Soft Crystals

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.

“Soft crystals” typically undergo structural transformations in response to weak stimuli while maintaining the crystalline structural order. These transformations are often manifested as visible phenomena, such as changes in optical properties (color and/or luminescence color). In this chapter, the structural order, activation energy, and softness of “soft crystals” are compared with those of conventional hard crystals and soft materials, including liquid crystals and gels. Based on the results of this comparison, “soft crystals” are defined more precisely.

In this chapter, we aim to explain the chromic phenomena observed in “soft crystals”. Accordingly, the basic principles for comprehending the fundamental photophysical properties of molecular monomers, such as electronic absorption spectra and luminescence properties, are introduced. Moreover, the photophysical properties of molecular dimers and molecular crystals are explained in terms of intermolecular interactions in excited states.

Vapochromism, a phenomenon in which the color or luminescence color of a substance changes in response to gaseous molecules, has potential for developing sensor materials to detect harmful substances in the environment. In addition, vapochromism is scientifically interesting for the direct visualization of interactions between gases and solids. The crystals of metal complexes involve diverse and flexible electronic interactions, such as metal–metal and metal–ligand interactions. It is expected that slight structural changes in such crystals will lead to distinct color or emission color changes, thus achieving highly sensitive and selective vapochromic responses. Consequently, highly ordered and flexible response systems (i.e., soft crystals) can be constructed. This chapter introduces the interesting and attractive features of vapor-responsive soft crystals by discussing platinum complexes that show color and luminescence changes in dilute vapor atmospheres while maintaining an ordered structure, nickel(II) complexes that change magnetic properties in conjunction with a color change, and copper(I) complexes that change luminescence color in response to N-heteroaromatic vapors.

A study of stimuli-responsive molecules that can change their physical properties or external shape owing to variations in the external environment has attracted much attention owing to potential application in sensors and actuators. Our group has intensively studied aryl gold(I) isocyanide complexes to develop stimuli-responsive molecular crystals that can show luminescent mechanochromism and crystal jumping through phase transitions induced by mechanical stimulation or photoirradiation. Interestingly, some of our gold(I) isocyanide complexes have crystalline or even single crystalline characteristic both before and after mechano-induced emission color changes or photoinduced crystal jump. Based on the detailed information on molecular arrangements of the aryl gold(I) isocyanide complexes, the underlying mechanism of the responses can be clearly identified. In the Sect. 5.2 of this chapter, we review luminescent mechanochromic aryl gold(I) isocyanide complexes that has unique characteristic such as multiple emission colors, infrared emission, and noncentrosymmetry/centrosymmetry switching. Section 5.3 describes the mechano-induced single-crystal-to-single-crystal phase transitions of aryl gold(I) isocyanide complexes with red- and blue-shifted emission color changes or reversibility. In Sect. 5.4, the photoinduced phase transition of a gold(I) complex which accompanied by mechanical motion, i.e., crystal jump is described.

Superelasticity is the ability of a plastically deformed solid to spontaneously recover its shape upon unloading due to stress loading. From its discovery in Au–Cd alloys in 1932, superelasticity had been believed to be limited to certain alloys until the discovery of “organic superelasticity” in 2014, which revealed it to be a general phenomenon observed in organic crystals along with ferroelasticity—reversible plastic deformability that is not accompanied by spontaneous shape recovery. In this chapter, we will introduce the discovery of organic superelasticity and the shape-memory effect, discuss superelasticity and ferroelasticity, and explain the properties and characteristics of various molecular crystals, including metal complexes. Furthermore, “organic superplasticity”—irreversible plastic deformation of several hundred percent or more, accompanied by the retention of crystallinity—has also been described.

The photoluminescence of lanthanide complexes originating from f–f transitions is generally sensitized through energy transfer from the ligand to the lanthanide ion in the excited state under UV irradiation. This phenomenon is known as the photo-antenna effect. Luminescence driven by mechanical stimuli, such as tapping or rubbing, is called mechanoluminescence or triboluminescence (TL). In recent years, reports on TL in rare-earth complexes, which have attracted attention as novel luminescent materials that do not require an electrical excitation source, have steadily increased. In this chapter, we focus on triboluminescent lanthanide complexes. Specifically, we introduce the history and detection methods of TL and cite recent examples of materials demonstrating this phenomenon, particularly coordination polymer-like and discrete molecular crystalline lanthanide complexes. Finally, we summarize the application prospects of these complexes as soft crystals.

Molecular crystals have a regularly packed structure, and their physical properties often depend on intramolecular and intermolecular interactions. Here, we review the crystal jumping phenomena under a thermal stimulus (thermosalient phenomenon). Thermosalient phenomena are characterized by thermal phase transitions and anisotropic lattice expansion/contraction at a microscopic scale and jumping behavior through bending/deformation/rotation/cleavage of crystals at a macroscopic scale. The absence of strong intermolecular interaction in the crystal and the misalignment of the crystal plane associated with the phase transition are explained as factors causing the thermosalient phenomena. In this chapter, various case studies with representative molecular crystals that exhibit the thermosalient phenomenon are explained in detail.

Chemiluminescence (CL) is a phenomenon in which a chemical reaction produces an excited-state product that emits light. Taking advantage of this property, several analytical methods to study the CL reactions by photon detection have been developed in the literature. By applying this methodology to molecular crystals, soft crystal CL systems have been constructed to analyze the intracrystalline reactions of chemiluminescent compounds. In this chapter, the fundamental concept and applications of CL are presented. Using the example of the CL reactions involving organic peroxides, important characteristics of CL such as chemiexcitation, quantum yield and emission wavelengths are discussed. Furthermore, CL in solid state and in molecular crystals are described. Finally, the application of organic peroxides as a soft crystal CL system and the characteristics of their intracrystalline reactions such as crystal structure-dependencies, reaction kinetics and inductions of phase transitions are elucidated. This chapter concludes with a brief outlook towards the future of soft crystal CL systems.

To establish the theory of soft crystals, computational chemistry must be applied to analyze the structural phase transitions of molecular crystals and develop new methodologies. The accuracy of first-principles calculations for molecular crystals has rapidly improved over the last decade with the contribution of the Cambridge Crystallographic Data Centre blind test, which predicts the crystal structure from the structural formula. However, it is often difficult to apply first-principles calculations to large molecular crystals, such as typical soft crystals, because of the computational cost. In this chapter, we review the applicability of crystal force field calculations as an alternative method for theoretically analyzing molecular crystals. We also introduce some examples of our previous collaborations and discuss the promising methodologies to elucidate the soft crystal phenomena.

Nowadays, the importance of molecular crystals and solids with regular structures is increasing in both basic chemistry and applied fields. However, theoretical studies of those systems based on electronic structure theories have been limited. Although density functional theory (DFT) calculations using generalized gradient approximation type functional under periodic boundary condition is effective for such theoretical studies, we need some improvements for calculating the dispersion interaction and the excited state of crystals. Accordingly, in this chapter, two methods for calculating the electronic structures of molecular crystals are discussed: cluster-model/periodic-model (CM/PM)-combined method and quantum mechanics/periodic-molecular mechanics (QM/periodic-MM) method. In the CM/PM-combined method, an infinite crystal system is calculated by the DFT method under periodic boundary condition, and important moieties, which are represented by CMs, are calculated by either DFT method with hybrid-type functionals or wave function theories such as the Møller–Plesset second-order perturbation theory (MP2), spin-component-scaled-MP2, and coupled-cluster singles and doubles theory with perturbative triples (CCSD(T)). This method is useful for gas adsorption into crystals such as metal–organic frameworks. In the QM/periodic-MM method, an important moiety is calculated using a QM method such as the DFT method with hybrid-type functionals and wave function theories, where the effects of the crystal are incorporated into the QM calculation via the periodic MM method using a classical force field. This method is useful for theoretical studies of excited states and chemical reactions. The applications of these methods in the following processes are described in this chapter: adsorption of gas molecules on metal–organic frameworks, chemical reactions in crystals, and luminescence of the crystals of transition metal complexes. To the best of our knowledge, the theoretical calculations conducted in this chapter show one of the successful approaches of electronic structure theories to molecular crystals, because of the reasonable and practical approximations.

In this chapter, to design functions of soft crystals, the relationships between gentle stimuli and the corresponding responses are classified based on the photofunctions, and the key terms are linked and then systematized. Also, trials of functionalization based on soft crystal-related phenomena are introduced toward the functionalization of soft crystals.