Lifetimes and quantum yields

A tetrameric structure with the Zn40 core can also be formed with the 7-azaindolate ligand, [Zn40(C7H5N2)6], and has been structurally characterized. The tetramer displays intense photoluminescence at 448 nm in the solid state and 425 nm in acetonitrile with a lifetime and quantum yield of 0.1 ps and 0.17 ps respectively.280... 

The measurement of fluorescent decay dynamics, i.e., fluorescence lifetime measurements, promise to overcome several of the challenges discussed above. Most importantly, lifetime and quantum yield are directly related through (11),... 

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

TABLE 4.3 Luminescence Lifetimes and Quantum Yields (qy) of Some Selected Compounds... 

In conclusion, lifetimes and quantum yields are characteristics of major importance. Obviously, the larger the fluorescence quantum yield, the easier it is to observe a fluorescent compound, especially a fluorescent probe. It should be emphasized that, in the condensed phase, many parameters can affect the quantum yields and lifetimes temperature, pH, polarity, viscosity, hydrogen bonding, presence of quenchers, etc. Attention should be paid to possible erroneous interpretation arising from the simultaneous effects of several factors (for instance, changes in viscosity due to a variation in temperature). 

The temperature dependence of luminescent metal complexes can be controlled by molecular design that affects the energy gap between the emitting state and the deactivating d-d or by altering the preexponential factor for thermal deactivation. The sometimes large temperature dependencies of lifetime and quantum yields for metal complexes also suggest their use as temperature sensors. 

It has been suggested that this increase in yield is due to the heteroatom which reduces the energy difference between the singlet and triplet states of the common, biphenyl nucleus. - - The lifetime and quantum yields of triplet state formation have been measured at 25° and interpreted in the context of spin-orbital coupling. -... 

What are the effects of a rigid polymer matrix and of oxygen on the fluorescence lifetime and quantum yield ... 

Interesting examples are found in substituted anthracenes. The lifetimes and quantum yields of fluorescence of substituted anthracenes show different dependencies on temperature. The position of the substituent is more important than its nature. For 9- and 9,10-substituted anthracenes, fluorescence quantum yields increase steeply with decrease of temperature, while side-substituted derivatives have low yield and small temperature dependence. The variation is of the form... 

We have seen that in the statistical limit simultaneous nonradiative and radiative processes proceed independently. The lifetimes and quantum yields characteristic of these several processes are then defined in terms of the relevant densities of states and matrix coupling elements connecting the initial state with the appropriate con tin ua and quasicontinua. 

In Table 1 we have collected data on measured lifetimes and quantum yields in various solvents for [Ru(bpy)3]2+. Also included is a calculated quantum yield from the observed lifetime, assuming a constant value for the radiative lifetime (14 ps174,192)). Demas and Crosby188 have argued that the intersystem crossing efficiency to populate the emitting MLCT triplet is unity and should be independent of solvent Bolletta et al.193)... 

Young et al.253 have observed that [Ru(trpy)2]2+ has a much more open structure than does [Ru(bpy)3]2+. Thus, solvents can more readily approach the d orbitals in the former. Van Houten and Watts174 made this proposition with regard to [Ru(bpy)3]2+ photochemistry and suggested that lowering the solvent polarity (and metal interaction) should enhance the energy of the CT states. Caspar and Meyer196 have observed a solvent dependence for both lifetime and quantum yield in [Ru(bpy)3]2+ however, it remains to be seen if [Ru(trpy)2]2+ luminescence can be enhanced by proper solvent/ matrix choice. 

Recently, the effect of the donor-acceptor separation has been studied.76 Both the fluorescence lifetime and quantum yield were found to decrease as the distance between the two porphyrins—Cmeso-Cmeso (cd) and CCmeso-CCmeso (ab)—decreases (Fig. 24). As the two rings get closer to each other, they interact more strongly, and hence nonradiative deactivation becomes more pronounced.75,76... 

---