Steady-state Photolysis Techniques Research Articles

Triphenylamine-functionalized chalcones as one-component type II visible-light-absorbing photoinitiators for free radical photopolymerization

In this study, four new one-component Type II visible-light-absorbing chalcones (CY6–CY9) that include triphenylamine substitute in one side and dimethylaniline in another side have been prepared and applied for free radical photopolymerization. The position and methoxy-substituted of the triphenylamines demonstrate significant influence on their properties. In addition, the triphenylamine-free one-component Type II photoinitiator (CYP2) was also prepared for comparing. All the new photoinitiators exhibit boarder and red-shifted absorption than did the CYP2 one. The maximum absorption peaks were in the range of 414–425 nm. The photochemical properties of the compounds were studied by the electron spin resonance (ESR), steady-state photolysis techniques, and theoretical calculations. Finally, all the one-component Type II photoinitiators were used to initiated the free radical photopolymerization of trimethylolpropane triacrylate (TMPTA) through a Photo-DSC experiment. Markedly, the double bond conversion efficiency for TMPTA in the presence of CY8 exhibits the highest values (UV for 34 % and LED@405 nm for 33 %) than other analogues (UV for 12–20 % and LED@405 nm for 8–13 %) and control sample (UV for 9 % and LED@405 nm for 9 %) under UV lamp and LED@405 nm irradiation source. This work thus demonstrates a judicious chemical design to fine-tune the photopolymerization character in one-component Type II triphenylamine-functionalized chalcones.

Synthesis and free radical photopolymerization of one-component type II photoinitiator based on benzophenone segment

In this study, novel visible light one-component photoinitiators (BPN-D, BPN-Ph, BPN-N, BPN-An and BPN-Py), based on benzophenone scaffold have been synthesized. Compared to the reference samples (BPN and EMK), these new photoinitiators not only contribute to red-shifted UV–Vis absorption regions but also strongly enhance the thermal properties. The free radical generation in one-component Type II photoinitiators were also confirmed by the electron spin resonance (ESR) and steady-state photolysis techniques, and their chemical reaction mechanisms were also discussed. Finally, a photopolymerization studied using those novel and reference photoinitiators as one-component Type II photoinitiator was performed. The photopolymerization of TMPTMA monomer initiated by various photoinitiators was monitored by photo-DSC experiment. Under UV lamp irradiation, the BPN-Ph compound could be promoted with the highest double bond conversion efficiency up to 30%, which was higher than the reference BPN (25%) and EMK (22%) based photoinitiating systems. In addition, the BPN-An based formulation exhibited highest double conversion efficiency (22%) upon LED@405 nm. The experimental results show that the synthesized compounds can effectively improve photosensitivity as well as enhanced their optical and thermal properties.

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.

Naphthalimide Derivatives: Substituent Effects on the Photoinitiating Ability in Polymerizations under Near UV, Purple, White and Blue LEDs (385, 395, 405, 455, or 470 nm)

Naphthalimide derivatives (NDAs) with amino‐ or alkylthio‐substituents have been synthesized and combined in photoinitiating systems (PIS) with an iodonium salt, N‐vinylcarbazole, an amine, or 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine to produce radicals, cation radicals, or cations. The photochemical mechanisms are investigated by fluorescence, cyclic voltammetry, laser flash photolysis, electron spin resonance spin trapping, and steady state photolysis techniques. The generated reactive species are capable of initiating the cationic polymerization (CP) of epoxides under air or the free radical polymerization (FRP) of acrylates in laminate upon exposure to visible lights delivered by a halogen lamp, a cold white light‐emitting diode (LED), or LEDs at 385, 395, 405, 455, or 470 nm. Compared to the well‐known camphorquinone based systems, the novel NDA containing PISs exhibit higher efficiencies both for CP and FRP. Remarkably, the photo­bleaching of some NDAs is significant, and colorless final polymer coatings are obtained. The structure/photochemical property relationships of NDAs are discussed in detail (redox potentials, fluorescence, rate constants of reactions, etc.) to shed some light on their respective photoinitiating ability. image

Naphthalimide‐phthalimide derivative based photoinitiating systems for polymerization reactions under blue lights

ABSTRACTNaphthalimide‐phthalimide derivatives (NDPDs) have been synthesized and combined with an iodonium salt, N‐vinylcarbazole, amine or 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine to produce reactive species (i.e., radicals and cations). These generated reactive species are capable of initiating the cationic polymerization of epoxides and/or the radical polymerization of acrylates upon exposure to very soft polychromatic visible lights or blue lights. Compared with the well‐known camphorquinone based systems used as references, the novel NDPD based combinations employed here demonstrate clearly higher efficiencies for the cationic polymerization of epoxides under air as well as the radical polymerization of acrylates. Remarkably, one of the NDPDs (i.e., NDPD2) based systems is characterized by an outstanding reactivity. The structure/reactivity/efficiency relationships of the investigated NDPDs were studied by fluorescence, cyclic voltammetry, laser flash photolysis, electron spin resonance spin trapping, and steady state photolysis techniques. The key parameters for their reactivity are provided. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 665–674

Design of High Performance Photoinitiators at 385–405 nm: Search around the Naphthalene Scaffold

Novel naphthalene derivatives have been designed to be used as versatile photoinitiators upon a laser diode (405 nm), a polychromatic halogen lamp, or an UV LED (385 nm) exposure. The reactive species produced from photoinitiating systems based on one particular naphthalene derivative (NA3) and an iodonium salt, N-vinylcarbazole, an amine or 2,4,6-tris(trichloromethyl)-1,3,5-triazine were particularly efficient for cationic, radical, IPN and thiol–ene photopolymerizations upon low light intensity exposure. The best proposed systems exhibit a higher efficiency than references systems for visible lights (i.e., camphorquinone CQ-based photoinitiating systems). The mechanisms for the photochemical generation of reactive species (i.e., radicals and cations) were studied by electron spin resonance spin-trapping, fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques.

Blue Light Sensitive Dyes for Various Photopolymerization Reactions: Naphthalimide and Naphthalic Anhydride Derivatives.

Novel naphthalimide derivatives (or naphthalic anhydride derivatives) have been prepared and combined with an iodonium salt, N-vinylcarbazole, amines or 2,4,6-tris(trichloromethyl)-1,3,5-triazine to produce radicals and cations upon exposure to low intensity blue lights (e.g., a household blue LED bulb). The photochemical mechanisms are studied by electron spin resonance spin trapping, fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques. The naphthalimide derivatives (ND4) or the naphthalic anhydride derivative (ND10) based photoinitiating systems are particularly efficient for cationic, radical and thiol–ene photopolymerizations; the synthesis of interpenetrated polymer networks IPNs can also be easily carried out. Compared to camphorquinone/amine or camphorquinone/iodonium salt, the new proposed combinations appear as highly versatile and high performance visible light photoinitiating systems. Some of these photoinitiating systems can also be used for UV LED ...

Structural Effects in the Indanedione Skeleton for the Design of Low Intensity 300–500 nm Light Sensitive Initiators.

Newly synthesized indanedione derivatives combined with an iodonium salt, N-vinylcarbazole, amine, phenacyl bromide, or 2,4,6-tris(trichloromethyl)-1,3,5-triazine have been used as photoinitiating systems upon very low visible light intensities: blue lights (e.g., household blue LED bulb at 462 nm) or even a halogen lamp exposure. One of them (ID2) is particularly efficient for cationic, radical and thiol–ene photopolymerizations as well as for the synthesis of interpenetrated polymer networks (IPNs). It can be useful to overcome the oxygen inhibition. ID2 based photoinitiating systems can also be selected for the reduction of Ag+ and the in situ formation of Ag(0) nanoparticles in the synthesized polymers. The (photo)chemical mechanisms are studied by electron spin resonance spin trapping, fluorescence, cyclic voltammetry, laser flash photolysis, and steady state photolysis techniques.

Photoinitiating free-radical polymerization electron-transfer pairs applying amino acids and sulfur-containing amino acids as electron donors

A series of free-radical polymerization initiation systems, based on xanthene dyes as the absorbing chromophores [Rose bengal derivative, 3-(3-methylbutoxy)-5,7-diiodo-6-fluorone and 3-acetoxy-2,4,5,7-tetraiodo-6-fluorone] and sulfur-containing amino acids as the electron donors, were investigated. The photoredox pair xanthene dye/sulfur-containing amino acid was effectively used for photoinitiation of free-radical polymerization of the mixture composed of poly(ethylene glycol)diacrylate–1% NH4OH (3 : 1). The highest initiating efficiencies were observed for the system composed of methionine derivatives as the electron donor. The mechanism of photoinduced electron transfer between sulfur-containing amino acids and triplet state of xanthene dye was investigated using laser-flash and steady-state photolysis techniques. Based on photochemistry of xanthene dyes, photochemistry of sulfur-containing amino acids, and obtained results, the mechanism describing the major processes occurring during the photoinitiated polymerization by a photoinduced intermolecular electron-transfer process was postulated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 358–365, 2005

Reactivity of the carbocation generated in the photolysis of 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline toward azide ion

The reaction of the azide ion with the carbocation generated in the photolysis of 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline in methanol was studied by pulse (conventional and laser) and steady-state photolysis techniques. The adduct of the azide ion was characterized by 1H NMR spectrum. Experimental results were interpreted taking into account a competition between the addition of methanol and azide ion to the carbocation. The rate constants for the reaction of the azide ion with the carbocation (kAz) were measured at 2—48 °C in a wide range of [N3–]0 concentrations from 2·10–7 to 0.1 mol L–1 at different ionic strengths (μ) of the solution. The resulting kAz values are more than an order of magnitude lower than those for diffusional-controlled reactions and vary from 3.2·108 (μ = 0) to 4.5·106 L mol–1 s–1 (μ = 0.8 mol L–1) in the presence of NaClO4 (18 °C). The activation energy of addition of the azide ion to the carbocation is 21 kJ mol–1, which is by 12 kJ mol–1 lower than the activation energy of the reaction of the carbocation with methanol. The features of the reaction under study are discussed from the viewpoint of the structures of carbocations generated in the photolysis of dihydroquinolines.

Structure/properties relationships in photoinitiators of polymerization: 10. The behaviour of bichromophoric derivatives

The photochemical behaviour and excited state processes of three new α-sulphonyl acetophenones exhibiting a high efficiency in the radical polymerization of acrylic coatings have been studied using time resolved laser spectroscopy and steady state photolysis techniques. The triplet state energy levels greatly depend on the substituent present on the compound. The influence of the environment viscosity on energy and electron transfers has been studied. A complete characterization of the excited states has been done. Effects of the bichromophoric structure compared to that of monochromophoric derivatives are discussed.

Mechanism of One-Electron Oxidation of β-, γ-, and δ-Hydroxyalkyl Sulfides. Catalysis through Intramolecular Proton Transfer and Sulfur−Oxygen Bond Formation

The mechanism of photoinduced electron transfer between sulfur-containing alcohols and the 4-carboxybenzophenone (CB) triplet state in aqueous solution was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by five hydroxyalkyl sulfides, 2-(methylthio)ethanol (2-MTE), 2,2‘-dihydroxydiethyl sulfide (2,2‘-DHE), 3-(methylthio)propanol (3-MTP), 3,3‘-dihydroxydipropyl sulfide (3,3‘-DHP), and 4-(methylthio)butanol (4-MTB), with varying numbers of OH groups and varying locations with respect to the sulfur atom, were determined to be in the range (3.3−4.8) × 109 M-1 s-1 for neutral solutions. The intermediates identified were the CB ketyl radical anion (CB•-), the CB ketyl radical (CBH•), and intermolecularly (S∴S)-bonded radical cations. An additional absorption band at approximately 400 nm in the transient spectra for some of the hydroxyalkyl sulfides was assigned to the intramolecularly (•S−O)-bonded species (only f...

Structure/properties relationships in photoinitiators of polymerization—9. The behaviour of a new bis α-sulphonyl ketone derivative

The photochemistry of a new α-sulphonyl acetophenone characterized by high efficiency in the radical polymerization of acrylic coatings is presented. Its photochemical behaviour and the excited-state processes have been studied using time resolved laser spectroscopy and steady-state photolysis techniques; the photopolymerization activity has been evaluated. The photoinitiator works according to a β cleavage process in the triplet state (rate constant ⩽ 1.8 × 10 5 sec −1). Interactions with e.g. isopropanol, methylacrylate and methyldiethanolamine led to bimolecular quenching rate constants of 10 3, 7.4 × 10 5 and 3.2 × 10 8mol −1 sec −1, respectively. The main applications are UV curable inks and can be found in UV thin layer pigmented systems. One of its important peculiarities is the absence of odour developed after curing. Primary toxicological evaluations show its safety in handling.

Spin–orbit relaxation kinetics of Br(4 2P1/2)

Pulsed and steady-state photolysis techniques have been employed to measure the rate coefficients for collisional deactivation of the spin–orbit excited state of atomic bromine, Br(4 2P1/2). Pulsed lifetime studies for quenching by Br2 and CO2 yielded absolute rate coefficients at room temperature of kBr2=1.2±0.1×10−12 and kCO2=1.5±0.3×10−11 cm3/molecule s. The rate coefficients for quenching by rare gases, N2, O2, NO, NO2, N2O, CO, CO2, COS, SO2, SF6, CF4, CH4, H2S, H2, D2, HBr, HCl, and HI, relative to that for Br2 were determined in a steady-state photolysis experiment. Correlation of the deactivation probabilities with energy defect for the case of electronic-to-vibrational energy transfer is demonstrated.

Photoinduced Electron Transfer between Sulfur-Containing Carboxylic Acids and the 4-Carboxybenzophenone Triplet State in Aqueous Solution

The mechanism of photoinduced electron transfer was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by six sulfur-containing acids, with varying numbers of COO[sup [minus]] groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3-2.1) x 10[sup 9]M[sup [minus]1] s[sup [minus]1] depending on the charge of the acid molecule. The observation of ketyl radical anions and intermolecular (S S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanism of quenching. An additional absorption band at approximately 410 nm in the transient absorption spectra for some of the acids was assigned to intramolecularly (S O)-bonded species (for acids for which a five-member ring structure was sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of CO[sub 2] formation and CB disappearance from the steady-state measurements were determined. The values of these quantum yields clearly indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is the main photochemical pathway. 29 refs., 4 figs., 3 tabs.

CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER REACTIONS IN LIPID BILAYER VESICLES: VECTORIAL ELECTRON TRANSFER ACROSS THE BILAYER FROM REDUCED CYTOCHROME c IN THE INNER COMPARTMENT TO OXIDIZED FERREDOXIN IN THE OUTER COMPARTMENT

A negatively charged large unilamellar vesicle system containing a membrane-bound photo-sensitizer (chlorophyll, Chi), a reduced redox protein [cytochrome c, cyt c(red)] in the inner aqueous compartment, an oxidized redox protein [ferredoxin, Fd(ox)] in the outer aqueous compartment, and propylene diquat (PDQ2+) as a mediator, was investigated using both flash and steady-state photolysis techniques. The results demonstrate that the light-generated triplet state of Chi (3Chl) was initially quenched by PDQ2+ at the outer membrane surface to form Chi cation radical (Chl+) and the reduced diquat (PDQ+). This was succeeded by a biphasic recombination between Chi+ and PDQ+. The slow phase of the recombination process, which represents reverse electron transfer between Chl+ and those PDQ+ molecules which escaped from the membrane surface, could be suppressed effectively both by the reduction of Chl^ in the inner monolayer of the vesicles by cyt c(red), and by the reoxidation of PDQ+ by Fd(ox) in the outer aqueous compartment. These reactions lead to the permanent accumulation of oxidized and reduced product proteins, i.e. cyt c(ox) in the inner compartment and Fd(red) in the outer compartment. The yields of such accumulation were 11%, based on the 3Chl quenched, and 1.4%, based on absorbed quanta, under the conditions used in the present study. This system mimics one of the key events in natural photosynthesis and results in an appreciable storage of electromagnetic energy in the reaction products.

FLAVIN PIGMENTS EMBEDDED IN LIPID MATRICES: A SPECTROSCOPIC AND PHOTOCHEMICAL INVESTIGATION

Abstract— –The spectroscopic properties of riboflavin tetrabutyrate have been studied in lecithin lipo‐somes and several detergent micellar systems. The results indicate that the flavin chromophore is located close to the liposome‐water interphase rather than in the hydrophobic region. By varying the number of flavins contained in each liposome and employing fluorescence spectroscopy and steady state photolysis techniques, the possible existence of two distinct environments for the flavin chromophore was demonstrated. Several basic photochemical properties of riboflavin tetrabutyrate in liposomes and micellar systems were investigated using laser flash and microsecond flash photolysis techniques.

Effect of solvent on the flash photolysis and laser photolysis of N,N,N?,N?-tetramethyl-paraphenylene diamine in solution

The photolysis of TMPD has been studied in cyclohexane, ethanol and water, by microsecond flash photolysis, by nanosecond laser flash photolysis and by conventional steady-state photolysis techniques. In cyclohexane the fluorescence of TMPD singlet is observed with t½= 3.2 ns. The triplet state of TMPD is also observed; it decays by a bimolecular triplet-triplet annihilation process accompanied by delayed fluorescence. Several solutes, e.g., N2O, SF6 and CO2 react with the excited singlet state; N2O also reacts with the triplet forming a charge transfer complex. Fluorescence with t½= 5.1 ns is observed in the flash photolysis of TMPD in ethanol. The triplet state of TMPD is also observed, but this decays rapidly in ethanol over several 100 ns. Another long-lived species is produced independent of the excited singlet or triplet states of TMPD. No fluorescence or is produced independent of the excited singlet or triplet states of TMPD. No fluorescence or triplet state is observed in the photolysis of TMPD in water, but the cation of TMPD and the hydrated electron e–aq are observed. Possible mechanisms leading to the ionization of TMPD in water are discussed.