Showing papers on "Flash photolysis published in 2001"

Photodimerization of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photocycloaddition and of the photochemical and thermal cleavage

Abstract: One of the classics in photochemistry, the photodimerization of anthracenes can be considered as a paradigm of the photocycloaddition of non saturated hydrocarbons. The historical steps of the mechanistic studies are reviewed: based on fluorescence quenching, cyclization quantum yields measurement, the influence of dioxygen and solvents, they support a singlet state pathway; the dimerization rate constants are found to be generally high for reactions occurring within a few nanoseconds unless they are slowed down or inhibited by steric strain. In several cases, excimers have been demonstrated to be intermediates and it is shown that excimer fluorescence and cyclization are competitive processes. Another intermediate known as pericyclic minimum (or conical intersection) is postulated to form a sort of floppy cycloadduct where the reacting centres are at mutual distances shorter than in excimers and longer than in dimers. For intermolecular dimerizations, the triplet state is also reactive but through triplet–triplet annihilation in dilute solutions. Intramolecular photocycloadditions have also been carefully examined, for the role of multiple excimer formation, regioselectivity (9,10∶1′,4′ and 9,10∶1′,2′ cyclization) and solvent polarity. The triplet state reactivity is shown to lead to 4π + 2π or 4π + 4π cycloadducts, depending on geometric factors. In the latter case when intersystem crossing is favoured by the substituents, cyclization quantum yields as high as 0.65–0.72 have been observed. Photodissociation quantum yields are generally high and the reactions are partly adiabatic, leading to excimer and monomer fluorescence, but the major part follows another pathway not fully elucidated by flash photolysis. Thermodynamic and kinetic parameters for the thermal cleavage are given; they reveal a large gamut of stability for the photocycloadducts.

Unusual Photophysical Properties of Coumarin-151

Abstract: The photophysical properties of 7-amino-4-trifluoromethyl-1,2-benzopyrone (coumarin-151; C151) in nonpolar (NP) solvents are drastically different than those in other solvents of moderate to higher polarities. Thus, the absorption and fluorescence maxima are largely blue-shifted and the Stokes' shifts unusually low in NP solvents compared to those in the other solvents. The fluorescence quantum yields (Φf) in NP solvents are also exceptionally lower. While in all other solvents the fluorescence decays of C151 are single-exponential, in NP solvents the decays show non-single-exponential behavior. The average fluorescence lifetimes (τf) in NP solvents are also substantially shorter. Low Φf and τf values in NP solvents indicate very fast nonradiative deactivation for the S1 state of C151. Investigating the triplet characteristics of C151 by using picosecond laser flash photolysis and pulse radiolysis techniques, it has been established that the fast nonradiative deactivation in NP solvents is not due to an e...

Alkylation of amino acids and glutathione in water by o-quinone methide. Reactivity and selectivity.

Abstract: o-Quinone methide (1) has been produced in water both thermally and photochemically from (2-hydroxybenzyl)trimethylammonium iodide (2). Michael addition reactions of 1 to various amines, and sulfides, including amino acids and glutathione have been carried out, obtaining alkylated adducts (3-16) in fairly good to quantitative yields. The reaction rate and selectivity of 1 toward nitrogen and sulfur nucleophiles, in competition with the hydration reaction, have been investigated at different pH by laser flash photolysis technique. The observed reactivity spans 7 orders of magnitude on passing from water (kNu = 5.8 M-1 s-1) to the most reactive nucleophile (2.8 x 10(8) M-1 s-1, 2-mercaptoethanol under alkaline conditions). These are the first direct reaction rate measurements of nucleophilic addition to the parent o-quinone methide (1). Competition experiments provided strong kinetic support to the involvement of free 1 as an intermediate in both thermal and photochemical reactions. Furthermore, several alkylation adducts regenerate 1 either by heating (9, 10, 13, and 14) or by irradiation (9, 11-13, 16). Such a thermal and photochemical reversibility of the alkylation process opens a new perspective for the use and application of such adducts as o-QM molecular carriers.

Kinetics and Mechanism of the Reversible Binding of Nitric Oxide to Reduced Cobalamin B12r (Cob(II)alamin)

Abstract: The reduced form of aquacobalamin binds nitric oxide very effectively to yield a nitrosyl adduct, Cbl(II)−NO. UV−vis, 1H-, 31P-, and 15N NMR data suggest that the reaction product under physiological conditions is a six-coordinate, “base-on” form of the vitamin with a weakly bound α-dimethylbenzimidazole base and a bent nitrosyl coordinated to cobalt at the β-site of the corrin ring. The nitrosyl adduct can formally be described as Cbl(III)−NO-. The kinetics of the binding and dissociation reactions was investigated by laser flash photolysis and stopped-flow techniques, respectively. The activation parameters, ΔH⧧, ΔS⧧, and ΔV⧧, for the forward and reverse reactions were estimated from the effect of temperature and pressure on the kinetics of these reactions. For the “on” reaction of Cbl(II) with NO, the small positive ΔS⧧ and ΔV⧧ values suggest the operation of a dissociative interchange (Id) substitution mechanism at the Co(II) center. Detailed laser flash photolysis and 17O NMR studies provide evidence...

Kinetic Study of the Reactions of Chlorine Atoms and Cl2•- Radical Anions in Aqueous Solutions. II. Toluene, Benzoic Acid, and Chlorobenzene†

Abstract: Laser and conventional flash photolysis of Na2S2O8 aqueous solutions containing Cl- ions were employed to investigate the reactions of chlorine atoms and Cl2•- radical ions with toluene, benzoic acid, and chlorobenzene. A mechanism is proposed which accounts for the faster decay of Cl2•- in aqueous solutions containing increasing concentrations of the organic substrates. Interpretation of the experimental data is supported by kinetic computer simulations. Chlorine atoms react with the three substituted aromatics studied here almost with diffusion-controlled rate constants, k = (1.8 ± 0.3) × 1010 M-1 s-1. The high reactivity observed for Cl atoms contrasts with that of the Cl2•- radical ions, for which the rate constant for its reactions with the substituted benzenes is ≤ 1 × 106 M-1 s-1. The organic radicals produced from these reactions, as well as the nature of the reaction products are discussed. The observed results seem to support an addition mechanism yielding chlorocyclohexadienyl radicals (Cl−CHD)...

Sensitised luminescence from phenanthridine appended lanthanide complexes: analysis of triplet mediated energy transfer processes in terbium, europium and neodymium complexes

Abstract: Time resolved luminescence spectroscopy and laser flash photolysis have been used to probe the energy transfer processes in the europium, neodymium and terbium complexes of ligand, 1. The ligand, chosen because of the high kinetic stability of lanthanide complexes derived from amide-functionalised azamacrocycles, bears a phenanthridine chromophore that absorbs light at 355 nm, and which can be protonated at low pH. We show that triplet mediated energy transfer to the metal centre occurs in the protonated complexes.

Photolabile Protecting Groups for Nucleosides: Mechanistic Studies of the 2-(2-Nitrophenyl)ethyl Group

Abstract: The photochemistry of several 2-(2-nitrophenyl)ethyl-caged compounds including caged thymidine nucleosides was studied by nanosecond laser flash photolysis and stationary illumination experiments with quantitative HPLC analysis for quantum yields and product distribution. Effects of solvent basicity and acidity were investigated by varying the H2O content and HCl concentration, respectively, in MeCN/H2O mixtures. For all compounds 1 ‐ 7 investigated, intramolecular H abstraction by the nitro group from the exocyclic a-position with respect to the aryl moiety was found to be the primary process. The protolytic dissociation equilibrium of the resulting aci-nitro compound was kinetically characterized in the 0.1 ‐ 10 ms time region. In general, two reaction channels compete for the aci-nitro compound and its anion: b-elimination of the caged compound occurs from the anion, while from the undissociated aci-nitro compound, a nitrosobenzene derivative is formed with no release of the caged compound. The yield ratio of these two reaction channels can be controlled through shifts in the protolytic dissociation equilibrium of the aci-nitro compound. In solutions with either low basicity (H2O-free MeCN) or high acidity (higher concentration of HCl in H2O/MeCN), two as yet unidentified products are formed, each one specifically for one of the mentioned conditions.

Substituent effects on the reactivity of the silicon-carbon double bond.

Abstract: Laser flash photolysis of various organosilicon compounds such as aryl-, vinyl-, and alkynyldisilanes, silacyclobutanes and silacyclobutenes, and alpha-silylketenes and -diazomethanes leads to the formation of reactive silenes which can be detected directly in solution, allowing detailed studies of the kinetics and mechanisms of their reactions with nucleophiles. Over 30 transient silenes have now been studied by these methods, providing the opportunity to systematically assess the effects of substituents at silicon and carbon on the reactivity of the Si=C bond.

Flash photolytic generation of ortho-quinone methide in aqueous solution and study of its chemistry in that medium.

Abstract: Flash photolysis of o-hydroxybenzyl alcohol, o-hydroxybenzyl p-cyanophenyl ether, and (o-hydroxybenzyl)trimethylammonium iodide in aqueous perchloric acid and sodium hydroxide solutions, and in acetic acid and biphosphate ion buffers, produced o-quinone methide as a short-lived transient species that underwent hydration back to benzyl alcohol in hydrogen-ion catalyzed (k(H+) = 8.4 x 10(5) M(-1) s(-1)) and hydroxide-ion catalyzed (k(HO)- = 3.0 x 10(4) M(-1) s(-1)) reactions as well as an uncatalyzed (k(UC) = 2.6 x 10(2) s(-1)) process. The hydrogen-ion catalyzed reaction gave the solvent isotope effect k(H+)/k(D)+ = 0.42, whose inverse nature indicates that this process occurs by rapid and reversible equilibrium protonation of the carbonyl oxygen atom of the quinone methide, followed by rate-determining capture of the carbocation so produced by water. The magnitude of the rate constant of the uncatalyzed reaction, on the other hand, indicates that this process occurs by simple nucleophilic addition of water to the methylene group of the quinone methide. Decay of the quinone methide is also accelerated by acetic acid buffers through both acid- and base-catalyzed pathways, and quantitative analysis of the reaction products formed in these solutions shows that this acceleration is caused by nucleophilic reactions of acetate ion rather than by acetate ion assisted hydration. Bromide and thiocyanate ions also accelerate decay of the quinone methide through both hydrogen-ion catalyzed and uncatalyzed pathways, and the inverse nature of solvent isotope effects on the hydrogen-ion catalyzed reactions shows that these reactions also occur by rapid equilibrium protonation of the quinone methide carbonyl oxygen followed by rate-determining nucleophilic capture of the ensuing carbocation. Assignment of an encounter-controlled value to the rate constant for the rate-determining step of the thiocyanate reaction leads to pK(a) = -1.7 for the acidity constant of the carbonyl-protonated quinone methide.

Photochemical Reaction Mechanisms of 2-Nitrobenzyl Compounds in Solution, I. 2-Nitrotoluene: Thermodynamic and Kinetic Parameters of the aci-Nitro Tautomer

Abstract: The largely reversible, light-induced tautomerization of 2-nitrotoluene (1) to the quinonoid aci-nitro tautomer aci-1 was studied by flash photolysis as a benchmark for comparison with the widely used nitrobenzyl phototriggers (`caged compounds'). The pH-rate profile for the decay of aci-1 in aqueous solution exhibits downward curvature at pH 3 – 4, which is attributed to pre-equilibrium ionization of the nitronic acid aci-1 to its anion 1− (pKa=3.57). Two regions of upward curvature, at pH ca. 6 and 6. A hird, irreversible Nef-type isomerization of aci-1 prevails in highly acidic solutions (pH<0). The equilibrium constant for the thermal tautomerization of 1 to aci-1 is estimated as pKT=17.0±0.2 based on kinetic data.

Water penetration and binding to ferric myoglobin.

Abstract: Flash photolysis investigations of horse heart metmyoglobin bound with NO (Mb3+NO) reveal the kinetics of water entry and binding to the heme iron. Photodissociation of NO leaves the sample in the dehydrated Mb3+ (5-coordinate) state. After NO photolysis and escape, a water molecule enters the heme pocket and binds to the heme iron, forming the 6-coordinate aquometMb state (Mb3+H2O). At longer times, NO displaces the H2O ligand to reestablish equilibrium. At 293 K, we determine a value kw ≈ 5.7 × 106 s-1 for the rate of H2O binding and estimate the H2O dissociation constant as 60 mM. The Arrhenius barrier height Hw = 42 ± 3 kJ/mol determined for H2O binding is identical to the barrier for CO escape after photolysis of Mb2+CO, within experimental uncertainty, consistent with a common mechanism for entry and exit of small molecules from the heme pocket. We propose that both processes are gated by displacement of His-64 from the heme pocket. We also observe that the bimolecular NO rebinding rate is enhanced ...

Photochemical studies of 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM).

Abstract: Kinetic UV-VIS absorption data following 355 and 266 nm nanosecond laser flash photolysis of 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM) solutions is presented. The kinetics of the decay of the non-chelated enol (NCE) produced following 355 nm excitation of BM-DBM solutions are analysed in terms of mixed 1st- and 2nd-order kinetics. The temperature dependences of the component rate constants are unusual and both 1st- and 2nd-order components display negative activation energies, which are explained by invoking pre-equilibria. In addition, it is shown for the first time that 266 nm laser photolysis of BM-DBM solutions leads to formation of the triplet state of the keto (K) form with a lifetime of approximately 500 ns. Under these conditions the triplet state of the K form is quenched by oxygen.

Efficient Generation of the Ligand Field Excited State of Tris-(2,2‘-bipyridine)-ruthenium(II) through Sequential Two-Photon Capture by [Ru(bpy)3]2+ or Electron Capture by [Ru(bpy)3]3+

Abstract: The relaxation dynamics and product distribution resulting from the decay of high lying excited states generated via sequential two-photon capture by [Ru(bpy)3]2+ or electron capture by [Ru(bpy)3]3+ have been investigated by flash photolysis and pulse radiolysis techniques. In comparison to the decay dynamics for monophotonic excitation, dramatically different relaxation dynamics have been observed. High-power flash excitation yields both the lowest lying metal-to-ligand charge transfer (3MLCT) state and a new transient photoproduct associated with nonradiative decay through the photodissociative metal-centered (3dd) excited state/s. The photoproduct is postulated to be [RuII(bpy)2(η1-bpy)]2+ where the pendant pyridine has rotated to yield a transient that is stabilized by a π−bonded or a three-centered Ru−C−H agostic interaction.

Reactions of the HO2 Radical with CH3CHO and CH3C(O)O2 in the Gas Phase

Abstract: Flash photolysis UV absorption techniques were used to study the HO2 + CH3C(O)O2 reaction. It was found that the reaction HO2 + CH3CHO ⇌ CH3CH(OH)O2 (2, −2) can interfere with the kinetic measureme...

Primary photoreaction of photoactive yellow protein studied by subpicosecond-nanosecond spectroscopy.

Abstract: The primary photochemical event of photoactive yellow protein (PYP) was studied by laser flash photolysis experiments on a subpicosecond−nanosecond time scale. PYP was excited by a 390-nm pulse, and the transient difference absorption spectra were recorded by a multichannel spectrometer for a more reliable spectral analysis than previously possible. Just after excitation, an absorbance decrease due to the stimulated emission at 500 nm and photoconversion of PYP at 450 nm were observed. The stimulated emission gradually shifted to 520 nm and was retained up to 4 ps. Then, the formation of a red-shifted intermediate with a broad absorption spectrum was observed from 20 ps to 1 ns. Another red-shifted intermediate with a narrow absorption spectrum was formed after 2 ns and was stable for at least 5 ns. The latter is therefore believed to correspond to I1 (PYPL), which has been detected on a nanosecond time scale or trapped at −80 °C. Singular value decomposition analysis demonstrated that the spectral shifts...

Photochemistry of 1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7- (piperazin-1-yl)quinoline-3-carboxylic acid (=ciprofloxacin) in aqueous solutions

Abstract: The 1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7-(piperazin-1-yl)quinoline-3-carboxylic acid (=ciprofloxacin; 1) undergoes low-efficiency (Φ=0.07) substitution of the 6-fluoro by an OH group on irradiation in H2O via the ππ* triplet (detected by flash photolysis, λmax 610 nm, τ 1.5 μs). Decarboxylation is a minor process (≤5%). The addition of sodium sulfite or phosphate changes the course of the reaction under neutral conditions. Reductive defluorination is the main process in the first case, while defluorination is accompanied by degradation of the piperazine moiety in the presence of phosphate. In both cases, the initial step is electron-transfer quenching of the triplet (kq=2.3⋅108M−1 s−1 and 2.2⋅107M−1 s−1, respectively). Oxoquinoline derivative 1 is much more photostable under acidic conditions, and in this case the F-atom is conserved, and the piperazine group is stepwise degraded (Φ=0.001).

Dynamics of structure and energy of horse carboxymyoglobin after photodissociation of carbon monoxide.

Abstract: The energetics and structural volume changes after photodissociation of carboxymyoglobin are quantitatively investigated by laser-induced transient grating (TG) and photoacoustic calorimetric techniques. Various origins of the TG signal are distinguished: the phase grating signals due to temperature change, due to absorption spectrum change, and due to volume change. We found a new kinetics of ∼700 ns (at room temperature), which was not observed by the flash photolysis technique. This kinetics should be attributed to the intermediate between the geminate pair and the fully dissociated state. The enthalpy of an intermediate species is determined to be 61 ± 10 kJ/mol, which is smaller than the expected Fe−CO bond energy. The volume of MbCO slightly contracts (5 ± 3 cm3/mol) during this process. CO is fully released from the protein by an exponential kinetics from 25 to −2 °C. During this escaping process, the volume expands by 14.7 ± 2 cm3/mol at room temperature and 14 ± 10 kJ/mol is released, which shou...

A laser flash photolysis and quantum chemical study of the fluorinated derivatives of singlet phenylnitrene.

Abstract: Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, kOBS. The observed rate constant in inert solvents is the sum of kR + kISC where kR is the absolute rate constant of rearrangement of singlet nitrene to an azirine and kISC is the absolute rate constant of nitrene intersystem crossing (ISC). Values of kR and kISC were deduced after assuming that kISC is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 ( 0.3, 5.5 ( 0.3, 6.7 ( 0.3, 8.0 ( 1.5, and 8.8 ( 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 ( 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluoro- substituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347). A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states converting the benzazirines to ketenimines were performed. The computational results are in good qualitative and quantitative agreement with the experimental findings.

P680, the primary electron donor of photosystem II

Abstract: The primary electron donor of photosystem II is a special form of chlorophyll a known as P680. Its detection and subsequent biophysical characterisation has relied heavily on the technique of flash photolysis of Norrish and Porter [Nature 164 (1949) 658] and on the physical principles which emerged from photochemical studies of isolated chlorophyll a using this technique. When oxidised the P680 radical has a midpoint redox potential estimated to be 1.17 V or more which is needed to drive the oxidising reactions of the water-splitting process. Such a high oxidising potential dictates special properties of P680 which are discussed in terms of robustness and structural organisation of photosystem II. Of particular importance has been the recent finding that P680 is not a ‘special pair’ of chlorophyll molecules as is the case for the primary electron donors of other types of photosynthetic reaction centres. Instead P680 is composed of a cluster of four weakly coupled monomeric chlorophylls which together with the local protein environment enables this primary donor to generate a redox potential capable of oxidising water.

Mechanism for formation of the lightstruck flavor in beer revealed by time-resolved electron paramagnetic resonance.

Abstract: Time-resolved electron paramagnetic resonance (TREPR) data collected during the photodegradation of iso-a-acids (isohumulones), the principal bittering agents from hops in beer, are presented and discussed, and, from the data, the photophysics leading to free-radical production as the primary step in the photodecomposition of iso-alpha-acids towards the development of "skunky" beer are explained. During laser flash photolysis of iso-alpha-acids at 308 nm in toluene/methylcyclohexane (1:1), TREPR spectra exhibit net emissive signals that are strongly spin polarized by the triplet mechanism of chemically induced electron spin polarization. From two potential photochemically active sites, the TREPR data show that although the first site, an enolized beta-triketone, is the primary light-absorbing chromophore, an uphill intramolecular triplet energy transfer process leads to Norrish type I alpha-cleavage at a second site, an alpha-hydroxycarbonyl. The energy transfer mechanism is supported by additional TREPR experiments with chemically modified hop compounds. Structural parameters (hyperfine coupling constants, g factors, line widths) for the observed free radicals, obtained from computer simulations, are presented and discussed.

Carbonate radical ion is the only observable intermediate in the reaction of peroxynitrite with CO2

Abstract: The reaction of ONOO- with CO2 at alkaline pH was recently reported to form a transient absorption with a maximum at 640 nm and a half-life of ca. 4 ms at 10 degreesC [Meli et al. (1999) Helv. Chim ...

Kinetics of the ClO Self-Reaction and 210 nm Absorption Cross Section of the ClO Dimer

Abstract: The kinetics of the dimerization of ClO radicals, ClO + ClO + M → Cl2O2 + M (1a), and the 210 nm absorption cross sections of the ClO dimer have been studied using the technique of flash photolysis with UV absorption spectroscopy, over the temperature range 183−245 K and pressure range 25−700 Torr. ClO radicals were generated following the photolysis of Cl2/Cl2O/N2 mixtures and were quantified via their differential absorption between the peak of the (12,0) band of the (A ← X) transition at 275.2 nm and the adjacent minimum to higher wavelengths, while Cl2O2 formation was simultaneously monitored at 210 nm. ClO differential absorption cross sections were measured under identical conditions to the kinetic experiments by four separate calibration schemes. The rate coefficient measured at the lower temperatures studied (T < 200 K) was found to be up to 40% faster than extrapolation of previous results would suggest, with the limiting low- and high-pressure rate coefficients for reaction 1 in nitrogen determ...

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer.

Abstract: The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.

Photogeneration and migration of electrons and holes in zeolite NaY

Abstract: Electron transfer and charge migration between electron donors and acceptors encapsulated within dry NaY zeolites are explored using nanosecond laser flash photolysis. The role of the zeolite in these redox processes is examined in order to characterize the intrazeolite mobility of electrons and holes. Electron migration is initiated by photoexciting trans-anethole in NaY containing coadsorbed 1,4-dicyanobenzene as an electron acceptor, while hole migration is initiated by photoexciting chloranil in NaY containing coadsorbed 4,4‘-dimethoxybicumene as an electron donor. The experimental results demonstrate that ultrafast redox reactions (>108 s-1) take place, leading to long-lived charge separated species within the zeolite cavities. The efficiency of these redox processes is examined as a function of donor−acceptor concentration and the presence of nitrous oxide as an electron trap. Interpretation of the experimental data with two independent models provides evidence that the redox chemistry occurring wit...

Pico- and nano-second laser flash photolysis study on photoinduced charge separation in oligothiophene-C 60 dyad molecules

Abstract: Photoinduced charge separation (CS) and charge recombination (CR) processes of octathiophene-C60 and dodecathiophene-C60 dyad molecules (8T-C60 and 12T-C60, respectively) have been investigated by time-resolved absorption spectroscopy in the visible and near-IR regions. In toluene, 18T*-C60 and 112T*-C60 showed energy transfer to 1C*-moiety predominantly, while 60 contribution of CS was small. In various polar solvents, on the other hand, CS states were predominantly formed from both singlet-excited oligothiophene and 1C6*0-moiety because of lower CS level in polar environments. The CR process generating both the triplet state of oligothiophene and the ground state was confirmed in anisole and anisole/toluene mixture within a few nanoseconds. In more polar solvents (dielectric constant (∈s) > 7), CS states showed two components decay: Slow decay component showed lifetime in the hundred nanosecond-region, while fast component decayed within a few nanoseconds. For the mechanism of the long-living CS state in polar solvents (∈s > 7), equilibrium between the CS state and the triplet state was proposed. Furthermore, effects of length of oligothiophene on the CS and CR processes were discussed on the basis of the free energy changes.

Photochemistry of the π‐Extended 9,10‐Bis(1,3‐dithiol‐2‐ylidene)‐ 9,10‐dihydroanthracene System: Generation and Characterisation of the Radical Cation, Dication, and Derived Products

Abstract: Flash photolysis of bis[4.5-di(methylsulfanyl) 1,3-dithiol-2-ylidene]-9,10(-dihydroanthracene (1) in chloroform leads to formation of the transient radical cation species 1.+ which has a diagnostic broad absorption band at lambdamax approximately 650 nm. This band decays to half its original intensity over a period of about 80 micros. Species 1.+ has also been characterised by resonance Raman spectroscopy. In degassed solution 1.+ disproportionates to give the dication 1(2+), whereas in aerated solutions the photodegradation product is the 10-[4,5-di(methylsulfanyl) 1,3-dithiol-2-ylidene]anthracene-9(10 H)one (2). The dication 1(2+) has been characterised by a spectroelectrochemical study [lambdamax (CH2Cl2) = 377, 392, 419, 479 nm] and by an X-ray crystal structure of the salt 1(2-) (ClO4)2, which was obtained by electrocrystallisation. The planar anthracene and 1,3-dithiolium rings in the dication form a dihedral angle of 77.2 degrees; this conformation is strikingly different from the saddle-shaped structure of neutral 1 reported previously.

Synthesis and Excited State Dynamics of μ-Oxo Group IV Metal Phthalocyanine Dimers: A Laser Photoexcitation Study

Abstract: The synthesis of two metal phthalocyanine monomers, GePc[OSi(n-C6H13)3]2 and SnPc[OSi(n-C6H13)3]2, and two μ-oxo-bridged dimers, (n-C6H13)3SiOSiPcOGePcOSi(n-C6H13)3 and (n-C6H13)3SiOSiPcOSnPcOH, are described. The ground-state absorption spectra and excited-state dynamics of these compounds together with those of (n-C6H13)3SiOSiPcOSiPcOSi(n-C6H13)3 have been measured. The absorption spectra of the dimers are blue shifted with respect to the monomers and whereas the latter exhibit a strong fluorescence in the visible, the dimers show only a weak emission in the near-IR. These observations are characteristic for the presence of exciton interactions in all three dimers, as had been reported earlier for the Si−O−Si dimer. Subnanosecond laser flash photolysis experiments on all five compounds yielded triplet−triplet absorption spectra, triplet lifetimes, triplet quantum yields, and bimolecular rate constants for quenching of the triplet states by O2. The triplet quantum yields and lifetime for the monomers and...

Evaluation of N-Aromatic Maleimides as Free Radical Photoinitiators: A Photophysical and Photopolymerization Characterization

Abstract: Photopolymerizable compositions were prepared using acrylate monomers in combination with various N-aromatic maleimides. N-aromatic maleimides were segregated into two groups: those that could adopt a planar conformation and those that could not adopt a planar conformation. The maleimides were characterized using single-crystal X-ray diffraction spectroscopy, laser flash photolysis spectroscopy, UV−vis absorption spectroscopy, and photodifferential scanning calorimetry. Planar N-aromatic maleimides were found to have a low relative excited-state triplet yield, showing significant shift of the primary maleimide UV absorption band with changes in solvent polarity, and did not initiate free radial polymerization upon direct UV excitation. Twisted N-aromatic maleimides have a higher relative triplet yield, show negligible shift of the primary maleimide UV absorption band, with solvent polarity, and initiate free radical polymerization upon direct excitation. Addition of benzophenone was found to dramatically...