Aggregation-induced emission (AIE) describes a photophysical phenomenon in which molecular aggregates exhibit stronger emission than the single molecules. Over the course of the last 20 years, AIE research has made great strides in material development, mechanistic study and high-tech applications. The achievements of AIE research demonstrate that molecular aggregates show many properties and functions that are absent in molecular species. In this review, we summarize the advances in the field of AIE and its related areas. We specifically focus on the new properties of materials attained by molecular aggregates beyond the microscopic molecular level. We hope this review will inspire more research into molecular ensembles at and beyond the meso level and lead to the significant progress in material and biological science.

Aggregation-Induced Emission: New Vistas at the Aggregate Level

Visualization of stress distribution has been realized by a nondestructive mechanoluminescence (ML) from SrAl2O4:Eu, which can emit three magnitudes higher visible light than that of well-known ML substance of quartz. A simulation result confirms that such a ML image successfully reflects the stress distribution. A kinetic model for ML of SrAl2O4:Eu is proposed.

Direct view of stress distribution in solid by mechanoluminescence

The idea and successful practice of a stress sensor to sense mechanical stress by an artificial skin, i.e., self-diagnosis thin film, has been realized, through the fabrication of a high-luminescence thin piezoelectric film which can reproducibly emit strong visible light upon stressing. The strongest luminescent film consists of nanosized crystallites of ZnS doped with 1.5 at. % Mn, in which Mn acts as the emitting center. The intensity of the emitted luminescence responds to stress applied directly onto the film or to the underlying material reversibly and reproducibly, so it can be used as an artificial skin to sense mechanical stress.

Artificial skin to sense mechanical stress by visible light emission

Publisher Summary This chapter presents a discussion on the coordination chemistry of 2,2’: 6’, 2”-terpyridine and higher oligopyridines. The potentially terdentate ligand 2,2’: 6’, 2”-terpyridine presented, was first isolated by Morgan and Burstall as one of the numerous products from the reaction of pyridine with iron (III) chloride. There has been a considerable increase in the use of these ligands in recent years, prompted in part by the attractive photochemical and photophysical properties exhibited by complexes of the related ligands 2,2’-bipyridine and 1,10-phenanthroline. The chapter discusses stabilization of unusual complexes and states that low oxidation states are characterized by an excess of electron density at the metal atom; stabilization may be achieved by the use of ligands capable of reducing that electron density. One of the simplest ways to reduce the electron density is to design ligands with low-lying vacant orbitals of suitable symmetry for overlap with filled metal orbitals. This results in the transfer of electron density from the metal to the ligand (back-donation). In general, ligand nonbonding or π antibonding orbitals are of the correct symmetry for such overlap. The oligopyridines are ideally suited to such roles; they possess a filled highest occupied molecular orbital (HOMO) and a vacant lowest unoccupied molecular orbital (LUMO) of suitable energies for interaction with metal d orbitals. The chapter also discusses monodentate, bidentate, and bridging; terdentate and higher polydentate; cyclometallated, and others.

The Coordination Chemistry of 2,2′:6′,2″-Terpyridine and Higher Oligopyridines

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Electro‐Mechano‐Optical Conversions in Pr3+‐Doped BaTiO3–CaTiO3 Ceramics

The luminescence induced by elastic deformation of solids, called the phenomenon of elastico-mechanoluminescence (EML), is observed in several materials. For applied pressure in the range of 17 MPa, certain crystals emit intense EML, which can be seen in day light with naked eye. In the present paper, we explore that, as the piezoelectric constant near the photo-generated electric dipoles formed by trapping of charge carriers in crystals is several times higher as compared to that at normal sites, the piezoelectrically induced detrapping of charge carriers and EML emission may take place for less value of the pressure applied onto the crystals.

Strong luminescence induced by elastic deformation of piezoelectric crystals

The concepts of piezoelectric field dependence of the detrapping probability and lifetime of charge carriers in traps are developed The charge carriers in traps become unstable after a particular piezoelectric field whereby more number of charge carriers are detrapped with increasing piezoelectric field The detrapped electrons tunnel to the energy level of the hole-captured activator ions lying adjacent to the conduction band or tunnel to conduction band Subsequently, luminescence is induced by the electron-hole recombination Using these concepts microscopic theory of elastico-mechanoluminescent smart materials is explored Present study may be useful in tailoring intense elastico-mechanoluminescent materials needed for different applications

Microscopic theory of elastico-mechanoluminescent smart materials

The mechanoluminescence spectra of (Zn, Cd)S phosphors shift towards shorter wavelength side as compared to the photoluminescence spectra, however, the photoluminescence spectra shift towards shorter wavelength side with increasing pressure with less pressure coefficient. This finding eliminates the thermal population mechanism and suggests the electrical excitation mechanism for the mechanoluminescence excitation. The decay of mechanoluminescence after the deformation (Zn, Cd)S phosphors may be controlled by the recombination rate of holes and electrons, i.e. by the finite times required for the liberation of the electrons from the traps and for the electron transport, and consequently the decay of mechanoluminescence may be similar to the decay of photoluminescence.

Mechanoluminescence and high pressure photoluminescence of (Zn, Cd) S phosphors

Emission of light is found during application as well as release of uniaxial pressure in X-ray irradiated alkali halide crystals. As unpinning of dislocations can not take place, however, dislocations can move in backward directions during release of the pressure; the emission of light during release of pressure gives a direct support that the interaction of moving dislocations with F-centers is responsible for the emission of light during deformation of colored alkali halide crystals.



Sowohl bei der Anwendung von Druck als auch bei Druckentlastung wird Lichtemission in rontgenbestrahlten Alkalihalogenidkristallen beobachtet. Da Versetzungsloslosung von F-Zentren jedoch nicht eintreten kann, konnen die Versetzungen wahrend der Druckentlastung zuruckwandern. Die Lichtemission wahrend der Druckentlastung unterstutzt die Annahme, das die Wechselwirkung der wandernden Versetzungen mit den F-Zentren fur die Lichtemission wahrend der Verformung von farbigen Alkalihalogenid-kristallen verantwortlich ist.

Luminescence during release of pressure in X-ray irradiated alkali halide crystals

When a crystal is cleaved, initially the mechanoluminescence (ML) intensity increases linearly with time, attains an optimum-value I m at a particular value of time t m , and then decays exponentially with time. Cleavage ML provides a new tool to determine the velocity, v of cracks in crystals, and it may be given by v = H/t m , where H is the thickness of the crystal. Both, the peak ML intensity I m and total ML intensity I T increase linearly with the area of newly created surfaces A as well as with the surface charge density γ. The ML intensity decreases with temperature primarily due to the decrease in the surface charge density. Beyond a particular temperature, the surface charge density may decrease to such a value where the breakdown of gases and solids may not be possible and thereby the ML may not appear. Depending on the prevailing conditions either the ML emission resembling gas discharge or other types of the luminescence of solids, or that having these two characters may be otained. There exists a good correlation between the theoretical and experimental results obtained for cleavage ML in crystals.

Cleavage Mechanoluminescence in Crystals

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