Biexponential decay

The transient at pH 5 decays biexponentially with time constants of 8.7 2.0 ps and 310 30 ps. In order to determine whether still slower decay components are present will require... 

Acridine and phenothiazine cocrystallized to give two kinds of hydrogen-bonded CT crystals. Both crystals showed some photoreactivity and appear to have given many photoproducts (Scheme 18) [44]. Although this crystalline complex is complicated in terms of stoichiometry, crystal structure, and photoreaction, a transient study by femtosecond diffuse reflectance spectroscopy was carried out, as had been done for durene-pyromellitic dianhydride cocrystal [45]. For the yellow cocrystal, a transient absorption spectrum with maxima around 600 and 520 nm was obtained, which decayed biexponentially with lifetimes of 2 and 50 ps. The two absorption maxima were ascribed to the acridine anion radical and the phenothiazine cation radical, respectively. 

This behavior is consistent with experimental data. For high-frequency excitation, no fluorescence rise-time and a biexponential decay is seen. The lack of rise-time corresponds to a very fast internal conversion, which is seen in the trajectory calculation. The biexponential decay indicates two mechanisms, a fast component due to direct crossing (not seen in the trajectory calculation but would be the result for other starting conditions) and a slow component that samples the excited-state minima (as seen in the tiajectory). Long wavelength excitation, in contrast, leads to an observable rise time and monoexponential decay. This corresponds to the dominance of the slow component, and more time spent on the upper surface. 

For vacuum sublimed thin films, Grabuzov et al. [138] reported a photoluminescence quantum efficiency of 32 2%. In the same paper, data on the absorption coefficient at the maximum, a = (4.4 0.1) x 104 cm 1, and the refractive index at 633 nm (n = 1.73 0.05) can be found. Other reported values for the photoluminescence quantum efficiency that can be found in the literature are 30 5% [124] and 25 5% [139]. Naito et al. [109] reported a quantum yield of 5% in the amorphous film compared to 35% in the crystalline state. The fluorescence lifetime is reported to be biexponential with x = 3.4 and 8.4 ns, which is much shorter than in the crystal (17.0 ns). In the amorphous state, the larger free volume allows more vibrations and rotations to take place, which favors nonradiative decay. 

Nafion is thought to form dimers giving rise to a biexponential decay. 44,46) Using diode laser excitation at 670 nm, the fluorescence of oxazine in Nafion and its quenching by copper ions has been shown to give rise to a complex fluorescence decay.(47) Despite such complications there is still room for optimism. For example, Zen and Patonay(48) have demonstrated a pH sensor based on cyanine dye fluorescence intensity in Nafion excited with 30 mW diode laser excitation at 780 nm. 

Becker.(49) The lifetimes were all reported as being single-exponential. Rousslang and his collaborators have recently reexamined a number of these compounds at pH 3 and 5.(50) In general, the phosphorescence decays are biexponential, but are dominated by a longer lived component of about 3 s which comprises 98 % or more of the decay. 

With the development of multifrequency phase-modulation technology, Lakowicz and co-workers(171) were able to examine the time dependence of the anisotropy decay of BPTI. They noted that the intensity decay of the fluorescence is best fit by a biexponential decay law and that the anisotropy decay is also complex. At 25 °C and pH 6.5, correlation times of 39 ps and 2.25 ns were recovered from analysis of data obtained over the range 20 MHz to 2 GHz. The longer correlation time is close to that predicted for the overall rotational motion of a molecule of the size of BPTI. They indicated, however, that additional experiments need to be done to resolve whether the 39-ps... 

Proteins having one chromophore per molecule are the simplest and most convenient in studies of fluorescence decay kinetics as well as in other spectroscopic studies of proteins. These were historically the first proteins for which the tryptophan fluorescence decay was analyzed. It was natural to expect that, for these proteins at least, the decay curves would be singleexponential. However, a more complex time dependence of the emission was observed. To describe the experimental data for almost all of the proteins studied, it was necessary to use a set of two or more exponents.(2) The decay is single-exponential only in the case of apoazurin.(41) Several authors(41,42) explained the biexponentiality of the decay by the existence of two protein conformers in equilibrium. Such an explanation is difficult to accept without additional analysis, since there are many other mechanisms leading to nonexponential decay and in view of the fact that deconvolution into exponential components is no more than a formal procedure for treatment of nonexponential curves. 

Schneider et al. [63] investigated the photochemistry of the spiro-oxazine merocyanines pumping and probing at 570 nm in acetonitrile. The found that the solution bleached within the <5-psec pulse duration. The bleached state recovered with at least a biexponential behavior, and from their fluorescence decay measurements, three exponentials were required to fit the decay. They attribute these findings to the possibility of three merocyanine isomers that are in equilibrium. Their compounds feature geminal ethyl groups on the indoline moieties and this may influence the system as compared to NOSIl. 

In comparison, photolysis of 83 in protic solvents such as methanol, ethanol, and water yields 84 as expected, but 84 forms mainly 87 rather than 85. Furthermore, in these solvents, the transient absorption (Amax 425 nm) due to 84 decays not with a second-order rate law but by biexponential decay. For example, the decay of transient absorption of 84 (A ax 420 nm) in water at pH 7 had rate constants of 2 x 10 and 3 x lO s Subsequent to the decay of 84, a transient absorption was formed with Amax 330 nm and a weak absorption band at 740 nm. However, this transient was formed much slower than 84 decayed. The absorption at 330 nm was described as a biexponential growth with rate constants of 584 and 21 s h The authors assigned this absorption to 88. Since 84 and 88 do not form and decay at the same rate, the authors theorized that 84 decays into 87, which then furnishes 88. Even though intermediate 87 does not absorb in the near UV, the authors characterized it with time-resolved IR spectroscopy. The authors demonstrated that, in hexane and a strongly acidic or basic aqueous solution, the photorelease from 83 goes through the formation of 87, whereas in near neutral aqueous solution, formation of 85 predominates. The authors concluded that the dehydration of intermediates 85 and... 

Figure 2.1. Transient absorption at 680 nm following the 355-nm excitation of 0.02 M. benzophenone in acetonitrile as a function of concentration of A, A -dimethylani-line. 14)M Fit to a rate of appearance of 6.9 x lO s and a biexponential decay with...

Siese and Zetzsch (1995) and Bohn and Zetzsch (1998) have studied the OH-C2H2 reaction using FP-RF (see Chapter 5.B.3) and observed biexponential decays of OH. They propose that the adduct I has two channels in its reaction with 02, rather than one as shown above, and that one of the two generates OH and glyoxal, a small portion of which is excited and decomposes to HCO. 

Values of kD which appear in Table 9 were obtained by the usual biexponential decay analysis37) adapted from the intermolecular version due to Birks 7l). Despite observations made for meJo-bis( 1 -(2-naphthyl)-ethyl) ether13) and l,3-bis(2-naphthyl) propane 159) of one rise time and two decay times in the transient fluorescence of the excimer, there have been no reports confirming this complication for the compounds... 

Obtained after analyzing the decay behavior by the biexponential scheme of Birks 71> and assuming that kM = l.8x 107 s 1 andQ = 0.27 ... 

Free cw-azobenzene, excited at 480 nm displays a biexponential decay of the excited state Si with time constants of 0.1 ps and 0.9 ps. Here the ultrafast kinetic component dominates the absorption change (it contains 90 % of the whole amplitude). A direct interpretation would relate the fast component to a free isomerizational motion, where the most direct reaction path on the Si and So potential energy surface is used without disturbance. The slower process may be assigned to a less direct motion due to hindrance by the surrounding solvent molecules. This interpretation is supported by the observation of the absorption changes in the APB and AMPB peptides. Here both reaction parts are slowed down by a factor of 2 - 3 and both show similar amplitudes The peptide molecules hinder the motion of the azobenzene switch and slow down considerably the initial kinetics. However, in all samples the transition to the ground state is finished within a few picoseconds. 

Fig. 3. (top) Species-associated difference spectra of WT PYP upon 475-nm excitation, fitted to the model depicted in the lower figure. The ES decay was fitted with a biexponential rate of (2 and 9 ps)1, the I0 to I, transition rate was (0.9 -1 ns)"1. Since the quantum yield of 10 and I, states was only 24%, their presence in the raw data was 4 times smaller relative to the ES spectrum. 

Kinetics extracted from the transient spectra were fitted to a biexponential function with 3 ps and 18 ps lifetimes, except in the spectral region located between the bleaching and the stimulated emission bands where the decays were fitted to one exponential function with a 1.5 ps lifetime. Decays in the stimulated emission band are biexponential with an average lifetime of about 10 ps, which is comparable to the fluorescence lifetime reported in [3]. 

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