Background and Overview

Abstract

In this chapter, the characteristics and potentials of “soft crystals” are compared with those of conventional hard crystals after providing a historical background. In addition, representative examples of “soft crystals” are discussed, and their thermodynamic models are qualitatively described.

1 Background and Significance

Crystals are solid structures with regularly arranged atoms, molecules, or ions that exhibit anisotropic properties unlike randomly oriented amorphous solids. The characteristic properties of crystals have been utilized to develop materials that require fine tuning of their electrical, magnetic, optical, and other parameters.

Historically, Max Theodor Felix von Laue from Germany observed X-ray diffraction from crystals in 1912, which confirmed that X-rays were electromagnetic waves. In 1913, William Henry Bragg and his son William Lawrence Bragg from England discovered Bragg's law, which formulated the relationship between the X-ray diffraction wavelength and the distance between diffracting planes in crystals. Based on this law, we can determine not only the structures of natural crystals such as minerals, but also those of single crystals consisting of artificially synthesized compounds. Similarly, the structures of complex biomolecules, such as proteins and deoxyribonucleic acids, were elucidated by preparing their single crystals. Thus, crystals and their analytical methods played important roles not only as functional materials with regular arrangements and/or anisotropic characteristics but also as materials that helped achieve a better understanding of various biological phenomena.

According to the definition provided by the International Union of Crystallography in 1992, “crystal” denotes any solid that produces an essentially discrete diffraction pattern [1]; however, many people think that crystals should be hard and stable like diamonds. The hardness of crystals mainly originates from stable interatomic bonds, which are regularly arranged in three dimensions. Meanwhile, in the past decade, a group of crystals with characteristic molecular rearrangements in the solid state caused by gentle external stimuli have attracted considerable attention from researchers. For example, single crystal ↔ single crystal phase transitions accompanied by color changes occurred in response to specific volatile organic compounds. In addition, remarkable photoluminescent color changes were detected during mechanical grinding. These phenomena were observed in a wide range of materials mainly consisting of molecules, such as organic and inorganic molecules, metal complexes, and coordination polymers. In contrast to the previously known crystals, which are thermodynamically stable and rigid, these new materials are classified based on their ability to undergo structural transformations in response to macroscopic gentle external stimuli, while their stable crystals can be synthesized. The class of these materials, which are different from conventional and liquid crystals, was named “soft crystals”, and their concept paper was published in 2019 [2]. Typical examples of “soft crystals” are provided in Fig. 1.1.

Fig. 1.1

Typical examples of “soft crystals”. Reprinted with permission from ref. [2]

How can macroscopic gentle external stimuli change the solid-state nanometer-scaled molecular assembled structures? This apparent conundrum cannot be solved by conventional science and is similar to the initial stages of previous scientific discoveries. The first example includes the discovery of liquid crystals serving as intermediates between liquid and crystals. F. Reinitzer, a botanist in Austria, discovered an unusual phenomenon, two melting points, when studying the functions of cholesterol in plants. In 1888, he wrote a letter to O. Lemann, a physicist in Germany, about this phenomenon, which was a landmark in the development of liquid crystals. After a press release about the birth of a liquid crystal display (LCD) was issued by RCA Laboratories in U.S.A. in 1968, more than 100 million LCD monitors per year have been manufactured in recent years. Another example is the discovery of metal complexes constituting various soft crystals. In the late nineteenth century, it was believed that salts consisted of atoms with simple ratios or their multiples; however, many exceptions from this rule were observed. Later, these exceptions were assigned to the metal complexes named “complex salts” because of their initial complexity. The first step in the discovery of coordination bonds and coordination chemistry was the systematization of the coordination theory by Alfred Werner in Switzerland. With the further development of metal complexes and supramolecules, organic light-emitting materials were constructed owing to the strong phosphorescence of metal complexes. Compared to the development of liquid crystals or metal complexes, which produced a significant impact on the society, “soft crystals” are expected to become a class of materials strongly influencing future technological innovations. Thus, it is extremely important to summarize, classify, and systematize their main properties.

2 Structure of This Book

This is the first book focusing on the science of “soft crystals” by classifying and systematizing various soft crystals’ phenomena because it is scientifically important to present new ways of interpreting newly discovered data. The syntheses of molecular crystals, which contain not only various types of atoms but also a mixture of “strong intramolecular atomic bonds” and “weak but non-negligible intermolecular atomic interactions”, are very complex and continue to depend on serendipity and screening. Therefore, because to clarify and control the formation and phase transitions of molecular crystals has remained the most challenging issue in molecular science and technology in recent decades, this book intends to address them to contribute to the future development of related fields.

The next Sect. 1.3 provides an overview of the initial representative examples of “soft crystals”, and their thermodynamic properties are qualitatively described in Sect. 1.4. In Chap. 2, “soft crystals” are compared with other materials and defined more precisely. In Chap. 3, the photophysical properties of molecules and molecular crystals are briefly explained by considering their colors or luminescence colors. In Part II, the latest studies on various “soft crystals” are discussed in detail. In Part III, potential applications of “soft crystals” as future functional materials are discussed with reference to boundary region studies related to soft matter and/or device engineering.

3 Soft Crystals: An Overview

If you search for “soft crystals” online, you will find multiple papers. Even hard metal crystals such as iron soften and eventually melt at high temperatures. Crystals near the melting point can be called soft crystals, which have been the subject of recent research studies because of their unique properties such as atomic diffusion [3]. We might even say that rock salt, i.e., the ionic crystal of sodium chloride, is soft compared to those of metal crystals because they shatter when crushed even at room temperature. Crystals of organic molecules or metal complexes are also weak in terms of mechanical strength. In particular, crystals composed of neutral molecules have low melting points and are relatively soft. In the field of liquid crystals, the soft crystal phase located at the boundary of the crystal phase was proposed as one of the smectic liquid crystal phases [4]. In addition, the flexible properties and mechanical functions of organic molecular crystals formed by van der Waals forces and hydrogen bonds have attracted considerable attention [5]. Metal–organic frameworks (MOFs) undergo flexible structural changes due to the adsorption and desorption of gas molecules; therefore, they are called porous soft crystals or flexible MOFs [6]. In the following sections, we discuss some specific examples that enabled defining certain materials as soft crystals and describe their properties.

3.1 Vapochromic Crystals

The reversible color change induced by organic vapors such as alcohol and ether or inorganic gases such as hydrogen chloride and sulfur dioxide is called vapochromism. As an example, a luminescent vapochromic platinum dinuclear complex discovered by Kato et al. in their early work is shown in Fig. 1.2 [7]. When a crystal of this compound is exposed to organic vapors, its color change occurs between bright red and dark red, and a visual ON–OFF change of luminescence is detected (Fig. 1.2b). This phenomenon is observed only for the structure depicted in Fig. 1.2a (which is called a syn-type isomer) and not for isomers with different arrangements of bridging ligands (anti type). The color change is caused by the crystal structural transformation induced by the entry and exit of vapor molecules. An analysis of the structural changes caused by the single crystal-to-single crystal conformational transition revealed that the arrangement of two dinuclear complexes in the crystal switched from a close arrangement between intermolecular Pt atoms (the right panel in Fig. 1.2c) to a distant arrangement between them (the left panel in Fig. 1.2c) upon the release of the enclosed crystalline solvent molecules (left). Vapochromism has attracted significant attention as a process that can be potentially used to easily and sensitively detect volatile organic compounds (which are considered the cause of a sick building syndrome) and acidic exhaust gases representing environmental pollutants. Since 2000, various vapochromic crystal systems have been developed [8]. Vapochromism is also scientifically important as an emergent phenomenon of solid–gas interactions.

Fig. 1.2

a Structural formula of the syn-isomer of a dinuclear Pt(II) complex. b Vapochromic behavior of crystals (the white scale bar in the left photograph has a length of 100 μm). c Vapor-induced structural transformation of the dimer-of-dimer. Reprinted with permission from ref. [7]

3.2 Mechanochromic Crystals

The color change of solid materials in response to weak mechanical stimuli such as touch, grinding, or scratching is called mechanochromism [9]. Sometimes, the terms tribochromism (friction) and piezochromism (pressure) are also used to describe similar phenomena. Mechanochromism has been known for a long time, and related processes such as the color change due to pulling and glowing caused by pressing occur not only in crystals but also in polymers and inorganic solid powders [10]. In recent years, the number of publications on mechanochromic crystals has increased dramatically because the recent advances in X-ray structural analysis methods have allowed easy determination of the three-dimensional structures of even very small crystals by synthetic chemists. As one of the starting points of this trend, the luminescent mechanochromic Au(I) complex synthesized by Ito et al. is shown in Fig. 1.3a [11]. A crystalline powder of this complex emits blue-colored luminescence under UV light; however, after gently grinding with a spatula, the luminescence color changes to yellow (Fig. 1.3b). This phenomenon is caused by the structural change from the crystalline to amorphous phase, and the observed luminescence color change is attributed to the change in the interactions between gold atoms (Fig. 1.3c). The luminescence returns to the original crystalline blue state after the dropwise addition of solvent. Araki et al. found that the luminescence color changed from blue to green after rubbing or pressing the crystalline sample of a tetraphenylene derivative and returned to the original crystalline state after heating, which were attributed to the changes in intermolecular interactions (Fig. 1.4) [12].

Fig. 1.3

Mechanochromic luminescence of a Au(I) complex. a Structural formula of the Au(I)-isocyanide complex and b luminescence color change accompanied by c the crystal-to-amorphous transformation during grinding. Reprinted with permission from reference [11]

Fig. 1.4

Piezochromic luminescence of a tetraphenylpyrene derivative. Reprinted with permission from reference [12]

3.3 Organic Crystals Exhibiting Superelasticity or Ferroelasticity

Superelasticity, a property in which a material is deformed by mechanical loading but returns to its original state upon removing the load, had been observed only for a limited number of metal alloys such as Ni–Ti. Takamizawa et al. reported the first superelastic crystal of an organic compound in 2014 [13]. As shown in Fig. 1.5a, a terephthalamide crystal is deformed by pressing the crystal’s right side with a needle tip but returns to its original shape after releasing the load. In the deformed area, the optical property of the crystal is changed because of the phase transition. The authors also found the shape memory effect of an ionic organic crystal that was plastically deformed but returned to the original crystalline shape after heating (Fig. 1.5b) [14]. Furthermore, crystals possessing ferroelasticity [15] or superplasticity [16] were reported as well. They undergo macroscopic morphological changes upon weak mechanical stimuli, which are literally soft crystals. Recently, the characteristic properties owing to microscopic structural changes in bent or twisted organic molecular crystals have attracted significant interest from organic crystal researchers worldwide [17]. In addition to crystallographic studies, a superelastic chromic crystal that changes luminescence color upon pressing the crystal has recently been discovered, and the functionalization of organic superelastic crystals is expected to be developed [18].

Fig. 1.5

a Structural transformation followed by phase transition upon the application of slight shear stress to a telephthalamide single crystal. b Shape recovery of the deformed zigzag tetrabutyl-n-phosphonium tetraphenylborate crystal induced by heat sweeping. Reprinted with permission from ref. [13, 14]

4 Thermodynamic Images: Stimulus Versus Potential Energy

When studying soft crystals that undergo structural transformations in response to macroscopic gentle external stimuli, it is important to thermodynamically consider potential energy changes corresponding to these transformations. For vapochromism, mechanochromism, and superelasticity, the qualitative images of potential energy changes are shown in Fig. 1.6.

Fig. 1.6

Schematic potential energy diagrams constructed for the models of vapochromic, mechanochromic, and superelastic crystals

The left panel of Fig. 1.6 qualitatively illustrates the vapochromic behavior of a crystal. Here, the horizontal q₁, vertical, and the third q₂ axes denote the uptake of vapor molecules by the crystal, Gibbs free energy of the entire system, and intermolecular arrangement, respectively. This indicates that the uptake of vapor molecules by the crystal leads to structural transformations including both intramolecular structural changes and intermolecular rearrangements accompanied by color and/or luminescence color changes. The middle panel of Fig. 1.6 thermodynamically describes typical mechanochromic behavior. Here, the horizontal and vertical axes denote the intermolecular arrangement and Gibbs free energy, respectively, and the initial potential energy surface (gray line) is compared with that obtained during macroscopic mechanical grinding (black line). In this model, the local minimum structure is varied under gentle grinding, after which structural transformations accompanied by color and/or luminescence color changes occur even without an additional heat energy because of the decrease in activation energy. The right panel of Fig. 1.6 shows the superelasticity of a crystal. Here, the horizontal q₁, vertical, and the third q₂ axes denote the bending of the crystal, the Gibbs free energy, and intermolecular arrangement, respectively. The molecular crystal is flexibly bended throughout structural transformations because the energy relationship between the two local minimum structures is changed under bending.

Importantly, “soft crystals” may undergo structural transformation even at room temperature (ΔG^‡/kT ~ 1) (i.e., approximately 2.5 kJ mol⁻¹) under intrinsic gentle external stimuli, although the as-synthesized crystals are stable without these stimuli. Thus, they can be potentially used as the next-generation stimuli-responsive materials with highly ordered structures for sensors and luminescent and/or electronic devices. In the next chapter, the question “what are soft crystals?” is addressed.