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Fluorescence decay constant

This agrees quite well with the rate constants for intramolecular proton transfer in 2,4-bis(dimethyl-amino )-6-(2-hydroxy-5-methylphenyl)-5-triazine which had been measured by Shizuka et al. ( l6) using laser picosecond spectroscopy. The fluorescence decay constant t of (TIN) was found to be 60 20 ps. Because of the weak intensity all fluorescence lifetimes refer to the pure substance in crystalline form at room temperature. [Pg.11]

Ionic dissociation is also responsible31,102 for an increase in the measured fluorescence decay constant (Fig. 12) now given by... [Pg.210]

FIG. 12 Simulation of fluorescent decays for dye species located in the aqueous phase following laser pulses in TIR from the water-DCE interface according to Eq. (38). A fast rate constant of excited state decay (10 s ) was assumed in (a). The results showed no difference between infinitely fast or slow kinetics of quenching. On the other hand, a much slower rate of decay can be observed for other sensitizers like Eu and porphyrin species. Under these conditions, heterogeneous quenching associated with the species Q can be readily observed as depicted in (b). (Reprinted with permission from Ref 127. Copyright 1997 American Chemical Society.)... [Pg.214]

Lifetime heterogeneity can be analyzed by fitting the fluorescence decays with appropriate model function (e.g., multiexponential, stretched exponential, and power-like models) [39], This, however, always requires the use of additional fitting parameters and a significantly higher number of photons should be collected to obtain meaningful results. For instance, two lifetime decays with time constants of 2 ns, 4 ns and a fractional contribution of the fast component of 10%, requires about 400,000 photons to be resolved at 5% confidence [33],... [Pg.133]

Comparison of the Experimental and Simulation Results. The preceding discussion has shown that both the experimental anthracene fluorescence profiles and the simulated anthracene concentration profiles decrease in a manner which closely follows an exponential decay. Therefore, the most convenient way to compare the simulation results to the experimental data is to define an effective overall photosensitization rate constant, kx or k2, as described above. Adoption of this lumped-parameter effective kinetic constant allows us to conveniently and efficiently compare the experimental data to the simulation results by contrasting the rate constant obtained from the steady-state fluorescence decay with the value obtained from the simulated decrease in the anthracene concentration. [Pg.103]

The following argument was used first by E. Y. Schweidler in 1905 to describe radioactive decay but it applies to all similar kinetic processes. The fundamental assumption is that the probability p of an event occurring over a time interval dt is independent of past history of a molecule it depends only on the length of time represented by dt and for sufficiently short intervals is just proportional to dL Thus, p — kdt where k is a constant of proportionality characteristic of the process being awaited. In fluorescence decay it is characteristic of the kind of molecule in chemical terms. [Pg.263]

Fluorescence Lifetimes. The fluorescence decay times of TIN in a number of solvents (11.14.16.18.19), low-temperature glasses (12.) and in the crystalline form (15.) have been measured previously. Values of the fluorescence lifetime, Tf, of the initially excited form of TIN and TINS in the various solvents investigated in this work are listed in Table III. Values of the radiative and non-radiative rate constants, kf and knr respectively, are also given in this table. A single exponential decay was observed for the room-temperature fluorescence emission of each of the derivatives examined. This indicates that only one excited-state species is responsible for the fluorescence in these systems. [Pg.76]

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. [Pg.323]

The fluorescence decay time is one of the most important characteristics of a fluorescent molecule because it defines the time window of observation of dynamic phenomena. As illustrated in Figure 3.2, no accurate information on the rate of phenomena occurring at time-scales shorter than about t/100 ( private life of the molecule) or longer than about 10t ( death of the molecule) can be obtained, whereas at intermediate times ( public life of the molecule) the time evolution of phenomena can be followed. It is interesting to note that a similar situation is found in the use of radioisotopes for dating the period (i.e. the time constant of the exponential radioactive decay) must be of the same order of magnitude as the age of the object to be dated (Figure 3.2). [Pg.44]

The case of several populations of fluorophores having their own fluorescence decay i (t) and time constants characterizing r (t) deserves particular attention. In Section 5.3, it was concluded that an apparent or a technical emission anisotropy r(t) can be obtained by considering that the measured polarized components, I(t) and I (t), are the sums of the individual components (i.e. of each population) and by using Eq. (6.43). Hence... [Pg.191]

Time-resolved method 1 decay of the donor fluorescence If the fluorescence decay of the donor following pulse excitation is a single exponential, the measurement of the decay time in the presence (td) and absence (t ) of transfer is a straightforward method of determining the transfer rate constant, the transfer efficiency and the donor-acceptor distance, by using the following relations ... [Pg.252]

From a practical point of view the consequences of TOF dispersion are important only for short intrinsic fluorescence decay times of to < 1 nsec. Figure 8.15 shows an example with to = 50 psec and realistic optical constants of the substrate. The intensity maximum in Fb(t) is formed at At 30 psec after (5-excitation. After this maximum, the fluorescence decays with an effective lifetime of r ff = 100 psec that increases after long times to t > > 500 psec. The long-lived tail disappears as soon as there is some fluorescence reabsorption, and for Ke = K there is practically no difference to the intrinsic decay curve (curve 3 in Figure 8.15). [Pg.243]

Lakowicz et al.(]7] VB) examined the intensity and anisotropy decays of the tyrosine fluorescence of oxytocin at pH 7 and 25 °C. They found that the fluorescence decay was best fit by a triple exponential having time constants of 80, 359, and 927 ps with respective amplitudes of 0.29, 0.27, and 0.43. It is difficult to compare these results with those of Ross et al,(68) because of the differences in pH (3 vs. 7) and temperature (5° vs. 25 °C). For example, whereas at pH 3 the amino terminus of oxytocin is fully protonated, at pH 7 it is partially ionized, and since the tyrosine is adjacent to the amino terminal residue, the state of ionization could affect the tyrosine emission. The anisotropy decay at 25 °C was well fit by a double exponential with rotational correlation times of 454 and 29 ps. Following the assumptions described previously for the anisotropy decay of enkephalin, the longer correlation time was ascribed to the overall rotational motion of oxytocin, and the shorter correlation time was ascribed to torsional motion of the tyrosine side chain. [Pg.43]

At binding ratios r > 0.27, both linear and supercoiled DNAs show evidence of a marked structural change. A component with intermediate lifetime (t 5 ns) appears in the ethidium fluorescence decay, which may represent a partially intercalated species. The apparent torsion constants become highly nonuniform and exhibit considerably altered values. The long-range torsion constant increases appreciably for the linear DNA, but decreases for the supercoiled DNAs, which are substantially positively supercoiled at that point.(53)... [Pg.199]

The iFi terms are the fluorescence lifetimes of fractional contributions a, and the xRJ indicate decay constants due to solvent relaxation (or other excited-state processes) of fractional contribution Pj. The negative sign is indicative of a relaxation process (red shift). Usually, the relaxation process is approximated to a single relaxation time x R by assuming an initial excited state and a final fully relaxed state (see, e.g., Ref. 128). A steady-state fluo-... [Pg.258]

Table II presents the vadues of v, the rate constant for the electron transfer reaction with the donor and acceptor in contact, calculated by deconvolution of the fluorescence decay curves for a number of excited porphyrin-cOkyl halide systems. It appears that the rate parauneter depends strongly on the calculated exothermicity for these reactions. Parauneter i/ contadns information about the Framck-Condon factor of the electron-tramsfer reaction, which is in itself dependent on the reaction exothermicity and reorgauiization energy (22.23). Whether the rate constauit for the electron-transfer reactions depends on the exothermicity in the manner predicted by theory, that is with a simple Gaussian dependence (22), cannot be ainswered at present because of the uncertainties in the energetics of the particular reactions studied here. Table II presents the vadues of v, the rate constant for the electron transfer reaction with the donor and acceptor in contact, calculated by deconvolution of the fluorescence decay curves for a number of excited porphyrin-cOkyl halide systems. It appears that the rate parauneter depends strongly on the calculated exothermicity for these reactions. Parauneter i/ contadns information about the Framck-Condon factor of the electron-tramsfer reaction, which is in itself dependent on the reaction exothermicity and reorgauiization energy (22.23). Whether the rate constauit for the electron-transfer reactions depends on the exothermicity in the manner predicted by theory, that is with a simple Gaussian dependence (22), cannot be ainswered at present because of the uncertainties in the energetics of the particular reactions studied here.
A kinetic technique for determining a fluorophore s excited state lifetime by using a light source whose intensity is modulated sinusoidally at a certain frequency, such that the intensity of the fluorescence emission likewise varies sinusoidally but with an added delay from the finite relaxation constant for fluorescence decay. The period of the sinusoidal modulation is chosen to be in the neighborhood of the magnitude of the fluorescence lifetime. [Pg.544]


See other pages where Fluorescence decay constant is mentioned: [Pg.182]    [Pg.201]    [Pg.23]    [Pg.50]    [Pg.1139]    [Pg.182]    [Pg.201]    [Pg.23]    [Pg.50]    [Pg.1139]    [Pg.209]    [Pg.89]    [Pg.11]    [Pg.45]    [Pg.46]    [Pg.276]    [Pg.227]    [Pg.229]    [Pg.138]    [Pg.77]    [Pg.78]    [Pg.168]    [Pg.338]    [Pg.343]    [Pg.384]    [Pg.393]    [Pg.7]    [Pg.10]    [Pg.24]    [Pg.26]    [Pg.33]    [Pg.42]    [Pg.31]    [Pg.165]    [Pg.298]    [Pg.299]    [Pg.100]    [Pg.61]   
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