Electron transfer flash photolysis

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

Some ingenious experimental innovations have now made it possible to conduct flash photolysis on time scales < 10-11 s. They are anything but routine, especially as they approach a resolution of some femtoseconds, which is the approximate current state of the art. The implementation of these methods allows the study of chemical and physical events on time scales approaching and even exceeding those of molecular vibrations. Indeed, it is studies of vibration, including ligand motion, and (especially) electron transfer that have benefited most. 

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... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

The electron transfer reaction from fluorenyl carbanions adjacent to sulfoxide or sulfone (9) to fluorenone (FIO) has been studied by means of flash photolysis. For n = 1 as well as 71 = 2 the transient FIO, M (contact ion pair) appeared in THF and FIO (free ion)... 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

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,... 

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. 

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. 

Co(II) and 02 to form Coin-C>2, the superoxo species. It is the latter process that accounts for the large volume reduction en route to the reaction products. Thus during flash-photolysis, electron transfer in the reverse direction occurs due to irradiation into the CT band, which is followed by the rapid release of dioxygen. 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

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

Laser flash photolysis was also applied to study the anion radicals of /ram-isomers of 4-nitro, 4,4 -dinitro- and 4-nitro-4 -methoxystilbenes, that are generated by triplet state quenching with l,4-diazabicyclo[2.2.2]octane (DABCO) in polar solvents at room temperature7. The study shows that electron transfer competes against the tram —> cis... 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

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). 

Fig. 13. Kinetic progress curves at 20°C for the electron transfer intermediate (I) formed upon flash photolysis of the [ZnCcP, Cc (horse)] complex. The absorbance was monitored at the 549 nm A /A isosbestic. The sample contained 4.7 pM ZnCcP and 14.2 pM Cc in 5 mM KPj, pH 7...

Flash photolysis and pulse radiolysis techniques have been developed to study Fe Ru ET in Ru-modified proteins [21,26,27]. A method that allows study of electron transfer from a surfaee ajRu(IIIXhistidine) to a protein redox center is outlined in the Scheme [21]. The ET reaction is initiated by photogenerated... 

The formation of the transient reduced complex can also be observed by flash photolysis in the presence of CT-DNA [1(X)]. This shows clearly the existence of a photoinduced electron transfer from a base of the polynucleotide to the excited complex. However, the relative amount of reduced complex which is photoproduced, is smaller in the presence of CT-DNA than in the presence of GMP this may be attributed to a more important back electron transfer process in the ion pair produced on the polynucleotide compared to that in solution with the mononucleotide. 

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-. ... 

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. 

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