Laser photolysis techniques

Ferrone FA, Hofrichter J, Eaton WA. Kinetics of sickle hemoglobin polymerization. I Studies using temperature jump and laser photolysis techniques. J Mol Biol 1985 183 591-610. 

The biradicals produced in the hydrogen transfer reaction can be monitored directly using laser photolysis techniques (8). 

The formation of the radical-cation, PQ+ was monitored using laser photolysis techniques at its absorption maxima at 603nm. A study of the rates of PQ+" formation at different PQ++ concentrations led to kg=l.Tx109 M-1s-1. Despite the fact that this reaction is extremely fast, the rate of electron transfer for the macrobiradical is significantly slower than those for the same group in small molecules (8,11). 

Rate Constants of Reactions of para-Disubstituted Diphenylaminyl Radicals with Phenols in Decane and Toluene Estimated by the Laser Photolysis Technique [31-33]... 

Photochemical reactions occur under the influence of radiation. Conventional sources of radiation, and modem flash and laser photolysis techniques, are both extensively used. 

Kurita et al. 2 Takeda et al. 3 and Parthenopoulos and Rentzepis11 used picosecond laser photolysis techniques to study the photochromic processes of furyl fulgide. They found that the excited states of furyl fulgide and its colored form were singlet states and had n,n characteristics. Takeda et al.13 reported from theoretical studies that the values of the oscillator strength and the radiation lifetime (/r) were... 

In 1984, Weller s group reported MFEs on the primary photochemical processes of polymethylene-linked compounds containing electron acceptor and donor groups (A and D) with a nanosecond-laser photolysis technique in the presence of magnetic fields below 0.3 T [3a], Their reaction scheme is represented as follows ... 

Ru -moiety described by a magnetic field independent relaxation time ( s) was also important for their MFEs. They carried out numerical simulation of the observed AR E) values with Eq. (12-39), where fe, and bet were only treated as empirical parameters. We can see from Fig. 12-13 that the simulated curves can well reproduce the observed AR B) values. The parameters determined with this method for the four complexes are listed in Table 12-6. It is noteworthy from this table that the reaction processes described by Reactions (12-34a) - (12-34e) occur in ps-time region. Although such ps-processes of the RIPs in fluid solutions can not be measured directly by ps-laser photolysis techniques due to diffusion-controlled formation of the RIPs, it is a great experimental challenge to observe directly the MFEs of the reaction processes of dyadic RIPs in ps time-resolved experiments. Such investigations have recently been carried out [19]... 

Interest has been restricted this year to studies of photoelimination of nitrogen from arenediazonium salts and attempts to characterize the resulting aryl cations. Nanosecond laser photolysis techniques have been used to examine the photodecomposition of diazonium tetrafluoroborates. Aryl cations could not directly be detected, but evidence from bleaching experiments indicates that, in water, the phenyl cation has a lifetime of about 500 ps. The isolation of the ether (131) as the sole product of photodecomposition of the diazonium salt (132) in methanol demonstrates that rearrangement of the intermediate aryl cation does not occur. Both heterolytic and homolytic pathways have been described for photodecompositions of arenediazonium salts complexed with crown ethers. A useful synthetic application of this conversion is the formation in 53% yield of ethyl 2,4-difluoroimidazole-5-carboxylate (133) on irradiation of the diazonium fluoroborate (134). ... 

While the significance of radicals in biological systems has been appreciated for decades, there is relatively little definitive experimental infonnation on the identity of the radicals and even less on the mechanisms by which they affect the physiology of living systems. The paucity of detailed information is a direct consequence of the fact that most radicals are highly reactive and, therefore, short-lived transient species. Despite the tremendous advances in spectroscopic and laser photolysis techniques, much less is known about radicals than about closed-shell species. The treatment of radicals by theoretical methods is, however, only marginally more difficult than that of closed-shell molecules. It is for these reasons that the numerous applications of quantum chemical techniques to radicals have proven to be complementary to experimental studies. 

The quenching mechanism has been investigated by a picosecond laser photolysis technique for the lumiflavin (Lf) and riboflavintet-rabutylate (RFTB) as fluorescers, and indole, N-methylindole and phenol as quenchers in various solvents of different polarity, and the electron transfer from the quencher to the fluorescer has been confirmed by transient absorption spectral measurements (5). The reaction scheme of the fluorescence quenching is given in... 

When riboflavin (RF) is mixed with riboflavin binding protein from egg white, the fluorescence intensity decreased to about two-thousandth. Transient absorption spectra of riboflavin binding protein were also measured with the picosecond laser photolysis technique (14), as shown in Fig.8. The transient behavior of the spectra was quite different from that of flavodoxin. 

The kinetics of this process was studied by laser photolysis technique. In Fig. 4.9 is shown the temporal behaviour of the absorbance of a solution containing 5 x 10-5 M ZnTMPyP4 and 10 3 M C14MV2+. The concentration of Ci4MV2+ is well below its CMC value, i.e. 7 X IQ-3 M. The absorbance at 890 nm reflects the behaviour of the triplet... 

A distinct advantage of the finely divided platinum dispersions is that the photolysis solutions remain completely transparent even at high catalyst concentration. This allows a direct study of the dynamics of the reaction of MV+ with Pt particles by employing laser photolysis technique. The upper part of Fig. 7.3 shows oscilloscope traces illustrating the temporal behaviour of the characteristic MV+ absorbance at 602 nm in the absence and presence of catalyst. The upward deflection of the signal after the laser pulse is due to the formation of MV+ via the photoredox process ... 

A major experimental development in the past two years has been in the application of laser photolysis techniques for producing transla-tionally hot hydrogen atoms to the H + D2 HD + D reaction . This has enabled the direct observation of product vibration/rotation distributions in this reaction for the first time. As might be expected. these experiments have prompted a flurry of theoretical activi-ty44,45,107-112 and by and large the comparison between theory and experiment has been very good. 

The reaction of "OH or "S04 radicals with CM-chitin and CM-chitosan were investigated in aqueous solutions using a laser photolysis technique (Zhai et al. 2004a). The rate constants of the reactions between "OH or "804" radicals with CM-chitosan are always higher than those for... 

Ferrone, F. A. Hofrichter, J. Eaton, W. A. 1985a. Kinetics of Sickle Hemoglobin Polymerization I. Studies Using Temperature-Jump and Laser Photolysis Techniques, J. Mol. Biol. 183, 591-610. 

The speed of AgO formation is so rapid that it cannot be resolved by ns-laser photolysis techniques. Obviously, this effect has to be attributed to the close proximity of photoactive donor and Ag" " sites on the surface of the micelle. The photoinduced reduction of Ag" can be performed with a number of other donor chromophores such as porphyrins. 

Frei et al. [34[ examined the surface chelation of phenylfluorene on Ti02 using FT-IR and laser photolysis techniques. The surface chelate has its visible absorption band maximum located at 476 nm (e = 3.6x10 M cm ). Electronic excitation in the visible absorption band results in extremely rapid and efficient injection in the conduction band of the semiconductor. A lower limit for the rate constant of interfacial electron transfer was determined as 10 s l and the back electron transfer was found to occur with a specific rate of 2.8x10 s l. The injected electrons in the conduction band readily reduce electron acceptors such as methyl viologen efficiently in the same manner as when they are produced by bandgap excitation of the semiconductor. 

When cyclobutene 5 is irradiated with an XeCl excimer laser (308 nm), compounds 6 and 7 are formed (Scheme 3). It was revealed that 6 is formed by the excitation of the triplet state of 5 whereas 7 was formed from higher singlet states. The transient spectrum of 5 shows a triplet absorption maximum at 440 nm with its maximum intensity appearing 3.9 ps after the laser pulse. The time-delayed, two-color pulse laser photolysis technique using an XeCl and 440-nm laser pulses showed the increase in the ratio of the yields 6 7. 

The reaction shown in Scheme 12 was studied using the time-delayed, two-color pulse laser photolysis technique. An unstable o-quinodimethane derivative 42 is photochemicaUy generated from substituted indanone 41. The o-quinodimethane derivative 42 is further photolyzed to 43 and 44. In the case of simple 308-nm laser photolysis, the ratio of the yields 44 43 is 2.54, whereas the ratio changes to 3.93 when the time-delayed, two-color photolysis technique is used. In the two-color photolysis, the first laser used is a 308-nm laser pulse and the second is a 480-nm laser pulse that is irradiated 2 is after the first laser pulse. The wavelength of the second laser is adjusted to the absorption maximum of 42. 

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