Nanosecond Flash Photolysis Research Articles

On the photochemical decomposition of aromatic α-azohydroperoxides

The photochemistry of some α-azohydroperoxides (phenyl(phenylazo)methyl hydroperoxide and its halogenated derivatives) was investigated in various solvents by microsecond and nanosecond flash photolysis methods and steady state product analysis. Photodecomposition of the molecule, demonstrated by the formation of benzaldehyde, is a major, but not the only, photochemical pathway. Investigation of the influence of solvent and substitution on the photoinduced transients provides strong evidence for alternative photoreactions. One of the species produced by flash photolysis was studied in detail and assigned to a transient zwitterion formed from the α-azohydroperoxide by proton transfer via an excited state pathway. Semiempirical calculations add further support to the proposed mechanism.

Kinetic spectroscopy of pyrazolotriazole azomethine dyes

The photophysical properties of pyrazolotriazole azomethine dyes have been investigated using both picosecond and nanosecond flash photolysis. On nanosecond timescales, prompt formation of a photoisomer is observed, the rate of decay of which shows a solvent dependence. In the presence of a triplet sensitiser a triplet pathway to the photoisomer has been established, the yield of isomer from the triplet state being considerably higher than from direct excitation. On picosecond timescales, two transients are observed; the first has a very short, solvent-independent lifetime, while the second has a longer solvent-dependent lifetime. These two transients are assigned as states formed during the relaxation of the molecules along the first excited singlet and the ground-state potential-energy surfaces, respectively. Similar kinetic behaviour is observed in a high concentration, high viscosity environment designed to mimic the photographic emulsion, indicating that the photophysical relaxation pathways are still very rapid even in this type of environment.

Photovernetzung von Siliconen. VII. Zum Einflu� der Siliconkette - eine kinetische Studie

Photocrosslinking of Silicones. VII. The Influence of the Silicone Backbone – a Kinetic Study The photo-induced radical crosslinking of silicone containing pendant acrylate groups was investigated by nanosecond flash photolysis and photocalorimetry. From the results of the flash photolysis is inferred that the photochemically formed benzoyl radicals react with the acrylate groups of the system. A possible H-abstraction from the methyl groups of the silicone backbone was not observed. The silicone backbone functions only as an internal solvent. Calorimetric measurements with model systems support these results. The polymerization process is inhibited by oxygen. From calorimetric results is inferred that oxygen both inhibits and terminates the polymerization process. Due to their high oxygen solubility the silicone units increase the oxygen influence on the crosslinking. The calorimetric measurements shows also that a direct bond between silicone backbone and the acrylate group reduces the oxygen influence. Also, in absence of oxygen a direct bonded acrylate group accelerates the polymerization rate.

Active site of DNA photolyase: tryptophan-306 is the intrinsic hydrogen atom donor essential for flavin radical photoreduction and DNA repair in vitro

DNA photolyases repair cyclobutadipyrimidines (Pyr()Pyr) in DNA by photoinduced electron transfer. The enzyme isolated from Escherichia coli contains methenyltetrahydrofolate (MTHF), which functions as photoantenna, and FADH2, which is the redox-active cofactor. During purification, FADH2 is oxidized to the blue neutral radical form, FADH., which has greatly diminished activity. Previous nanosecond flash photolysis studies [Heelis, P.F., Okamura, T., & Sancar, A. (1990) Biochemistry 29, 5694-5698] indicated that excitation of FADH. either directly by absorbing a photon or indirectly by electronic energy transfer from MTHF excited singlet state yielded an FADH. quartet which abstracted a hydrogen atom from a nearby tryptophan to generate the catalytically competent FADH2 from of the enzyme. Using site-directed mutagenesis, we replaced all 15 photolyase tryptophan residues by phenylalanine, individually, in order to identify the internal hydrogen atom donor responsible for photoreduction. We found that W306F mutation abolished photoreduction of FADH. without affecting the excited-state properties of FADH. or the substrate binding (KA approximately 10(9) M-1) of the enzyme. The specificity constant (kcat/km) was approximately 0 for the mutant enzyme in the absence of reducing agents in the reaction mixture, indicating that photoreduction of FADH. is an essential step for photorepair by photolyase in vitro. Chemical reduction of FADH. of the mutant enzyme restored the specificity constant to the wild-type level.

The photolysis of acylphosphine oxides IV. investigations with di- n-butyl-2,6-dichlorobenzoyl phosphine oxide and dicyclohexyl-2,4,6-trimethylbenzoyl phosphine oxide

Di- n-butyl-2,6-dichlorobenzoyl phosphine oxide (DCBPO) and dicyclohexyl-2,4,6-trimethylbenzoyl phosphine oxide (TMCPO) undergo α scission on irradiation with UV light: φ(α)  0.3 (DCBPO) and φ(α)  0.2 (TMCPO). Picosecond and nanosecond flash photolysis studies, in conjunction with electron spin resonance (ESR) and optical absorption detection, provide evidence for the formation of phosphinoyl radicals R 2·  O (R  n-butyl or cyclohexyl respectively). From the results, it is inferred that excited singlet states deactivate by two routes: α scission and intersystem crossing to the triplet manifold (φ(T)  0.5 (DCBPO) and φ(T)  0.7 (TMCPO)). The triplets deactivate essentially by intramolecular hydrogen abstraction resulting, to a small extent, in the formation of olefins: φ(1-butene)  0.04 (DCBPO) and φ(cyclohexene)  0.03 (TMCPO). Enolization with subsequent reketonization is the main triplet deactivation route in the case of TMCPO.

Photoinduced electron transfer and charge separation in anthraquinone-substituted porphyrin–phthalocyanine heterodimer

Photoinduced intramolecular electron transfer in anthraquinone-substituted porphyrin–phthalocyanine heterodimer has been investigated by fluorescence quenching experiments and nanosecond flash photolysis techniques, giving a final charge separated state Zn(ttp)+˙–Zn(pc)–aq–(tpp = tetraphenyl porphyrin; pc = phthalocyanine; aq = anthraquine) dianion by exciting the phthalocyanine moiety.

Excited-state properties of Escherichia coli DNA photolyase in the picosecond to millisecond time scale

Escherichia coli DNA photolyase contains a stable flavin radical that is readily photoreduced in the presence of added electron donors. Picosecond, nanosecond, and conventional flash photolysis technique have been employed to investigate the events leading to photoreduction from 40 ps to tens of milliseconds following flash excitation. Direct light absorption by the flavin radical produces the first excited doublet state which undergoes rapid (within 100 ps) intersystem crossing to yield the lowest excited quartet (n pi*) state. In contrast, light absorption by the folate chromophore produces a new intermediate state via interaction of the folate excited singlet state with the ground-state flavin radical, leading to an enhanced yield of the excited radical doublet state and hence quartet state. Subsequent reaction of the excited quartet state involves hydrogen atom abstraction from a tryptophan residue. Secondary electron transfer from added electron donors occurs to the oxidized tryptophan radical with rate constants ranging from 10(4) (dithiothreitol) to 4 x 10(6) M-1 s-1 (n-propyl gallate). The low value of the latter rate compared to reduction of the tryptophan radical in lysozyme suggests that the reactive tryptophan is highly buried in photolyase. A redox potential diagram has been constructed for the ground and excited states involved. It is concluded that the one-electron reduction potential of the excited quartet state of the flavin radical must be at least 1.23 V more positive than the ground state, in agreement with the value of delta E greater than 1.77 V calculated from spectroscopic data.

Intramolecular photochemical electron transfer. 4. Singlet and triplet mechanisms of electron transfer in a covalently linked porphyrin-amide-quinone molecule

The authors have carried out an extensive photophysical analysis of a tetraarylporphine linked through a single amide bridge to either methyl-p-benzoquinone (PAQ) or the corresponding hydroquinone (PAQH/sub 2/) in benzonitrile as the solvent. The photophysical properties of PAQH/sub 2/ are closely similar to those of nonlinked tetraarylporphine species, while for PAQ significant lifetime quenching of both the lowest excited singlet and triplet states is observed. Picosecond transient absorption spectroscopy and fluorescence lifetime measurements were used top show that quenching of the excited singlet state of PAQ is due to intramolecular electron transfer to the singlet radical ion pair /sup 1/(P/sup .+/AQ/sup .-/) with the rate constant of 4.1 (+-0.3) x 10/sup 8/ s/sup -1/. /sup 1/(P/sup .+/AQ/sup .-/) subsequently decays to the ground state by reverse electron transfer with a rate constant of 1.6 (+- 0.2) x 10/sup 8/ s/sup -1/. This reaction has ..delta..G/sup 0/ approx. = -1.4 eV and is predicted by Marcus theory. Nanosecond flash photolysis studies show that the lowest triplet state of PAQ is also quenched, most likely by electron transfer to the triplet radical ion pair /sup 3/(P/sup .+/ AQ/sup .-/, with a rate constant of 4.6 (+-0.2) x 10/sup 4/ s/sup -1/. Theymore » suggest that /sup 3/(P/sup .+/AQ/sup .-/), which then decays rapidly to the ground state.« less

Stereochemistry and mechanism of the [2 + 2] and [4 + 2] photocycloaddition of alkenes and dienes to pummerer's ketone

Irradiation of Pummerer's ketone (PK) in furan gave the trans-fused [4 + 2] cycloadducts 2 and 3, whose structures were determined by their 1H NMR spectra and confirmed by X-ray crystallography. Irradiation of PK in the presence of tetramethylethylene (TME) gave cycloadduct 4 and with 1,1-dimethoxyethylene (DME) gave [2 + 2] cycloadducts 5,6 and 7, respectively. The X-ray crystal structures of 4—7 show that the four- and six-membered rings of 4 and 7 are trans-fused, and 5 and 6 are cis-fused, respectively. Differential quenching of the formation of cycloadducts 2–5 and 7 by the free radical TEMPO was observed. Formation of cycloadducts 2–4 was also quenched differentially by O 2. A short lived (15 nsec) transient observed on nanosecond flash photolysis of PK is suggested to be a highly twisted triplet excited state, which is the likely precursor of both [4 + 2] and [2 + 2] adducts. It is proposed that stepwise addition of furan and alkenes to PK triplets gives triplet biradicals, which can be intercepted by TEMPO and O 2 , and that these reactions probably do not involve a ground state trans-cyclohexenone intermediate.

Implementation of an optical multichannel analyzer controller for nanosecond flash photolysis measurements

Traditionally optical multichannel analyzers (OMAs) have been turnkey instruments. Controllers are now available which replace the OMA console with the users existing computer. The increased flexibility of this approach comes at the expense of increased involvement in the details of system operation. This is particularly the case for pulsed experiments where the detector is gated on for short periods. We report here implementation of an OMA controller hosted by an IBM personal computer using IEEE-488 communication. A microchannel-plate intensified detector is used in gated mode to collect spectra of intermediates formed by nanosecond laser photolysis. Three different pulsers for intensifier gating are compared. Both conventional absorbance measurements on transparent samples and diffuse reflectance measurements on opaque low-temperature matrices are described. In both cases, however, the samples are sufficiently photostable that noise can be reduced by signal averaging using repeated photolysis of the same sample volume. A second paper describes the additional complexity required for high signal-to-noise measurements on small quantities of photolabile samples.

Mechanism for photoisomerization of an olefin in the presence of electron acceptors as studied by magnetic field effects

An external magnetic field is found to retard the photoisomerization of trans-3,3-dimethyl-1-(2-naphthyl)-1-butene ( t-DMNB) in degassed acetonitrile in the presence of electron acceptors. From this and other observations, the precursor of the isomerization is indicated to be the olefin triplet arising from intersystem crossing of a singlet radical pair consisting of a t-DMNB cation and an acceptor anion, identified by nanosecond flash photolysis.

Trans to cis photoisomerization of N,N′-disubstituted indigo dyes via excited singlet states; a laser flash photolysis and steady state irradiation study

The relaxation pathways of excited trans-N,N′-diacylindigo dyes (trans-1–trans-4) and of the rigid trans (5) and cis (6) indigo analogues have been studied by nanosecond flash photolysis and steady state measurements. In fluid solution the trans to cis photoisomerization of 1–4 occurs via a singlet excited state mechanism on direct excitation, and the triplet state appears to be reached only by sensitized reaction. The lower quantum yield for trans → cis isomerization of 2–4 at room temperature, as compared to 1, is due to a thermally activated internal conversion step leading to the trans ground state. This pathway competes with fluorescence and twisting of the molecule around the central double bond in the excited singlet state. Increasing the viscosity of the medium decreases the twisting of the flexible molecules and, as a consequence, increases the population of the lowest triplet state. A triplet transient, the intensity of which increases with the viscosity of the medium, could be characterized at low temperature under direct excitation. A transient species, which can be assigned to a triplet state on the basis of quenching experiments and on the fact that the same transient could be found under sensitized conditions, also has been observed at room temperature for the rigid 5 and 6 molecules. Quenching measurements of triplet sensitizers by 5, trans-1, and trans-2 enables the localization of the triplet energy at around 40 kcal/mol.

Complex formation of spiropyrans with metal cations in solution: a study by laser flash photolysis

Nanosecond flash photolysis was used to study the complexation reactions of a spiropyran of the indoline series (1′,3′,3′-trimethyl-6-nitro-8-methoxyspiro [2 H-1-benzopyran-2,2′-(2 H)-indol]) with lead(II), cobalt(II), cadmium(II) and zinc(II) in an acetone—water mixture. Two relatively unstable trans isomers of the spiropyran merocyanine form were found to be involved in the complexation. A decrease in the yield of complex was found to occur in the presence of acid. Several transient forms of the complex were revealed. The lifetime of the transient forms of the complex is the same as that of the corresponding trans isomers of the spiropyran merocyanine form. A reaction scheme of complexation is proposed.

ChemInform Abstract: Nanosecond Flash Photolysis Studies of Intersystem Crossing Rate Constants in Biradicals: Structural Effects Brought About by Spin ‐ Orbit Coupling.

AbstractThe lifetimes (r) of the triplet biradicals (II) are governed by intersystem crossing (isc) to the singlet biradicals.

Acid effect on photochromism of spiropyrans: A study by microsecond and nanosecond flash photolysis

Microsecond and nanosecond flash photolysis was used to study the photochromic reactions of spiropyrans of the indoline series 1′,3′,3′-trimethyl-6-nitro, 6-nitro-8-methoxy, 6-methoxyspiro[2 H-1-benzopyran-2′,2′-(2 H)-indol] in acetone in the presence of the acids HNO 3, HCl and H 2SO 4. Two relatively unstable trans isomers of spiropyran (merocyanine form) were found to be involved in the protolytic reaction. A reaction mechanism is proposed.

Nanosecond flash photolysis studies of intersystem crossing rate constants in biradicals: structural effects brought about by spin-orbit coupling

On montre que le couplage spin-orbite est renforce dans les diradicaux ayant un groupe acyle terminal. Effet de la longueur de la chaine et de la conformation

Picosecond photochemistry of methyl viologen

The photochemistry of the dithiocyanate complex of methylviologen is very similar to that of the chloride and yields MV/sup +/ and (SCN/sup -/)/sub 2/ with a quantum yield of 0.16 as observed by nanosecond flash photolysis. Picosecond flash photolysis reveals that both the monothiocyanate and the dithiocyanate complexes give charge transfer upon excitation with a quantum yield of approx. 0.3. The radical pair (MV/sup +/SCN) undergoes geminate recombination with a rate constant greater than or equal to x 10/sup 9/ s/sup -1/ while (MV/sup +/SCNSCN/sup -/) rearranges to (MV/sup +/(SCN/sup -/)/sub 2/) in less than 30 ps. (MV/sup +/(SCN/sup -/)/sub 2/) forms a bonded radical pair with an association constant of 120 M/sup -1/. Half of the (MV/sup +/(SCN/sup -/)/sub 2/) formed undergoes geminate recombination (5 x 10/sup 8/ s/sup -1/) while for the other half the two radicals diffuse apart (5 x 10/sup 8/ s/sup -1/). MV/sup +/ and (SCN/sup -/)/sub 2/ reencounter by second-order kinetics. The picosecond photochemistry of MV/sup 2 +/(Cl)/sub 2/ is also analyzed and compared with that of MV/sup 2 +/(SCN/sup -/)/sub 2/. The energetics of these reactions are considered, and the general consequences of these results are discussed.

Nanosecond flash photolysis study of carbon monoxide binding to the beta chain of hemoglobin Zürich [beta 63(E7)His leads to Arg

Binding of carbon monoxide to beta chains of hemoglobin Zürich has been studied by flash photolysis over the time range of nanoseconds to seconds at temperatures from 20 to 300 K. From 20 to 200 K a single rebinding process (process I) is seen, characterized by a distribution of barrier heights with a peak enthalpy of 2.3 kJ/mol. Above 200 K some ligands escape from the pocket into the matrix, and above 260 K recombination from the solvent sets in. Process I is visible up to 300 K, but above 200 K its rate remains essentially constant at about 4 X 10(8)s -1. Above about 250 K, process I is exponential in time, indicating rapid conformational relaxation. The results are discussed within the framework of a sequential model for ligand binding.

Benzophenone photoreduction by heteroatom-containing donors

Nanosecond flash photolysis has been carried out on benzophenone solutions in benzene and acetonitrile in presence of oxygen- and sulfur-containing reductants such as anisole, thioanisole, dibutyl sulfide and methyl 2-octyl ether. The quantum yield of ketyl formation is compared to previous results on the quantum yields of the final benzophenone disappearance. This comparison is also related to previous data on benzophenone reduction by nitrogen-containing reductants such as amines and amides and suggests a general mechanism for benzophenone photoreduction by heteroatom-containing donors.

Cis ⇌ trans photoisomerization of thioindigoid dyes

The mechanism of the cis ⇌ trans photoisomerization of perinaphtho-thioindigoid dyes was investigated by nanosecond flash photolysis. It is proved that the trans ⇌ cis isomerization occurs via the trans-triplet whereas the cis → trans isomerization follows a singlet route. This mechanism seems to be valid for all thioindigoid dyes.