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Racemic low-conjugated non-emissive THPs 1–3 can form highly emissive RS- and RR/SS-packing polymorphs with mixed through-bond and through-space conjugation., Racemic C6-unsubstituted tetrahydropyrimidines (THPs) are a series of fluorophores with a strong aggregation-induced emission (AIE) effect. However, they do not possess the structural features of conventional AIE compounds. In order to understand their AIE mechanism, here, the influences of the molecular packing mode and the conformation on the optical properties of THPs were investigated using seven crystalline polymorphs of three THPs (1–3). The racemic THPs 1–3 have low-conjugated and highly flexible molecular structures, and hence show practically no emission in different organic solvents. However, the fluorescence quantum yields of their polymorphs are up to 93%, and the maximum excitation (λ ex) and emission (λ em) wavelengths of the polymorphs are long at 409 and 484 nm, respectively. Single-crystal structures and theoretical calculation of the HOMOs and LUMOs based on the molecular conformations of these polymorphs indicate that the polymorphs with the shortest λ ex and λ em values possess a RS-packing mode (R- and S-enantiomers self-assemble as paired anti-parallel lines) and a more twisted conformation without through-space conjugation between the dicarboxylates, but the polymorphs with longer λ ex and λ em values adopt a RR/SS-packing mode (R- and S-enantiomers self-assemble as unpaired zigzag lines) and a less twisted conformation with through-space conjugation between the dicarboxylates. The molecular conformations of 1–3 in all these polymorphs are stereo and more twisted than those in solution. Although 1–3 are poorly conjugated, the radiative rate constants (k r) of their polymorphs are as large as conventional fluorophores (0.41–1.03 × 108 s–1) because of improved electronic conjugation by both through-bond and through-space interactions. Based on the obtained results, it can be deduced that the strong AIE arises not only from the restriction of intramolecular motion but also from enhanced electronic coupling and radiatively-favored inter-crossed local excitation (LE) and intramolecular charge transfer (ICT) excitation states. The abnormal molecular structures, easily-controllable self-assembly of the R- and S-enantiomers, and the strong AIE effect make THPs very useful fluorophores for applications and theoretical research.

Chiral nematic liquid crystals (N*LCs) are a kind of smart material that can be obtained directly from cholesteric LCs or from nematic LCs by adding a chiral dopant. As a 1D photonic crystal, [ 1 ] the helical and periodic structure of a N*LC leads to unique optical properties, including selective reflection, circular dichroism (CD), optical rotation, and so on. [ 2–5 ] The most important property of N*LCs is the selective reflection of circularly polarized (CP) light. When N*LCs are sandwiched between two substrates with planar anchoring surfaces, CP light with the same handedness as the helix will be selectively reflected whereas light with opposite handedness will be transmitted.

Self-assembling of luminescent molecules into one-dimensional nanostructures is of particular interest in fabricating nanoscale electronic and photonic devices. Herein, we report the rational design and synthesis of a chiral fluorescent tetraphenylethylene derivative containing L-leucine methyl ester moiety (TPE-Leu). In solution, TPE-Leu is non-emissive and CD silent, but becomes highly emissive and CD active upon aggregation, exhibiting aggregation-induced emission (AIE) and chirality (AIC). Upon evaporation of its solution, TPE-Leu readily self-assembles into helical fluorescent micro/nanofibers, which show circularly polarized luminescence (CPL) and have the CPL dissymmetry factors in the range of 0.02–0.07. This molecular design combines the AIE effect, chirality, and self-assembling capability together, and is highly efficient in constructing novel functional micro/nanomaterials with well-defined structures and enhanced emission.

In this work, a heteroatom-containing luminogen (TPE-Py) with multi-functionalities was synthesized in a reasonable yield by melding a pyridinium unit with tetraphenylethene through vinyl functionality. TPE-Py is weakly emissive in solution but becomes a strong emitter when aggregated as nanoparticle suspensions in poor solvents or in the solid state, displaying a phenomenon of aggregation-induced emission. Crystallization generally weakens and red-shifts the light emission. The crystalline aggregates of TPE-Py, however, emit stronger and bluer light than their amorphous counterparts. The solid-state emission of TPE-Py can be reversibly switched between green and yellow color by grinding–fuming and grinding–heating processes with a high contrast due to the transformation from the crystalline to the amorphous state and vice versa. The large Stokes shift and well-ordered molecular arrangement of the crystalline microrods of TPE-Py make it promising as an optical waveguide material with a low optical loss coefficient of ∼0.032 dB μm−1. TPE-Py works as a good fluorescent visualizer for specific staining of mitochondria in living cells with a high photostability, thanks to its hydrophobic and cationic features.

Tetraphenylethene-containing diyne (1), named 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene, was synthesized and polymerized by TaBr5 catalyst, affording the hyperbranched polymer hb-P1 with a high molecular weight (Mw up to 157[thin space (1/6-em)]800) in a nearly quantitative yield. The cyclotrimerization of 1-(4-ethynylphenyl)-1,2,2-triphenylethene was also carried out to give 1,3,5- and 1,2,4-tris[4-(1,2,2-triphenylvinyl)phenyl]benzene (1,3,5-2 and 1,2,4-2) that served as model compounds for structural characterization and properties investigation. The structures of hb-P1 and 2 are characterized by MS, IR, and NMR spectroscopies with satisfactory results. All the molecules and polymer are completely soluble in common organic solvents such as chloroform, toluene and THF. They show high thermal stability with degradation temperatures up to 443 and 446 °C, respectively, in nitrogen and air. Whereas their solutions are almost non-emissive or weakly fluorescent, their aggregates in poor solvents and solid powders emit intensely with fluorescence quantum yields up to 81%, demonstrating a novel phenomenon of aggregation-induced or enhanced emission. The unreacted triple bonds on the periphery of hb-P1 allow its thin film to cross-link under UV irradiation, generating well-resolved photopatterns. The hyperbranched polymer can limit harsh laser pulses. Its emission can be quenched efficiently by picric acid in solution and aggregated states with large quenching constants, suggesting that it is a promising fluorescent chemosensor for explosives detection.

Benzene-cored luminogens with multiple triarylvinyl units are designed and synthesized. These propeller-shaped molecules are nonemissive when dissolved in good solvents, but become highly emissive when aggregated in poor solvents or in the solid state, showing the novel phenomenon of aggregation-induced emission. Restriction of intramolecular motion is identified as the main cause for this effect. Thanks to their high solid-state fluorescence quantum yields (up to unity) and high thermal and morphological stabilities, light-emitting diodes with the luminogens as emitters give sky-blue to greenish-blue light in high luminance and efficiencies of 10800 cd m−2, 5.8 cd A−1, and 2.7%, respectively. The emissions of the nanoaggregates of the luminogens can be quenched exponentially by picric acid, or selectively by Ru3+, with quenching constants up to 105 and ∼2.0 × 105 L mol−1, respectively, making them highly sensitive (and selective) chemosensors for explosives and metal ions.

Sensors based on fluorescent conjugated polymers (FCPs) have attracted much attention, due to their amplified response and superior sensitivity to analytes, in comparison to their low molar mass congeners.1 Among many potential applications of FCPs, the detection of explosives is of great current interest because of its anti-terrorism implications.2 Much effort has been devoted to the development of linear FCPs with an amplification effect for sensitive detection of explosives. It has been shown that a FCP with a fast rate of exciton migration over a long sequence of its repeat units along the one-dimensional (1D) chain affords a high sensitivity (Fig. 1A).1 The sensing performance of the FCP, however, is often deteriorated by the aggregation of its linear chains. This notorious aggregation-caused quenching (ACQ) effect interferes with the detection of the analytes that induce the polymer chains to aggregate in solutions.