Photoacid Generator Research Articles

Supramolecularly Assembled Nanocomposites as Biomimetic Chloroplasts for Enhancement of Photophosphorylation.

Prototypes of natural biosystems provide opportunities for artificial biomimetic systems to break the limits of natural reactions and achieve output control. However, mimicking unique natural structures and ingenious functions remains a challenge. Now, multiple biochemical reactions were integrated into artificially designed compartments via molecular assembly. First, multicompartmental silica nanoparticles with hierarchical structures that mimic the chloroplasts were obtained by a templated synthesis. Then, photoacid generators and ATPase-liposomes were assembled inside and outside of silica compartments, respectively. Upon light illumination, protons produced by a photoacid generator in the confined space can drive the liposome-embedded enzyme ATPase towards ATP synthesis, which mimics the photophosphorylation process in vitro. The method enables fabrication of bioinspired nanoreactors for photobiocatalysis and provides insight for understanding sophisticated biochemical reactions.

Supramolecularly Assembled Nanocomposites as Biomimetic Chloroplasts for Enhancement of Photophosphorylation

AbstractPrototypes of natural biosystems provide opportunities for artificial biomimetic systems to break the limits of natural reactions and achieve output control. However, mimicking unique natural structures and ingenious functions remains a challenge. Now, multiple biochemical reactions were integrated into artificially designed compartments via molecular assembly. First, multicompartmental silica nanoparticles with hierarchical structures that mimic the chloroplasts were obtained by a templated synthesis. Then, photoacid generators and ATPase‐liposomes were assembled inside and outside of silica compartments, respectively. Upon light illumination, protons produced by a photoacid generator in the confined space can drive the liposome‐embedded enzyme ATPase towards ATP synthesis, which mimics the photophosphorylation process in vitro. The method enables fabrication of bioinspired nanoreactors for photobiocatalysis and provides insight for understanding sophisticated biochemical reactions.

Adhesion Control of Branched Catecholic Polymers by Acid Stimulation.

Biomimetic material design is a useful method for producing new functional materials. In recent years, catecholic polymers inspired from the adhesion mechanism of marine organisms have attracted attention. Here, we demonstrated the preparation of catecholic polymers by reversible addition–fragmentation chain transfer (RAFT) polymerization of an acetonide-protected catecholic monomer, that is, N-(2-(2,2-dimethylbenzo-1,3-dioxol-5-yl)ethyl)-acrylamide (DDEA). By selecting the specific RAFT reagents, well-defined branched PDDEA and linear PDDEA were obtained. These PDDEA samples showed stronger adhesion strength after deprotection by acid stimulation compared with that before deprotection. In addition, we demonstrated the adhesion control of synthetic polymers by photoirradiation in the presence of photoacid generators, which decompose under light and release an acid.

Visible light 3D printing with epoxidized vegetable oils

Stereolithography is a 3D printing technique in which a liquid monomer is photopolymerized to produce a solid object. The most widely used materials usually belong to the family of acrylate monomers, and photopolymerization occurs through a radical pathway. Photoinitiators can absorb UV or (less often) visible light, producing radicals for direct decomposition or hydrogen abstraction. Due to the toxicity of acrylates, vegetable oil-derived monomers were used in this study. In fact, vegetable oils contain unsaturations, and thus, they can be exploited as monomers. In particular, linseed oil, tung oil or edible oils (soybean, sunflower or corn) could be good candidates as raw materials. Unfortunately, the photoinduced radical polymerization of these oils either does not occur or is too slow for 3D printing applications. For this reason, the oils were modified as epoxides. Epoxides are monomers that are more reactive than natural oils, and they can be polymerized via a cationic mechanism. The aim of this work was to exploit visible light generated by a common digital projector (like those used in classrooms) as a light source. Since the tested photoacid generators working under visible light are ineffective for the polymerization of epoxidized oils, a multi-component photo-initiating mixture was used.

Photoresist materials comprising photobase generators and epoxy resins bearing carboxylic acids to lead to marked enhancement of base‐catalyzed cross‐linking reactions

Most of the photopatterning materials based on epoxy resins utilize photoacid generators (PAGs), which generate superacids as catalysts. They have been used for high aspect ratio photoresists in the fabrication of MEMS devices. However, there is a drawback, in that the acidic species from PAGs will induce metal corrosion. One of the approaches to overcome this problem is the use of photobase generators (PBGs) because organic bases would induce no corrosion. Although there have been many previous investigations of PBGs, only a few articles have mentioned photoreactive materials relying on PBGs because of their low photosensitivity. We report here highly sensitive photopatterning materials comprising PBGs and an epoxy resin bearing carboxylic acid groups. As a result, the photopatterning materials showed higher photosensitivity than conventional epoxy resin systems. We obtained high‐photosensitivity (up to 900 mJ/cm2), high‐resolution (10‐μm line‐and‐space) patterning materials in films, 10 μm in thickness.

Phenol-functionalized polymerization control additives for negative tone epoxide crosslinking molecular resists

Controlling undesired polymerization in nominally unexposed regions is critical to achieving high-resolution, defect-free patterns when using negative tone molecular resists based on the crosslinking of epoxides. Two onium salt additives, a photodecomposable nucleophile (PDN) and a photoacid generator (PAG), were functionalized with phenols in order to investigate their use as generalized additives capable of slowing crosslinking and improving the resolution of a variety of epoxide resists. Presented here is a phenol-functionalized PDN [tris(4-hydroxyphenyl)sulfonium triflate (TPS-OH-Tf)] and a phenol-functionalized PAG [tris(4-hydroxyphenyl)sulfonium antimonate (TPS-OH-SbF6)] used in combination with a model epoxide resist (4-Ep). Utilizing additives that contained phenols resulted in a decrease in resist sensitivity, but enabled higher additive loadings which could be used to offset this loss in sensitivity. Using TPS-OH-SbF6 did not provide enough polymerization control to prevent line broadening, and the use of TPS-OH-Tf was still required to achieve sub 35 nm 1:1 line:space patterns. Adding TPS-OH-Tf was also found to improve pattern collapse behavior at reduced (<25 nm) feature sizes. Initial patterning using 100 keV electron-beam lithography showed that the resolution of 4-Ep was improved to 15 nm 1:1 line:space patterns using these phenol-functionalized additives and demonstrate the potential of these additives to improve the resolution of a variety of epoxide crosslinking molecular resists.

Sunlight photodepolymerization of transient polymers

ABSTRACTThe study and development of transient devices is an emerging field where the disposal of a device after use is desired to avoid reverse engineering and minimize the environmental impact. Polyaldehydes with phototriggers have been investigated because the radiation wavelength can be adjusted to meet the transient application. Polynuclear aromatic hydrocarbons (PAHs) were used as the optical sensitizer for photoacid generators (PAGs). Photoinduced electron transfer (PET) with an iodonium‐based PAG was used to expand the spectral sensitivity range. Anthracene, tetracene, and pentacene derivatives were synthesized with appended phenylethynyl groups to improve the solubility of the sensitizer and adjust the absorption wavelength. Sensitization of the iodonium‐based PAG with the PAH derivatives was found to have thermodynamically favorable PET reactions for depolymerization of poly(propylene carbonate) and poly(phthalaldehyde) (PPHA). The Rehm–Weller equation and Stern–Volmer analysis were used to study the electron transfer and the fluorescence quenching rates of the PAHs with the iodonium salts, respectively. The photosensitivity, efficiency, and byproducts of the PET reactions in the decomposable polymer films are reported. A rapid photoreaction is reported for the depolymerization of PPHA exposed to a sunlight dose of <6 J cm−2 (i.e., 1 min of direct sunlight) with a pentacene‐based sensitizer. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47141.

High aspect ratio nanotubes assembled from macrocyclic iminium salts

One-dimensional nanostructures such as carbon nanotubes and actin filaments rely on strong and directional interactions to stabilize their high aspect ratio shapes. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>103), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that, in organic solvent (tetrahydrofuran), are two orders of magnitude stronger than the neutral macrocycles, as explained by physical arguments and demonstrated by molecular dynamics simulations. Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by cross-linking their pendant alkenes. As large macrocyclic chromonic liquid crystals, these iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.

Phototriggered Depolymerization of Flexible Poly(phthalaldehyde) Substrates by Integrated Organic Light-Emitting Diodes.

We demonstrate phototriggered depolymerization of a low ceiling temperature ( Tc) polymer, poly(phthalaldehyde) (PPHA), via internal light emission from integrated organic light-emitting diodes (OLEDs) fabricated directly on flexible PPHA substrates with silver nanowire electrodes. The depolymerization of the PPHA substrates is triggered by absorption of the OLED emission by a sensitizer that activates a photoacid generator via energetically favorable electron transfer. We confirm with Fourier-transform infrared spectroscopy that the photon doses delivered by the integrated OLED are sufficient to depolymerize the PPHA substrates. We determine this critical dosage by measuring the operating lifetimes of the OLEDs whose failure is believed to be due to significant mechanical softening during the liquefaction of decomposed phthalaldehyde monomers.

Electron trapping: a mechanism for acid production in extreme ultraviolet photoresists

The photomechanism of extreme ultraviolet (EUV) exposures in chemically amplified photoresists is much different than that of previous lithographic wavelengths. Electrons generated during EUV exposure are demonstrated to be a source of acid production through a process referred to as electron trapping. Density functional theory modeling indicates that it is energetically favorable for the photoacid generator (PAG) molecule to decompose if an electron is trapped. Low-energy electrons (<10 eV) that are unlikely to produce holes and secondary electrons generate acid-indicating electron–PAG interactions that are capable of inducing decomposition. Additionally, solution phase reduction in PAGs via electrolysis is shown to produce acid. Furthermore, a more easily reduced PAG (i.e., higher likelihood of trapping an electron) produces a higher acid yield, further supporting electron trapping as a process of acid production regardless of the polymer matrix. An acid indicator, Coumarin 6, was used to determine the number of acids generated per absorbed EUV photon. The results of these measurements indicate that electron–PAG interactions are a source of acid production through electron trapping; thus, an increase in the number of electron-hole pairs available to induce chemical reactions would improve sensitivity.

A triboelectric nanogenerator based on polypropylene carbonate and photoacid generator

A simple and cost-effective paper-based triboelectric nanogenerator (TENG) using polypropylene carbonate (PPC) and photoacid generator (PAG) is proposed in this work. Paper is used as the supporting platform for triboelectric friction between commercial Teflon tape and PAG-PPC. The PAG-PPC layer is proved to be a novel and promising material for TENG, and brings fast termination of functionality of TENG after thermal triggering. The proposed TENG has potential application in transient sensor networks.

Laser-Induced CO2 Generation from Gold Nanorod-Containing Poly(propylene carbonate)-Based Block Polymer Micelles for Ultrasound Contrast Enhancement.

Poly(propylene carbonate) (PPC) decomposes at high temperature to release CO2. This CO2-generation temperature of PPC can be reduced down to less than 80 °C with the aid of a photoacid generator (PAG). In the present work, we demonstrate that using an additional helper component, surface plasmonic gold nanorods (GNRs), the PPC degradation reaction can also be initiated by infrared (IR) irradiation. For this purpose, a PPC-containing nanoparticle formulation was developed in which PPC-based amphiphilic block copolymers (BCPs), poly(poly(ethylene glycol) methacrylate- b-propylene carbonate- b-poly(ethylene glycol) methacrylate) (PPEGMA-PPC-PPEGMA), were self-assembled with GNRs and PAG molecules via solvent exchange. Under IR irradiation, GNRs produce heat that can cause PPC to decompose into CO2, and PAG (after UV pretreatment) catalyzes this PPC degradation process. Two PPEGMA-PPC-PPEGMA materials were used for this study: PPEGMA7.3K-PPC5.6K-PPEGMA7.3K ("G7C6G7") and PPEGMA2.1K-PPC5.6K-PPEGMA2.1K ("G2C6G2"). Addition of CTAB-coated GNRs dispersed in water to a G2C6G2 solution in DMF produced individually G2C6G2-encapsulated GNRs, whereas the same solvent exchange procedure resulted in the formation of polymer-coated GNR clusters when G7C6G7 was used as the encapsulating material. GNR/G2C6G2 NPs exhibited a surface plasmon resonance peak at 697 nm. The clustered morphology of G7C6G7-encapsulated GNRs caused a blue shift of the absorbance maximum to 511 nm. As a consequence, GNR/G2C6G2 NPs showed a greater absorbance/heat generation rate under IR irradiation than did GNR/G7C6G7 NPs. The IR-induced CO2 generation rate was about 4.2 times higher with the GNR/G2C6G2+PAG sample than that with the GNR/G7C6G7+PAG sample. Both GNR/G7C6G7+PAG and GNR/G2C6G2+PAG systems produced ultrasound contrast enhancement effects under continuous exposure to IR light for >20 min; contrast enhancement was more spatially uniform for the GNR/G2C6G2+PAG sample. These results support the potential utility of PPC as a CO2-generating contrast agent in ultrasound imaging applications.

Photolithographic Fabrication of Pocket-like Hydrogel Microstructures for Novel Cell Culture System

Various functional hydrogels have been developed, and can be applied to the fields of bioengineering, such as cell culture substrates and microfluidic systems. It is very important to provide these materials with some functional shapes in a scale of micrometers when these applications are examined. In this study, we proposed the fabrication of unique hydrogel microstructures through photolithography, and examined its application to novel cell culture system. They are expected to be applied to compose novel cell culture system which can maintain physiological activity of living cells, and to contribute to increasing precision of drug screening through providing totally different culture environment from conventional systems composed of rigid materials. First, we coated a solution containing a UV responsive photo acid generator (PAG) on polystylene (PS) surface, and then coated a methanol solution containing poly (acrylic acid) (PAAc). The micro-patterned light irradiation (wavelength: 365nm) was carried out to induce area-selective cross-linking. After flushing with water, ethanol and 1M NaOH aqueous solution, a micro-patterned cross-linked PAAc layer was obtained. Next, we prepared a pre-gel layer by coating a solution containing hydroxypropyl cellulose (HPC) 1,3,4,6-tetrakis (methoxymethyl) glycoluril, an acid catalyzed cross-linker, and PAG, on the cross-linked PAAc layer. The pre-gel layer was irradiated with the light in a different micro-pattern, and was baked at 85 ˚C to promote acid-catalyzed cross-linking of HPC. After flushing with water, a micro-patterned cross-linked HPC layer was obtained on the cross-linked PAAc layer. By soaking the patterned bilayer in 1M NaOH aqueous solution, HPC gel sheet lifted off of the substrate and swelled in the areas where the cross-linked PAAc had remained while the sheet was immobilized on the PS surface in the other areas. As a result, pocket-like semi-3D microstructures were obtained. To provided the hydrogel with the property of cell adhesion, we examined the coating of several polymer materials on HPC.

(Invited) Micro-Manipulation of Hydrogel Sheet through Photoresponsive Polymer Dissolution

Various functional hydrogels have been developed, and their applications to the fields of bioengineering have been examined actively. In these situations, we developed novel technologies to manipulate hydrogel sheet and living cells through photoresponsive polymer dissolution. Firstly, we proposed the fabrication of unique hydrogel microstructures through photolithography. As a photo-acid-generation (PAG) polymer, we synthesized a poly(methylmethacrylate) functionalized with visible- light-responsive PAG group (pPAGMMA), and spin-coated polystyrene surface with the polymer solution. Then we overcoated pregel solution containing hydrophilic polymer (hydroxypropyl cellulose) (HPC) and 1,3,4,6-Tetrakis(methoxymethyl)glycoluril, and carried out micro-patterned light projection. After post-bake and flushing, we obtained crosslinked hydrophilic polymer sheet only in the irradiated area. Due to another property of pPAGMMA getting dissolved in ethanol/water mixture by further irradiation, the cross-linked hydrophilic polymer sheet was lifted from the substrate surface and became swollen in the desired areas by the second micro-patterned projection in ethanol/water mixture. As a result, we obtained various microstructures which were composed of patterned swollen hydrogel sheet partially attached to the substrate in the patterned areas. Secondly, we developed a novel photoresponsive polymer materials by functionalizing poly(N-isopropylacrylamide) with o-nitrobenzaldehyde (pNBANIPAAm) to implement in situ photocontrol in aqueous systems containing bio-compatible environment. We observed that the polymer exhibited sharp aqueous dissolution upon light irradiation in wide pH range including biological conditions. We prepared closs-linked HPC (cHPC) layer on pNBANIPAAm thin layer and carried out micropatterned light projection in PBS. As a result, we observed that the cHPC layer lifted off of the substrate and swelled while nothing happened in unirradiated areas. Also living cells were manipulated by light in a similar manner.

A new type of sulfonium salt copolymers generating polymeric photoacid: Preparation, properties and application

Photoacid generators (PAGs) are critical components in chemically amplified (CA) photoresists system, among which small molecular sulfonium salts are commonly used but possess inherent incompatibility with polymer matrixes and acid migration during the post exposure baking (PEB) processes. PAG-bound polymers would be better choices for CA photoresists to acquire excellent lithographic performance. In this work, a new and convenient way is designed to prepare polymeric PAGs which generate polymeric photoacids and with this method the existing small molecular sulfonium salts can be transformed into polymeric compounds with convenience. Poly (sodium p-styrenesulfonate-co-tertiary-butyl methacrylate) (p(SSNa-co-t-BMA)) was prepared through free radical polymerization and then was further reacted with various sulfonium halides to give sulfonium salt copolymers. Different molecular weights of the copolymers can be obtained from Mn 5.5 × 103–2.5 × 104 with distribution coefficient between 1.18‐1.88. The ionic copolymers display good solubilities in common resist solvents. The thermal decomposition temperatures (Td) are around 150–155 °C and the glass transition temperatures (Tg) are above 130 °C. After exposed to light, the copolymers can generate polymeric photoacids. The photoacid generation efficiencies are determined around 0.31–0.34. UV spectra show big difference between the polymeric PAGs and the mixture of the corresponding small molecular sulfonium salts and framework polymers and demonstrate much better transparency of polymeric PAGs. Thick film 248-nm CA photoresist can be formed by one of the polymeric PAGs and partly protected poly(4-hydroxystyrene-co- tertiary-butyl methacrylate) (p(4-HS-co-t-BMA)), and display excellent performance with high height/width (H/W) ratio of 5:1 and resolution of 0.35 μm. These polymeric PAGs also have potentiality to be used in CA photoresists for other photolithography technologies such as 193-nm immersion technology.

Synthesis and properties of new UVA-sensitive photoacid generators of super acid for near-UV LED applications

Thioxanthone sulfonate photoacid generators (PAGs) for super acid sensitive to UVA were synthesized using thioxanthone derivatives with perfluoroalkyl sulfonate units with good thermal stability (over 240℃). The PAGs exhibited good photoreactivity with photoacid generation quantum yields (Φa) between 0.10 to 0.14, and well utilized the energy of the UV-LED at 365, 385 and 395 nm.

Enhancement of conductivity and transparency for of poly(3,4-ethylenedioxythiophene) films using photo-acid generator as dopant

We introduced the photoacid generator as dopant of photon energy to improve the conductivity of PEDOT:PSS films and form patterned film. The PEDOT:PSS film using PAG-001 exhibited the lowest sheet resistance of 156 Ω/sq at the photon energy of 1 J (10 wt% of PAG-001 in IPA). Also, the PEDOT:PSS film using PAG-301s exhibited the lowest sheet resistance of 154 Ω/sq at the photon energy of 1 J (10 wt% of PAG-301s in IPA). In terms of durability, the sheet resistance of the PEDOT:PSS film with PAG increased to 10% after 10 days, which shows better stability than that of PEDOT:PSS film with DMSO (10% 3 days). We introduced piperidine to increase sheet resistance gaps between conductive and insulating region. In the case of piperidine treatment, the sheet resistance of the conductive region showed 243 Ω/sq, and that of the insulating region showed 2.6*107 Ω/sq. In one film, the difference of sheet resistance between the conductive and insulating regions increased from 103 to 105 times.

Light-Switchable Self-Assembly of Non-Photoresponsive Gold Nanoparticles.

Herein, an interesting light-induced self-assembly behavior from non-photoresponsive gold nanoparticles (Au NPs) was reported. Specifically, a pH-responsive amphiphile SPBwas developed that contained a particular phenylboronic acid moiety and showed excellent surface activity at the neutral and basic conditions, thereby stabilizing Au NPs well. Accordingly, the SPB-functionalized Au NPs showed strong pH dependence that there presented the pH-induced reversible self-assembly behavior. Furthermore, the introduction of a small amount of commercially available photoacid generator named diphenyliodonium nitrate (DIN) into the system could endow it with apparent light-switchable self-assembly behavior. The pH- and light-induced self-assembly behaviors of SPB-functionalized Au NPs in the absence and presence of DIN, respectively, were systematically studied by various techniques including UV-vis spectrum, transmission electron microscope, nuclear magnetic resonance, and Fourier transform infrared spectroscopy, which evidently confirmed that the stimuli-responsive self-assembly was controlled by the hydrogen-bonding interactions between phenylboronic acid moieties. Attributing to the light-induced obvious color change from bright-red to deep purple, the system was applied in particle imprinting successfully.

Ultrafast Dynamics of the Metal-to-Ligand Charge Transfer Excited States of Ir(III) Proteo and Deutero Dihydrides.

For decades, transition metal hydrides have been at the forefront of numerous photocatalytic reactions leveraging either photoacid or photohydride generation. Of upmost importance is the nature of the M-H bond itself, which is typically the major site of photochemical reactivity, particularly in Ir(III) hydrides featuring metal-to-ligand charge transfer (MLCT) excited states. As a departure point for understanding the fundamental spectroscopy and photophysics of the MLCT excited states of Ir(III) diimine hydrides, cis-[Ir(bpy)2H2]+ (bpy = 2,2'-bipyridine) and its deuterated analogue cis-[Ir(bpy)2D2]+ were prepared and investigated. The robust nature of these molecules enabled detailed solution-based photophysical studies using ultrafast transient absorption and infrared spectroscopy, executed without the generation of permanent photoproducts. Static Fourier transform infrared and Raman spectra (λex = 785 nm) of these two molecules revealed weak but measurable Ir-H and Ir-D stretching vibrations centered at 2120 and 1510 cm-1, respectively. Short-lived (τ = 25 ps) MLCT excited states were observed for both cis-[Ir(bpy)2H2]+ and cis-[Ir(bpy)2D2]+ following femtosecond pulsed laser excitation at 480 nm in visible and near-IR transient absorption experiments. A similar time constant was measured for the in-phase and out-of-phase Ir-H stretching modes of the triplet excited state between 1900 and 2200 cm-1 using transient IR spectroscopy. The Ir-D stretching modes in the MLCT excited state were masked by bpy-localized vibrations rendering quantitative evaluation of these modes difficult. The time-resolved infrared data were consistent with density functional theory calculated mid-IR difference spectra in both of these molecules, yielding quantitative matches to the measured IR difference spectra. The information presented here provides valuable insight for understanding the primary photophysical events and transient absorption and IR spectroscopic signatures likely to be encountered throughout metal hydride photochemistry.

I-line photoresist composed of multifunctional acrylate, photo initiator, and photo acid generator, which can be patterned after g-line photo-crosslinking

A positive-tone photoresist which can be patterned after photo-crosslinking was prepared. The photoresist is mainly composed of a multifunctional acrylate having a tertiary ester structure, a photo radical initiator, and a photo-acid generator (PAG). Photolysis of the initiator induces crosslink of the acrylate, and acid generated from the PAG catalyzes decomposition of the crosslinked acrylate. By loading the acrylate with both the initiator, that is sensitive to g-line, and the PAG, that is sensitive to i-line, the crosslink of the coating was promoted by g-line exposure. The subsequent i-line photolithography yielded the patterns of the resulting crosslinked film. Photolithography of both conventional negative- and positive-tone photoresists was performed on the crosslinked films. The patterned film stacks could be removed after additional i-line exposure followed by post-exposure bake and development.