Decay of fluorescence

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

Fig. 3.2. Decay of fluorescence intensity and period 7 is the time needed for the number analogy with radioactive decay. Note that the of radioactive entities to decrease to 1 of its...

One sees that the result is the same as before, namely, a rise and decay of fluorescence. In this case, however, the rise and decay is from an ion different from that initially excited. 

The measurement of the growth and decay of fluorescence requires essentially two items (a) modulated excitation source and (b) a detector. The modulation of an excitation source may be accomplished in various ways. These range from simple mechanical choppers to highly sophisticated electronic pulsers. Detectors may be phototubes or semiconducting devices, or even the human eye. The detector itself, in some instances, may be modulated. Of course, the detector chosen must depend upon the spectral range to be studied and the response time desired. 

Figure 21. Rise and decay of fluorescence from the Tb 5Z)4 state. Peak A is the stray light from the flash [from Ref. (92)].

In a low-concentration sample, with both levels stable, they applied bursts of excitation to the 5D3 level while the emissions from the 5Z>4 were monitored. As expected, a rise and decay of fluorescence was observed. This again indicates that a slow internal-conversion process is occurring from the 5Z)3 to the 5D4 states. The rise time of the fluorescence turned out to be 189 /xsec whereas the decay time was 566 /xsec. 

Nemzek and Ware [7] have studied the fluorescence decay of 1,2-benzanthracene (and naphthalene) in 1,2-propanediol or purified mineral oil by the single photon counting technique over the temperature range 10—45°C. The fluorescence lifetimes, t0, were measured. In further experiments, which included a heavy atom fluorescence quencher, carbon tetrabromide in concentration [Q] 0.05—0.29 mol dm-3, no longer could the decay be characterised by an exponential with a constant lifetime. However, the decay of fluorescence was well described by an expression of the form... 

No changes in the absorption spectra of LB films were observed in argon during irradiation for up to 225 hr. The decay of fluorescence during irradiation in argon was also found to depend on the temperatures. It became slower at lower temperatures and only 16% decayed at 80°K after 90 min irradiation, which indicated some contribution of thermal process to the fluorescence de-... 

Figure 1.74 Decay of fluorescence intensity at selected points in the bifurcation laminating mixing element. These data are taken as a measure for spatially judging the mixing efficiency [42] (by courtesy of RSC).

Bucci, E., and Steiner, R. F. (1988). Anisotropy decay of fluorescence as an experimental approach to protein dynamics. Biophys. Chem. 30, 199—224. 

Anisotropy measurements yield information on molecular motions taking place during the fluorescence lifetime. Thus, measuring the time-dependent decay of fluorescence anisotropy provides information regarding rotational and diffusive motions of macromolecules (Wahl and Weber, 1967). Time-resolved anisotropy is determined by placing polarizers in the excitation and emission channels, and measuring the fluorescence decay of the parallel and perpendicular components of the emission. 

Wahl, Ph., Paoletti, J. and Le Pecq, J.-B. (1970). Decay of fluorescence emission anisotropy of the ethidium bromide-DNA complex evidence for an internal motion in DNA. Proceedings of the National Academy of Sciences, USA, 65, 417-421. 

Popovic et al. (1987) studied photogeneration of N,N-bis(methyl)perylene-3,4,9,10-tetracarboxyldiimide by field modulation of the time-resolved fluorescence. The results show that the field increases the rate of decay of fluorescence but leaves the initial intensity unchanged. The results were described by a process which occurs by the field-assisted dissociation of the first-excited singlet state. The absence of quenching of the initial fluorescence (amplitude quenching) was interpreted as evidence that the photogeneration process cannot be described by Onsager theories. 

Wahl, P. Decay of Fluorescence Anisotropy in Concepts in Biochemical Fluorescence , (R. F. Chen and H. Edelhoch Eds.), Marcel Dekker, New York, 1975... 

The global resolution of heterogeneous decay of fluorescence lifetime data is a powerful technique capable of extracting exponential parameters in such systems as liver alcohol dehydrogenase. The procedure and its sensitivity have been briefly discussed. This methodology represents an important develop-... 

A spectroscopic investigation of the formation of THF-Cu"Cl2 complexes has been described, and irradiation of [Cu(Dto)2] (Dto = dithiooxalate) has been found to induce an intramolecular Dto Cu two-electron transfer with cleavage of the C-C bond in the Dto ligand and the formation of SCO. The kinetics of photo-oxidation of pyrene by Cu" in SDS micelles have been measured, but oxidants such as Eu" and Hg do not produce pyrene cations. A non-exponential decay of fluorescence is observed, and this is interpreted in terms of a model due to Tachiya which restricts the numbers of quenchers in a micelle. Transient Cu"-alkyl species are formed on flash photolysis of Cu -bis(amino-acid) complexes such as those of serine and valine, and pseudo first order rate constants for the decay of the transients have been obtained. 

Figure 9. The time profile for the decay of fluorescence from a solution of anthracene in degassed hexane. The data were obtained with a time-correlated single-photon-counting instrument such as that shown in Figure 8. The upper panel shows the raw data with a superimposed linear fit and the instrument-response function. The extracted lifetime was 5.14 ns. The lower panel shows the residuals. (Courtesy of Dr. F. N. Castellano).

Energy migration may reveal itself as a fast component in the decay of fluorescence of M, being often in the pfcosecond region, but possibly in some polymer systems on the nanosecond time scale. The phenomenon can certainly contribute greatly to the observed depolarization of fluorescence ... 

Fig. 47. Decay of fluorescence anisotropy for polystyrene labelled with anthracene in various viscosities. Theoretical curves (drawn as bold lows) from Eq. 107. For clarity each curve has been displaced by 1.5 ns with respect to the preceding one.

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