Xiqi Zhang

Recent advances in mechanochromic luminescent metal complexes

Mechanochromic luminescent metal complexes have recently attracted considerable interest as smart materials for use in the fields of luminescence switches, mechanosensors, mechanohistory indicators, security papers, optoelectronic devices, and data storage. In this review, recent advancements in metal complex mechanochromic luminescence are summarized. The majority of the reported mechanochromic luminescent metal complex systems are discussed, including Zn(II), Au(I), Pt(II), Ag(I), Cu(I), Al(III), and Ir(III) complexes. Aggregation-induced emission (AIE) complexes are described based on the existence of a structural relationship between AIE compounds and mechanochromism. It is expected that AIE complexes may become important alternative sources of mechanochromic luminescent complexes.

Piezofluorochromism of an Aggregation‐Induced Emission Compound Derived from Tetraphenylethylene

The development of organic fluorescence materials is of great interest for both fundamental research and practical applications. The modification or alteration of molecular chemical structures is the most-common approach for controlling fluorescence properties. However, there are a limited number of effective materials for the dynamic control of solid-state fluorescence with high efficiency and reversibility, because most chemical reactions in the solid state involve insufficient conversion and irreversible reactions, or result in the loss of fluorescence capability. To overcome this problem, a very attractive approach is proposed which dynamically controls solid fluorescence properties by altering the mode of solid-state molecular stacking without changing the chemical structure of the constituting molecules. Found recently, piezofluorochromism is a change in the fluorescence color induced by mechanical stress, accompanied with a reversion to the original fluorescent color by heating, recrystallization, or exposure to solvent vapor. Many piezochromic materials have been reported based on the change in absorption characteristics under pressure. Fluorescence can be detected with high sensitivity, thus enabling materials with piezofluorochromism to have a wide variety of applications in optical recording and strainor pressure-sensing systems. However, organic piezofluorochromic materials are exceedingly rare. Recently, many aggregation-induced emission (AIE) compounds with various chemical structures have been synthesized in our laboratory, and it is very interesting to find that almost all of these compounds are piezofluorochromic materials. We would thus suggest calling these compounds piezofluorochromic aggregation-induced emission (PAIE) materials as they possess both piezofluorochromic and aggregation-induced emission properties. Aggregation-induced emission materials, first reported by Tang and co-workers, are one of an important class of luminescent materials and exhibit many special properties, such as strong solid emission, excellent device performance, and highly stimuli-sensitive fluorescence. PAIE materials have the advantages of both piezofluorochromic materials and aggregation-induced emission materials and can be more-widely used. However, to the best of our knowledge, there has been only one compound (DBDCS, Scheme 1 c) that has both piezofluorochro-

Piezofluorochromic Properties and Mechanism of an Aggregation-Induced Emission Enhancement Compound Containing N-Hexyl-phenothiazine and Anthracene Moieties

A fluorescent compound, 9,10-bis(2-(10-hexyl-10H-phenothiazin-3-yl)vinyl) anthracene, has been synthesized and studied. The results show that the compound possesses piezofluorochromic properties as well as aggregation-induced emission enhancement effect. The spectroscopic properties and morphological structures are reversibly exhibited upon pressing (or grinding) or annealing (or fuming). The piezofluorochromic nature is generated through phase transformation under the stimulus of external pressure. The reason for the phase transformation caused by external pressure is ascribed to the twisted conformation of the molecule which leads to poor solid molecular packing and weak interactions in the interfaces of lamellar layers confirmed by its single-crystal X-ray diffraction analysis.

Aggregation induced emission-based fluorescent nanoparticles: fabrication methodologies and biomedical applications

Novel fluorescent nanoparticle (FNP)-based bioprobes are expected to generate new medical diagnostic techniques in biomedical and biological areas for their superior brightness and photostability compared with conventional molecular probes including small organic dyes and fluorescent proteins. Potentially interesting nanoscale platforms for various biomedical applications are greatly attractive due to the potential to avoid exposure of human tissues to toxic drugs, enhancing delivery of hydrophobic therapeutics and fabricating multifunctional imaging, targeting and delivery system. In this review, recent progress in the area of novel aggregation induced emission (AIE)-based FNPs is summarized over the past few years (2007-2013), and the reported fabrication methodologies of these fluorescent systems including non-covalent and covalent strategies are mainly discussed, and the biomedical applications of AIE-based FNPs are also summarized.

End-group effects of piezofluorochromic aggregation-induced enhanced emission compounds containing distyrylanthracene

Distyrylanthracene derivatives, AnP3 and AnP3P with triphenylethylene end-groups, and AnP4 and AnP4P with tetraphenylethylene end-groups, were synthesized and characterized using nuclear magnetic resonance, mass spectrometry, elemental analysis, photoluminescence, ultraviolet-visible absorption, wide-angle X-ray diffraction, differential scanning calorimetry and other techniques. The results show that the piezofluorochromic properties of these four aggregation-induced enhanced emission (AIEE) compounds were influenced significantly by the structures of the end-groups in the molecules. The AnP4 and AnP4P derivatives with tetraphenylethylene end-groups exhibited distinct piezofluorochromic properties, which might be switched reversibly upon pressing or annealing because of crystalline-amorphous phase transformation. Thus, we propose that those AIEE compounds that contain steric hindrance groups may exhibit evident piezofluorochromic activities.

New thermally stable piezofluorochromic aggregation-induced emission compounds.

New piezofluorochromic compounds with high thermal stabilities and aggregation-induced emission behavior were developed. The spectroscopic properties and morphological structures of these compounds were reversed upon pressing (or grinding)/annealing (or fuming). The switchable color change feature and aggregation-induced emission make the compounds promising candidates for optical recording, pressure-sensing, and light-emitting systems.

Triphenylethylene carbazole derivatives as a new class of AIE materials with strong blue light emission and high glass transition temperature

A new class of aggregation-induced emission (AIE) compounds with strong blue-light-emitting properties and a high thermal stability, derived from triphenylethylene carbazole, has been synthesized. Their glass transition temperatures range from 126–151 °C and the maximum fluorescence emission wavelengths are 451–466 nm.

Facile Incorporation of Aggregation-Induced Emission Materials into Mesoporous Silica Nanoparticles for Intracellular Imaging and Cancer Therapy

Aggregation-induced emission (AIE) materials were facilely incorporated into mesoporous silica nanoparticles (MSNs) via one-pot surfactant templated method. Cell imaging and cancer therapy applications of such fluorescent MSNs were further explored. We demonstrated that AIE-MSN nanocomposites showed strong fluorescence and uniform morphology, making them promising for both cell imaging and cancer therapy.

Synthesis and properties of novel aggregation-induced emission compounds with combined tetraphenylethylene and dicarbazolyl triphenylethylene moieties

A novel class of compounds containing tetraphenylethylene and dicarbazolyl triphenylethylene moieties combined with different spacers is synthesized. The compounds exhibit high thermal stabilities and aggregation-induced emission properties. The different spacers lead to differences in spatial conformation structures and spatial electron distributions, and markedly affect the thermal and photophysical properties of the compounds. The size of the aggregates formed in water–THF mixtures decreases with increasing water fractions. The time-resolved emission decay behaviors of the synthesized compounds in water–THF mixtures with water fractions from 60% to 99% show three relaxation pathways in their fluorescence decays.

Aggregation-induced emission enhancement compounds containing triphenylamine-anthrylenevinylene and tetraphenylethene moieties

New compounds containing triphenylamine-anthrylenevinylene and tetraphenylethene moieties are synthesized by the Wittig–Horner reaction. The compounds exhibit weak emission in good solvents, but strong fluorescence in aggregates or in the solid state, indicating that they are aggregation-induced emission enhancement compounds. The symmetry of their molecular structure affects their aggregation structures and photoluminescence emission properties in aggregation. These compounds show high thermal stabilities with glass transition temperatures between 168–177 °C and decomposition temperatures between 521–537 °C. The HOMO levels of the compounds are in the region of 4.92–4.94 eV, very close to the work function of indium tin oxide anode (4.8–5.0 eV). One of the compounds exhibits obvious piezofluorochromic behavior. The piezofluorochromic nature is generated through changing the mode of molecular packing under external pressure.