Flash photolysis derivatives

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

Using nanosecond laser flash photolysis techniques, Leigh80 observed transient absorption spectra which he attributed to the silenes derived from photolysis of various methylphenyldisilylbenzenes. Thus the silenes 52,53, and 54 were found to absorb at 425,460, and 490 nm, respectively, in isooctane, and 55 was also found to absorb at 490 nm.75 In other studies, the silene Ph2Si=CH2 derived by laser flash photolysis was found to absorb at 323 nm.111... 

Midinger and Wilkinson<54> have used flash photolysis and fluorescence quenching by heavy atoms to determine the intersystem crossing efficiencies of anthracene and a number of its derivatives. As discussed in Section 5.2b, heavy atoms present as molecular substituents or in the solvent serve to promote multiplicity forbidden transitions. When anthracene is excited the following processes can occur ... 

Nitroxyl radicals produced in the reactions of R02 with aminyl radicals react with peroxyl radicals. The latter reaction is considerably slower than the reaction of peroxyl with aminyl radicals, which can be seen from the following data derived by flash photolysis (the photolysis of bis(l,l-dimethylethyl)peroxide in toluene was performed in the presence of... 

Laser flash photolysis (LFP) of quinone diazide 2d in Freon-113 at room temperature produces carbene Id, which could be monitored indirectly by addition of trapping reagents.25 At 2.0 xs the lifetime of Id is slightly longer than that of la (1.65 xs), otherwise the reactivities of these carbenes are very similar. The Id —> 11 rearrangement is not observed in the LFP experiments. All trapping products with a variety of reagents (O2, acetonitrile, pyridine etc.) are derived from carbene Id. 

Recent advances in measuring the kinetics of the various electron-transfer steps in this system have been achieved by use of flash photolysis of ruthenated derivatives of cytochrome c (Ru-Cc) (17-19). In these studies [Ru(bpy)3]2+ is covalently bound to a surface residue at a site that does not interfere with the docking of cytochrome c to cytochrome c oxidase. Solutions are then prepared containing both Ru-Cc and cytochrome c oxidase, and the two proteins associate to form a 1 1 complex. Flash photolysis of the solution leads directly to the excitation of the RuII(bpy)3 site, which then reduces heme c very rapidly. This method thus provides a convenient means to observe the subsequent intracomplex electron transfer from heme c to cytochrome c oxidase and further stages in the process. 

An alternative application of flash photolysis to study myoglobin electron transfer kinetics has been employed by Hofifinan and co-workers 156). In this approach, the photoactive zinc-substituted derivative of Mb is mixed with an equivalent amoimt of ferricytochrome bs to form an electrostatically stabilized binary complex. Upon transient irradiation, the strongly reducing Zn-Mb intermediate is formed, and the kinetics of ferricytochrome reduction within the preformed complex can be monitored spectrophotometrically. The resulting kinetics represents a mixed-order process consistent with electron transfer both within the electrostatically stabilized complex and between the dissociated components of the complex. 

The iron(II) complexes of the hexadentate 2-pyridylmethyl derivatives of bipy (94) and (95) are spin cross-over compounds, whose light-induced high-spin to low-spin conversion has been monitored in solution by laser flash photolysis. Single exponential kinetics (A h l = 6.7 x 10 s , at 273 K) were observed for [Fe(94)] ", but for [Fe(95)] " kinetics were biphasic, with the spin-conversion step (ku i = 2.5 x 10 s ) followed by a slower step (k = 3.7 x 10 s ) involving rearrangement of the pyridylmethyl pendant arms. ... 

Several researchers have investigated the photoenolization of various o-methyl acetophenone and o-methyl benzophenone derivatives. The mechanism of photoenolization of o-methyl benzophenone, 1, has been studied with laser flash photolysis and can be described as follows (Scheme 2) Irradiating 1 forms its first... 

The flash photolysis technique for generating radicals with fast spectroscopic observation was pioneered by Porter and Wright, " who observed benzyl, anilino, and phenoxyl radicals in the gas phase. Further applications of this technique include diradicals from diazenes, " nucleoside-derived radicals, and ultrafast radical clocks. ... 

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... 

We have investigated the reactions of the COs " radicals with double-stranded DNA by laser flash photolysis techniques [15]. In these time-re-solved experiments, the COs radicals were generated by one-electron oxidation of HCOs by sulfate radical anions, SO4 the latter were derived from the photodissociation of persulfate anions, S20s initiated by 308-nm XeCl excimer laser pulse excitation. In air-equilibrated buffer solution containing the self-complementary oligonucleotide duplex d(AACGCGAATTCGCGTT), 208 , and an excess of HCO3., the decay of the CO3 radical anion absorption band at 600 nm is associated with the concomitant formation of the characteristic narrow absorption band of the G(-H) radicals near 310 nm. 

Laser flash photolysis has been performed with the halogenated derivatives in ethanol [374], Measurements of the triplet lifetimes as a function of dye concentration, laser power, and in the presence of electron donors allowed us to determine the rate constants collected in Table 17. 

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