Luminescence Decays

The mechanisms of luminescence decay from an optical center are of critical importance. In particular we have to know if there are any processes internal to the center or external to it, which reduce the luminescence efficiency. It is possible to define two decay times, ir, the true radiative decay time which a transition would have in absence of all non-radiative processes, and r, the actual observed decay time, which maybe temperature dependent, as will usually occur when there are internal non-radiative channels, and which may also be specimen dependent, as when there is energy transfer to other impurities in the mineral. The quantum yield may be close to unity if the radiationless decay rate is much smaller than the radiative decay. 

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The triplet-state energy level of oxytetracycline, the excitation maximum (412 nm), lifetimes of Eu-OxTc (58 p.s) and Eu-OxTc-Cit (158 p.s), were determined. A 25-fold luminescence enhancement at 615 nm occurs upon addition of citrate within a short 5-min incubation time at neutral pH. It s accompanied by a threefold increase of the luminescence decay time. The optimal conditions for determination of OxTc are equal concentrations of Eu(III) and citrate (C = T lO mol-E ), pH 7.2. Eor determination of citrate, the optimal conditions concentrations of Eu(HI) and OxTc are 1 0,5 (Cg = MO Huol-E-i, = 5-10-HuohE-i) at pH 7.2. 

In the mechanism of an interfacial catalysis, the structure and reactivity of the interfacial complex is very important, as well as those of the ligand itself. Recently, a powerful technique to measure the dynamic property of the interfacial complex was developed time resolved total reflection fluorometry. This technique was applied for the detection of the interfacial complex of Eu(lII), which was formed at the evanescent region of the interface when bathophenanthroline sulfate (bps) was added to the Eu(lII) with 2-thenoyl-trifuluoroacetone (Htta) extraction system [11]. The experimental observation of the double component luminescence decay profile showed the presence of dinuclear complex at the interface as illustrated in Scheme 5. The lifetime (31 /as) of the dinuclear complex was much shorter than the lifetime (98 /as) for an aqua-Eu(III) ion which has nine co-ordinating water molecules, because of a charge transfer deactivation. 

Two principal methods are employed for the determination of triplet lifetimes flash photolysis and triplet luminescence decay. Since both of these techniques have been discussed previously in some detail, only a short treatment of the application of these techniques to the determination of triplet lifetimes will be given in this section. 

A linear plot indicates that the luminescence decay is exponential. The slope of the line gives kt, and rt can be calculated as above. The lifetime obtained by measuring the decay of P-type delayed fluorescence is equal to one-half the lifetime of the triplet state (see Section 5.2). Since in fluid solution at room temperature phosphorescence is generally much weaker than delayed fluorescence, the measurement of delayed fluorescence decay offers a convenient method for determining the lifetime of triplets at room temperature. 

The value of k so determined could then be compared with the theoretical value of 4ttN aD. However, when viscosity is considerable and/or for short lifetimes, the transient effect in diffusion is not negligible and -30% of the transfer may be attributable to the transient phase. In such a case, the luminescence decay is not simply exponential (Sveshnikov, 1935). For a brief pulse excitation, a complicated decay ensures on the other hand, for so prolonged an excitation as to generate a steady state, the resultant decay curve in many cases is indistinguishable from an exponential (Yguerabide et ah, 1964). 

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

Figure 4. Luminescence decay profile of an oxygen indicator dye excited by a short flash of light, in (a) solution and (b) embedded into a gas-permeable film used to fabricate fiber-optic sensors for such species. The logarithmic scale of the Y-axis allows to compare the exponential emission decay in homogeneous solution and the strongly non-exponential profile of the photoexcited dye after immobilization in a polymer matrix.

Kosch U., Klimant I., Werner T., Wolfbeis O.S., Strategies To Design pH Optodes With Luminescence Decay Times In The Microsecond Time Regime, Anal. Chem. 1998 70 3892-3897. 

Transient UV-vis absorption spectra showed that theTi02/Ru(II) films yield prompt electron injection upon photolysis ( >108s 1) These same films displayed photoluminescence decays with parallel first- and second-order components, the first-order component having a rate constant of about lxl06s-1. These two sets of results provide further support for the existence of at least two populations of adsorbed Ru(II), one of which injects electrons rapidly and another which does not inject electrons and is thus capable of luminescing on a longer time scale. The second-order component of the luminescence decay is attributed to bimolecular triplet-triplet annihilation of surface-bound Ru(II). (Note that the second-order rate constants reported for luminescence decay have units of s-1 because they are actually values for k2(Asi))... 

Fig. 2 Comparison of the luminescence decays of QDs and organic dyes. InP and CdTe QDs decay multiexponentially with a mean lifetime (ii/e) of 17 and 6 ns, respectively. The organic dye Cy5 shows monoexponential decay with tf of 1.5 ns...

Wolfbeis OS, Klimant I, Werner T, Huber C, Kosch U, Krause C, Neurauter G, Diirkop A (1998) Set of luminescence decay time based chemical sensors for clinical applications. Sens Actuators B 51 17-24... 

Measurement of luminescence decay time represents another method of self-referencing. The method is widely used for the indicators that possess luminescence decay times in the microsecond and millisecond domain which can be interrogated... 

Mayr T, Moser C, Klimant I (2007) Luminescence decay time encoding of magnetic micro spheres for multiplexed analysis. Anal Chim Acta 597 137-144... 

Figure 1.24. (a) Excitation distribution along the channel axis of a zeolite L crystal consisting of 90 slabs under the condition of equal excitation probability at t = 0 calculated for front-back trapping. Fluorescence of the donors is assumed. (1) t = 5 ps, (2) t— 10 ps, (3) t — 50 ps, and (4) t = 100 ps after irradiation. (b) Luminescence decay of the donors in absence of traps (dotted), in the presence of traps at both ends (solid), and luminescence decay of the acceptors (dashed). 

The quantum recombination model. The photogeneration of an exciton and its luminescent decay occur within a quantum-confined silicon crystallite [Ca6]. 

The luminescence decays are somewhat nonexponential for the ionically bound metal complexes nonexponentiality is exacerbated by the presence of 02.We suggest that nonexponential decays reflect a persistent microheterogeneity around the complex. This was our first clear evidence of spectroscopically different binding sites. In this case, oxygen enhances heterogeneity detection by differentially quenching different sites. 

Our simplest continuous microheterogeneous model assumes that the luminophore exists in a distribution of spectroscopically different environmental sites. For a tractable, yet plausible, model each site is assumed to be quenched by normal Stem-Volmer quenching kinetics. For luminescence decays each individual component is assumed to give a single exponential decay with the following impulse response ... 

Fitting luminescence decay data to sums of exponentials, even with rather good statistics, can present very serious problems in data fitting and in uniqueness of the solutions. This difficulty can severely cloud interpretation of data from... 

Nonexponential luminescence decays are not well understood. However, regardless of the lack of understanding, it is a tradition to fit complex decays to sums of exponential functions either discrete or continuous (lifetime distributions). An important limitation of this approach is introduced by the nonorthogonal nature of the exponential function. The practice of fitting nonexponential luminescence decays to... 

In order to implement frequency domain based sensing systems capable of monitoring the temporal luminescence of sensors, in few seconds, data must be collected at multiple frequencies simultaneously. Single-frequency techniques have been used to make frequency domain measurements of luminescent decays. 14, 23 28) This approach is unsuitable for real-time applications since data must be acquired at several frequencies in order to precisely and accurately determine the temporal variables of luminescent systems. 1 Each frequency requires a separate measurement, which makes the single frequency approach too slow to monitor the evolution... 

Platinum and palladium porphyrins in silicon rubber resins are typical oxygen sensors and carriers, respectively. An analysis of the characteristics of these types of polymer films to sense oxygen is given in Ref. 34. For the sake of simplicity the luminescence decay of most phosphorescence sensors may be fitted to a double exponential function. The first component gives the excited state lifetime of the sensor phosphorescence while the second component, with a zero lifetime, yields the excitation backscatter seen by the detector. The excitation backscatter is usually about three orders of magnitude more intense in small optical fibers (100 than the sensor luminescence. The use of interference filters reduce the excitation substantially but does not eliminate it. The sine and cosine Fourier transforms of/(f) yield the following results ... 

The previous formula indicates that the radiative lifetime tq (and hence the radiative rate A) can be determined from luminescence decay-time measurements if the quantum efficiency rj is measured by an independent experiment. Methods devoted to the measurement of quantum efficiencies are given in Section 5.7. 

At this point it is important to mention that the experimental setup used for luminescence decay-time measurements is similar to that of Figure 1.8, although the light source must be pulsed (alternatively, a pulsed laser can be used) and the detector must be connected to a time-sensitive system, such as an oscilloscope, a multichannel analyzer, or a boxcar integrator (see Chapter 2). 

Most optical centers show luminescence decay times in the nanoseconds-milliseconds range. However, many other physical processes involved in optical spectroscopy are produced in the picoseconds-femtoseconds range, and mnch more complicated instrumentation becomes necessary. For instance, interband Inminescence in solids, which is of particular interest in semiconductors, can involve decay times in the range of picoseconds. Pulses generated from solid state lasers have already reached this femtosecond domain. 

At low temperature, the emission spectra of the complexes are well-structured and assigned to ligand-localised nn phosphorescences , responsible for multiexponential luminescence decays observed with mixed-ligand compounds [125,126],... 

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