Radiative decay rate constant

The photophysical properties of a xanthene dye have been extensively studied. In order to explain a relationship between the non-radiative decay rate constant (k r) of the dye and a solvent polarity parameter ( t(30)), Quitevis et al. proposed a two-state model [36,37]. According to the model, kot is given by... 

The fluorescence lifetimes (t determined at 580 nm) and quantum yields () of SRB were determined in water-dioxane mixtures and a series of alcohols at 25°C. The km value varied with the medium in the range of (4.1-0.7) x 10 s whereas the radiative decay rate constant (kr) was rather insensitive to the medium properties (2.8-1.7) X 10 s" . The relationship between In km and t(30) fall on a straight line and the slope value of the plot was 0.074 0.01. Therefore, the photophysical properties of SRB and Equation (20) are applicable to probing the polarity at a water/oil interface. 

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... 

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. 

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 other spin sublevels are non-active in the radiative decay of the Ti state of free-base porphin in accord with the ODMR measurements [6, 29]. The spin sublevel is completely dark by symmetry selection rules for spin-orbit coupling and electric dipole operators. For the out-of-plane spin sublevel, T, the calculated radiative decay rate constant is equal to 0.8 x 10 s , thus it is practically negligible. 

In Table 1.2 the experimental rate constants measured for different spin-sublevels by the fluorescent-detected ODMR techniques [6] are also presented. They are much higher than the calculated radiative decay rate constants. This means that the experimental rate constants, presented in the third row of Table 1.2, refer to the nonra-diative decay. A good quantitative agreement between our calculated value and the radiative rate constant of the spin-sublevel, extracted from kinetic analysis of the ODMR signals [6], indicates, without doubt, the correct interpretation of the observed decay. It is also important that an identical interpretation of the most active spin-sublevel is obtained in our theory and in the ODMR experiment [6], where the z-axis coincides with the N-H...H-N bond direction. 

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