Photolysis studies, flash

Methods used for the direct characterisation of reactive intermediates must have a timescale shorter than or comparable to the lifetime of the reactive intermediate (Table 10.1). Because of the very short lifetimes of these transient intermediates, this can be brought about by  

The basis of flash photolysis is to irradiate the system with a very short, intense pulse of light and then, as soon as the pulse is over, to monitor the changes in the system with time by some spectroscopic 

With the invention of the laser in 1960 and the subsequent development of pulsed lasers using Q-switching (Chapter 1), monochromatic and highly-collimated light sources became available with pulse durations in the nanosecond timescale. These Q-switched pulsed lasers allow the study of photo-induced processes that occur some 103 times faster than events measured by flash lamp-based flash photolysis. 

By the late 1960s the development of mode locking (Chapter 1) allowed the study of picosecond laser techniques. Excited-state processes carried out in the picosecond domain allow such processes as intersystem crossing, energy transfer, electron transfer and many pho-toinduced unimolecular reactions to be investigated. 

Now the concentration of a transient absorbing species varies with time, c(t), and so A is a function of two variables, namely time and 

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The flash lamp teclmology first used to photolyse samples has since been superseded by successive generations of increasingly faster pulsed laser teclmologies, leading to a time resolution for optical perturbation metliods tliat now extends to femtoseconds. This time scale approaches tlie ultimate limit on time resolution (At) available to flash photolysis studies, tlie limit imposed by chemical bond energies (AA) tlirough tlie uncertainty principle, AAAt > 2/j. 

Nanosecond flash photolysis studies of Ct-nitronaphthalene in polar and nonpolar solvents led to the detection of the trip let-triplet absorption spectrum of this nitrocompd (Ref 26). 

In attempts to understand the photochemical reactions of Fischer carbene complexes, several matrix isolation and flash photolysis studies have been conducted using both Cr and W (but not Mo) complexes [5-11]. Although the complexes studied and conditions used varied, several general conclusions were drawn ... 

Despite the unpromising UV-visible spectra and flash photolysis studies, the carbene complexes presented in this chapter have a rich photochemistry at wavelengths exceeding 300 nm. A wide range of synthetically useful transformations has been developed, and continued studies are likely to reveal more. 

Somlyo, A., Somlyo, A.V. (1989). Flash photolysis studies of excitation-contraction coupling. Arm. Rev. Physiol. 52, 857-875. 

Lu, C. Y. Lui, Y.Y. (2002). Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study. Biochimica et Biophysica Acta (BBA) - General Subjects, Vol. 1571, No.l, (May 2002), pp. 71-76, ISSN 0304-4165... 

Despite the above, there is also considerable evidence to suggest that oxazole formation proceeds via an intermediate nitrile ylide, particularly in the catalysed reactions (see below). Nitrile ylides have been detected in laser flash photolysis studies of diazo compounds in the presence of nitriles, and stable nitrile ylides can be isolated in some cases.<94CRV1091>... 

Pina, R, Caffeine interaction with synthetic flavylium salts. A flash photolysis study for the adduct involving 4,7-dihydroxyflavylium, J. Photochem. Photobiol. A Chem., 117, 51, 1998. 

Various Flash Photolysis Studies on TiOj Sols.155... 

Gratzel and Serpone and co-workers recently reported on a picosecond laser flash photolysis study of TiO. They observed the absorption spectrum immediately after the 30 ps flash and attributed it to electrons trapped on Ti" " ions at the surface of the colloidal particles. The absorption decayed within nanoseconds, the rate being faster as the number of photons absorbed per colloidal particle increased. This decay was attributed to the recombination of the trapped electrons with holes. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

In a flash photolysis study of a series of diazo carbonyl compounds, a correlation was found between the amount of carbene that could be trapped by pyridine and the amount of s-trans ketone.240... 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

Flash photolysis studies<22) have indicated singlet methylene to be produced from the diazomethane-excited singlet upon loss of nitrogen followed by collisional deactivation to the triplet, the ground state multiplicity for this molecule. 

El-Agamey, A. and McGarvey, D.J. 2003. Evidence for a lack of reactivity of carotenoid radicals towards oxygen A laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents. J. Am. Chem. Soc. 125 3330-3340. 

Mortensen, A. 2000. Mechanism and kinetics of scavenging of the phenylthiyl radical by carotenoids. A laser flash photolysis study. Asian Chem. Lett. 4 135-143. 

Ikegami M, Arai T (2000) Laser flash photolysis study on hydrogen atom transfer of 2-(2-hydn>xyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole in the triplet excited state. Chem Lett 9 996-997... 

Evidence from nanosecond flash photolysis studies indicates that an oxa-ziridine precursor to the diazoketone is unlikely. Photoelimination of nitrogen is also observed in 4-methyl-1,2,3-benzotriazine 3-oxide and affords 3-methylanthranil a mechanism involving loss of nitrogen from an intermediate oxaziridine has been proposed.74... 

Alvaro, M., Atienzar, P., Bourdelande, J.L., and Garda, H. (2004) An organically modified single wall carbon nanotube containing a pyrene chromophore Fluorescence and diffuse reflectance laser flash photolysis study. Chem. Phys. Lett. 384, 119-123. 

Schnapp, K.A., and Platz, M.S. (1993) A laser flash photolysis study of di-, tri- and tetrafluorinated phe-nylnitrenes Implications for photoaffinity labeling. Bioconjugate Chem. 4, 178-183. 

The flash photolysis study of the reaction of Ph2N with cumyl hydroperoxide showed the more sophisticated mechanism of this reaction [99]. When [ROOH] is low (less than 0.01 mol L-1), the reaction proceeds as bimolecular. The mechanism changes at the hydroperoxide concentration greater than 0.02mol L-1. The diphenylaminyl radical forms complex with hydroperoxide, and the reaction proceeds through the electron transfer. 

Platz, M.S. Maloney, V.M. Laser Flash Photolysis Studies of Triplet Carbenes. In Kinetics and Spectroscopy of Carbenes and Biradicals, M.S. Platz, Ed. Plenum New York, 1990 ... 

Jackson, J.E. Platz, M.S. Laser Flash Photolysis Studies of Ylide-Forming Reactions of Carbenes. In Advances in Carbene Chemistry, U.H. Brinker, Ed. JAI Greenwich, CT, 1994 pp 89-160. 

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

The vivid interest in hydrazine as a powerful propellant has stimulated many investigations both of its thermal decomposition and of its oxidation. Although hydrazine decomposes much more readily than ammonia, the study of its homogeneous decomposition by classical means using a static system is complicated considerably by wall catalysis. Thus, other experimental techniques have had to be applied, e.g. decomposition flames, flash photolysis, studies of explosion characteristics and the shock-tube technique. 

The flash photolysis study of Norrish et al,415 has confirmed the suggestions of earlier workers that CIO is an important reaction intermediate. It reaches its maximum concentration during the photolytic flash and is clearly generated in... 

Bhattacharyya, A.K., Lipka, J.J., Waskell, L. and Tollin, G. (1991) Laser flash photolysis studies of the reduction kinetics of NADPH cytochrome P-450 reductase. Biochemistry, 30 (3), 759-765. 

Flash Photolysis Studies in Bimolecular Electron-transfer Processes... 

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