Laser Flash Photolysis Techniques Research Articles

Photosensitivity to Triflusal: Formation of a Photoadduct with Ubiquitin Demonstrated by Photophysical and Proteomic Techniques.

Triflusal is a platelet aggregation inhibitor chemically related to acetylsalicylic acid, which is used for the prevention and/or treatment of vascular thromboembolisms, which acts as a prodrug. Actually, after oral administration it is absorbed primarily in the small intestine, binds to plasma proteins (99%) and is rapidly biotransformed in the liver into its deacetylated active metabolite 2-hydroxy-4-trifluoromethylbenzoic acid (HTB). In healthy humans, the half-life of triflusal is ca. 0.5 h, whereas for HTB it is ca. 35 h. From a pharmacological point of view, it is interesting to note that HTB is itself highly active as a platelet anti-aggregant agent. Indeed, studies on the clinical profile of both drug and metabolite have shown no significant differences between them. It has been evidenced that HTB displays ability to induce photoallergy in humans. This phenomenon involves a cell-mediated immune response, which is initiated by covalent binding of a light-activated photosensitizer (or a species derived therefrom) to a protein. In this context, small proteins like ubiquitin could be appropriate models for investigating covalent binding by means of MS/MS and peptide fingerprint analysis. In previous work, it was shown that HTB forms covalent photoadducts with isolated lysine. Interestingly, ubiquitin contains seven lysine residues that could be modified by a similar reaction. With this background, the aim of the present work is to explore adduct formation between the triflusal metabolite and ubiquitin as model protein upon sunlight irradiation, combining proteomic and photophysical (fluorescence and laser flash photolysis) techniques. Photophysical and proteomic analysis demonstrates monoadduct formation as the major outcome of the reaction. Interestingly, addition can take place at any of the ε-amino groups of the lysine residues of the protein and involves replacement of the trifluoromethyl moiety with a new amide function. This process can in principle occur with other trifluoroaromatic compounds and may be responsible for the appearance of undesired photoallergic side effects.

Photochemical study of Eosin-Y with PAMAM dendrimers in aqueous solution

A steady-state and time-resolved study of the xanthene dye Eosin-Y (Eos) in the presence of amino-terminated polyamidoamine (PAMAM) dendrimers in alkaline aqueous solution is presented. Stationary photolysis experiments in deaerated and air equilibrated solutions were carried out. The absorption spectra of the photolyzed solutions in the absence of oxygen showed the formation of several intermediate species. With the aid of laser flash photolysis technique (LFP) those transients could be ascribed to semireduced and partially debrominated species of Eos. The triplet quenching rate-constant (3kq) and radical quantum yields (ΦR) were determined by LFP. In the presence of PAMAM, the triplet lifetime of Eos is greatly decreased and the data fitted to a Stern–Volmer treatment. These experiments indicated that the electron transfer from dendrimers to Eos is a highly favored process. Based on the results found in this work, we concluded that Eos/PAMAM couple might act as a promising photoinitiator of vinyl polymerization with low ecological impact.

Molecular oxygen migration through the xenon docking sites of human hemoglobin in the R-state

A nanosecond laser flash-photolysis technique was used to study bimolecular and geminate molecular oxygen (O2) rebinding to tetrameric human hemoglobin and its isolated α and β chains in buffer solutions equilibrated with 1atm of air and up to 25atm of xenon. Xenon binding to the isolated α chains and to the α subunits within tetrameric hemoglobin was found to cause a decrease in the efficiency of O2 escape by a factor of ~1.30 and 3.3, respectively. A kinetic model for O2 dissociation, rebinding, and migration through two alternative pathways in the hemoglobin subunits was introduced and discussed. It was shown that, in the isolated α chains and α subunits within tetrameric hemoglobin, nearly one- and two-third escaping molecules of O2 leave the protein via xenon docking sites, respectively. The present experimental data support the idea that O2 molecule escapes from the β subunits mainly through the His(E7) gate, and show unambiguously that, in the α subunits, in addition to the direct E7 channel, there is at least one alternative escape route leading to the exterior via the xenon docking sites.

Iron Complexes in Visible‐Light‐Sensitive Photoredox Catalysis: Effect of Ligands on Their Photoinitiation Efficiencies

AbstractThe novel role of metal‐based complexes as photoinitiator catalysts fits well into the concept of green chemistry, as it realizes the activation of polymer synthesis processes by visible light that are abundant in the solar light and allows the marked reduction of photoinitiator amount in the systems. In the present paper, a series of iron complexes (FeC_x) with various ligands have been proposed as new photoinitiator catalysts to initiate the cationic polymerization of epoxides or the free radical polymerization of acrylates upon a near‐UV or visible‐light LED exposure. The ligands play an important role on the light absorption properties and the photoinitiation ability of the iron complexes. In combination with one or two additives, FeC_x are capable to efficiently generate radicals, cations, and radical cations through an oxidative or a reductive path. Two of the newly developed FeC_x‐based photoinitiating systems exhibited comparable photoinitiation efficiency with the commercial Type I photoinitiator bis(2,4,6‐trimethylbenzoyl)‐phenylphosphineoxide (BAPO). Owing to the photocatalytic effect, remarkable photoinitiation efficiencies have been achieved by using very low concentration of iron complexes (0.02 wt %) in the systems. The involved photochemical mechanisms have been studied using electron spin resonance spin trapping, steady state photolysis, cyclic voltammetry, and laser flash photolysis techniques.

Laser Flash Photolysis Mechanism of Anthraquinone-2-Sodium Sulfonate in Pyridine Ionic Liquid/Water Mixed System

The photochemical reaction process of anthraquinone-2-sodium sulfonate (AQS) in the mixture of water (H2O) and N-butylpyridinium tetrafluoroborate ([BPy][BF4]) was studied using the laser flash photolysis technique. Experimental results show that the excited triplet of AQS (3AQS*) could react rapidly with H2O and the transient absorption spectra greatly changed by increasing the volume fraction of the ionic liquid (VIL) in [BPy][BF4]/H2O mixtures. The absorbance at 510 nm increased gradually with increasing VIL when 0<VIL<0.1. By contrast, the absorbance decreased gradually when VIL>0.1. Otherwise, the absorbance of the band near 380 nm steadily increased. The apparent kinetic parameters of transient species B and 3AQS* are obtained approximately. 3AQS* abstracting hydrogen from [BPy]+ was also explored. It was deduced that the 350–420 nm band was the superposition of the peaks of 3AQS* and AQSH·. The two reactions of 3AQS* with [BPy][BF4] and H2O are a pair of competitive reactions. We also concluded that the entire reaction processes slow down in the case of high [BPy][BF4] concentrations.

Molecular interactions, characterization and photoactivity of Chlorophyll a/chitosan/2-HP-β-cyclodextrin composite films as functional and active surfaces for ROS production

Novel photosensitizing film based on the natural hybrid polymer Chitosan/2-hydroxy-propyl-β-Cyclodextrin (CH/CD) is synthesized introducing Chlorophyll a (CH/CD/Chla) as a photoactive agent for possible application in antimicrobial photodynamic therapy (PDT). The polymer absorbs visible light, in turn able to generate reactive oxygen species (ROS) and, therefore it can be used as environmental friendly and biodegradable polymeric photosensitizer (PS). The modified film is characterized by means of different spectroscopic, calorimetric, diffraction techniques and microscopic imaging methods including time-resolved absorption spectroscopy. UV–Vis, FTIR-ATR and X-ray Photoelectron Spectroscopy (XPS) analyses suggest that Chla shows a strong affinity toward Chitosan introducing interactions with amino groups present on the polymer chains. Nanosecond laser flash photolysis technique provides evidence for the population of the excited triplet state of Chla. Photogeneration of singlet oxygen is demonstrated by both direct detection by using infrared luminescence spectroscopy and chemical methods based on the use of suitable traps. Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Differential Scanning Calorimetry (DSC) analyses confirm also the occurrence of structural changes both on the film surface and within the film layer induced by the insertion of the pigment. Moreover, X-ray Diffraction data (XRD) shows the existence of an amorphous phase for the chitosan films in all the compared conditions.

Analysis of mebendazole binding to its target biomolecule by laser flash photolysis

Mebendazole (MBZ) and related anticancer benzimidazoles act binding the β-subunit of Tubulin (TU) before dimerization with α-TU with subsequent blocking microtubule formation. Laser flash photolysis (LFP) is a new tool to investigate drug–albumin interactions and to determine binding parameters such as affinity constant or population of binding sites. The aim of this study was to evaluate the interactions between the nonfluorescent mebendazole (MBZ) and its target biomolecule TU using this technique. Before analyzing the MBZ@TU complex it was needed to determine the photophysical properties of MBZ triplet excited state (3MBZ⁎) in different media. Hence, 3MBZ⁎ showed a transient absorption spectrum with maxima at 520 and 375nm and a lifetime much longer in acetonitrile (12.5μs) than in water (260ns). The binding of MBZ to TU produces a greater increase of the lifetime of 3MBZ⁎ (25μs). This fact and the strong electron acceptor capability observed for 3MBZ* evidence that MBZ must not be located close to any electron donor amino acid of TU such as its tryptophan or cysteine residues. Adding increasing amounts of MBZ to aqueous TU was determined the MBZ-TU binding constant (2.0±0.5×105M−1 at 298K) which decreased with increasing temperature. The LFP technique has proven to be a powerful tool to analyze the binding of drug–TU systems when the drug has a detectable triplet excited state. Results indicate that LFP could be the technique of choice to study the interactions of non-fluorescent drugs with their target biomolecules.

Steady state and laser photolysis studies of keto-enol tautomerizations in 2-alkyl-1,3-diketones having five-membered rings in acetonitrile: Temporal UV-A sunscreen

Abstract Keto-enol tautomerization in 2-alkyl-1,3-diketons having five-membered rings was studied using steady state and laser flash photolysis techniques in solution. The alkyl keto-diketones undergo photoinduced tautomerization mainly in the triplet state to the enol in acetonitrile. The alkyl enol-diketones, thus formed, undergo thermal tautomerization to the original keto forms in a few days. The alkyl enol-diketones show fast internal conversion from the excited singlet state to the ground state without yielding the corresponding isomeric forms (rotamer). Based on the computation for state energies of the keto, enol and isomer forms, a schematic energy diagram for the tautomerization was drawn.

Thioxanthonation of fluorenone: Visible photoinitiator for radical polymerization

ABSTRACTThioxanthonation of fluorenone resulted in a new visible light initiator namely, indeno [1,2‐b]thioxanthene‐7,13‐dione (TX–FN). The detailed photophysical properties of the singlet and the triplet excited states of TX–FN are reported using steady‐state absorption, fluorescence, and phosphorescence, as well as laser flash photolysis techniques. Photoinitiated polymerization of MMA with TX–FN and CQ has been investigated in the presence and absence of a co‐initiator (MDEA). Additionally, Photo‐DSC studies were performed both in mono‐ and multiacrylate systems with TX–FN and parent compounds TX and FN. The obtained results were compared for formulations consisting of CQ. At low initiator concentrations, the conversions were lower than those obtained with TX–FN. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1012–1019

The carbazole-bound ferrocenium salt as a specific cationic photoinitiator upon near-UV and visible LEDs (365–405 nm)

A carbazole-bound ferrocenium salt [i.e., (η6-Carbazole) (η5-cyclopentadienyl) iron hexafluorophosphate—FS] is proposed as an efficient photoinitiator for the cationic ring-opening polymerization of epoxides under air upon the exposure to a near-UV LED at 385 nm or a visible LED at 405 nm. When using this ferrocenium salt FS (0.2 wt%)/diphenyliodonium hexafluorophosphate or FS (0.2 wt%)/diphenyliodonium hexafluorophosphate/N-vinylcarbazole, final epoxide conversions of 55–66 % can be obtained after 800 s of irradiation at 385 or 405 nm. Reference cationic photoinitiators (i.e., diphenyliodonium hexafluorophosphate; 9-(4-hydroxyethoxyphenyl) thianthrenium hexafluorophosphate and triphenylsulfonium hexafluorophosphate) are unable to initiate the epoxide polymerization under the same conditions. The photochemical mechanisms for the formation of the initiating species are studied using steady-state photolysis, cyclic voltammetry, laser flash photolysis and electron spin resonance spin-trapping techniques. Molecular orbital calculations help to describe the absorption properties and the initiation step. The performance attained when using FS alone is really promising for applications under soft near-UV or visible light-emitting diode irradiation.

Panchromatic photoinitiators for radical, cationic and thiol-ene polymerization reactions: A search in the diketopyrrolopyrrole or indigo dye series

A series of diketopyrrolopyrrole derivatives (DPPs) and indigo derivatives (IDGs) are synthesized and applied as photoinitiators (PIs) for the cationic polymerization (CP) of epoxides, the free radical polymerization (FRP) of acrylates or the thiol-ene polymerization under different lights (purple, blue, green, yellow, red LEDs and green laser diode). The photochemical mechanisms involved in the presence of these PIs and additives (iodonium salt (Iod), N-vinylcarbazole (NVK), amine (MDEA) or 2,4,6-tris(trichloromethyl)-1,3,5-triazine (R-Cl)) are investigated by steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques. Real-time infrared spectroscopy studies reveal that DPP4/Iod/NVK is very efficient for both CP and FRP and, compared to two references (Eosin-Y and camphorquinone), displays noticeably higher polymerization efficiencies. Panchromatic curable formulations exhibiting an almost constant photosensitivity from the blue to the red are proposed.

Excited-state absorption of meso-tetrasulfonatophenyl porphyrin: Effects of pH and micelles

The influence of the environment on the excited state transitions of meso-tetrakis(p-sulfonatophenyl) porphyrin (TPPS) is reported. TPPS was investigated in protonated and non-protonated forms, and in the presence of the cationic cetyltrimethylammonium bromide (CTAB) micelles. The singlet excited-state absorption spectra were measured by using the white-light continuum Z-scan technique and the triplet–triplet absorption spectra were acquired employing an association of laser flash photolysis and Z-scan techniques. Our results show that the perseveration of the molecular symmetry, upon excitation, depends on the state of multiplicity of the molecules, as well as on the environment and structural characteristics of the porphyrin. Additionally, it was observed that for excited molecules, the ring distortion caused by the protonation of porphyrin ring has great influence on the changes observed for the symmetry and vibronic structure. The results clearly show that the porphyrin investigated is a promising candidate for optical limiting applications for all investigated environments.

Synthesis, photophysical and photochemical properties of novel phthalocyanines substituted with triptycene moieties

New metal-free and zinc(II) phthalocyanines containing four triptycene moieties were synthesized and characterized by 1H NMR, mass, IR, electronic spectroscopies and elemental analysis as well. The steady-state absorption spectra of the investigated compounds exhibited the main absorption bands in the visible region, at around 700nm. The fluorescence spectra and quantum yields were recorded by steady-state emission measurements. The powerful nanosecond laser flash photolysis technique was employed to record the transient absorption spectra of the long-lived triplet states of the investigated compounds. The singlet oxygen generation and photodegradation properties of the target phthalocyanines under light irradiation were also investigated in tetrahydrofuran solution.

Triplet excited States as a source of relevant (bio)chemical information.

The properties of triplet excited states are markedly medium-dependent, which turns this species into valuable tools for investigating the microenvironments existing in protein binding pockets. Monitoring of the triplet excited state behavior of drugs within transport proteins (serum albumins and α1-acid glycoproteins) by laser flash photolysis constitutes a valuable source of information on the strength of interaction, conformational freedom and protection from oxygen or other external quenchers. With proteins, formation of spatially confined triplet excited states is favored over competitive processes affording ionic species. Remarkably, under aerobic atmosphere, the triplet decay of drug@protein complexes is dramatically longer than in bulk solution. This offers a convenient dynamic range for assignment of different triplet populations or for stereochemical discrimination. In this review, selected examples of the application of the laser flash photolysis technique are described, including drug distribution between the bulk solution and the protein cavities, or between two types of proteins, detection of drug-drug interactions inside proteins, and enzyme-like activity processes mediated by proteins. Finally, protein encapsulation can also modify the photoreactivity of the guest. This is illustrated by presenting an example of retarded photooxidation.

Photochemical Reaction Kinetics of Duroquinone in a Mixture of Ionic Liquid [BPy][BF<sub>4</sub>] and Acetonitrile

Photochemical reactions of duroquinone (DQ) in solutions consisting of mixtures of the ionic liquid N-butylpyridinium tetrafluoroborate ([BPy][BF4]) and acetonitrile (MeCN) were studied using the 355 nm laser flash photolysis technique. Increasing the ratio of [BPy][BF4] had no obvious impact on the absorption peak of the triplet excited state of DQ ((3)Da), which followed monoexponential kinetics in the N-2-saturated solution. However, increasing the ratio of [BPy] [BF4] did affect the photoinduced electron transfer process from triethylamine to (3)DQ in the mixture by decreasing the reaction rate and the quantum yield of the transient radical. The results highlight the ability to tune the efficiency and the rate of the photoinduced electron transfer by changing the proportion of [BPy][BF4] in the mixture.

The effect of leaving radical on the formation of tetrahydroselenophene by SHi ring closure: an experimental and computational study.

Competition kinetic studies augmented with laser-flash photolysis and high-level computational techniques [G3(MP2)-RAD], with [COSMO-RS, SMD] and without solvent correction, provide kinetic parameters for the ring closures of a series of 4-(alkylseleno)butyl radicals 1. At 22 °C rate constants (kc) that lie between 10(4)-10(7) s(-1) were determined experimentally and correlate with expectations based on leaving group ability. Activation energies (Eact) were determined to lie between 10.6 (R = Ph2CH) and 28.0 (R = n-Bu) kJ mol(-1), while log(A/s(-1)) values were generally between 9 and 10 in benzene. Computationally determined rate constants were in good-to-excellent agreement with those determined experimentally, with the COSMO-RS solvation model providing values that more closely resemble those from experiment than SMD.

Photochemical studies of new benzothiazole- and benzoselenazole-derived aminosquarylium dyes

In this work, new modified squarylium dyes, namely two new groups of benzothiazole- and benzoselenazole-derived squarylium dyes, were studied. An extensive photochemical characterization of all dyes was performed and the following parameters were studied: quantum yields of singlet oxygen formation, fluorescence quantum yields and fluorescence lifetimes. Depending on the nature of the substituent located in the squaric ring, significant results were obtained. The laser flash photolysis technique was used to determine triplet–triplet absorption spectra, in ethanol. In chloroform, photoproducts resulting from a reaction of the excited squarylium dye in the singlet state with CHCl3 were observed. A solute–solvent complex was detected, the primary product formed after excitation, being the complex [SQ+ ⋯ CHCl2 ⋯ Cl−].The effect of two different counterions (CF3SO3− and I−) was also addressed by these techniques providing new insights about the external heavy atom effect mechanism.

Laser flash photolysis study of the photochemistry of 4,5-diaza-9-fluorenone

The triplet excited state of 4,5-diaza-9-fluorenone (1) shows absorption maxima at 410 and 470 nm and a lifetime of 3 μs, in acetonitrile. Its intersystem crossing quantum yield was determined using 9-fluorenone as a secondary standard and a value of 0.41 ± 0.01 was obtained. The reactivity of the triplet excited state of 1 towards several quenchers, in acetonitrile, was investigated employing the laser flash photolysis technique quenching rate constants ranging from 7.9 × 104 M−1 s−1 (2-propanol) to 1.0 × 1010 M−1 s−1 (triethylamine) were obtained. From the quenching rate constants obtained one can conclude that 4,5-diaza-9-fluorenone has a ππ* triplet excited state. A Hammett plot for the quenching rate constants of triplet 1 by phenols containing polar substituents against σ + gave a reaction constant ρ of –1.54 ± 0.10, which demonstrates the electrophilic character of the 4,5-diaza-9-fluorenone triplet excited state.

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

ABSTRACTA 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

UV-violet-blue LED induced polymerizations: Specific photoinitiating systems at 365, 385, 395 and 405 nm

Two naphthalimide derivatives (DMAENs) containing tertiary amine groups have been designed and synthesized. Upon exposure to near UV and visible LEDs (365 nm–455 nm), they lead to radicals without adding a hydrogen donor and, in combination with 2,4,6-tris(trichloromethyl)-1,3,5-triazine, an iodonium salt or N-vinylcarbazole, they produce radicals and cations. Compared to the well-known camphorquinone or bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide photoinitiator, the novel DMAENs containing photoinitiating systems are characterized by a very high reactivity for both the free radical polymerization of acrylates and the cationic polymerization of epoxides. This outstanding performance paves the way for polymerization in soft conditions (e.g. upon LED irradiation). The photochemical mechanisms are studied by Molecular Orbitals MO calculations, steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping and laser flash photolysis techniques.