Introduction to Supramolecular Chemistry

Supramolecular chemistry has been one of the most interesting fields in modern chemistry. In 1987, J.-M. Lehn, C. J. Pederson and D. J. Cram received the Nobel Prize for their pioneering work.¹ Self-recognition and self-assembly processes represent the basic concept of the supramolecular chemistry and the interactions involved are mainly of non-covalent nature (e.g. van der Waals, hydrogen bonding, ionic or coordinative interaction). Compared to covalent bonds, these interactions are weaker and usually reversible. Natural is the model for artificial supramolecular processes. Inter- and intramolecular non-covalent interactions are of the major importance for most biological processes such as highly selective catalytic reactions and information storage; ^2,3 different non-covalent interactions are present in proteins giving them their specific structures. DNA represents one of the most famous examples, where the self-recognition and complementary base-pairs by hydrogen bonding leads to the self-assembly of the double helix. Today, many synthetic supramolecular systems are known.^1,4 The resulting compounds are expected to reveal new chemical and physical as well as biological properties. Starting from biomimetic systems, the concept was extended to “molecular machines” ⁵ and supramolecular polymers.

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1.2.1 Introduction to Supramolecular Polymers

An area of special interest in recent years is supramolecular polymer chemistry. ⁷ Polymers that are synthesized by non-covalent interactions, and not by conventional covalent polymerization, offer new possibilities because such interactions can be favorably influenced

by external parameters such as the temperature or mechanical stimuli causing drastic changes in the polymer properties, particularly the elasticity and solution viscosity. A large number of supramolecular polymers can be built by hydrogen bonding, ⁸ in some cases in combination with further interactions such as π-π stacking that significantly determine the structures of the polymers. Also, more exotic interactions such as dipolar aggregation have been successfully applied for the formation of highly complex polymeric dye aggregates.⁹ Metal-ligand coordination provide an excellent means for the synthesis of supramolecular systems as the coordination bond is highly directional, the ligand structures can be varied in a desired manner by established organic chemistry, and the thermodynamic and kinetic stability can be fine-tuned with the appropriate ligand types and metal ions. Supramolecular systems constructed from metal-ligand bond include lattice, cyclic and filamentous motives as well as interlaced systems.¹⁰

1.2.2 Introduction to Coordination Polymers: Definition, Formation and Interactions

Definition

The key word coordination polymer (metallo-supramolecular polymer) is abundantly found in advanced chemistry literature. However, care most be taken because the term coordination polymer is defined quite differently in the organic and supramolecular chemistry communities. Inorganic chemists consider infinite one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) coordination networks as coordination polymers. These systems are in the great majority crystalline solid-state materials that fragment into molecular building blocks upon dissolution. Alternative terms for these types of compounds are metal-organic coordination network and metal-organic frameworks. ¹¹ One highlight of recent achievements in this filed is the construction of functional porous coordination polymer.¹² In the field of supramolecular chemistry, the definition of

coordination polymer is more precise and is related to macromolecular chemistry. A coordination polymer is an entity constructed by a supramoecular approach through metal coordination and consisting of a backbone, which is held together by metal-ligand interactions. These interactions have to be strong enough to retain the polymer chain also in solution. The coordination polymers should exhibit properties that are characteristic for polymers, monomeric building blocks and/or a glassy solid state. In an outstanding review on organic/inorganic hybrid polymers, Rehahn¹³ proposed a classification of different structures of coordination polymers.

Formation

The main difference between classical covalent polymers and supramolecular polymers is the dependence of the chain length [degree of polymerization (DP)] on the solvent and temperature dependent bonding constant (K) and related therewith, the concentration. For reversible coordination polymers, the relationship between DP and K is given by the following expression:¹⁴

DP ~ (K[M])^1/2

According to this relationship, high DP values can be achieved only at high monomer concentration ([M]) and with a metal-ligand coordinative bond with a large K value. This relationship, of course, holds true only if the complexation is a fully reversible process. For intermolecular interactions, the combination high bonding constants and reversibility is challenging because these properties are highly divergent; that is, high binding constants often hamper reversibility.¹⁵ Therefore, a detailed investigation of both the thermodynamic and kinetic properties of the individual metal complexes is necessary when a suitable metal-ligand is being chosen. The thermodynamic properties, expressed in the individual binding constants or overall binding constant,¹⁶ provide the information on whether the complex is stable or unstable. These expressions have to be clearly distinguished from the terms inert or labile, which describe the kinetic properties. The inertness or lability of metal

complex is determined by ligand-exchange experiments and expressed, for example, in the half-life of the complex species (Table 1).¹⁷ When a suitable metal-ligand is being chosen for the construction of a reversible coordination polymer, the ideal combination is, therefore, a system that provides a complex of high thermodynamic stability combined with high kinetic lability. This combination ensures the formation of a high-molecular-weight coordination polymer because of the stable complex bond together with the inherent advantages of reversible (labile) noncolvant supramolecular systems, that is, error correction, the identical ligand units, and switching between the polymeric and monomeric states.

Table 1.1 Binding and Functional Properties of Selected Terpyridine (tpy) Complexes

Ion Type

Fe²⁺ 1.3 × 10⁷ 8400 Paramagnetic Strong

Co²⁺ 2.5 × 10⁸ 50 Paramagnetic Medium

Ni²⁺ 5.0 × 10¹⁰ 610 Paramagnetic Medium

Zn²⁺ 1.0 × 10⁶ <0.1 Dimagnetic Weak

Interactions

As discussed previously, the nature of the metal-ligand interaction has a significant effect on the mechanical properties of the resulting coordination polymer, and in this regard, the binding constant is the most important parameter. Because most applications demand a high DP and increase in the concentration cannot be realized in many cases because of practical limitations or insufficient solution, control of the binding strength is of paramount importance. An increase K value may be achieved by the combination of multiple interacting binding sites, the simplest one being the use of the chelate ligands and multivalent metal ions.

The increase in the binding constant upon application of chelating ligands is illustrated by a comparison of Zn²⁺ complexation with different types of pyridine donor ligands (Figure 1.1 and Table 1.2)^18-20

Figure 1.1 Examples of compounds with bidentate or tridetate pyridine/Zn²⁺interactions.

Table 1.2 Binding Constants for Complexes of Zn²⁺ with Aromatic N-Donor Ligands of Increasing Chelation

aK1 is the binding constant for the first binding event, and βn is the overall binding constant.

bWhere the bpy is 2,2’-bipyridine, phen is 1,10-phenanthroline, tpy is 2,2’:6’,2”-terpyridines

cDetermined for a 4’-substituted terpyridine unit.

dDetermined for a 4’-substituted unit (TBAPF = 0.01M tetrabutylammonium hexafluorophosphate).

1.3 Introduction to Functional polymers and Materials Based on 2,2’:6’,2”-Terpyridine Metal Complexes

2,2’:6’,2”-Terpyridines are among the N-heterocycles that have very high binding affinity towards transition metal ions due to dπ-pπ^* back binding of the metal to pyridine ring and the chelate effect.²¹ Complexation of one or two 2,2’:6’,2”-terpyeidine (Figure 1.3) ligands can lead to a metal complex, and in many case bis-complexs thus formed have octahedral coordination geometries.²² These complexes posses distinct photophyscial, electrochemical, and magnetic prorperties.²³ The complex binding can be reversed under certain conditions, e.g., varying PH, temperature, or applying even stronger competitive ligands, which makes such compounds interesting for the design of new functional materials.⁵ In the search of new functional materials, metallo-supramolecular polymers, dendrimers, or micelles have been of special interest for the last few yeas, but the combination of such stable complexes with “biomolecules” such as DNA/RNA, peptides, and enzymes for labeling, intercalation, and inhibition purposes is also promising.²⁴Another field which is rapidly growing due to the technical advances made is the build-up of ordered structures on a molecular scale on different kind of surfaces.²⁵ Here also, terpyridine complexes play an increasing role for applications such as solar-cell devices or electrode catalysis. Furthermore, such easily detectable and multifunctional entities are of great use for gaining a more fundamental understanding of self-assembly phenomena or organic or inorganic-organic hybrid materials on surfaces. Recent advances in synthesis of functionalized terpyeidine have open new possibilities for the introduction of metal complexs into polymers and onto surfaces. In particular, functionalization in the 4’-positon, by using, e.g., substitution reactions with (nowadays) commercially available 4’-chloroterpyridine²⁶ or 4’-hydroxyterpyridine,²⁷ leads to symmetrical bis-cpmplexes with ether bridged functional groups (R) which, upon complexation, do not give additional chiral products (Figure 1.2).

Figure 1.2 Left: unfunctionalized 2,2’:6’,2”-terpyeidine; right: symmetric 4’-functionalized bis-terpyridine-metal complex (charge and anions omitted).

1.3.1 Introduction to Polymeric Architectures Containing 2,2’:6’,2”-Terpyeidine Metal Complexes

Metallo-Polymers from Terpyridines-Fuctionalized Monodisperse Monomers

One way of classing polymers that contain terpyridine metal complexes is to distinguish between metallo-polymers built up from monodisperse or polydisperse monomers.

Metallo-polymer systems, starting from monodisperse building blocks, are discussed first. In 1995, Constable presented the general concept of bis-terpyridine functionalized telechelics, which, upon addition of metal ions, should give coordination polymers (Figure 1.3).²⁸

Figure 1.3 Polymeric bis-terpyridine-metal complex (charge and anions omitted).

It was mainly work by Kurth et al. and Rehahn et al. in the late 1990s that pioneered this concept,²⁹ using small monodiesperse di-terpyridines as monomers in order to create, for example, coordination polyelectrolyte layers. In the last few years, there have been a number of different approaches, mainly focusing on iron(II), zinc(II) and ruthenium(II) as the ”metal glue” for coordination polymerization. An example of Meijer and co-workers reported the synthesis of rigid iron(II)-bis-terpyridine polymer including oligo(phenylene/vinylene) (OPV)

units (Figure 1.4).³⁰Due to this rigidity, the formation of small cycles in this case is unlikely, and the DP was estimated to be 100 at applied millimolar concentration, derived from kinetic data obtained from Uv-visible titration experiment.

Figure 1.4 Combination of a bis-terpyridine-iron(II) complex and an oligo(phenylene vinylene) (OPV) unit.

Polydisperse System Cotaining 2,2’:6’,2”-Terpyridines Metal Complexes

The combination of properties of conventional polymers with those of bis-2,2’;6’,2”-terpyridine metal complex has become of increasing interest over the last few years. The three main approaches to chemically introducing terpyridines and their complexes into polymeric systems are: a) by functionalizing properly modified polymers with terpyridine ligands; b) by using a functionalized terpyridine as an initiator; and c) by using a terpyridine with a polymerizable group as the monomer or co-monomer (these can be classified as convergent approaches, staring from uncomplexed terpyridine). These main approaches also apply to corresponding bis-terpyridine metal complexes (a divergent approach, in which metallo-polymers are formed starting from the complex). Having a functionalized polymer with non-complexed terpyridine ligands subsequently allows different combinations upon bis-complexation with different metals, leading to a rich variety of possible new structures. For a detailed insight into these strategies, the reader should turn to recent overviews.^6,31

The first two approaches are especially appealing for gaining access to extended polymer chains through metal complexation by a terpyridine-functionalized polymer. These

polymeric terpyridine starters can be either mono-functionalized or of telechelic nature, processing more than one terpyridine unit per chain. Having terpyridine units at both ends of each chain allow access to linearly extended chains cotaining metal “linkers”. Such a system, consisting of a high molecular weight poly(enthylene oxide) polymer end-capped with terpyridines, gave upon addition of iron(II) or nickel(II) acetate an extended polymer, which was especially demonstrated through the increase in viscosity (Figure 1.5).³²

Figure 1.5 Right: synthesis of metal-linked poly(ethylene glycol)180(charge and anions omitted); Left: viscosity increase when adding nickel(II) acetate.

In order to investigate a variety of such polymers were synthesized using a combinatorial approach.³³ Another possibility for creating and investigating such systems is to first functionalize only one chain end with terpyridine, and than to apply directed as well as undirected coupling methods, in order to obtain AA, AB, and ABA block copolymer systems (Figure 1.6). Directed coupling can be achieved by first forming a mono-terpyridine metal complex, the most common metal for this strategy being ruthenium(Ш). Subsequent reduction to ruthenium(II) in the presence of a differently functionalized terpyridine leads to a heteroleptic complex. In contrast, undirected coupling use the same ligand for bis-complexation with bivalent metal salts. Looking at AA homopolymer systems, this concept was recently realized using poly(ethylene oxide) functionalized with one terpyridine, which, upon complexation with various transition metal ions, gave water soluble polymers

with double the mass of the starting polymer ligand plus the metal and counter ions.³⁴

Figure 1.6 Schematic representation of AA-, AB-, and ABA-type metallo- supramolecular block copolymer systems.

Concerning AB- or ABA-type structures, their preparation requires directed coupling techniques. A well-known strategy for creating such hetero-complexes is the ruthenium(Ш)/ruthenium(II) coupling method. First the dark brown mono-terpyridine-Ru(Ш) chloride in dimethylformamide (DMF) with the ligand. This is then further than reacted with a different ligand under reducing condition (ethanol/cat. N-ethyl-morpholine), resulting in the formation of the red ruthenium(II) hetero-complex. Applying this strategy to different terpyridine end-capped polymers led to the hetero-ruthenium-complex polymers (Figure 1.7).³⁵Such AB-type structures combing two different polymer chains have up to now only been accessible using or controlled polymerization procedures. The same strategy has also been applied for ABA-type systems. Here, a bis-terpyridine end-capped poly(propylene oxide) telechelic was first complexed at both end with ruthenium(Ш) chloride and subsequently complexed symmetrically on both sides with a 4’-functionalized terpyridine.³⁶

Figure 1.7 Synthesized AB-type bis-terpyridine –Ru(II) complexes combing different polymer blocks.

The third main approach for including terpyridines and their complexes in polymeric structures is to use terpyridine (complexes) functionalized with a polymerizable group as the monomer or co-monomer. As a convergent approach, this was demonstrated over 10 years ago, utilizing vinyl and acrylic groups as functional moieties for polymerization.⁶ Recently, Calzia and Tew prepared a random copolymer using a methyl methacrylate-functionalized terpyridine as co-monomer (Figure 1.8).³⁷ Upon addition of cobalt(II) nitrate the authors observed a rise in viscosity which did not occur in the case of the homopolymer. An example for the divergent approach of polymerization bis-terpyridine complexes, which are functionalized on one side with a polymerizable group, was reported, recently.³⁸ Hetero bis-terpyridineu-Ru(II) complexes bearing a 4-vinyl-phenyl substituent on one of the 4’-position were copolymerized with the styrene using radical polymerization. Through possible further functionalization on the hydroxymethyl function of the ligands, new possibilities for crosslinked and grafted systems become available.

Figure 1.8 Tew’s methacrylate copolymer for metal complexations.

Cho et al. used the convergent approach to produce side-chain-functionalized ruthenium(II)-complexes.³⁹Here, first a conventional ABA tri-block copolymer was found by anionic polymerization to yield poly(CzMA-b-2VP-b-CzMA), with CzMA = 2-(N-carbazoly)ethyl metahacrylate, and 2VP = 2-vinylpyridine. The middle block, consisting of 0-20 2VP units, was then complexed with the mixed Ru(II)(tpy)-(dmbpy) chloride (dmbpy = 4,4’-dimethyl-2,2’-bipyridine) in order to yield the octahedral six-coordinative ruthenium(II) complexes as the grafted species (Figure 1.9)

Figure 1.9 Cho’s Ru-coordinated block copolymer.

1.3.2 Introduction to Surfaces Modified with 2,2’:6’,2”-Terpyridines Metal Complexes

Assemblies and Layers

Research concerning the modification of surface properties on a molecular level has increased since Binning and Rohrer invented scanning probe techniques STM (scanning tunneling microscopy) or AFM (atomic force microscopy) in the mid-1980s. However, there is still much to learn in terms of ordering and orientation of substances on surfaces.

Metallo-supramolecular structures in particular add a whole range of possibilities, not least because of possible interactions between the complexed metal and a metal surface. Recently, there has been increasing interest in the investigation adsorbed ordered structures on surfaces containing bis-terpyridine metal complexes. For an overview on layer-by-layer self-assemblies containing terpyridine complexes, the reader is referred a review.⁴⁰ For

example, Abruña and co-workers described the synthesis of chair and dendritic multi-terpyridine molecules, which, upon complexation with iron(II) or cobalt(II) and deposition on a surface, led to well-ordered two-dimensional (2D) arrays, recently.⁴¹ In case of terpyridine functionalized second-generation poly(amido amine) (PAMAM) starburst dendrimer, the the author states that instead of more thermodynamically stable (2D) arrays, chains stacked next to each other (“pearl necklace” formation) were found by STM investigation on highly ordered pyrolytic graphite (HOPG) (Figure 1.10).

Figure 1.10 Top: structure of the terpyridine dendrimer (dend-8-tpy); bottom: unfiltered images of dend-8-tpy/Fe²⁺ on HOPG, a) 550 nm×550 nm, b) 200 nm×200 nm, c) 304 nm×304 nm, d) 69 nm×69 nm.

Photoactive Materials

Another major area of research, which includes the study of terpyridine-complex-surface interactions, is that of interfacial photophysical processes, especially systems concerning solar light to energy conversion. First example include photoelectrodes based on electropolymerized molecular ruthenium diads reported by Collin et al.⁴² They described polymer films incorporating molecular diads of the type V²⁺-[Ru(II)-(ptpy)2]²⁺ (V = methylviologen, ptpy = 4’-phenyl-terpyridine) (Figure 1.11, left). Thin films were prepared by anodic electropolymerization of the pyrrol groups on the ligand opposite to the ligand containing the methylviologen on an indium oxide (ITO) electrode. Upon irradiation using visible light and in the presence of triethanolamine (TEOA, irreversible electron donor), an anodic photocurrent is observed when the electrode is potentiostated to 0 V. The photoactive center is first excited by visible light upon which the charge-separated state with the methylviologen is formed. Ruthenium(Ш) then irreversibly oxidizes TEOA an the photocurrent is produced by electron transfer into the polymer an to the electrode. The steady-state photoresponse was moderately stable with time (loss of 19% after 30 min) in accordance with stability with modified electrode (Figure 1.12, right).

Figure 1.11 Left: donor-acceptor system DA with pyrrol (anchor), a ruthenium(II) or osmium(II) bis-terpyridine complex (photochemical center) and a methylviologen (MV²⁺,

donor); right: photocurrent response of ITO/poly-(DA) modified electrode.

Apart from solar-cell research the other photophyscially interesting example have been reported: organic light-emitting devices (OLEDs). Elliott and co-workers investigated an

electropolymerized [Ru(tpy)2]⁰ film which was vapor deposited onto an AlQ/TPD/ITO (ALQ = tris(8-hydroxyquinoline) aluminum(Ш) complex, emissive and electron transport layer/TPD = triarylamine derivative, hole transport layer) substrate in order to create an electroluminescent device.⁴³ Another example of blue LED was shown very recently by Che and co-workers.⁴⁴ They used a bis-terpyridine zinc(II) polymer spin-coated on ITO with the device structure: ITO/PEDOT:PSS/zinc-terpyridine-polymer/Ca/Al. A peak maximum of 450 nm is observed in the electroluminescence spectrum with the blue EL intensity increasing with the increasing bias voltage. These examples demonstrate the versatility of terpyridine metal complex, leading to, in this case, encouraging results towards the search for stable and intense blue-light emission, which is currently of particular interest for many photo-optical application.

Finally, terpyridine-metal complexs are gaining increasing interest as a type of new functional materials. Their reversibility under certain conditions, as well as photophyscial properties led to a number of research activities combining these complexes with polymers and/or surfaces.

Chapter 2

Novel Light-Emitting Metallo-Homopolymers and Metallo-alt-copolymer Containing Terpyridyl Zinc(II) Moieties

2.1 Abstract

A series of novel terpyridyl Zn(II))-based metallo-polymers, including metallo-homopolymers and metallo-alt-copolymer, containing carbazole pendants attached to the C-9 position of fluorene by long alkyl spacers were constructed by self-assembled reaction. The integrated ratios of¹H NMR reveal a facile result to distinguish the differences between metallo-homopolymers and copolymers. To further investigate these polymers, UV–vis and PL spectral titration experiments were also carried out by varying the molar ratios of zinc(II) ions to monomers. The photophysical properties of these polymers exhibited blue PL emissions (around 420 nm) with quantum yields of 11–23% (in DMF) and the PL results revealed that the formation of excimers were suppressed by the incorporation of carbazole pendant groups. In addition, the EL results showed green EL emissions (around 550 nm) with turn-on voltages of 6.0–6.5 V, maximum efficiencies of 0.85–1.1 cd A^–1(at 100 mA cm^–2), and maximum luminances of 1704–2819 cd/m² (around 15 V), correspondingly.

2.2 Introduction

Metal-ligand coordination seems to be particularly attractive in past few decades because

Metal-ligand coordination seems to be particularly attractive in past few decades because