INTRODUCTION - Handbook on the Physics and Chemistry of Rare Earths

2. Overview of LCs and

Mesophases 3

2.1. Different Types of LCs 3 2.2. Thermotropic

Mesophases 5

2.3. Lyotropic Mesophases 14 3. Characterization of

Mesophases 15

3.1. Polarizing Optical Microscopy (POM) 15 3.2. Differential Scanning

Calorimetry (DSC) 19 3.3. X-ray Diffraction (XRD) 21 4. Schiff’s Base Complexes 28 5. b-Enaminoketonate

Complexes 63

6. b-Diketonate Complexes 65

7. Bis(benzimidazolyl)pyridine

Complexes 73

8. Phthalocyanine Complexes 84 9. Porphyrin Complexes 99 10. Lanthanide Alkanoates 101 11. Polyoxometalate-Surfactant

Complexes 108

12. Miscellaneous Thermotropic Lanthanidomesogens 112 13. Lyotropic

Lanthanidomesogens 117 14. Physical Properties 119 14.1. Luminescence 119 14.2. Magnetism 127 14.3. Electrical Properties 135 15. Actinidomesogens 140 16. Conclusions and Outlook 144

1. INTRODUCTION

Lanthanidomesogens can be defined as liquid-crystalline lanthanide complexes or lanthanide-containing liquid crystals (Piguet et al., 2006). Liquid crystals(LCs) are substances that are observed to flow like a liquid, but they also show the direction-dependent (anisotropic) physical properties of crystalline solids. LCs can be considered to be crystals in which the molecules have lost some or all of their positional order, while maintaining most of their

Handbook on the Physics and Chemistry of Rare Earths, Vol. 43.

http://dx.doi.org/10.1016/B978-0-444-59536-2.00001-5

stances in liquid crystal displays (LCDs). Research efforts to combine the properties of LCs and metal complexes have led to the development of liquid-crystalline metal complexes (metallomesogens, Bruce, 1993). The development of the field of metallomesogens has been described in several reviews (Binnemans, 2005c; Binnemans and Go¨rller-Walrand, 2002; Bruce, 1996, 2000; Collinson and Bruce, 1999; Davidson and Gabriel, 2005; Donnio, 2002; Donnio and Bruce, 1999; Donnio et al., 2003; Espinet et al., 1992;

Gabriel and Davidson, 2000, 2003; Giroud-Godquin, 1998; Giroud-Godquin and Maitlis, 1991; Hoshino, 1998; Hudson and Maitlis, 1993; Neve, 1996; Oriol and Serrano, 1995; Oriol et al., 1997; Polishchuk and Timofeeva, 1993;

Serrano, 1996; Serrano and Sierra, 2003; Sonin, 1998). Lanthanidomesogens are a subclass of the metallomesogens. Whereas the first examples of metallomesogens mimicked the rodlike- or disklike-shape of conventional organic LCs, it gradually became clear that mesomorphism can also be observed for coordination geometries other than linear or square-planar ones. However, obtaining high coordination number LCs remains a challenge (Bruce, 1994). The next logical step toward metallomesogens with a coordination number higher than six, was the design of LCs containing lanthanide ions (or rare-earth ions in general). The coordination number of the trivalent lanthanide ions is typically eight or nine. The search for metallomesogens with a high coordination number is not the only reason why scientists are looking for new lanthanide-containing LCs. The driving forces for the development of this type of advanced materials are the unique physical properties of lanthanide ions. Several of the lanthanide ions show a very intense luminescence. By incorporating lanthanide ions into LCs, one can obtain luminescent LCs, which might be useful for the design of emissive LCDs or other devices. Most of the trivalent lanthanide ions are paramagnetic, due to unpaired electrons in their 4f subshell. They have a high magnetic moment, and more importantly, often a large magnetic anisotropy. A large magnetic anisotropy and a low viscosity are factors which reduce the thresh- old magnetic field strength for the alignment of molecules in an external magnetic field. Switching of molecules by a magnetic field can have some advantages over switching by an electric field. First of all, whereas due to the presence of the electrode on the walls of the LC cell, molecules in an external electric field can only be oriented with their long molecular axis either parallel or perpendicular to the LC cell walls. In principle, a magnetic field can be applied in any given direction with respect to the LC cell walls, and thus magnetic alignment is possible in any desired direction as well.

Unwanted charge separation or redox reactions can occur when metallomesogens are placed in strong electric fields. These problems are absent in the case of magnetic switching. Although magnetic switching will never replace electric switching in LCDs, one can think of possibilities in the field of mag- netooptical data storage.

Until about 1997, only a very limited number of lanthanidomesogens had been reported in the literature and from then on several research groups world- wide, including the team of the author, have been working on these fascinating materials, resulting in the discovery of several new classes of lanthanidomesogens with different molecular shapes and with different types of mesophases.

This body of work has also resulted in a better understanding of the structure- property relationships of these materials and it allows at present to design lanthanide complexes that exhibit liquid-crystalline behavior at room temperature or even at lower temperatures. The aim of this chapter is to introduce the reader to the different aspects of lanthanidomesogens: their synthesis, thermal behavior, mesophase structure, luminescence, electrical properties and switching by external magnetic fields. The first part of the review consists of an introduction to LCs and to methods used to identify mesophases. Besides the pure lanthanidomesogens, also host-guest systems consisting of lanthanide complexes dis- solved in a LC host matrix will be discussed. The reader will be familiarized with methodologies of designing new lanthanide-containing LCs and will be informed about how ligands of a given lanthanide complex can be modified in order to introduce LC behavior. Also an overview of actinide-containing LCs (actinidomesogens) is given. This chapter is the first comprehensive review on lanthanidomesogens since the publication of the Chemical Reviews review written by the author in 2002 (Binnemans and Go¨rller-Walrand, 2002). Since that time, several shorter reviews have appeared, but none of them covers the whole field of lanthanidomesogens (Binnemans, 2005b, 2009, 2010; Eliseeva and Bu¨nzli, 2010; Piguet et al., 2006; Terazzi et al., 2006a). Present review covers the literature to the end of 2011.

2. OVERVIEW OF LCs AND MESOPHASES 2.1. Different Types of LCs

A main difference between crystals and liquids is that the molecules in a crystal are well ordered in a three-dimensional (3D) lattice, whereas the molecules in a liquid are totally disordered. A molecular crystal consists of a more or less rigid arrangement of molecules, which possess both orientational and positional order. The molecules are constrained to occupy specific positions in the crystal lattice. At low temperatures, the attractive intermolecular forces in a crystal are strong enough to hold the molecules firmly in place, even though they all exhibit a random motion due to thermal vibrations. When a crystalline compound is heated, the thermal motions of the molecules increase and eventually become so strong that the intermolecular forces cannot keep the molecules in their position and the solid melts (Fig. 1). The long-range orientational and positional order is lost to yield a disordered isotropic liquid.

However, this melting process, which transforms a compound in one step from a highly ordered to a totally disordered phase, is a very destructive one

and is not universal for all types of compounds. There exist phases that are called “orientationally ordered liquids” or “positionally disordered crystals.”

In other words, these phases have more order than present in liquids, but less order than in typical molecular crystals. Compounds that exhibit such phases are called liquid crystals, since they share properties generally asso- ciated with both liquids and crystals. A more proper name for a LC molecule ismesogenand the phase it forms is known asmesophase(from the Greek “mesos”¼middle, between, intermediate). The terms “mesogenic” and

“mesomorphic” are often used as synonyms, but strictly spoken, they are not.

A mesogenic compoundhas all the structural characteristics that are required to form a mesophase, but when a mesogenic compound is heated, it does not necessarily form a mesophase. The term “mesogenic” tells something about the shape of the molecule, but nothing about the thermal behavior.Mesomor- phic compounds have the shape required for the formation of a mesophase and effectively exhibit a mesophase. The expression “mesomorphic” tells something about both the shape and the thermal behavior of a compound.

The motion of the molecules in liquid-crystalline phases is comparable with that of the molecules in an ordinary liquid, but the molecules maintain some degree of orientational order and sometimes some positional order as well (Fig. 2). A vector, called thedirector(n) of the LC, represents the pre-~ ferred orientation of the molecules.

In the case of thermotropic liquid crystals, a liquid-crystalline phase is obtained by heating a solid mesomorphic compound. At the melting point (Tm), the thermal motion of the molecules has increased to such an extent that the material passes from the crystalline phase to the liquid-crystalline phase.

A mesomorphic compound that exists in the glass state will enter the liquid- crystalline phase at the glass-transition temperature (Tg). On further heating, the orientational order of the molecules is lost as well. The LC transforms into an isotropic, clear liquid at the clearing point (or isotropization point; T_c).

Many materials are liquid-crystalline at room temperature. Several types of liquid-crystalline phases can occur between the solid state and the isotropic liquid state. Sometimes decomposition of the material occurs before the

Temperature

FIGURE 1 Schematic representation of the melting process of a nonmesomorphic molecular compound.

clearing point is reached. The LC-to-LC transitions and the LC-to-liquid transitions are essentially reversible and usually occur with only little hysteresis in temperature, in contrast to the recrystallization process, which can be subject to supercooling (due to the induction period of nucleation and crystallization).

When the mesophase is formed on both heating and cooling the material, the mesophase is thermodynamically stable and is termed enantiotropic. When the mesophase only appears on cooling a material below its melting point, and is therefore metastable (in fact, thermodynamically less stable than the crystalline phase), the mesophase is termed monotropic. One and the same compound can exhibit both enantiotropic and monotropic mesophases. For example, it is possible to have an enantiotropic nematic phase with an accom- panying, monotropic smectic A phase. The liquid-crystalline order can sometimes be frozen into a glassy solid state, by cooling the liquid-crystalline phase very rapidly, thus preventing the formation of nucleation sites to initiate crystallization. Phase transitions of a thermotropic LC are usually presented in the following way: Cr 50 Cr⁰60 M₁90 M₂120 I (C), for a material that exhibits a crystalline phase Cr between room temperature and 50C, another (distinct) crystalline phase Cr⁰ between 50 and 60C, one type of liquid- crystalline mesophase M1between 60 and 90C and another type of liquid- crystalline mesophase M2 between 90 and 120C, and that clears out into an isotropic liquid at 120C. Transitions to monotropic phases appear between round parentheses, for example, Cr(N 90)120 I. Upon heating, the compound melts at 120C into the isotropic phase, without going through a mesophase. Below 90C, the isotropic melts is transformed in a monotropic mesophase. The notation Cr 60 M 120 dec. designates a compound that melts at 60C to a mesophase M, and that decomposes at 120C before the clearing is reached. M is a general symbol for a mesophase. The use of the symbol M implicates that nothing can be said about the structure of the mesophase, oth- erwise more specific symbols have to be used (vide infra). In Table 1, an overview of the different mesophases formed by thermotropic LCs is given.

Another way to induce mesomorphism is by adding a solvent to the solid phase, which has a disruptive effect on the crystal lattice (lyotropic liquid crystals). The types of lyotropic mesophases formed depend on the type of

Crystal Liquid crystal

Liquid

Temperature FIGURE 2 Schematic representation of the melting behavior of a liquid crystal.

solvent, on the concentration of the compound in the solution and on the temperature. Usually lyotropic LCs areamphiphiles. In the compounds, different parts of the molecules have different solubility properties: the molecules con- sist of a very polar (hydrophilic) part (ionic or nonionic) and a nonpolar (hydrophobic or lipophilic) part (often hydrocarbon chains). One of the driving forces for lyotropic mesophase formation is the entropy-driven hydrophobic effect. Compounds that exhibit both thermotropic and lyotropic mesomorphism are termed amphotropic liquid crystals.

TABLE 1 Overview of Mesophases Formed By Thermotropic Liquid Crystals

Symbol Name Molecular shape

N Nematic phase Rod

N* Chiral nematic phase (cholesteric phase) Rod

SmA Smectic A phase Rod

SmA* Chiral smectic A phase Rod

SmC Smectic C phase Rod

SmC* Chiral smectic C phase Rod

SmB Smectic B phase Rod

SmI Smectic I phase Rod

SmF Smectic F phase Rod

B Crystal B phase Rod

J Crystal J phase Rod

G Crystal G phase Rod

E Crystal E phase Rod

K Crystal K phase Rod

H Crystal H phase Rod

cub Cubic phase Rod

ND Discotic nematic phase Disk

NC Nematic columnar phase Disk

Colh Hexagonal columnar phase Disk

Colr Rectangular columnar phase Disk

Colo Oblique columnar phase Disk

A detailed description of the molecular order in the mesophases can be found in the text.

2.2. Thermotropic Mesophases

Many factors determine the type of mesophase formed by thermotropic LCs, but the molecular shape is by far the most important one. The most common classification scheme of liquid-crystalline phases is therefore based on the specific anisometric shape of the constituent mesogenic molecules (Demus et al., 1998; Kelker and Hatz, 1980). This shape is usually rod-like (calamitic mesogens), disk-like (discoticmesogens), or V-shaped (bent-coreorbanana- shaped mesogens). Polycatenar LCs have a structure intermediate between that of purely rod-like and disk-like molecules and are also considered as a separate class of thermotropic LCs (Nguyen et al., 1997). Some kind of anal- ogy can be drawn between the mesophases exhibited by calamitic mesogens and the phases exhibited by discotic mesogens (Fig. 3). In both cases, the molecules can be described as cylinders with a high degree of structural

Nematic phase

Discotic nematic phase

Discotic mesogen Calamitic

mesogen

Columnar nematic

phase

Column (1D fluid) Layer

(2D fluid)

Columnar phases Smectic

phases

FIGURE 3 Mesophases exhibited by calamitic mesogens (rod-like shape) versus mesophases exhibited by discotic mesogens (disk-like shape).

by a simple cylinder seems to be a rather crude simplification, this approach turned out to be quite successful since mesogens in fluid phases are free to rotate (at different rates) about their molecular axis. The cylinder thus represents the average (or effective) shape of the molecule in the liquid-crystalline structure. Both calamitic and discotic mesogens are able to form nematic phases, which show only orientational order (vide infra). On the other hand, it is possible that these compounds show a stacking of the molecules, which leads to the formation of layers or columns. In this way, smectic or lamellar phases and columnar phases are formed, respectively (vide infra). Besides long-range orientational order, fluid smectic phases and columnar phases also show long-range translational order. It should be noted that discotic mesogens can also form a columnar nematic phase (NC), in which the column motif can be found. Liquid-crystalline mesophases originate from the self-organization of mesogenic molecules. The concept is called supramolecular self-assembly:

an internal system spontaneously opens a new route to increased complexity via molecular recognition processes. In the case of liquid-crystalline mesophases, the self-assembly process is driven by the (anisotropy of the)intermolecular interactions (dispersion forces, dipole-dipole interactions, p–p stacking between aromatic moieties, steric interactions, Coulomb forces or ionic interactions between charged moieties, hydrogen bonding or halogen bonding, charge-transfer interactions between electron donor and electron acceptor components, . . .) between the constituent molecules. By increasing the temperature, the molecular mobility increases, and as a consequence the weak interactions break up and the stronger interactions remain. The latter interactions dominate liquid state superstructures such as liquid-crystalline phases (but also micellar structures, etc.). Another, related driving force is so-calledmicrosegregation, caused by chemical or structural contrasts within the mesomorphic molecules (Tschierske, 1998). Examples are the incompati- bility between hydrophilic and hydrophobic moieties, polar and nonpolar regions, hydrocarbon and fluorocarbon parts, hydrocarbon and oligosiloxane parts, rigid and flexible moieties, etc. Compatible regions will interact with one another, while incompatible molecular segments segregate into distinct subspaces, thus causing microphase segregation. This driving force does not apply to nematic phases, but it is of prime importance in the case of amphi- philic mesogens, which are not necessarily anisometric. Owing to the chemical bonding between the incompatible regions, segregation does not lead to macroscopic phase separation, but it results in the formation of different regions which are separated by interfaces at a molecular scale.Macrophase separation gives rise to thermodynamically different phases, while microphase separation occurs in a single phase. The relative size and volume frac- tion of the incompatible segments determine the interface curvature and strongly influence the mesophase morphology. Another factor which deter- mines the type of mesophase is space filling. The whole space must be

“reachable” and filled up by the mesogenic molecules. The desire of rigid anisometric units to minimize the excluded volume (which is in fact a general organization principle of matter) adds extra limitations to the structures that can be displayed by thermotropic LCs.

The optically uniaxialnematic phase (N) is the least ordered mesophase exhibited by calamitic mesogens. Its name is derived from ancient Greek nematos (¼thread-like), because of the characteristic thread-like texture observed by polarizing optical microscopy. It can be formed when rod-like mesogens align their long molecular axes to a certain degree along a common direction defined by a vector, the director n~ (Fig. 4). This results in long- range orientational order with full rotational symmetry aroundn. However,~ the molecular centers of mass are, as in an ordinary fluid, arranged randomly in all directions without long-range correlation, so there is no long-range positional or translational ordering. The nematic phase can be considered as a one- dimensionally ordered elastic fluid, wherein the molecules can move freely.

While the molecules are free to rotate about their long molecular axis, rotation about their short molecular axis can occur to a certain degree as well. In the bulk nematic phase, there are as many molecules pointing in one direction relative to the director as there are pointing in the opposite direction (i.e., the molecules have a disordered head-to-tail arrangement in the phase). The director’s orientation varies with temperature, pressure, applied electric and magnetic fields, and—for mixtures—composition. It is also influenced by the thickness of the cell containing the LC sample, and by possible treatments of the cell surfaces. Many practical applications of LCs rely on the control of the molecular orientation by external stimuli. As the nematic phase is the least viscous mesophase displayed by calamitic mesogens, fast switching of the molecules (i.e., changing the director’s orientation, usually by application of an external electric field) is possible. This is the reason why the nematic phase is of interest for display applications. In thechiral nematic phase (N*), the director precesses through the material, describing a helix (Fig. 4). The pitch of a chiral nematic phase is the distance along the helix over which the director

n^~

FIGURE 4 Schematic representation of the molecular order in the nematic phase (left), and schematic representation of the molecular order in the chiral nematic phase (right).

because it was first observed for cholesterol derivatives. Chirality can directly be introduced within the mesomorphic molecules by incorporation of chiral elements, mostly chiral centers (asymmetric carbon atoms).

Smectic phases are characterized by a one-dimensional (1D) periodic stacking of “layers” formed by orientationally ordered LC molecules. The name is derived from ancient Greeksmegma(¼soap), because it was already known in the beginning of the twentieth century that soaps formed smectic mesophases. Smectic phases are also calledlamellar phases. In the most typical cases, each layer can be seen as a two-dimensional (2D) anisotropic fluid.

Smectic phases thus have a fluid-like structure in two dimensions combined with a long-range positional/translational order in the third dimension, normal to the smectic layers. The thickness of the layers is the so-called layer period- icityd. The smectic layers are not necessarily well defined, but can be rather flexible and often show a curved arrangement. The formation of smectic layers is mainly driven by microsegregation.

Thesmectic A phase(SmA) is the least ordered smectic modification, and therefore it shows the lowest viscosity of all smectic mesophases. The molecules in the smectic layers have, on average, their long molecular axes parallel to the normal to the layer planes (Fig. 5). The lateral distribution of the molecules within each layer is random, and the molecules have a considerable free- dom for rotation about their long molecular axis and even for translation in the smectic layer. The molecules can also be tilted to some degree within the individual layers. However, since the tilt direction is random, the situation is averaged out to give a mean orthogonal, uniaxial arrangement of the long molecular axes. In general, the layer thickness is not constant, but fluctuates as a function of the local average tilt angle and as a function of the temperature. The smectic layers are free to slide over one another and relatively dif- fuse. In the smectic C phase (SmC), in contrast to the SmA phase, the molecules are tilted within the layers, along a preferred direction (Fig. 5).

Consequently, the directorn~is not parallel anymore to the normal to the layer planes. Thetilt angleytis the angle between the director and the normal to the layer planes. Just like the SmA phase, the SmC phase is a fluid smectic phase:

the molecules are arranged randomly within each layer and can move within the layer, and the layers are free to slide over one another. This inherent

n^~ n^~

FIGURE 5 Schematic representation of the SmA phase (left) and the SmC phase (right).

Dalam dokumen Handbook on the Physics and Chemistry of Rare Earths (Halaman 38-52)