Decay nonexponential emission

The mixed ligand complexes Rh(bipy)2(phen) and Rh(phen)2 (bipy) " " exhibit nonexponential emission decay at 77 K (184)... 

Persistent microheterogeneity exists when the luminescence and quenching decay are fast compared to conformational-environmental rearrangement. Excited molecules can then emit and be quenched in distinctly different chemical environments. Decays can then be nonexponential, multiple emissions may be present, and simple Stern-Volmer kinetics can break down. 

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. 

In the majority of cases, fluorescent labels and probes, when studied in different liquid solvents, display single-exponential fluorescence decay kinetics. However, when they are bound to proteins, their emission exhibits more complicated, nonexponential character. Thus, two decay components were observed for the complex of 8-anilinonaphthalene-l-sulfonate (1,8-ANS) with phosphorylase(49) as well as for 5-diethylamino-l-naphthalenesulfonic acid (DNS)-labeled dehydrogenases.(50) Three decay components were determined for complexes of 1,8-ANS with low-density lipoproteins.1 51 1 On the basis of only the data on the kinetics of the fluorescence decay, the origin of these multiple decay components (whether they are associated with structural heterogeneity in the ground state or arise due to dynamic processes in the excited state) is difficult to ascertain. 

This model permits xR to be determined using information on the fluorescence decay in a very simple way. If unrelaxed fluorophores are excited, the decay is exponential beyond the relaxation range and, in this range, consists of two components t, and r2. These components will be simple functions of xR and t>. If we assume that emission on the short-wavelength side occurs only from the unrelaxed state and that the simultaneous loss of emitting quanta occurs due to relaxation, then the longer component, t, equals xF, and the shorter one, t2, equals 1(1/t + jxF). Unfortunately, this approach is difficult to apply when the decay is nonexponential, which is almost always the case with proteins (see Section 2.3.1.). 

There should exist a correlation between the two time-resolved functions the decay of the fluorescence intensity and the decay of the emission anisotropy. If the fluorophore undergoes intramolecular rotation with some potential energy and the quenching of its emission has an angular dependence, then the intensity decay function is predicted to be strongly dependent on the rotational diffusion coefficient of the fluorophore.(112) It is expected to be single-exponential only in the case when the internal rotation is fast as compared with an averaged decay rate. As the internal rotation becomes slower, the intensity decay function should exhibit nonexponential behavior. 

The decay-time curve of the acceptor centers (A) under excitation in the donor centers (D) is also nonexponential. This is evident in Figure 5.20(b), where the time evolution of the emission intensity of the Yb + ions (the acceptors) in YAlj (603)4 is shown under excitation with light absorbed by the Nd + ions (the donors). The Yb + decay-time curve shows an initial rise time, due to the excitation via energy transfer from the Nd + ions, followed by the characteristic exponential decay of the Yb + ions. 

The result is shown as the dot-dash curve of Figure 9.11. It is shown that the spontaneous emission has been effectively suppressed, with the suppression becoming more effective, the smaller is the Autler-Townes splitting A due to the CW laser. Also shown in Figure 9.11 (as the dashed line) are the natural decay curves, arising when we start with one of the eigenstates. As can be seen, this decay, which is nonexponential due to the interaction between the resonances, is still much faster than the suppressed decay aided by the interruptions. 

The compound bis-(4,4 -dimethylaminophenyl)-sulfone (DMAPS) and related compounds show multiple fluorescences in polar solvents due to excited state charge transfer (Rettig and Chandross [144]). Su and Simon [84,85] have examined the intramolecular electron transfer reaction in DMAPS, in alcohol solution over the temperature range from — 50°C to + 30°C. They observe that the decay of the local excited state is nonexponential and significantly faster than the longitudinal relaxation time of the solvent. In addition, they observed that the emission spectrum of the TICT state... 

From the theoretical expressions (4.218) and (4.219) valid for a totally efficient sink and a 8-shaped initial distribution, it comes out that the general feature of the B emission, proportional to the survival probability p(t), is a nonexponential decay with two limit behaviors characterized by [see Eqs. (4.222) and (4.223)]... 

The appearance of the second emission was attributed to (1) strongly bound Ru(bpy) + molecules or (2) a chemical change in Ru(bpy) + during the adsorption process. No significant differences were observed in the diffuse reflectance absorption spectra. A bathochromic shift of the absorption band, a hypsochromic shift of the fluorescence spectrum, and nonexponential luminescence decays are a few other interesting changes observed with these samples. 

An unusual feature is that the luminescence intensity and lifetime of 12b increases substantially in nonpolar solvents relative to the lifetime in polar solvents. Moreover, as the emission lifetime increases, the decay kinetics become distinctly nonexponential. The explanation for the unusual solvent-dependent luminescence properties of 12b is that in a low dielectric solvent the MLCT and charge separated states (14 and 15, respectively, in Scheme 8) are similar in energy and equilibrium is established between the two states. In polar solvents 15 is stabilized with respect to 14, and photoinduced ET is irreversible. Similar, but attenuated, solvent dependent luminescence is observed for 12c and 12d. The effect is attenuated in these complexes because the amine donors are easier to oxidize and thus the charge separated state is stabilized with respect to the MLCT state. 

Collisions can also cause small changes in v and / levels within the Estate, an effect that can lead to nonexponential decay curves since the emission rates vary somewhat with vibrational and rotational level. In the present experiment, these effects of vibrational and rotational relaxation should be minor since the total emission is measured and the pressure of collision partners is kept low. At higher collision pressures however, clear deviations from single exponential decay curves can be observed and the simplified analysis presented here is inadequate. 

Nature of the Lower Temperature Transition. The complex nonexponential phosphorescence decays, apparent under the higher time resolution afforded by use of the modified 199 spectrometer are a consequence of interjection by the polymer matrix in the photophysies experienced by the chromophore. Non-exponential decays of triplet naphthalene (and other chromophore) emissions have been observed in PMMA. Horie et al.(18-20) ascribe such effects to dynamic, intermolecular quenching of the excited state by the polymer whereas MacCallum et al(21-23) invoke an energy migrative process within the polymer following quenching of the triplet state of the naphthalene. 

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