Radiative decay rate efficiency

When more conjugated diimine or pyridine ligands are used, the excited states of rhenium(I) carbonyl complexes can have substantial IL character. While the MLCT emission is often broad, with a lifetime in the submicrosecond to microsecond timescale, the IL emission usually has noticeable structural features, even in fluid solutions at ambient temperature. The emission lifetime is usually very long. A simple and widely applicable approach is to evaluate the ratio of the emission quantum yield and the emission lifetime (the product of the intersystem crossing efficiency and radiative decay-rate constant). Experimental values of... 

For efficient lasing on intraband transitions one need not only to depress nomadiative relaxation but also to enhance the radiative decay rate of the transitions. 

Raithby and co-workers further compared the photophysical properties of several platinum(II) polyynes P25, P27, and P29 with their organic copolyynes.59 Since the nonradiative decay rate for the triplet emission, (Lm)p, is equal or larger than the corresponding radiative decay rate, (Ljjp, the PL quantum efficiencies of the platinum polyynes are reduced from those for the organic polymers. Optical data reveal that the anchoring of octyl side chains on the fluorenyl spacer reduces interchain interaction in the polyynes, while a fluorenonyl spacer affords a donor-acceptor motif along the rodlike backbone. 

The FRET efficiency (E) is the quantum yield of the energy transfer transition or the fraction of energy transfer event occurring per donor excitation event where ET is the rate of energy transfer,. the radiative decay rate, and the k. are the rate constants of any other de-excitation pathway. 

Excited-State Kinetics. A principal emphasis of this chapter is concerned with how the application of hydrostatic pressures influences rates of ES processes such as those illustrated in Figure 9. In this simple model, it is assumed that electronic excitation leads efficiently to the formation of a single, bound state, which can decay by unimolecular radiative decay (rate constant kr), nonradiative decay (fc ), or chemical reaction to give products (kp). Alternatively, there may be bimolecular quenching of the ES dependent on the nature and concentration of some quencher Q (fcq [Q]). Each of these processes may be pressure dependent. 

The excited-state lifetime of the molecule in absence of any radiationless deeay processes is the natural fluorescence lifetime", r . The natural lifetime is a constant for a given molecule and given refraction index of the solvent. Because the absorbed energy can also be dissipated by internal conversion, the effective fluorescence lifetime, is shorter than the natural lifetime, The fluorescence quantum efficiency", i.e. the ratio of the number of emitted photons to absorbed photons, reflects the ratio of the radiative decay rate to the total decay rate. Most dyes of high quantum efficiency, such as laser dyes and fluorescence markers for biological samples, have natural fluorescence decay times of the order of 1 to 10 ns. There are a few exceptions, such as pyrene or coronene, with lifetimes of 400 ns and 200 ns, and rare-earth chelates with lifetimes in the ps range. 

In this case the radiative decay rate constant is obtained from the ratio of the emission yield and lifetime. However, even in this simple case the observed nonradiative decay is the sum of the rate constants for nonradiative relaxation of the singlet state and intersystem crossing. If emission is observed exclusively from a state with a spin multiplicity differing from the ground state (i.e., Ti in Figure 1), the observed emission quantum yield will reflect the efficiency for populating the emissive excited state (Equation (4)). 

The time-resolved fluorescence spectrum of a pure Lissamine solution (0.37 /iM) is shown by the uppor curve in figure 8. A straight line is observed on the s ni-logarithmic plot revealing a fluorescence lifetime of 1.54 ns. As lissamine molecules have a quantum efficiency of 33 % in aqueous soluticm, the pure radiative decay rate is... 

By this approach, in Ref. [57] it was found that even for a very small nanoparticle, where excitations are of molecular character, quenching efficiency is as high as when plasmon resonances are present. We report in Fig. 5.12 the non-radiative decay rate of a dye molecule (PDl) as a function of its distance from a metal cluster composed of 20 Au atoms. For such a small cluster, ab initio results predict a molecular-like excitation spectrum, while applying a continuum model for the molecule is the same as assuming that the Au surface plasmon excitations of larger metal nanoparticle are conserved also for this size. 

To test the above ideas, Weitz etal.(i2) performed experiments on the fluorescence decay from a thin layer of europium(III) thenoyltrifluoracetonate (ETA) deposited on a glass slide covered with Ag particles approximately 200 A in diameter. The fluorescence decay rate was found to increase by three orders of magnitude in comparison with that of ETA in solid form. In addition to the large increase in decay rate, there was also evidence for an increase in overall fluorescence quantum efficiency. It is not possible from Eq. (8.11) to say anything about the manner in which is partitioned between radiative and nonradiative processes, y should be written in terms of a radiative part yr and a nonradiative part ynr y = yr + y r. Since the radiative rate for dipole emission is given by... 

Here, is the rate constant for radiative decay (fluorescence), while k r is the combined rate constant for aU non-radiative decay processes, is virtually constant and is an inherent property of the material in question, and for this material is significantly greater than k r, given the high fluorescence efficiency. When a fluorescence quencher, such as TNT, is introduced, km increases because an additional efficient non-radiative pathway now exists. This, via Eq. (4), makes r smaller. 

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