Laser induced photochromic mechanism of 3-phenyl-3-[1,2-dimethylindol-3-YL]-3H-naphtho[2,1-B]pyran

The photochromic process of 3-phenyl-3-[1,2-dimethylindol-3-yl]-3H-naphtho[2,1-b]pyran [I] has been examined with nanosecond laser flash photolysis techniques in cyclohexane and acetonitrile respectively. Both excited singlet state and triplet state are involved in the photocoloration process. The decay kinetics of photoproducts are also studied. The maximum absorption wavelength and lifetime of the transient species are solvent dependent.     

Photophysical Properties of Some Methyl-Substituted Angelicins: Fluorometric and Flash Photolytic Studies

This paper describes the results of a study of the photophysical properties of various methyl-angelicins (MA) in solvents of different polarity and proticity. The behavior of their excited singlet and triplet states was investigated by fluorometry and nanosecond laser flash photolysis. On the basis of semiempirical (ZINDO/S-CI) calculations and the solvent effect on the absorption and fluorescence properties, the lowest excited singlet state (S₁) is assigned to a partially allowed π, π* state. The close lying S₂ state is n,π* in nature. The efficiency of the decay pathways of S₁ (fluorescence, intersystem crossing and internal conversion) strongly depends on the energy gap between the S₁ and S₂ states consistent with the manifestation of “proximity effect.” Thus, MA in cyclohexane decay only through S₁→ S₀ internal conversion, while in acetonitrile and ethanol, where the n, π* state is located at higher energy, their fluorescence and intersystem crossing increase significantly. The lowest excited triplet states (T₁) were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yields. The interaction of T₁ MA with molecular oxygen leads to an efficient formation of singlet oxygen, as evidenced by the appearance of characteristic IR phosphorescence centered at 1269 nm.     

A pulse radiolysis and pulsed laser study of the pyrromethene 567 triplet state

The triplet state of pyrromethene 567, a molecule with potential as a solid state laser dye, has been characterized in benzene by pulse radiolysis in terms of its absorption spectrum, lifetime, self-quenching, electronic excitation energy, triplet–triplet extinction coefficient and oxygen quenching rate constant. The use of laser flash photolysis has then allowed determination of the triplet quantum yield, efficiency of formation of singlet oxygen (¹Δ_g), and the rate constant for reaction of the latter species with the ground state. The affects of oxygen on the fluorescence and triplet yields demonstrate that oxygen-induced intersystem crossing is important, the sum of these parameters being unity within experimental error. The mechanism of reaction of P-567 with ¹Δ_g in benzene is predominantly physical in character with only a small (6%) contribution from chemical reaction.     

萘酰亚胺及其含二茂铁二元化合物的分子间(内)三重态能量传递

近年来萘酰亚胺类化合物作为超分子体系中的功能单元逐渐引起了人们的重视,如快速响应的光开关体系[1]、生物荧光探针体系[2]、激光染料天线分子[3]等.     

The photophysical properties of menthyl anthranilate: a UV-A sunscreen

The results of a comprehensive investigation of the photophysical properties of the sunscreen agent menthyl anthranilate in various solvent systems are reported. Luminescence studies reveal that this ester is highly fluorescent (phi f = 0.64 +/- 0.06 in ethanol) and has a solvent-dependent emission maximum in the range of 390-405 nm. Phosphorescence has also been detected in low-temperature glasses with an emission maximum at 445 nm and a lifetime of 2.5 s. Kinetic UV-visible absorption measurements revealed a transient species with absorption maxima at 480 nm and solvent-dependent lifetimes of 26-200 microseconds that are attributed to the triplet state. The triplet state is efficiently quenched by oxygen, leading to the formation of singlet oxygen in all of the solvent systems studied. The singlet-oxygen quantum yields (phi delta) determined by time-resolved near-infrared luminescence measurements were determined to be in the range 0.09-0.12 for all systems.     

SINGLET OXYGEN YIELDS AND RADICAL CONTRIBUTIONS IN THE DYE-SENSITISED PHOTO-OXIDATION IN METHANOL OF ESTERS OF POLYUNSATURATED FATTY ACIDS (OLEIC, LINOLEIC, LINOLENIC AND ARACHIDONIC)

Abstract— The triplet state characteristics (spectrum, lifetime and quantum yield) for four dye sensi tisers [methylene blue (MB), erythrosin (ER), haematoporphyrin (HP) and riboflavin (RF)] were determined in methanol by laser flash photolysis and singlet oxygen yields (0.60 to 0.48) from time-resolved measurements of the 1270 nm near infrared emission. The reaction of singlet oxygen with four long chain unsaturated phenyl esters [oleate (18: 1), linoleate (18: 2), linolenate (18: 3) and arachidonate (20: 4)] was followed quantitatively using the singlet oxygen luminescence technique and also, after continuous420–700 nm irradiation, by HPLC and other analysis of the isomeric product monohydroperoxides. The overall quantum yield of photooxidation (∼10^-2) was shown to be consistent with the observed singlet oxygen quenching constants(2–12 times 10⁴ dm³ mol^-1 s^-1) for the four esters studied and the singlet oxygen lifetime in methanol (τ∼ 9 μs). The isomer product distribution was interpreted in terms of a dual singlet oxygen and radical mechanism, the radical contribution increasing with sensitiser in the order ER = MB < HP ≪ RF, but also showing some dependence on substrate unsaturation. Evidence is presented for singlet oxygen quenching by MB and RF ( k_O = 1.6 and 6.0 times 10⁷ dm³ mol^-1 s^-1) and for the accelerated photobleaching of the dye sensitisers in the presence of the unsaturated esters.     

PHOTOPHYSICAL PROPERTIES OF meso-TETRAPHENYLPORPHYRIN and SOME meso-TETRA(HYDROXYPHENYL)PORPHYRINS

Abstract— The o-, m-, and p-isomers of 5, 10, 15, 20- tetra(hydroxyphenyl)-porphyrin have been of recent interest as potential second-generation sensitisers in tumour phototherapy. Fluorescence spectroscopy, nanosecond laser flash photolysis and pulse radiolysis have been used to characterise the singlet and triplet excited states of tetraphenylporphyrin and the o, m-, and p-isomers of tetra(hydroxyphenyl)porphyrin. This has included evaluation of fluorescence yields and lifetimes, triplet spectra, lifetimes, oxygen quenching rate constants, extinction coefficients, and yields and singlet oxygen yields. Whilst the fluorescence quantum yields were low, the triplet yields were all 0.7 ± 10% and the singlet oxygen yields 0.6 ± 10%: all these parameters are in the ranges shown by other efficient porphyrin photosensitisers. The similar photophysical properties found for these compounds suggest that their differing tumour sensitising efficiencies are likely to be due to other factors.     

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.     

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm^?3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA‐acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet‐state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet‐triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile.     

pH DEPENDENCE OF SINGLET OXYGEN PRODUCTION IN AQUEOUS SOLUTIONS USING THIAZINE DYES AS PHOTOSENSITIZERS

Abstract— The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5–9. For methylene blue in aerated solutions, the production of ¹O₂* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the p K 's of the triplet dyes, by evaluating the yields of ¹O₂* from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.     

Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution

The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 +/- 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.     

Photochemistry and photophysics of thienocarbazoles

Two methylated thienocarbazoles and two of their synthetic nitro-precursors have been examined by absorption, luminescence, laser flash photolysis and photoacoustic techniques. Their spectroscopic and photophysical characterization involves fluorescence spectra, fluorescence quantum yields and lifetimes, and phosphorescence spectra and phosphorescence lifetimes for all the compounds. Triplet-singlet difference absorption spectra, triplet molar absorption coefficients, triplet lifetimes, intersystem crossing S1 --> T1 and singlet molecular oxygen yields were obtained for the thienocarbazoles. In the case of the thienocarbazoles it was found that the lowest-lying singlet and triplet excited states, S1 and T1, are of pi,pi* origin, whereas for their precursors S1 is n,pi*, and T1 is pi,pi*. In both thienocarbazoles it appears that the thianaphthene ring dictates the S1 --> T1 yield, albeit there is less predominance of that ring in the triplet state of the linear thienocarbazole, which leads to a decrease in the observed phiT value.     

TRIPLET EXCITATION TRANSFER INVOLVING β-IONONE. A KINETIC STUDY BY LASER FLASH PHOTOLYSIS

Abstract— Employing nanosecond laser flash photolysis, β-ionone (BI) has been examined as an acceptor and as a donor of triplet excitation. In the limit of diffusion-control as well as below it, the rate constants for the quenching of a series of sensitizer triplets by BI are2–3 times smaller than those by 2,4-hexadienal (HD), although the triplet energies (spectroscopic) of the two carbonyl-containing dienes are estimated to be the same (∼55 kcal mol^-1). We attribute the difference to a steric effect arising from ground-state geometric distortion and heavy alkyl-substitution in BI. In spite of possible exothermic energy transfer, BI triplet is nearly nonquenchable by azulene and ferrocene; this is explainable by torsional relaxation to an equilibrium geometry at which the vertical energy gap is smaller than 40 kcal mol^-1. The singlet oxygen yield from the interaction of BI triplet with oxygen in benzene is estimated to be 0.5, suggesting that spin-exchange and energy-transfer may be involved to the same extent in the oxygen quenching process.     

Laser study of photooxidation of chloranyl-sensitized 1,2,3,4-tetrachlorodibenzo-p-dioxine

The kinetics of quenching of the triplet state of chloranyl (CA) by 1,2,3,4-tetrachlorodibenzo-p-dioxine (TCD) in benzene and acetonitrile was studied by nanosecond laser flash photolysis. The reaction proceedsvia electron transfer (ET) with the rate constants of 1.5·10⁹ and 3.7·10⁹ L mol^?1 s^?1, respectively. In benzen ET results in the formation of short-lived triplet radical ion pairs (lifetime 100 ns). In acetonitrile, relatively long-lived (lifetime ≥10 μs) radical anion CA^.? and radical cation TCD^.? are formed and decay due to bimolecular reactions in the bulk of the solvent accompanied by the consumption of TCD.     

Effect of aggregation on bacteriochlorin a triplet-state formation: a laser flash photolysis study

Bacteriochlorin a (BCA) is a potential photosensitizer for photodynamic therapy of cancer. It has been shown previously that the photoefficiency of the dye is mainly dependent on singlet oxygen (1O2) generation. Nanosecond laser flash photolysis was used to produce and to investigate the excited triplet state of the dye in methanol, phosphate buffer and dimiristoyl-L-alpha-phosphatidylcholine (DMPC) liposomes. The transients were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yield of singlet-triplet intercrossing. The lifetime of the BCA triplet state was measured at room temperature. The triplet-state quantum yield is quite high in methanol (0.7) and in DMPC (0.4) but only 0.095 in phosphate buffer. In the last case, BCA is in a monomer-dimer equilibrium, and the low value of the quantum yield observed was ascribed to the fact the triplet state is only formed by the monomers.     

Photosensitizing properties of triplet beta-lapachones in acetonitrile solution

The photochemical reactivity of β-lapachone ( 1 ), nor -β-lapachone ( 2 ) and 1,2-naphthoquinone ( 3 ) towards amino acids and nucleobases or nucleosides has been examined employing the nanosecond laser flash photolysis technique. Excitation (λ = 355 nm) of degassed solutions of 1 – 3 , in acetonitrile, resulted in the formation of their corresponding triplet excited states. These transients were efficiently quenched by l -tryptophan, l -tryptophan methyl ester, l -tyrosine, l -tyrosine methyl ester and l -cysteine ( k _q  ≈ 10⁹ L mol⁻¹ s⁻¹). For l -tryptophan, l -tyrosine and their methyl esters new transients were formed in the quenching process, which were assigned to the corresponding radical pair resulting from an initial electron transfer from the amino acids or their esters to the excited quinone, followed by a fast proton transfer. No measurable quenching rate constants could be observed in the presence of thymine and thymidine. On the other hand, efficient rate constants were obtained when 1 – 3 were quenched by 2'-deoxyguanosine ( k _q  ≈ 10⁹ L mol⁻¹ s⁻¹). The quantum efficiency of singlet oxygen (¹O₂) formation from 1 to 3 was determined employing time-resolved near-IR emission studies upon laser excitation and showed considerably high values in all cases (Φ_Δ = 0.6), which are fully in accord with the ππ* character of these triplets in acetonitrile.     

Proton and electron transfer in the excited state quenching of phenosafranine by aliphatic amines

The quenching of the excited singlet and triplet states of phenosafranine by aliphatic amines was investigated in acetonitrile and methanol. The rate constants for the quenching of the excited singlet state depend on the one-electron redox potential of the amine suggesting a charge transfer process. However, for the triplet state, quenching dependence on the redox potential either is opposite to the expectation or there is not dependence at all. Moreover, in MeOH the first-order rate constant for the decay of the triplet state, k(obs) presents a downward curvature as a function of the amine concentration. This behavior was interpreted in terms of the reversible formation of an intermediate excited complex, and from a kinetic analysis the equilibrium constant K(exc) could be extracted. The log K(exc) shows a linear relationship with the pKb of the amine. On the other hand, for the triplet state quenching in acetonitrile k(obs) varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with pKb and not with the redox potential. The results were interpreted in terms of a proton transfer quenching, reversible in the case of MeOH and irreversible in MeCN. This was further confirmed by the transient absorption spectra obtained by laser flash photolysis. The transient absorption immediately after the triplet state quenching could be assigned to the unprotonated form of the dye. At later times the spectrum matches the semireduced form of the dye. The overall process corresponds to a one-electron reduction of the dye mediated by the deprotonated triplet state.     

Photochemistry and photopolymerization activity of novel perester derivatives of fluorenone: a conventional and laser flash photolysis study

The photochemistry of three novel t-butylperester derivatives of fluorenone was examined and compared with unsubstituted fluorenone and a mono-t-butylperester of benzophenone using both conventional microsecond and nanosecond laser flash photolysis. On conventional microsecond flash photolysis in 2-propanol, all four fluorenone compounds gave transient absorption in the region 300–400 nm due to a ketyl radical formed from the abstraction of a hydrogen atom from the solvent by the upper excited triplet n—π* state of the fluorenone chromophore. This assignment was confirmed by a pH-dependent study on the transient absorption spectra. The nitro-t-butylperester derivative of fluorenone gave additional absorption above 400 nm due to species associated with the nitro group. No evidence for benzoyloxy radical formation could be found in non-hydrogen-atom-donating solvents with microsecond flash photolysis which is associated with homolysis of the perester groups. On nanosecond laser flash photolysis of the fluorenone compounds at 355 nm excitation in acetonitrile and hexa-fluorobenzene, transient absorptions were observed in the region 320–640 nm due to the corresponding triplet states. All the t-butylperester derivatives showed residual absorbances at longer time delays which were tentatively assigned to the corresponding benzoyloxy radicals produced by homolysis of the perester groups. In contrast, the mono-t-butylperester of benzophenone, included for comparison only, showed very weak transient absorption in the region 320–640 nm compared with that of the strong triplet of benzophenone under the same excitation conditions. The triplet absorptions and lifetimes of the fluorenone compounds were correlated with their photopolymerization activities in bulk methylmethacrylate monomer. In oxygenated solutions, the triplet absorptions of fluorenone and benzophenone were effectively quenched; however, long-lived transient growths were observed for all the t-butylperester derivatives. The intensities of these novel transient absorptions appear to correlate with the total number of t-butylperester groups in the fluorenone molecule and tentative assignments are discussed.     

Photochemical studies of 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM).

Kinetic UV-VIS absorption data following 355 and 266 nm nanosecond laser flash photolysis of 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM) solutions is presented. The kinetics of the decay of the non-chelated enol (NCE) produced following 355 nm excitation of BM-DBM solutions are analysed in terms of mixed 1st- and 2nd-order kinetics. The temperature dependences of the component rate constants are unusual and both 1st- and 2nd-order components display negative activation energies, which are explained by invoking pre-equilibria. In addition, it is shown for the first time that 266 nm laser photolysis of BM-DBM solutions leads to formation of the triplet state of the keto (K) form with a lifetime of approximately 500 ns. Under these conditions the triplet state of the K form is quenched by oxygen.