Photochemistry in solution-XX : Triplet reactivity of aliphatic aldehydes

The triplet self-quenching process of three aliphatic aldehydes has been investigated by inhibition with dienes (taking into account the singlet interaction with the dienes) and by laser flash photolysis. The results obtained for intersystem crossing, the setf-quenching process and product formation have been rationalized. The main reactivity observed for the three aldehydes is the self-quenching process which occurs from both the singlet and triplet state. The laser flash photolysis experiments carried out with butanal show two absorptions of a transient at 320 aod 355 nm; no evidence for two different species could be put forward. The similar decay of the two absorption maximas of the transient, as the concentration of aldehyde is increased, would be indicative of only one single absorbing species which could be either the triplet state of the aldehyde or a radical-pair formed by the self-quenching process or the 1,4-biradical resulting from γ-H abstraction. The fact that both the quenching experiments (by dienes or by 1-methylnaphthalene) and the laser flash measurements lead to about the same lifetime also indicates only one species.The products formed from the triplet setf-quenching process have also been obtained by a different method: excitation of benzophenone at 365 nm in the presence of butanal. The quantum yields for product formation is about the same as those obtained for the triplet by direct irradiation of butanal, except that of octane-4,5-dione which is increased if the photoreaction is carried out at 365 nm in the presence of beazophenone.     

Photochemistry of thioxanthones VI: A polymerization,spectroscopic and flash photolysis study on novel water-soluble substituted 3-(9-oxo-9H-thioxanthene-2,3-γ-4-yloxy)-N,N,N-trimethyl-1-propanaminium salts

The photopolymerization activity and photochemistry of nine novel water-soluble 3-(9-oxo-9H-thioxanthene-2,3-γ-4-yloxy)-N,N,N-trimethyl-1-propanaminium salts is examined in water and 2-propanol using absorption, luminescence and conventional microsecond flash photolysis techniques. For both the chloro and the methylsulphonate salts, photopolymerization activity in water follows the order of substitution 4- > 2- > 3-. Methyl substitution in the 1-position deactivates the molecule and this is reflected by a reduction in both the photopolymerization activity and luminescence (fluorescence and phosphorescence) quantum yields and the triplet lifetime. Intramolecular hydrogen-atom abstraction is the main competitive process suppressing photochemical activity. The longest wavelength absorption maxima and extinction coefficients and luminescence quantum yields are similar to those of water-soluble 2-substituted derivatives studied previously, indicating a lowest excited singlet state with strong n̄n̄^* character. On changing the solvent from 2-propanol to water all the fluorescence quantum yields are enhanced and this is consistent with a strong degree of charge transfer character in the lowest excited singlet state. From conventional microsecond flash photolysis results, transient absorptions below 400 nm are associated with the intermediate ketyl radical formed by the lowest excited singlet/triplet states of the thioxanthene molecule abstracting a hydrogen atom from the amine synergist (used in photopolymerization) while absorptions above 400 nm are associated with the radical anion intermediate formed by a concurrent electron abstraction process from the amine. The latter is confirmed through a detailed study on the effect of pH and amine concentration on transient production, as well as the effect of various amines of increasing ionization potential. Transient formation is only partly reduced in aerobic conditions and is consistent with our earlier findings and conclusions that photopolymerization activity is associated, in the main, with electron abstraction by the lowest excited singlet state of the thioxanthone molecule from the tertiary amine synergist. This is confirmed by a correlation between the photopolymerization activity and the ionization potential of different amines.     

Intermolecular electron transfer from excited benzophenone ketyl radical

The electron transfer from the benzophenone ketyl radical in the excited state (BPH(.-)(D(1))) to several quenchers (Qs) was investigated using nanosecond/picosecond two-color two-laser flash photolysis and nanosecond/nanosecond two-color two-laser flash photolysis. The electron transfer from BPH(.-)(D(1)) to Qs was confirmed by the transient absorption and fluorescence quenching measurements. The intermolecular electron-transfer rate constants were determined using the Stern-Volmer analysis. The driving force dependence of the electron-transfer rate was revealed.     

Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones

Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino- 3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron (e_aq⁻) has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pK_a values of ketylanion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized.     

萘基衍生物的光敏化瞬态吸收光谱

本文利用激光闪光光解技术对二苯甲酮光敏化一系列萘基烷烃衍生物的三重态—三重态吸光光谱及他们之间的三重态能量传递进行了研究. 计算了三重态能量传递速度常数和传递效率, 二苯甲酮在不同体系中的三重态寿命, 探讨了分子结构对光敏化能量传递的影响.     

Pressure dependence of triplet formation in acridine and phenazine vapours

The pressure dependence of T₁ formation in acridine and phenazine vapours was studied between 0.05 torr and 40 torr of an added buffer gas, by microsecond flash photolysis. In contrast to the pressure dependence of anthracene triplet formation, in the heterocyclic compounds the triplet absorption increases slightly with pressure up to 1 torr and then decreases strongly with an increase in pressure up to 30–40 torr. These effects were confirmed by the slight increase followed by a decrease with time of the integrated areas of the T₁ acridine spectra observed at much shorter times by means of nanosecond flash photolysis. The absorption of the lowest triplet was found to have an induction period more than one order of magnitude longer than the laser pulse or the singlet lifetimes revealing that the T₁ state is not populated directly from S₁ but via other electronic triplet states. The pressure effects on triplet formation are interpreted in terms of a reversible intersystem crossing S₁ ? T₁ associated with vibrational relaxation in the triplet states.     

PHOTOPHYSICAL STUDIES ON ORGANIC COMPOUNDS——TIME-RESOLVED TRANSIENT ABSORPTION SPECTRA STUDIES ON TRIPHENYLAMINE IN VARIOUS SOLVENTS

The transient absorption spectra of triphenylamine (TPA) in various solvents have beeninvestigated by the methods of nanosecond laser photolysis. The lifetimes of TPA were meas-ured. In hexane and ethanol, the transient absorptions of triplet state and triplet excimer ofTPA were observed. However, the transient absorptions of the TPA cation radical was alsoobserved in acetonitrile besides that of the triplet TPA monomer and excimer. According tothe experimental facts, the transient photophysical mechanism of TPA in various solvents hasbeen proposed.     

Laser Flash Photolysis Studies of the UVA Sunscreen Mexoryl SX

Abstract The results of a nanosecond laser flash photolysis investigation of the UVA sunscreen Mexoryl* SX in various solvent environments and within a commercial sunscreen formulation are reported. To the best of our knowledge this is the first laser flash photolysis study of a commercial suncare formulation. In each of these environments kinetic UV-visible absorption measurements following nanosecond 355 nm laser excitation reveals a short-lived species with a solvent-dependent absorption maximum around 470–500 nm and a solvent-dependent lifetime of 50–120 ns. This transient absorption is attributed to the triplet state of Mexoryl* SX on the basis that it is quenched by molecular oxygen leading to the formation of singlet oxygen in acetonitrile. The singlet oxygen quantum yield (φ_Δ), determined by comparative time-resolved near-infrared luminescence measurements and extrapolated to the limit of complete triplet state quenching, is estimated as 0.09 ± 0.03 in acetonitrile. In aqueous solution the shorter triplet state lifetime combined with lower ambient oxygen concentrations precludes significant triplet state quenching. For the commercial sunscreen formulation there was no observable difference in the measured triplet lifetime between samples exposed to oxygen or argon, suggesting that the singlet oxygen quantum yield in such environments is likely to be orders of magnitude lower than that measured in acetonitrile.     

Combined Transient Spectra and Semiempirical Calculation of [60]Fullerenes Attached with Piperidinodithiocarboxylate and 7-Chloro-phenazine

[60]Fullerenes attached with piperidinodithiocarboxylate dyad （1） and 7-chloro-1,2,3,4-tetrahydrophenazine （2） were efficiently synthesized through Diels-Atder cycloaddition with dienes. The physical properties of the triplet states of these compounds, in which strong electron acceptor moieties were covalently attached to C60 cores, were investigated by nanosecond laser flash photolysis. The excited triplet states in benzonitrite have been evaluated by observing the transient absorption bands in the near-IR region. The HOMO and LUMO were calculated by semiempirical methods AM1, which could predict the intramolecular photoinduced electron transfer in 1 and 2, and the nanosecond transient absorption spectra observed experimentally in solution were in excellent agreement with the calculated ones.     

Photoinduced electron transfer in crystal violet (CV)–bovine serum albumin (BSA) system: evaluation of reaction paths and radical intermediates

Photoinduced electron transfer (PET) from excited probes attached to proteins is of considerable current interest. Photochemical processes following 532 nm excitation of triphenyl methane dye, crystal violet (CV⁺) bound to a protein, bovine serum albumin (BSA), have been investigated in picosecond (ps) to microseconds (μs) time scales by flash photolysis technique. The excited singlet state lifetime of CV⁺ is found to be increased to 130 ps as compared to 1–5 ps for the unbound dye in low viscosity solvents. From flash photolysis studies in microsecond region, transient absorption in the region 650 nm is observed which is attributed to the dication radical CV√²⁺ formed by electron transfer from ³CV^+* to BSA, contrary to electron transfer from BSA to the excited dye as proposed in a recent report. Supporting spectral evidence for the electron transfer from ³CV^+* to BSA is obtained from pulse radiolysis studies.     

Micellar effects on photoprocesses in retinyl polyenes

The effects of micellar solubilization on excited-state properties of several retinyl polyenes have been examined primarily by nanosecond laser flash photolysis. The relative intensity of band system III (254–256 nm) in the ground state absorption spectrum of 11-cis retinal decreases significantly on going from methanol to micellar solutions, suggesting that the 12-s-trans form of 11-cis retinal is relatively favored in the organized media. In addition to microsecond transient phenomena due to triplets, the laser flash photolysis of all-trans and 11-cis retinal and all-trans retinyl Schiff base incorporated into micelles leads to ‘permanent’ absorption changes attributable to photoisomerization (in the case of retinals) and protonation and/or complexation with water (in the case of Schiff base). All-trans retinol and retinyl acetate in micellar solutions undergo ionic photodissociation leading to long-lived retinyl carbocation (λ_max = 585–600 nm), the process being monophotonic in the case of retinyl acetate and predominantly bipho-tonic in the case of retinol. The trends in the location of ground-state absorption maxima (^IB_u^+*←^IA_g) and triplet yield of retinals, and photodissociation yield of retinyl acetate suggest that the polarity of the environment probed by the polyene systems increases in the order: Triton X-100 < CTAB < NaLS.     

INTERMEDIATES IN THE ROOM TEMPERATURE FLASH PHOTOLYSIS OF ADENINE AND SOME OF ITS DERIVATIVES

The transient absorption spectra of aqueous solutions of adenine, 2′-deoxyadenosine, 2′-deoxyadenosine-5′-phosphate and 2′-deoxyadenylyl-(3′-5′)-2′-deoxyadenosine have been determinated at different pH values using conventional flash photolysis. Reactives intermediates produced in the flash photolysis of these adenine derivatives present similar absorption regions: two higher intensity bands in the UV and 560–720 nm wavelength region and a third weaker band at 420–560 nm. On the basis of the effects produced by triplet quenchers and/or electron scavengers the bands have been assigned to hydrated electrons, radical cations, radical anions and/or neutral radicals resulting from neutralization reactions of the charged radicals. The results indicate that the bases photoionize via a triplet state under these conditions.     

PHOTOCHEMICAL REACTIONS OF AROMATIC KETONES WITH NUCLEIC ACIDS AND THEIR COMPONENTS I—. PURINE AND PYRIMIDINE BASES AND NUCLEOSIDES*.

Abstract— The photochemical reactions of benzophenone and acetophenone with purine and pyrimidine derivatives in aqueous solutions have been investigated by flash photolysis and steady-state experiments. Upon excitation of these two ketones in aqueous solutions, two transient species are observed: molecules in their triplet state and ketyl radicals. The triplet state lifetimes are 65 μsec for benzophenone and 125 μsec for acetophenone. The ketyl radicals disappear by a second order reaction, controlled by diffusion. In the presence of pyrimidine derivatives, the triplet state is quenched and the ketyl radical concentration is decreased without any change in its kinetics of disappearance. Ketone molecules in their triplet state react with purine derivatives leading to an increase in the yield of ketyl radicals due to H-atom abstraction from the purines. Steady-state experiments show that benzophenone and acetophenone irradiated in aqueous solution at wavelengths longer than 290 nm undergo photochemical reactions. The rate of these photochemical reactions is increased in the presence of pyrimidine derivatives and even more in the presence of purine derivatives. Following energy transfer from the triplet state of benzophenone to diketopyrimidines, cyclobutane dimers are formed. The energy transfer rate decreases in the order orotic acid > thymine > uracil. Benzophenone molecules in their triplet state can also react chemically with pyrimidine derivatives to give addition photoproducts. All these results are discussed with respect to photosensitized reactions in nucleic acids involving ketones as sensitizers.     

Accessing the long-lived triplet excited states in bodipy-conjugated 2-(2-hydroxyphenyl) benzothiazole/benzoxazoles and applications as organic triplet photosensitizers for photooxidations

Bodipy derivatives containing excited state intramolecular proton transfer (ESIPT) chromophores 2-(2-hydroxyphenyl) benzothiazole and benzoxazole (HBT and HBO) subunits were prepared (7-10). The compounds show red-shifted UV-vis absorption (530-580 nm; ε up to 50000 M(-1) cm(-1)) and emission compared to both HBT/HBO and Bodipy. The new chromophores show small Stokes shift (45 nm) and high fluorescence quantum yields (Φ(F) up to 36%), which are in stark contrast to HBT and HBO (Stokes shift up to 180 nm and Φ(F) as low as 0.6%). On the basis of steady state and time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7-9 do not undergo ESIPT upon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipy-localized triplet excited states were populated for 7-10 upon photoexcitation; the lifetimes of the triplet excited states (τ(T)) are up to 195 μs. DFT calculations confirm the transient absorptions are due to the triplet state. Different from the previous report, we demonstrated that population of the triplet excited states is not the result of ESIPT. The compounds were used as organic triplet photosensitizers for photooxidation of 1,5-dihydroxylnaphthalene. One of the compounds is more efficient than the conventional [Ir(ppy)(2)(phen)][PF(6)] triplet photosensitizer. Our result will be useful for design of new Bodipy derivatives, ESIPT compounds, and organic triplet photosensitizers, as well as for applications of these compounds in photovoltaics, photocatalysis and luminescent materials, etc.     

A laser flash photolysis study of curcumin in dioxane-water mixtures.

Curcumin [bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was studied by means of UV-VIS absorption spectroscopy and nanosecond laser flash photolysis in 1,4-dioxane-water mixtures in a series of dioxane-water volume ratios. The transient characteristics were found to be dependent on the amount of water. In pure dioxane the triplet state of the molecule in its enolic form was detected (lambda(max) = 720 nm, tau = 3.2 micros), whereas upon water addition, the diketo form was found to prevail, because of the perturbation of intramolecular H-bonded structure. This led to hydrogen abstraction from dioxane by curcumin triplet state and the formation of the corresponding ketyl radical (lambda(max) = 490 nm, tau approximately 10 micros). Laser flash photolysis measurements, carried out in solvents of different polarity and proticity (benzene, cyclohexane and various alcohols), allowed the transient assignments to be confirmed, supporting our interpretation.     

不同极性溶剂中2-蒽醌磺酸钠三重态特征瞬态吸收谱及动力学研究

利用纳秒级激光光解瞬态吸收光谱装置－248nm(KrF）激光光解研究了水和乙腈溶液中2－蒽醌磺酸钠激发三重态的瞬态吸收光谱。水溶液中纯将的AQS激发三重态的特征吸收在580nm处；确定了AQS激发三重态的特征吸收普形、范围在水和乙腈溶液中的相似性；2－蒽醌磺酸钠激发三重态的半衰期在乙腈中为10.7μs，在水中却为为0.5μs；测定了2－蒽醌磺酸钠激发三重态衰减动力学参数。     

INTERFACING FOR LASER FLASH PHOTOLYSIS EXPERIMENTS: A NEW APPROACH

Abstract— A computer-controlled nanosecond laser flash photolysis system incorporating a CAMAC interface system is described. The resulting system provides many benefits, particularly with regard to ease of operation and flexibility. The use of readily available, off-the-shelf components results in rapid installation, high-level language programability, expandability, and moderate cost. Transient absorption spectra of the benzophenone triplet, ketyl radical and ketyl radical anion in aqueous acetonitrile, demonstrate the utility of the apparatus described.     

THE PHOTOREDUCTION OF FLAVINS BY AMINO ACIDS AND EDTA. A CONTINUOUS AND FLASH PHOTOLYSIS STUDY

Abstract— Primary and secondary photochemical processes in oxygen-free aqueous solution have been characterised for FMN alone and in the presence of EDTA and four amino acids using nanosecond and microsecond flash photolysis and continuous photolysis techniques. The relative contributions of oneelectron and two-electron (group or hydride transfer) reactions to the deactivation of the triplet has been determined by comparing the radical concentration (560 nm) with the bleaching of the ground state (446 nm). It was concluded that one-electron reactions (hydrogen atom or electron abstraction) are the major mode of reactivity of the flavin triplet state with all the suhstrates studied.
The nature of the reactions of the flavin semiquinone radical have been studied quantitatively by microsecond flash photolysis. These secondary reactions consist of either a 'back reaction' between the flavin and substrate radicals (tryptophan or glycyl-tyrosine) or the transfer of a second electron (or hydrogen atom) from the substrate radical to the flavin radical (EDTA, methionine and possibly cysteine) to form reduced flavin and oxidised substrate. From a comparison of the quantum yields of formation of reduced flavin using 'flash' and continuous irradiation, an additional pathway for the decay of the flavin radical is suggested to occur at low light intensities in the presence of glycyl-tyrosine or histidine.     

Photo-oxidation of polymers—VII: A re-investigation of the photo-oxidation of polystyrene based on a model compound study

Fluorescence quenching experiments indicate that energy transfer occurs from cumene excited at 254 nm to cumene hydroperoxide. Quantum yields show that the sensitized decomposition of the hydroperoxide occurs quantitatively and that 2-phenylpropanol-2 is the main photoproduct. In the presence of oxygen, this process plays a dominant role in the initiation of the photo-oxidation. When benzophenone is excited to the first triplet state by irradiation at 365 nm in the presence of cumene hydroperoxide, phosphorescence quenching experiments and laser flash photolysis suggest that an exciplex is formed. This exciplex dissociates into cumylperoxy and ketyl radicals in such a way that 80% of the excited ketone molecules are transformed into the corresponding pinacol. In the presence of oxygen, benzophenone primarily initiates the photo-oxidation of cumene by hydrogen abstraction but, as cumene hydroperoxide is formed, formation and reaction of the exciplex become progressively more and more important. The photochemical behaviour a fluorenone is quite different from that of benzophenone. The sensitized decomposition of cumene hydroperoxide occurs in the presence of that ketone. Surprisingly, fluorenone also initiates the photo-oxidation of cumene; the mechanism of that reaction is discussed. The whole set of results provides a sound basis for the interpretation of the photo-oxidation of polystyrene in various conditions.     

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二苯硒醚(TPDASe)间光诱导电子转移过程的激光光解研究

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二 苯硒醚(TPDASe)间在激光光诱导条件下，发生了分子间的电子转移过程．在可见- 近红外区(600-1200nm)，观测到了TPDASe阳离子自由基、富勒烯(C60／C70)激发三 线态和阴离子自由基，在苯腈溶液中，观测瞬态谱测定了电子从TPDASe转移到富勒 烯(C60／C70)激发三线态的量子转化产率(Φet^T)和电子转移常数(Ket)．