Chemistry of Polythiols and Their Industrial Applications.

Thiols can react with readily available organic substrates under benign conditions, making them suitable for use in chemical, biological, physical, and materials and engineering research areas. In particular, the highly efficient thiol-based click reaction includes the reaction of radicals with electron-rich enes, Michael addition with electron-poor enes, carbonyl addition with isocyanate SN2 ring opening with epoxies, and SN2 nucleophilic substitution with halogens. This mini review provides insights into emerging venues for their industrial applications, especially for the applications of thiol-ene, thiol-isocyanate, and thiol-epoxy reactions, highlighting a brief chemistry of thiols as well as various approaches to polythiol synthesis.

A Polyolefin Elastomer Encapsulant Modified by an Ethylene-Propylene-Diene Terpolymer for Photovoltaic Applications.

In this study, a newly designed adhesion promoter, a modified ethylene-propylene-diene terpolymer (m-EPDM), was constructed via a simple thiol-ene click reaction between the ethylene-propylene-diene terpolymer (EPDM) and 3-mercaptopropyltrimethoxysilane (MPTS) to employ polyolefin elastomer (POE) encapsulants in photovoltaic modules. The grafting reaction of MPTS on an EPDM backbone (thiol-ene click reaction) was verified using 1H NMR, 29Si NMR, and SEM/EDX. The thermal and mechanical characteristics of the POE compounds did not significantly change with an increasing m-EPDM content irrespective of the cross-linking state. Interestingly, the adhesion strength to the glass substrate increased linearly with an increasing m-EPDM content until 9 phr. Also, the POE compounds containing more than 12 phr m-EPDM showed cohesion failure of the encapsulant layer, remaining as a residue of the encapsulant layer on the glass surface after peel testing. The damp-heat test was conducted to evaluate the long-term durability of the photovoltaic module encapsulated with m-EPDM, and no significant power loss was found even after 1000 h under the test conditions.

Construction and Characterization of Polyolefin Elastomer Blends with Chemically Modified Hydrocarbon Resin as a Photovoltaic Module Encapsulant.

In this study, polyolefin elastomer (POE) was blended with a chemically modified hydrocarbon resin (m-HCR), which was modified through a simple radical grafting reaction using γ-methacryloxypropyl trimethoxy silane (MTS) as an adhesion promotor to the glass surface, to design an adhesion-enhanced polyolefin encapsulant material for photovoltaic modules. Its chemical modification was confirmed by 1H and 29Si NMR and FT-IR. Interestingly, the POE blends with the m-HCR showed that the melting peak temperature (Tm) was not changed. However, Tm shifted to lower values with increasing m-HCR content after crosslinking. Additionally, the mechanical properties did not significantly differ with increasing m-HCR content. Meanwhile, with increasing m-HCR content in the POE blend, the peel strength increased linearly without sacrificing their transmittance. The test photovoltaic modules comprising the crosslinked POE blend encapsulants showed little difference in the electrical performance after manufacturing. After 1000 h of damp-heat exposure, no significant power loss was observed.

Construction, Physical Properties and Foaming Behavior of High-Content Lignin Reinforced Low-Density Polyethylene Biocomposites.

Lignin was chemically modified with oligomeric polyethylene (oPE) to form oPE-grafted lignin (oPE-g-lignin) via lignin surface acylation and a radical coupling reaction with oPE. Then, pristine lignin and oPE-g-lignin were successfully compounded with low-density polyethylene (LDPE) through a typical compounding technique. Due to the oligomeric polyethylene chains grafted to the lignin's surface, the interfacial adhesion between the lignin particles and the LDPE matrix was considerably better in the oPE-g-lignin/LDPE biocomposite than in the pristine-lignin/LDPE one. This demonstrated that oPE-g-lignin can serve as both a biodegradable reinforcing filler, which can be loaded with a higher lignin content at 50 wt-%, and a nucleating agent to increase the crystallization temperature and improve the tensile characteristics of its LDPE biocomposites. Moreover, the foamability of the lignin-reinforced LDPE biocomposites was studied in the presence of a chemical blowing agent (azodicarbonamide) with dicumyl peroxide; for an oPE-g-lignin content up to 20 wt-%, the cell size distribution was quite uniform, and the foam expansion ratios (17.69 ± 0.92) were similar to those of the neat LDPE foam (17.04 ± 0.44).

Synthesis and Characterization of Multifunctional Secondary Thiol Hardeners Using 3-Mercaptobutanoic Acid and Their Thiol-Epoxy Curing Behavior.

3-Mercaptobutanoic acid (3-MBA) was synthesized by the less odorous Michael addition pathway using an isothiouronium salt intermediate. Using the synthesized 3-MBA, multifunctional secondary thiol (sec-thiol) compounds were obtained and applied to thiol-epoxy curing systems as hardeners. As the functionality of the sec-thiol hardeners increased, the purity of the product obtained after distillation decreased. The equivalent epoxy mixtures with multifunctional sec-thiol hardeners were evaluated based on their impact on the curing behavior in thiol-epoxy click reactions by differential scanning calorimetry. The thermal features of sec-thiol-epoxy click reactions in the presence of a base catalyst were assessed according to the functionality of the sec-thiol hardeners. Our results showed that sec-thiol hardeners with less reactivity to the epoxy group provide long-term storage stability for the formulated epoxy resin, promising for industrial applications.

Preparation of Chemically Modified Lignin-Reinforced PLA Biocomposites and Their 3D Printing Performance.

Using a simple esterification reaction of a hydroxyl group with an anhydride group, pristine lignin was successfully converted to a new lignin (COOH-lignin) modified with a terminal carboxyl group. This chemical modification of pristine lignin was confirmed by the appearance of new absorption bands in the FT-IR spectrum. Then, the pristine lignin and COOH-lignin were successfully incorporated into a poly(lactic acid) (PLA) matrix by a typical melt-mixing process. When applied to the COOH-lignin, interfacial adhesion performance between the lignin filler and PLA matrix was better and stronger than pristine lignin. Based on these results for the COOH-lignin/PLA biocomposites, the cost of printing PLA 3D filaments can be reduced without changing their thermal and mechanical properties. Furthermore, the potential of lignin as a component in PLA biocomposites adequate for 3D printing was demonstrated.

Optimization of synthetic parameters of high-purity trifunctional mercaptoesters and their curing behavior for the thiol-epoxy click reaction.

The direct esterification reaction between 3-mercaptopropionic acid (3-MPA) and trimethylolpropane (TMP) was conducted in the presence of various catalyst concentrations of p-toluenesulfonic acid (p-TSA) to examine the optimized synthetic conditions needed to produce high-purity trimethylolpropane-tris(3-mercaptopropionate) (TMPMP). The purity of the desired TMPMP and uncompleted side-product reduced as the acid catalyst concentration in this esterification reaction increased while the generation of thioester-based side-product increased. The equivalent ratio between epoxy and the manufactured TMPMP was maintained at 1 : 1 to monitor the curing behavior of the thiol-epoxy click reaction using the DSC technique. The thermal features of the base-catalyzed TMPMP-cured epoxy resin were assessed according to the purity of the TMPMP curing agent.

Highly efficient and color tunable thermally activated delayed fluorescent emitters using a "twin emitter" molecular design.

High efficiency and color tuning of thermally activated delayed fluorescent emitters were achieved at the same time by designing emitters with a twin emitter molecular design. The control of the interconnect position between two emitters could manage the emission spectrum of the thermally activated delayed fluorescent emitters without affecting the quantum efficiency.

Design strategy for 25% external quantum efficiency in green and blue thermally activated delayed fluorescent devices.

Carbazole- and triazine-derived thermally activated delayed fluorescent (TADF) emitters, with three donor units and an even distribution of the highest occupied molecular orbital, achieve high external quantum efficiencies of above 25% in blue and green TADF devices.

Butterfly-Shaped Diphenylpyrimidine Molecule: Tunable Photophysical Properties by Molecular Self-Assembly Pathways.

To understand the relationships between chemical structures, molecular packing structures, and photophysical properties of organic materials, a butterfly shaped diphenylpyrimidine molecule (abbreviated as DPP-6C12) was newly synthesized [Park, M.; Choi, Y.-J.; Kim, D.-Y.; Hwang, S.-H.; Jeong, K.-U. Cryst. Growth Des. 2015, 15, 900-906]. By breaking the molecular symmetry and coplanarity of DPP-6C12, peculiar monotropic phase transitions were observed. Based on two-dimensional wide-angle X-ray diffraction and selected area electron diffraction, the molecular packing structures of ordered phases were identified, which were further confirmed by the computer simulations in the real and reciprocal spaces. Finally, we demonstrated that the photophysical properties of DPP-6C12 can be tuned by controlling the molecular packing structures with simple thermal treatments.

Rational design of host materials for phosphorescent organic light-emitting diodes by modifying the 1-position of carbazole.

A novel carbazole moiety with bromine at the 1-position of carbazole was synthesized and four carbazole compounds derived from the 1-position modified carbazole were developed as the host materials for phosphorescent organic light-emitting diodes. The 1-position modified carbazole was coupled with another carbazole to prepare bicarbazole intermediates, which were substituted with 4,6-diphenyltriazine to yield four bicarbazole derivatives modified with the electron deficient diphenyltriazine unit. The triplet host materials showed high quantum efficiency above 20% and low driving voltage below 5.0 V at 1000 cd m(-2) in green phosphorescent organic light-emitting diodes.

Molecular design of triazine and carbazole based host materials for blue phosphorescent organic emitting diodes.

We synthesized 3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-methylphenyl)-9-phenyl-9H-carbazole (TrzmPCz) as a new bipolar host material for blue phosphorescent organic light-emitting devices and investigated the electro-optical properties of the blue devices fabricated using the TrzmPCz host. We managed the triplet energy of the host by inserting a methyl substituent in the phenyl linkage between triazine and carbazole. The methyl substituent distorted the backbone structure of TrzmPCz and lead to high triplet energy of 2.79 eV. After optimization of the device structure, the TrzmPCz based organic light-emitting diodes achieved the maximum quantum efficiency of 16.4%, a current efficiency of 32 cd A(-1), and a power efficiency of 21.5 lm W(-1).

Benzofurocarbazole and benzothienocarbazole as donors for improved quantum efficiency in blue thermally activated delayed fluorescent devices.

Benzofurocarbazole and benzothienocarbazole were used as electron donors of thermally activated delayed fluorescence (TADF) emitters and the performances of the TADF devices were examined. The benzofurocarbazole and benzothienocarbazole donor moieties were better than carbazole as the electron donors of the TADF emitters.

Above 30% external quantum efficiency in green delayed fluorescent organic light-emitting diodes.

Highly efficient green thermally activated delayed fluorescent organic light-emitting diodes with an external quantum efficiency of 31.2% were investigated by using 3-(3-(carbazole-9-yl)phenyl) pyrido[3',2':4,5]furo[2,3-b]pyridine (3CzPFP) derived from carbazole and pyrido[3',2':4,5]furo[2,3-b]pyridine. The host material showed well-matched photoluminescence emission with absorption of the green dopant material, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) and harvested all excitons of 4CzIPN. The 3CzPFP:4CzIPN film exhibited high photoluminescence quantum yield of 100%, and the green delayed fluorescence device employing the 3CzPFP host showed high maximum quantum efficiency of 31.2 ± 0.5% at 1% doping after optimization of the device structure.

Synthesis of dibenzothiophene-based host materials with a dimesitylborane substituent and their green PHOLED performances.

In the two isomeric dibenzothiophene-based host materials, the different linkages between dibenzothiophene and dimesitylborane on a phenyl spacer dictated their photophysical properties. The performances of Ir(ppy)2(acac)-based green phosphorescent devices with two isomeric host materials were similar regardless of different linkage positions.

Stable blue thermally activated delayed fluorescent organic light-emitting diodes with three times longer lifetime than phosphorescent organic light-emitting diodes.

High quantum efficiency above 18% and extended lifetime three times longer than that of phosphorescent organic light-emitting diodes (OLEDs) are demonstrated in blue thermally activated delayed fluorescent OLEDs.

Synthesis of dimesitylborane-substituted phenylcarbazoles as bipolar host materials and the variation of the green PHOLED performance with the substituent position of the boron atom.

Two structural isomers of the bipolar host material containing the dimesitylborane moiety were synthesized and their device performance was investigated. The quantum efficiency of the devices depends on the substituent position of the dimesitylborane moiety. The maximum external quantum efficiency of the device reached as high as 23.8% with a green color coordinate of (0.30, 0.63).

Thermal- and photo-induced phase-transition behaviors of a tapered dendritic liquid crystal with photochromic azobenzene mesogens and a bicyclic chiral center.

A ribbon-shaped chiral liquid crystalline (LC) dendrimer with photochromic azobenzene mesogens and an isosorbide chiral center (abbreviated as AZ3 DLC) was successfully synthesized and its major phase transitions were studied by using differential scanning calorimetry (DSC) and linear polarized optical microscopy (POM). Its ordered structures at different temperatures were further identified through structure-sensitive diffraction techniques. Based on the experimental results, it was found that the AZ3 DLC molecule exhibited the low-ordered chiral smectic (Sm*) LC phase with 6.31 nm periodicity at a high-temperature phase region. AZ3 DLC showed the reversible photoisomerization in both organic solvents and nematic (N) LC media. As a chiral-inducing agent, it exhibited a good solubility, a high helical-twisting power, and a large change in the helical-twisting power due to its photochemical isomerization in the commercially available N LC hosts. Therefore, we were able to reversibly "remote-control" the colors in the whole visible region by finely tuning the helical pitch of the spontaneously formed helical superstructures.

Columnar structures from asymmetrically tapered biphenylamide.

An asymmetrically tapered N,N'-tris[[(2-dodecylaminocarbonyl)ethyl]methyl]-4-biphenylamide (asym-C(12)PhA, where n is the number of carbon atoms in the alkyl chains, n = 12) was newly designed and synthesized. In this asymmetrically tapered asym-C(12)PhA biphenylamide, H-bondable hydrophilic amide moieties are located at between a rigid hydrophobic biphenyl rod and three flexible hydrophobic alkyl chains. Computer energy minimization indicated that three-dimensional (3D) geometry of asym-C(12)PhA biphenylamide looks like a cone with dimensions of 3.01 nm in height and 1.44 nm in bottom radius. Phase transitions and supra-molecular structures were identified utilizing the combined techniques of differential scanning calorimetry, 1D wide-angle X-ray diffraction (1D WAXD), Fourier-transform infrared spectroscopy, and solid-state (13)C nuclear magnetic resonance analyses. The asym-C(12)PhA self-assembled into a highly ordered columnar mesophase just below the isotropization temperature and then transformed to 3D columnar crystalline phase (Phi(Cr)) on further cooling. Selected area electron diffractions in transmission electron microscopy (TEM) along with 1D WAXD and cross-polarized optical microscopy suggested that discotic building blocks were constructed by rotating 120 degrees of three asym-C(12)PhA with respect to neighboring ones and the tmb (top-middle-bottom) stacked discotic building blocks further self-organized into columns. These columns are laterally intercalated to form the Phi(Cr) phase. On the basis of the TEM image and polyethylene surface decoration technology, it was identified that the self-assembled asym-C(12)PhA fibers with approximately 1 mum in diameter and several millimeters in length were braids of tiny single crystals.