Luminescence lifetime measurements

Ultraviolet absorption spectra were obtained from a Cary 118C Spectrophotometer. Luminescence measurements were obtained from a Perkin-Elmer Model MPF-3 Fluorescence Spectrophotometer equipped with Corrected Spectra, Phosphorescence and Front Surface Accessories. A Tektronix Model 510N Storage Oscilloscope was used for luminescence lifetime measurements. Fiber irradiation photolyses were carried out in a Rayonet Type RS Model RPR-208 Preparative Photochemical Reactor equipped with a MGR-100 Merry-go-Round assembly. 

A strong decrease in relaxivity (from 12.8mM-1s-1 to 2mM-1s-1) between pH 6 and 11 has been reported for a positively charged macrocyclic Gdm complex (Scheme 10), which was explained by the successive deprotonation of the coordinated water molecules.167 Luminescence lifetime measurements of a Yb111 analogue proved that the complex possesses three bound waters at pH 5.5. Above pH 11, a di-oxo-bridged dimer is formed that has no more bound water or OH groups. 

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence lifetimes measured in D20 and H20 can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced 170 chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equilibrium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

Structure determinations of salts containing [R c P5W30O110]12-, R = Y, Eu, and a redetermination of the sodium derivative, showed that the central cavity also contained a water molecule coordinated to the encrypted cation (Dickman et al., 1996 Kim et al., 1999). This observation, also noted for the Ca and structures, provides an explanation for the fact that the encrypted cation does not lie in the pseudo-equatorial mirror plane of the polytungstate framework. Luminescence lifetime measurements on the europium complex have led to ambiguous conclusions regarding the number of coordinated water molecules (Soderholm et al., 1995 Lis et al., 1996). As shown in Table 9 the 31P chemical shifts are significantly affected by the acidity of the solution. Remarkably, at intermediate acidities... 

For supramolecular assemblies, intramolecular processes may quench the emission of A to a degree which depends on the relative efficiency of the process when compared with emission. It is often useful to compare the photophysical and chemical behavior of the supramolecular species, e.g. A -L-B, with an appropriate model compound, for instance, AH, which contains the photochemically active component, A, in the absence of any units capable of interacting with A. For example, from luminescence lifetime measurements, the rate of electron transfer may be estimated by comparing the excited-state lifetime of the mononuclear model complex, tModei, with that of the supramolecular species, rsupra, by using the following equation ... 

The energy barriers associated with the tautomerism have been determined from quantum yields and luminescence lifetime measurements <84JST(l 14)329). Table 25 lists the activation energies and solvent viscosity activation energies associated with the tautomer in a variety of alcoholic solvents. The data suggest a correlation between the energy barrier associated with proton transfer and the viscosity of the solvent. 

Faulkner and coworkers (27., ), have also studied the interaction of Ru(bpy)32+ with zeolite X. Luminescence lifetime measurements and emission spectra were used to study electron transfer quenching of the electron donors N,N,N, N -tetramethyl-p-phenylenediamine and 10-phenyl-phenothiazine. Lifetime measurements show at least two modes of quenching for the interaction of Ru(bpy)32+ ions with these donors. Products of these electron transfer reactions were isolated and these experiments show that the zeolite can separate the products of light induced electron transfer. 

Luminescence Lifetime Measurements on Various Terbium Complexes (100). 

Ce (a2-P2Wi706i)2] ) appeared about ten years later." Recent investigations have confirmed the earlier structures and provide more detailed metrical information. The intermediate 1 1 complexes, e.g. [Ce(H20)x(SiWn039)] > which have been characterized in solution (electrochemistry, NMR spectroscopy, luminescence lifetime measurements, etc)," often associate into dimeric or polymeric assemblies upon crystallization (Figure 1). "... 

Luminescent lifetime measurements in H2O and D2O allow determination of the number of coordinated water molecules, q, according to the following equation. 

Luminescence lifetimes measured using pulsed laser excitation involve either direct detection of emission decays with time or a technique known as time-correlated single photon counting (TCSPC). The latter technique involves repeated measurement of the delay time between the excitation pulse and the arrival of an emitted photon packet above a given discrimination level the intensity-time decay profile accumulates over many millions of excitation pulses. The TCSPC experiment has the advantage that much better signal to noise can be obtained relative to the direct capture of the luminescence decay. 

The X-ray techniques have limited applications in actinide systems because the high concentration required for the measurements (ca. 2-3 M) does not allow study of elements for which only isotopes of high specific radioactivity are available. Beitz (1991) calculated the hydration number of some curium complexes using luminescence lifetime measurements and assuming a hydration number of nine of the free ion. Electrophoretic measurements using tracer concentrations (Lundqvist 1981, Lundqvist et al. 1981) have provided estimates of the hydrated radii (r ) for Eu, Am, Cm, Es, Fm and Md from Stokes law. The hydrated radii were used, in turn, to calculate hydration numbers, h, by dividing the volume of the hydrated sphere by the volume of a water molecule (si 30 A ). Since the volume occupied by the bare metal ion is small [2-5 A for Ln(III) and An(III)], it was neglected in comparison with the volume of the water molecules. The results are listed in table 3. 

Eu(III) is preferentially solvated by DMSO in water-DMSO mixtures over the whole range of solvent composition, as demonstrated by luminescence lifetime measurements (Tanaka et al. 1988). Similarly, solutions of europium chloride, nitrate and acetate in DMSO exhibit a considerable luminescence enhancement (Chrysochoos and Evers 1973a). Addition of large amounts of water has no effect at all, while adding very small quantities of DMSO to aqueous solutions of Eu(III) induces drastic changes in the transition probabilities. This clearly demonstrates the stronger coordination properties of DMSO as compared to water. 

A major advantage of luminescence lifetime measurements is their use for spectral assignment. Specifically, the assignment of the luminescence bands as fluorescence or phosphorescence is primarily determined via luminescence lifetime measurements. As a rule of thumb, lifetimes on the order of microseconds and longer (milliseconds, seconds) are normally indicative of phosphorescence, while fluorescence lifetimes are normally on the sub-microsecond level (nanoseconds, picoseconds etc). It should be noted, however, that fluorescence and phosphorescence lifetime values vary from one case to another, depending on the system under study as well as... 

Table 7.2 Selected standards for luminescence lifetime measurement ...

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