Study of spectral and kinetic characteristics of products of photolysis of a trifunctional compound with light of different wavelengths

Spectral and kinetic characteristics of intermediate products of photolysis of a novel phototrifunctional compound (PTC), whose molecule combines spironaphthoxazine, hydroxyazomethine, and azobenzene fragments, have been studied by microsecond flash photolysis with UV and visible light in toluene and methanol solutions. Three products of PTC photolysis have been detected. It has been found that the efficiency of the photoprocesses substantially depends on the solvent and the wavelength of excitation light.     

Kinetics of photochemical reactions of a new biphotochromic compound upon photolysis with light of different wavelengths

Using the method of microsecond flash photolysis with UV and visible light, the spectral and kinetic characteristics of intermediate products of photolysis in toluene and methanol solutions of a new biphotochromic compound have been studied, in which two photochromic moieties, spironaphthoxazine and azomethine, are linked to each other in such a way that there is conjugation between π-electronic systems of the moieties in the ground state of the molecule. Two intermediate products have detected, whose relative efficiency of formation substantially depends on the solvent and the wavelength of excitation light.     

Synthesis and kinetics of photochemical reactions of novel bifunctional salicylideneiminospironaphthoxazines

A series of novel photochemically bifunctional compounds was synthesized. Their molecules combine the photochromic spironaphthoxazine fragment with a salicylideneimine fragment containing various substituents. The latter can undergo intramolecular proton transfer in the electron-excited state. Photolysis products of three types, namely, open merocyanine forms having an enol or cis-keto form of the salicylideneimine moiety and a trans-keto form with the closed spiro fragment, were detected using pulse photolysis technique in toluene and methanol solutions. The spectral kinetic characteristics of the photoproducts were studied, and their quantum yields were measured. The effects of substituents in the salicylideneimine fragment and the solvent nature were discussed.     

Spectral kinetic characteristics of the photoisomerization products of naphthylmethylideneiminospironaphthopyran induced by photolysis at different wavelengths

The spectral kinetic characteristics of intermediates generated by photolysis with light at the wavelengths 337 and 430 or 470 nm of the photobifunctional compound (PBC), 1,3-dihydro-5-(2-hydroxy-1-naphthylmethylideneimino)-1,3,3-trimethylspiro[2H-indole-2,3-[3H]-naphtho[2,1-b]pyran], whose molecule combines the spironaphthopyran and hydroxyazomethine fragments, and also parameters of model compounds, viz., naphthylmethylideneimine and spironaphthopyran, were studied in methanol and toluene. The relative quantum yields of formation of different intermediates of PBC were measured relatively to model compounds, namely, trans-keto isomer formed due to cis-trans-isomerization and prototropic equiliration in the azomethine fragment, and in the merocyanine form generated by spiro bond opening. It was found that the photolysis of the PBC with light at the wavelengths ?? = 430 or 470 nm nearly no produces the merocyanine form, whereas the relative yield of the trans-keto tautomer is ??0.6. For PBC photolysis at ?? = 337 nm, the yield of the merocyanine form is ??0.2 and the yield of the trans-keto isomer decreases substantially (??0.2). The solvent nature affects the kinetic behavior of the system. The consistency of the isomerization and proton transfer processes is discussed.     

Spectral and kinetic parameters of transient species in the photolysis of naphthylmethylideneiminospironaphthopyran by excitation at different wavelengths: Nano- and femtosecond laser photolysis

Intramolecular processes occurring in a photobifunctional compound (PBC) comprising the spironaphthopyran and hydroxyazomethine moieties have been studied in methanol solutions by femtosecond laser photolysis using light with wavelengths of 340 and 490 nm. At the excitation wavelength of 490 nm, the cis-trans photoisomerization in the azomethine moiety occurs in the S₁ state. In the case of PBC photolysis with 340-nm light, the opening of the spiro bond of the spiropyran moiety (formation of the X form) also takes place during relaxation of the S_n state to the S₁ state followed by isomerization to the merocyanine form. The spectral and kinetic characteristics of different electronically excited have been were determined. The data have been compared with those of nanosecond laser photolysis.     

Further investigation of the photodissociation dynamics of dichlorocarbene near 248 nm

A further investigation of the 248 nm photodissociation of CCl(2), which expands upon our original study of this process [S. K. Shin and P. J. Dagdigian, Phys. Chem. Chem. Phys. 8, 3446 (2006)], is presented. The CCl(2) parent molecule and the CCl photofragment were detected by laser fluorescence excitation in a molecular beam experiment. From the dependence of the CCl(2) signals on the photolysis laser fluence, attenuation cross sections of the 0(0), 1(1), and 2(1) vibrational levels were determined; the cross sections for the excited vibrational levels were found to be significantly smaller than those for the ground vibrational level. The previously observed fragment CCl bimodal rotational state distribution was found to arise from the photolysis of more than one parent molecule. At low CHCl(3) mole fractions in the gas supplied to the pyrolysis beam source, it was concluded that CCl(2) is the photolysis precursor for both low-J and high-J CCl fragments. On the basis of the dependence of the CCl signals on the photolysis laser fluence, ground and vibrationally excited CCl(2), respectively, were assigned as the precursors to these two classes of fragments. The photofragment excitation spectra for low-J and high-J CCl fragments from the photolysis of CCl(2) were recorded in the wavelength range around 248 nm; both were found to be structureless. The 248 nm photodissociation dynamics of CCl(2) is discussed in light of our experimental observations and quantum chemical calculations of the CCl(2) excited electronic states.     

Dynamics of photoprocesses in novel bifunctional salicylideneiminospironaphthoxazines

The dynamics of photochemical processes in a number of novel photobifunctional compounds, whose molecules combine a photochromic spironaphthoxazine fragment with the salicylideneimine moiety containing different substituents, was studied by the flash photolysis technique. Three products of photolysis of these compounds were detected, and the effects of the substituents and the solvent nature (toluene and methanol) on their spectral and kinetic characteristics and quantum yields were studied.     

THE USE OF SPECIFIC RADIOIMMUNOASSAYS TO DETERMINE ACTION SPECTRA FOR THE PHOTOLYSIS OF (6-4) PHOTOPRODUCTS

Abstract. A radioimmunoassay (RIA) was developed which specifically detects a photoproduct produced by the near-UV photolysis of pyrimidine(6-4)pyrimidone photoproducts. This assay was used in conjunction with a previously characterized RIA which specifically detects (6-4) photoproducts to determine the relative efficiency of wavelengths between 265 and 435 nm for photolysing these lesions. The rate of loss of antibody-binding sites associated with (6-4) photoproducts correlates with the production of those associated with its photolysis product. Action spectra for both the loss of (6-4) photoproducts and the induction of the photolysis product parallel the absorption spectrum of the (6-4) photoproduct.     

High-performance liquid chromatographic mass spectrometric identification of the photoproducts of cymoxanil

The photochemical degradation of the fungicide cymoxanil {2-cyano-N-[(ethylamino)carbonyl]-2-(methoxyimino) acetamide} was studied in aqueous buffer solution (pH 5.9+/- 0.1) under UV light and in laboratory conditions. The degradation followed a pseudo- first-order kinetic with significant correlation coefficient. The main photoproducts were separated and tentatively identified by HPLC/UV and HPLC/MS data, as 3-ethyl-4-(methoxyamino)-2,5-dioxo-4-imidazolidinecarbonitrile, 1-ethyl-5-(methoxyimino)-2,4-imidazolidin-2,4-dione, ethylimidazolidinetrione and {[(ethylamino)carbonyl]-amino}oxoacetic acid. A photolysis pathway of cymoxanil is proposed.     

Formation of the CH fragment in the 193 nm photodissociation of CHCl

The CH fragment from the 193 nm photodissociation of CHCl is observed in a molecular beam experiment. This fragment is formed in the higher-energy dissociation pathway, the lower pathway involving formation of CCl. Both the CHCl parent molecule and the CH fragment were detected by laser-induced fluorescence. The 193 nm CHCl absorption cross section was estimated from the reduction of the CHCl signal as a function of the photolysis laser fluence. The CH internal state distribution was derived from the analysis of laser-induced fluorescence spectra of the A-X Deltav=0 sequence. A modest degree of rotational excitation was found in the CH fragment; the most probable rotational level is N=1, but the distribution has a tail extending to N>25. Also observed is a slight preference for formation of Lambda-doublets of A(") symmetry, which appears to increase with increasing rotational angular momentum N. Vibrationally excited CH was observed, and the degree of vibrational excitation was found to be low. The energy available to the photofragments is predominantly released as translational excitation. The preferential formation of A(") Lambda-doublets suggests that dissociation occurs through a nonlinear excited state.     

离子速度影像法研究n-C7H15Br分子光解反应动力学

利用离子速度成像方法, 研究n-C7H15Br分子在231～239 nm范围内几个波长处的光解离动力学. 通过同一束激光经(2+1)共振多光子电离(REMPI)过程探测光解碎片Br(2P3/2)和Br*(2P1/2), 得到了不同激光波长处的离子速度分布图像, 从而获得C7H15Br光解产物的能量分配和角度分布. 结合各向异性参数和量子产率, 计算了n-C7H15Br分子在234 nm波长下不同解离通道的比例. 实验表明光解产物的能量分配可以用冲击模型中的软碰撞模型来解释. 实验还发现, 各向异性参数β(Br*)的值对光波长变化很敏感, 这是由电子激发态的绝热和非绝热过程决定的.     

A study of photochemistry of flavins in pyridine and with a donor

Abstract— Both anaerobic and aerobic photolysis of riboflavin in pyridine yielded several photoproducts, analogous to the photochemical reaction in aqueous solution. Lumiflavin was also photoreduced in pyridine (an electron donor) without decomposition of the isoall-oxazine ring Differences in the reactivity of excited singlet and triplet states with respect to formation of photoproducts have been confirmed in pyridine. In the photoreduction of riboflavin by N, N'-dimethyl-N-benzylethylene diamine, an initial rate with a higher relative quantum yield than that of the reaction at a later stage was observed both in water and pyridine. Photolysis in D₂O with the donor showed no significant solvent isotope effect. These results strongly suggest that the photolysis of riboflavin in water does not involve the water molecule in the primary photochemical act. A detailed mechanism of flavin photoreaction in water to account for solvent oxygen incorporation into a photoproduct (benzaldehyde) from N, N'-dimethyl-N-benzylethylene diamine has been proposed without involving photochemical splitting of water based on our results and molecular orbital computations. Preliminary results on the kinetics of free radical decay were obtained using an ESR spectrometer and the significance of the results are discussed.     

Asulam in Aqueous Solutions: Fate and Removal Under Solar Irradiation

The photodegradation of the pesticide asulam (methyl[(4-aminophenyl)sulfonyl]carbamate) in aqueous solutions (1.0 2 10 m 4 mol L m 1 = 23 mg L m 1 ) has been investigated with and without Fe(III). The asulam disappearance were measured by direct photolysis at 254 nm as a function of pH and oxygen concentration. Different photoproducts have been identified, among them a blue condensation product which was only observed upon selective direct excitation of asulam. In the presence of Fe(III) and by excitation at 365 nm, we obtained the complete mineralisation of asulam, while no complete transformation of organic carbons into CO 2 was observed by direct photolysis. The continuous formation of ” OH radicals generated from the excitation of Fe(III) species allowed the total mineralisation of asulam. Information is also given about the fate of asulam in aqueous solutions under solar irradiation.     

Near-infrared two-photon excitation of protoporphyrin IX: photodynamics and photoproduct generation

The spectroscopy and photochemistry of protoporphyrin IX in ethanol and in Triton X-100 micelle solution have been examined using near-infrared two-photon excitation (TPE). TPE will allow photodynamic therapy with highly localized light dosage. We have determined that the photochemistry subsequent to TPE is very similar to that found for one-photon excitation. Moreover, the photoproducts observed possess very intense TPE fluorescence spectra, which allows their detection at low relative concentrations.     

The photodissociation of NO(2) by visible and ultraviolet light

We present velocity map images of the NO, O((3)P(J)) and O((1)S(0)) photofragments from NO(2) excited in the range 7.6 to 9.0 eV. The molecule was initially pumped with a visible photon between 2.82-2.95 eV (440-420 nm), below the first dissociation threshold. A second ultraviolet laser with photon energies between 4.77 and 6.05 eV (260-205 nm) was used to pump high-lying excited states of neutral NO(2) and/or probe neutral photoproducts. Analysis of the kinetic energy release spectra revealed that the NO photofragments were predominantly formed in their ground electronic state with little kinetic energy. The O((3)P(J)) and O((1)S(0)) kinetic energy distributions were also dominated by kinetically 'cold' fragments. We discuss the possible excitation schemes and conclude that the unstable photoexcited states probed in the experiment were Rydberg states coupled to dissociative valence states. We compare our results with recent time-resolved studies using similar excitation and probe photon energies.     

Ice photolysis of 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100): Laboratory investigations using solid phase microextraction

Here, we report for the first time a laboratory investigation into the photochemical degradation of 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100) in ice solid samples using an artificial UV light source. Solid phase microextraction (SPME) was used as a sensitive extraction technique for monitoring trace amounts of the hydrophobic pollutant and its photoproducts. The results showed that ice photolysis kinetics for BDE-100 is similar to the one observed in the aqueous counterpart. The eight photoproducts identified consisted of brominated diphenyl ethers with lower bromine content and polybrominated dibenzofurans, suggesting two important photodegradation pathways for BDE-100 in ice solid samples: (i) stepwise reductive debromination and (ii) intramolecular elimination of HBr. Similarities in photochemical product arrays observed in the ice and water photolysis of BDE-100 were attributed to a similar mechanism for photochemical decomposition for both phases. Possible involvement of the water molecules in the reactions has been excluded by performing photolysis in D₂O ice solid and water samples. Taking advantage of the high preconcentration factor obtained with SPME at low temperatures, a SPME fiber cooled with liquid carbon dioxide down to 0 °C was used as a photoreaction support for BDE-100 allowing the identification of a greater number of photoproducts.     

Photolysis of Spiropentane with Xenon Resonance Line

Spiropentane was photolyzed using xenon 1470Å resonance light source at room temperature. The photolysis products were analyzed by means of FID-gas-chromatographic technique. Through the information about the yields of products, the major primary process of this light-molecule interaction was assigned as C-C bond breakage. Methylenecyclobutane, which has been an identified isomerization product of excited spiropentane molecule, was not able to be produced at present high excitation energy.     

THE PHOTOCHEMISTRY OF ADENOSINE: INTERMEDIATES CONTRIBUTING TO ITS PHOTODEGRADATION MECHANISM IN AQUEOUS SOLUTION AT 298 K AND CHARACTERIZATION OF THE MAJOR PRODUCT

Abstract— The steady-state (254 nm) photolysis of 9–(β-d-erythropentofuranosyl)adenine (adenosine) in aqueous solution was studied. Photodestruction yields on the order of 1.3 × 10⁻³⁾ were determined at room temperature by measuring the initial decrease in the absorption maximum as a function of irradiation time. The use of high performance liquid chromatography (HPLC) permitted a more exact determination of the yield (2.5 × 10³). The formation of photoproducts was also studied using HPLC. In the photolysis of 50 μ M aqueous solutions of adenosine under anaerobic conditions at least 11 stable photoproducts are formed that absorb at 260 nm, the wavelength of maximum absorption of adenosine. The major photoproduct was also isolated and characterized as adenine; its formation yield was determined to be 4.5 × 10⁴. This yield is affected by the presence of oxygen and by the initial concentration of adenosine employed. Fluorescence emission and excitation spectra were used to monitor the formation of highly fluorescent photoproducts that emit with maxima at 365, 398 , and 430 nm and absorb in the wavelength region of 240–380 nm.
The reactive species in the photodestruction mechanism were established using substrates that react selectively with the respective short-lived species. Photoionization is a primary photoprocess implied by these studies. The triplet state of adenosine also contributes to the photodestruction mechanism.     

Dynamics of the 193 nm photodissociation of dichlorocarbene

The dynamics of the 193 nm photodissociation of the CCl2 molecule have been investigated in a molecular beam experiment. The CCl2 parent molecule was generated in a molecular beam by pyrolysis of CHCl3, and both CCl2 and the CCl photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section was estimated from the reduction of the CCl2 signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A 2Delta-X 2Pi band system. Most of the energy available to the CCl(X 2Pi)+Cl fragments appears as translational energy. The CCl fragment rotational energy is much less than predicted in an impulsive model. The excited electronic state appears to dissociate indirectly, through coupling with a repulsive state arising from the ground-state CCl(X 2Pi)+Cl asymptote. The identity of the initially excited electronic state is discussed on the basis of what is known about the CCl2 electronic states.     

PHOTOSENSITIZATION BY METHOTREXATE PHOTOPRODUCTS

Abstract— Photoproducts induced upon excitation of methotrexate by UV light have been separated by ion exchange chromatography. They include 2,4-diamino-6-pteridinecarboxylic acid, 2,4-diamino-6-pteridine-carboxaldehyde and other unidentified pteridine derivatives. The same photoproducts can be also formed upon photodynamic reaction using hematoporphyrin as photosensitizer. In oxygen saturated aqueous solutions (pH∼7), methotrexate photoproducts sensitize the oxidation of histidine and tryptophan by UV light by a process involving singlet oxygen. In aqueous solutions containing albumin or in human serum, the same photoproducts are formed from free methotrexate but not from albumin-bound methotrexate. In the latter case the results may suggest that methotrexate covalently binds to albumin upon excitation with UV light either in absence or in presence of oxygen. These results could explain the photosensitization accompanying cancer chemotherapy with high dose methotrexate and also the synergistic effects of PUVA + low dose methotrexate in psoriasis therapy.