摘要:

AbstractIn the presence of double helical polynucleotides (sodium poly(dA‐dT)‐poly(dA‐dT) or calf thymus DNA), the efficiency of oxidative or reductive electron transfer between photoexcited ruthenium(II) chelates Ru(tap)2(hat)2+or Ru(phen)⅔+(where tap =1,4,5,8‐tetraazaphenanthrene, hat = 1,4,5,8,9,12‐hexaazatriphenylene, and phen = 1,10‐phenanthroline) and appropriate cationic quenchers (ethidium, Ru(NH3)3/6+, methyl viologen, or M(phen)3/3+, where M = Co, Rh, Cr) increases1–2 orders of magnitude compared to the efficiency of the same quenching in microhomogeneous aqueous medium(kq= 0.3‐1.8 × 109M's−1). The enhancement is more pronounced when the binding constant of the quencher (103

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02049.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

ReferencesPhotoexcitation of 1‐pyrenyldiazomethane (1) leads to carbenes which attach to various molecules,e.g.surfactants, polymers, hydroxylated surfaces. This provides a convenient method of labelling these compounds with pyrene for probe studies of complex structures and surfaces. Photophysical and photochemical studies of the various events resulting from the photoexcitation of 1, show that the quantum yield for disappearance of 1 and of carbene formation is low (∼ 5%) and that other processes lead to relaxation of excited 1, in particular fluorescence and intersystem crossing. The mechanism involves a mixing of low lying π‐π* states of the pyrene chromaphore with diazo n‐π+states, and is discussed in the light of other similar systems and the prese

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02050.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThetrans → cisphotoisomerization of fran.s‐6‐styrylquinoxaline (6‐StQx) has been investigated. Fluorescence quantum yields increase with increasing solvent polarity, whereas internal conversion and intersystem crossing decrease because of the weakening of vibronic coupling of the lowest1(π,π*) state and1(n,π*) state in polar solvents. Laser flash photolysis studies show an efficient singlet‐triplet intersystem crossing on direct irradiation at room temperature. Azulene quenching of fluorescence, and direct and sensitized photoisomerization indicate that the direct photoisomerization proceeds through both the singlet and triplet manifolds, mainlyviathe triplet state (75‐80%) at room temperature, in contrast to correspondin

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02051.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractPhotosensitizers trapped in porous sol‐gel glasses are shown to undergo photoinduced electron transfer with molecules present in the intra‐pore liquid phase. The phenomenon appears to be general, independent of molecular charge and size. The predominant mechanism involves long range electron transfer from, or to, an adsorbed solute molecule and a photosensitizer embedded in the glass. Such long range interactions are also the primary factor leading to a residual photogenerated radical ion pair which is extremely long lived (4–5orders of magnitude longer with respect to the same process in homogeneous solutions). The results bear on artificial photosynthetic devices in which solar energy conversion and storage depends on inhibition of the energy‐wasting back electron transfer r

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02052.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA general approach to using crystal chirality in asymmetric synthesis is described. An example consists of the preparation of crystalline salts of dimethyl 9‐amino‐9,10‐ethenoanthracene‐11, 12‐dicarboxylate (1) with optically active sulfonic acids. Photolysis of the salts in the solid state generates a product that has chiral centers in the amine‐containing portion of the molecule (di‐π‐methane photorearrangement). The optical activity in the photoproduct was determined by polarimetry and NMR chiral shift reagent analysis, which revealed enantiomeric excesses ranging from 30 to 68% depending on the optically active sulfonic acid employed. In contrast to the results in the crystalline state, irradiation of the salts in acetonitrile gave race

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02053.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA series of polycondensed aromatic N‐heterocycles (acridine, benzo‐f‐quinoline 1,2,7,8‐dibenzacridine and 3,4,5,6‐dibenzacridine) were adsorbed from the gas phase and from liquid solution on highly dispersed silica gels with very different specific surface areas and pore sizes. The translational mobility of the adsorbed species was quantified by the triplet decay and the delayed fluorescence following bimolecular triplet‐triplet annihilation after pulsed laser irradiation. The decay kinetics were analyzed with conventional second order rate equations and with the fractal approach. The first method is reliable without limitations on adsorbents with large pore diameters. It yields second order annihilation constants of 4 times 1012‐6 times 1011dm2mol−1s−1depending on the masses and sterical requirements of the adsorbates. For the second method a spectral dimensionds= 4/3 was used. This method is quantitatively applicable to all heterocycles adsorbed on silica gel 60 that have very small pore sizes. An activation energy of 4.9 ± 0.5 kcal mol−1was obtained for the translational diffusion of acridine on hyd

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02054.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe study of photosensitization processes in aqueous solution is complicated by the unsuit‐ability of water as solvent for the commonly used techniques for monitoring radiative and non‐radiative relaxation processes. The infrared luminescence method is hampered by the relatively weak intensity of emission and short lifetime of singlet molecular oxygen, O2(1Δg), in water. Photoacoustic and photothermal detection of the dominant non‐radiative relaxation process for O2(1Δg) is also inhibited by the unfavorable physico‐chemical properties of water which reduce the sensitivities of these techniques for the study of sensitizers in aqueous with respect to non‐aqueous media. Such problems have been alleviated by incorporating the sensitizer in the aqueous core of aerosol OT reverse micelles. It is demonstrated that the sensitivity of both radiative and non‐radiative detection depends not on the local environment of the sensitizer but on the overall composition of the medium, which, in the case of reverse micelle solutions, is predominantly hydrocarbon. This produces a 4.5 and 6‐fold enhancement of the sensitivity of luminescence and optoacoustic detection, respectively, in comparison to purely aqueous solution. The utility of the method is demonstrated for Rose Bengal, where quantum yields of intersystem crossing (φisc) of 0.78 (LIOAC) and singlet oxygen generation (φδ) of 0.81 (LIOAC) and 0.80 (TRLD) were measured. Phenazine was also studied for comparative purposes and to corroborate values obtain

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02055.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAn investigation of the photoreactivity of the amphiphilic α‐oxoamide, N‐(12‐dodecanoic acid)benzoylformamide, (1), in aqueous (pH 10) solution and its inclusion complexes with β‐cyclodextrin (β‐CD) and carboxymethylamylose (CMA). In all three media photolysis of 1 yields predominantly the fragmentation products from hydrolysis, mandelamide and the corresponding aldehyde. The most reasonable mechanism for the reaction is an intramolecular quenching of a ketone‐localized excited state by electron transfer from the amide nitrogen followed by proton transfer and a second electron transfer (zwitterion‐1 → diradical ⇄ zwitterion‐2) to yield ultimately the mandelamide‐imine precursor which hydrolyzes to the observed products. The quantum yields for 1 decomposition and mandelamide appearance in aqueous solution (pH 10) are 0.21 and 0.16, respectively. Compound 1 forms a 1:1 and 1:2 complex with β‐CD and shows enhanced quantum efficiencies for reaction, especially the 1:2 complex. 1 also forms a complex with CMA which shows a similar binding constant to other similar chain length amphiphiles in other studies. Here again reactivity is enhanced for the photoreaction of 1 upon complexation, indicating that neither the electron transfer quenching of the excited states nor reaction of subsequent intermediates is prohibited in the complexes. Mechanistic implications of

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02056.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe consequences of solvent order, imposed by the two smectic phases of 1,1,1,2,2,3,3,4, 4,5,5,6,6,7,7,8,8,9,9,10,10‐heneicosylfluoroeicosane (F10H10, a “diblock” molecule) on the photochemistry of 1,1,1,2,2,3,3,4,4,5,5.6,6,7,7‐pentadecylfluoro‐8‐hexadecanone (F7COH8) were investigated. Results from irradiations of F7COH8 in smectic F10H10 are compared to those obtained in hexane, perfluorohexane, and the isotropic phase of F10H10. They are complemented by2H‐NMR experiments on F10H10 solutions of F7COH8 deuterated at the methylene alpha to the carbonyl, and differential scanning calorimetry thermograms. More than 85% of the photoproducts are derived from carbonyl reduction and Norrish II processes. Based upon the2H‐NMR spectra and Norrish II photoproduct ratios, the microscopic environment experienced by the locus of reaction of F7COH8 in smectic F10H10 is not very restrictive towards the solute shape changes that must occur. This conclusion was not anticipated because themacroscopicorder of the smectic phases of

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02057.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe Norrish Type I photofragmentation of two dibenzylic ketones (l,3‐diphenyl‐2‐pro‐panone and l‐(4‐methylphenyl)‐3‐phenyl‐2‐propanone) in supercritical ethane and carbon dioxide proceeds without evidence for cage recombination of the photogenerated radical pair. A statistical mixture of the bibenzyls formed by random coupling of benzyl and p‐xylyl radicals was observed and the first order rate constant for the depletion of the reactant was independent of pressure. These results indicate that solvent cage effects are not operative in these low viscosity supercritical fluids even in the near critical region where solute‐solvent clusterin

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1991.tb02058.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY