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Flash Photolysis (flash + photolysi)
Kinds of Flash Photolysis Selected AbstractsThe influence of temperature and osmolyte on the catalytic cycle of cytochrome c oxidaseFEBS JOURNAL, Issue 2 2003Jack A. Kornblatt The influence of temperature on cytochrome c oxidase (CCO) catalytic activity was studied in the temperature range 240,308 K. Temperatures below 273 K required the inclusion of the osmolyte ethylene glycol. For steady-state activity between 278 and 308 K the activation energy was 12 kcal·mol,1; the molecular activity or turnover number was 12 s,1 at 280 K in the absence of ethylene glycol. CCO activity was studied between 240 and 277 K in the presence of ethylene glycol. The activation energy was 30 kcal·mol,1; the molecular activity was 1 s,1 at 280 K. Ethylene glycol inhibits CCO by lowering the activity of water. The rate limitation in electron transfer (ET) was not associated with ET into the CCO as cytochrome a was predominantly reduced in the aerobic steady state. The activity of CCO in flash-induced oxidation experiments was studied in the low temperature range in the presence of ethylene glycol. Flash photolysis of the reduced CO complex in the presence of oxygen resulted in three discernable processes. At 273 K the rate constants were 1500 s,1, 150 s,1 and 30 s,1 and these dropped to 220 s,1, 27 s,1 and 3 s,1 at 240 K. The activation energies were 5 kcal·mol,1, 7 kcal·mol,1, and 8 kcal·mol,1, respectively. The fastest rate we ascribe to the oxidation of cytochrome a3, the intermediate rate to cytochrome a oxidation and the slowest rate to the re-reduction of cytochrome a followed by its oxidation. There are two comparisons that are important: (a) with vs. without ethylene glycol and (b) steady state vs. flash-induced oxidation. When one makes these two comparisons it is clear that the CCO only senses the presence of osmolyte during the reductive portion of the catalytic cycle. In the present work that would mean after a flash-induced oxidation and the start of the next reduction/oxidation cycle. [source] Modeling the Photochemistry of the Reference Phototoxic Drug Lomefloxacin by Steady-State and Time-Resolved Experiments, and DFT and Post-HF CalculationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2008Mauro Freccero Prof. Abstract The irradiation in water of 1-ethyl-6,8-difluoro-7(3-methylpiperazino)3-quinolone-2-carboxylic acid (lomefloxacin), a bactericidal agent whose use is limited by its serious phototoxicity (and photomutagenicity in the mouse), leads to formation of the aryl cation in position eight that inserts into the 1-ethyl chain. Trapping of the cation was examined and it was found that chloride and bromide straightforwardly add in position eight, but with iodide and with pyrrole the 1-(2-iodoethyl) and the 1-[2-(2-pyrrolyl)ethyl] derivatives are formed. Flash photolysis reveals the triplet of lomefloxacin, a short-lived species (,max=370,nm, ,=40,ns) that generates the triplet cation (,max=480,nm, ,,120,ns). The last intermediate is quenched both by halides and by pyrrole. DFT and post-HF methods have shown that the triplet is the lowest state of the cation (,GST=13.3,kcal,mol,1) and intersystem crossing (ISC) to the singlet has no role because a less endothermic process occurs, that is, intramolecular hydrogen abstraction from the N -ethyl chain (9.2,kcal,mol,1) that finally leads to cyclization. The halides form weak complexes with the triplet cation (kq from 4.9×108 for Cl, to 7.0×109,m,1,s,1 for I,). With Cl, and Br, ISC occurs in the complex along with C8X bond formation. However, this latter process is slow with bulky iodide and with neutral pyrrole, and in these cases moderately endothermic electron transfer (ca. 7,kcal,mol,1) yielding the 8-quinolinyl radical occurs. Hydrogen exchange leads to a new radical on the 1-ethyl chain and to the observed products. These findings suggest that the mutagenic activity of the DNA-intercalated drug involves attack of the photogenerated cation to the heterocyclic bases. [source] No evidence for calcium electrogenic exchanger in frog semicircular canal hair cellsEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 9 2002M. Martini Abstract We investigated the possibility that, in hair cells mechanically isolated from frog semicircular canals, Ca2+ extrusion occurs via a Na+ : Ca2+ (cardiac type) or a Na+ : Ca2+,K+ (retinal type) exchanger. Cells concurrently imaged during whole-cell patch-clamp recordings using the Ca2+ sensitive fluorescent dye Oregon Green 488 BAPTA-1 (100 µm) showed no voltage dependence of Ca2+ clearance dynamics following a Ca2+ load through voltage-gated Ca2+ channels. Reverse exchange was probed in hair cells dialyzed with a Ca2+ - and K+ -free solution, containing a Na+ concentration that saturates the exchanger, after zeroing the contribution to the whole-cell current from Ca2+ and K+ conductances. In these conditions, no reverse exchange current was detected upon switching from a Ca2+ -free external solution to a solution containing concentrations of Ca2+ alone, or Ca2+ + K+ that saturated the exchanger. By contrast, the same experimental protocol elicited peak exchange currents exceeding 100 pA in gecko rod photoreceptors, used as positive controls. In both cell types, we also probed the forward mode of the exchanger by rapidly increasing the intracellular Ca2+ concentration using flash photolysis of two novel caged Ca2+ complexes, calcium 2,2,-{[1-(2-nitrophenyl)ethane-1,2-diyl]bis(oxy)}bis(acetate) and calcium 2,2,-{[1-(4,5-dimethoxy-2-nitrophenyl)ethane-1,2-diyl]bis(oxy)} bis(acetate), in the presence of internal K+ and external Na+. No currents were evoked by UV-triggered Ca2+ jumps in hair cells, whereas exchanger conformational currents up to 400 pA, followed by saturating forward exchange currents up to 40 pA, were recorded in rod photoreceptors subjected to the same experimental conditions. We conclude that no functional electrogenic exchanger is present in this hair cell population, which leaves the abundant plasma membrane Ca2+ -ATPases as the primary contributors to Ca2+ extrusion. [source] Laser Flash Photolysis of Disulfonyldiazomethanes: Partitioning between Hetero-Wolff Rearrangement and Intramolecular Carbene Oxidation by a Sulfonyl GroupEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 11 2003Götz Bucher Abstract Laser flash photolysis of bis(phenylsulfonyl)diazomethane (1a) and ditosyldiazomethane (1b) leads to the formation of sulfonylsulfenes 8a/8b (,max = 310 nm). Non-first-order kinetics and non-linear Stern,Volmer behavior suggest the formation of another transient species also absorbing at , = 310 nm. Based on the results of DFT calculations, these species are tentatively identified as dithiocarbonate S,S,S, -trioxides 12a/b, which are formed from the oxathiirene S -oxides 9a/b. The latter compounds bear the characteristics of masked carbenes (, estimated as 700 ps) and are quenched by methanol and cyclooctene. Singlet disulfonylcarbenes 2a/b are not predicted to be minima, since all attempts at optimizing the geometry of 2a led to 9a instead. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] The Effect of Nanoparticle Shape on the Photocarrier Dynamics and Photovoltaic Device Performance of Poly(3-hexylthiophene):CdSe Nanoparticle Bulk Heterojunction Solar CellsADVANCED FUNCTIONAL MATERIALS, Issue 16 2010Smita Dayal Abstract The charge separation and transport dynamics in CdSe nanoparticle:poly(3-hexylthiophene) (P3HT) blends are reported as a function of the shape of the CdSe-nanoparticle electron acceptor (dot, rod, and tetrapod). For optimization of organic photovoltaic device performance it is crucial to understand the role of various nanostructures in the generation and transport of charge carriers. The sample processing conditions are carefully controlled to eliminate any processing-related effects on the carrier generation and on device performance with the aim of keeping the conjugated polymer phase constant and only varying the shape of the inorganic nanoparticle acceptor phase. The electrodeless, flash photolysis time-resolved microwave conductivity (FP-TRMC) technique is used and the results are compared to the efficiency of photovoltaic devices that incorporate the same active layer. It is observed that in nanorods and tetrapods blended with P3HT, the high aspect ratios provide a pathway for the electrons to move away from the dissociation site even in the absence of an applied electric field, resulting in enhanced carrier lifetimes that correlate to increased efficiencies in devices. The processing conditions that yield optimum performance in high aspect ratio CdSe nanoparticles blended with P3HT result in poorly performing quantum dot CdSe:P3HT devices, indicating that the latter devices are inherently limited by the absence of the dimensionality that allows for efficient, prolonged charge separation at the polymer:CdSe interface. [source] Functional characterization of the evolutionarily divergent fern plastocyaninFEBS JOURNAL, Issue 16 2004José A. Navarro Plastocyanin (Pc) is a soluble copper protein that transfers electrons from cytochrome b6f to photosystem I (PSI), two protein complexes that are localized in the thylakoid membranes in chloroplasts. The surface electrostatic potential distribution of Pc plays a key role in complex formation with the membrane-bound partners. It is practically identical for Pcs from plants and green algae, but is quite different for Pc from ferns. Here we report on a laser flash kinetic analysis of PSI reduction by Pc from various eukaryotic and prokaryotic organisms. The reaction of fern Pc with fern PSI fits a two-step kinetic model, consisting of complex formation and electron transfer, whereas other plant systems exhibit a mechanism that requires an additional intracomplex rearrangement step. The fern Pc interacts inefficiently with spinach PSI, showing no detectable complex formation. This can be explained by assuming that the unusual surface charge distribution of fern Pc impairs the interaction. Fern PSI behaves in a similar way as spinach PSI in reaction with other Pcs. The reactivity of fern Pc towards several soluble c -type cytochromes, including cytochrome f, has been analysed by flavin-photosensitized laser flash photolysis, demonstrating that the specific surface motifs for the interaction with cytochrome f are conserved in fern Pc. [source] Role of electrostatics in the interaction between plastocyanin and photosystem I of the cyanobacterium Phormidium laminosumFEBS JOURNAL, Issue 23 2002Beatrix G. Schlarb-Ridley The interactions between photosystem I and five charge mutants of plastocyanin from the cyanobacterium Phormidium laminosum were investigated in vitro. The dependence of the overall rate constant of reaction, k2, on ionic strength was investigated using laser flash photolysis. The rate constant of the wild-type reaction increased with ionic strength, indicating repulsion between the reaction partners. Removing a negative charge on plastocyanin (D44A) accelerated the reaction and made it independent of ionic strength; removing a positive charge adjacent to D44 (K53A) had little effect. Neutralizing and inverting the charge on R93 slowed the reaction down and increased the repulsion. Specific effects of MgCl2 were observed for mutants K53A, R93Q and R93E. Thermodynamic analysis of the transition state revealed positive activation entropies, suggesting partial desolvation of the interface in the transition state. In comparison with plants, plastocyanin and photosystem I of Phormidium laminosum react slowly at low ionic strength, whereas the two systems have similar rates in the range of physiological salt concentrations. We conclude that in P. laminosum, in contrast with plants in vitro, hydrophobic interactions are more important than electrostatics for the reactions of plastocyanin, both with photosystem I (this paper) and with cytochrome f[Schlarb-Ridley, B.G., Bendall, D.S. & Howe, C.J. (2002) Biochemistry41, 3279,3285]. We discuss the implications of this conclusion for the divergent evolution of cyanobacterial and plant plastocyanins. [source] 2-(2,4-Dinitrobenzyl)pyridine (DNBP): A Potential Light-Activated Proton ShuttleHELVETICA CHIMICA ACTA, Issue 10 2009Catrin Goeschen Abstract The well-known photochromic tautomerism of 2-(2,4-dinitrobenzyl)pyridine (1; CH; Scheme,1) was re-investigated by flash photolysis in aqueous solution in view of its potential application as a light-activated proton pump. Irradiation of 1 yields the enamine tautomer NH (,max=520,nm) that rapidly equilibrates with its conjugate base CNO, (,max=420,nm). The pH,rate profile for the first-order decay of NH and CNO, provides a direct determination of the acidity constant of NH, pK=5.94±0.12 (I=0.1M) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH+), pK=4.18±0.02, was determined by spectrophotometric titration. [source] Hydroxyl radical reactions with halogenated ethanols in aqueous solution: Kinetics and thermochemistryINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2008I. Morozov Laser flash photolysis combined with competition kinetics with SCN, as the reference substance has been used to determine the rate constants of OH radicals with three fluorinated and three chlorinated ethanols in water as a function of temperature. The following Arrhenius expressions have been obtained for the reactions of OH radicals with (1) 2-fluoroethanol, k1(T) = (5.7 ± 0.8) × 1011 exp((,2047 ± 1202)/T) M,1 s,1, (2) 2,2-difluoroethanol, k2(T) = (4.5 ± 0.5) × 109 exp((,855 ± 796)/T) M,1 s,1, (3) 2,2,2-trifluoroethanol, k3(T) = (2.0 ± 0.1) × 1011 exp((,2400 ± 790)/T) M,1 s,1, (4) 2-chloroethanol, k4(T) = (3.0 ± 0.2) × 1010 exp((,1067 ± 440)/T) M,1 s,1, (5) 2, 2-dichloroethanol, k5(T) = (2.1 ± 0.2) × 1010 exp((,1179 ± 517)/T) M,1 s,1, and (6) 2,2,2-trichloroethanol, k6(T) = (1.6 ± 0.1) × 1010 exp((,1237 ± 550)/T) M,1 s,1. All experiments were carried out at temperatures between 288 and 328 K and at pH = 5.5,6.5. This set of compounds has been chosen for a detailed study because of their possible environmental impact as alternatives to chlorofluorocarbon and hydrogen-containing chlorofluorocarbon compounds in the case of the fluorinated alcohols and due to the demonstrated toxicity when chlorinated alcohols are considered. The observed rate constants and derived activation energies of the reactions are correlated with the corresponding bond dissociation energy (BDE) and ionization potential (IP), where the BDEs and IPs of the chlorinated ethanols have been calculated using quantum mechanical calculations. The errors stated in this study are statistical errors for a confidence interval of 95%. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 174,188, 2008 [source] Effects of unidirectional and mutual interactions between microstructures and azo dyes as "kinetic" probe molecules on cis,trans isomerization rate constants in aqueous P85 and F88 triblock copolymer solutionsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2008Helmut Knoll Rate constants of the thermal cis , trans isomerization of azo dyes have been studied in aqueous P85 and F88 triblock copolymer solutions by means of microsecond flash photolysis. Inflection points or maxima of the nonlinear Arrhenius plots indicate microstructure changes around the "kinetic" probe molecules. Unidirectional interactions of micelle formation on reactivity are reflected by discontinuous change of rate constants at temperatures near the critical micellization temperatures (cmT) determined by means of probe-free methods. Mutual interactions have been identified by means of significant differences between cmT and temperatures, where rate constants change discontinuously. The type of interactions depends on the properties of probe molecules and amphiphiles. Systems as studied in this work might be models of more complex biological reaction systems with temperature-dependent microstructure changes. © 2007 Wiley Periodicals, Inc. 40: 59,65, 2007 [source] Rate constants for the gas-phase reaction of CF3CF2CF2CF2CF2CHF2 with OH radicals at 250,430 KINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2004L. Chen The rate constants k1 for the reaction of CF3CF2CF2CF2CF2CHF2 with OH radicals were determined by using both absolute and relative rate methods. The absolute rate constants were measured at 250,430 K using the flash photolysis,laser-induced fluorescence (FP-LIF) technique and the laser photolysis,laser-induced fluorescence (LP-LIF) technique to monitor the OH radical concentration. The relative rate constants were measured at 253,328 K in an 11.5-dm3 reaction chamber with either CHF2Cl or CH2FCF3 as a reference compound. OH radicals were produced by UV photolysis of an O3,H2O,He mixture at an initial pressure of 200 Torr. Ozone was continuously introduced into the reaction chamber during the UV irradiation. The k1 (298 K) values determined by the absolute method were (1.69 ± 0.07) × 10,15 cm3 molecule,1 s,1 (FP-LIF method) and (1.72 ± 0.07) × 10,15 cm3 molecule,1 s,1 (LP-LIF method), whereas the K1 (298 K) values determined by the relative method were (1.87 ± 0.11) × 10,15 cm3 molecule,1 s,1 (CHF2Cl reference) and (2.12 ± 0.11) × 10,15 cm3 molecule,1 s,1 (CH2FCF3 reference). These data are in agreement with each other within the estimated experimental uncertainties. The Arrhenius rate constant determined from the kinetic data was K1 = (4.71 ± 0.94) × 10,13 exp[,(1630 ± 80)/T] cm3 molecule,1 s,1. Using kinetic data for the reaction of tropospheric CH3CCl3 with OH radicals [k1 (272 K) = 6.0 × 10,15 cm3 molecule,1 s,1, tropospheric lifetime of CH3CCl3 = 6.0 years], we estimated the tropospheric lifetime of CF3CF2CF2CF2CF2CHF2 through reaction with OH radicals to be 31 years. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 26,33, 2004 [source] A temperature-dependent kinetics study of the reaction of O(3PJ) with (CH3)2SOINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 3 2002F. D. Pope A laser flash photolysis,resonance fluorescence technique has been employed to investigate the kinetics of the reaction of ground state oxygen atoms, O(3PJ), with (CH3)2SO (dimethylsulfoxide) as a function of temperature (266,383 K) and pressure (20,100 Torr N2). The rate coefficient (kR1) for the O(3PJ) + (CH3)2SO reaction is found to be independent of pressure and to increase with decreasing temperature. The following Arrhenius expression adequately describes the observed temperature dependence: kR1(T) = (1.68 ± 0.76) × 10,12 exp[(445 ± 141)/T] cm3 molecule,1 s,1, where the uncertainties in Arrhenius parameters are 2, and represent precision only. The absolute accuracy of each measured rate coefficient is estimated to be ±30%, and is limited predominantly by the uncertainties in measured (CH3)2SO concentrations. The observed temperature and pressure dependencies suggest that, as in the case of O(3PJ) reactions with CH3SH and (CH3)2S, reaction occurs by addition of O(3PJ) to the sulfur atom followed by rapid fragmentation of the energized adduct to products. The O(3PJ) + (CH3)2SO reaction is fast enough so that it could be a useful laboratory source of the CH3SO2 radical if this species is produced in significant yield. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 156,161, 2002; DOI 10.1002/kin.10040 [source] Kinetic and mechanistic study of the reaction of O(1D) with CF2HBrINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2001R. S. Strekowski A laser flash photolysis,resonance fluorescence technique has been employed to investigate the kinetics and mechanism of the reaction of electronically excited oxygen atoms, O(1D), with CF2HBr. Absolute rate coefficients (k1) for the deactivation of O(1D) by CF2HBr have been measured as a function of temperature over the range 211,425 K. The results are well described by the Arrhenius expression k1(T) = 1.72 × 10,10 exp(+72/T) cm3molecule,1 s,1; the accuracy of each reported rate coefficient is estimated to be ±15% (2,). The branching ratio for nonreactive quenching of O(1D) to the ground state, O(3P), is found to be 0.39 ± 0.06 independent of temperature, while the branching ratio for production of hydrogen atoms at 298 K is found to be 0.02,0.02+0.01. The above results are considered in conjunction with other published information to examine reactivity trends in O(1D) + CF2XY reactions (X,Y = H, F, Cl, Br). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 262,270, 2001 [source] Synthesis and characterisation of 13C and 15N isotopomers of a 1-acyl-7-nitroindolineJOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 9 2001John E. T. Corrie Abstract Efficient methods are described for synthesis of isotopomers of the water-soluble, photolabile 1-acyl-7-nitroindoline 5 with either 13C in the carbonyl of the acyl substituent or 15N in the nitro group. The isotopic incorporations were verified by IR difference spectroscopy coupled with flash photolysis. [source] Effect of Lewis base coordination on boryl radical reactivity: investigation using laser flash photolysis and kinetic ESRJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2009Jacques Lalevée Abstract The effect of Lewis base coordination on boryl radical reactivity (L,BH where L is triethylamine, 2-picoline, 4-dimethylaminopyridine, quinoline and diphenyl phosphine) is examined. Direct detection of the different boryl radicals, their transient absorption spectra, their interaction with double or triple bonds, oxygen, oxidizing agent, alkyl halides and their hydrogen lability have been studied using laser flash photolysis (LFP), kinetic ESR (KESR), ESR spin trapping and MO calculations. For example, a strong decrease of both the bond dissociation energy (BDE)(BH) (33.8,kcal,mol,1) and the addition rate constant to MA (1300 105 to,>105 M,1,s,1) was noted when going from the triethylamine borane to the quinoline borane complex, in line with the spin delocalization (1.04 vs. 0.19). These structures are also proposed as new highly efficient co-initiators for the acrylate photopolymerization. Copyright © 2009 John Wiley & Sons, Ltd. [source] Ionic and radical fragmentations of alkoxyhalocarbenes , a perspectiveJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2009Robert A. Moss Abstract Fragmentations of secondary or tertiary alkoxyhalocarbenes in polar solvents generate carbocations as components of ion pairs. A variety of carbocations can be produced including acyclic, alicyclic, benzyl, bridgehead, cyclopropyl, cyclopropylcarbinyl, and norbornyl examples. Laser flash photolysis (LFP) studies provide kinetics and activation parameters for the carbene fragmentations, which are orders of magnitude faster, and require considerably reduced activation energies, compared to analogous solvolytic carbocation-generative processes. In some cases, the time required for solvent and anion equilibration of the ion pairs can be estimated. In nonpolar solvents, the gas phase, or cryogenic matrices, homolytic carbene fragmentation may, in certain cases, supplant heterolytic fragmentation. Copyright © 2008 John Wiley & Sons, Ltd. [source] Photochromism of dihydroindolizines part VII: multiaddressable photophysical properties of new photochromic dihydroindolizines bearing substituted benzo[i]phenanthridine as a fluorescing moiety,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2007Saleh Abdel-Mgeed Ahmed Abstract Sixteen benzo[i]phenanthridine derivatives 8a-p were prepared via photocyclization of cis-trans substituted 4-styrylquinolines in low-to-moderate yields. The chemical structure of the photocyclized benzo[i]phenanthridine derivatives was unambiguously elucidated by means of both spectral and analytical tools. The photochromic (PC) dihydroindolizines (DHIs) 8a-p based on benzo[i]phenanthridines were prepared in 19,57% yields via nucleophilic addition of benzo[i]phenanthridines 4a-p to spirocyclopropenes 5. The 1D, 2D, NOESY NMR spectra, mass spectrometry, and elemental analysis were used for characterization of the chemical structures of the newly synthesized DHIs 8a-p. Developing and tuning of the photophysical properties of the synthesized compounds by substituents in the base part have been achieved. The absorption maxima (,max) and the half-lives (t1/2) of the colored zwitterionic forms 7a-p were detected in all cases by flash-photolysis measurements due to the fast 1,5-electrocyclization back to the DHI system. Irradiation of DHI 8a-p in'CH2Cl2 solution with polychromatic light leads to the formation of green to green,blue colored betaines 7a-p after cooling with liquid nitrogen. The kinetics of the fast bleaching process of betaines 7a-p to DHIs 8a-p, studied by flash photolysis as well as low temperature FT-UV/VIS, were found to take place in the millisecond range (432,2675,ms) in dichloromethane solution and fitted well a first-order thermal back reaction. The fluorescence spectra as well as the fluorescence quantum yield were studied. Noticeable bathochromic and hypsochromic shifts in the emission spectra by changing the substituents in the base part were monitored. Interestingly, the photo-fatigue resistance of some studied betaines 7a-p showed a higher t30 -value than the standard one (dicyanopyridazine DHI). Large solvatochromic effects on the absorption maxima (,max) as well as a substantial increase in the half-lives (t1/2) with solvent polarity of betaines 7a-p were also observed. The multiaddressable PC properties of DHIs 8a-p will help these compounds to find applications. Copyright © 2007 John Wiley & Sons, Ltd. [source] Triplet state and phenoxyl radical formation of 3, 4-methylenedioxy phenol: a combined laser flash photolysis and pulse radiolysis studyJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2007Ravi Joshi Abstract Formations of triplet state, molecular cation radical, and phenoxyl radical of 3,4-methylenedioxy phenol (sesamol, SOH) in organic solvents have been investigated by laser photolysis as well as pulse radiolysis techniques. Photolysis of SOH in cyclohexane has been found to produce both triplet state (,max,,,480,nm) and phenoxyl radical (425,430,nm) of SOH by mono-photonic processes. However, radical cation (,max,=,450,nm) and phenoxyl radical of SOH have been observed on radiolysis in cyclohexane. Further, radiolysis of SOH in benzene has been found to produce phenoxyl radical only. Mechanism of phenoxyl radical formation by photo-excitation of SOH has been studied and triplet energy level of SOH is estimated to lie between 1.85 and 2.64,eV. Copyright © 2007 John Wiley & Sons, Ltd. [source] Self-initiation of the UV photopolymerization of brominated acrylatesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 14 2008Tom Scherzer Abstract Brominated aromatic acrylates were found to polymerize rapidly upon exposure to UV light. Moreover, they are able to initiate the UV-induced polymerization of acrylic formulations that do not contain a conventional photoinitiator. In contrast, the corresponding unbrominated homologues are not effective as initiators. Investigations by real-time FTIR spectroscopy have shown that the addition of only 1 wt % of a brominated acrylate is sufficient for an efficient initiation. Fast photopolymerization is achieved even if irradiation is carried out at , > 300 nm where most acrylates do not absorb. Short-lived transients were studied by laser flash photolysis. The triplet was found to show low sensitivity to oxygen which is because of its very short lifetime. Bromine radicals split of from the acrylates were trapped with bromine ions from tetraethyl ammonium bromide and detected as Br. The resulting quantum yields for the formation of bromine radicals are in the range of up to 0.3. Quantum chemical modeling was carried out to establish a mechanism for the release of bromine radicals. Both bromine and bromophenyl radicals are able to initiate the polymerization reaction. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4905,4916, 2008 [source] Photopolymerization of 1,6-hexanedioldiacrylate initiated by three-component systems based on N -arylphthalimidesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 16 2004T. Brian Cavitt Abstract Three-component photoinitiators comprised of an N -arylphthalimide, a diarylketone, and a tertiary amine were investigated for their initiation efficiency of acrylate polymerization. The use of an electron-deficient N -arylphthalimide resulted in a greater acrylate polymerization rate than an electron-rich N -arylphthalimide. Triplet energies of each N -arylphthalimide, determined from their phosphorescence spectra, and the respective rate constants for triplet quenching by the N -arylphthalimide derivatives (acquired via laser flash photolysis) indicated that an electron,proton transfer from an intermediate radical species to the N -arylphthalimide (not energy transfer from triplet sensitization) is responsible for generating the initiating radicals under the conditions and species concentrations used for polymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4009,4015, 2004 [source] Photoinitiated grafting of maleic anhydride onto polypropyleneJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 8 2004Bo Pan Abstract The photoinitiated grafting of maleic anhydride (MAH) onto polypropylene with the use of benzophenone (BP) as the initiator has been investigated. In comparison with the process of thermally initiated grafting with peroxide as the initiator, photoinitiated grafting affords a higher grafting efficiency. The efficient photografting sensitized by BP can be explained by two possible mechanistic processes: the sensitization of the formation of the excited triplet state of MAH by BP and electron transfer followed by proton transfer between MAH and the benzopinacol radical, which may operate together. In the former case, the generated MAH excited triplet state abstracts a hydrogen from the polymer substrate to initiate grafting. A rate constant of 3.6 × 109 M ,1 s ,1 has been determined by laser flash photolysis for the process of quenching the excited triplet state of BP with ground-state MAH. In comparison, the rate constant for the quenching of the excited triplet state of BP by hydrogen abstraction has been determined to be 4.1 × 105 M ,1 s ,1. In a study of photografting using a model compound, 2,4-dimethylpentane, as a small-molecule analogue of polypropylene, the loss of BP was significantly reduced upon the addition of MAH, and this is consistent with the proposed mechanistic processes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1953,1962, 2004 [source] Tetraalkylammonium salt as photoinitiator of vinyl polymerization in organic and aqueous media: A mechanistic and laser flash photolysis studyJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2002María L. Gómez Abstract N -Dimethyl- N -[2-(N,N -dimethylamino)ethyl]- N -(1-methylnaphthyl)ammonium tetrafluoroborate (I) was synthesized with the aim of obtaining a versatile photoinitiator for vinyl polymerization in organic solvents and water. Salt I was able to trigger the polymerization of acrylamide, 2-hydroxyethylmethacrylate and styrene even at very low concentrations of the salt (,1.0 × 10,5 M). Using laser flash photolysis and fluorescence techniques and analyzing the photoproduct distribution, we were able to postulate a mechanism for the photodecomposition of the salt. With irradiation, I undergoes an intramolecular electron-transfer reaction to form a radical ion pair (RIP). The RIP intermediate decomposes into free radicals. The RIP and the free radicals are active species for initiating the polymerization. Depending on the concentration of the vinyl monomers studied, the initiation mechanism of the polymerization reaction changes. At large monomer concentrations, the RIP state is postulated to trigger the reaction by generating the anion radical of the olefin substrate. At a low monomer concentration, the free radicals produced by the decomposition of I are believed to start the chain reaction. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 901,913, 2002; DOI 10.1002/pola.10166 [source] Primary Photoprocesses in a Fluoroquinolone Antibiotic Sarafloxacin,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2009Fernando Lorenzo The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (, = 36800 and 17000 dm3 mol,1 cm,1, respectively). Both the absorption and emission properties of SFX are pH-dependent; pKa values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet,triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000 ± 1000 dm3 mol,1 cm,1. The quantum yield of triplet formation has been determined to be 0.35 ± 0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (3A*) was found to react with phosphate buffer to form triplet 3B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens. [source] Primary Photophysical Properties of Moxifloxacin, A Fluoroquinolone AntibioticPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 5 2008Fernando Lorenzo The photophysical properties of the fluoroquinolone antibiotic moxifloxacin (MOX) were investigated in aqueous media. MOX in water, at pH 7.4, shows two intense absorption bands at 287 and 338 nm (, = 44 000 and 17 000 dm3 mol,1 cm,1, respectively). The absorption and emission properties of MOX are pH-dependent, pKa values for the protonation equilibria of both the ground (6.1 and 9.6) and excited singlet states (6.8 and 9.1) of MOX were determined spectroscopically. MOX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Phosphorescence from the excited triplet state in frozen ethanol solution has a quantum yield of 0.046. Laser flash photolysis and pulse radiolysis studies have been carried out to characterize the transient species of MOX in aqueous solution. On laser excitation, MOX undergoes monophotonic photoionization with a quantum yield of 0.14. This leads to the formation of a long-lived cation radical whose absorption is maximum at 470 nm (,470 = 3400 dm3 mol,1 cm,1). The photoionization process releases hydrated electron which rapidly reacts (k = 2.8 × 1010 dm3 mol,1 s,1) with ground state MOX, yielding a long-lived anion radical with maximum absorption at 390 nm (,390 = 2400 dm3 mol,1 cm,1). The cation radical of MOX is able to oxidize protein components tryptophan and tyrosine. The bimolecular rate constants for these reactions are 2.3 × 108 dm3 mol,1 s,1 and 1.3 × 108 dm3 mol,1 s,1, respectively. Singlet oxygen sensitized by the MOX triplet state was also detected only in oxygen-saturated D2O solutions, with a quantum yield of 0.075. [source] Photoreaction Between Benzoylthiophenes and N -BOC-Tryptophan Methyl Ester,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2006Julia Pérez-Prieto ABSTRACT Drug-induced photoallergy requires as the first step formation of covalent drug-protein photoadducts. One of the key amino acids involved in this process is tryptophan (Trp). In this context, several diaryl ketones, including 2-benzoylthiophene (BT), [2-(5-benzoyl-5-thienyl)]-2-methylpropanoic methyl ester (TPA methyl ester) and 4-(2-thienylcarbonyl)phenyl]-2-methylpropanoic methyl ester (SUP methyl ester) have been irradiated in the presence of N -BOC-(L)-tryptophan methyl ester. Laser flash photolysis has allowed to detect three neutral radicals (ketyl, indolyl and skatolyl radicals) resulting from formal hydrogen-atom abstraction. This correlates well with the isolation of homodimers, as well as with cross-coupling products, in the preparative irradiation. The main cross-coupling products were in all cases lactones arising from the reaction of the Trp-derived skatolyl radicals with the corresponding ketyl radicals. These lactones were obtained as the (4R) stereoisomers with remarkable diasteroselectivity. No coupling products through the phenyl p -position of BT or TPA methyl ester were found. By contrast, ketone homodimers and cross-coupling products arising from reaction through the thienyl 5-position were obtained when using BT and SUP methyl ester; this is very interesting, because stable LAT-derived products are difficult to isolate. [source] A Laser Flash Photolysis Study of Curcumin in Dioxane,Water Mixtures,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2001F. Ortica ABSTRACT Curcumin [bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was studied by means of UV,VIS absorption spectroscopy and nanosecond laser flash photolysis in 1,4-dioxane,water mixtures in a series of dioxane,water volume ratios. The transient characteristics were found to be dependent on the amount of water. In pure dioxane the triplet state of the molecule in its enolic form was detected (,max= 720 nm, ,= 3.2 ,s), whereas upon water addition, the diketo form was found to prevail, because of the perturbation of intramolecular H-bonded structure. This led to hydrogen abstraction from dioxane by curcumin triplet state and the formation of the corresponding ketyl radical (,max= 490 nm, ,, 10 ,s). Laser flash photolysis measurements, carried out in solvents of different polarity and proticity (benzene, cyclohexane and various alcohols), allowed the transient assignments to be confirmed, supporting our interpretation. [source] Photoreactivity of the Nonsteroidal Anti-inflammatory 2-Arylpropionic Acids with Photosensitizing Side Effects,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 5 2001Francisco Boscá ABSTRACT The photoreactivity of the nonsteroidal anti-inflammatory 2-arylpropionic acids benoxaprofen, carprofen, naproxen, ketoprofen, tiaprofenic acid, and suprofen is reviewed with special emphasis on fundamental photophysical and photochemical properties. The absorption and emission properties of the excited states of these drugs as well as their main photodegradation routes are summarized. The photochemical mechanisms are discussed on the basis of product studies and detection of short-lived intermediates by means of laser flash photolysis. After dealing with the unimolecular processes, attention is focused on the photosensitized reactions of key biomolecules, such as lipids, proteins or nucleic acids. Finally, a short section on the photobiological effects on simple biological models is also included. Although some earlier citations are included, the literature coverage is in general limited to the last decade. [source] Protein dynamics control proton transfer from bulk solvent to protein interior: A case study with a green fluorescent proteinPROTEIN SCIENCE, Issue 7 2005Anoop M. Saxena Abstract The kinetics of proton transfer in Green Fluorescent Protein (GFP) have been studied as a model system for characterizing the correlation between dynamics and function of proteins in general. The kinetics in EGFP (a variant of GFP) were monitored by using a laser-induced pH jump method. The pH was jumped from 8 to 5 by nanosecond flash photolysis of the "caged proton," o -nitrobenzaldehyde, and subsequent proton transfer was monitored by following the decrease in fluorescence intensity. The modulation of proton transfer kinetics by external perturbants such as viscosity, pH, and subdenaturing concentrations of GdnHCl as well as of salts was studied. The rate of proton transfer was inversely proportional to solvent viscosity, suggesting that the rate-limiting step is the transfer of protons through the protein matrix. The rate is accelerated at lower pH values, and measurements of the fluorescence properties of tryptophan 57 suggest that the enhancement in rate is associated with an enhancement in protein dynamics. The rate of proton transfer is nearly independent of temperature, unlike the rate of the reverse process. When the stability of the protein was either decreased or increased by the addition of co-solutes, including the salts KCl, KNO3, and K2SO4, a significant decrease in the rate of proton transfer was observed in all cases. The lack of correlation between the rate of proton transfer and the stability of the protein suggests that the structure is tuned to ensure maximum efficiency of the dynamics that control the proton transfer function of the protein. [source] Influence of ionic strength on the time course of force development and phosphate release by dogfish muscle fibresTHE JOURNAL OF PHYSIOLOGY, Issue 3 2005Timothy G. West We measured the effects of ionic strength (IS), 200 (standard) and 400 mmol l,1 (high), on force and ATP hydrolysis during isometric contractions of permeabilized white fibres from dogfish myotomal muscle at their physiological temperature, 12°C. One goal was to test the validity of our kinetic scheme that accounts for energy release, work production and ATP hydrolysis. Fibres were activated by flash photolysis of the P3 -1-(2 nitrophenyl) ethyl ester of ATP (NPE-caged ATP), and time-resolved phosphate (Pi) release was detected with the fluorescent protein MDCC-PBP, N -(2[1-maleimidyl]ethyl)-7-diethylamino-coumarin-3-carboxamide phosphate binding protein. High IS slowed the transition from rest to contraction, but as the fibres approached the isometric force plateau they showed little IS sensitivity. By 0.5 s of contraction, the force and the rate of Pi release at standard and high IS values were not significantly different. A five-step reaction mechanism was used to account for the observed time courses of force and Pi release in all conditions explored here. Only the rate constants for reactions of ATP, ADP and Pi with the contractile proteins varied with IS, thus suggesting that the actin,myosin interactions are largely non-ionic. Our reaction scheme also fits previous results for intact fibres. [source] Selective Inclusion of Electron-Donating Molecules into Porphyrin Nanochannels Derived from the Self-Assembly of Saddle-Distorted, Protonated Porphyrins and Photoinduced Electron Transfer from Guest Molecules to Porphyrin DicationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2007Takahiko Kojima Prof. Abstract A doubly protonated hydrochloride salt of a saddle-distorted dodecaphenylporphyrin (H2DPP), [H4DPPP]Cl2, forms a porphyrin nanochannel (PNC). X-ray crystallography was used to determine the structure of the molecule, which revealed the inclusion of guest molecules within the PNC. Electron-donating molecules, such as p -hydroquinone and p -xylene, were selectively included within the PNC in sharp contrast to electron acceptors, such as the corresponding quinones, which were not encapsulated. This result indicates that the PNC can recognize the electronic character and steric hindrance of the guest molecules during the course of inclusion. ESR measurements (photoirradiation at ,>340,nm at room temperature) of the PNC that contains p -hydroquinone, catechol, and tetrafluorohydroquinone guest molecules gave well-resolved signals, which were assigned to cation radicals formed without deprotonation based on results from computer simulations of the ESR spectra and density functional theory (DFT) calculations. The radicals are derived from photoinduced electron transfer from the guest molecules to the singlet state of H4DPP2+. Transient absorption spectroscopy by femtosecond laser flash photolysis allowed us to observe the formation of 1(H4DPP2+)*, which is converted to H4DPP+. by electron transfer from the guest molecules to 1(H4DPP2+)*, followed by fast disproportionation of H4DPP+., and charge recombination to give diamagnetic species and the triplet excited state 3(H4DPP2+)*, respectively. [source] | ||||||||||||||||