Intrinsic radiative rate

This enhancement of intersystem crossing by combining heavy atom and paramagnetic effects explains the relative insensitivity of the Gd phosphorescence lifetime (Table IV) to any additional heavy atom effect (as in the chelate with iodo-BTFA), or to deuteration of solvent or ligand which, by inhibiting nonradiative deactivation, usually increases the lifetime of organic phosphorescence. This insensitivity of the lifetime of the Gd chelate permits us to assign the value of ca. 3 X sec." as the intrinsic radiative rate for the triplet state for Gd BTFA chelates, and a similar value should apply for the Eu compounds. 

Dispersion of the radiative rate constant by local variations of the refractive index at the solid/gas interface. This could explain the tailing of the decay curves even at very low loadings, with lifetime components that are two to three times as long as the intrinsic radiative lifetimes in solution/85 This could also explain the disappearance... 

In about 2000, my laboratory started to study the interactions of fluorophores with metallic nanoparticles, both solution-based and surface-immobilized. Our findings agreed with other workers whom had observed increases in fluorescence emission coupled with a decrease in the fluorophores radiative lifetime. Subsequently, we applied classical far-field fluorescence descriptions to these experimental observations, which ultimately suggested a modification in the fluorophores s intrinsic radiative decay rate, a rate thought to be mostly unchanged and only weakly dependent on external environmental factors. This simple description, coupled with what seemed like a limitless amount of applications led to a paper published by our laboratory in 2001 entitled Metal-Enhanced Fluorescence , or MEF, a term now widely used today almost a decade later. 

Gryczynski, L, et al. (2002) The CFS Engineers the Intrinsic Radiative Decay Rate of Low Quantum Yield Fluorophores. J Fluoresc 12 11-13. 

There is no sensitivity to individual molecular trajectories or dipole orientations, but one ends up directly with global figures to characterize the emitted fluorescence. Besides, distinguishing between the contributicms of the radiative rate and the collection efficiency remains a challenge, mainly because of the intrinsic difficulty to reliably measure collection efficiency. Lastly, the fluorescence enhancement factors are spectrally averaged within the fluorescence bandpass detection window. However, further investigations can provide some additional knowledge on these last two points, as we will discuss hereafter. 

Figure 3. Energy level diagram and restrictions on kinetic parameters for the series of chromo-phores C, C2, C3. k C ) represents the intrinsic decay rates for C , regardless the radiative or nonradiative nature of the processes k and k2 are energy transfer rate constants.

In this approach, the excited-state lifetime of molecules is measured by following the decay of their fluorescence. The electron injection rate ( j) is calculated from the measured fluorescence decay rate (kobs) and the intrinsic radiative (kf) and non-radiative (A nr) excited decay rates through the relationship... 

The radiative and nonradiative (fc ) rate constants estimated using the emission lifetimes (Tobs) and the intrinsic emission quantum yields ( Ln) are summarized in Table 6.1. The radiative rate constant for Eu(hfa)3(fBu-xantpo) in acetone was estimated to be 5.4 x 10 s This value is much similar to that for Eu(hfa)3(fBu-xantpo) in acetone-t/e (5.5 x 10 s ). The nonradiative rate constant for Eu(hfa)3(fBu-xantpo) in acetone-t/s (2.7 x 10 s ) is smaller than that for Eu(hfa)3(fBu-xantpo) in acetone (3.0 x 10 s ). The relatively smaller kai for Eu(hfa)3(fBu-xantpo) in acetone-t/s is attributed to the suppression of vibrational relaxation surroundings of the Eu(Ill) complex. The nonradiative transitions of lanthanide complexes are affected by the high-vibrational frequency of C-H and O-H bonds of solvent. The author consider that introduction of deuterated solvent is effective for enhancement of emission quantum yield of octa-coordinated Eu(lll) complexes. 

The radiative k and nonradiative rate constants, the emission lifetimes (Tobs), and the intrinsic emission quantum yields (< n) of Eu(hfa)2(xantpo)2 and Eu(hfa)3(tBu-xantpo) are summarized in Table 6.2. The radiative rate constants of Eu(hfa)3(fBu-xantpo) k = 3.6-S.6 x 10 s ) were larger than that for Eu(hfa)2(xantpo)2 (fer = 3.0. 9 x 10 s ). Generally, the radiative rate constants of lanthanide complexes are directly linked to their geometrical structures. The symmetrical point groups of Eu(hfa)2(xantpo)2 and Eu(hfa)3(fBu-xantpo) are D4d and D2, respectively [7], The larger radiative rate constants for... 

There is a fluorophore-metal interaction, which results in an increase in the intrinsic radiative decay rate of the fluorophore. 

Certain chromophore systems are intrinsically predisposed for ultrafast single molecule microscopy. Among these, emitters coupled to metal surfaces stand out as exceptionally well-suited subjects. Numerous observations of substantial radiative rate enhancement at the surface or in the vicinity of the surface of a metal were reported. Radiative rate enhancements as large as 10 have been predicted for molecular fluorophores and for semiconductor quantum dots coupled to optimized nanoantennae.Such accelerated emission rates put these systems well within the reach of the emerging femtosecond microscopy techniques. As a result, we decided to apply the Kerr-gated microscope to study of fluorescence dynamics of individual core-shell quantum dots in contact with smooth and nanostructured metal surfaces. 

As mentioned earlier, certain single chromophore systems are intrinsically predisposed to ultrafast microscopy. Enhanced emission rates observed for molecules and quantum dots in the vicinity of a metal surface, as well as the theoretical predictions of the radiative rate acceleration factors in excess of 10 for optimized plasmonic nanoantennae, make these systems ideally suited to femtosecond imaging. " With the emission lifetime compressed (not quenched) from nanoseconds to picoseconds, the probabihty of emitting a photon within a 100 fs window increases dramatically and fadhtates the measurements. 

In the case of lanthanides, foUowing direct excitation of the metal ion, the efficiency of emission is called the intrinsic emission efficiency which is directly related to the overall rate at which the emissive state is depopulated through radiative/ and non-radiative NR pathways, and the radiative rate constant, k/, or their corresponding... 

From the practical point of view, the radiative decay rate kr may be assumed to be independent of the external parameters surrounding the excited sensor molecule. Its value is determined by the intrinsic inability of the molecule to remain in the excited state. The radiative decay rate kr is a function of the unperturbed electronic configuration of the molecule. In summary, for a given luminescent molecule, its unperturbed fluorescent or phosphorescent decay rate (or lifetime) may be regarded to be only a function of the nature of the molecule. 

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