Theoretical Study on Reactions of Triplet Excited State Thioxanthone with Indole

In the present work, a theoretical study on the deactivation of triplet excited (T₁) state thioxanthone (TX) by indole (INH) was performed, based on density functional theory calculations. Three feasible pathways, namely direct electron transfer from INH to T₁ state TX, electron transfer followed by proton transfer from INH^.+ to TX^.−, and H-atom transfer from nitrogen of INH to keto oxygen of T₁ state TX, were proposed theoretically to be involved in T₁ state TX deactivation by INH.     

油橄榄叶中多酚和黄酮的含量分析

以儿茶素为标准品,采用Folin试剂分光光度法建立油橄榄叶中多酚的分析方法;以芦丁标准品为对照,建立油橄榄叶中黄酮的分析方法,研究不同树龄和干燥方式对油橄榄叶中黄酮和多酚含量的影响。结果表明:3年生油橄榄叶中黄酮和多酚的含量明显高于20年以上树龄的油橄榄叶。高温或阳光能降低黄酮和多酚的含量,采用大孔树脂精制油橄榄叶提取物,可获得多酚含量在80%以上的油橄榄叶提取物。     

Elektrochemische erzeugung von triplettzustanden V.[1] elektrochemische lumineszenz des systems 2,4,6-triphenylpyryliumperchlorat-rubren

The electrochemical reduction of 2,4,6-triphenylpyrylium cations in acetonitrile and dimethyl-formamide yields triphenylpyranyl radicals existing in a reversible equilibrium with their dimers. The homegeneous electron transfer reaction between these radicals and the cation radicals of rubrene gives rise to electrochemical luminescence via the triplet state of rubrene. This leads to the conclusion that in addition to the well known Anion-Cation-Annihilation also electron transfer processes involving radicals can occur with formation of excited states.     

Photopolymerization and photocycloaddition reactions in the 2-vinylnaphthalene — Maleic anhydride system

Summary Photochemical reaction of the 2-vinylnaphthalene (VN) (electron-donor monomer) — maleic anhydride (MAn) (electron-acceptor monomer) system has been studied in order to elucidate reaction pathways and the correlation between them and the multiplicity of an excited-state molecule. It was found that direct irradiation of the VN — MAn system both in benzene and acetonitrile produces predominantly copolymers probably via ion radicals generated by electron transfer in the electronically excited singlet state, whereas benzophenone-sensitized reaction yields a cycloadduct of VN with MAn as a main product via the electronically excited triplet state.     

Photochemistry and photoinitiation activity of radical polymerization of 2-substituted anthraquinone derivatives. III. Nanosecond laser flash photolysis study

A nanosecond laser flash photolysis study was undertaken on a selected range of 2-substituted anthraquinone derivatives and the data are discussed in relation to the photoactivities in industrial photopolymerization. All the compounds give rise to a triplet-triplet absorption that is quenched by ground-state molecular oxygen. The transient half-lives range from 0.64 to 11.3 μs of which the 2-(1,1 -dibromomethyl) anthraquinone exhibits the longest life-time. In a reductive solvent, 2-propanol, hydrogen atom abstraction takes place with the compounds haveing the lowest-lying triplet ³nπ^* state, whereas those with a low-lying triplet³ππ^* state show mixed kinetics. In the latter case, a disproportionation reaction involving the semianthraquinone radical may be taking place, competing with the direct hydrogen atom abstraction reaction. In addition, investigation of the halogenated derivatives has indicated the possibility of the corresponding halo radicals being formed. In the presence of a tertiary amine, triethylamine, all anthraquinone derivatives show the formation of stable species related to either the exciplex or the radical ion pair. The extent of exciplex formation is more effective with compounds possessing a lowest-lying triplet ³ππ^* excited state than those with a triplet ³nπ^* excited state. The results from the nanosecond laser flash photolysis study show the differences in behavior toward hydrogen atom abstraction and electron transfer processes that is dependent on the nature of the low-lying triplet state and the type of substituent present, i.e., electron-donating or electron-withdrawing. © 1996 John Wiley & Sons, Inc.     

Wirkungsweise von stabilisatoren in polymeren unter berücksichtigung der 2-hydroxyphenylbenzotriazole

In the first part a survey is given on UV screeners, UV absorbers, quenchers of excited states, antioxidants and peroxide decomposers. In the second part UV absorbers, e. g. Tinuvin P (2-(2'-hydroxy-5'-methylphenyl) benzotriazole, Scheme 8) are described in detail where intramolecular proton transfer occurs in the excited state (TIN(intra) ). This proton transfer may be accompanied and/or followed by very fast and effective temperature dependent radiationless deactivation processes which are promoted by internal vibrations and librations of the molecule. - In polar solvents or in polymers with polar groups some of the molecules convert their intramolecular hydrogen bond into an intermolecular one (TIN (inter) ). TIN (inter) cannot be considered any longer as UV stabilizer. Due to its high triplet quantum yield it may even initiate photodegradation processes of itself and of the polymer. From this follows that the TIN molecule can only act as stabilizer if the intramolecular hydrogen bond is intact. This can be answered from the UV absorption spectrum.     

金属酞菁激发态与氧的相互作用

比较了几种金属酞菁光敏产生单重态氧和超氧负离子的能力,结果表明它们产生~1O_2的能力与中心金属的电子结构有关,取决于三重态寿命和量子产率。顺序如下:Zn>Ga>Cu>H_2>Al>Co。产生O_2~-的能力不仅与三重态寿命和量子产率有关,也与激发能和氧化还原电位有关。其顺序如下:Ga>Al>Cu>Zn。还研究了酪氨酸与镓酞菁激发态相互作用,酪氨酸猝灭镓酞菁荧光。在除氧条件光激发下,酪氨酸猝灭镓酞菁的激发三重态发生电子转移,检测到GaTSPc~-在560nm处的瞬态吸收,在氧的存在下进一步反应生成O_i~-。     

Spectroscopic and Theoretical Study on Electronically Modified Chromophores in LOV Domains: 8‐Bromo‐ and 8‐Trifluoromethyl‐Substituted Flavins

Two chemically synthesized flavin derivatives, 8‐trifluoromethyl‐ and 8‐bromoriboflavin (8‐CF₃RF and 8‐BrRF), were photochemically characterized in H₂O and studied spectroscopically after incorporation into the LOV domain of the blue light photoreceptor YtvA from Bacillus subtilis. The spectroscopic studies were paralleled by high‐level quantum chemical calculations. In solution, 8‐BrRF showed a remarkably high triplet quantum yield (0.97, parent compound riboflavin, RF: 0.6) and a small fluorescence quantum yield (0.07, RF: 0.27). For 8‐CF₃RF, the triplet yield was 0.12, and the fluorescence quantum yield was 0.7. The high triplet yield of 8‐BrRF is due to the bromine heavy atom effect causing a stronger spin–orbit coupling. Theoretical calculations reveal that the decreased triplet yield of 8‐CF₃RF is due to a smaller charge transfer and a less favorable energetic position of T₂, required for intersystem crossing from S₁ to T₁, as an effect of the electron‐withdrawing CF₃ group. The reconstitution of the LOV domain with the new flavins resulted in the typical LOV photochemistry, consisting of triplet state formation and covalent binding of the chromophore, followed by a thermal recovery of the parent state, albeit with different kinetics and photophysical properties.     

Study of Photoinduced Electron Transfer Process in Ruthenium Complex Modified Zinc Oxide Nanoparticles by Ultrafast Time-Resolved Transient Absorption Spectroscopy

We report the preparation of ruthenium complex functionalized zinc oxide nanoparticles and the studies of the photophysical properties by ultrafast time-resolved transient absorption spectroscopy. Diazonium group was used as the anchoring unit on ZnO surface and covalent linkage was formed between the ruthenium complex and ZnO upon UV irradiation. The morphological and electron transfer properties of the ruthenium complex modified ZnO were studied. XPS and EDX results confirmed the presence of covalent linkage. The charge generation and the transport dynamics of this light harvesting system were probed by ultrafast transient absorption (TA) spectroscopy. No positive TA absorption band was observed, which suggested an ultrafast direct electron injection from the singlet ¹MLCT excited state of the ruthenium complex to the conduction band of ZnO without going via the triplet ³MLCT excited state. The time constants for the ground states bleaching of the complex modified ZnO are 8.1 and 167 ps, both of which are shorter than those of the pure ruthenium complex (21.7 and 360 ps). This further suggests a strong electronic coupling between the ruthenium complex and ZnO.     

Benzophenone photosensitized DNA damage

Although the carcinogenic potential of ultraviolet radiation is well-known, UV light may interact with DNA by direct absorption or through photosensitization by endogenous or exogenous chromophores. These chromophores can extend the "active" fraction of the solar spectrum to the UVA region and beyond, which means that photosensitizers increase the probability of developing skin cancer upon exposure to sunlight. Therefore researchers would like to understand the mechanisms involved in photosensitized DNA damage both to anticipate possible photobiological risks and to design tailor-made photoprotection strategies. In this context, photosensitized DNA damage can occur through a variety of processes including electron transfer, hydrogen abstraction, triplet-triplet energy transfer, or generation of reactive oxygen species. In this Account, we have chosen benzophenone (BP) as a classical and paradigmatic chromophore to illustrate the different lesions that photosensitization may prompt in nucleosides, in oligonucleotides, or in DNA. Thus, we discuss in detail the accumulated mechanistic evidence of the BP-photosensitized reactions of DNA or its building blocks obtained by our group and others. We also include ketoprofen (KP), a BP-derivative that possesses a chiral center, to highlight the stereodifferentiation in the key photochemical events, revealed through the dynamics of the reactive triplet excited state ((3)KP*). Our results show that irradiation of the BP chromophore in the presence of DNA or its components leads to nucleobase oxidations, cyclobutane pyrimidine dimer formation, single strand breaks, DNA-protein cross-links, or abasic sites. We attribute the manifold photoreactivity of BP to its well established photophysical properties: (i) it absorbs UV light, up to 360 nm; (ii) its intersystem crossing quantum yield (?(ISC)) is almost 1; (iii) the energy of its nπ* lowest triplet excited state (E(T)) is ca. 290 kJ mol(-1); (iv) it produces singlet oxygen ((1)O(2)) with a quantum yield (?(Δ)) of ca. 0.3. For electron transfer and singlet oxygen reactions, we focused on guanine, the nucleobase with the lowest oxidation potential. Among the possible oxidative processes, electron transfer predominates. Conversely, triplet-triplet energy transfer occurs mainly from (3)BP* to thymine, the base with the lowest lying triplet state in DNA. This process results in the formation of cyclobutane pyrimidine dimers, but it also competes with the Paternò-Büchi reaction in nucleobases or nucleosides, giving rise to oxetanes as a result of crossed cycloadditions. Interestingly, we have found significant stereodifferentiation in the quenching of the KP triplet excited state by both 2'-deoxyguanosine and thymidine. Based on these results, this chromophore shows potential as a (chiral) probe for the investigation of electron and triplet energy transport in DNA.     

Synthesis,photodegradation, and energy transfer in a series of poly(ethylene terephthalate–co–4,4′-biphenyldicarboxylate) copolymers

Poly(ethylene terephthalate) (PET) filament yarns were photostabilized by addition of 0.5–4.0 mole % dimethyl 4,4′-biphenyldicarboxylate (4,4′-BPDC) to the polymerization feed. The mechanism of photostabilization is proposed to be a triplet–triplet energy transfer from excited terephthalate units to ground-state biphenyldicarboxylate units. The mechanism of transfer is reported to be via an electron exchange mechanism, with the “quenching sphere” calculated to be 14.9 Å. Kinetic studies show the “pseudo” zero-order rate constant of initial photodegradation to decrease from 3.4 × 10^?19 for the PET homopolymer to 2.0 × 10^?19% breaking strength loss/quantum exposure/cm² for the copolyester containing 4.0 mole % of the 4,4′-biphenyldicarboxyl moieties. The photophysical processes available to the dimethyl 4,4′-biphenyldicarboxylate monomer were characterized by absorption and luminescence studies. In solution, dimethyl 4,4′-biphenyldicarboxylate was shown to emit an intense fluorescence from a ²(π,π*) state and a weaker (～10^?2×) phosphorescence from a ¹(π,π*) state derived from the ¹A→supn1L_b absorption. The copolymer yarns were shown to exhibit both fluorescence and phosphorescence from the biphenyldicarboxylate units: the fluorescence from direct excitation, the phosphorescence by sensitized transfer.     

Phenyltrialkylborates as co-initiators with cyanine dyes in visible light polymerization of acrylates

Photoredox pairs consisting of selenocarbocyanine dye cations and phenyltrialkylborate anions were employed as the novel, effective visible-wavelength initiators of the radical polymerization of acrylic monomer. The influence of the sensitizers and electron donor structure on the photopolymerization kinetics of multiacrylate monomer was investigated by photo-DSC. It was found that the polymerization rate and the final conversion degree were dependent on both dye and borate structure. The kinetic studies of the free radical polymerization revealed an increase in the polymerization rate with a decrease of the borate oxidation potentials which was additionally reflected by the linear relationship between the Hammett constant and rate of polymerization. The efficiency of these initiators was discussed on the basis of the free energy change for electron transfer from an excited cyanine dye cation to a borate anion. The ΔG_el values were estimated for photoredox pairs containing a series of phenyltrialkylborate anions and one selenocarbocyanine dye cation. The relationship between the rate of polymerization and the free energy of activation for electron transfer reaction gives the dependence predicted by the classical theory of electron transfer. The photoreduction of cyanine phenyltrialkylborate complex was studied using nanosecond laser flash photolysis. The dye triplet was found to be quenched by the electron donors via an electron transfer process. Rate constants (k_q) for the quenching of the excited states were high and approached diffusion-controlled limits and were found to depend on the borate structure.     

Photoprocesses in Derivatives of 1,4- and 1,3-Diazadistyryldibenzenes

Photoprocesses in 1,4-diazadistyrylbenzene (1) and 1,3-diazadistyrylbenzene derivative (2) diperchlorates in MeCN were studied by absorption, luminescence, and kinetic laser spectroscopies. For compound 1, trans-cis-photoisomerization and intersystem crossing to a triplet state are observed. For compound 2, photoelectrocyclization is suggested. Quantum chemical calculations of diazadistyrylbenzene structures in the ground and excited states were carried out. The schemes for photoreactions were proposed.     

Assembly,crystal structure and luminescent properties of coordination polymer of europium nitrate with an amide type bridging podand

A novel one-dimensional coordination polymeric structure comprised of dinuclear metallorings connected by ligands has been rationally assembled by the reaction of europium nitrate and an amide type bridging podand, 1,3-bis{[(2′-benzylaminoformyl)phenoxyl]methyl}benzene (L). At the same time, the luminescent properties of the Eu(III) complex were also investigated. Under the UV excitation, the complex exhibited characteristic emissions of europium ion and could be significant in the field of supramolecular photonic devices. The triplet excited state T₁ of the ligand is well placed to allow energy transfer to Eu³⁺ excited states.     

Specific Polyphenols and Tannins are Associated with Defense Against Insect Herbivores in the Tropical Oak Quercus oleoides

The role of plant polyphenols as defenses against insect herbivores is controversial. We combined correlative field studies across three geographic regions (Northern Mexico, Southern Mexico, and Costa Rica) with induction experiments under controlled conditions to search for candidate compounds that might play a defensive role in the foliage of the tropical oak, Quercus oleoides. We quantified leaf damage caused by four herbivore guilds (chewers, skeletonizers, leaf miners, and gall forming insects) and analyzed the content of 18 polyphenols (including hydrolyzable tannins, flavan-3-ols, and flavonol glycosides) in the same set of leaves using high performance liquid chromatography and mass spectrometry. Foliar damage ranged from two to eight percent per region, and nearly 90% of all the damage was caused by chewing herbivores. Damage due to chewing herbivores was positively correlated with acutissimin B, catechin, and catechin dimer, and damage by mining herbivores was positively correlated with mongolinin A. By contrast, gall presence was negatively correlated with vescalagin and acutissimin B. By using redundancy analysis, we searched for the combinations of polyphenols that were associated to natural herbivory: the combination of mongolinin A and acutissimin B had the highest association to herbivory. In a common garden experiment with oak saplings, artificial damage increased the content of acutissimin B, mongolinin A, and vescalagin, whereas the content of catechin decreased. Specific polyphenols, either individually or in combination, rather than total polyphenols, were associated with standing leaf damage in this tropical oak. Future studies aimed at understanding the ecological role of polyphenols can use similar correlative studies to identify candidate compounds that could be used individually and in biologically meaningful combinations in tests with herbivores and pathogens.     

The influence of the photophysics of 2-substituted thioxanthones on their activity as photoinitiators

The polymerization of methyl methacrylate photoinitiated by thioxanthones (TXs) substituted in the position 2 of the chromophore ring in the presence of 2-(N,N-diethylamino)ethanol has been investigated. The photoinitiation efficiency of these systems is highly dependent on the structure of the 2-substituent. At high amine concentrations, compounds with electron-withdrawing substituents appear as more efficient photoinitiators. The photophysics of the ketones was studied in the polymerization medium. These data show that the singlet excited state of TXs is deactivated by the amine with a rate constant near the diffusional control limit. The triplet rate constants for quenching by the amine and the monomer are highly dependent on the ketone structure. Compounds with electron-withdrawing substituents in the position 2 are more reactive. These studies allow to simulate the dependence of the photoinitiation efficiency with the amine concentration and indicate that the active radicals are produced by the interaction of the ketone triplet with the amine.     

Antioxidant activities of six natural phenolics against lipid oxidation induced by Fe2+ or ultraviolet light

The mechanisms and antioxidant activities of six natural phenolics against lipid oxidation induced by ultraviolet (UV) radiation or Fe²⁺ were studied. An oil emulsion was prepared with flax oil and the thiobarbituric acid-reactive substances (TBARS) method was used to determine lipid oxidation. The antioxidant activities of the six phenolics against UV-induced lipid oxidation were as follows: quercetin > rutin = caffeic acid = ferulic acid = sesamol > catechin. The inhibitory concentrations (IC₅₀) showed that the effectiveness of these antioxidants against Fe²⁺-induced lipid oxidation was in the order quercetin (1.7 μM) > rutin (10.3 μM) > catechin (14.9 μM) > sesamol (18.5 μM) > caffeic acid (19 μM) > ferulic acid (>250 μM), and quercetin was more efficient than butylated hydroxytoluene (BHT) (2.9 μM). Quercetin and rutin had absorption maxima at the UV-A (320–380 nm) region, while the other phenolics tested had absorption maxima near (catechin, 278 nm) or at the UV-B (280–320 nm) region. The stoichiometric ratios of quercetin, rutin, catechin, and caffeic acid to Fe²⁺ were 3:1, 2:1, 1:1, and 1:1, respectively. Although free-radical scavenging capability of antioxidants was the most critical, UV absorption and/or Fe²⁺-chelation properties of natural phenolics also contributed significantly to the control of lipid oxidation induced by UV or Fe²⁺ in oil systems.     

香豆素酮类化合物的瞬态研究——Ⅲ.香豆素酮和碘鎓盐间的电子转移

本工作采用激光闪光光解法对香豆素酮类化合物敏化碘鎓盐化合物问题进行了研究。结果表明:碘鎓盐化合物能通过电子转移机理猝灭香豆素酮的激发三重态。工作中还用甲基紫精(PQ~(2+))为模型化物,观察到它也能使香豆素酮的三重态猝灭,同时可看到在位于610nm处的PQ~+生成。这些结果说明,在发生电子转移的反应中香豆素酮是电子给体,按Weller公式的计算结果也表明它们之间可发生电子转移反应。     

Direkte und sensibilisierte Photoisomerisierung der α,δ-Phenylfulgide

Direct and Sensitized Photoisomerization of α,δ-Phenylfulgides The photoreactions starting from the E- and E-E-isomers of mono-, di-, tri- and tetra-(substituted-)phenylfulgides are investigated. The phenylfulgides show strong steric interactions in the molecular framework. For that reason internal conversion is the main deactivation process which increases in the series of mono-, tri- and tetraphenylfulgides. The α,δ-di-(substituted-)-phenylfulgides show the highest photoreactivity. At room temperature in no case luminescence is observed from any E(E-E)-, Z(E-Z, Z-E)- or Z-Z-phenylfulgid isomer of the substances under study. Intersystem crossing cannot compete neither with thermal deactivation nor can it be observed by applying both the internal and external heavy atom effect. The phenylfulgides show typical ππ*-photoreactivity: in the singlet state E-Z-isomerization about one double bond competes with electrocyclic ring closure; in the triplet state only simultaneous E-Z-isomerization about the two double bonds is observed. The steric and electronic substituent influences on the photoreactivity are interpreted. A first qualitative model of the isomerization reactions of the fulgides in the excited states is established.     

Three-component photo radical initiating system—the effect of 2-mercaptobenzothiazole as a co-initiator in polymer matrix

We studied the sensitization mechanism in a photo initiating system that consists of an aminostyryl sensitizing dye, 2-[p-(diethylamino)styryl]naphto[1,2-d]thiazole (NAS), and a radical generator, 2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1, 1′-bi-1H-imidazole (HABI). In a photopolymer bearing NAS/HABI system, an improvement of the photosensitivity was observed by the addition of co-initiator, 2-mercaptobenzothiazole (MBT), to the initiating system. Photochemical and photophysical behavior of the three-component initiating system, NAS/HABI/MBT, in the PMMA film was studied by means of a nanosecond laser flash photolysis method employing a total reflection sapphire plate. It is considered that MBT can enhance to generate the triplet state of NAS, which is hardly observed by the direct excitation, and the mechanism of both singlet and triplet electron transfer from NAS to HABI is suggested.