Bingjia Xu

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.

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.

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.

A new ligand and its complex with multi-stimuli-responsive and aggregation-induced emission effects

A new ligand containing tetraphenylethylene and terpyridine moieties, and its zinc ion complex were synthesized. Both of them exhibit an aggregation-induced emission effect. Their colors and emissions can be smartly switched by various external stimuli including grinding, heating and solvent-fuming, as well as exposure to acid and base vapors.

Metal-free organic dyes derived from triphenylethylene for dye-sensitized solar cells: tuning of the performance by phenothiazine and carbazole

Four novel D–D–π–A configuration metal-free organic dyes (C3, P2, C2 and P3) with triphenylethylene phenothiazine moieties or triphenylethylene carbazole moieties as additional electron donors for dye-sensitized solar cells (DSSCs) have been synthesized. The cells based on C3, P2, C2 and P3 dyes with efficiencies of 2.14%, 2.69%, 5.51% and 6.55%, respectively, are obtained. The P3 based cell exhibits the highest efficiency of 6.55% accompanied by a short-circuit current density (Jsc) of 12.18 mA cm−2, a rather high open-circuit photovoltage (Voc) of 826 mV, and a fill factor (ff) of 0.65, performances which are remarkable in the DSSCs based on metal-free organic dyes. The twisted non-planar configuration in P3 decelerates the charge recombination in the charge-separated state and hence contributes to the improvement of the performance of DSSCs.

Piezofluorochromic and Aggregation‐Induced‐Emission Compounds Containing Triphenylethylene and Tetraphenylethylene Moieties

New fluorescent compounds containing triphenylethylene and tetraphenylethylene moieties were synthesized, and their piezofluorochromic and aggregation-induced emission behaviors were investigated. The results show that all compounds exhibit aggregation-induced emission characteristics and only the crystalline compound possesses piezofluorochromic properties. The color, emission spectra, and morphological structures of the one piezofluorochromic compound exhibit reversibility upon grinding and annealing (or fuming) treatments. The piezofluorochromic behaviors are caused by a change between different modes of solid state molecular packing under external pressure. The single crystal X-ray diffraction analysis reveals that the twisted conformation of the aggregation-induced emission compound leads to the formation of metastable crystal lattice with cavity which is readily destroyed under external pressure. A possible mechanism of piezofluorochromic phenomenon has been proposed.