Search

The immobilized propeller-like conformation of tetraphenylethylene (TPE) can be used to emit strong circularly polarized luminescence (CPL) light, but the immobilized propeller-like conformation must be resolved. A single-handed helical conformation of TPE units induced by chiral group(s) that do not need resolution can also enable CPL emission, but related research is very rare. In this paper, TPE dicycle tetracholesterol is synthesized which could self-assemble into nano-tubes with a double molecule-layer shell, and which showed a fluorescence quantum yield of >76%. One film composed of nano-tubes displayed an enhanced first positive circular dichroism (CD) effect and emitted strong positive CPL light. A suspension or solution of the TPE dicycle tetracholesterol in DCE had a first negative CD band and negative CPL emission. In contrast, TPE tetracholesterol without intramolecular cyclization self-assembled into very soft “noodle-like” aggregations and showed very weak CD signals and CPL emission. The TPE dicycle tetracholesterol had a large CPL dissymmetric factor (≤3.0 × 10−3), which was 30 times larger than the TPE tetracholesterol without cycles. Due to the strong bisignate band in the CD spectrum of TPE dicycle tetracholesterol and the absence of a helical structure in aggregates, the CD and CPL signals of the TPE dicycle tetracholesterol could be ascribed to the prevailing single-handed propeller-like conformation of TPE units induced by chiral cholesterol groups.

Tetraphenylethylene (TPE) foldamers with double hairpin-turn linkers showing an aggregation-induced emission (AIE) effect have been synthesized for the first time. A crystal structure of a foldamer-TNT complex has been obtained, enabling unprecedented direct observation of the interactions between TNT molecules and the chromophores of the foldamer. Instead of π-π stacking interactions, which have often been considered to be the key mechanism in the binding of TNT by chromophoric receptors, strong n-π interactions between the nitro groups of TNT and the aromatic rings of the foldamer have been found. Exceptionally, by addition of 1 % NaF to a suspension of the foldamer in H2 O/THF (95:5), the fluorescence quenching efficiency by TNT vapor significantly increased from about 20 % to more than 90 %. Even after diluting TNT-saturated air at 25 °C by a factor of 2×104 , an obvious quenching response was observed, indicating that ultratrace TNT vapor (down to 3.4 fg per mL of air) could be detected.

A new [3 + 3] trimeric macrocycle, based on tetraphenylethylene having an aggregation-induced emission effect, could form a interdigitation molecular cage with a big void by host–guest interactions. The cage could accommodate two TNT molecules and detect TNT at the 1.7 fg level per mL of air. Moreover, the cage could give permanent pores that had a BET surface area of 347 m2/g and could adsorb 7.8 wt % CO2 with high CO2/N2 selectivity up to 32 (273 K, 1 bar).

This paper reports that cis-TPE dicycles emit strong fluorescence, while the gem dicycles show almost no emission in solution, demonstrating that the free rotation restriction of the double bond at the excited state is the key factor for AIE effects.

Organic molecules with an aggregation-induced emission (AIE) effect have recently been attracting more and more attention due to their colossal potential in solid emitters and chemo/biosensors. The number and variety of AIEgen compounds are expanding very rapidly to obtain better application performance and a wider area of application. Among AIEgen systems, tetraphenylethylene (TPE) and its derivatives are the class that have received the most extensive study and the most rapid development because of their facile synthesis. Due to its C2 symmetry and at least tetratopic reaction positions, the TPE unit is also an ideal building block for constructing macrocycles and cages. The resultant cyclic TPE compounds have exhibited many exceptional performances that are difficult to access in their open chain counterparts, such as AIE enhancement, improvement in selectivity and sensitivity as sensors, emission tuning by guests, supramolecular catalysis, further disclosure of the AIE mechanism, molecular adsorption, storage and release, the propeller-like conformation exploitation of the TPE unit in chiral materials and so on. Recently, therefore, a large variety of studies about the synthesis, properties and application research of TPE macrocycles and cages have been reported. These TPE macrocycles and cages significantly expand the research area for the AIE phenomenon and its applications, and represent a development of the AIE area. However, up to now, no review of TPE macrocycles and cages has been available. Thus, this review serves as a summary of the designs, synthesis, photophysical properties, self-assembly, applications and prospects of TPE macrocycles and cages.

Herein, novel tetraphenylethylene (TPE) triangular macrocycles bearing three crown ether rings were synthesized by connecting three TPE crown ether units at the cis-position with three ethynylene bridging linkers. The macrocycles showed typical aggregation-induced emission (AIE) effect and a high fluorescence quantum yield up to 92%. They could exhibit both circular dichroism (CD) and circularly polarized luminescence (CPL) signals in the presence of enantiomers of chiral acids. Moreover, the chiroptical signals in a film became stronger with the increasing rigidity of the macrocycles. Exceptionally, the chiroptical signals of the macrocycles in solid were ascribed to the induced prevailing single-handed propeller-like conformations of TPE units rather than to the supramolecular helical micro-/nanofiber self-assembled structure.

Tetraphenylethylene (TPE) and its derivatives are the most typical and most widely studied organic compounds showing aggregation-induced emission (AIE). Due to their propeller-like structures, V-like clefts exist between the aryl rings, which make them promising host compounds. However, such a possibility is seldom explored. Herein, it is reported that TPE derivatives bearing two or four pyridine rings at the para positions of the phenyl rings (TPE-Pys) can selectively include triangular (Δ-like) m-phthalic acid from a mixture of o-, m-, and p-phthalic acids due to their shape complementary to form host-guest co-crystals, which showed redder emission than the TPE-Pys themselves. The emission of co-crystals 1-5 could be reversibly switched between yellow and red by alternating exposure to HCl and ammonia vapor. The host-guest co-crystals not only exhibited great potential for selectively recognizing and separating m-phthalic acid and as multicolor emission materials, but are also suitable for use as secret ink due to their reversible color change on varying the host-guest interactions.

A neutral chiral receptor based on TPE cyclohexylbisurea was synthesized and could discriminate the enantiomers of many different kinds of chiral reagents, including chiral acidic compounds, basic compounds, amino acids, and even neutral alcohols. The (1)H NMR spectra disclosed that the ability of chiral recognition could be ascribed to the multiple hydrogen bonds and CH-π interactions between the TPE urea receptor and the enantiomer of the chiral guest, which led to the selective aggregation of the receptor with one of the two enantiomers. This result exhibited a great potential in enantiomer discernment and high-throughput analysis of enantiomer composition of these chiral analytes by one chiral AIE molecule.

A new imidazolium macrocycle based on tetraphenylethylene (TPE) has been synthesized. This positively charged macrocycle showed a typical aggregation-induced emission (AIE) effect but fluorescence emission could not be induced by common inorganic anions in aqueous solution, including pyrophosphate bearing four negative charges. However, in the presence of a half equivalent of zinc(II) ions, the addition of pyrophosphate anions could arouse a strong fluorescence while other common inorganic anions gave almost no response. The macrocycle has an inherent cavity with the correct size for binding a pyrophosphate anion, so that the pyrophosphate anion can be easily aggregated together with the macrocycle and a zinc cation, which selectively triggers a turn-on fluorescence.

Planar molecules usually display aggregation-caused quenching (ACQ). Here, it was found that a completely planar organic compound, 5,7,12,14-tetraoxapentacene A-D-A triad with carbon-oxygen bonds, had no fluorescence in high polar solvents but emitted strong light in suspension and in solid state, showed typical AIE effect. One novel AIE mechanism was disclosed by molecular packing in crystal state. In addition, its exceptional AIE effect showed great potential applications in fluorescent thermometer and highly sensitive sensor for selective detection of nitrophenolic compounds. The detection limit for 2,4,6-trinitrophenol was low to 0.168 nM.

The propeller-like conformation of tetraphenylethylene (TPE) with aggregation-induced emission (AIE) effect was partially and completely fixed by intramolecular cyclization for the first time. The immobilization of propeller-like conformation was found to show great advantages in determining the enantiomer purity, identifying the chiral amines. The completely fixed conformers are resolved into M- and P-enantiomer, which showed mirror imaged CD and almost quantitative fluorescence quantum yield. Furthermore, it also showed a mirror and large circularly polarized luminescence dissymmetric factor, depending on the helicity of the enantiomer. The result provides the most direct and persuasive evidence for AIE via the restriction of intramolecular rotation and finds the new insight of the compounds in chiroptical property.