Photoinitiated Reaction Research Articles

Photoinitiated Polymerization for Active Packaging of Light-Emitting Diodes

The kinetics of photoinitiated polymerization for active packaging of light-emitting diodes (LEDs) is presented. The LED chip in our model was considered to consist of an upper emission surface and a lateral emission surface. Light emission from the LED chip triggers the photoinitiated polymerization reaction, leading to the formation of a mushroom-shaped encapsulant lens on the LED chip. With our proposed simulation process, the concentrations of both the photoinitiator and monomer, and the photolysis as time evolves can be calculated by the finite-difference method. The time evolution of the lens profile during polymerization can be analyzed as the monomer concentration varies. The proposed technique can depict the shape and precise size of the mushroom-shaped lens for any specific parameters in the polymerization reaction, e.g., LED power, polymerization time, absorptivity of the epoxy, and photolysis. LED encapsulation experiments with different LED power and polymerization time were made to support our simulation results, and the simulation results agree well with the experiment results. Finally, the radiation patterns of the LEDs with the encapsulant lenses are measured to prove the light focusing characteristics of the mushroom-shaped lenses.

Hydrogel scaffolds to study cell biology in four dimensions.

Poly(ethylene glycol) (PEG) hydrogels represent a versatile material scaffold for culturing cells in two or three dimensions with the advantages of limited protein fouling and cytocompatible polymerization to enable cell encapsulation. By using light-based chemistries for gelation and for incorporating biomolecules into the network, dynamic niches can be created that facilitate the study of how cells respond to user-dictated or cell-dictated changes in environmental signals. Specifically, we demonstrate integration of a photo-cleavable molecule into network cross-links and into pendant functional groups to construct gels with biophysical and biochemical properties that are spatiotemporally tunable with light. Complementary to this approach, an enzymatically cleavable peptide sequence can be introduced within hydrogel networks, in this case through photoinitiated addition reactions between thiol-containing biomacromolecules and ene-containing synthetic polymers, to enable cellular remodeling of their surrounding hydrogel microenvironment. With such tunable material platforms, researchers can employ a systematic approach for 3D cell culture experiments, spatially and temporally modulating physical properties (e.g., stiffness) as well as biological signals (e.g., adhesive ligands) to study cell behavior in response to environmental stimuli. Collectively, these material systems suggest routes for new experimentation to study and manipulate cellular functions in four dimensions.

Photo-initiated Thiol-ene Click Reactions as a Potential Strategy for Incorporation of [MI(CO)3]+(M = Re,99mTc) Complexes

Click reactions offer a rapid technique to covalently assemble two molecules. In radiopharmaceutical construction, these reactions can be utilized to combine a radioactive metal complex with a biological targeting molecule to yield a potent tool for imaging or therapy applications. The photo-initiated radical thiol-ene click reaction between a thiol and an alkene was examined for the incorporation of [M(I)(CO)3](+) (M = Re, (99m)Tc) systems for conjugating biologically active targeting molecules containing a thiol. In this strategy, a potent chelate system, 2,2'-dipicolylamine (DPA), for [M(I)(CO)3](+) was functionalized at the central amine with a terminal alkene linker that was explored with two synthetic approaches, click then chelate and chelate then click, to determine the flexibility and applicability of the thiol-ene click reaction to specifically incorporate ligand systems and metal complexes with a thiol containing molecule. In the click then chelate approach, the thiol-ene click reaction was carried out with the DPA chelate followed by complexation with [M(I)(CO)3](+). In the chelate then click approach, the alkene functionalized DPA chelate was first complexed with [M(I)(CO)3](+) followed by the conduction of the thiol-ene click reaction. Initial studies utilized benzyl mercaptan as a model thiol for both strategies to generate the identical product from either route to provide information on reactivity and product formation. DPA ligands functionalized with two unique linker systems (allyl and propyl allyl ether) were prepared to examine the effect of the proximity of the chelate or complex on the thiol-ene click reaction. Both the thiol-ene click and coordination reactions with Re, (99m)Tc were performed in moderate to high yields demonstrating the potential of the thiol-ene click reaction for [M(I)(CO)3](+) incorporation into thiol containing biomolecules.

Photoinitiated release of an aziridinium ion precursor for the temporally controlled alkylation of nucleophiles

A photo-activatable aziridinium precursor has been developed to investigate the possibility of a photo-initiated traditional nucleophilic reaction. The photolysis of a quaternary amine yields a tertiary amine and has allowed us to temporally control aziridinium formation and subsequent alkylation of a colorimetric nucleophilic reporter molecule. We have also used this photo-initiated reaction to alkylate a sulfhydryl group. This new photo-initiated alkylation strategy is water-soluble and expands the toolkit of photo-activated crosslinkers for protein labeling research.

Exploring quantum phenomena and vibrational control in σ* mediated photochemistry

Non-adiabatic dynamics involving 1πσ* or 1nσ* excited electronic states play a key role in the photochemistry of numerous heteroatom containing aromatic (bio-)molecules. In this contribution, we investigate more exotic phenomena involved in σ* mediated dynamics, namely: (i) the role of purely quantum mechanical behavior; and (ii) manipulating non-adiabatic photochemistry through conical intersections (CIs) with ‘vibration-specific control’. This is achieved by investigating S–CH3 bond fission via a 1nσ* potential energy surface (PES) in thioanisole (C6H5SCH3). Using a combination of time- and frequency-resolved velocity map ion imaging techniques, together with ab initio calculations, we demonstrate that excitation to the 1ππ* ← S0 origin [1ππ*(ν = 0)] results in S–CH3 bond fission on the 1nσ* PES, despite an (apparent) energetic barrier to dissociation formed by a CI between the 1ππ* and 1nσ* PESs. This process occurs by accessing ‘classically forbidden’ regions of the excited state potential energy landscape where the barrier to dissociation becomes negligible, aided by torsional motion of the S–CH3 group out of the plane of the phenyl ring. Control over these dynamics is attained by populating a single quantum of the S–CH3 stretch mode in the 1ππ* state [1ππ*(ν7a = 1)], which mirrors the nuclear motion required to promote coupling through the 1ππ*/1nσ* CI, resulting in a marked change in the electronic branching in the C6H5S radical products. This observation offers an elegant contribution towards a vision of ‘quantum control’ in photo-initiated chemical reaction dynamics.

Nucleophilic substitution of bromonorbornenes and derivatives by electron transfer reactions

The photoinitiated substitution reactions of anti-7-bromobenzonorbornadiene (5), its syn isomer 6, exo-anti-13-bromobenzocyclobutanorbornene (7), syn-7-bromonorbornene (8) and bromonorbornane (9) with Me(3)Sn(-) and Ph(2)P(-) anions, in liquid ammonia, are here informed to occur with good yields of substitution. The stereochemical outcome is discussed in terms of calculations with the B3LYP functional and the 6-31+G* basis set; the solvent being included as a continuum through the PCM model. The experimental relative chemical reactivity of pairs of substrates toward a given anion is also presented.

A smart hydrogel-based time bomb triggers drug release mediated by pH-jump reaction

We demonstrate a timed explosive drug release from smart pH-responsive hydrogels by utilizing a phototriggered spatial pH-jump reaction. A photoinitiated proton-releasing reaction of o-nitrobenzaldehyde (o-NBA) was integrated into poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide) (P(NIPAAm-co-CIPAAm)) hydrogels. o-NBA-hydrogels demonstrated the rapid release of protons upon UV irradiation, allowing the pH inside the gel to decrease to below the pKa value of P(NIPAAm-co-CIPAAm). The generated protons diffused gradually toward the non-illuminated area, and the diffusion kinetics could be controlled by adjusting the UV irradiation time and intensity. After irradiation, we observed the enhanced release of entrapped L-3,4-dihydroxyphenylalanine (DOPA) from the gels, which was driven by the dissociation of DOPA from CIPAAm. Local UV irradiation also triggered the release of DOPA from the non-illuminated area in the gel via the diffusion of protons. Conventional systems can activate only the illuminated region, and their response is discontinuous when the light is turned off. The ability of the proposed pH-jump system to permit gradual activation via proton diffusion may be beneficial for the design of predictive and programmable devices for drug delivery.

Cyclic Transmembrane Charge Transport Mediated by Low-Potential Pyrylium Ions

We have investigated the capacity of a series of N-dialkylaminophenyl-substituted pyrylium and thiopyrylium ions to act as photosensitizers and redox mediators between reactants separated by bilayer membranes. These studies were prompted by earlier results indicating that simple trimethy- and triphenyl-substituted analogues could promote efficient photosensitized transmembrane redox between vectorially organized reactants by an electroneutral e(-)/OH(-) antiport mechanism. Unlike the dyes used in the earlier studies, the ions investigated herein absorb strongly throughout the visible absorption region and are therefore potentially useful in solar photoconversion processes. We demonstrate that these ions can carry out cyclic electron transport between phase-separated electron donors and occluded Co(bpy)(3)(3+) in several transversely organized vesicles. The quantum yields obtained were relatively low, but were independent of the membrane microviscosity, suggesting that transmembrane diffusion was not rate-limiting. Triphenylpyrylium and triphenylthiopyrylium ions were shown to be capable of acting as combined photosensitizers/redox relays, apparently by direct oxidation of either solvent (water) or buffer (acetate) ions from their triplet-excited state. These reactions did not require addition of separate photosensitizers and electron donors; as such, they represent a minimal photochemical scheme for effecting transmembrane charge separation. The low-potential visible-absorbing pyrylium ions were unable to function in this dual capacity, consistent with thermodynamic limitations. However, redox titrations established that the pyranyl radicals of these dyes should be capable of reducing H(+) to H(2) in weakly acidic solutions. Consistent with their strongly reducing nature, these dyes were shown to be capable of forming methyl viologen radical in photoinitiated transmembrane redox reactions.

Snowballing Radical Generation Leads to Ultrahigh Molecular Weight Polymers

Styrene is the classical monomer obeying zero-one kinetics in radical emulsion polymerization. Accordingly, particles that are less than 100 nm in diameter contain either one or no growing radical(s). We describe a unique photoinitiated polymerization reaction accelerated by snowballing radical generation in a continuous flow reactor. Even in comparison to classical emulsion polymerization, these unprecedented snowballing reactions are rapid and high-yielding, with each particle simultaneously containing more than one growing radical. This is a consequence of photoinitiator incorporation into the nascent polymer backbone and repeated radical generation upon photo-irradiation.

Photoinitiated quenching reactions of excited triplet duroquinone by vitamin C in homogeneous and AOT (SDS) micelle solutions

Time-resolved EPR has been used to study the photoinitiated reactions of the excited triplet of duroquinone (3DQ*) quenched by the antioxidant vitamin C (VC) in homogeneous solutions of ethylene glycol/water (EG/H2O) and micelle solutions of aerosol OT (AOT) and sodium dodecyl sulphate (SDS). During the photolysis reactions of DQ and VC in homogeneous solutions of EG/H2O, 3DQ* abstracts hydrogen atoms from the solvent EG and the antioxidant VC. The rate constant for the quenching of 3DQ* by VC is 4.90 × 107 L mol−1 s−1, close to being diffusion-controlled. In AOT and SDS micelle solutions, rate constants for the quenching of 3DQ* by VC are 3.28 × 107 and 3.15 × 107 L mol−1 s−1, respectively. Lipid-soluble 3DQ* and water-soluble VC need to diffuse to the w/o interface to react, which reduces the reaction rate between 3DQ* and VC. The charge repulsion interaction between the anionic shell of AOT (SDS) micelles and the VC monoanions AsH− also slows the reaction.

Photoinitiated Alkyne–Azide Click and Radical Cross-Linking Reactions for the Patterning of PEG Hydrogels

The photolithographical patterning of hydrogels based solely on the surface immobilization and cross-linking of alkyne-functionalized poly(ethylene glycol) (PEG-tetraalkyne) is described. Photogenerated radicals as well as UV absorption by a copper chelating ligand result in the photochemical redox reduction of Cu(II) to Cu(I). This catalyzes the alkyne-azide click reaction to graft the hydrogels onto an azide-functionalized plasma polymer (N(3)PP) film. The photogenerated radicals were also able to abstract hydrogen atoms from PEG-tetraalkyne to form poly(α-alkoxy) radicals. These radicals can initiate cross-linking by addition to the alkynes and intermolecular recombination to form the PEG hydrogels. Spatially controlling the two photoinitiated reactions by UV exposure through a photomask leads to surface patterned hydrogels, with thicknesses that were tunable from tens to several hundreds of nanometers. The patterned PEG hydrogels (ca. 60 μm wide lines) were capable of resisting the attachment of L929 mouse fibroblast cells, resulting in surfaces with spatially controlled cell attachment. The patterned hydrogel surface also demonstrated spatially resolved chemical functionality, as postsynthetic modification of the hydrogels was successfully carried out with azide-functionalized fluorescent dyes via subsequent alkyne-azide click reactions.

Interaction of NO2 with TiO2 Surface Under UV Irradiation: Products Study

The interaction of NO(2) with TiO(2) solid films was studied under UV irradiation using a low pressure flow reactor (1-10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. HONO, NO, and N(2)O were observed as the products of the reactive uptake of NO(2) to the illuminated TiO(2) surface with the sum of their yields corresponding nearly to 100% of the nitrogen mass balance. The yield of the products was measured as a function of different parameters such as irradiance intensity, relative humidity (RH), temperature, and concentrations of NO(2) and O(2). The yield of N(2)O was found to be 0.15 ± 0.05 independent of the experimental conditions. The distribution of the products between NO and HONO was found to be independent of temperature in the range T = 280-320 K and was governed by relative humidity: increase in RH led to lower NO and higher HONO yield, with a maximum of nearly 65% reached at ~5% RH. Presence of molecular oxygen was shown to shift the HONO/NO distribution to HONO at low RH (<5%) with no effect at higher RH where the HONO yield is maximum. The following values for the yield of the products of NO(2) interaction with pure TiO(2) under real atmospheric conditions can be recommended from this work: 0.65 ± 0.10, 0.05 ± 0.05, and 0.15 ± 0.05 for HONO, NO, and N(2)O, respectively. The mechanism of the photoinitiated heterogeneous reaction and possible atmospheric implications of the obtained results are discussed.

Photogenerated lectin sensors produced by thiol-ene/yne photo-click chemistry in aqueous solution

The photoinitiated radical reactions between thiols and alkenes/alkynes (thiol-ene and thiol-yne chemistry) have been applied to a functionalization methodology to produce carbohydrate-presenting surfaces for analyses of biomolecular interactions. Polymer-coated quartz surfaces were functionalized with alkenes or alkynes in a straightforward photochemical procedure utilizing perfluorophenylazide (PFPA) chemistry. The alkene/alkyne surfaces were subsequently allowed to react with carbohydrate thiols in water under UV-irradiation. The reaction can be carried out in a drop of water directly on the surface without photoinitiator, and any disulfide side products were easily washed away after the functionalization process. The resulting carbohydrate-presenting surfaces were evaluated in real-time studies of protein–carbohydrate interactions using a quartz crystal microbalance (QCM) flow-through system with recurring injections of selected lectins, with intermediate regeneration steps using low pH buffer. The resulting methodology proved fast, efficient and scalable to high-throughput analysis formats, and the produced surfaces showed significant protein binding with expected selectivities of the lectins used in the study.

Model-based analysis of photoinitiated coating degradation under artificial exposure conditions

Coating degradation mechanisms of thermoset coatings exposed to ultraviolet radiation and humidity at constant temperature are investigated. The essential processes, photoinitiated oxidation reactions, intrafilm oxygen permeability, water absorption and diffusion, reduction of crosslink density, and development of a thin surface oxidation zone are quantified and a mathematical model for degrading coatings developed. Front-tracking techniques are used to determine the rate of movement of the oxidation and ablation fronts, the positions of which define the extension of the surface oxidation zone. Three previous and independent experimental investigations with two-component, densely crosslinked, epoxy–amine model coatings were selected for verification of the mathematical model. Simulations can match and explain transient mass loss and coating thickness reduction data and are in agreement with infrared measurements of carbonyl groups formed in the surface zone. The thickness of the stable surface oxidation zone, which is established after an initial ablation lag time, is estimated by the model to 0.5–2 μm in good agreement with previous measurements. Simulated concentration profiles of active groups, oxygen, and radicals in the stable surface oxidation zone are presented and analyzed. The mathematical model can be used for obtaining a quantitative insight into the degradation of thermoset coatings and has potential, after further development, to complete commercial coatings and dynamic exposure conditions, to become a supplementing tool for predicting in-service coating behavior based on accelerated laboratory measurements.

Progress in Thiol-Ene/Yne Click Chemistry

Click chemistry has attracted great interests due to a wide variety of superior merits, including a wide range of applications, simplified experimental condition, rapid reaction speed, higher functional group conversion, environmental friendly and selectivity. And conception of click chemistry contributed to the synthetic chemistry apparently, in particular, with respect to pharmaceutical manufacture and biomedical materials, resulting in the most useful and promising synthetic method. Furthermore, thiol-ene/yne click chemistry, utilizing inclusion of visible light or UV light as catalytic medium, is a comparatively novel kind of click chemistry in comparison to several types of conventional reactions. In addition, the proper- ties of the creative reaction stems from the combination the photoinitiated chemical reaction with the forementioned traditional click chemistry, providing a flexible approach to occur reaction within the oriented region and functional group. This review highlights the recent research on the preparation of functional polymeric microsphere, amphipathic block well-defined poly- mer, materials employed in the molecular device, desired dendritic polymers and chemistry modification via thiol-ene/yne click chemistry. Moreover, the problems and solutions are given for the future development of thiol-ene/yne click chemistry. Keywords click chemistry; thiol-ene/yne; functional polymeric microsphere; amphipathic block polymer; molecular device; dendritic polymers; surface modification

Light-induced spatial control of pH-jump reaction at smart gel interface

We proposed here a ‘smart’ control of an interface movement of proton diffusion in temperature- and pH-responsive hydrogels using a light-induced spatial pH-jump reaction. A photoinitiated proton-releasing reaction of o-nitrobenzaldehyde (NBA) was integrated into poly(N-isopropylacrylamide-o-2-carboxyisopropylacrylamide) (P(NIPAAm-co-CIPAAm)) hydrogels. NBA-integrated hydrogels demonstrated quick release of proton upon UV irradiation, allowing the pH inside the gel to decrease below the pKa of P(NIPAAm-co-CIPAAm) within a minute. The NBA-integrated gel was shown to shrink rapidly upon UV irradiation without polymer “skin layer” formation due to a uniform decrease of pH inside the gel. Spatial control of gel shrinking was also created by irradiating UV light to a limited region of the gel through a photomask. The interface of proton diffusion (“active interface”) gradually moved toward non-illuminated area. The apparent position of “active interface”, however, did not change remarkably above the LCST, while protons continuously diffused outward direction. This is because the “active interface” also moved inward direction as gel shrank above the LCST. As a result, slow movement of the apparent interface was observed. The NBA-integrated gel was also successfully employed for the controlled release of an entrapped dextran in a light controlled manner. This system is highly promising as smart platforms for triggered and programmed transportation of drugs.

Thiol–ene Click Reaction as a General Route to Functional Trialkoxysilanes for Surface Coating Applications

Functionalized trialkoxysilanes are widely used to modify the surface properties of materials and devices. It will be shown that the photoinitiated radical-based thiol-ene "click" reaction provides a simple and efficient route to diverse trialkoxysilanes. A total of 15 trialkoxysilanes were synthesized by reacting either alkenes with 3-mercaptopropyltrialkoxysilane or thiols with allyltrialkoxysilanes in the presence of a photoinitiator. The functionalized trialkoxysilanes were obtained in quantitative to near-quantitative yields with high purity. The photochemical reactions can be run neat in standard borosilicate glassware using a low power 15-W blacklight. A wide range of functional groups is tolerated in this approach, and even complex alkenes click with the silane precursors. To demonstrate that these silanes can be used as surface coating agents, several were reacted with iron oxide superparamagnetic nanoparticles and the loadings quantified. The photoinitiated thiol-ene reaction thus offers a facile and efficient method for preparing surface-active functional trialkoxysilanes.

Concise Photochemical Synthesis of the Antimalarial Indole Alkaloid Decursivine

A four-step synthesis of the extracyclic, antimalarial indole natural product decursivine is described starting from commercial piperonyl bromide and serotonin (see scheme). A photoinitiated reaction cascade involving indole radical cation formation, rearrangement, radical recombination, rearomatization, elimination, and diastereoselective auto-acid-catalyzed closure of the dihydrofuran ring combine in a single step to conclude this remarkably efficient synthesis.

Bioorthogonal dual functionalization of self-assembling peptide fibers

The ability to modify peptide- and protein-based biomaterials selectively under mild conditions and in aqueous buffers is essential to the development of certain areas of bionanotechnology, tissue engineering and synthetic biology. Here we show that Self-Assembling peptide Fibers (SAFs) can incorporate multiple modified peptides non-covalently, stoichiometrically and without disrupting their structure or stability. The modified peptides contain groups suitable for post-assembly click reactions in water, namely azides and alkenes. Labeling of these groups is achieved using the orthogonal Cu(I)-catalyzed azide-alkyne and photoinitiated thiol-ene reactions, respectively. Functionalization is demonstrated through the conjugation of biotin followed by streptavidin-nanogold particles, or rhodamine, and visualized by electron and light microscopy, respectively. This has been shown for fibers harboring either or both of the modified peptides. Furthermore, the amounts of each modified peptide in the fibers can be varied with concomitant changes in decoration. This approach allows the design and assembly of fibers with multiple functional components, paving the way for the development of multi-component functionalized systems.

Covalently Assembled Gold Nanoparticle-Carbon Nanotube Hybrids via a Photoinitiated Carbene Addition Reaction

A nanohybrid consisting of monolayer protected gold nanoparticles (AuNPs) covalently attached to multiwalled carbon nanotubes (CNT) was prepared using a photoinitiated carbene addition approach. Photolysis of 3-aryl-3-(trifluoromethyl)diazirine-modified gold nanoparticles (Diaz-AuNPs) of two core sizes (3.9 ± 0.9 nm and 1.8 ± 0.3 nm in diameter) resulted in the generation of reactive carbene intermediates on the monolayer of the AuNPs. In the presence of untreated CNT the reactive carbenes undergo an addition reaction with the π-conjugated carbon skeleton. The AuNPs are well dispersed on the sidewall of the CNT and the hybrids are robust enough to survive vigorous washing in a variety of solvents. In the absence of photolysis no covalent attachment occurs with the same AuNP. The nanohybrid AuNP-CNTs were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman, and UV−visible spectroscopy. All of the characterization studies confirm the presence of the AuNPs. This methodo...