Photochemical generation and trapping of radicals from bisphenyl carbodiazone

UV irradiation at 350 nm of bisphenyl carbodiazone creates nitrogen, phenyl and (phenyldiazenyl)carbonyl radicals under mild conditions. The radicals formed were trapped by olefins and tetramethylpiperidinoxyl radicals.     

Photochemical generation of nitrenium ions from protonated 1,1-diarylhydrazines

[reaction: see text] Laser flash photolysis experiments, chemical trapping studies, and time-dependent density functional theory calculations demonstrate that photolysis of protonated 1,1-diarylhydrazines generates N,N-diarylnitrenium ions.     

Photochemical generation of benzene-naphthalene biplanemers

A direct biplanemer synthesis could be achieved by a quantitative [4π+4π]photocycloaddition of benzyloxy substituted benzene and naphthalene rings (3d,e ? 4d,e). Since the products do not show a thermal Cope rearrangement—in contrast to the related longicyclic conjugated 9,10-benzotricyclo[4.2.2.2^2,5]dodeca-3,7,9-trienes—but a quantitative reverse reaction, they are interesting systems for an optical switching.     

Photochemical generation of a new, highly fluorescent compound from non-fluorescent resveratrol

UV irradiation of trans-resveratrol leads to its photochemical transformation to a new, highly fluorescent compound, whose chemical structure was unambiguously identified. The new compound has large values of fluorescence quantum yield, Stokes' shift, and two-photon absorption cross section, which make it suitable for bio-imaging, multi-color labeling, and two-photon microscopy.     

Photochemical generation of nitric oxide from 6-nitrobenzo[a]pyrene

Photolabile 6-nitrobenzo[a]pyrene (6-nitroBaP) released nitric oxide (NO) under visible-light irradiation. The generation of NO and the concomitant formation of the 6-oxyBaP radical were confirmed by ESR. BaP quinones were also detected as further oxidized products of the 6-oxyBaP radical. No such photodegradation was observed with other nitrated BaPs, such as 1-nitroBaP and 3-nitroBaP. DNA-strand breakage, caused by photoexcited 6-nitroBaP, was closely related to its NO-releasing activity. MO calculations of nitrated BaP suggest that the perpendicular conformation of the nitro substituent to the aromatic ring is important for the release of NO with light. These findings may be useful for the development of a new type of NO donor.     

Photochemical generation of tetramethyldisilene and photoinduced 1,2-silyl-migration

Described herein is the first clear evidence of the photochemical generation of tetramethyldisilene which can be efficiently trapped by dienes. An intriguing 1,2-photo-induced silyl migration is also reported.     

Photochemical generation and matrix-isolation detection of dimethylvinylidene.

We report the spectroscopic characterization of dimethylvinylidene, (CH3)2C=C:, generated within an argon matrix at 14 K from a bisperoxyester precursor. The carbene was identified by comparison of the experimental IR spectrum with vibrational frequencies computed at the B3LYP/6-31G(d) level. Chemical trapping of the carbene within a 9% CO/Ar matrix to form dimethylpropadienone supports this analysis. Additional products produced during photolysis were identified by comparison to the appropriate computed vibrational frequencies. The potential energy surface of dimethylvinylidene and its intramolecular rearrangement products, 2-butyne and methylcyclopropene, were also investigated computationally at the B3LYP/6-31G(d) level. A spin-state analysis of this carbene using a variety of computational methods (CCSD(T), B3LYP, MP2) indicates the singlet state is more stable than the triplet by approximately 45 kcal mol-1. We anticipate the bisperoxyester precursor used here will be a convenient and general way for initiating future studies of alkylvinylidenes under matrix-isolation conditions.     

Photochemical upconversion approach to broad-band visible light generation

The sensitized triplet-triplet annihilation (TTA) of 9,10-dimethylanthracene (DMA) upon selective excitation of [Ru(dmb)3]2+ (dmb = 4,4'-dimethyl-2,2'-bipyridine) at 514.5 nm in dimethylformamide (DMF) resulted in upconverted and downconverted DMA excimer photoluminescence. The triplet excited state of [Ru(dmb)3]2+ is efficiently quenched by 11 mM DMA in DMF resulting in photon upconversion but no excimer formation. The bimolecular quenching constant of the dynamic quenching process is 1.4 x 109 M-1 s-1. At 90 mM DMA, both upconversion and downconversion processes are readily observed in aerated DMF solutions. The TTA process was confirmed by the quadratic dependence of the upconverted and downconverted emission emanating from the entire integrated photoluminescence profile (400-800 nm) of DMA measured with respect to incident light power. Time-resolved emission spectra of [Ru(dmb)3]2+ and 90 mM DMA in both aerated and deaerated DMF clearly illustrates the time-delayed nature of both types of singlet-state emission, which interestingly shows similar decay kinetics on the order of 14 mus. The emission quantum yields (Phi) measured using relative actinometry increased with increasing DMA concentrations, reaching a plateau at 3.0 mM DMA (Phi = 4.0%), while at 90 mM DMA, the overall quantum yield diminished to 0.5%. The dominant process occurring at 3.0 mM DMA is upconversion from the singlet excited state of DMA, whereas at 90 mM DMA, both upconversion and excimeric emission are observed in almost equal portions, thereby resulting in an overall broad-band visible light-emission profile.     

Photochemical generation of dihydrogen complexes of chromium and tungsten

Photolysis of solutions of M(CO)(6) (M = Cr, W) at low temperature in the presence of hydrogen gas affords Cr(CO)(5)(H(2)) (1) and W(CO)(5)(H(2)) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T(1) values for the hydride resonances and the observation of a large HD coupling in the HD derivatives. Irradiation of a phosphine-substituted derivative (PMe(3))Cr(CO)(5) in the presence of hydrogen gas gave similar results. Thus cis-(PMe(3))Cr(CO)(4)(H(2)) (3) and trans-(PMe(3))Cr(CO)(4)(H(2)) (4) were prepared and characterized by (1)H and (31)P NMR spectroscopy. When the photolysis reactions were carried out in methylene chloride, solvent binding competitive with hydrogen binding was observed. This was not observed in less coordinating solvents such as alkanes. Subsequent displacement of solvent by H(2) leads to the dihydrogen complexes. Complexes 1 and 2 are moderately acidic, with deprotonation effected by mild bases.     

Photochemical generation of vinylketenes by electrocyclic opening of cyclobutenones

4,4-Dich1oro-2-cyclobutenones , available by cycloaddition of acetylenes to dichloroketene, can be photolyzed in the presence of olefins or dienes to produce regiospecifically 2-vinylcyclobutanones . The photochemical reaction succeeds even in some cases where thermolysis fails and was shown to proceed by electrocyclic ring opening to a vinylketene. Stereochemically the results of photolysis of differ from thermolysis with identical olefinic substrates.     

Photochemical generation of ribose abasic sites in RNA oligonucleotides

[reaction: see text] A general method for the photochemical generation of ribose abasic sites within RNA oligonucleotides is reported. Photochemically caged nucleoside phosphoramidite analogues were prepared and incorporated into RNA oligonucleotides by automated RNA synthesis. Irradiation of the modified RNA at 350 nm efficiently produced ribose abasic sites at specific sites within RNA sequences. The current approach offers a chemical route to RNA abasic lesions for RNA biochemical studies.     

Photochemical vapor generation of selenium: Mechanisms and applications

Selenium was the first reported element that could be converted into its volatile compounds via photochemical vapor generation (photo-CVG) process before its atomic spectrometric detection. Photo-CVG is a newly emerging vapor generation technique, offering its inherent advantages of matrix interferences eliminated and high vapor efficiency etc., photo-CVG has been combined with various methods for selenium determination and mechanism exploration. Herein, we summarize the development of selenium in photo-CVG from the first report in 2003, the mechanisms of selenium with or without TiO₂ were discussed and its applications for selenium determination, speciation analysis and prereduction were summarized.     

Photochemical generation of six- and five-membered cyclic vinyl cations

The photochemical solvolyses of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate (1) and cyclopent-1-enyl(phenyl)iodonium tetrafluoroborate (2) in methanol yield vinylic ethers and vinylic cycloalkenyliodobenzenes and cycloalkenylbenzene, which are the trapping products of the geometrically destabilized C6-ring and C5-ring vinyl cation with the solvent and with the leaving group iodobenzene. Iodonium salt 2 also yields an allylic ether and allylic cyclopentenyliodobenzenes and cyclopentenylbenzene, which are the trapping products of the C5-ring allylic cation produced from the C5-ring vinyl cation by a hydride shift in a typical carbocationic rearrangement.     

Photochemical generation and reactivity of carbenes within an organic cavitand and capsule: photochemistry of adamantanediazirines

Chemical behavior of carbenes (adamantylidenes) generated by photolysis of adamantanediazirines has been investigated while they were incarcerated within an organic container in water and on silica surfaces. Confined carbenes behave differently from the free ones, and their behavior is dictated by the nature and the structure of the host-guest complexes. The substituent present on the adamantyl skeleton controls the stoichiometry (1:1 or 2:2) and the orientation of guest molecules within the host.     

Photochemical generation of carbonate radicals and their reactivity with phenol

The reaction of carbonate radical with phenol in aqueous solution has been investigated in systems in which carbonate radicals were generated by UV irradiation of an aqueous solution of [Co(NH₃)₅CO₃]⁺ (pH 8.0 phosphate buffer). Both steady state and time resolved photolysis experiments were performed. Upon continuous irradiation of complex phenol mixtures, phenol was converted into benzoquinone and dihydroxybenzenes. Benzoquinone was the major by-product in the early stages of the reaction. Laser flash excitation (266 and 355 nm) of the cobalt complex clearly showed the formation of the carbonate radical. When phenol was added to the solution of the complex, a second species was observed which was assigned to the phenoxyl radical. The second-order rate constant of reaction between phenol and carbonate radical was found to be equal to 1.6 × 10⁷ M⁻¹ s⁻¹, in agreement with literature data of 2.2 × 10⁷ M⁻¹ s⁻¹.     

光化学蒸气发生方法用于水样中汞的可视化检测

汞与碘化亚铜的络合显色反应已被广泛应用于汞的可视化检测。然而,样品中高盐、强酸或强碱性的复杂基质对碘化亚铜及络合物的稳定性影响较大,可视化测定受到了严重干扰。光化学蒸气发生是一种绿色、高效的样品引入技术,可实现待测离子与样品基质的有效分离,并且在蒸气发生过程中不需要添加有毒、强还原性试剂。因此,本实验建立了一种光化学蒸气发生和自制碘化亚铜试纸结合用于可视化检测水样中汞的方法,对反应时间、低分子量有机酸浓度等条件进行了考察。在最优的条件下实现了1 ng·mL^-1～1μg·mL^-1浓度范围内水样中汞的可视化半定量分析,并通过原子荧光光谱法对分析结果进行了验证。     

Photochemical generation of aliphatic radicals from benzophenone oxime esters: simple synthesis of alkylbenzenes and alkylpyridines

Photolysis of benzophenone oxime esters, prepared with aliphatic carboxylic acids and benzophenone oxime, in benzene and pyridine generates various primary, secondary and tertiary aliphatic radicals selectively, and corresponding alkylbenzenes and alkylpyridines are produced in good yields, respectively.     

Photochemical generation of cationic lewis acids via deligation of metal-sandwich complexes

Photoinduced loss of one or both ligands from bis(arene) metal complexes provides an attractive route to cationic Lewis acids that can function as initiators for polymerization and crosslinking reactions. In this article, we examine the spectroscopy, photochemistry and photoinitiation activity of two such complexes: Fe(mes)₂²⁺ (mes is mesitylene) and Cr(tol)₂⁺ (tol is toluene). Acetonitrile solutions of Fe(mes)₂²⁺ undergo photochemical loss of both mesitylene rings to yield solvated Fe²⁺. The quantum efficiency of deligation ranges from 0.6 to 1.0 depending upon excitation wavelength, but is independent of complex concentration, solvent, oxygen concentration and excess counter-ion. Reaction appears to occur from both ligand field and ligand-to-metal charge transfer excited states. Low thermal stability, however, limits the use of Fe(mes)₂²⁺ as a practical photoinitiator. Deligation of both arene rings also occurs upon 366 nm irradiation of Cr(tol)₂⁺ in acetonitrile. The quantum efficiency for this process is 0.04 and is independent of oxygen concentration. Photoinitiated crosslinking and insolubilization occur in a thin film of COP (copolymer of glycidyl methacrylate and ethyl acrylate) containing Cr(tol)₂⁺.     

Photochemical generation of radical anions. An access to novel structures

The preparation of the radical anions of tetrabenzocyclooctatetraene (TBCOT) and fluorene (FLH) by photochemical methods leads to the observation of novel structures.