Ching Mui Cho

Polyhedral oligomeric silsesquioxanes tethered with perfluoroalkylthioether corner groups: Facile synthesis and enhancement of hydrophobicity of their polymer blends

Octa(vinyl)-POSS or octa(vinyldimethylsilyloxy)-POSS reacts with perfluoroalkyl thiol in the presence of 2,2′-azobisisobutyronitrile to afford perfluoroalkylthioether substituted POSS compounds in high yields. 29Si NMR and MALDI-TOF mass spectroscopic analyses indicate that the eight perfluoroalkylthioether chains are attached to the POSS core structure. Studies on poly(methyl methacrylate) (PMMA) blends with 5 wt% fluorinated POSS (fluoroPOSS) reveal that the POSS molecules considerably enhance the hydrophobicity of polymer blend surfaces as evidenced by significant increases in water contact angles from 71° for pure PMMA to 114–124° for fluoroPOSS-blends, making this type of fluoroPOSS a promising new class of highly hydrophobic hybrid materials.

Modulating high-energy visible light absorption to attain neutral-state black electrochromic polymers

Neutral-state black diketopyrrolopyrrole (DPP)-based conjugated polymers with the colour channels a* from −3 to 0 and b* from −2 to −7 are achieved by incorporating an additional donor segment – dialkoxythiophene (DalkOT), which leads to the enhancement in blue-wavelength absorption. The polymers display black-to-transmissive grey electrochromic switching with good redox stability.

Synthesis of Ultrahighly Electron-Deficient Pyrrolo[3,4-d]pyridazine-5,7-dione by Inverse Electron Demand Diels–Alder Reaction and Its Application as Electrochromic Materials

A new electron acceptor 6-alkylpyrrolo[3,4-d]pyridazine-5,7-dione (PPD) with a very low LUMO level has been synthesized via a challenging inverse electron demand Diels-Alder reaction between thiophene and furan-decorated tetrazine substrates and an electron-deficient 1-alkyl-1H-pyrrole-2,5-dione unit. The PPD monomer has been incorporated into a series of donor-acceptor-type conjugated polymers as electrochromic materials with good optical contrast, fast switching speed, and high coloration efficiency.

Organic–inorganic hybrid liquid crystals derived from octameric silsesquioxanes. Effect of the peripheral groups in mesogens on the formation of liquid crystals

A series of octameric silsesquioxane-based organic–inorganic hybrid compounds 7a–h was synthesised by reacting octakis(dimethylsiloxy)silsesquioxane with 4′-(undec-10-enyloxy)biphenyl-4-yl mono-, di-, or trialkoxybenzoateviaplatinum-catalysed hydrosilylation reaction. The chemical structures of 7a–h were characterised by 1H, 13C, and 29Si nuclear magnetic resonance spectroscopy, mass spectroscopy and elemental analysis. Liquid crystallinity of 7a–h was investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). Temperature-dependent X-ray diffraction was used to verify liquid crystal phases, revealing that 7a–d assembled to smectic C and smectic A phases. The effects of the number and length of peripheral flexible alkoxy groups in 7a–h on the formation of liquid crystals were studied using DSC and POM, which revealed that 7a–d bore one alkoxy terminal group in each mesogen exhibited a mesophase, 7e–f and 7g–h bore two and three flexible alkoxy terminal groups in each mesogen, respectively, and did not show mesomorphic behaviour. The concept of “flexible group density” (FGD), which was defined as the number of the CH2 groups per unit volume in the periphery of the POSS molecule, was introduced to interpret the formation of liquid crystals. The results revealed that the POSS molecules with di- and tri-substituted mesogens exhibited much larger FGD than those with mono-substituted mesogens, hence the absence of mesophases for 7e–f and 7g–h. This study provides a useful archetype to predict the formation of liquid crystals when designing and synthesising liquid crystalline materials, particularly star-like or dendrimer-like molecular materials.

A thermally stable and reversible microporous hydrogen-bonded organic framework: aggregation induced emission and metal ion-sensing properties

A microporous hydrogen-bonded organic framework (HOF) derived from a polyhedral oligomeric silsesquioxane (POSS) intermediate and an aggregation-induced emission (AIE) luminogen tetraphenylethene (TPE) derivative has been synthesized and structurally characterized by various methods. This unique HOF exhibits a permanent porosity with a Brunauer–Emmett–Teller (BET) surface area of 101.9 m2 g−1. This HOF could be well dispersed in organic solvents in the form of nanoparticles with a size of a few hundred nanometers. These nanoparticles are highly fluorescent in organic solution, and exhibit a high fluorescence quenching selectivity towards copper ions. Furthermore, the fluorescence of this HOF could be recovered by the removal of copper ions upon addition of cyanide and, more interestingly, this process could be repeated several times without considerably sacrificing the sensing activity towards copper ions.

Solution-processable blue-to-transmissive electrochromic benzotriazole-containing conjugated polymers

A series of donor–acceptor type electrochromic (EC) conjugated polymers P1–P4, poly(4,7-bis(3′,4′-ethylenedioxythiophen-2′-yl)-2-alkylbenzotriazole-5′,5′-diyl-alt-(3,4-dialkoxy-thiophen-2,5-diyl), was synthesized via a Stille coupling reaction. The polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The electrochemical, spectroelectrochemical and EC properties of the polymers were studied. The polymers displayed improved solubility in common organic solvents. Upon oxidation, the polymers switched from a saturated blue to a transmissive light blue state. In particular, P4 exhibited exceptional optical contrasts of 46% and 72% in the visible and near-infrared regions, respectively. Furthermore, the effects of the side chains on the EC properties were examined and the results revealed that polymers with a long alkyl chain in the benzotriazole moiety had faster switching speed both in the visible and near infrared regions in the polymer film state. The absorption–transmission ECDs were fabricated, revealing that under ambient conditions and without any encapsulation of ECDs, the polymers exhibited good optical contrasts and coloration efficiencies with improved stability.

Synthesis and self-assembly of halogen-bond donor–spacer–hydrogen-bond donor molecules: polymeric liquid crystals induced by combination of intermolecular halogen- and hydrogen-bonding interactions

A series of linear molecules of 4-(n-(2,3,5,6-tetrafluoro-4-iodophenoxy)alkoxy)benzoic acid (4a–d) bearing a hydrogen-bond donor on one end of the molecule and a halogen-bond donor on another end was synthesised. Compounds 4a–d formed composite halogen- and hydrogen-bonded polymeric complexes with bis(4-trans-4′-stilbazolyl)-oligo(ethylene glycol) (5a-c), as verified by Fourier transform infrared spectroscopy and differential scanning calorimetry (DSC). The degree of polymerisation of complexes 4a–d/5a–c was estimated to be in the range of 11–25 using the Flory equation, suggesting the polymeric nature of the complexes. The complexes 4a–d/5a–c showed thermotropic mesophases upon cooling, confirmed by polarised optical microscopy and DSC. The liquid crystals exhibited significantly broader mesophase temperature ranges than those induced by the halogen bond alone, indicating that the introduction of a more flexible hydrogen-bonded mesogenic core in halogen-bonded polymeric assembly likely led to a marked improvement in the stability of the mesophase by way of modulating the flexibility of the polymeric chains. The novel halogen- and hydrogen-bonded assembly of this type is thought to provide an excellent model upon which to build supramolecular polymeric liquid crystals through a precise manipulation of the two types of non-covalent interactions.

Ultrahigh electron-deficient pyrrolo-acenaphtho-pyridazine-dione based donor–acceptor conjugated polymers for electrochromic applications

Novel electron acceptors 2-alkyl-6,9-di(thiophen-2-yl)-1H-pyridazino[4′,5′:2,3]indeno[6,7,1-def]isoquinoline-1,3(2H)-dione and 2-alkyl-6,9-di(furan-2-yl)-1H-pyridazino[4′,5′:2,3]indeno[6,7,1-def]isoquinoline-1,3(2H)-dione derived from pyrrolo-acenaphtho-pyridazine-diones (PAPD) with a very low-lying lowest unoccupied molecular orbital (LUMO) level have been synthesized via a regioselective inverse electron demand Diels–Alder reaction between thiophene- and furan-substituted 1,2,4,5-tetrazine and an electron-deficient compound 2-(2-alkyl)-1H-indeno[6,7,1-def]isoquinoline-1,3(2H)-dione. The chemical structures of two PAPD monomers were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry as well as single-crystal X-ray structural analysis. The time-dependent density functional theory (TD-DFT) calculations were performed to show that PAPD series monomers have a LUMO energy level down to −3.42 eV, much lower than the popular electron acceptors such as benzotriazole, benzothiadiazole and its fluorinated derivatives (−2.19–−2.98 eV). The PAPD based monomer was incorporated into a series of donor–acceptor type conjugated polymers comprising 3,3-bis((dodecyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and thieno[3,2-b]thiophene as co-monomers through Stille coupling polymerization to give electrochromic conjugated polymers P1–P5 with high number-average molecular weights in the range of 42 000–67 000 (g mol−1). The polymers showed optical bandgaps between 1.90–1.99 eV. Electrochromic devices displayed reversible color changes between purple/red in the neutral state and greyish blue/grey in the oxidized state with an outstanding redox stability of less than 1% decrease in contrast after 800 cycles using polymer P3 as an example, and high optical contrasts of up to 80% at 1500 nm in the near infrared region.

Fluorinated polyhedral oligomeric silsesquioxanes

A series of fluorinated polyhedral oligomeric silsesquioxane (POSS) derivatives were prepared via hydrosilylation reaction and they were characterized by 1H, 13C, 29Si and 19F nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The thermal stability of POSS derivatives was studied using thermogravimetric analysis (TGA) and they exhibited high decomposition temperatures ranging from 327 to 380 °C in air and inert atmosphere. At ambient temperature, fluorinated POSS (FluoroPOSS) compounds 3a–b and 3d–e appear as fluidic oils with a dynamic viscosity of 140–430 mPas and it is noteworthy to mention that 3a, 3d and 3e do not solidify even below −80 °C. The thermal aging experiments of these FluoroPOSS oils were performed, and results revealed that these oils exhibited excellent long-term thermal stability with minimal weight loss over 48 hours in air, rendering them as promising lubricant candidates for unique applications in a wide temperature range from −80 to 300 °C. These fluorinated POSS derivatives have also shown potential in polymer surface modification as evidenced by the great increase in water contact angle from ∼68° for a neat hydrophilic PMMA film to 94–105° for the blended film with 5 wt% fluorinated derivatives. The rise in water contact angle is likely to be due to migration of the hydrophobic POSS core from the interior to the surface, which has been observed by atomic force microscopy (AFM).

Engineering polydimethylsiloxane with two-dimensional graphene oxide for an extremely durable superhydrophobic fabric coating

A type of nanostructured material comprising reduced graphene oxide (RGO) modified polydimethylsiloxane (PDMS) for fabric coating is described. RGO modified PDMS was prepared through an aryl radical assisted addition reaction, followed by hydrosilylation with Si–H terminated PDMS or poly(dimethylsiloxane-co-methylhydrosiloxane) (PDMS-co-PHMS) in the presence of a catalytic amount of Pt catalyst. The introduction of a trace amount of RGO to cross-link with PDMS or PDMS-co-PHMS led to a significant improvement in the hydrophobicity (water contact angle > 150°) of textile fabrics. The superhydrophobicity of the fabric coating, which was readily achieved by a facile dipping coating process, is long-term resistant to strong acids, corrosive alkalis, hot water and 200 cycles of laundry. This type of RGO modified PDMS or PDMS-co-PHMS coating materials makes use of the inherent morphological anisotropy of fabrics which latter provides a hierarchical roughness on the micro scale to further enhance the surface hydrophobicity, showing potential use in various textile applications.