Decay kinetics

Sub-picosecond photoinduced absorption studies were employed to demonstrate the speed of the photoinduced electron transfer. Upon addition of C(M to P30T, the P1A spectrum, decay kinetics, and intensity dependence all change dramatically 36J. Already at 1 ps after photoexcitation by a 100 fs pump pulse at... 

For QPh-x, the decay of SPV- was faster than that for the APh-x-SPV system, and obeyed first-order kinetics [76]. The addition of NaCl (0.2 M) caused the decay profile to change i.e. the back ET was considerably slowed and the decay kinetics changed from first-order to second-order, with a reaction rate constant of kb = 1.8 x 109 M-1 s-1. These findings suggest the impossibility of escape of... 

Here, the addition of NaCl also changes the decay kinetics from first-order to second-order. 

Carbonaceous Deoxygenation. In this process microorganisms, principally bacteria, enzymatically mediate oxidation of simple and complex organic substances according to first order decay kinetics. 

The permanganate oxidation of oxalic acid has been studied exhaustively and has been reviewed by Ladbury and Cullis . It is characterised by an induction period and a sigmoid dependence of rate upon time. Addition of manganous ions eliminates the induction period and produces first-order decay kinetics . Addition of fluoride ions, however, practically eliminates reaction . ... 

Flash photolysis with microwave detection of charge carriers could become an additional technique in the future. The method has not yet been applied to colloids but has been used with small suspended particles. Immediately after the laser flash a conductivity signal was observed which decayed in the 0.1 to 1 microsecond range. The signal was longer-lived for a suspension of TiOj in para-dioxane than in Decalin. Such an effect of the surrounding medium on the decay kinetics of the conductivity indicates that surface states are involved... 

M. Sponsler, R. Jain, F. Corns, and D. A. Dougherty, Matrix-isolation decay kinetics of triplet cyclobutanediyls. Ohservation of both Arrhenius behavior and heavy-atom tunneling in C-C bond-forming reactions, J. Am. Chem. Soc. 1989, 111, 2240. 

The absorption bands measured by the flash spectrographic method are often assigned by (a) comparison with known singlet-singlet absorption spectra, (b) comparison of the lifetime of the species responsible for the absorption with the phosphorescence lifetime, (c) comparison with calculated energies and intensities of the various possible absorptions by semi-empirical molecular orbital methods, and (d) comparison with published triplet absorption spectra and decay kinetics of model compounds. 

Fig. 2 The changes in fluorescence decay kinetics on binding the analyte, (a) The analyte is the dynamic quencher. The decay becomes shorter gradually as a function of its concentration, (b) The analyte binding changes the lifetime. Superposition of decay kinetics of bound and unbound forms is observed...

Padilha LA, Webster S, Hu H, Przhonska OV, Hagan DJ, Van Stryland EW, Bondar MV, Davydenko IG, Slominsky YL, Kachkovski AD (2008) Excited state absorption and decay kinetics of near IR polymethine dyes. Chem Phys 352 97-105... 

Broos J, Maddalena F, Hesp BH (2004) In vivo synthesized proteins with monoexponential fluorescence decay kinetics. J Am Chem Soc 126 22-23... 

The pseudo-first order decay kinetics of this absorption are consistent with the decay of Cr(CO)5. Thus, in cyclohexane solution under 1 atm pressure CO, the half-life of decay is 25 /xseconds k = 2.8 x 10-4 second-1) 30). 

In general, intramolecular isomerization in coordinatively unsaturated species would be expected to occur much faster than bimolecular processes. Some isomerizations, like those occurring with W(CO)4CS (47) are anticipated to be very fast, because they are associated with electronic relaxation. Assuming reasonable values for activation energies and A-factors, one predicts that, in solution, many isomerizations will have half-lives at room temperature in the range 10 7 to 10 6 seconds. The principal means of identifying transients in uv-visible flash photolysis is decay kinetics and their variation with reaction conditions. Such identification will be difficult if not impossible with unimolecular isomerization, particularly since uv-visible absorptions are not very sensitive to structural changes (see Section I,B). These restrictions do not apply to time-resolved IR measurements, which should have wide applications in this area. 

With some further assumptions, it is possible to use single frequency FLIM data to fit a two-component model, and calculate the relative concentration of each species, in each pixel [16], To simplify the analysis, we will assume that in each pixel of the sample we have a mixture of two components with single exponential decay kinetics. We assume that the unknown fluorescence lifetimes, iq and r2, are invariant in the sample. In each pixel, the relative concentrations of species may be different and are unknown. We first seek to estimate the two spatially invariant lifetimes, iq and t2. We make a transformation of the estimated phase-shifts and demodulations as follows ... 

The only unknowns in these equations are the two fluorescence lifetimes, which considerably reduces the complexity of the problem. Figure 2.3 shows a plot of N versus D for all possible monoexponential decays, and for all possible mixtures of two monoexponential species with lifetimes equal to 2.5 and 1 ns. The half-circle though (0,0) and (0,1) represents the values of N and D that correspond to all possible monoexponential decay kinetics [13, 16, 43], All the values of /V, and D, for a mixture of two species he on a straight line connecting the two points on the half-circle that correspond to the lifetimes of the two species. The offset and the slope of this straight line are given by Eq. (2.21). 

FIGURE 7.10 Comparison of long-time decay kinetics for ion-pair recombination. The authors find r 0 ) decay describes the kinetics over a wider range of time than r05. Here T is normalized time. See text for details. Reproduced from Bratczak et al. (1997), with permission of Elsevier . 

D. Behar, G. Czapski, J. Rabani, L.M. Dorfman, and H.A. Schwarz, Acid dissociation constant and decay kinetics of the perhydroxyl radical. J. Phys. Chem. 74, 3209-3213 (1970). 

Table 1. Decay Kinetics and Yields for Various Carbocations 141 Produced by Protonation of Diarylcarbenes 139...

In the 1988-1999 period, almost all absolute kinetic studies of carbenic reactions employed LFP with UV detection. Carbenes that contain a UV chromophore (e.g., PhCCl) are easily observed, and their decay kinetics during reaction can be readily followed by LFP.11 However, alkyl, alkylhalo, and alkylacyloxycarbenes are generally transparent in the most useful UV region. To follow their kinetics, Jackson et al. made use of the ylide method, 12 in which the laser-generated carbene (2) is competitively captured by (e.g.) pyridine, forming a chromophoric ylide (3, cf. Scheme 1). The observed pseudo first order rate constants (kobs) for the growth of ylide 3 at various concentrations of pyridine are monitored by UV spectroscopy, and obey Eq. 1. 

Denisov, I.G., Grinkova, Y.V., Baas, B.J. and Sligar, S.G. (2006) The ferrous-dioxygen intermediate in human cytochrome P450 3 A4. Substrate dependence of formation and decay kinetics. Journal of Biological Chemistry, 281 (33), 23313-23318. 

The fluorescence decay kinetics of exemplary chosen QDs and small organic dyes are compared in Fig. 2. The size of the fluorescence parameter luminescence lifetime is determined by the electronic nature of the transitions involved. As a rule... 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

D. J. Morantz and J. W. Wigley, The contrasting phosphorescence decay kinetics of diacetyl and aromatic ketone phosphors in polymeric matrices, Polymer Communication 26, 170-171 (1985). 

J. R. Lakowicz, G. Laczko, H. Cherek, E. Gratton, and M. Limkeman, Analysis of fluorescence decay kinetics from variable-frequency phase shift and modulation data, Biophys. J. 46, 463—+77 (1984). 

S. Yu. Egorov, V. F. Kamalov, N. I. Koroteev, A. A. Krasnovsky Jr, B. N.Toleutaev,andS. V. Zinukov, Rise and decay kinetics of photosensitized singlet oxygen luminescence in water measurements with nanosecond time-correlated single photon counting technique, Chem. Phys. Lett. 421 —424 (1989). 

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