Flash photolysis experiments

Miller R J and Closs G L 1981 Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments Rev. Sc/. Instrum. 52 1876-85... 

Two kinds of flash photolysis experiments on the interfacial reactions of electrons and holes have been reported. In the first case, the electrons and holes themselves were detected by their optical absorptions, and in the second case the products of their reactions were traced. 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

The H20 exchange mechanism was studied by Hunt et al. (32) who reported that exchange between aqueous solvent and Fein(TPPS)(H20)2 occurs with a first-order rate constant (kex = 1.4xl07s-1 in water at 298 K) far exceeding the k0 s values determined at any [NO]. If the steady state approximation was applied with regard to the intermediate Fem(Por)(H20), the kohs for the exponential relaxation of the non-equilibrium mixture generated by the flash photolysis experiment would be,... 

In flash photolysis experiments at pressure <0.1 torr reactions (1) and (2) presumably still occur, but the subsequent reactions involve vibrationally excited ethyl radicals91, viz. 

Future work will be concerned with quantitative measurement of hydrogen abstraction rates of labile hydrogens in the carbamate moieties of several aromatic diisocyanate based polyurethanes. It is expected that experimental conditions will alter significantly the hydrogen abstraction rate. Emphasis will also be placed on measurement of transient intermediates in polyurethane films. Finally, extensive laser flash photolysis experiments will be conducted on polyurethanes based on both 2,A-toluenediisocyanate and 2,6-toluenediisocyanate. Preliminary data suggest that the placement of the methyl substituent can alter the nature of the transient intermediates formed. 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

By using pulsed laser sources and fast measuring devices, direct observation of redox products in flash photolysis experiments provides evidence regarding oxidative and reductive quenching mechanisms in Ru(bpy)3+. 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

The time course of the charge-separated intermediate I can be measured in a flash photolysis experiment that monitors the (I — A) transient absorbance difference at a ground state/triplet state isosbestic point (e.g., 432 nm for Mg, and 435 nm for Zn). We have observed this intermediate for the [M, Fe] hybrids with M = Mg, Zn representative kinetic progress curves are shown in Fig. 3 [7a]. In a kinetic scheme that includes Eqs. (1) and (2) as the only electron-transfer processes, when the I A step is slow (kb < kp) the intermediate builds up (exponentially) during the lifetime of A and exponentially disappears with rate constant kb (Fig. 4A). This behavior is not observed for the hybrids, where the I - A process is more rapid than A - I, with kb > kp. In this case, I appears exponentially at early times with rate-constant kb and is expected to disappear completely in synchrony with A in an exponential fall with rate-constant kp (Fig. 4B). 

The following data were obtained for the decay of methyl radicals in the presence of argon diluent in a flash photolysis experiment. 

The photoreactions of Ru(TAP)3 in the presence of different mononucleotides have been used as models for the photoreactivity with CT-DNA and polynucleotides and for the photoreactions of other oxidising polyazaaromatic Ru(II) complexes [100]. Flash photolysis experiments with RufTAP) in the presence of GMP or AMP, demonstrate clearly the presence of a photo-induced... 

The second category comprises the flash photolysis experiments using the short high power light pulses from Q-switched lasers, furthermore all investigations of time-dependent behavior of excited dye molecules, which play an important role as active material in dye lasers or as saturable absorbers in passive Q-switched giant pulse lasers. 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Laser flash photolysis experiments showed that the Ceo triplet state ( C q), obtained by 532 nm excitation, reacts with poly(methylphenylsilane) by an... 

Laser flash photolysis experiments showed that the (Reaction 8.17) reacts with cyclic oligosilanes 42, 43 and 44 in benzonitrile by an electron transfer mechanism [43]. The rate constant ( et) for the three-membered cyclic compound 42 is found to be 7.0 x 10 M s, whereas for the other two compounds it was more than two orders of magnitute lower, i.e., (1-2) X 10 M- s-. ... 

Nakamura and co-workers provided detailed mechanistic information for the photoinduced electron transfer from tri-1 -naphthyl phosphate and related compounds to 9,10-dicyanoanthracene yielding binaphthyls The intramolecular nature of the reaction could be established by using laser flash photolysis experiments as well as fluorescence measurements [17],... 

The product is proposed to arise from the coupling of the [Os (N)(NH3)4(CO)] intermediate (see also Section 5.6.5.2). The first direct observation of N-"N coupling of Os=N came from laser flash photolysis experiments on [Os (N)(NH3)4] in the presence of electron donors such as 1,4-dimethoxybenzene. This produces the nucleophilic Os =N which undergoes N- N coupling with the electrophilic Os (Equation (13)). 

A mixture of 2-naphthoylazide and sensitizer 2-isopropylthioxanthone ( r = 65 kcal/mol) was irradiated (k > 385 nm), conditions under which the sensitizer absorbed >95% of the light. Laser flash photolysis experiments demonstrated that the triplet sensitizer is quenched by 2-naphthoylazide at a diffusion controlled rate. 

McClelland and co-workers identified the initial adduct detected in laser flash photolysis experiments involving the reaction of 75g with d-G as 111 (Ar = 2-fluorenyl, Y = H, R = 2 -deoxyribose)." This identification was based on the absorption spectrum of the intermediate, which extends out to 400 nm suggesting a highly conjugated species, by the observed pXa of 3.9 of the intermediate, which is consistent with deprotonation of 111 to form 112, by the lack of dependence of the rate constant for decomposition of the intermediate on the nature of Ar for the intermediates derived from 75g, 75n, 75p, and 75q, and by the kinetics of the decomposition of the intermediate into the stable C-8 adduct 102, which includes a pH-rate profile that showed both ionization states were reactive, buffer catalysis of decomposition of the... 

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