Novel visible photoinitiators systems for free-radical/cationic hybrid photopolymerization

Free radical/cationic hybrid photopolymerization of acrylates and epoxides was induced using an initiator system comprised of the dye derivative 5,12-dihydroquinoxalino[2,3-b]quinoxaline or 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine as the sensitizers and hexafluoroantimonate triarylsulfonium salt as the initiators. The curing experiments were carried out in the presence of air and the consumption of each monomer upon VIS-radiation was monitored in situ by real-time infrared spectroscopy. Hardness (H), elastic modulus (E) and the H/E ratio of the coatings obtained by visible-initiated acrylate/epoxide hybrid photopolymerization were determined using nanoindentation. DSC measurements show that the initiator system presented here may produce an interpenetrating polymer network. The pencil hardness of the obtained coatings indicates that the dye/hexafluoroantimonate triarylsulfonium salts systems studied here may have practical applications as visible-light hybrid initiators.     

The synthesis of novel,visible-wavelength,oxidizable polymerization sensitizers based on the 8-halogeno-5,12-dihydroquinoxalino[2,3-b]quinoxaline skeleton

Novel dyes, based on the 8-halogeno-5,12-dihydroquinoxalino[2,3-b]quinoxaline skeleton, were synthesized and characterized using ¹H NMR spectroscopy and chemical ionization mass spectroscopy. Their electrochemical and spectral properties, such as absorption and emission spectra, quantum yield of fluorescence and quantum yield of singlet oxygen generation, were also measured. These dyes were used as oxidizable sensitizers for diphenyliodonium and N-alkoxypyridinium salts. Photoredox pairs, consisting of dyes and pyridinium or iodonium salts, were found to be effective visible-wavelength initiators of free radical or cationic polymerization, respectively. The ability of each dye to act as a photoinitiator strongly depended upon its chemical structure. The heavy atoms present in the chemical structure could lead to excited triplet states within the dye, thereby facilitating electron transfer from these states.     

The synthesis of novel,visible-wavelength oxidizable polymerization sensitizers based on the 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine skeleton

Novel dyes based on the 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine skeleton were synthesized and subsequently characterized using ¹H NMR. Their electrochemical and spectral properties, such as absorption and emission spectra, quantum yield of fluorescence, and quantum yield of singlet oxygen generation, were also measured. The dyes were useful as photoinitiators for sensitizing compounds to photooxidation. Photoredox pairs comprising dyes and onium salts were found to be effective visible-wavelength initiators of free radical or cationic polymerization.     

Photocationic and radical polymerizations by novel N‐phenacylammonium salts

Novel N‐phenacylammonium salts [N‐(α‐benzoylbenzyl)‐, N‐(1‐benzoylethyl)‐, N‐phenacyl‐, pyrazinium, 3‐bromoquinolinium, benzothiazolium, or p‐cyanopyridinium hexafluoroantimonates] were synthesized by the reaction of α‐benzoylbenzyl bromide, 2‐bromopropiophenone or phenacylbromide, and the corresponding N‐hetero aromatic compound, followed by anion exchange with KSbF₆. These N‐phenacylammonium salts showed higher activity than previously reported N‐benzyl‐2‐cyanopyridinium hexafluoroantimonate in the photopolymerization of epoxides. Further, these N‐phenacylammonium salts served as photoradical and thermal cationic initiators as well as photocationic initiators. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3470–3476, 2004     

A nitrogen rich Ni(II)–dithiolate system exhibiting acid–base behavior: Synthesis,supramolecular structure and spectroscopy of [Bu4N]2[NiII(ppdt)2] (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate)

The compound [Bu₄N]₂[Ni(ppdt)₂] (1) (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate) has been synthesized, starting from pyrido[2,3-b]pyrazine-2,3-dithiol, nickel chloride and tetrabutylammonium bromide in methanol. Compound 1 crystallizes in P2₁/c space group (monoclinic system). Its crystal structure is characterized by interesting C–H?S and C–H?N supramolecular weak interactions. Its solution state has been described with acid–base (protonation–deprotonation) behavior, that has rarely been investigated for a metal–dithiolene system. Compound 1 is first instance of a metal–dithiolene compound that has three ring nitrogen on each dithiolate ligand. The pH dependent changes in the charge-transfer absorption band are attributed to the protonation on an imine nitrogen of the ppdt ligand. The complex is electrochemically quasi-reversible with an oxidation potential of E_1/2 = +0.46 V vs. Ag/AgCl in methanol.     

Novel magnesium and zinc porphyrazines containing galactose moieties: Synthesis via click reaction and characterization

In this study, galactose conjugated new magnesium and zinc porphyrazines were synthesized by the cyclotetramerization reaction of 2,3-bis[1-(2,2,7,7-tetramethyltetra-hydro-bis[1,3]dioxolo[4,5-b;4′,5′-d]pyran-5,methyl)-1H-[1,2,3]triazol-4-yl methylsulfanyl]-but 2-enedinitrile. This substituted dicyano compound was prepared via two different routes. One started from cis-1,2-dicyano-1,2-ethylenedithiolate disodium, [1-(2, 2, 7,7-tetramethyltetrahydrobis[1,3]dioxolo[4,5-b;4′,5′-d]pyran-5-yl-methyl)-1H-[1,2,3]triazo-l-4-yl]methanol and ended in a multi-step reaction sequence via Click procedures. The other reaction was between 5-azidomethyl-2,2,7,7-tetramethyltetrahydro-bis[1,3]dioxolo[4,5-b;4′,5′-d]pyran and (2Z)-2,3-bis(prop-2-yl-1-yl-thio)but-2-enedinitrile. A very soluble galactose linked magnesium porphyrazine derivative in common polar solvents and water was achieved by the deprotected isopropylidene groups in TFA and water media. It is first time, zinc porphyrazine complex has been achieved at one-step reaction by using Zn(BuO)₂ as template agent. The new compounds have been characterized by a combination of elemental analysis, ¹H, ¹³C NMR, IR, UV–vis and MS spectral data.     

Polyacrylate/SiO2 nanocomposites prepared by combining non-aqueous sol–gel process and miniemulsion polymerization

Cationically photopolymerizable phenyl epoxy-silicone monomers (Ep-Ph-Si) with a high RI and high photopolymerization activity have been synthesized via sequential hydrosilylation reactions of 1,3,5,7- tetramethylcyclotetrasiloxane by attaching first styrene, followed by 4-vinyl-1-cyclohexene-1,2-epoxide in the presence of Lamoreaux catalyst. The content of benzene ring in the monomers was determined by ¹HNMR; the epoxy value was examed by chemical titration. The RI of the obtained monomers showed that the benzene ring could improve the refractive indices (RIs) and the RIs of the phenyl epoxy-silicone monomers ranged from 1.50 to 1.52. The cationic photopolymerization of these monomers initiated by triarylsulfonium hexafluorophosphate (P-S) showed that phenyl epoxy-silicone monomers with little amount of benzene ring still exhibited high photopolymerization rate. Those properties are well suitable for the application in UV-curing optical coatings and encapsulation materials.     

Photo‐differential scanning calorimetry study on photopolymerization of nanosized titanium dioxide/polyacrylate hybrid materials

Nanosized titania–polyacrylate hybrid films were prepared by controlled hydrolysis of titanium tetrabutoxide (TTB) in Span 85/Tween 80 reverse micelles as well as rapid entrapment of nanosized particles by subsequent photopolymerization. This work focused on the characterization of the photopolymerization of the transparent hybrid films via photocalorimetry. The polymerization kinetics of several mixtures of mono‐ and di‐functional acrylate monomers were compared to examine the effect of average functionality. Further, Effects of [H₂O]/[surfactant] molar value and TTB concentration on photopolymerization kinetics were also studied. X‐ray photoelectron spectra and FTIR demonstrated the formation of TiO₂ in the hybrid films. Atomic Force Microscope (AFM) images showed the inorganic domains with mean sizes of 25 nm were uniformly dispersed in the polymeric networks. Copyright © 2006 Society of Chemical Industry     

Preparation and characterization of hybrid nanocomposite coatings by photopolymerization and sol-gel process

Organic-inorganic nanocomposite hybrid coatings were prepared through a dual-cure process involving cationic photopolymerization of epoxy groups and subsequent condensation of alkoxysilane groups. The kinetics of the photopolymerization and condensation reactions were investigated; suitable conditions for obtaining a complete conversion of both reactive groups were found. The obtained films are transparent to visible light. The T_g values of the hybrids increase by increasing the TEOS content in the photocurable formulation. Also, a significant increase in surface hardness could be obtained for the hybrid systems. AFM analysis gave evidence of a strong interaction between the organic and inorganic phase with the formation of silica domains in the nanoscale range.     

Substrate effect on wear resistant transition metal nitride hard coatings: Microstructure and tribo-mechanical properties

Four types of different hard transition metal nitrides (TMN:ZrN, CrN, WN and TiN) coatings were deposited on Si (100) and 316LN stainless steel substrates using DC magnetron sputtering. A comprehensive study of microstructure and substrate dependent tribo-mechanical properties of TMN coatings was carried out. Higher hardness (H) and elastic modulus (E) were obtained for WN (H=40 GPa and E=440 GPa) and TiN (H=30 GPa and E=399 GPa) coatings. This is related to the formation of (100) and (111) preferred orientations in WN and TiN coatings, respectively. However, the less hardness and elastic modulus were obtained for ZrN and CrN coatings where (200) orientation is preferred. Remarkably, low friction coefficient (0.06–0.57) and higher wear resistance in the coatings deposited on steel substrates are directly associated with the higher resistance to plastic deformation (H³/E²) and the presence of intrinsic compressive stress. Three body wear modes enhanced the friction coefficient (0.15–0.62) and the wear rate in the coatings deposited on Si substrates. This is primarily associated with low fracture toughness of brittle single crystalline Si (100) substrates. Steel-on-steel contact was dominated in ZrN/steel sliding system. This occurs due to the severe adhesive wear mode of steel ball, whereas, the abrasive wear modes were attained for the CrN, WN and TiN coatings sliding against steel balls.     

Dense yttria-stabilized zirconia coatings fabricated by plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition (PS-PVD) is a novel technique which can offer new opportunities to obtain advanced microstructures. In this study, dense yttria-stabilized zirconia (YSZ) coatings were deposited using PS-PVD with proper parameters. The dependencies of the microstructure of the as-sprayed coatings on axial spray distance and radial position were discussed. Four typical coatings were selected for the current work. The corresponding microstructures, phase compositions and mechanical properties were studied in detail. These as-sprayed coatings had dense lamellar or lamellar/columnar hybrid microstructures, the lowest porosity was 0.44%. Both monoclinic and tetragonal zirconia were detected in the as-sprayed coatings. The distribution of yttrium was not homogeneous, especially in the coatings deposited at shorter spray distance. Furthermore, oxygen got lost partly during the spraying process with decreasing spray distance. The hardness (H) and Young's modulus (E) of the coatings changed simultaneously with the different microstructures. The maximum values of H and E were 16.6±0.6 and 234.3±10.1 GPa respectively, which were found in the densest coatings. The dense PS-PVD ceramic coatings might be applicable in anti-corrosion fields (such as alkali corrosion, marine corrosion and hot corrosion).     

2-Arylamino-thiophen-3-carbonsäurederivate

2-Arylamino-thiophene-3-carboxylic Acid Derivatives The title compounds 3 are synthesized by reaction of 2-amino-thiophene-3-carboxylic acid derivatives 1 with anilines. The nucleophilic 5-position in 3 allowed the synthesis of 5-aryliden-Δ³-thiolen-2-one-imines 5 , 6 and the oxidative coupling to yield the azo compound 7 . 1-Aryl-thieno-[2,3-d]pyrimid-4-ones 9 , the thieno[2,3-b]chinolin-4-one 10 and the 4-chloro thieno[2,3-b]chinolines 10 , 11 can be obtained from 3 .     

Phenolation of (+)-Catechin with Mineral Acids. II. Identification of New Reaction Products

To investigate the reactions that occur in the flavanoid unit during the liquefaction of tannin in phenol, the phenolysis of (+)-catechin was studied using either H₂SO₄, HCl, or BF₃ 2H₂O as acid catalyst. In addition to 2-[3-(3,4-dihydroxyphenyl)-2-hydroxy-3-(4-hydroxyphenyl)propyl]-1,3,5-benzenetriol (1) and 2-[(3,4-dihydroxyphenyl)(4-hydroxyphenyl)methyl]-2,3-dihydro-4,6-benzofurandiol (3) that have been described previously, eight additional reaction products were isolated, four of which were compounds that have not been described previously. The novel compounds described here are: 2-[3-(3,4-dihydroxyphenyl)-2-hydroxy-3-(2-hydroxyphenyl)propyl]-1,3,5-benzenetriol (2), 2-[(3,4-dihydroxyphenyl)(2-hydroxyphenyl)methyl]-2,3-dihydro-4,6-benzofurandiol (4), 2-[(3,4-dihydroxyphenyl)(4-hydroxyphenyl)methyl]-2,3-dihydro-7-(4-hydroxyphenyl)methyl-4,6-benzofurandiol (5), and 2-(1,3,5-trihydroxyphenyl)methyl-3-(3,4-dihydroxyphenyl)-6-[(3,4-dihydroxyphenyl)(4-hydroxyphenyl)methyl]-2,3,5,6-tetrahydrobenzo[1,2-b:5,4-b′]-difuran-4-ol (6). The structures of these and other previously described products are consistent with opening of the pyran ring of catechin and reaction at C-2 by either the para or the ortho position of phenol. Additional products resulting from reaction between pyran ring cleavage products and catechin, and from reaction of cleavage products were found. Similar reactions would be expected to take place during the phenolysis of condensed tannins.     

Chemical defense of giant springtailTetrodontophora bielanensis (Waga) (Insecta: Collembola)

The giant springtail,Tetrodontophora bielanensis (Waga), is characterized by integumental openings (pseudocells) from which small droplets of a sticky defensive fluid are secreted after molestation. The secretion originates initially from secretory cells below the pseudocellae; subsequent irritations result in release of hemolymph, which was identified by both chemical and microscopical methods as well as by scanning electron microscopy. Bioassays with topically treated ground beetlesNebria brevicollis showed that the pseudocellular fluid evokes a total disorientation and cleansing behavior of the beetle. The main constituents were identified as the following pyridopyrazines: 2,3-dimethoxpyrido[2,3-b]pyrazine (1), 3-isopropyl-2-methoxypyrido[2,3-b]pyrazine (2), and 2-methoxy-4H-pyrido[2,3-b]pyrazine-3-one (3). These alkaloids are mainly present in the pseudocellar fluids of female and male springtails but are absent in their food or feces. Minor amounts are found in the hemolymph of adults, while larvae contain traces of2 only. All compounds were synthesized and tested for activity. In natural concentrations, the synthetic alkaloids elicited the same effects from the ground beetles as the pseudocellar fluid.     

Coupling of cyclohexene oxide (CHO) and carbon disulfide (CS2) catalyzed by the asymmetric bis-Schiff-base Zn(II) complex

Based on the self-assembly of the asymmetric bis-Schiff-base ligand H₂L (H₂L = 4-((E)-(2-((E)-3,5-dibromo-2-hydroxybenzylideneamino)phenylimino)(phenyl)methyl-1-(4-chlorophenyl)-3-methyl-1H-pyrazol-5-ol) and Zn(OAc)₂·2H₂O, a new [Zn(L)] (1) was obtained and shown to efficiently catalyze the coupling of CHO (cyclohexene oxide) and CS₂ (carbon disulfide) in activation with [PPN]Cl (PPN⁺ = bis(triphenylphosphoranyidene)-ammonium), n-Bu₄NBr or n-Bu₄NI, where both poly[thio]carbonates and cyclic [thio]carbonates were produced, and the strong O/S exchange afforded the limited formation of trithiocarbonate in the cyclic [thio]carbonate byproducts.     

The synthesis and photophysical studies of quinoxaline and pyridopyrazine derivatives

A simple and facile method for the synthesis of 2,3-bis[(E)-2-aryl vinyl]-quinoxaline and 2,3-bis[(E)-2-aryl vinyl]-pyrido(2,3-b)pyrazine is described. Condensation of cinnamils with 1,2-diaminobenzene and 2,3-diaminopyridine in water using conventional heating and microwave irradiation afforded high yields (71–92%) of 2,3-bis[(E)-2-aryl vinyl]-quinoxalines and 2,3-bis[(E)-2-arylvinyl]-pyrido(2,3-b)pyrazines. The photophysical properties of the resultant quinoxaline and pyrazine derivatives were studied; the pyridopyrazine derivatives were found to exhibit halochromism.     

Critical energy for photopolymerization of ceramic suspensions in acrylate monomers

The critical energy dose E_c required for photopolymerization of silica and alumina suspensions in acrylate monomers was measured for a large series of suspensions, and contrasted with a model that based on the assumption that E_c is the dose necessary to exhaust the polymerization inhibitors. The major predictions of the model were in good agreement with the data: E_c is inversely proportional to photoinitiator concentration, directly proportional to inert dye concentration, and directly proportional to the concentration of inhibitors.     

Comprehensive and Facile Synthesis of Some Functionalized Bis-Heterocyclic Compounds Containing a Thieno[2,3-b]thiophene Motif

A comprehensive and facile method for the synthesis of new functionalized bis-heterocyclic compounds containing a thieno[2,3-b]thiophene motif is described. The hitherto unknown bis-pyrazolothieno[2,3-b]thiophene derivatives 2a-c, bis-pyridazin othieno[2,3-b]thiophene derivatives 4, bis-pyridinothieno[2,3-b]thiophene derivatives 6a,b, and to an analogous bis-pyridinothieno[2,3-b]thiophene nitrile derivatives 7 are obtained. Additionally, the novel bis-pyradazinonothieno[2,3-b]thiophene derivatives 9, and nicotinic acid derivatives 10, 11 are obtained via bis-dienamide 8. The structures of all newly synthesized compounds have been elucidated by (1)H, (13)C NMR, GCMS, and IR spectrometry. These compounds represent a new class of sulfur and Nitrogen containing heterocycles that should also be of interest as new materials.     

Synthesis and characterization of Cu(I) and Zn(II) complexes with new sulfur-bearing isoxazole- or pyrazole-based ligands

The synthesis of the new ligands 6-(5-methyl-1,2-oxazol-3-yl)-2,3-dihydro-5H-[1,4] dithiino[2,3-c]pyrrole-5,7(6H)-dione (isox′) and 6-(3-methyl-1H-pyrazol-5-yl)-2,3-dihydro-5H-[1,4]dithiino[2,3-c]pyrrole-5,7(6H)-dione (pyraz′) and their coordination chemistry toward Cu(I) and Zn(II), was studied. The ligands and their complexes were characterized using a combination of either multinuclear NMR (¹H and ¹³C{¹H}), HRMS, FTIR or Uv–Vis spectroscopy. The solid state structures of ligand isox′ and complexes [Cu(pyraz′)₂]OTf and [Zn(OOCCF₃)₂(pyraz′)₂] were determined. Interestingly, isox′ presents a yellow luminescence in its free form. Additionally, the ability of isox′ to coordinate as an N–O bidentate ligand or as an N–S bridge between two copper centers, forming a coordination polymer, is studied. The solid state structure of this Cu(I)-isox′ 1D coordination polymer is also reported.