Flash excitation

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

Flash excitation of solutions of 3,5-dinitroanisole in the presence of nucleophiles produces a short-lived species with an absorption maximum at 550-570 nm (Figure 11, curve a). Decay of this species follows first order kinetics. The lifetime varies rather strongly with the nature of the nucleophile, from 40 ms with hydroxide ion to 0 1 ms with thiocyanate (Table 4). The absorption spectrum is independent of the reagent used. 

Direct flash excitation (>400 nm) or the triplet-acetone-sensitization of nitrosamide X in degassed water or benzene solutions gives the amidyl radical transient exhibiting lax 335-350 nm. This transient is not observed with undegassed solution of X, indicating that oxygen has intercepted the precursor of the amidyl radical at least, with the diffusion controlled rate... 

Fig. 7. Schematic molecular /"(/) and excimer /D(f) fluorescence response curves following pulsed-flash excitation p(t) (after Birks, Dyson, and Munro1).

Experimentally the key to the method is determining the relative population of triplet molecules by the optical density of triplet-triplet absorption following flash excitation. To derive the kinetic expressions needed to determine O , the scheme is the same as that written previously with the additional proviso that the singlet interaction lead to intersystem crossing by the sensitizer. 

But more recent work using flash excitation has shown that Hg(63P0) may not be the final state in all cases. It is true for N2, but CO deactivates the excited mercury to ground state Hg (6 S0), itself being transferred to high vibrational level (v = 91 through the intermediary of mercury-carbonvl complex formation. 

Figure 7.32 Kinetics of luminescence of pyrene following laser flash excitation. L, laser pulse profile M, monomer emission, E, excimer emission rise and decay. Horizontal axis, time in ns vertical axis, light intensity in arbitrary units. The three kinetic curves are normalized to a common maximum...

Flash-photolysis of potassium amide solutions gives different photochemical patterns, depending on the excitation wavelength. Flash excitation through Coming 9863 filters (passing 240-400 m/z and far red... 

In conclusion, the photochemical behavior is in agreement with that observed in pH-jump experiments. Although Cc is obviously the primary product of flash excitation, the observed species and their survival time (from seconds to years) before reversion to the thermodynamically stable form Ct depend on temperature and pH. 

Manganese. Photooxidation of Mn2+ upon flash excitation in an aqueous solution has been reported278. The redox interaction between Mn2+ and thionine278, in our opinion, is unlikely because Mn2+ is expected to be very short lived and the thionine concentration was very low. 

The time-resolved spectroscopic studies of the transient intermediates provide the keys to understanding the charge-transfer photochemistry of contact ion pairs. Thus, the unambiguous identification of the carbonylmetal radical Mn(CO)ja immediately following the 10-nsecond laser-flash excitation of the contact ion pair Cp2Co+ Mn(CO)s- at A = 532 nm relates directly to the photoredox process,... 

When excited states of a molecule are created in solution by continuous or flash excitation, the excited-state molecule interacts to a varying degree with the surrounding solvent molecules, depending on their polarity, before returning to the ground state. These excited-state solute/solvent interactions found in fluorescent molecules are often reflected in the spectral position and shape of the emission bands as well as in the lifetimes of the excited-state molecules. The solvent-dependence of the position of emission bands in fluorescence spectra is commonly included in the term solvatochromism. Sometimes, the solvent-dependence of fluorescence spectra has been called fluoro-solvatochromism [26] or solvatofluorochromism [27], Because of the close connection between spectral absorption and emission (see Figs. 6A and 6-7), there is no need for special terms for the fluorescence-based solvatochromism [16],... 

At room temperature, flash absorption studies revealed that an electron acceptor designated Aj was functioning under conditions where F and Fg were presumably reduced [37]. The state (P-700, A2 ) is formed upon flash excitation and recombines with tiu — 250 jUS. The difference spectrum due to its formation was analysed into contributions of P-700 and A2. The latter includes mainly a small and broad bleaching around 430 nm, and perhaps some absorption shifts in the red. These absorption properties, together with the disappearance of the A2 absorption signal when iron-sulfur proteins are denatured [38,39], indicate that Aj may be an iron-sulfur centre. 

Fig. 5 shows a kinetic model of the bacteriorhodopsin photocycle. It accounts quantitatively for time-resolved difference spectra measured between 100 ns and 100 ms after flash excitation at temperatures between 5 and 30°C. The kinetic coimection between M, L, and K through reversible reactions introduces two additional time constants into the rise kinetics of M, and the reverse reactions which connect M, N, and O likewise affect the decay of M. An important part of this scheme is the existence of two consecutive M substates, Mi and M2, connected by an irreversible reaction [114]. 

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