Showing papers on "Flash photolysis published in 2003"
TL;DR: The results of these experiments indicate that photooxidation mediated by 1O2 is the likely degradation pathway for cimetidine in most natural waters, and photodegradation by direct photolysis is expected to be the major pathway for ranitidine, with some degradation caused by1O2.
Abstract: The photochemical fates of the histamine H2-receptor antagonists cimetidine and ranitidine were studied. Each of the two environmentally relevant pharmaceuticals displayed high rates of reaction with both singlet oxygen (1O2, O2(1delta(g))) and hydroxyl radical (*OH), two transient oxidants formed in sunlit natural waters. For cimetidine, the bimolecular rate constant for reaction with *OH in water is 6.5 +/- 0.5 x 10(9) M(-1) s(-1). Over the pH range 4-10, cimetidine reacts with 1O2 with bimolecular rate constants ranging from 3.3 +/- 0.3 x 10(6) M(-1) s(-1) at low pH to 2.5 +/- 0.2 x 10(8) M(-1) s(-1) in alkaline solutions. The bimolecular rate constants for ranitidine reacting with 1O2 in water ranges from 1.6 +/- 0.2 x 10(7) M(-1) s(-1) at pH 6-6.4 +/- 0.2 x 10(7) M(-1) s(-1) at pH 10. Reaction of ranitidine hydrochloride with *OH proceeds with a rate constant of 1.5 +/- 0.2 x 10(10) M(-1) s(-1). Ranitidine was also degraded in direct photolysis experiments with a half-life of 35 min under noon summertime sunlight at 45 degrees latitude, while cimetidine was shown to be resistant to direct photolysis. The results of these experiments, combined with the expected steady-state near surface concentrations of 1O2 and *OH, indicate that photooxidation mediated by 1O2 is the likely degradation pathway for cimetidine in most natural waters, and photodegradation by direct photolysis is expected to be the major pathway for ranitidine, with some degradation caused by 1O2. These predictions were verified in studies using Mississippi River water. Model compounds were analyzed by laser flash photolysis experiments to assess which functionalities within ranitidine and cimetidine are most susceptible to singlet-oxygenation and direct photolysis. The heterocyclic moieties of the pharmaceuticals were clearly implicated as the sites of reaction with 1O2, as evidenced by the high relative rate constants of the furan and imidazole models. The nitroacetamidine portion of ranitidine has been shown to be the moiety active in direct photolysis.
258 citations
TL;DR: Differences in the fluorescence intensity as a function of dimer–monomer ratio as well as in the resonance light scattering spectra indicate that distinct types of dimers are induced in sodium dodecyl sulfate micelles and aerosol-OT (sodium dioctyl sulfoxinate, AOT) reversed micells.
Abstract: Properties of the ground and excited states of methylene blue (MB) were studied in negatively charged vesicles, normal and reverse micelles and sodium chloride solutions. All these systems induce dimer formation as attested by the appearance of the dimer band in the absorption spectra (lamdaD approximately 600 nm). In reverse micelles the dimerization constant (KD) corrected for the aqueous pseudophase volume fraction is two-three orders of magnitude smaller than KD of MB in water, and it does not change when W0 is increased from 0.5 to 10. Differences in the fluorescence intensity as a function of dimer-monomer ratio as well as in the resonance light scattering spectra indicate that distinct types of dimers are induced in sodium dodecyl sulfate (SDS) micelles and aerosol-OT (sodium dioctyl sulfoxinate, AOT) reversed micelles. The properties of the photoinduced transient species of MB in these systems were studied by time-resolved near infrared (NIR) emission (efficiency of singlet oxygen generation), by laser flash photolysis (transient spectra, yield and decay rate of triplets) and by thermal lensing (amount of heat deposited in the medium). The competition between electron transfer (dye*-dye) and energy transfer (dye*-O2) reactions was accessed as a function of the dimer-monomer ratio. The lower yield of electron transfer observed for dimers in AOT reverse micelles and intact vesicles compared with SDS micelles and frozen vesicles at similar dimer-monomer ratios is related with the different types of aggregates induced by each interface.
176 citations
TL;DR: In this article, a joint computational and experimental study for a series of annulated metalloporphyrins was performed to establish structure-property relationships, and the effects of substitution by meso-tetraphenylation, tetrabenzo and tetranaphtho annulation, and effects of changing the central metal from zinc (Zn) to palladium (Pd).
Abstract: We present results of a joint computational and experimental study for a series of annulated metalloporphyrins in order to establish structure−property relationships. Specifically, we have examined the effects of substitution by meso-tetraphenylation, tetrabenzo and tetranaphtho annulation, and effects of changing the central metal from zinc (Zn) to palladium (Pd). Utilizing absorption and emission spectroscopy and laser flash photolysis techniques, the photophysical properties of these porphyrins have been determined. Upon the addition of benzo or naphtho groups, we observed an overall red shift in the ground state absorption spectra of both the B-bands and the Q-bands with increased conjugation and an increase in the Q-band to B-band intensity ratios. Time-dependent density functional theory calculations were performed on both series of porphyrins to identify the effects of phenyl, benzo, and naphtho substituents on the spectra. The benzo and naphtho adducts provide a larger contribution (typically 40−9...
158 citations
TL;DR: This study presents clear evidence for the Marcus inverted region in both the electron-transfer reduction and oxidation of fullerenes in a truly intermolecular electron transfer.
Abstract: Pulse-radiolytic studies were performed to determine the rate constants of intermolecular electron transfer (k e t ) from fullerenes (C 6 0 , C 7 6 , and C 7 8 ) to a series of arene radical cations in dichloromethane. The one-electron oxidation potentials of the employed arenes-corresponding to the one-electron reduction potentials of arene π-radical cations-were determined in dichloromethane to evaluate the driving forces of electron-transfer oxidation of fullerenes with arene π-radical cations. The driving force dependence of log k e t shows a pronounced decrease towards the highly exothermic region, representing the first definitive confirmation of the existence of the Marcus inverted region in a truly intermolecular electron transfer. Electron-transfer reduction of fullerenes with anthracene radical anion was also examined by laser flash photolysis in benzonitrile. The anthracene radical anion was produced by photo-induced electron transfer from 10,10'-dimethyl-9,9',10,10'-tetrahydro-9,9'-bi-acridine [(AcrH) 2 ] to the singlet excited state of anthracene in benzonitrile. The rate constants of electron transfer (k e t ) from anthracene radical anion to C 6 0 , C 7 0 , and a C 6 0 derivative were determined from the decay of anthracene radical anion in the presence of various concentrations of the fullerene. Importantly, a significant decrease in the k e t value was observed at large driving forces (1.50 eV) as compared to the diffusion-limited value seen at smaller driving forces (0.96eV). In conclusion, our study presents clear evidence for the Marcus inverted region in both the electron-transfer reduction and oxidation of fullerenes.
151 citations
TL;DR: It was conclusively shown that with both sensitizers, the photooxygenations of PhSMe occur by an electron transfer (ET) mechanism, as no sulfoxidation was observed in the presence of benzoquinone (BQ), which is a trap for O2-*, NMQ*, and DCA-*.
Abstract: Photooxygenations of PhSMe and Bu2S sensitized by N-methylquinolinium (NMQ+) and 9,10-dicyanoanthracene (DCA) in O2-saturated MeCN have been investigated by laser and steady-state photolysis. Laser photolysis experiments showed that excited NMQ+ promotes the efficient formation of sulfide radical cations with both substrates either in the presence or in absence of a cosensitizer (toluene). In contrast, excited DCA promotes the formation of radical ions with PhSMe, but not with Bu2S. To observe radical ions with the latter substrate, the presence of a cosensitizer (biphenyl) was necessary. With Bu2S, only the dimeric form of the radical cation, (Bu2S)2+•, was observed, while the absorptions of both PhSMe+• and (PhSMe)2+• were present in the PhSMe time-resolved spectra. The decay of the radical cations followed second-order kinetics, which in the presence of O2, was attributed to the reaction of the radical cation (presumably in the monomeric form) with O2-• generated in the reaction between NMQ• or DCA-• a...
141 citations
TL;DR: Time-resolved FTIR spectroscopic studies of the flash photolysis of several 1-(2-nitrophenyl)ethyl ethers derived from aliphatic alcohols showed that a long-lived hemiacetal intermediate was formed during the reaction, and AMSOL calculations indicate that the pK(a) of the nitronic acid isomer formed byphotolysis is likely to determine partition into the alternate paths.
Abstract: Time-resolved FTIR spectroscopic studies of the flash photolysis of several 1-(2-nitrophenyl)ethyl ethers derived from aliphatic alcohols showed that a long-lived hemiacetal intermediate was formed during the reaction. Breakdown of this intermediate was rate-limiting for product release. One of these compounds (methyl 2-[1-(2-nitrophenyl)ethoxy]ethyl phosphate, 9) was studied in detail by a combination of time-resolved FTIR and UV−vis spectroscopy. In addition, product studies confirmed clean photolytic decomposition to the expected alcohol, 2-hydroxyethyl methyl phosphate, and the 2-nitrosoacetophenone byproduct. At pH 7.0, 1 °C, the rate constant for product release was 0.11 s-1, very much slower than the 5020 s-1 rate constant for decay of the photochemically generated aci-nitro intermediate (pH 7.0, 2 °C). Time-resolved UV−vis measurements showed that the hemiacetal intermediate is formed by two competing pathways, with fast (∼80% of the reaction flux) and slow (∼20% of the flux) components. Only the ...
116 citations
TL;DR: In this paper, the photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and nanosecond laser flash photolysis (LFP) techniques.
Abstract: Photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and picosecond laser flash photolysis (LFP) and nanosecond pulse radiolysis (PR) techniques. C120 shows unusual photophysical properties in nonpolar solvents compared to those in other solvents of moderate to higher polarities. Where the Stokes shifts (Δν=νabs−νfl), fluorescence quantum yields (Φf), and fluorescence lifetimes (τf) show more or less linear correlation with the solvent polarity function Δf={(e−1)/(2e+1)−(n2−1)/(2n2+1)}, all these parameters are unusually lower in nonpolar solvents. Unlike in other solvents, both Φf and τf in nonpolar solvents are also strongly temperature dependent. It is indicated that the excited singlet (S1) state of C120 undergoes a fast activation-controlled nonradiative deexcitation in nonpolar solvents, which is absent in all other solvents. LFP and PR studies indicate that the intersystem crossing process is negligible for the present dye in all the solvents studied. Photophysical behavior of C120 in nonpolar solvent has been rationalized assuming that in these solvents the dye exists in a nonpolar structure, with its 7-NH2 group in a pyramidal configuration. In this structure, since the 7-NH2 group is bonded to the 1,2-benzopyrone moiety by a single bond, the former group can undergo a fast flip-flop motion, which in effect causes the fast nonradiative deexcitation of the dye excited state. In moderate to higher polarity solvents, it is indicated that the dye exists in an intramolecular charge-transfer structure, where the bond between 7-NH2 group and the 1,2-benzopyrone moiety attains substantial double bond character. In this structure, the flip-flop motion of the 7-NH2 group is highly restricted and thus there is no fast nonradiative deexcitation process for the excited dye.
114 citations
TL;DR: The site selective oxidation of G and 8-oxo-G residues by carbonate radicals results in the formation of unique end products, the diastereomeric spiroiminodihydantoin (Sp) lesions, the products of a four electron oxidation of guanine.
Abstract: Carbonate radical anions are potentially important oxidants of nucleic acids in physiological environments. However, the mechanisms of action are poorly understood, and the end products of oxidation of DNA by carbonate radicals have not been characterized. These oxidation pathways were explored in this work, starting from the laser pulse-induced generation of the primary radical species to the identification of the stable oxidative modifications (lesions). The cascade of events was initiated by utilizing 308 nm XeCl excimer laser pulses to generate carbonate radical anions on submicrosecond time scales. This laser flash photolysis method involved the photodissociation of persulfate to sulfate radical anions and the one electron oxidation of bicarbonate anions by the sulfate radicals to yield the carbonate radical anions. The latter were monitored by their characteristic transient absorption band at 600 nm. The rate constants of reactions of carbonate radicals with oligonucleotides increase in the ascendin...
105 citations
TL;DR: The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus.
Abstract: The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 μs. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 ± 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.
105 citations
TL;DR: In this article, the quenching of excited singlet and triplet states of riboflavin, lumiflavin and lumichrome was investigated in methanol. The quenchers were aromatic electron donors and aliphatic amines.
Abstract: The quenching of excited singlet and triplet states of riboflavin, lumiflavin and lumichrome was investigated in methanol. The quenchers were aromatic electron donors and aliphatic amines. Bimolecular quenching rate constants were determined from static and dynamic fluorescence measurements. Triplet quenching was studied by laser flash photolysis. Transient absorption spectra showed the presence of semireduced flavins and lumichrome, and the radical cation of quenchers. The results confirm that the quenching rate constants for aliphatic donors are lower than those of aromatic donors of similar oxidation potential. Plots of the quenching rate constants vs. the free energy for the electron transfer reaction, ΔG°, were fitted by the Rehm–Weller model of electron transfer quenching. The aliphatic quenchers needed a higher intrinsic barrier for the fitting that was ascribed to the internal reorganisation of the amines. For the aromatic donors the singlet quenching rate constants reach the diffusional limit at highly negative ΔG°. However, for the triplet quenching the limiting value of the rate constants is lower than the plateau of the singlet quenching. This is explained in terms of the non-adiabaticity of the triplet quenching process.
95 citations
TL;DR: In this article, the photophysical properties of fac-[Re(CO)3(dppz)(py)]+======(1, where dppz = dipyrido[3,2-a:2′,3′-c]phenazine) in CH3CN have been investigated using a series of complementary techniques including visible and infrared transient absorption and resonance Raman spectroscopy on the picosecond and nanosecond timescales.
Abstract: The photophysical properties of fac-[Re(CO)3(dppz)(py)]+
(1, where dppz = dipyrido[3,2-a:2′,3′-c]phenazine) in CH3CN have been investigated using a series of complementary techniques including visible and infrared transient absorption and resonance Raman spectroscopy on the picosecond and nanosecond timescales. The results confirm previous reports that the lowest-lying emissive state in 1 is a triplet intra-ligand (3IL) state localised on the dppz ligand and have provided detailed information on the dynamics of 1 upon photoexcitation, including the relative energies of the excited state species encountered and the electronic distribution within these. If the dppz ligand is viewed in terms of phenanthroline (phen) and phenazine (phz) moieties, the emissive state is probably more accurately described as a 3π→π*(phz) IL state. The picosecond studies have shown that this emissive state is formed, at least in part, within 30 ps of excitation from a precursor, which is possibly a 3π→π*(phen) IL state. On the nanosecond timescale, TRIR has been employed to elucidate further dynamics and reveal the presence of an energetically close-lying state in equilibrium with the emissive state. This has tentatively been assigned as being 3dπ(Re)→π*(phz) metal-to-ligand charge transfer (MLCT) in nature. A summary of the photophysics is proposed in the form of a Jablonski scheme. Time dependent density functional theory (TD-DFT) calculations support the relative ordering and suggested electronic character of the excited state species involved.
TL;DR: Irradiation of porphyrin-manganese(III) perchlorate complexes in acetonitrile with 355 nm laser light gave MnV-oxo intermediates that were characterized by their UV-vis spectra and reactivities.
Abstract: Irradiation of porphyrin-manganese(III) perchlorate complexes in acetonitrile with 355 nm laser light gave MnV-oxo intermediates that were characterized by their UV−vis spectra and reactivities. The MnV-oxo species of tetrakis(pentafluorophenyl)porphyrin (2), tetraphenylporphyrin (4), and tetra-(4-(N-methylpyridiniumyl))porphyrin (6) were generated. Second-order rate constants for reactions of 2 with substrates were as follows: 6.1 × 105 M-1 s-1 (cis-stilbene), 1.3 × 105 M-1 s-1 (diphenylmethane), 1.3 × 105 M-1 s-1 (ethylbenzene), and 0.55 × 105 M-1 s-1 (ethylbenzene-d10). In oxidations of cis-stilbene and diphenylmethane, the order of reactivity of the MnV-oxo species was 2 > 6 > 4.
TL;DR: Despite their high photoreactivity, cyclopropenones were found to be thermally stable compounds with the exception of hydroxy- and methoxy-substituted cycloprostenones, which undergo rapid solvolysis in hydroxylic solvents even at room temperature.
Abstract: UV irradiation of alkyl-, aryl-, and heteroatom-substituted cyclopropenones results in the loss of carbon monoxide and the formation of quantitative yields of corresponding alkynes. The quantum yield of the photochemical decarbonylation reaction ranges from 20% to 30% for alkyl-substituted cyclopropenones to above 70% for the diphenyl- and dinaphthylcyclorpopenones. Rapid formation (<5 ns) and then a somewhat slower decay (ca. 40 ns) of an intermediate in this reaction was observed by using laser flash photolysis. The DFT calculations allowed us to identify this intermediate as a zwitterionic species formed by a cleavage of one of the carbon−carbon bonds of the cyclopropenone ring. The latter then rapidly loses carbon monoxide to produce the ultimate acetylenic product. Despite their high photoreactivity, cyclopropenones were found to be thermally stable compounds with the exception of hydroxy- and methoxy-substituted cyclopropenones. The latter undergo rapid solvolysis in hydroxylic solvents even at room...
TL;DR: Nanosecond flash photolysis studies of Fe(2)(SR)(2)(NO)(4) indicate that the initial photoreaction is the reversible dissociation of NO and under aerated conditions the intermediate reacts with oxygen to give permanent photochemistry.
Abstract: The photochemistry of various Roussin's red ester compounds of the general formula Fe2(SR)2(NO)4, where R = CH3, CH2CH3, CH2C6H5, CH2CH2OH, and CH2CH2SO3-, were investigated. Continuous photolyses of these ester compounds in aerated solutions led to the release of NO with moderate quantum yields for the photodecomposition of the ester (ΦRSE = 0.02−0.13). Electrochemical studies using an NO electrode demonstrated that 4 mol of NO are generated for each mole of ester undergoing photodecomposition. Nanosecond flash photolysis studies of Fe2(SR)2(NO)4 (where R = CH2CH2OH and CH2CH2SO3-) indicate that the initial photoreaction is the reversible dissociation of NO. In the absence of oxygen, the presumed intermediate, Fe2(SR)2(NO)3, undergoes second-order reaction with NO to regenerate the parent cluster with a rate constant of kNO = 1.1 × 109 M-1 s-1 for R = CH2CH2OH. Under aerated conditions the intermediate reacts with oxygen to give permanent photochemistry.
TL;DR: The hydrogen-bonding dynamics in the photoinduced electron transfer from the ferrocene (Fc) to the quinone moiety (Q) in Fc-Q have been successfully detected in the femtosecond laser flash photolysis experiments.
Abstract: A ferrocene-quinone dyad (Fc-Q) with a rigid amide spacer and Fc-(Me)Q dyad, in which the amide proton acting as a hydrogen-bonding acceptor is replaced by the methyl group, are employed to examine the effects of hydrogen bonding on both the thermal and the photoinduced electron-transfer reactions The hydrogen bonding of the semiquinone radical anion with the amide proton in Fc-Q•- produced by the electron-transfer reduction of Fc-Q is indicated by the significant positive shift of the one-electron reduction potential of Fc-Q The hyperfine coupling constants of Fc-Q•- also indicate the existence of hydrogen bonding, agreeing with those predicted by the density functional calculation The hydrogen-bonding dynamics in the photoinduced electron transfer from the ferrocene (Fc) to the quinone moiety (Q) in Fc-Q have been successfully detected in the femtosecond laser flash photolysis experiments Thermal intramolecular electron transfer from Fc to Q in Fc-Q and Fc-(Me)Q also occurs efficiently in the presen
TL;DR: The electronic and vibrational spectra of the intermediates, computed by density functional theory, support the assignment of the transient spectra, observed in the formation of 9 from 1a, and successfully predicted by CASPT2/6-31G calculations.
Abstract: The photochemistry of ortho-biphenyl azide (1a) has been studied by laser flash photolysis (LFP), with UV−vis and IR detection of the transient intermediates formed. LFP (266 nm) of 1a in glassy 3-methylpentane at 77 K releases singlet ortho-biphenylnitrene (1b) (λmax = 410 nm, τ = 59 ± 6 ns), which under these conditions decays cleanly to the lower energy triplet state. In fluid solution at 298 K, 1b rapidly (τ < 10 ns) partitions between formation of isocarbazole (4) (λmax = 430 nm, τ = 70 ns) and benzazirine (1e) (λmax = 305 nm, τ = 12 ns). Isocarbazole 4 undergoes a 1,5-hydrogen shift, with kH/kD = 3.4 at 298 K to form carbazole 9 and smaller amounts of two other isocarbazoles (7 and 8). Benzazirine 1e ring-opens reversibly to azacycloheptatetraene (1f), which serves as a reservoir for singlet nitrene 1b. Azacycloheptatetraene 1f ultimately forms carbazole 9 on the millisecond time scale by the pathway 1f → 1e → 1b → 4 → 9. The energies of the transient intermediates and of the transition structures c...
TL;DR: It is concluded that a synergy between the spin prohibition and the reaction energetics creates an intersystem barrier and is responsible for slowness of the spin-forbidden deprotonation of (1)HNO by OH(-); thespin prohibition alone plays a minor role.
Abstract: The first mechanistic study of a spin-forbidden proton-transfer reaction in aqueous solution is reported. Laser flash photolysis of alkaline trioxodinitrate (N2O32-, Angeli's anion) is used to generate a nitroxyl anion in its excited singlet state (1NO-). Through rapid partitioning between protonation by water and electronic relaxation, 1NO- produces 1HNO (ground state, yield 96%) and 3NO- (ground state, yield 4%), which comprise a unique conjugate acid−base couple with different ground-state multiplicities. Using the large difference between reactivities of 1HNO and 3NO- in the peroxynitrite-forming reaction with 3O2, the kinetics of spin-forbidden deprotonation reaction 1HNO + OH- → 3NO- + H2O is investigated in H2O and D2O. Consistent with proton transfer, this reaction exhibits primary kinetic hydrogen isotope effect k(H)/k(D) = 3.1 at 298 K, which is found to be temperature-dependent. Arrhenius pre-exponential factors and activation energies of the second-order rate constant are found to be: log(A, ...
TL;DR: In this article, the authors used laser flash photolysis at 248 nm to generate OH radicals and laser-induced fluorescence to monitor their decay under pseudo-first-order conditions, [OH] ≪ [SO2].
Abstract: The kinetics of the association reaction OH + SO2 have been studied using laser flash photolysis at 248 nm to generate OH radicals and laser-induced fluorescence to monitor their decay under pseudo-first-order conditions, [OH] ≪ [SO2]. The removal kinetics of OH(v = 1) + SO2 have been measured over the temperature range of 295 to 673 K. Master equation calculations were performed to demonstrate that, provided intramolecular vibrational redistribution is fast, OH(v = 1) + SO2 is a good approximation for the high-pressure rate coefficient of the OH(v = 0) + SO2 + M reaction, giving k1∞(T) = (2.04 ± 0.10) × 10-12 (T/300 K)-0.27 ± 0.11 cm3 molecule-1 s-1. This temperature dependence of the rate coefficient suggests that the reaction occurs on a barrierless surface. The kinetics of the reaction OH(v = 0) + SO2 + M, k1, were also studied. At room temperature, the kinetic data were in good agreement with literature values. At elevated temperatures, 523 to 603 K, equilibrium behavior was observed between OH + SO2...
TL;DR: In this article, the structures of these composites were established by TEM, elemental analysis, and 1 H NMR spectroscopy, and changes in optical and physical properties caused by attachment of the fluorenyl groups to the gold nanoparticles were studied by absorption, Fourier transform infrared, and fluorescence spectroscopies.
Abstract: 9-(9-Fluorenyl)-nonane-1-thiolate and 12-(9-fluorenyl)-dodecane-1-thiolate (fluorenyl-alkane-1-thiolates) monolayer-protected gold clusters (Au-MPCs) have been prepared by place exchange of fluorenyl-alkane-1-thiol with alkane-1-thiolate Au-MPCs. The structures of these composites were established by TEM, elemental analysis, and 1 H NMR spectroscopy. Changes in optical and physical properties caused by attachment of the fluorenyl groups to the gold nanoparticles were studied by absorption, Fourier transform infrared, and fluorescence spectroscopies. Emission from the end-attached fluorenyl groups is substantially quenched upon attachment to a gold cluster so that less than 5% of emission intensity from an optically matched solution of the analogous freely dissolved fluorene and alkane-1-thiolate Au-MPCs is retained in the fluorenyl-alkane-1-thiolate Au-MPCs. Nanosecond flash photolysis shows that intersystem crossing (from the fluorenyl singlet to the corresponding triplet) was suppressed because energy transfer was much faster. No direct evidence of electron transfer from the excited fluorenyl group to the gold cluster could be observed, implying that electronic coupling through energy transfer accounts for most of the observed emission quenching and suppressed intersystem crossing.
TL;DR: In this article, the authors highlight the application of time-resolved infrared spectroscopy to excited states by probing the nature of the lowest excited states of fac-[Re(CO)3(dppz-Cl2)(R)]n+======¯¯(R = Cl−======�
Abstract: Time-resolved infrared (TRIR) spectroscopy, a combination of UV flash photolysis and fast infrared detection, is a powerful technique for probing excited states and detecting reaction intermediates. In this Perspective we highlight the application of TRIR to excited states by probing the nature of the lowest excited states of fac-[Re(CO)3(dppz-Cl2)(R)]n+
(R = Cl−
(n
= 0), py (n
= 1) and 4-Me2N-py (n
= 1); dppz-Cl2
= 11,12-dichlorodipyrido[3,2-a:2′,3′-c]phenazine) in CH3CN. The characterisation of [Cr(η6-C6H6)(CO)2Xe] and [Re(η5-C5H5)(CO)2(C2H6)] in supercritical Xe and liquid ethane solution exemplifies how this technique can be applied to detect new organometallic species.
TL;DR: The results show, for the first time, that carotenoid addition radicals do not react with oxygen to form carOTenoid peroxyl radicals; an observation which is of significance in relation to antioxidant/pro-oxidant properties of carotanoids.
Abstract: In this paper, we report the results of a laser flash photolysis study of the reactions of a range of carotenoids with acylperoxyl radicals in polar and nonpolar solvents. The results show, for the first time, that carotenoid addition radicals do not react with oxygen to form carotenoid peroxyl radicals; an observation which is of significance in relation to antioxidant/pro-oxidant properties of carotenoids. Acylperoxyl radicals, generated by photolysis of ketone precursors in oxygenated solvents, display high reactivity toward carotenoids in both polar and nonpolar solvents, but the nature of the carotenoid radicals formed is dependent on solvent polarity. In hexane, acylperoxyl radicals react with carotenoids with rate constants in the region of 109 M-1 s-1 and give rise to transient absorption changes in the visible region that are attributed to the formation of addition radicals. All of the carotenoids show bleaching in the region of ground-state absorption and, with the exception of 7,7‘-dihydro-β-ca...
TL;DR: In this article, the chromophores Ru(bpy)3(PF6)2 and Os (bpy)-polymethyl methacrylate (PMMA) thin films on glass substrates were used as actinometers for nanosecond flash photolysis.
Abstract: The chromophores Ru(bpy)3(PF6)2 and Os(bpy)3(PF6)2 were immobilized within poly(methyl methacrylate) (PMMA) thin films on glass substrates for applications as actinometers for nanosecond flash photolysis. Transient absorption difference spectra of M(bpy)3(PF6)2 (M = Ru, Os), at ambient temperature and in an argon atmosphere, were the same when imbedded in PMMA films as in solution, within experimental error. Linear ranges of ΔA versus 532 nm pulsed laser energy where these actinometers were applicable were identified, up to 25 mJ/(cm2 pulse) for Ru(bpy)32+/PMMA and up to 5 mJ/(cm2 pulse) for Os(bpy)32+/PMMA. Laser energy measurements were used to estimate the difference between the excited- and ground-state extinction coefficients at 450 nm for Os(bpy)32+, Δe450nm, which is −7300 M-1 cm-1. The Ru(bpy)32+/PMMA actinometer was useful from 300 to ∼550 nm, while the Os(bpy)32+/PMMA actinometer extends the sensitivity to ∼700 nm. An application of these actinometers for dye-sensitized solar cells is described,...
TL;DR: The conditions determining the efficiency and mode of dehalogenation have been defined and are significant for devising synthetic methods via photogenerated phenyl cations and for rationalizing the photodegradation of halogenated aromatic pollutants and the phototoxic effect of some fluorinated drugs.
Abstract: The photochemistry of 4-haloanilines and 4-halo-N,N-dimethylanilines has been studied in apolar, polar aprotic, and protic solvents. Photophysical and flash photolysis experiments show that the reaction proceeds in any case from the triplet state. It is rather unreactive in apolar media, the highest value being Phi = 0.05 for the iodoanilines in cyclohexane. Changing the solvent has little effect for iodoanilines and for the poorly reacting bromo analogue, while it leads to a variation of over 2 orders of magnitude in the quantum yield for the chloro and fluoro derivatives. The triplets have been characterized at the UB3LYP/6-31G(d) level of theory, evidencing a deformation and an elongation (except for C-F) of the C-X bond. Homolytic fragmentation is in every case endothermic, but calculations in acetonitrile solution show that heterolytic cleavage of C-Cl and C-Br is exothermic. Experimentally, the occurrence of heterolytic fragmentation has been monitored through selective trapping of the resulting phenyl cation by allyltrimethylsilane. Heterolytic dechlorination occurs efficiently in polar media (e.g., Phi = 0.77 in MeCN), while debromination remains ineffective due to the short lifetime of the triplet. Heterolytic defluorination is efficient only in protic solvents (Phi = 0.48 in MeOH), in accord with calculations showing that in the presence of an ancillary molecule of water fragmentation is exothermic due to the formation of the strong H-F bond. The energy profile for both homo- and heterolytic dissociation paths has been mapped along the reaction coordinates in the gas phase and in acetonitrile. The conditions determining the efficiency and mode of dehalogenation have been defined. This is significant for devising synthetic methods via photogenerated phenyl cations and for rationalizing the photodegradation of halogenated aromatic pollutants and the phototoxic effect of some fluorinated drugs.
TL;DR: The photopolymerization of acrylamide (AA) initiated by the synthetic dye safranine-T in the presence of triethanolamine (TEOHA) as co-initiator has been investigated in aqueous solution as discussed by the authors.
TL;DR: In this paper, the dynamics of the proton-transfer reactions of three anthocyanins in the ground and first excited singlet state were determined using laser flash photolysis and single-photon counting.
Abstract: Malvidin-3,5-diglucoside (malvin), cyanidin-3,5-diglucoside (cyanin), and pelargonidin-3,5-diglucoside (pelargonin) are among the most representative anthocyanins because of their abundance in the most common red flowers and fruits. Anthocyanin color is directly affected by the pH-dependent chemistry of the red (acid) form of these compounds, while anthocyanin photostability is a function of the photophysics of the first excited singlet state. In the present work, we employ laser flash photolysis and picosecond time-correlated single-photon counting to determine the dynamics of the proton-transfer reactions of these three anthocyanins in the ground [deprotonation rate constants, kd = 1.3 × 106 s-1 (pelargonin), 1.8 × 106 s-1 (cyanin), and 3.8 × 106 s-1 (malvin)] and first excited singlet state [deprotonation rate constants, kd = 4.3 × 1010 s-1 (pelargonin), 4.0 × 1010 s-1 (cyanin), and 1.6 × 1011 s-1 (malvin)], respectively. The ground- and excited-state proton-transfer rate constants for anthocyanins and...
TL;DR: The photo-induced Diels−Alder reaction of anthracene and its derivatives with dienophiles (p-benzoquinone and fumaronitrile) occurs in competition with the dimerization of Anthracenes in chloroform at 298 K as discussed by the authors.
Abstract: Photochemical Diels−Alder reaction of anthracene and its derivatives with dienophiles (p-benzoquinone and fumaronitrile) occurs in competition with the dimerization of anthracenes in chloroform at 298 K. The dependence of the quantum yields on the concentrations of dienophiles has revealed that the photochemical Diels−Alder reaction proceeds via electron transfer from the singlet excited states of anthracenes to dienophiles. The rates of photoinduced electron transfer are diffusion limited, agreeing with the largely negative free energy change of electron-transfer judging from the more negative one-electron oxidation potentials of the singlet excited states of anthracenes than the one-electron reduction potentials of dienophiles. The radical ion pair produced in the photoinduced Diels−Alder from the singlet excited states of anthracenes to p-benzoquinone and fumaronitrile has been detected as the transient absorption spectrum at 298 K with use of laser flash photolysis. The diradical intermediates prior t...
TL;DR: These viologen-linked acridines are quite stable in aqueous solutions and oxidize DNA efficiently and hence can be useful as photoactivated DNA-cleaving agents which function purely by the co-sensitization mechanism.
Abstract: A new series of photoactivated DNA oxidizing agents in which an acridine moiety is covalently linked to viologen by an alkylidene spacer was synthesized, and their photophysical properties and interactions with DNA, including DNA cleaving properties, were investigated. The fluorescence quantum yields of the viologen-linked acridines were found to be lower than that of the model compound 9-methylacridine (MA). The changes in free energy for the electron transfer reactions were found to be favorable, and the fluorescence quenching observed in these systems is explained by an electron transfer mechanism. Intramolecular electron transfer rate constants were calculated from the observed fluorescence quantum yields and singlet lifetime of MA and are in the range from 1.06x10(10) s(-1) for 1 a (n=1) to 6x10(8) s(-1) for 1 c (n=11), that is, the rate decreases with increasing spacer length. Nanosecond laser flash photolysis of these systems in aqueous solutions showed no transient absorption, but in the presence of guanosine or calf thymus DNA, transient absorption due to the reduced viologen radical cation was observed. Studies on DNA binding demonstrated that the viologen-linked acridines bind effectively to DNA in both intercalative and electrostatic modes. Results of PM2 DNA cleavage studies indicate that, on photoexcitation, these molecules induce DNA damage that is sensitive to formamidopyrimidine DNA glycosylase. These viologen-linked acridines are quite stable in aqueous solutions and oxidize DNA efficiently and hence can be useful as photoactivated DNA-cleaving agents which function purely by the co-sensitization mechanism.
TL;DR: In this article, photo-induced electron transfer from α-methylstyrene to the singlet excited state of AcrH+ (1AcrH+*) was shown to proceed via photo induced electron transfer via laser flash photolysis and ESR measurements.
Abstract: Photooxygenation of α-methylstyrene with oxygen occurs efficiently in the presence of 10-methylacridinium perchlorate (AcrH+ClO4-) under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield acetophenone as the main oxygenated product No photoinduced oxygenation of α-methylstyrene occurs in the absence of AcrH+ under otherwise the same experimental conditions Little photodegradation of AcrH+ occurs in the present photocatalytic system, which provides a clean method of photoinduced oxygenation reaction with molecular oxygen, alternate to the ene reaction of singlet oxygen The photocatalytic oxygenation of α-methylstyrene with oxygen is shown to proceed via photoinduced electron transfer from α-methylstyrene to the singlet excited state of AcrH+ (1AcrH+*) on the basis of the fluorescence quenching of 1AcrH+* by α-methylstyrene, the quantum yield determination, and the detection of radical intermediates by laser flash photolysis and ESR measurements
TL;DR: A minimal ligand binding model is proposed that suggests that Met 95 displacement from the heme is facilitated by the presence of an external ligand in the heMe environment, suggesting substantial rearrangement of the heME environment associated with ligand sensing.
Abstract: In the heme-based oxygen sensor Dos from Escherichia coli, one of the axial ligands (Met 95) of a six-coordinate heme can be replaced by external ligands such as O2, NO, and CO, which causes a switch in phosphodiesterase activity. To gain insight into the bidirectional switching mechanism, we have studied the interaction of ligands with the sensor domain DosH by flash photolysis experiments with femtosecond time resolution. The internal ligand can be photodissociated from the ferrous heme and recombines with time constants of 7 and 35 ps. This is somewhat slower than recombination of the external ligands NO, with which picosecond rebinding occurs with unprecedented efficiency (>99%) with a predominant phase of ∼5 ps, and O2 (97% in 5 ps, Liebl, U., Bouzhir-Sima, L., Negrerie, M., Martin, J.-L., and Vos, M. H. (2002) Proc. Natl. Acad. Sci. U.S.A. 99, 12771−12776). Dissociated CO displays geminate rebinding in 1.5 ns with a very high yield (60%). Together these results indicate that the heme environment pro...
TL;DR: Time-resolved experiments showed that BT ketyl (BTH) and phenoxy (PhO) or indolyl (In) radicals are generated with high rate constants and quantum yields close to 1.5%, indicating that recombination within the radical pairs must occur to a large extent, regenerating the starting materials.
Abstract: Laser flash photolysis, density functional theory (DFT) calculations, and product studies have been performed to understand the mechanism of photoreduction of 2-benzoylthiophene (BT) in the presence of phenol or indole. Time-resolved experiments showed that BT ketyl (BTH) and phenoxy (PhO) or indolyl (In) radicals are generated with high rate constants and quantum yields close to 1. However, low conversions (specially in the case of indole) of the starting reagents are obtained upon prolonged lamp irradiation, indicating that recombination within the radical pairs must occur to a large extent, regenerating the starting materials. The solvent-dependence of the quenching rate constants, together with DFT theoretical studies, indicate fundamental differences between the mechanisms of the reaction of BT triplet with phenol and indole. Thus, data for phenol agree with the involvement of a hydrogen-bonded exciplex BT...HOPh, where concerted electron and proton transfer leads to the BTH...OPh radical pair. Howev...