Steady state luminescence quenching

TABLE 8.3 Results of Steady-state Luminescence Quenching and Emission Lifetime of the [Cr(phen)3]3+-Guanosine Reaction... 

The violet emission of the radiation-induced center (COs) " is well known in steady-state luminescence spectra of calcite (Tarashchan 1978 Kasyanenko, Matveeva 1987). The problem is that Ce also has emission in the UV part of the spectrum. In time-resolved luminescence spectroscopy it is possible to differentiate between these two centers because of the longer decay time of the radiation-induced center. Its luminescence peaking at 405 nm becomes dominant after a delay time of 100-200 ns while emission of Ce is already quenched (Fig. 4.14f). 

Table IV compares this result to the steady-state luminescence of related species. For example, the addition of the doubly modified duplex to the ruthenium-modified duplex does not quench the luminescence from the ruthenium-modified duplex. The lack of luminescence demonstrates the absence of any adventitious quenchers in the rhodium(III) sample. Addition of an equimolar amount of rhodium-modified duplex to ruthenium-modified duplex also does not promote significant quenching of the ruthenium duplex, consistent with the quenching being substantially intramolecular at these concentrations. These studies complement the photocleavage experiments (Figure 6), from which we estimate less than 15% intermolecular interaction at these concentrations.

It is also useful to consider the luminescence from metallated oligonucleotides in the presence of noncovalent metallointercalator. Adding one equivalent of free [Ru(phen)2(dppz)]2+ to the ruthenium-modified duplex doubles the intensity in luminescence, consistent with independent intercalation by the two species. As described earlier, steady-state luminescence reaches saturation at approximately three times the luminescence of the ruthenium-modified duplex when two equivalents of [Ru(phen)2(dppz)]2+ have been added. It is not surprising, then, that addition of a stoichiometric amount of [Rh(phi)2(phen)]3+ to the ruthenium-modified duplex leads to substantial but not complete quenching of the ruthenium emission. Statistically, some duplexes will accommodate two rhodium(III) complexes, leaving a few ruthenium-modified duplexes unoccupied and therefore unquenched. Thus, complete quenching is observed only when the acceptor is covalently bound to the same duplex as the donor. 

In this section we deal with quantitative steady-state luminescence intensity determination in solution, at a fixed emission wavelength, using a commercial spectrofluorimeter with right-angle excitation (perpendicular geometry). These kind of measurements are particularly important in analyte detection, titrations, quenching and sensitization experiments, photoreaction and photoluminescence quantum yield determination, and whenever a luminescence signal is used to monitor a chemical process. 

Of the many strategies for optical detection, some of the most powerful are based on luminescence, which could provide exquisite sensitivity and selectivity. A useful type of luminescent probe is based on the quenching of a reporter molecule by the analyte [122-124], The presence of a quencher in the system results in more rapid depletion of the excited state population which is detected either as a concomitant decrease in steady-state luminescence intensity or as a shorter emission decay time. Change in either intensity or decay time could be used to quantitate the amount of analyte present [125]. 

The presence of Pr in apatite samples, up to 424.4 ppm in the blue apatite sample, was confirmed by induced-coupled plasma analysis (Table 1.3). The luminescence spectrum of apatite with a broad gate width of 9 ms is shown in Fig. 4.2a where the delay time of500 ns is used in order to quench the short-lived luminescence of Ce + and Eu +. The broad yellow band is connected with Mn " " luminescence, while the narrow lines at 485 and 579 nm are usually ascribed to Dy and the fines at 604 and 652 nm, to Sm +. Only those luminescence centers are detected by steady-state spectroscopy. Nevertheless, with a shorter gate width of 100 ps, when the relative contribution of the short lived centers is larger, the characteristic fines of Sm " at 652 nm and Dy + at 579 nm disappear while the fines at 485 and 607 nm remain (Fig. 4.2b). It is known that such luminescence is characteristic of Pr in apatite, which was proved by the study of synthetic apatite artificially activated by Pr (Gaft et al. 1997a Gaft... 

The spectral-kinetic parameters of the narrow band at 375 nm enable its confident identification as Eu + luminescence, which is confirmed by emission of synthetic BaS04 artificially activated by Eu (Fig. 5.15a). Such emission was also detected and interpreted by steady-state spectroscopy (Tarashchan 1978). It is interesting to note that very often such a band is absent in natural barite and appears only after heating in air at 600-700 °C (Fig. 4.31b). Such a transformation is reversible, at least partly. Under X-ray excitation the intensity of the UV band diminishes, and a new blue-green emission appears (Fig. 5.16). This shows some kind of transformation, which takes place in the barite lattice under these conditions. Several possibihties exist. It is possible that in barite the luminescence is quenched by the components with high-energy phonons. The water and organic matter may represent the latter. They are removed after... 

Quenching mechanism. Steady state measurements of the luminescence quantum yield of Ru(byp)3 intercalated in clay films brings about more detailed information with respect to the possible role of electron transfer in luminescence quenching. The quantum yield is dependent upon the amount of co-adsorbed water and s strongly depleted by transition metal impurities, such as Fe5 or Cr in the lattice (28). 

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

Figure 3 Steady-state quenching of A-[Ru(phen)2dppz]2+ luminescence by metal complexes in the presence of CT-DNA. (Data compiled from Ref. 27.) ( ) A-[Rh(phi)2phen]3+ and ( ) [Ru(NH3)6]3+ [Ru(phen)2dppz]2 t = 10 p.M [bp] = 500 p.M buffer is 5 mM Tris-HCy50 mM NaCl (pH 7.2).

Figure 5 Quenching of A-[Ru(phen)2dppz]2+ luminescence by viologens in the presence of poly(dA-dT)-poly(dA-dT)—comparison of steady-state and lifetime quenching. ( ) Short lifetime, ( ) long lifetime, ( ) steady-state intensity, and (o) intensity calculated from lifetimes (Sot). Upper panel MV2+ and lower panel Me2DAP2+. [Ru(phen)2dppz]2+ = 20 pM bp = 500 pM buffer is 5 mM sodium phosphate (pH 7).

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...

The validity of Eqn V-3 was checked by measuring the transient photoconductivity, dark current, and steady-state field-induced luminescence quenching at T = 77 K on the same PPV sample (tensile drawn to l/lo = 2) [166]. Because the excited state lifetime in PPV is a few hundred picoseconds, and the transient photoconductivity also spans a few hundred picoseconds, the experiment was carried out at times particularly sensitive to the photogeneration process. 

Identification of the photoactive state is a prerequisite to the calculation of the photochemical rates by comparison of the photochemical yields and luminescence lifetimes. Although measurements of steady state quenching and the dynamics of intermediate formation have been used to infer the reactive states in several complexes, there is still much to be done in this area. The effect of thermally accessible higher energy states can be minimized by reducing the temperature. 

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