Keywords

1 Introduction

We live by responding to various stimuli sensitively, adapting to the environment, and communicating with other people. Biosystems have various wonderful stimuli-responsive functions to be living matters. Various efforts have focused on developing materials that mimic how living systems interact with the environment. In the material world, crystals may look like far from biosystems. However, soft crystals, that show visible and distinct color and luminescence changes in response to weak stimuli while maintaining structural order are attracting increasing attention as new stimuli-responsive systems [1]. Various external stimuli, such as heat, light, pressure, chemicals, and mechanical stimuli, can change the structure and properties of crystals, and a wide range of stimuli-responsive materials are known, including metals, polymers, and liquid crystals. Soft crystals are expected to offer new possibilities because their structures and properties can be transformed by mild stimuli while maintaining three-dimensional order, and the original order can also be recovered. This chapter focuses on metal complex crystals that respond to gaseous molecules, such as water vapor, organic vapor, and inorganic gases, by changing their color or emission color.

2 Soft Crystals and Vapochromism

Vapochromism involves the color change of a material in response to gas molecules. This phenomenon is relatively new compared with other types of chromism, such as thermochromism, photochromism, and piezochromism. Many systems showing chromism have flexible structures that undergo structural transformations and exhibit multichromic properties in response to various external stimuli. It has long been known that platinum complexes exhibit temperature- and pressure-dependent color changes in the solid state. However, the term vapochromism was not used until the end of the twentieth century to describe double-salt-type platinum(II) complexes that change color when exposed to chloroform or alcohol vapors [2]. Subsequently, various systems have been constructed as volatile organic compound (VOC) sensors to visually detect the vapor of harmful organic solvents [3]. Vapochromism is also of considerable scientific interest as a visible manifestation of the interaction between a gas and a solid. Vapochromic materials are not limited to crystals, with amorphous materials, liquid crystals, and ionic liquids also known to show color changes in response to vapor. However, the highly ordered three-dimensional nature of crystals enables this phenomenon to be traced at the molecular level.

3 Interactions Between Vapor Molecules (Gases) and Crystals

The interactions between gases and crystals are commonly exploited in gas storage materials. Palladium is used for hydrogen purification owing to its high hydrogen mobility and excellent hydrogen storage capacity. In this case, stable hydrides form via interactions between small hydrogen molecules and palladium metal crystals. In addition, inorganic porous materials such as zeolites, metal–organic frameworks (MOFs), and porous coordination polymers (PCPs) have been extensively studied. Porous materials have attracted attention as rigid and stable materials with nanospaces. However, some porous materials undergo structural changes as the framework expands and contracts with gas adsorption and desorption, and the mechanism of structural change in such soft porous crystals has been studied [4]. Materials containing nanospaces, including supramolecular and covalent structures, have been widely developed to have not only gas storage properties but also novel physical properties such as conductivity and magnetism. Furthermore, the nanospaces in such materials have been used to modulate reactivity. Here, we focus on vapor-responsive photofunctional crystals that show distinct visible changes in color or luminescence [5]. Although such vapochromic crystals are found in coordination polymers (so-called flexible MOFs), more flexible systems with electronic interactions, such as supramolecular crystals constructed using weak intermolecular interactions and molecular crystals constructed using van der Waals forces, are the main targets. Vapor can also affect soft matter and thin-film surfaces. For example, it has been reported that vapor exposure can induce a liquid-crystalline-to-crystalline phase transition, and such vapor annealing has attracted attention as a strategy for constructing highly ordered systems [6].

To achieve vapochromism, a gas molecule must induce a change in the electronic state of a crystal that results in a change in color or luminescence. In metal complex crystals, the effects of gas molecules on vapochromism can be classified into three main categories (Fig. 4.1). First and most commonly, a new crystalline phase is formed by the inclusion of gas molecules (Fig. 4.1a). In this case, the color change is caused by a structural change in the complex, and the included gas molecules are involved in stabilizing the new crystal phase, which is similar to a solvate in the crystallization from a solution. If the molecules are released by heating or decompressing, the original structure is recovered. Second, gas molecules are directly coordinated to the metal center (Fig. 4.1b). Here, the mobility of the gas molecules in the crystal is critical for changing the crystalline state. There is an interesting example reported recently for manganese(II)-halide complexes which shows reversible tetrahedral-octahedral geometrical change by water–vapor exposure and heating, followed by the luminescence color change between green and red [7]. Third, the vapor molecules change the crystalline environment (dielectric constant, pressure, etc.), which induces a crystal phase transformation, but are not incorporated into the crystal (Fig. 4.1c). It can be said to be real polymorphism induced by vapor, although those cases are not so many compared with the others [8]. In all cases, the crystal phase transition should be easily induced under mild conditions. In other words, soft crystals with low activation barriers for structural changes and crystal polymorphism should be designed. In the next section, typical vapochromic metal complex crystals are introduced as representative examples of these cases.

Fig. 4.1
Three illustrations, a to c, are of two square-shaped structures in different shades, with two sets of three horizontal lines on both sides with tiny balls attached to one end. Illustration C with no balls are indicated.

Schematic of the effects of gas molecules on vapochromism

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4 Vapochromic Metal Complexes

4.1 Vapochromism in Integrated Luminescent Platinum(II) Complexes

Commonly, d8 and d10 metal complexes containing soft metal ions show unique coloration and luminescence. In particular, the luminescence properties of planar tetracoordinated platinum(II) complexes have long been studied. The luminescence originates from electronic interactions between stacked platinum(II) complexes. The stacking of platinum(II) complexes with appropriate π-conjugated ligands results in the overlap of platinum ion \({d_{{z^2}}}\) orbitals, which split into dσ and dσ* states. Consequently, a metal–metal-to-ligand charge transfer (MMLCT) state is formed, which has the lowest energy transition, and luminescence occurs from the excited triplet state (3MMLCT) (Fig. 4.2) [9]. Because the structure constructed by weak metal–metal interactions is flexible, the MMLCT transition energy is sensitive to small changes caused by external stimuli. Unsurprisingly, MMLCT-derived platinum and gold complexes have recently been reported to have excellent stimuli-responsive properties and remain the subject of active research. Incidentally, aggregation-induced emission (AIE) has attracted much attention in organic crystals and organic polymers [10]. AIE is a phenomenon in which the luminescence of organic molecules is switched on and off using orientation changes caused by hydrogen bonding, π–π interactions, CH–π interactions, etc. Here, the luminescence based on metal–metal interactions caused by the accumulation of complexes is called assembly-induced emission and is considered distinct from aggregation-induced emission.

Fig. 4.2
An illustration depicts plates of P subscript z of p t with P sigma superscript asterisk and p sigma. Pi superscript asterisk of ligand, Pi of ligand, and d subscript z superscript 2 of p t with d sigma superscript asterisk and d sigma. At the bottom, isolated has one, stacked has two, and isolated has one rectangular plate with a ball at the center.

Schematic MO diagram for Pt(II) complexes with aromatic ligands showing effective Pt···Pt electronic interactions by stacking. Reprinted with permission from ref. [9]

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As a typical vapochromic platinum(II) complex, syn-[Pt2(pyt)2(bpy)2](PF6)2 (pyt = pyridine-2-thiolate, bpy = 2,2′-bipyridine) (Chap. 1, Fig. 1.2), which exhibits reversible on/off luminescence in response to vapor molecules such as ethanol, was introduced in Chap. 1 [11]. In this dinuclear complex, vapochromism is accompanied by a structural transition between a dark-red form with incorporated vapor molecules and a light-red form without vapor molecules. The two stable configurations of this complex (tetranuclear platinum interaction configuration and stacked bpy ligand configuration) are easily converted by vapor molecules, providing a guideline for interaction conversion in soft crystals.

Mononuclear platinum(II) complexes with planar structures can self-assemble and form Pt···Pt interactions. Complexes that are colorless when dispersed in dilute solution often show bright colors and luminescence when crystallized via self-assembly. A typical example is the platinum(II) complex [Pt(bpy)(CN)2], which comprises bpy, a typical aromatic bidentate chelate ligand, and cyanide ligands that provide a strong ligand field. Two forms of this complex can be produced by a slight change in the stacking structure (Fig. 4.3): a red hydrated crystal and a yellow anhydrous crystal [12]. Interestingly, this complex shows environmental sensitivity. When the yellow form (monohydrate) is exposed to organic solvents such as DMF or ethanol, it instantaneously changes to the red form (anhydrate), whereas the red form returns to the yellow form upon exposure to water. This change can be controlled using water vapor and is accompanied by a change in the luminescence color, which is typical of vapochromic platinum complexes. The color change has been attributed to changes in the interplatinum interactions, as shown in the stacking structures of the crystals. The correlation between the interplatinum distance and the MMLCT transition energy has been studied in detail based on temperature-dependent data [13], but this is not discussed here. Note that the correlation between the interplatinum distance and the excited state energy in the ground state can be explained by the delocalization of the excited state. The theoretical interpretation of the emission state change is given in Chap. 11.

Fig. 4.3
Two illustrations of ball and stick structures, which are aligned in four layers, are connected by a line on both sides. In the first illustration, two P T and 3.34 angstrom at R T are marked, while in the second illustration, 3.33 and 4.68 angstroms of measurement are marked.

Stacking structures of the red and yellow forms of [Pt(bpy)(CN)2]. Reprinted with permission from ref. [12]

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To develop a distinct vapochromic response using platinum interactions, it is effective to construct a soft crystal structure in which vapor molecules can easily form gaps to allow their entry and exit. Based on this strategy, a variety of vapochromic platinum(II) complexes with long-chain alkyl groups or bulky substituents have been reported [3]. In the author's group, a hydrogen-bonded network structure was constructed using a complex with carboxy groups, [Pt(CN)2(H2dcbpy)] (H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine), which produced a variety of colors ranging from white to yellow, red, blue, and purple depending on the vapor type (Fig. 4.4a) [14]. Figure 4.4b shows a schematic of the response of this system to vapor. Vapor molecules incorporated in the crystal pores affect the three-dimensional framework of the platinum complexes, which induces changes in the Pt···Pt interactions and causes the 3MMLCT luminescence color to change. Interestingly, the supramolecular framework is stable enough to retain its structure upon the release of enclosed water molecules and the emission color remains essentially unchanged. However, the framework structure is easily collapsed by macroscopic mechanical abrasion, and the sample shows a distinct color change related to changes in intermolecular interactions. Moreover, the collapsed framework structure can be reconstructed by exposure to methanol vapor, thus restoring the original red luminescent crystals. The process of crystal structure construction by the penetration of gas molecules from the surface was analyzed in detail using super-resolution microscopic luminescence observations of single particles [15]. Thus, this system is notable for exhibiting both mechanochromism (structural destruction) and vapochromism (structural restoration and self-healing phenomena). Chromic luminescence based on similar metal–metal interactions has also been observed in gold(I) complexes. The vapochromic phenomena of gold(I) complexes are introduced together with their mechanochromic phenomena in the next chapter (Chap. 5).

Fig. 4.4
2 illustrations. a. A group of connected chemical structures with a highlighted square at the center. b. A diagram illustrates vapochromic behaviors including molecular recognition.

a Porous supramolecular structure of [Pt(CN)2(H2dcbpy)] crystals, and b schematic diagram of vapochromic behavior, including vapor-induced crystallization, self-healing, and molecular recognition, depending on the state. G1 and G2 represent guest molecules. Reprinted with permission from refs. [14a, b]

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4.2 Vapochromism and Single-Crystal-To-Single-Crystal Structural Transitions

As mentioned above, the energy of 3MMLCT luminescent state formed via platinum interactions depends on the strength of the interaction between the metal \({d_{{z^2}}}\) orbitals, but the relationship with the energy level of the ligand π* orbital, which is the LUMO, must also be considered. In platinum(II) complexes with 3MMLCT emission, diimine and cyclometalating ligands have been successfully used as aromatic ligands. As shown in Fig. 4.2, the 3MMLCT transition energy based on Pt···Pt interactions is lower in energy than the π–π* (LC) and d–π* (MLCT) transitions of a mononuclear complex. Thus, for conventional systems, 3MMLCT emission generally occurs in the red region. An assembled system with very weak Pt···Pt interactions and an unstable LUMO could realize MMLCT emission at a higher energy. However, the precise control of weak Pt···Pt interactions is difficult in flexible media such as solutions and even in crystals. An integrated system with yellow to green 3MMLCT emission has been reported using platinum complexes with N-heterocyclic carbene (NHC) ligands, which make the LUMO level relatively unstable; however, shorter-wavelength blue emission could not be realized. Recently, the author’s group found that the emission color can be controlled from red to blue by systematically changing the substituents of the NHC ligands. This realization of strong blue 3MMLCT luminescence breaks with conventional wisdom [16]. Specifically, a series of Pt(II)-NHC complexes, [Pt(CN)2(R-impy)] (R-impyH+  = 1-alkyl-3-(2-pyridyl)-1H-imidazolium, R = Me, Et, iPr, tBu), form similar stacking structures (Fig. 4.5a) with average interplatinum distances varying systematically from 3.2 Å (R = Me) to 3.5 Å (R = tBu). All these crystals show high luminescence quantum yields (ϕ = 0.5–0.7), and the monotonic spectral shapes suggest that the emission originates from charge-transfer-type luminescence (Fig. 4.5b). Single-crystal X-ray diffraction measurements revealed that the Pt···Pt distance decreases with decreasing temperature from room temperature (298 K) to 77 K. This trend is similar to that observed in self-assembled platinum complexes and corresponds to the anisotropic shrinkage of the crystal lattice at lower temperatures. Therefore, the Pt···Pt interaction is expected to become stronger at lower temperatures. However, the temperature dependence of the emission spectra was found to depend on the substituent. For all four complexes, the average interplatinum distance (Rave) increases with increasing temperature from 77 to 298 K, whereas the 3MMLCT luminescence energies show different behavior, with decreasing slope of the emission energy against the distance (Rave) from the R = Me complex to the Et and iPr complexes, and no emission spectral shift occurs for the R = tBu complex (Fig. 4.5c). This important result, which is related to the nature of integrated luminescence, reveals the specific limits of interplatinum interactions. Although the relative energy difference cannot be changed using conventional ligands, changing the substituents of the NHC ligands allows the Pt···Pt interactions to be systematically controlled, and consequently, the luminescence can be modulated.

Fig. 4.5
3 illustrations. a. A four molecular structures highlighting the joint of P t 1. Graphs b and c plots normal curves and a line curve for a corresponding emission spectrum.

a Stacking structures of platinum(II)-NHC complexes, b photographs of luminescent crystalline powders and corresponding emission spectra at RT, and c correlation between the emission energy and average Pt···Pt distance (Rave). Reprinted with permission from ref. [16]

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Interestingly, in such a system with controlled platinum interactions, a vapochromic phenomenon based on a single-crystal-to-single-crystal (SCSC) structural transformation has been observed [17]. The blue-emitting crystal (R = tBu), which is a trihydrate, is stable in a sealed environment but immediately releases water upon exposure to air, and forms a yellowish-green-emitting anhydrous crystals when dried under reduced pressure (Fig. 4.6). Upon exposure to water vapor to the anhydrous form, a dihydrate is formed at saturated water vapor pressure. Through in-situ tracking of single-crystal X-ray diffractions, it was found that an atmospheric pressure environment is necessary to return to the original trihydrate. Notably, all these changes occur while maintaining a single-crystal structure. In the present system, the self-assembled platinum complexes can change flexibly in response to the movement of water molecules by reconstructing hydrogen bonds, which is considered to enable the SCSC structural transformation.

Fig. 4.6
An illustration with a molecular structures depicts the relationship between oblique, parallel, and intermediate stacking.

Vapor-induced reversible and stepwise single-crystal-to-single-crystal (SCSC) transformations of [Pt(CN)2(tBu-impy)] (tBu-impyH+  = 1-tBu-3-(2-pyridyl)-1H-imidazolium) crystals. Reprinted with permission from ref. [17]

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4.3 Cooperation Between Vapochromism and Spin-State Changes in a Nickel(II) Complex

Nickel(II)-quinonoid complexes show distinct changes in physical properties in conjunction with vapochromism [18]. Quinonoid compounds have also attracted attention as redox-active ligands, and various noninnocent metal complexes have been constructed in which the electronic states are spread between the ligand and the metal. The nickel(II)-quinonoid complex [Ni(HLMe)2] (H2LMe = 4-methylamino-6-methyliminio-3-oxocyclohexa-1,4-dien-1-olate) has been found to selectively respond to methanol vapor with a distinct purple to orange color change (Fig. 4.7). This phenomenon is caused by methanol molecules coordinating to nickel ions in the supramolecular crystal structure of the complex, which is an example of the direct coordination of gaseous molecules. The color change is caused by a perturbation of the electronic state of the quinonoid by the coordination of a methanol molecule to the axial position of nickel. Interestingly, the spin state of the nickel ion also changes upon coordination and desorption of the methanol molecule. The low-spin square-planar four-coordinated nickel(II) complex is diamagnetic and purple in color, whereas methanol molecule coordination induces a high-spin state of the nickel(II) ion, resulting in the orange crystal being paramagnetic (Fig. 4.7). Although several examples of the vapor-induced spin-switching of nickel complexes have been previously reported, they were based on ionic liquids or liquid crystals. In the present system, the selective response to methanol was achieved by a structural transformation between crystal states that retain precise arrangements. This system provides a new direction for developing highly ordered, flexible, and multifunctional vapochromic complexes.

Fig. 4.7
An organic hexagonal chemical structure with O, H N, N i, and N H molecules is conversely connected to another structure with O, H N, N H, and N i molecules.

Coordination-induced spin-state switching of a Ni-quinonoid complex by methanol vapor. Reprinted with permission from ref. [18]

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4.4 Strongly Luminescent Copper(I) Complexes

Most strongly luminescent metal complexes contain noble metals such as platinum, iridium, gold, and europium, or rare earths. The development of highly luminescent complexes using copper, which is an abundant and inexpensive element, is of great interest from both academic and elements strategy points of view. Luminescent copper(I) complexes have long been known to show temperature-dependent luminescence color changes, and recently, strongly luminescent copper(I) complex crystals that exhibit vapor responsivity have been constructed [19]. For example, a mononuclear copper(I) complex with a phosphine ligand and an N-heteroaromatic compound, [CuI(PPh3)2L] (L = N-heteroaromatic compound), is strongly luminescent and can be easily synthesized by simply mixing and grinding. Moreover, the ligand can be changed by exposure to the vapor of various N-heteroaromatic compounds. This ligand-substitution behavior is accompanied by a change in the luminescence color change (Fig. 4.8) [20]. This phenomenon is attributed to the direct exchange of coordinated vapor molecules, which is unique to these ligand-substituted copper(I) complexes.

Fig. 4.8
An illustration of a vapor exchange circle with a chemical structure in the middle points to four different colors of substances with their hexagonal chemical structures that contain N molecules.

Reversible luminescence color changes of [CuCl(PPh3)2(L)] based on ligand exchange reactions with N-heteroaromatic vapors. Reprinted with permission from ref. [20]

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5 Conclusion

Recently, vapochromic phenomena have been reported in not only complex crystals but also organic crystals and liquid crystals [3e]. Thus, it may be possible that any soft crystal or soft material with color and luminescence properties can exhibit vapochromism. However, to verify this suggestion, research on this phenomenon must advance from the stage of serendipitous discoveries and observations. Soft crystals are advantageous because they enable the precise elucidation of three-dimensional structures, including dynamic structural changes, at the molecular level. In addition, they exhibit visually perceptible changes in properties such as color and luminescence, which can be precisely evaluated spectroscopically. Thus, the information about small molecule–solid-state interactions obtained through soft crystal studies may provide useful molecular insights into more complex systems, for example, into the interactions and mechanisms of enzyme–protein–substrate systems.

From the viewpoint of function, various advances can be expected for vapor-responsive crystalline materials. High selectivity and high sensitivity are important issues for humidity and VOC sensors. Vapor-selective vapochromism as well as high-sensitivity and fast-response vapochromism have been achieved via the skillful molecular design of MOFs and supramolecules [21]. However, further development is needed to realize high-precision devices utilizing the characteristics of crystalline materials. Multifunctional vapochromic materials, in which other physical properties change simultaneously with color or luminescence changes are also attracting attention. The aforementioned nickel complex is a good example of such a system, in which concurrent changes in color and spin state are induced by methanol vapor. As a stimuli-responsive material that can undergo stable spin state changes in a wide temperature range, this complex is expected to be applicable to memory functions. Vapochromism is a visible manifestation of the interaction between a gas and a solid, which is of broad interest. A multifaceted approach based on the observation of bulk changes combined with the tracking of microscopic and dynamic behavior can advance the understanding of these complex phenomena. Such insights are expected to lead to the development of novel materials with outstanding functions.