Xinbing Chen

Cyclic thiourea/urea functionalized triphenylamine-based dyes for high-performance dye-sensitized solar cells.

Six cyclic thiourea/urea functionalized triphenylamine-based dyes (AZ1-AZ6) containing 2-cyanoacrylic acid as an acceptor and various linkers (phenyl, biphenyl, and bithiophene) were synthesized. They exhibited high photovoltaic performance owing to an improved short-circuit photocurrent density (J(sc)) and open-circuit voltage (V(oc)). Among them, AZ6 bearing a cyclic thiourea group and bithiophene linker showed the highest power conversion efficiency (PCE) up to 7.29%, which was comparable to that of N719 (PCE = 7.36%).

Synthesis and properties of substituted benzoxazole-terminated liquid crystals

The synthesis and characterisation, and mesomorphic and photophysical properties of a new type of benzoxazole-terminated heterocyclic compounds, 2-(4ʹ-alkoxybiphenyl-4-yl)-benzoxazole derivatives (nB-x), are reported. For better understanding of the structure–property relationships, compounds bearing a variety of polar substituents in the 5-position of the benzoxazole moiety were prepared, including H (nB-H), NO2 (nB-N), CH3 (nB-M) and Cl (nB-C). Their phase transition behaviour was investigated by differential scanning calorimetry and polarising optical microscopy. In the case of carbon atoms in the alkoxy chain between 2 and 10 they exhibited enantiotropic smectic mesophases, for which the mesophase ranges were 101–175°C and 159–182°C on heating and cooling for nB-N, 5–63°C and 19–63°C for nB-H, 81–90°C and 78–96°C for nB-M, and 131–132°C and 131–132°C for nB-C, respectively. The effect of the length of the alkoxy chain on mesomorphic properties was studied. Compounds nB-N, nB-M and nB-C exhibited a much wider mesophase range either on heating or cooling than the corresponding nB-H series, which indicated that the substituent on the benzoxazole moiety was helpful in increasing the mesophase stability. Substituents with stronger electron-withdrawing properties gave a wider mesomorphic temperature domain, in the order NO2 > Cl > CH3 > H, which was similar to the effect of the substituents on the UV absorption bands of nB-x. The opposite effect was found for the corresponding Schiff base liquid crystals (nS-x). With the exception of nB-N, the nB-x series displayed intense photoluminescence emission in methylene chloride solution, with absorption maxima within the range 395–404 nm.

Dielectric and optical anisotropy enhanced by 1,3-dioxolane terminal substitution on tolane-liquid crystals

Four series of 1,3-dioxolane-terminated liquid crystals were prepared via multi-step reactions based on 4-alkylcyclohexanecarboxylic acids. Their structures were confirmed by infrared spectrometry (IR), nuclear magnetic resonance (NMR) and mass spectrometry (MS). Their properties were measured by differential scanning calorimetry (DSC), polarising optical microscopy (POM), X-ray diffraction (XRD), Abbe refractometer and an electrical constants instrument. The results show that the positive dielectric anisotropy and birefringence of the tolane-liquid crystals are increased significantly by an introduction of 1,3-dioxolane as a terminal group. The application of the 1,3-dioxolane-terminated liquid crystals in the mixture was investigated and the results show that lower threshold voltage, wider nematic range and higher birefringence are obtained when compared to the mixture containing liquid crystals with buty-3-enyl as a terminal group. The lateral fluoro substituent, alkyne-bridge and 1,3-dioxolane terminal group enhance the stability of the nematic mesophase and the solubility of the corresponding compounds in the common host liquid crystal mixture. Meanwhile, DFT calculations of dipole moments and molecular polarizabilities were used to correlate the experimental findings.

Synthesis and characterisation of benzoxazole-based liquid crystals possessing 3,5-difluorophenyl unit

Series of fluorinated compounds, 2-(3′,5′-difluoro-4′-alkoxybiphenyl-4-yl)-benzoxazole derivatives (nFBx), were prepared and characterised. Their phase transition behaviour was investigated by differential scanning calorimetry and polarising optical microscopy. In the case of carbon atoms in the alkoxy chain between 4 and 10, they exhibited enantiotropic mesophases with the mesophase ranges of 12–119°C and 23–152°C on heating and cooling for compounds bearing different substituents (H, CH3, Cl, and NO2). With the exception of nitro-substituted compounds, the nFBx series displayed intense photoluminescence emission at 380–385 nm in methylene chloride solution when they were excited at their absorption maxima. Compared to non-fluorinated analogues, fluorinated compounds nFBx (apart from nitro-substituted compounds) exhibited much lower melting points, but comparable or slightly narrower mesophase ranges during both heating and cooling, which were attributed to the disruption of the side-to-side intermolecular packing caused by the two lateral fluoro substituents.

Synthesis and mesomorphic properties of but-3-enyl-based fluorinated biphenyl liquid crystals

But-3-enyl-based fluorinated biphenyl liquid crystals, composed of alkyl chain, trans-cyclohexane and biphenyl mesogenic core, ethylene linking group, lateral fluoro substitute, and but-3-enyl terminal group, were prepared via nine step reactions based on 4-alkylcyclohexanecarboxylic acids, and their structures were confirmed via infrared spectrometry, nuclear magnetic resonance, and mass spectrometry. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry and polarising optical microscopy. The results showed that these compounds exhibited a broad nematic mesophase with low melting points and high clearing points. The effects of the but-3-enyl terminal group and the lateral fluoro substituent on the mesomorphic properties were discussed. These but-3-enyl-based biphenyl liquid crystals have potential for use in liquid crystal display mixtures.

Synthesis and mesomorphic properties of 2-(4′-alkoxybiphenyl-4-yl)-1H-benzimidazole derivatives

Three series of 2-(4′-alkoxybiphenyl-4-yl)-1H-benzimidazole derivatives (nM-x), which possessed 5-nitrobenzimidazole (nM-N series), benzimidazole (nM-H series) or 5-methylbenzimidazole (nM-M series) units at the end of the molecule, were synthesised and characterised by infrared, 1H- and 13C-nuclear magnetic resonance spectra, electrospray ionisation-mass spectrometry and elemental analysis. Their phase transition behaviour was investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. All the compounds exhibited enantiotropic smectic mesophases with wide temperature domains for a carbon number in the alkoxy chain from 6 to 16, where the mesophase ranges were 14–91°C and 17–99°C during heating and cooling processes for the nM-N compounds, 7–25°C and 8–49°C for the nM-H compounds and 48–81°C and 52–85°C for the nM-M compounds, respectively. The effect of the length of alkoxy chain on mesomorphic properties was discussed. The nM-N and nM-M exhibited a much wider mesophase range whether during heating or cooling process than the corresponding nM-H series, especially for the longer terminal chain (n > 8), which indicated that the substituent in the benzimidazole moiety was helpful in increasing the mesophase stability.

Approach to tune short-circuit current and open-circuit voltage of dye-sensitized solar cells: π-linker modification and photoanode selection

The modification of π-linkers represents a viable strategy to improve the performance of organic dye in dye-sensitized solar cells (DSSCs). On the basis of the π-linker modification, four cyclic thiourea functionalized dyes (AZ260, AZ261, AZ262, and AZ263) were synthesized and their effects on the short-circuit current density (Jsc) and open-circuit voltage (Voc) were investigated. The results showed that AZ261 bearing an n-hexyl chain on the π-linker generated higher Jsc (16.02 mA cm−2) and Voc (739.4 mV) than AZ6 without a substituent on the bithiophene (Jsc = 15.89 mA cm−2, Voc = 729.2 mV, PCE = 7.20%), and a higher PCE of 7.42% was obtained from AZ261. Furthermore, insertion of a double-bond into the π-linker (AZ263) yielded the highest Jsc (17.83 mA cm−2) with a somewhat lower Voc (719.2 mV), and delivered the highest PCE up to 8.24%. In addition, the function of mesoporous spherical TiO2 on the photoanodes were also systematically examined and proven to be beneficial in increasing the Jsc.

Synthesis and Characterization of Novel Sulfonated Polyimides Derived from Naphthalenic Dianhydride

4,4′-Ketone dinaphthalene 1,1′,8,8′-tetracarboxylic dianhydride (KDNTDA) was prepared by a new route including six steps with an overall yield of 19%. The structure was confirmed by IR, 1H NMR and UV spectroscopy. Several novel sulfonated polyimides (SPIs) derived from KDNTDA were synthesized and their structures were characterized by IR and 1H NMR. Their reduced viscosity ranged from 0.8 to 4.5 dL g-1 at 35°C. They had good solubility in common aprotic solvents not only in triethylamine salt form but also in proton form. The co-SPIs showed anisotropic membrane swelling with larger swelling in thickness than in plane, while the homo-polymer displayed isotropic membrane swelling. The thermal stability measurements revealed that sulfonic acid group in KDNTDA-based SPIs were stable up to 300°C. They displayed reasonably high proton conductivity, water stability and mechanical properties.

Synthesis and properties of allyloxy-based biphenyl liquid crystals with multiple lateral fluoro substituents

Allyloxy-based biphenyl liquid crystals, composed of alkyl chain, trans-cyclohexane and biphenyl mesogenic cores, ethylene linking groups, lateral fluoro substituents and allyloxy terminal groups were prepared via a six-step reaction based on 4-alkylcyclohexanecarbonitrile. Their structures were confirmed via infrared spectrometry, mass spectra and nuclear magnetic resonance. The mesomorphic properties of the present compounds were investigated by differential scanning calorimetry and polarising optical microscopy. The compounds exhibited negative dielectric anisotropy from −3.36 to −4.29 and a broad nematic mesophase with low melting points and high clearing points. The effects of the allyloxy terminal group and the lateral fluoro substituents on the mesomorphic properties and negative dielectric anisotropies are discussed. The present allyloxy-based biphenyl liquid crystals have potential for use in liquid crystal display mixtures.

Synthesis, mesomorphic and gelation properties of 7-alkoxycoumarin-3-carbonyl hydrazine

A series of coumarin derivatives, 7-alkoxycoumarin-3-carbonyl hydrazine (nCCH), which possessed a hydrazine moiety at the end of the molecule was prepared and was characterised by infrared and 1H-nuclear magnetic resonance spectra, electrospray ionisation-mass spectrometry and elemental analysis. All the compounds exhibited enantiotropic mesophases with temperature domains of 67–82°C and 46–82°C on heating and cooling processes, as well as favourable gelability to kinds of organic solvents. The effect of the length of the alkoxy chain on mesomorphic and gelation properties was discussed. The studies revealed that hydrogen bonding between the nCCH molecules was one of the main driving forces for the formation of the gel and thermotropic mesophases.