DNA-grafted supramolecular polymers: synthesis, self-assembly and potential applications

Exciton-polarons in self-assembling helical aggregates: relating optical properties to supramolecular structure

Conventional amphiphilic block copolymers are macromolecules consisting of at least a hydrophilic segment covalently attached to a hydrophobic segment. They are unique and versatile building blocks in supramolecular polymer chemistry, both for the generation of highly organized, self-assembled structures and for the structural control of material interfaces. In the absence of solvents, the phase-behavior of block-copolymers is sparingly described. In aqueous solutions block copolymers self-assemble into nanoscopic objects. These structures are gaining more and more attention for technical formulations. Self-assembly is simply induced by dissolving dry powder of amphiphilic copolymers in water. This self-assembly process, however, cannot be described solely based on bulk phase behavior and very few experimental phase diagrams of block copolymer/water mixtures have been reported. In order to control, predict and eventually achieve a comprehensive understanding of this process and in particular the formation of vesicles, worm/rod-like micelles and spherical micelles it is important to systematically investigate the phase behavior of block copolymers across the whole water concentration range. We therefore undertook investigations of the self-assembly of two block copolymers. Preliminary encapsulation studies reveal their potential for applications relevant for drug delivery. 
\nThe concentration profile of the identified amphiphilic block-copolymers diluted in aqueous solution was examined across the whole concentration range with four poly(ethylene oxide)-block-poly(γ-methyl-ε-caprolactone) (PEO-b-PMCL) block copolymers keeping the hydrophilic block length constant but varying the hydrophobic lengths. Bulk polymers did not display any ordered morphology. With increasing water concentrations the polymers underwent transitions from lamellar phases to packed vesicles and eventually all polymers self-assembled into vesicles in dilute aqueous solutions. At high concentration the largest polymer organized further into an inverse hexagonal phase prior to self-assembly into a lamellar phase. The smallest block-copolymer also self-aggregated into rod-like micelles and exhibited a hexagonal phase between the packing of the rod-like micelles/vesicles and the lamellar phase.
\nThe concentration profile of phases assembled by amphiphilic block-copolymers diluted in aqueous solution was examined as well across the whole concentration range with three poly(isobutylene)-block-poly(ethylene oxide) block copolymers having the same hydrophobic block length but varying in their hydrophilic lengths. In contrast to PEO-b-PMCL these polymers self-aggregated depending on the hydrophilic length into micelles, worm-like micelles and for the shortest polymer into vesicles. Thus, increasing the polymer concentration lead to a different phase behavior. The largest polymer which solely formed micelles, underwent transitions from packed micelles with a few packed worm-like micelles to a hexagonal phase and finally into a lamellar phase, in contrary to the middle length polymer, which self-aggregated into worm-like micelles. The best to our knowledge, it is the first time that an experimental phase diagram is built such that it displays the formation of worm/rod-like micelles. With the composition studied in here, the worms, with increasing polymer concentration, pack and undergo afterwards into a hexagonal and finally a lamellar phase has been observed. For the shortest polymer the phase behavior was similar to that of vesicles self-assembled by PEO-b-PMCL. In contrast, the pure polymer formed an inverse hexagonal phase.
\nIt was eventually shown that PEO23-PMCl32 can be used to encapsulate both small and large molecules. By adjusting the pH towards acidic conditions the responsiveness of the vesicles could be triggered to release the encapsulated material.

The self-assembly of 3,6-disubstituted amine-linked phenanthrene trimers with a positively charged backbone leads to the formation of vesicles in aqueous medium. Previously, it was shown that 1,6-disubstituted phosphodiester-linked pyrene trimers - possessing a negatively charged backbone - assemble into sheets. Due to electrostatic interactions of the cationic and anionic species, alternated layering of the self-assembled structures is possible. The supramolecular polymers are usually preformed in their specific environment, in order to avoid problems because of different ambient conditions. Therefore, the layering on a mica substrate is done in a stepwise procedure. The structures formed by layer-by-layer self-assembly are analyzed by microscopy.

Regenerative medicine claims for a better understanding of the cause-effect relation between cell behaviour and environment signals. The latter encompasses topographical, chemical and mechanical stimuli, electromagnetic fields, gradients of chemo-attractants and haptotaxis. In this perspective, a spatial control of the structures composing the environment is required. In this thesis I describe a novel approach for the multiscale patterning of biocompatible functional materials in order to provide systems able to accurately control cell adhesion and proliferation. The behaviour of different neural cell lines in response to several stimuli, specifically chemical, topographical and electrical gradients is presented. For each of the three kind of signals, I chose properly tailored materials and fabrication and characterization techniques. After a brief introduction on the state of art of nanotechnology, nanofabrication techniques and regenerative medicine in Chapter 1 and a detailed description of the main fabrication and characterization techniques employed in this work in Chapter 2, in Chapter 3 an easy route to obtain accurate control over cell proliferation close to 100% is described (chemical control). In Chapter 4 (topographical control) it is shown how the multiscale patterning of a well-established biocompatible material as titanium dioxide provides a versatile and robust method to study the effect of local topography on cell adhesion and growth. The third signal, viz. electric field, is investigated in Chapter 5 (electrical control), where the very early stages of neural cell adhesion are studied in the presence of modest steady electric fields. In Chapter 6 (appendix) a new patterning technique, called Lithographically Controlled Etching (LCE), is proposed. It is shown how LCE can provide at the same time the micro/nanostructuring and functionalization of a surface with nanosized objects, thus being suitable for applications both in regenerative medicine in biosensing.

ConspectusSince its inception in the early 1990s, the field of supramolecular polymers (SPs) has grown into an interdisciplinary field of chemistry. It expanded from the self-assembly of molecular building blocks based on H-bonding into the realm of complex dynamic material, encompassing both supramolecular noncovalent and molecular covalent regimes. It has paved the path for a more diverse field of research into a new class of polymeric materials, coined dynamic polymers or dynamers. Dynamers are bringing a paradigm shift not only in material science research but also in a broad field of applications from self-healing materials to biocompatible polymeric materials. The present Account presents the evolution of supramolecular polymer chemistry from simple linear polymeric chains to complex dynamic polymers imparting novel functional properties, such as component exchange and self-healing. We explore how SPs led to materials of increasing complexity, starting from simple main-chain polymers to the formation of more complex columnar SPs and lateral SPs. The field has experienced three partially overlapping periods. The main goal was first the generation of polymeric entities from various molecular components connected through noncovalent interactions, especially complementary hydrogen bonding recognition patterns as well as stacked columnar SPs. Thereafter, attention was directed in parallel to the exploration of the properties of SPs and their applications as novel materials. In a third period, the dynamic properties of supramolecular polymers were explored, taking advantage of the lability of noncovalent interactions to perform component rearrangement and exchange. We illustrate how the field of SPs has emerged as a multidisciplinary field of chemistry, biology, and materials science with selected examples from the literature. The SPs, specifically dynamic owing to their inherent reversibility, also pave the path to easier sorting and recycling, as desired in the plastics industry.One of the biggest challenges that the plastics industry is facing today is the end-of-life fate of plastics. Plastics that cannot be recycled end up in landfills or are improperly disposed of in rivers and oceans, polluting and damaging the environmental balance irreversibly. Dynamic polymeric materials presenting inherent dynamicity could pave the way for addressing this long-standing challenge of nonrecyclability of plastics. Dynamers formed via noncovalent interactions or reversible covalent bonds can be broken into components that could be easily recycled and reused. Therefore, dynamers could play a pivotal role toward closing the loop for the plastics industry and provide a solution to an elusive circular economy with plastics being an integral part.

Supramolecular phenanthrene polymers with a negative charged phosphate backbone have been synthesized in our group. These assembled oligomers can act as light harvesting antennas. Herein we replaced the negative charged phosphate backbone by a positive charged amine one and analyzed the aggregation behavior and light harvesting properties. The self-assembly of 1,8-disubstituted amine-linked phenanthrene trimers in aqueous solution formed mono- and even multilayered vesicles based on atomic-force microscopy. The incorporation of pyrene into the vesicles allows aninvestigation of light harvesting properties.

A low optical bandgap and fullerene-like electron affinity of functionalized cyclopenta[hi]aceanthrylene (cypac) was reported and potential applications as electron acceptors in organic solar cells are suggested among others.[1] In this work, a water-soluble, bolaamphiphilic cypac derivative with a low-lying LUMO and intense absorption bands over an extended range of the UV-visible spectral region has been synthesized.[2] Self-assembly of cypac-bis-phosphate in aqueous medium leads to the formation of a supramolecular polymer with a vesicular morphology. The electronic properties of the monomer remain largely intact upon supramolecular polymer formation, as evidenced by UV-vis and cyclic voltammetry measurements.

Cells as functional units from algae to mammals demonstrate the most remarkable degree of self-organization. Processes like membrane formation, protein folding and signal cascades excel in selectivity and control. Nanotechnology is often inspired by biological properties but, despite Nature’s seductive elegance and putative simplicity, often fails at prediction of complex self-assembly. Even weak forces, multiplied by the large number of subunits, contribute to the assemblies and frequently lead to unforeseen results. Membranes are prominent and well understood examples for self-organization and since there is a rising interest in vesicular self-assemblies, the number of potential applications increased with the complexity of the membrane material. The controlled variation of structure and dimension in supramolecular assemblies is a desirable feature for medical and technical applications. From lipids to polymers to the incorporation of proteins: today we are able to tailor membrane properties desirable for many purposes. Highly specific interactions in between membrane constituents are a desirable feature. And when it comes to the discipline of self-assembly, barely a process compares to the specificity and control that is represented by proteins folding into their biologically active state. Thus, it is tempting to exploit this specificity not only in terms of intramolecular but also intermolecular interactions. However, the controlled formation of a membrane from short peptides has not been accomplished to this day. The aim of this work was to construct membranes from peptides, shorter than 30 amino acids in primary structure. The main challenge of the project is the hydrophilic contribution of every amino acid’s backbone that usually constrains the hydrophobic property of peptides. As a consequence, we considered secondary structure as the key to the formation of an entirely peptidic membrane constituent, an assumption that was confirmed by the helical conformation of the antibiotic peptide gramicidin. We present the formation of membranes based on its secondary structure motif and complemented it with varying lengths of positively charged oligo-lysine. The functional property of membrane formation could be assigned to the last seven amino acids of the gramicidin sequence, which allowed us to construct membranes out of peptides only eight amino acids in length. The results are unpreceded both in terms of controlled peptide self-assembly as well as abstraction from the peptides’ biological purpose.

Cyclodextrins functionalized polyvinylidene fluoride membranes: Strategies and diverse applications

Polymers have long been known to us for their versatile utility in our daily lives. With many years of successful advancement in controlled polymer synthesis and supramolecular chemistry, currently an amalgamation of these two fields is becoming increasingly important for generating modern‐day emerging polymeric materials. Hydrogen bonding and other noncovalent interactions like π‐stacking, host‐guest interaction, electrostatic interaction, metal‐ligand coordination have been explored much to this context, unlike the recently emerged halogen bonding, which indeed shows advantageous features like superior hydrophobicity, directionality and polar solvent resistance compared to ubiquitous hydrogen bonding. Hence, rendering these properties of halogen bonding into polymeric domain for generating functional materials for application in advance nanotechnologies is highly desirable. In the recent years, substantial progress has been made in fulfilling this challenging goal. This is evident from the escalating number of scientific publications in this research area every year. In this minireview, we have summarized the most recent developments in the field of halogen bonded polymeric assemblies and supramolecular polymers and discussed them in the context of their emerging materials properties and functions. Emphasis is given on highlighting various design criteria adopted in crystal engineering, covalent and supramolecular polymers in the past five years for constructing diverse organic functional materials, viz. liquid crystals, photoresponsive and photochromic materials, biomaterials and gels, self‐healing materials and many others by virtue of this highly directional supramolecular tool. In conclusion, a brief perspective on the advantages, challenges and future prospects in this rapidly growing field of research is discussed.

The work presented in this thesis entails the synthesis and characterisation of new iron(II) tris(dioxime) clathrochelate complexes functionalised with apical capping groups suitable for supramolecular applications, and their subsequent incorporation into self-assembled structures. These complexes possess a number of desirable characteristics as supramolecular building blocks, such as their synthetic accessibility and broad scope for structural and functional modification, which are highlighted throughout. Chapter 1 introduces certain key concepts and background literature relevant to this thesis. Clathrochelate-based 4,4'-bipyridyl metalloligands are presented in Chapter 2. Simple complexes bearing 4-pyridyl capping groups were prepared from commercially available starting materials and their potential as building blocks for supramolecular architectures demonstrated through the preparation of a discrete molecular square and a 3D coordination polymer (CP). Furthermore, bent and extended linear 4,4'-bipyridyl metalloligands up to 5.4 nm in length were prepared by two distinct homologation strategies. Clathrochelate-based 3,3'-bipyridyl metalloligands are presented in Chapter 3. Simple complexes were prepared from commercially available starting materials and subsequently combined with Pd(II) ions to give unprecedented [Pd6L12]12+ octahedral cage structures by coordination-driven self-assembly. These assemblies were shown to bind the large, non-coordinating anion BPh4- in aqueous solvent mixtures. Furthermore, linear and bent bis(clathrochelate) complexes bearing 3-pyridyl apical capping groups were also prepared by a one-pot approach. Clathrochelate complexes bearing carboxylic acid-functionalised capping groups are presented in Chapter 4. Ditopic and tetratopic metalloligands were synthesised in good yields. A heterometallic 2D CP was prepared through combination of Zn(II) ions with a clathrochelate complex functionalised with phenyl-4-carboxylic acid groups and the structure of this material determined by single crystal X-ray diffraction. Clathrochelates bearing functional groups suitable for imine-based self-assembly are presented in Chapter 5. Complexes functionalised with amine or aldehyde moieties as well as complexes functionalised with 3-pyridine-4-carboxaldehyde groups, were prepared in either one or two steps. Preliminary investigations into the preparation of metal-organic cage architectures through a well-established subcomponent self-assembly approach combining imine-based DCC and coordination-driven self-assembly gave promising results, including the synthesis of an [Fe4L6]8+ cage, which was characterised by single crystal X-ray crystallography.

This work describes the use of boron-nitrogen dative bonds in supramolecular chemistry. Molecular tectons have been synthesized for crystal engineering. These tectons are linked via B-N dative bonds to form 1-dimensional polymers and macrocycles. B-N dative bonds were further used for the formation of crystalline organic networks by assembly of a tritopic boronate ester with ditopic N-donor ligands. These networks could be structurally characterized by single crystal X-ray diffraction to reveal 2-dimensional polymers. These structures were observed by PXRD to collapse upon removal of solvent. The reaction of a larger triboronate ester with 4,4'-bipyridine resulted in an organogel which formed with as little as 0.5 wt% of the network in toluene. The orange gel exhibited thermochromic properties, with a Tgel of 60 °C. Association constants for the formation of B-N dative bonds between dioxaborole and pyridyl ligands were examined by 1H NMR and ITC titrations. Ka values in excess of 106 M-1 can be obtained by varying the electronics of the reaction partners. The adducts have been characterized comprehensively by single crystal X-ray diffraction. Finally, the dative B-N bond was used for the synthesis of main-chain supramolecular polymers. Self-complementary monomers were synthesized containing a dialkylaminopyridine and a dioxaborole. These monomers assembled in chloroform to form polymers which were characterized by viscosity and DOSY NMR experiments.

Thin coatings applied to surfaces dramatically improve the properties of materials. Thus, they have found application in a broad variety of areas including biomedicine, nanotechnology, bioelectronics and optics, amongst others. Polymer brushes prepared via surface-initiated controlled radical polymerization (SI-CRP) techniques represent a versatile class of thin polymer coatings that provide manifold opportunities to engineer surface and interface properties. Densely grafted polymer brushes prepared via SI-CRP possess several attractive material properties such as low friction coefficients and the ability to prevent biofouling, which are related to the stretched conformation of the surface tethered polymer chains. Those distinctive characteristics of polymer brushes make them highly appealing to engineer the properties of functional interfaces. A first aim of this Thesis is to investigate the mechanochemistry of hydrophilic polymer brushes prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) from silica substrates. The second aim of the Thesis is to explore poly(2-hydroxyethyl methacrylate) (PHEMA) based polymer brushes as soft-tissue adhesives in wet tissue environment. This Thesis is divided into four chapters, which are briefly summarized below. Chapter 1 provides a summary of recent literature that has reported detachment or degrafting of densely grafted hydrophilic polymer brushes from substrates upon exposure to aqueous media. The collective results from these reports suggest that swelling-induced stretching of hydrophilic polymer brushes lead to mechanochemical activation of the bonds near the brush-substrate interface and subsequently facilitates hydrolytic chain cleavage. Chapter 2 aims to shed light on the influence of two fundamental structural polymer brush parameters on the degrafting of surface-tethered, densely grafted polymer chains, viz. (i) the chemical composition of the ATRP initiator that is used to grow the polymer brushes and (ii) the effect of surface curvature. To this end, a series of poly(poly(ethylene glycol) methacrylate) (PPEGMA) and poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMEMA) brushes were grafted from both flat silica substrates as well as silica nanoparticles. Subsequently, the degrafting of the surface-tethered polymer chains was investigated in cell culture medium and phosphate buffered saline (PBS) using various techniques. The aim of the final two Chapters of this Thesis is less on exploring the chain conformation of polymer chains grafted by SI-CRP, but more to use the ability of SI-CRP to prepare thin polymer films that present a high surface concentration of functional groups to mediate soft tissue adhesion. Chapter 3 describes the preparation of photo-active poly(2-hydroxyethyl methacrylate) (PHEMA) brushes as soft tissue bioadhesives. Tissue adhesion in this Chapter is induced as UV-irradiation of PHEMA brushes converts the initially bio-inert side chain hydroxyl groups of these brushes into (putatively) aldehyde groups, which can react with amine groups present in the tissue. The prepared PHEMA brush coated substrates are biocompatible, which avoids fibrous capsule formation and presents functionalities that can be transformed, on-demand, from a non-reactive, non-adhesive state to a reactive, adhesive state in wet tissue environment. While the photoactivation of the PHEMA side chain functional groups is very efficient, it is not very well defined from a chemistry point of view. Chapter 4 therefore, explores an alternative photoactivation chemistry, which is based on the use of the diazirine motif. These diazirine motifs were incorporated in poly(2-hydroxyethyl methacrylate-co-poly(ethylene glycol) methyl ether methacrylate) (P(HEMA-co-PEGMEMA)) copolymer brushes via post-polymerization modification. These diazirine modified brushes are attractive as they may not only be activated by UV-light but also by electrochemical means.

Inspired by the vast array of assemblies present in nature, supramolecular chemistry has attracted significant attention on account of its diverse supra-structures, which include micelles, vesicles, and fibers, in addition to its extensive applications in luminescent materials, sensors, bioimaging, and drug delivery over the past decades. Supramolecular polymers, which represent a combination of supramolecular chemistry and polymer science, are constructed by non-covalent interactions, such as host-guest interactions, hydrogen bonding, hydrophobic or hydrophilic interactions, metal-ligand interactions, π-π stacking, and electrostatic interactions. To date, numerous host-guest recognition systems have been reported, including crown ethers, cyclodextrins, calixarenes, cucurbituril, pillararenes, and other macrocyclic hosts. Among them, crown ethers, as the first generation of macrocyclic hosts, provide a promising and facile alternative route to supramolecular polymers. In addition, the incorporation of fluorophores into supramolecular polymers could endow them with multiple properties and functions, thereby presenting potential advantages in the context of smart materials. Thus, this review focuses on the fabrication strategies, interesting properties, and potential applications of fluorescent supramolecular polymers based on crown ethers. Typical examples are presented and discussed in terms of three different types of building blocks, namely covalently bonded low-molecular-weight compounds, polymers modified by hosts or guests, and supramolecular coordination complexes.

Candida antarctica Lipase B catalysis in organic, polymer and supramolecular chemistry