Long-decay luminescent probes

Long-decay luminescent dyes and probes that are effectively quenched by molecular oxygen can be used for its quantitation. Examples of such probes include ruthe-nium(II)-rm(diphenyl phenanthroline) and phosphorescent platinum(II) porphyrins. Their long emission lifetimes facilitate quantitation by lifetime or intensity measurements. Other chemical specie, such as heavy-metal ions and heterocyclic compounds, can be quantified by luminescence quenching, according to Eq. 3. 

Probes based on long-decay luminescent complexes of lanthanides Sm " ) and transition metals... 

Let us now consider in general terms the effects of the confinement of a probe and quencher in a space that is finite in at least one dimension. Let us suppose that a relatively long lived luminescence probe has been excited with a brief pulse of light in a fluid solution containing an effective quencher. Neglecting transient effects, as is normally possible, the fluorescence decay will be exponential, and if we choose to represent it as in Fig. 3, i.e., on a logarithmic scale and with the intensities corrected for the natural decay by multiplication with a factor of exp(A o/), then a constant intensity level is obtained without quenchers, and a linear decay is obtained when quenchers are present. 

Time resolved fluorescence measurements have been used for decades because they are such a powerful tool to investigate fluorophore-metal composites. Due to insufficient time resolution, mostly long lived luminescence like that from triplet states has been investigated. When fluorophOTes are attached to metal nanostructures, fluorescence decay times are in the sub nanosecond time range. To measure those dect times accurately, techniques such as time correlated single photon counting, frequency domain fluorescence measurements, streak camera measuremets, and femtosecond pump SHG-probe have been used. 

A more sensitive way of probing the effect of the internal electric field is to study the luminescence decay time. Indeed for polar QDs, the reduction in spatial overlap of the electron and hole wavefunctions induced by the internal electric field has a dramatic effect on the radiative recombination rate [4, 44]. For three-dimensionally confined wavefunctions with negligible excitonic effect, the radiative decay rate is proportional to the electron-hole wavefunction overlap. This leads to very long decay times ranging from 5 ns to... 

In iron bearing clays the time law of the luminescence decay of Ru(byp)3 + is multiexponential (29). In the long time limit (>1 /is) the decay approaches an exponential law, but at shorter time ( 300 ns), a more complex decay has been observed. The decay mechanism has been explained by a simple localized model based on the assumption that on each particle the quenching process can be described as occurring in an ensemble of small independent subsystems. Each subsystem is composed of an excited probe system (with a very reduced mobility) and of the nearest lattice sites of the solid which may be occupied by the quencher ions. For the sake of simplicity, it has been assumed that the adsorbed probe molecules occupy the sites of a superlattice which matches the lattice containing randomly distributed quenchers (Fe, for instance)... 

The minimum prerequisite for generation of upconversion luminescence by any material is the presence of at least two metastable excited states. In order for upconversion to be efficient, these states must have lifetimes sufficiently long for ions to participate in either luminescence or other photophysical processes with reasonably high probabilities, as opposed to relaxing through nonradiative multiphonon pathways. The observed decay of an excited state in the simplest case scenario, as probed for example by monitoring its luminescence intensity I, behaves as an exponential ... 

Rotational relaxation times of polymers in solution are generally such that probes with nanosecond fluorescence decay times suffice for measurement of p. For faster relaxation, picosecond time resolution is required, and this may have applications in the polymer field. One should therefore pay tribute to the extraordinarily elegant instrumentation that is now available for picosecond rotational relaxation measurements on smaller molecules (85-87). Much more viscous solutions can be studied if long-lived phosphorescence is used as the luminescence monitor, and such studies on a millisecond time scale have recently been carried out on poly(vinyl alcohol), poly(ethylmethacrylate), poly-(styrene), poly(butylmethacrylate), and poly(methylmethacrylate) using benzophenone and anthrone as probes (88). 

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