Photochemical Reaction Pathways Research Articles

Femtosecond laser-induced permanent anisotropy in bacteriorhodopsin films and applications in optical data storage

In polymeric films of bacteriorhodopsin (BR) a photoconversion product named F540-state, which is excited by 790 nm femtosecond laser pulses, is stable either for photochemical reaction or thermal pathway. The optical properties of the F540-state were studied, and Jones-matrix theory was adopted to analyze the photoinduced anisotropy of the F540-state. Based on the permanently photoinduced anisotropy, write-once-read-many (WORM) optical data storage was demonstrated by using two polarization states of femtosecond pulsed laser. Since the polarization information is also written on the storage media, it is impossible to copy it in a common way. This storage technique has a potential application in advanced optical security.

Detection of Unusual Reaction Intermediates during the Conversion of W(N2)2(dppe)2 to W(H)4(dppe)2 and of H2O into H2

W(N(2))(2)(dppe-κ(2)P)(2) reacts with H(2) to form WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P) and then W(H)(4)(dppe-κ(2)P)(2). When para-hydrogen is used in this study, polarized hydride signals are seen for these two species. The reaction is complicated by the fact that trace amounts of water lead to the formation of H(2), PPh(2)CH(2)CH(2)Ph(2)P(O) and W(H)(3)(OH)(dppe-κ(2)P)(2), the latter of which reacts further via H(2)O elimination to form W(H)(4)(dppe-κ(2)P)(2) and [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)]. These studies demonstrate a role for the 14-electron intermediate W(dppe-κ(2)P)(2) in the CH activation reaction pathway leading to [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-k(2)P}(dppe-k(2)P)]. UV irradiation of W(H)(4)(dppe-κ(2)P)(2) under H(2) led to phosphine dechelation and the formation of W(H)(6)(dppe-k(2)P)(dppe-k(1)P) rather than H(2) loss and W(H)(2)(dppe-κ(2)P)(2) as expected. Parallel DFT studies using the simplified model system W(N(2))(2)((Ph)HPCH(2)CH(2)PH(2)-κ(2)P)(H(2)PCH(2)CH(2)PH(2)-κ(2)P) confirm that ortho-metalation is viable via both W(dppe-κ(2)P)(2) and W(H)(2)(dppe-κ(2)P)(2) with explicit THF solvation being necessary to produce the electronic singlet-based reaction pathway that matches with the observation of para-hydrogen induced polarization in the hydride signals of [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)], W(H)(3)(OH)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) during this study. These studies therefore reveal the existence of differentiated and previously unsuspected thermal and photochemical reaction pathways in the chemistry of both W(N(2))(2)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) which have implications for their reported role in N(2) fixation.

Photochemical Degradation of Polybrominated Diphenyl Ethers in Micro Photo-Reactor

Polybrominated diphenyl ethers (PBDE) were widely used as flame retardants in plastics. There are highly persistent and bio-accumulative toxicants that came to the attention after their huge quantities had been released into the environment. Due to distinct similarity of PBDE to the polychlorinated biphenyls (PCB), the PBDE present a potential health hazard. In order to decrease the environmental impact of PBDEs and to stop their spread in the food chain the different degradation methods have been investigated. This work is focused on verification and comparison of the various dehalogenation methods of PBDE. In particular, the photochemical degradation reaction pathways of different selected congeners substituted with 1 to 10 bromines, in solvents are examined. Photochemical reaction is a non-invasive method with a high degradation potential. The photochemical reaction is conducted in batch or continuous reactors. Comparison of experimental results based on determination of individual reaction products shows the efficiency of the studied photochemical methods running in both reactor arrangements. New photochemical method employing the photochemical microreactor was investigated that has not been reported in the literature so far. The original experimental results confirm the significantly faster and deeper degradation rate of PBDE's compared to the published results of the photocatalysis over nanoporous TiO 2 in microwave field and the conventional photochemical reactors.

A new photoproduct of 5-methylcytosine and adenine: a theoretical study

Both the cycloaddition mechanism of 5-methylcytosine with adenine and the deamination mechanism of the cycloaddition product have been studied using density functional theory method. The results suggest that the cycloaddition reaction could occur more easily through photochemical reaction pathway than through thermal reaction pathway. The obtained four-member ring structure could be easily transformed to an eight-member ring structure through bond cleavage of C5–C6 (the energy barrier is <2 kcal/mol). Then hydrolytic deamination reaction takes place with water assistance. The hydroxyl group of one water molecule attacks the C4′ atom and the hydrogen atom of another water molecule attacks N3′ atom to form a tetrahedral intermediate. Subsequently, the hydrogen atom of hydroxyl group transfers to N8′ to produce ammonia, and the amino group of the former 5-methylcytosine changes to carboxyl oxygen. Our calculations explain the phenomena that 5-methylcytosine and adenine could obtain the same photoproduct as thymine and adenine from theoretical aspects.

A short overview of the microbial population in clouds: Potential roles in atmospheric chemistry and nucleation processes

Recent studies showed that living microorganisms, including bacteria, fungi and yeasts, are present in the atmospheric water phase (fog and clouds) and their role in chemical processes may have been underestimated. At the interface between atmospheric science and microbiology, information about this field of science suffers from the fact that not all recent findings are efficiently conveyed to both scientific communities. The purpose of this paper is therefore to provide a short overview of recent work linked to living organisms in the atmospheric water phase, from their activation to cloud droplets and ice crystal, to their potential impact on atmospheric chemical processes. This paper is focused on the microorganisms present in clouds and on the role they could play in atmospheric chemistry and nucleation processes. First, the life cycle of microorganisms via the atmosphere is examined, including their aerosolization from sources, their integration into clouds and their wet deposition on the ground. Second, special attention is paid to the possible impacts of microorganisms on liquid and ice nucleation processes. Third, a short description of the microorganisms that have been found in clouds and their variability in numbers and diversity is presented, emphasizing some specific characteristics that could favour their occurrence in cloud droplets. In the last section, the potential role of microbial activity as an alternative route to photochemical reaction pathways in cloud chemistry is discussed.

Photochemistry of six‐ and five‐membered‐ring α,β‐unsaturated lactones in cryogenic matrices

AbstractThe photochemistry of representative six and five‐membered α,β‐unsaturated lactones [α‐pyrone and some of its derivatives, including coumarin and 3‐acetamidocoumarin, 2(5H)‐furanone] isolated in cryogenic inert matrices has been investigated by infrared spectroscopy and quantum chemical calculations. In these types of molecules, two main competitive photochemical reaction pathways could be identified: ring opening, leading to formation of the isomeric aldehyde‐ketenes, and ring contraction to the corresponding Dewar isomers. For α‐pyrone and 2(5H)‐furanone, the ring‐opening process dominates over the ring‐contraction reaction, the same occurring for derivatives of these compounds bearing a voluminous substituent at position 3. In 2(5H)‐furanone, the ring‐opening reaction requires the simultaneous occurrence of a [1,2]‐hydrogen atom migration. Nevertheless, it was found to be an easy process upon excitation at λ &gt; 235 nm. The ring‐opening reaction was also found to occur much easily in α‐pyrone than in coumarin, and factors explaining this observation were discussed. In turn, the Dewar forms of the studied compounds resulting from the ring‐contraction photoreaction were found to undergo subsequent photo‐elimination of CO2, with formation of the corresponding cycloalkenes. In the matrices, CO2 and the simultaneously formed cycloalkenes were found to exist as associated forms, in which the CO2 molecule is preferentially placed over the cycloalkene ring in a stacked‐type geometry. For coumarin, a third photoreaction channel was observed, leading to the formation of benzofurane and CO. This additional reaction channel corresponds to the photoreaction previously observed for the compound in the gaseous phase. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Photochemical Reaction Pathways of Carbon Tetrabromide in Solution Probed by Picosecond X-ray Diffraction

We report a liquid-phase time-resolved X-ray diffraction study that resolves the molecular structures of the short-lived intermediates formed in the photodissociation of tetrabromomethane in methanol. Time-resolved X-ray diffraction can detect all chemical species simultaneously, and the diffraction signal from each chemical species can be quantitatively calculated from molecular structures and compared with experimental data with high accuracy and precision. The photochemistry of carbon tetrahalides has long been explored to describe their reactions in the natural environment due to its relevance to ozone depletion. Excited with an ultraviolet optical pulse, the complicated photodissociation dynamics of CBr4 was followed in a wide temporal range from picoseconds up to microseconds and associated rate coefficients were determined by analyzing time-resolved diffraction patterns accumulated from 100 ps X-ray pulses. The homolytic cleavage of one C-Br bond in the parent CBr4 molecule yields the CBr3 and Br radicals, which escape from the solvent cage and combine nongeminately to form C2Br6 and Br2, respectively. C2Br6 eventually decays to give C2Br4 and Br2 as final stable products. Our diffraction data at the current signal-to-noise ratio could not provide any evidence for the geminate recombination of the CBr3 and Br radicals to form the Br2CBr-Br isomer or the solvated ion pair, implying that their formation is a minor channel compared with those observed clearly by time-resolved diffraction in this work.

Multiple Photochemical Reaction Pathways in a Ni(II) Coordination Compound

The gas-phase photofragmentation of the mixed-ligand coordination compound trans-bis(trifluoroacetato)bis(N,N'-dimethylethylenediamine)nickel(II) (Ni(tfa)2(dmen)2) detected via time-of-flight mass spectrometry is reported. In contrast to most gas-phase studies of metal-containing compounds where fragmentation of weak metal-ligand bonds dominates, the data here show that the dmen ligands fragment while still coordinated to nickel. The manner in which these ligands fragment is highly specific, leading to mono- and diimine species that remain coordinated to nickel. Uncoordinated mono- and diimine species and various small dmen fragments are also observed with high intensities in the low mass region of the spectra. NiF+, a fragment that is formed by fluorine abstraction, is always observed, even though no Ni-F bonds exist in the starting material.

Alkali Ion Exchanged Nafion as a Confining Medium for Photochemical Reactions†

Methanol-swollen Nafion beads were used as microreactors to control the photochemical reaction pathways. Product selectivity in three unimolecular reactions, namely, the photo-Fries rearrangement of naphthyl esters, Norrish Type I reaction of 1-phenyl-3-p-tolyl-propan-2-one and Norrish Type I and Type II reactions of benzoin alkyl ethers were examined. The influence of cations over the photodimerization of acenaphthylene and cross-photodimerization between acenaphthylene and N-benzyl maleimide included within Nafion were also examined. The photochemical behaviors of the above substrates were significantly altered within Nafion compared with their solution photochemistry. Of particular interest, the product distributions were found to depend on the counter cations of Nafion.

Photochemical processes on Titan: Irradiation of mixtures of gases that simulate Titan's atmosphere

Photochemical reaction pathways in Titan's atmosphere were investigated by irradiation of the individual components and the mixture containing nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene. The quantum yields for the loss of the reactants and the formation of products were determined. Photolysis of ethylene yields mainly saturated compounds (ethane, propane, and butane) while photolysis of acetylene yields the same saturated compounds as well as ethylene and diacetylene. Irradiation of cyanoacetylene yields mainly hydrogen cyanide and small amounts of acetonitrile. When an amount of methane corresponding to its mixing ratio on Titan was added to these mixtures the quantum yields for the loss of reactants decreased and the quantum yields for hydrocarbon formation increased indicative of a hydrogen atom abstraction from methane by the photochemically generated radicals. GC/MS analysis of the products formed by irradiation of mixtures of all these gases generated over 120 compounds which were mainly aliphatic hydrocarbons containing double and triple bonds along with much smaller amounts of aromatic compounds like benzene, toluene and phenylacetylene. The reaction pathways were investigated by the use of 13C acetylene in these gas mixtures. No polycyclic aromatic compounds were detected. Vapor pressures of these compounds under conditions present in Titan's atmosphere were calculated. The low molecular weight compounds likely to be present in the atmosphere and aerosols of Titan as a result of photochemical processes are proposed.

Ab Initio Study of the Photochemistry of c-C2H2Si

The photochemical reaction pathway of silacyclopropenylidene (c-C2H2Si) has been investigated using the eight electrons in eight orbitals complete active space with the 6-311++G(3df,3pd) basis sets. The mechanism of drastic structural change in the reaction of c-C2H2Si and the difference from the reaction of cyclyopropenylidene (c-C3H2) are elucidated. The photochemically active relaxation path of c-C2H2Si on the S1 excited-state potential surface leads to an S1/S0 conical intersection where the photoexcited system decays nonradiatively to S0. The relaxation of c-C2H2Si on the S1 surface causes the cleavage of the Si−C single bond, while that of c-C3H2 causes the cleavage of the C−C double bond. The difference in photochemical cleavage sites is well explained by the difference in the electronic nature of the S1 excited state. In the dark reaction following the relaxation on the S1 surface, hydrogen migration from carbon to silicon occurs together with ring opening at the Si−C bond.

Control of thermal and photochemical reaction pathways by NO2 pendants in macrocyclic complexes: The redox photochemistry of Cu(10-methyl-1,4,8,12-tetraazacyclopentadecan-10-Y)2+, Y = NO2, NH2, in charge transfer and ligand field excited states

The irradiation of Cu(10-methyl-1,4,8,12-tetraazacyclopentadecan-10-NO2)X+, X = halide, HCO2−, CH3CO2− and C6H5CH2CO2−, in deaerated CH3CN or CH3OH at 350nm resulted in the reduction of the pendent NO2 to NO. Flash photolysis revealed the formation of the nitroso products via the photo-induced oxidation of the axial ligand X−. Intermediates in a 10ns–1s time scale have been tentatively assigned as species formed by the addition of the X radicals to the NO2. Similar redox processes were observed when the irradiation of the complexes was carried out in the ligand field band, λexc = 560nm. Conversion of the ligand field excited states to X− to Cu(II) charge transfer excited state account for the photoreactivity of the former excited states.

Control of thermal and photochemical reaction pathways by &amp;z.sbnd;NO2 pendants in macrocyclic complexesThe redox photochemistry of Cu(10-methyl-1,4,8,12-tetraazacyclopentadecan-10-Y)2+, Y = NO2, NH2, in charge transfer and ligand field excited states

Abstract The irradiation of Cu(10-methyl-1,4,8,12-tetraazacyclopentadecan-10-NO 2 )X + , X = halide, HCO 2 − , CH 3 CO 2 − and C 6 H 5 CH 2 CO 2 − , in deaerated CH 3 CN or CH 3 OH at 350 nm resulted in the reduction of the pendent NO 2 to NO. Flash photolysis revealed the formation of the nitroso products via the photo-induced oxidation of the axial ligand X − . Intermediates in a 10 ns–1 s time scale have been tentatively assigned as species formed by the addition of the X radicals to the NO 2 . Similar redox processes were observed when the irradiation of the complexes was carried out in the ligand field band, λ exc = 560 nm. Conversion of the ligand field excited states to X − to Cu(II) charge transfer excited state account for the photoreactivity of the former excited states.

Photochemical Modification of Cross-Linked Poly(dimethylsiloxane) by Irradiation at 172 nm

Cross-linked poly(dimethylsiloxane) (PDMS) was irradiated with a Xe2*-excimer lamp (172 nm) under ambient conditions. The irradiation in combination with the formed ozone results in an oxidation of PDMS to SiO2 at the polymer-air interface. The surface properties of the irradiated surfaces were studied by means of contact angle measurements, infrared spectroscopy, and X-ray photoelectron spectroscopy. The photochemical conversion of surface methylsilane groups to silanol groups is responsible for the large increase in surface free energy. Subsequent degradation of the polymer and formation of SiOx was monitored by infrared spectroscopy. As determined by X-ray photoelectron spectroscopy, the binding energy shifts reach values corresponding to SiO2. The atomic ratio concentration O:Si changes from about 1:1 (PDMS) to about 2:1 (SiO2). On the basis of the XPS and IR results, the photochemical reaction pathway from PDMS to silicon oxide via surface silanol groups is discussed. The strict linearity of the contact angle versus irradiation time and the clear dependence from irradiation intensity allows the tuning of the chemical surface functionalities.

On the symmetry of the xyz vibration in the photochemistry of Pt(II) D4h complexes

A symmetry concept is proposed that links the presence of vibronic structure in UV-visible spectra in d-systems, with the identification of a single photochemical reaction pathway. It is shown that the vibronic structure on some bands of the Pt(II) D4h complexes is due to the non- degenerate, ungerade b1u (xyz) vibration that points on an intramolecular twisting process for the photochemical cis-trans isomerization. The reaction coordinate is unambiguously defined in a coordinate system where x and y bisect the metal-ligand ML2 angle.

New caged coumarin fluorophores with extraordinary uncaging cross sections suitable for biological imaging applications.

Photocaged fluorescent molecules are important research tools for tracking molecular dynamics with high spatiotemporal resolution in biological systems. We have designed and synthesized a new class of caged coumarin fluorophores. These coumarin cages displayed more than 200-fold fluorescence enhancement after UV photolysis. Remarkably, the uncaging cross section of a 1-(2-nitrophenyl)ethyl (NPE)-caged coumarin is 6600 at wavelength of 365 nm, about 2 orders of magnitude higher than previously described caged fluorophores. Product analysis of the photolytic reaction showed clean conversion of NPE-caged coumarin to 2-nitrosoacetophenone and the parent coumarin, suggesting that the mechanism of the photolysis follows the known photochemical reaction pathway of the 2-nitrobenzyl group. We have also measured the two-photon uncaging cross sections of NPE-caged coumarins 2a and 5 at 740 nm to be near 1 Goeppert-Mayer (GM). The mechanistic study, together with the two-photon uncaging data, suggested that the coumarin moiety serves as an antenna to enhance the light harvesting efficiency of the coumarin cage and that the photonic energy absorbed by coumarin was utilized efficiently to photolyze the NPE group. Future explorations of this type of "substrate-assisted photolysis" may yield other cages of high uncaging cross sections. For cellular imaging applications, we prepared a cell permeable and caged coumarin fluorophore, NPE-HCCC2/AM (10), which can be loaded into fully intact cells to high concentrations. Initial tests of this probe in a number of cultured mammalian cells showed desired properties for the in vivo imaging applications. The combined advantages of robust fluorescence contrast enhancement, remarkably high uncaging cross sections, noninvasive cellular delivery, and flexible chemistry for bioconjugations should generate broad applications of these caged coumarins in biochemical and biological research.

Studies on enamides. Part-7: a novel photochemical reaction pathway of N-aroyl-N-1-cyclohexenylanilines

UV irradiation of N-aroyl-N-1-cyclohexenyl-anilines involves a cleavage of vinyl nitrogen bond under oxidative condition, whereas 6H-5-phenyl/aryl-1,2,3,4-tetrahydrophenanthridine-6-ones are obtained when the reaction is performed non-oxidatively.

Photochemical reactivity of siderophores produced by marine heterotrophic bacteria and cyanobacteria based on characteristic Fe(III) binding groups

Siderophores, high‐affinity Fe(III) ligands produced by microorganisms to facilitate iron acquisition, might contribute significantly to dissolved Fe(III) complexation in ocean surface waters. In previous work, we demonstrated the photoreactivity of the ferric ion complexes of several α‐hydroxy carboxylic acid—containing siderophores produced by heterotrophic marine bacteria. Here, we expand on our earlier studies and detail the photoreactivity of additional siderophores produced by both heterotrophic marine bacteria and marine cyanobacteria, making comparisons to synthetic and terrestrial siderophores that lack the α‐hydroxy carboxylate group. Our results suggest that, in addition to secondary photochemical reaction pathways involving reactive oxygen species, direct photolysis of Fe(III)‐siderophore complexes might be a significant source of Fe(II) and reactive Fe(III) in ocean surface waters. Our findings further indicate that the photoreactivity of siderophores is primarily determined by the chemical structure of the Fe(III) binding groups that they possess—hydroxamate, catecholate, or α‐hydroxy carboxylate moieties. Hydroxamate groups are photochemically resistant regardless of Fe(III) complexation. Catecholates, in contrast, are susceptible to photooxidation in the uncomplexed form but stabilized against photooxidation when ferrated. α‐Hydroxy carboxylate groups are stable as the uncomplexed acid, but when coordinated to Fe(III), these moieties undergo light‐induced ligand oxidation and reduction of Fe(III) to Fe(II). These photochemical properties appear to determine the reactivity and fate of Fe(III)‐binding siderophores in ocean surface waters, which in turn might significantly influence the biogeochemical cycling of iron.

Quantum-chemical study of competitive photochemical reaction pathways in N-methyldiphenylamine derivatives

The electronic structures of diphenylamine derivatives Ph 2NCH 3− n Ph n , where n=0–3, in the ground and lowest excited states were studied by semi-empirical PM3 method. A priori these compounds can show two kinds of photochemical reactivity: photocyclization of the diphenylamine group and photodissociation of the N–C bond. To discriminate these possibilities, two cross-sections of the potential energy surface of the triplet excited state for every compound were studied, and the activation energies ( E a) for the two competing reactions were calculated. Comparison of E a values shows, that activation energy for the cyclization reaction remains nearly constant within 20.5±0.6 kcal/mol for all the compounds studied, whereas E a value for the dissociation reaction reduces from 27.6 kcal/mol for Ph 2NCH 3 to 12.6 kcal/mol for Ph 2NCPh 3. Thus, PM3 calculations predict predominantly photocyclization of the diphenylamine group for Ph 2NCH 3 and predominantly photodissociation of the N–C bond for Ph 2NCPh 3, in full accordance with the experimental data.

Controlled photocyclization, photodimerization, and photoisomerization of stilbazole salts within Nafion membranes.

[reaction: see text] Water- and methanol-swollen Nafion membranes were used as microreactors to successfully control the photochemical reaction pathway of stilbazole derivatives. Of particular interest is the production of azaphenanthrene (the product not obtainable in homogeneous solution) in high yield under high bulk concentration.