Time-resolved emission, luminescent

Figure 1.13 The time-resolved emission spectrum of anhydrite (CaS04) at two different delay times. The scale on the emission intensity axis of (b) has been enlarged by a factor of 1000 in order to clearly observe the remaining luminescence of the Eu and ions. Excitation wavelength at 266 nm (reproduced with permission from Gaft et al., 2001).

Among other examples, time-resolved luminescence has recently been applied to the detection of different trace elements (i.e., elements in very low concentrations) in minerals. Figure 1.13 shows two time-resolved emission spectra of anhydrite (CaS04). The emission spectrum just after the excitation pulse (delay 0 ms) shows an emission band peaking at 385 nm, characteristic of Eu + ions. When the emission spectmm is taken 4 ms after the pulse, the Eu + luminescence has completely disappeared, as this luminescence has a lifetime of about 10/rs. This allows us to observe the weak emission signals of the Eu + and Sm + ions present in this mineral, which in short time intervals are masked by the En + Inminescence. The trivalent ions have larger lifetimes and their luminescence still remains in the ms delay range. 

Most of the time-resolved emission spectroscopy setups are home made in the sense that they are built from individual devices (laser, detection system,. ..) hence they are not of a plug and press type, so that their exact characteristics may vary from one installation to the other. Some of these differences have no impact on the overall capabilities of the system but some have a drastic influence on the way the collected data are processed and analysed. This aspect will be detailed in the next section, while this section deals with a general description of the apparatus. The most basic type of apparatus will be described, with no reference to sophisticated techniques such as Time Correlated Single Photon Counting or Circularly Polarized Luminescence devices. 

Time-resolved emission spectroscopy is gaining importance in the study of various chemical aspects of luminescent lanthanide and actinide ions in solution. Here, the author describes the theoretical background of this analytical technique and discusses potential applications. Changes in the solution composition and/or in the metal-ion inner coordination sphere induce modifications of the spectroscopic properties of the luminescent species. Both time-resolved spectra and luminescence decays convey useful information. Several models, which are commonly used to extract physico-chemical information from the spectroscopic data, are presented and critically compared. Applications of time-resolved emission spectroscopy are numerous and range from the characterization of the... 

Table I shows examples of the steady-state and time-resolved emission characteristics of [Ru(phen)2(dppz)]2+ upon binding to various DNAs. The time-resolved luminescence of DNA-bound Ru(II) is characterized by a biexponential decay, consistent with the presence of at least two binding modes for the complex (47, 48). Previous photophysical studies conducted with tris(phenanthroline)ruthenium(II) also showed biexponential decays in emission and led to the proposal of two non-covalent binding modes for the complex (i) a surface-bound mode in which the ancillary ligands of the metal complex rest against the minor groove of DNA and (ii) an intercalative stacking mode in which one of the ligands inserts partially between adjacent base pairs in the double helix (36, 37). In contrast, quenching studies using both cationic quenchers such as [Ru(NH3)6]3+ and anionic quenchers such as [Fe(CN)6]4 have indicated that for the dppz complex both binding modes...

Several hetero-bischelated complexes of Ir(III) with 1,10-phenanthroline and substituted 1,10-phenanthroline have also been reported to have non-exponential luminescence decay curves (19). Although the individual emission spectra of the non-equilibrated levels of these complexes are again too close to resolve by conventional emission spectroscopy, partial resolution has been accomplished by time-resolved emission spectroscopy via box-car averaging techniques (20). Complete resolution has been accomplished by computer analysis of luminescence decay curves as a function of emission wavelength (20). In these complexes, the luminescent levels appear to arise from both ligand-localized ( tttt ) states and charge-transfer ( ) states. 

To obtain more information about the kinetics of luminescence, the time-resolved emission spectra of ScVO4 1.0%Bi were recorded on excitation into the absorption of V04, e.g., at 265 nm at room temperature. The collected delay time ranges from 0 to 50 ps as plotted in Fig. 14.16. It shows there are at least... 

In a series of papers, Ford and co-workers described the photophysical and photochemical properties of these luminescent clusters in detail [46-58]. In addition to complexes 4a and 4b, time-resolved emission spectra of the tetranu-clear copper(I) iodide clusters [Qi4l4(L)4] with a series of substituted pyridines [L = 4-tert-butylpyridine (4c), 4-benzylpyridine (4d), pyridine-dj (4e), 4-phe-nylpyridine (4f), 3-chloropyridine (4g), piperidine (4h), P"Bu3 (4i)] have also been studied [49]. The photophysical data are summarized in Table 1. In general, in toluene solution at 294 K, the complexes revealed a low-energy emission at 678-698 nm and a weaker, higher energy emission at 473-537 nm. The emission spectrum of 4a in toluene at 294 K is shown in Fig. 2... 

Fig. 10. Time resolved emission spectra of luminescence from 22 k CdS clusters in a frozen organic glass at 10 K. Short delay refers to the period 0 1.S 10 s, long delay to the period (17-34)X 10. There is no resolvable vibronic structure at = 1 A resolution under the x- ent conditions [12]...

Superimposed on these microcompositional effects are those of intramolecular concentration effects revealed in molecular weight effects upon time-resolved emission data [73,90] and the influence of intramolecular chromophore concentration upon the luminescence characteristics of copolymers. The self-consistency of rate-parameter data derived from time-resolved measurements on series of copolymers is good evidence for the assignment (in the copolymers studied to date) of observed decay times as averages representative of chromophore distributions centred on species of the type M, M and D in kinetic scheme (3). 

Figures 8.4 and 8.5 show the luminescence spectrum and the time-resolved emission decay of MEH-PPV/ Cgo composites compared with MEH-PPV alone. The strong photoluminescence of MEH-PPV is quenched by a factor in excess of 10, and the luminescence decay time is reduced from 7o 550 picoseconds to Trad 60 picoseconds (the instrumental resolution) indicating the existence of a rapid quenching process [53,54, 63]. An estimate of the transfer rate, l/td, is given by decay rate of the photoluminescence in the MEH-PPV/Cgo composite (charge transfer will cut off the radiative decay).

Different lanthanide metals also produce different emission spectrums and different intensities of luminescence at their emission maximums. Therefore, the relative sensitivity of time-resolved fluorescence also is dependent on the particular lanthanide element complexed in the chelate. The most popular metals along with the order of brightness for lanthanide chelate fluorescence are europium(III) > terbium(III) > samarium(III) > dysprosium(III). For instance, Huhtinen et al. (2005) found that lanthanide chelate nanoparticles used in the detection of human prostate antigen produced relative signals for detection using europium, terbium, samarium, and dysprosium of approximately 1.0 0.67 0.16 0.01, respectively. The emission... 

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