A benzophenone‐naphthalimide derivative as versatile photoinitiator of polymerization under near UV and visible lights

A benzophenone‐naphthalimide derivative (BPND) bearing tertiary amine groups has been developed as a high‐performance photoinitiator in combination with 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine or an iodonium salt for both the free radical polymerization (FRP) of acrylates and the cationic polymerization (CP) of epoxides upon exposure to near UV and visible LEDs (385–470 nm). BPND can even produce radicals without any added hydrogen donor. The photochemical mechanisms are studied by molecular orbital calculations, steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry and laser flash photolysis techniques. These novel BPND based photoinitiating systems exhibit an efficiency higher than that of the well‐known camphorquinone‐based systems (FRP and CP) or comparable to that of bis(2,4,6‐trimethylbenzoyl)‐phenylphosphineoxide (FRP at λ ≤ 455 nm). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 445–451     

Blue LED light‐sensitive benzo pyrazolo (or imidazo) isoquinolinone derivatives in high‐performance photoinitiating systems for polymerization reactions

Isoquinolinone derivatives (IQ) have been synthesized and combined with different additives (an amine, 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine, an iodonium salt, or N‐vinylcarbazole) to produce reactive species (i.e. radicals and cations) being able to initiate the radical polymerization of acrylates, the cationic polymerization of epoxides, the thiol‐ene polymerization of trifunctional thiol/divinylether, and the synthesis of epoxide/acrylate interpenetrated polymer network IPN upon exposure to very soft polychromatic visible lights, blue laser diode or blue LED lights. Compared with the use of camphorquinone based systems, the novel combinations employed here ensures higher monomer conversions (～50–60% vs. ～15–35%) and better polymerization rates in radical polymerization. The chemical mechanisms are studied by steady‐state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin trapping techniques. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 567–575     

Visible light‐emitting diode‐sensitive thioxanthone derivatives used in versatile photoinitiating systems for photopolymerizations

Novel thioxanthone (TX) derivatives are used as versatile photoinitiators upon visible light‐emitting diode (LED; e.g., 405, 425, and 450 nm) exposure. The mechanisms for the photochemical generation of reactive species (i.e., cations and free radicals) produced from photoinitiating systems based on the photoinitiator and an iodonium salt, tris(trimethylsilyl)silane, or an amine, were studied by UV–vis spectroscopy, fluorescence, cyclic voltammetry, steady‐state photolysis, and electron spin resonance spin‐trapping techniques. The reactive species are particularly efficient for cationic, free radical, hybrid, and thiol‐ene photopolymerizations upon LED exposure. The optimized photoinitiating systems exhibit higher efficiency than those of reference systems (i.e., isopropyl TX‐based photoinitiating systems), especially in the visible range. According to their beneficial features, these photoinitiating systems have considerable potential in photocuring applications. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 4037–4045     

2,2,2‐trifluoroacetophenone‐based D‐π‐A type photoinitiators for radical and cationic photopolymerizations under near‐UV and visible LEDs

Two D‐π‐A‐type 2,2,2‐trifluoroacetophenone derivatives, namely, 4′‐(4‐( N,N‐diphenyl)amino‐phenyl)‐phenyl‐2,2,2‐trifluoroacetophenone (PI‐Ben) and 4′‐(4‐(7‐(N,N‐diphenylamino)‐9,9‐dimethyl‐9H‐fluoren‐2‐yl)‐phenyl‐2,2,2‐trifluoroacetophenone (PI‐Flu), are developed as high‐performance photoinitiators combined with an amine or an iodonium salt for both the free‐radical polymerization of acrylates and the cationic polymerization of epoxides and vinyl ether upon exposure to near‐UV and visible light‐emitting diodes (LEDs; e.g., 365, 385, 405, and 450 nm). The photochemical mechanisms are investigated by UV‐Vis spectra, molecular‐orbital calculations, fluorescence, cyclic voltammetry, photolysis, and electron‐spin‐resonance spin‐trapping techniques. Compared with 2,2,2‐trifluoroacetophenone, both photoinitiators exhibit larger redshift of the absorption spectra and higher molar‐extinction coefficients. PI‐Ben and PI‐Flu themselves can produce free radicals to initiate the polymerization of acrylate without any added hydrogen donor. These novel D‐π‐A type trifluoroacetophenone‐based photoinitiating systems exhibit good efficiencies (acrylate conversion = 48%–66%; epoxide conversion = 85%–95%; LEDs at 365–450 nm exposure) even in low‐concentration initiators (0.5%, w/w) and very low curing light intensities (1–2 mW cm^?2). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1945–1954     

Azahelicenes as visible light photoinitiators for cationic and radical polymerization: Preparation of photoluminescent polymers and use in high performance LED projector 3D printing resins

In this article, novel azahelicenes (AZs) were synthesized and proposed as high performance visible light photoinitiators for both the free radical polymerization of acrylates and the cationic polymerization (CP) of epoxides upon visible light exposure using Light Emitting Diodes (LEDs) @405, @455, and @470 nm. Excellent polymerization initiating abilities are found and high final conversions were obtained. Remarkably, an exceptional long lifetime photoluminescence property of the polymer films was observed when synthesized in presence of AZs. A full picture of the involved chemical mechanisms is given. AZs being high performance photoinitiators, their use in new cationic LED 3D printing resins will be also presented, that is, the cationic process upon LED projector @405 nm can be useful to reduce the shrinkage usually observed for radical polymerization. LED projector printing is very interesting compared to laser writing as this technology projects the profile of an entire layer of a 3D object at one time. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 55, 1189–1199     

Organic light‐emitting diodes with multiple photocrosslinkable hole‐transport layers

We report on photocrosslinkable hole‐transport polymers and their use as photodefinable hole‐transport layers in organic light‐emitting diodes. The polymers were obtained by copolymerization of bis(diarylamino)biphenyl‐based acrylate monomers with cinnamate‐functionalized acrylate moieties. Polymers with a range of redox potentials were obtained by varying the substitution patterns of the bis(diarylamino)biphenyl units. The 2 + 2 cycloaddition of the cinnamate moieties following UV irradiation renders the material insoluble. This allows for patterning of the polymer and simultaneously enables the fabrication of multilayer structures from solution. Hole mobilities were measured in these copolymers with the time‐of‐flight technique. Their performance as hole‐transport layers in light‐emitting diodes, with tris(8‐hydroxyquinolinato)aluminum as the emitter and electron‐transport layer, is evaluated. Electroluminescent devices with multiple hole‐transport layers having different ionization potentials were fabricated from solution, and the quantum efficiency of these devices was greater than that for devices based on a single hole‐transport layer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2726–2732, 2003     

In situ access to white light‐emitting fluorescent polymer nanocomposites via plasma‐ignited frontal polymerization

We describe a facile fabrication of white light‐emitting cadmium sulfide (CdS)‐poly(HEA‐co‐NVK) nanocomposites [2‐hydroxyethyl acrylate (HEA) and N‐vinylcarbazole (NVK)] via plasma‐ignited frontal polymerization (PIFP), a novel and rapid reaction mode of converting monomers into polymers in minutes. Frontal polymerization was initiated by igniting the upper side of the reactant with plasma. Once initiated, no additional energy was required for the polymerization to occur. The chemical functional groups of the as‐prepared nanocomposites were thoroughly investigated using Fourier transform infrared spectra. The dependence of the front velocity and front temperature on the initiator concentration and weight ratios of HEA/NVK was also investigated in detail. Perhaps more interestingly, the white light‐emitting materials synthesized by ingeniously incorporating the compensating colors of yellow emitting from 3‐(trimethoxysilyl)‐1‐propanethiol‐capped CdS nanocrystals and blue emitting from carbazole‐containing polymer were conveniently applied onto a commercial UV light‐emitting diode (LED) to generate white LEDs. The subtle change in the weight ratios of CdS/NVK can significantly impact the color hue. The white light becomes gradually colder with the increase of NVK, but becomes gradually warmer with the increase concentration of CdS nanocrystals. In a broad perspective, these white light‐emitting materials designed by PIFP approach will open a new pathway to develop “QD‐polymer nanocomposite down‐conversion LED” in a fast and efficient way. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Fluorescent Brighteners as Visible LED‐Light Sensitive Photoinitiators for Free Radical Photopolymerizations

The photochemical and electrochemical investigations of commercially available, safe, and cheap fluorescent brighteners, namely, triazinylstilbene (commercial name: fluorescent brightener 28) and 2,5‐bis(5‐tert‐butyl‐benzoxazol‐2‐yl)thiophene, as well as their original use as photoinitiators of polymerization upon light emitting diode (LED) irradiation are reported. Remarkably, their excellent near‐UV–visible absorption properties combined with outstanding fluorescent properties allow them to act as high‐performance photoinitiators when used in combination with diaryliodonium salt. These two‐component photoinitiating systems can be employed for free radical polymerizations of acrylate. In addition, this brightener‐initiated photopolymerization is able to overcome oxygen inhibition even upon irradiation with low LED light intensity. The underlying photochemical mechanisms are investigated by electron‐spin resonance‐spin trapping, fluorescence, cyclic voltammetry, and steady‐state photolysis techniques.

     

Iodonium sulfonates as high‐performance coinitiators and additives for CQ‐based systems: Toward aromatic amine‐free photoinitiating systems

Iodonium sulfonates are proposed here as a new class of high‐performance coinitiators for camphorquinone (CQ)‐based systems for the polymerization of methacrylates under blue light irradiation. When combined with CQ, the new proposed coinitiators present excellent polymerization performances and are excellent candidates for the replacement of tertiary aromatic amines subjected to toxicological concerns in the well‐established CQ/amine photoinitiating system (PIS). Remarkably, good bleaching properties are obtained after polymerization. The use of the new PIS for dental adhesives is also investigated. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1664–1669     

Synthesis and optoelectronic properties of luminescent poly(p‐phenylenevinylene) derivatives containing electron‐transporting 1,3,4‐oxadiazole groups

Three random copolymers ( P1–P3 ) comprising phenylenevinylene and electron‐transporting aromatic 1,3,4‐oxadiazole segments (11, 18, 28 mol %, respectively) were prepared by Gilch polymerization to investigate the influence of oxadiazole content on their photophysical, electrochemical, and electroluminescent properties. For comparative study, homopolymer poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐p‐phenylenevinylene] ( P0 ) was also prepared by the same process. The polymers ( P0–P3 ) are soluble in common organic solvents and thermally stable up to 410 °C under a nitrogen atmosphere. Their optical properties were investigated by absorption and photoluminescence spectroscopy. The optical results reveal that the aromatic 1,3,4‐oxadiazole chromophores in P1–P3 suppress the intermolecular interactions. The HOMO and LUMO levels of these polymers were estimated from their cyclic voltammograms. The HOMO levels of P0–P3 are very similar (?5.02 to ?5.03 eV), whereas their LUMO levels decrease readily with increasing oxadiazole content (?2.7, ?3.08, ?3.11, and ?3.19 eV, respectively). Therefore, the electron affinity of the poly(p‐phenylenevinylene) chain can be gradually enhanced by incorporating 1,3,4‐oxadiazole segments. Among the polymers, P1 (11 mol % 1,3,4‐oxadiazole) shows the best EL performance (maximal luminance: 3490 cd/m², maximal current efficiency: 0.1 cd/A). Further increase in oxadiazole content results in micro‐phase separation that leads to performance deterioration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4377–4388, 2007     

Radical and cationic photopolymerization: New pyrylium and thiopyrylium salt‐based photoinitiating systems

New thiopyrylium and pyrylium salt‐based photoinitiating systems for visible light induced free radical polymerization (FRP) or free radical promoted cationic polymerization (FRPCP) under visible lights are presented. The reaction mechanisms are investigated by laser flash photolysis and the structure/reactivity trend is discussed. The abilities of two different classes of coinitiators are investigated (thiols/disulfides and silanes). In FRP, upon irradiation with a xenon lamp (λ > 390 nm), the (thio)pyrylium salts in combination with thiols or disulfides lead to very high polymerization rates, compared to the reference eosin Y/methyldiethanolamine system. In FRPCP, silanes are found much better coinitiators: a high efficiency of the photopolymerization under air is noted. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7369–7375, 2008     

Bis‐substituted thiophene‐containing oxime sulfonates photoacid generators for cationic polymerization under UV–visible LED irradiation

Three novel types of thiophene‐containing oxime sulfonates with a big π‐conjugated system were reported as non‐ionic photoacid generators. The irradiation of the newly synthesized photoacid generators using near UV–visible light‐emitting diodes (LEDs) (365–475 nm) results in the cleavage of two weak N O bonds in single molecules, which lead to the generation of different sulfonic acids in good quantum and chemical yields. The mechanism for the N O bond cleavage for acid generation was supported by the UV–visible spectra and real‐time ¹H NMR spectra. They are developed as high‐performance photoinitiators without any additives for the cationic polymerization of epoxide and vinyl ether upon exposure to near‐UV and visible LEDs (365–475 nm) at low concentration. In the field of photopolymerization, especially visible light polymerization, it has great potential for application. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 776–782     

Aminothiazonaphthalic anhydride derivatives as photoinitiators for violet/blue LED‐Induced cationic and radical photopolymerizations and 3D‐Printing resins

The cations and radicals produced in aminothiazonaphthalic anhydride derivatives (ATNAs) combined with an iodonium salt, N‐vinylcarbazole, amine, or chloro triazine initiate the ring‐opening cationic polymerization of epoxides and the free radical polymerization of acrylates under LEDs at 405 or 455 nm. The photoinitiating ability of these novel photoinitiating systems is higher than that of the well‐known camphorquinone‐based systems. An example of the high reactivity of the new proposed photoinitiator is also provided in resins for 3D‐printing using a LED projector@405 nm. The chemical mechanisms are investigated by steady‐state photolysis, cyclic voltammetry, fluorescence, laser flash photolysis, and electron spin resonance spin‐trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1189–1196     

Encapsulants for light‐emitting diodes from visible light‐cured epoxy monomers

During the last decade, light‐emitting diodes (LEDs) have replaced incandescent, fluorescent, and neon lamps due to their ability to produce high luminosity at low currents and voltages. LEDs are currently encapsulated by thermally curable epoxy resins. However, long periods of curing at high temperature result in high consumption of energy and require stringent process control to avoid failure of the devices. In addition, the thermal cure results in yellowing of the encapsulant, which decreases the efficiency of the LED. In recent years, photoinitiated polymerization has received much interest as it congregates a wide range of economic and ecological benefits. Cationic photoinitiators, such as diaryliodonium salts, generate Brønsted acid in situ, which initiates polymerization. The process can be triggered on demand by irradiating the mixture with light. Results from the present research reveal that cycloaliphatic epoxy monomers, photoactivated with an iodonium salt and Camphorquinone, polymerize readily under visible light irradiation (470 nm) in the absence of external heating. The partial replacement of cycloaliphatic epoxy with aromatic diglycidyl ether of bisphenol‐A (DGEBA) is an effective means of improving the refractive index of the material and consequently the efficiency of the photoemission. Visible light polymerization of DGEBA pure proceeds at a slow rate; however, it is enhanced by the increase in temperature during the polymerization of the highly reactive cycloaliphatic monomer. From results obtained in the present research, it may be concluded that visible light polymerization of epoxy monomers is a promising route for the processing of LED encapsulants. Copyright © 2013 John Wiley & Sons, Ltd.     

New series of highly phenyl‐substituted polyfluorene derivatives for polymer light‐emitting diodes

A new series of highly phenyl‐substituted polyfluorene derivatives were synthesized and characterized. The resulting polymers were amorphous and showed excellent solubility in common organic solvents, such as chloroform, tetrahydrofuran, xylene, toluene, chlorobenzene, and so forth. All possessed satisfied thermal stability with glass‐transition temperatures (T_g's) in the range of 79–115 °C. They emitted blue light with photoluminescent (PL) maximum peaks at about 408–412 nm in thin films. The PL efficiencies of the polymer films were measured around 30–33%. The highly phenylated pendants improved the T_g of polyfluorene without forming defects in the polymers and reduced their tendency to form aggregate/excimers. Polymer light‐emitting diodes were fabricated from these polymers with the configuration of indium tin oxide/polyethylenedioxythiophene:polystyrene sulfonic acid/polymer/Ba/Al, which emitted bright blue light with maximum peaks at 418–420 nm. The maximum external quantum efficiencies of these devices were 0.41–0.6%. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2985–2993, 2004     

A Multicolor Photoinitiator for Cationic Polymerization and Interpenetrated Polymer Network Synthesis: 2,7‐Di‐tert‐butyldimethyldihydropyrene

For polymer synthesis upon visible light, actual photoinitiator operates in a restricted part of the spectrum. As a consequence, several photoinitiators are necessary to harvest all of the emitted visible photons. Herein, 2,7‐di‐tert‐butyldimethyldihydropyrene is used for the first time as a multicolor photoinitiator for the cationic polymerization of epoxides. Upon addition of diphenyliodonium hexafluorophosphate and optionally N‐vinylcarbazole, the originality of this approach is to allow efficient monomer conversions under various excitation light sources in the 360–650 nm wavelength range: halogen lamps, and light‐emitting and laser diodes. The synthesis of an interpenetrated polymer network from an epoxide/acrylate blend using a red light at 635 nm is also feasible. The formed polymer material exhibits a photochromic character.

     

Visible‐light‐sensitive photoredox catalysis by iron complexes: Applications in cationic and radical polymerization reactions

The development of iron complexes for the photoredox catalysis is a huge challenge. Indeed, Iron complexes can be ideal candidates due to their potential visible light absorption and redox properties but also because they are less toxic, inexpensive and environmentally friendly compared to other catalysts. In the present paper, a series of novel iron complexes have been synthesized and utilized to initiate the free radical promoted cationic polymerization of epoxides or the free radical polymerization of acrylates through photoredox catalysis processes upon exposure to near UV (385 nm) or visible violet (405 nm) light emitting diodes (LEDs). When combined with an iodonium salt and N‐vinylcarbazole, the iron complex‐based photoinitiating systems are able to generate radicals, cations, and radical cations. The initiation efficiency is investigated through real‐time Fourier transform infrared spectroscopy and a satisfactory initiating ability is found. The mechanisms for the generation of the reactive initiating species through photoredox catalysis are studied by different methods (steady state photolysis, cyclic voltammetry and electron spin resonance spin trapping techniques) and discussed in detail. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2247–2253     

Versatile preparation of poly(1,4‐phenylenevinylene‐co‐1,4‐phenylene‐1,2‐ethanediyl) by CVD polymerization of p‐(methoxymethyl)benzyl chloride

It was demonstrated that a series of copolymers consisting of 1,4‐phenylenevinylene (PV) and 1,4‐phenylene‐1,2‐ethanediyl (PE) units could be prepared from a single monomer, p‐(methoxymethyl)benzyl chloride, via the chemical vapor deposition polymerization (CVDP) method. The composition of the copolymers could be varied simply by altering the monomer activation temperature. The higher the temperature, the lower the content of the PV unit. The photo (PL)‐ and electroluminescence (EL) properties of the copolymers that revealed a blueshift when compared with PPV strongly depend on the amount of the PE units incorporated. The external quantum efficiencies of the electroluminescence devices having the configuration of ITO/PEDOT‐PSS/copolymer/Al‐Li were higher than that of PPV, which can be ascribed to the improved confinement of excitons. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 742–751, 2005     

Novel Hole‐Transport Material for Efficient Polymer Light‐Emitting Diodes by Photoreaction

Summary: The photo‐crosslinking of carbazole dendrimers was analyzed by UV and IR spectroscopic methods. Photoirradiation results in the formation of a film that is insoluble in toluene and benzene. Time‐of‐flight mass spectrometry studies revealed that the photoirradiation lead to an oligomerization of the dendrimer through crosslinking. The resulting insoluble dendrimer film could be applied as a hole‐transport layer in efficient polymer electroluminescence devices (PLEDs).

Luminance‐voltage characteristics for PLEDs wherein PEDOT:PSS and CbzG3 complex with SnCl₂ were employed as the hole transport layer (ITO/HTL/EML/Ca/Ag).     

Naphthalic anhydride derivatives: Structural effects on their initiating abilities in radical and/or cationic photopolymerizations under visible light

Only one naphthalic anhydride derivative has been reported as light sensitive photoinitiator, this prompted us to further explore the possibility to prepare a new family of photoinitiators based on this scaffold. Therefore, eight naphthalic Naphthalic anhydride derivatives (ANH1‐ANH8) have been prepared and combined with an iodonium salt (and optionally N‐vinylcarbazole) or an amine (and optionally 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine) to initiate the cationic polymerization of epoxides and the free radical polymerization of acrylates under different irradiation sources, that is, very soft halogen lamp (～ 12 mW cm^?2), laser diode at 405 nm (～1.5 mW cm^?2) or blue LED centered at 455 nm (80 mW cm^?2). The ANH6 based photoinitiating systems are particularly efficient for the cationic and the radical photopolymerizations, and even better than that of the well‐known camphorquinone based systems. The photochemical mechanisms associated with the chemical structure/photopolymerization efficiency relationships are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin‐trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2860–2866