Here we focus only on the entities with extended π-conjugation, where π-π stacking and hydrophobic-hydrophilic effects due to amphiphilicity drive the folding of the backbone.
The most popular π-conjugated foldamers in aqueous media are meta-phenylene substi-tuted ethylene based derivatives. The folding of such systems is very well defined and explained now.76-79 The meta-substituted phenyl has an angle of 120° between the ethinyl substituents,
meaning that to create one turn, six monomers are needed. To increase the water solubility of these hydrophobic π-conjugated backbones, amphiphilicity is introduced by substituting the third meta-position with conventional hydrophilic groups, such as tetra ethylene glycol chains.
Iverson et al. have reported a variety of foldamers, where π-conjugated naphthalene de-rivatives are linked by L-aspartic acid spacers, which promote the solubility in water.80, 81
Vari-ous studies confirmed that designed molecule F1 forms well defined pleated structures in water, where the aromatic rings are stacked on each other. Interestingly, the electron donating naphtha-lene is always stacking on the electron withdrawing naphthanaphtha-lene bis-imide (Figure 2.6).
Figure 2.6. (a) Chemical structures of foldamers F1, F2 and F3. (b) Schematic representation of the polymer
18 folded structure. [Reproduced from ref. 80].
Strikingly, a lysine linkage in compound F2 enhanced electrostatic interactions with DNA. The ability to integrate into a major groove of DNA in a cooperative fashion shows the compatibility of these derivatives with various biological systems. Derivative F3, possessing hy-drophilic aspartic units lines on the one side and hydrophobic leucine units on the other, was synthesized to mimic the leucine scissor motif. These motifs are participating in gene expression in eukaryotic and prokaryotic cells, as a DNA binding domain. Strikingly, the derivative exhibit-ed a thermal behaviour similar to triple-helix collagen. Upon heating an aqueous solution of F3 above 80° C, the structure unfolds into an entangled conformation. The same conformational changes can be provoked by addition of the preformed aggregate to the solution of oligomer. These transitions are followed by a vast color change - from dark red to colorless. Such phenom-ena can be used in various thermal sensors.
DNA-assisted self-assembly of pyrene foldamers was reported by Malinovskii et al.82
Double-stranded helical structures were formed from bi- and tri-segmental oligomers, composed of nucleotides and achiral pyrene monomers (Figure 2.7).
The domains of hydrophobic π-conjugated pyrenes are substituted with oligodeoxynucle-otides, bearing hydrophilic phosphate groups. This gives a possibility to use the oligomers in aqueous media, which favors the folding of the aromatic units. There is a distinct chain-length dependence in this system. The helical structure forms only in extended stretches of more than twelve pyrene units. The DNA drives the right-handed twisting of pyrenes and the formation of a double helix
a) b)
F1
F2
Figure 2.7. The helical arrangement of achiral pyrene molecules in tri- and bi-segmental DNA/pyrene
chime-ras (top and bottom, respectively). The different colors represent the inter-strand stacked pyrene units of the two strands. [Reproduced from ref. 82].
Moreover, the presented compounds turned out to be dynamic foldamers, responsive to external parameters and stimuli, like solvent, temperature and chirality. Such function can be successfully harvested in various sensors and detectors.
2.3.4 Self-assembling π-conjugated amphiphiles
π-Conjugated systems with hydrophilic (and aliphatic) substituents can be used to form nano-architectures in water. They are formed through the stacking of aromatic planes and hydro-phobic-hydrophilic interactions.
Lee et al extensively investigated amphiphilic oligophenylenes during the past decade. They have successfully designed and synthesized novel facial π-conjugated amphiphiles, which were able to form tunable nanostructures in water. The compounds turned to form nanofibers that act like liquid crystals and micelles (Figure 2.8) or multilayer vesicles and gels.83-85 Such molecules, due to their ability to form a well-defined supramolecular self-assemblies, are inter-esting for biological application.86
Figure 2.8. (a) Chemical structures of the studied oligophenylenes. (b) Stimulus-responsive sol–gel phase
transition of self-assembled supramolecular nanofibers of the 1a T-shaped aromatic amphiphilic dendrone. [Reproduced from ref. 83].
Wurthner et al.87, 88 reported amphiphilic perylene bisimides, where the nano-aggregate morphology was directed by the shape control of the amphiphile itself as well (Figure 2.9).
Figure 2.9. Formation of Micelles from Wedge-Shaped PBI1 (top), Bilayer Vesicles from the
Co-self-assembly of PBI1 and Dumbbell-Shaped PBI3 (middle), and Rod Aggregates from Dumbbell-Shaped PBI4 (bottom). [Reproduced from ref. 87].
a) b)
1a
1b
2b
The self-assembly of different ratios of triangular prism and dumbbell perylene bisimides led to the formation of hollow vesicles or rod aggregates. These shape and size differences can be rationalized by considering a spontaneous curvature. Once triangular prism shaped PBI1 co-self assembles with dumbbell shaped PBI3, the average hydrophobic part volume increases. At this point, the hydrophilic-hydrophobic interface changes from curved to a more flat one. How-ever, the decrease of the spontaneous curvature releases the strain in the formed highly curved micelle, it starts to grow and eventually transforms into a vesicle. Such curvature control of spontaneous self-assembly is present in biology, for instance, the deformation of flat lipid mem-branes with integral proteins.89, 90
Maeda et al.91 designed amphiphilic π-conjugated acyclic oligopyrroles and investigated
the solvent-assisted aggregation. The compounds are based on the BF2 complexes of
1,3-dipyrrolylpropane-1,3-diones. To increase the solubility in water, aryl rings are substituted with water-soluble ethylene glycol chains (Figure 2.10a).
Figure 2.10. (a) Chemical structures of amphiphilic acyclic oligopyrroles; (b) Possible proposed transition
pathway (for 2 c) from H-aggregates to J-aggregates through removal of water molecules, and (b) slipped stacking structures of 2a in the solid state. [Reproduced from ref. 91].
It turned out that depending on the peripheral substituents, the molecules can form nano-networks or vesicles in water. UV/Vis and fluorescence spectroscopy confirmed the formation of H-aggregates. These aggregates can be disassembled by the introduction of miscible solvents such as alcohols. Freeze-drying and evaporating water caused the transformation from H- to J- aggregates. Even after this procedure, the architectures remained stable and could bind anions (Figure 2.10b). Strikingly, extraction with dichloromethane led to new, metastable assembling modes. These soft materials are unique and versatile because they can form highly organized, stimuli responsive structures with various interesting properties like exciton and carrier transport.
a) b)
properties are highly influenced by the self-assembling processes. This phenomenon can be used for various sensoric and organic electronic applications.
2.4 Thiophene derivatives for aqueous systems
Thiophenes are among the most widely used π-conjugated materials. These compounds, including the widely applied poly-3-hexylthiophene, are excellent candidates for application in molecular electronic devices, like light emitting diodes, field effect transistors and photovoltaic devices due to their chemical stability in various redox states and good electronic and charge transport properties.92, 93 α-Linked thiophenes are widely used as model compounds or starting
monomers for the preparation of polymers due to their defined chemical structure.92-94 The
search for soluble materials has led to the development of α-linked thiophenes with solubilizing groups on their β-positions.95-101 During the past decade various water-soluble and amphiphilic thiophene derivatives have been designed and fabricated. Introduction of water-soluble side chains to the α position of the hydrophobic backbones yields head/tail amphiphiles, while to the β positions - facial thiophene amphiphiles. The most interesting studies are reviewed here.
2.4.1 Water soluble polythiophenes
To apply the distinctive optical and electronic features of polythiophenes in aqueous me-dia102, great advances in the fabrication of water soluble polythiophenes has been made during
the past decades. These compounds, possessing cationic side chains, have been used for the de-tection of human thrombin, nucleic acid and DNA hybridization events.103-105
Specific protein aggregate staining is the most common application for water-soluble ion-ic polythiophenes (Figure 2.11).106-108
Figure 2.11. Chemical structures of conventional water-soluble polythiophenes.
Typically, the fluorescence emission of the thiophene backbone is changed, when rotational freedom and the geometry of thiophene heterocycles are restricted upon binding with a protein aggregate (Figure 2.12). Conventional sterically rigid dyes, like thioflavin T or Congo red do not have this unique property.
Figure 2.12. Biosensing schemes for the detection of DNA-hybridization using water soluble luminescent
polythiophenes. Detection using superquenching or energy transfer (left) or detection based on conformation-al transitions of the luminescent conjugated polyelectrolytes (LCP) chains (right). [Reproduced from ref. 109].
For this reason mentioned thiophenes offer a specific spectroscopic signature for individual pro-tein aggregates. Moreover, poly(thiophene acetic acid) shows a change in its emission wave-length, when bound to protein deposits associated with a prion strain, a specific infectious agent composed of protein in a misfolded form.109 Again, this is an unique feature of the flexible
thio-phene backbone that gives an opportunity to correlate a distinct prion strain to a protein aggre-gate via optical fingerprint.
Along with ionic water-soluble polythiophenes, non-ionic ones are also extensively in-vestigated. Highly-water soluble and thermally responsive polythiophene brushes have been de-signed and prepared by McCarley et al.110 Here the thiophene backbone was substituted with thermosensitive N-isopropylacrylamide side chains (Figure 2.13).
Figure 2.13. (a) Chemical structure of water soluble polythiophene; (b) Conformational transition of
polythi-ophene from extended random conformation above its lowest critical solution temperature, to collapsed
a) b)
This side group was chosen due to its lower critical solution temperature (30-32 °C) and easy synthesis. Synthesized poly(thiophene-ɣ-isopropylacrylamide) has an ability to reversibly change its conformation, when passing this temperature (Figure 2.13). The extended hydrated coil, when below the lower critical solution temperature, turns into a collapsed, hydrophobic, water insoluble globule. The reported highly water-soluble polymer showed unique electronic and optical properties due to extension and collapse of the side chain brushes towards the back-bone. This can be used in responsive soft nanodevices, light emitting diodes, biosensors, actua-tors, bioelectronics and fluorescent thermometers.
Meijer et al.111 demonstrated an easy way of making non-aggregating folded
polythio-phene structures in water. It was achieved by substituting a hydrophobic thiopolythio-phene backbone with hydrophilic ethylene glycol units (Figure 2.14).
Figure 2.14. Chemical structure of a non-ionic, non-aggregating, water-soluble polythiophene. [Reproduced
from ref. 111].
It turned out that this compound could provide a basis for the development of materials with con-trolled secondary structures, which can improve optical and electronic properties.
Recently, a very similar compound was used for the fabrication of environmentally-friendly organic field effect transistors.45 These non-ionic, highly water-soluble polythiophenes,
bearing ethylene glycol units at β-positions, were able to form highly ordered crystalline struc-tures on mica surfaces (Figure 2.15).
Figure 2.15. (a) Chemical structure of highly water-soluble polythiophene semiconductor; (b) Tapping mode AFM (1x1 mm) image of annealed polythiophene film from water solution. [Reproduced from ref. 45].
The charge mobility of a field-effect transistor made using this polymer was measured at 3.5 x 10-5 cm2 V-1 s-1, significantly lower than conventional poly(3-hexylthiophene). However,
solu-tion-based processing and solubility in water gives an opportunity to develop true low-cost, green solvent processed and flexible electronic devices.45
a) b)
n
2.4.2 Self-assembling thiophene low molecular weight amphiphiles
Low molecular weight thiophene amphiphiles are the target of our studies. It is due to the fact that their properties can be easily tuned upon substitution of various positions along the backbone. Inclusion of functional groups opens a way to further enhancement of functionality.
Sexithiophenes bearing water-soluble cationic alkyl substituents at the terminal α-positions of the hydrophobic backbone, were reported by Advincula et al.112, 113 Layer-by-layer self-assembly from molecularly dissolved or aggregated state yielded ordered ultra-thin films on various substrates.
Amphiphilic sexithiophene derivatives substituted with a non-ionic hydrophilic chiral penta(ethylene oxide) chains114-116 were reported by Schenning et al (Figure 2.16).
Figure 2.16. Structures of amphiphilic sexithiophenes. [Reproduced from ref. 32].
The substituents control the interplay of hydrophilic and hydrophobic interactions and guide π-π stacking. The materials were used for the fabrication of thin film transistors. Chiral substituents led to the formation of left-handed supramolecular helical ropes on silicon wafers from water, while achiral aciral substituents gave fibrile-like structures.32
Few years ago, Arslanov et al.117 reported terthiophene bolaamphiphiles, which were forming Langmuir-Blodgett films at the air-water interfase.
The presence of a water-soluble substituent at the α-position of the terthiophenes has led to the formation of H-aggregates, where the conjugated segments have an orientation perpendic-ular to a packing axis (Figure 2.17a). When the hydrophilic crown-ether segment was placed at the β-positions of terminal thiophenes, the molecules were forming J-aggregates (Figure 2.17b).
Figure 2.17. (a) Chemical structure and fluorescence microscopy image of α-substituted thiophene
am-phiphile. (b) Chemical structure and fluorescence microscopy image of β-substituted thiophene amam-phiphile. [Reproduced from ref. 117].
This study shows how it is important to investigate the relationship between the position of wa-ter-soluble substituent in the hydrophobic oligothiophene backbone and the morphology of the aggregates. Such materials can serve as a model to investigate structure-property relationship, which can lead to the discovery of promising materials for organic electronics.117
Recently Matile et al. reported an exciting investigation on planarizable and polarizable oligothiophene amphiphiles.118 It is well known that the responsiveness of oligothiophenes to the
environment stems from the flexibility of their backbone. The energy cost to rotate around the single α,α’ bond is very low in oligothiophenes.119 Based on this knowledge, the authors
de-signed and fabricated head/tail amphiphiles, responsive to the fluidity of surfactant membranes in which they are embeded (Figure 2.18).
Figure 2.18. Schematic representation of the thiophene planarization upon external stimuli. [Reproduced from
ref. 118].
a) b)
These push-pull quarterthiophene scaffolds are highly solvatochromic and show bathochromic shifts upon a decrease of membrane fluidity or an increase of membrane potential. This effect stems from the fact that the thiophene backbone is planarized due to the confining surrounding or a stabilized microdipole in the molecule. This phenomenon is a great platform for the devel-opment of membrane tension and heterogeneity sensors.
Finally, our group has performed extensive studies on variously substituted oligothio-phene facial amphiphiles. It was found that such compounds are able to form well-defined ag-gregates in water, depending on their shape and substitution.120 The manipulations of the nature
and positions of the side groups on the backbone of such compounds can lead to an implementa-tion of an addiimplementa-tional structural motif, the curvature.121 Moreover, the aggregation of these
thio-phenes in water can drive the formation of dynamic, but efficiently covalently bonded, novel imine polymers in water.122 In general, these thiophene amphiphiles can be successfully applied
for self-assembling antenna systems123, cell membrane curvature recognition and efficient
charge-transport platforms in water124. 2.5 Conclusions
Conjugated molecules are attractive materials for constructing self-assembled structures in water. The variety of hydrophobic π-conjugated backbones and functions available gives a freedom of choosing suitable systems for desired application. The position and nature of the substituents, that enhance the water solubility of the hydrophobic backbone, can result in various morphologies in water. The application of π-conjugated and water compatible compounds is very broad. It opens a way to use environmentally friendly processing steps and solvents in the fabrication of organic electronic devices. Moreover, it expands the conventional application field of these materials to biological systems. Conformational changes and self-assembly processes result in the chromic response of π-conjugated compounds in water. This ability allows the de-velopment advanced sensing systems, able to respond to various analytes and stimuli in aqueous environments. In general, this literature review shows that a broad variety of π-conjugated com-pounds with water-soluble groups is accessible. The future research should be focused on gain-ing an ability to understand and predict the self-assemblgain-ing processes and possible morphologies formed. This would open a way towards new functional materials for various application fields.
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