•This review provides a comprehensive overview of fluorescent chemosensors for copper(II) ion.•Three main types, namely, “on-off”, “off-on” and ratiometric fluorescence chemosensors for copper(II) ion are discussed systemically.•Cu2+-promoted reaction based chemosensors are summarized briefly.Among the transition metal ions, copper is the third most abundant essential trace metal element in human body, and is also required by many living organisms for normal physiological processes, while excessive levels of Cu2+ are highly toxic to organisms and generate enormous quantities of pollution to our living environment. It is an essential and key issue to devise more sensitive and selective fluorescent chemosensors to efficiently evaluate the Cu2+ levels in environmental and biological systems. This review summarizes the important advances in fluorescent chemosensors for Cu2+, mainly made in the recent five years. Three types of chemosensors, namely ‘on-off’, ‘off-on’ and ‘ratiometric’, are categorized according to fluorescence signal changes, and each type is further classified into several sections according to the molecular structure features and/or recognition mechanisms. Additionally, the Cu2+-promoted reaction based chemosensors are also discussed by the different reactions like hydrolysis, oxidation and reduction. The molecular structures, recognition mechanism and applications of the fluorescent chemosensors are emphatically discussed, and the future perspective is overviewed briefly.Download high-res image (94KB)Download full-size image

•A novel fluorescent chemosensor 1 for the detection of copper(II) ion has been developed.•1 showed high selectivity and good antiinterference properties.•The binding between 1 and Cu2+ shows 1:2 stoichiometry on basis of the mole ratio method.A new calix[4]arene derivative in 1,3-alternate conformation bearing four fluorescent 1,3,4-oxadiazole units was synthesized as a fluorescent chemosensor. This chemosensor displayed a highly selective recognition of Cu2+ among various metal ions by the fluorescence quenching. The binding between the chemosensor and Cu2+ showed 1:2 stoichiometry which was investigated by means of mole ratio method.

A side-chain polymer POXD and the corresponding monomer MOXD based on discotic 1,3,4-oxadiazole have been synthesized and characterized. Both POXD and MOXD exhibit liquid crystalline behaviors at room temperature with excellent thermal stability, while the mesomorphic temperature range of POXD is much wider than that of MOXD. In addition, they are photoluminescent in solution, in the solid state and in liquid crystalline phases. Notably, the fluorescent intensity of POXD is almost 12 times that of MOXD in both the solid state and liquid crystalline phases.

A triphenylene-based conjugated microporous polymer (TP-CMP) has been synthesized and shows high surface area (SBET = 1104 m2 g−1) and remarkable uptakes of H2 (7.08 mmol g−1 at 77 K and 900 mmHg) and CO2 (3.37 mmol g−1 at 273 K and 900 mmHg). In addition, it could be utilized to detect nitrobenzene selectively via fluorescence quenching.

Liquid crystals with 1,3,4-oxadiazole units in the aromatic core have attracted considerable attention due to their rich mesophases, high photoluminescence quantum yields, good electron transporting ability and excellent thermal stabilities. In this feature article, 1,3,4-oxadiazole-based liquid crystals are classified into three categories according to their different molecular shapes and molecular weight: calamitic and discotic monomers, symmetric and non-symmetric dimers, and polymers including main-chain, side-chain and mesogen-jacketed types. The structure–property relationships of this kind of liquid crystal are discussed in-depth, with an emphasis on strategies to devise columnar and biaxial nematic mesophases with a low temperature range.

Two C2-symmetrical hexaazatriphenylene (HAT) derivatives, 2,3-diphenyl-6,7,10,11-tetra(pyridin-2-yl)dipyrazino[2,3-f:2′,3′-h]quinoxaline (1) and 2,3,6,7-tetraphenyl-10,11-di(pyridin-2-yl)dipyrazino[2,3-f:2′,3′-h]quinoxaline (2), were designed and synthesized by the condensation reaction of 1,2-diamines and 1,2-diketones. Both compounds 1 and 2 exhibit sensitive, ratiometric and colorimetric fluorescence selectivity for Zn2+ ion over alkali ions, alkaline earth ions and a wide range of transition metal ions upon excitation at 350 nm in acetonitrile/water. The interactions between 1 or 2 and Zn2+ can be observed by naked eyes with an obvious color change of the solution from colorless to yellow. For fluorescence intensity of 2 toward Zn2+, a good linearity (correlation coefficient R2=0.993) was established with a detection limit of 0.095 μM, which is more sensitive than that of 1 (0.2 μM). The binding modes of the free ligands 1 and 2 with Zn2+ are discussed in context to their photophysical and electrochemical properties as well as single X-ray crystallographic structures of 1, 2 and 1-Zn.Highlights► C2-symmetrical hexaazatriphenylene based chemsensors 1 and 2 were synthesized. ► 1 and 2 exhibit ratiometric and colorimetric fluorescene selectivity for Zn2+. ► The color changes between 1 or 2 with Zn2+ can be observed by naked eyes. ► 1, 2 are sensitive to Zn2+ with detection limits of 0.2 and 0.095 μM, respectively.

Two bipolar molecules 1a and 1b, which incorporate both electron-transporting oxadiazole and hole-transporting carbazolyl or N,N′-dimethylphenyl moieties within the conjugation system, were synthesized and characterized by 1H NMR, 13C NMR, elemental analysis and mass spectrometry. The molecular structure of compound 1a was further confirmed by single X-ray crystallography. The solutions of both compounds in CH2Cl2 individually displayed two absorption bands with λmax values at 376–378 nm and emitted with λmax values at 478–479 nm and quantum yields of 0.61–0.63. The cyclic voltammograms results showed that the carbazole-containing compound 1a has better hole injection ability than the N,N′-dimethylphenyl end-capped 1b.

A new biindenylidenedione compound, named [trans-anti-3,3′-Di-p-hydroxyl phenyl-3,3′-dihydroxy-[2,2′-bi-1H-indene]-1,1′-dione] (2) was synthesized and characterized by means of 1H-NMR, IR, MS and X-ray crystallographic diffraction techniques. The photochromic behavior was investigated by means of UV–Vis spectroscopy, and the results showed that the inclusion complex of 2 with guest solvent N,N-dimethylformamide (DMF) did not exhibit photochromism in crystal state. The structure–property relation was discussed briefly in context to the intermolecular interactions and the solvent-dependent effect on photochromism.

A series of aromatically 2,5-disubstituted 1,3,4-thia(oxa)diazoles with a terminal methoxy or methylthio group were synthesized and characterized by means of 1H NMR, 13C NMR, MS and elemental analysis. The X-ray crystal structures of compounds 1a and 3b revealed that both of them adopted layered arrangement without short intermolecular interactions. The liquid crystal properties have been investigated by means of polarizing optical microscopy (POM), differential scanning calorimetry (DSC), variable temperature X-ray diffraction (VXRD) and thermogravimetric analysis (TGA). All compounds (except 2a and 2b) exhibited various mesophases (nematic, smectic C and/or smectic A) with wide temperature ranges and good thermal stability. In CH2Cl2 solution, all these compounds displayed a room temperature emission with λmax at 371–425 nm and quantum yields of 0.25–0.83. The effect of the sulfur and oxygen atoms on mesomorphic and photo-luminescent properties was discussed according to the positions in the central heterocyclic rings and the terminal groups.

Biindenylidenedione derivatives simultaneously exhibit photochromism and photo-generated organic radicals upon irradiation with UV light in solid and/or crystalline state, and they may be used as potential bi-functional materials in optoelectronic devices. This review article deals with recent progress in synthesis, relationship between molecular structures and phototochromic and photomagnetic properties of this kind of compounds.

Reactions of 2,2′-biindanylidene-1,1′,3,3′-tetraone with arylmagnesium bromide gave 3,3′-diaryl-3,3′-dihydroxy biindenylidenedione derivatives. These compounds exhibited photochromism in solid state, in which the formation of the isomeric radical species was confirmed by the election spin resonance spectroscopy, and the structures of new compounds 1 and 4 were determined by X-ray crystallography. The relationship between structures and properties was also discussed.

Liquid crystals with 1,3,4-oxadiazole units in the aromatic core have attracted considerable attention due to their rich mesophases, high photoluminescence quantum yields, good electron transporting ability and excellent thermal stabilities. In this feature article, 1,3,4-oxadiazole-based liquid crystals are classified into three categories according to their different molecular shapes and molecular weight: calamitic and discotic monomers, symmetric and non-symmetric dimers, and polymers including main-chain, side-chain and mesogen-jacketed types. The structure–property relationships of this kind of liquid crystal are discussed in-depth, with an emphasis on strategies to devise columnar and biaxial nematic mesophases with a low temperature range.