Smart materials are designed materials possessing one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH and electric or magnetic fields. Thus, smart materials are also called stimuli-responsive materials. Smart materials have been considered to be the fourth generation of materials after natural materials, synthetic polymer materials and artificial design materials, which is one of the important developing trends in high-tech new materials and will support the development of modern high technology, resulting in the disappearance of the boundaries of the traditional functional and structural materials to realize the functionalization of structural materials and the diversification of functional materials. It is believed that the development and large-scale application of smart materials will lead to a great revolution in the development of material sciences. Smart materials and systems have been widely investigated as physical atomic-level switches, memories to chemical sensing systems and bio-related controlled release systems. In particular, organic and biological soft materials, which are generally not restricted by crystal lattices or inflexible bonding, are anticipated to find various applications as functional materials because of their flexible responses to the applied stimuli. Some prototypical smart materials could be operated through rather ambiguous mechanisms, where precise control of the component molecules was often neglected. Because of this, these materials might be used in a variety of applications but are usually inferior in their specific response to a selected stimulus. To develop more specifically responsive materials, control over particular molecular activity is critical. The stimulus selectivity through molecular recognition or related interactions, together with the specific changes of molecular properties, defined by the component molecular structures would lead to materials with appropriate outputs upon application of selected inputs. Such materials are often referred to as molecular smart materials.¹ 

Generally, the external stimuli mainly fall into one of two categories: one is physical stimulus, which is used to promote alteration of molecular properties, including optical, electrical, magnetic and thermal inputs, and the other is chemical/biological stimulus, such as the addition of chemicals and biomolecules. Research into the molecular responses to these various stimuli has been extensive, and the design of molecular-responsive smart materials have been well explored. Most of these stimuli are suitable for direct interaction with the molecules dispersed in appropriate media. Also, several of these stimuli, including magnetic and thermal effects, can propagate through space and interact at a distance. Under these circumstances, the behaviors of the molecules upon application of these stimuli can be easily observed using conventional analytical tools.

A simple but important stimulus for controlling the properties of smart materials is mechanical stimulus. In fact, smart materials can response to external mechanical stimuli such as shearing, grinding, rubbing, stretching and bending, and provide controlled functions.

Mechanochromism is an overarching term that describes the phenomenon of color change caused by mechanical grinding, crushing or rubbing, which has also been known as tribochromism or pressing, and also termed piezochromism,²  of the solid sample and reversion to the original color by, for example, heating or recrystallization. The grinding process usually generates a metastable form. One example of a typical piezochromic organic compounds is diphenylflavylene.³  Here, piezochromism means the change of sample colors under mechanical force, not the change of the emission colors of the sample.

On the other hand, mechanochromic fluorescent (or mechanofluorochromic) materials change their emission colors (spectra) when an appropriate external mechanical force stimulus is applied. As a class of “smart” materials, they possess mechanical responsiveness that provides a fundamental basis for fluorescence switches, mechanosensors, indicators of mechanohistory, security papers, optoelectronic devices and data storage in various fields.^4–7 

Mechanofluorochromic behavior can be generally achieved by either chemical or physical structural changes to the materials. Although the modification of molecular structures containing open/closed cyclic forms⁸  and double-bond E/Z configurations³  is the most common approach for tuning the emissions of fluorescent compounds, limited success has also been achieved in switching the fluorescence of the solid-state materials with high efficiency and reproducibility.⁹ 

Chemical structural change is implemented using chemical reactions, such as bond breaking or forming at the molecular level. In these cases, a relatively high pressure or rigorous conditions would be necessary to promote the chemical reactions.¹⁰  Moreover, insufficient conversion, irreversible reactions, or loss of the fluorescence capability of the compound may frequently occur during the solid-state chemical reactions, which have been considered a drawback in such systems. Thus, the successful example reported for mechanofluorochromism based on the chemical structural change is very limited (Figure 1.1A).⁸ 

Control of the molecular phenomena by macroscopic mechanical stimuli is not well appreciated. For example, it may be claimed that elongation, deformation, disentanglements and ruptures of molecules may occur when mechanical stress is applied, but what is really occurring at the molecular level is not well understood. Although the activation of covalent bonds has primarily been demonstrated in solution, activation in the solid state has not been thoroughly investigated. To date, there are still many experimental and theoretical challenges to be overcome for a better comprehension of molecular behaviors in the solid state under mechanical stimuli.

By contrast, the fluorescent properties of molecules in the solid state depend on the molecular arrangement, conformational flexibility and intermolecular interactions in the materials. Any modification or alteration of the molecular packing and conformation of the fluorophore would affect the HOMO-LUMO energy levels and alter the fluorescent properties (Figure 1.1B). Thus, controlling the mode of molecular packing (aggregation states) to achieve the dynamic control of highly efficient and reversible solid-state fluorescence is more attractive for both fundamental research and practical applications because of low pressure demand and good reversibility for these compounds. In this book, recent progress in the development of mechanofluorochromic materials is reviewed and discussed.

By now, compounds reported to have mechanofluorochromic properties mainly include organic compounds, organometallic complexes, polymers and dye-doped polymer composites.

Dye-doped polymer composites are a representative family of mechanofluorochromic materials. The typical characteristic of these systems is that the fluorescence colors may change drastically along with the tensile deformation of matrix polymers. On the other hand, a great deal of attention has been paid to the mechanochromic properties of single-component small organic dye molecules. In comparison with the packing changes in the organic compounds, metal complexes can utilize metal-metal interactions to adjust their optoelectronic properties.

However, the mechanochromic fluorescent materials that are dependent on changes in the physical molecular packing modes are still extremely rare to this day. This rarity may be attributed to two major issues.⁵  Firstly, predicting and designing for such materials exhibiting polymorphism with different luminescent properties are difficult. Each identified compound reported in the literature seems an isolated event, which makes identification of a general characteristic and regularity difficult. Secondly, the luminescent efficiency of organic fluorescent materials often becomes very weak when they are in the solid state because of the so-called aggregation-caused quenching (ACQ) effect. Consequently, observation of the mechanochromic fluorescent phenomenon in the solid samples becomes difficult.

The discovery of so-called aggregation-induced emission (AIE) materials, first reported by Tang et al. in 2001,¹¹  opened up an important opportunity for the exploitation of new mechanochromic fluorescent materials. AIE materials are a class of anti-ACQ materials that emit more efficiently when they are in the aggregated state than when they are in the dissolved form, and have attracted considerable research attention for their potential application in various fields, such as organic light-emitting devices and chemosensors.^12–17  Meanwhile, a number of AIE compounds with different AIE moieties have been found to possess mechanochromic emission properties. Hence, the use of AIE becomes important in the synthesis of various mechanochromic fluorescent materials.

The miraculous properties and potential applications of mechanochromic fluorescent materials comprise a new branch of modern smart materials, and more mechanochromic luminescent systems are expected to be discovered.

Thus, seven chapters are included in this book. Chapter 1 serves as an introduction. Chapter 2 will describe organic small molecule mechanofluorochromic materials, which are considered to be a major component of the mechanochromic fluorescent materials reported in the literature. In Chapter 3, mechanochromic luminescent metal complexes are described. Although the number of mechanochromic luminescent metal complexes is still limited to date, the unique properties of organometallic or coordination compounds should make these materials a novel source for mechanochromic fluorescent materials. Mechanofluorochromic polymers and dye-doped polymer composites are described in Chapter 4 and Chapter 5, respectively, some of the systems are genetic examples of mechanochromic fluorescent materials. Chapter 6 will deal with the relatively new types of mechanofluorochromic AIE compounds. Many of the compounds have been prepared by our group, and this chapter can be considered a comprehensive summary by the authors in this field. In the last chapter, Chapter 7, the major mechanofluorochromic mechanisms are summarized, which is vital to understanding how mechanofluorochromism happens.

Readers are also encouraged to refer to other published valuable reference reviews about mechanochromic luminescent materials,^{1,4,5,18–31}  which are extremely helpful for understanding the concepts, mechanism and applications of these new materials.