Emission quantum yield values

Table 28.2 Molecular structure of some organic-inorganic hybrid precursors and emission quantum yield values of the corresponding materials.

The quantum yield (Q) represents the ratio between the number of photons absorbed and photons emitted as fluorescence. It is a measure of brightness of the fluorophore and represents the efficiency of the emission process. The determination of absolute quantum yield for a fluorophore is experimentally difficult. Therefore, usually relative quantum yield values are determined. To measure the relative quantum yield of a fluorophore, the sample is compared to a standard fluorophore with an established quantum yield that does not show variations in the excitation wavelength [5, 6]. 

To assess the effect of ligand structure, several measurements are necessary the emission energy, the lifetime, and the emission quantum yield. Furthermore, it is essential that details on precisely how the data have been obtained be given. Demas and Crosby220) have pointed out the necessity for this reporting. In the absence of specific details, discrepancies between reported values are difficult to assess. From the data in the following sections it will be seen that there is a wide variation in the quantities reported. 

In the mixed complexes, [Ru(i-bq)n(bpy)3 n]2+, i-bq is not involved in the emission252. The absorption spectra show shoulders where [Ru(bpy)3]2+ has its maximum and e at this point is slightly less than that of this compound. These i-bq mixed complexes do show room temperature-fluid emission, but unfortunately quantum yield values are not available. 

Since the emission quantum yield accounts for no more than 1.8% of the total depopulation pathways of Sp, the value of tF is mainly dictated by the nonradiative rate constant. By using the f values of Gandini and Kutschke (88), obtained at 265 and... 

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... 

Moreover, it is possible to determine the emission efficiency r] = Arad/ nrad of the emitting level, where the total decay rate is Atot = 1/r = Arad + nrad. and where the non-radiative component A rad depends on the vibronic couphng between the Eu ion and its chemical environment and r is the lifetime of the Do emitting level. It may be determined from lifetime measurements and the experimentally determined radiative component Arad- The hydrated Eu + S-diketonates tend to present lower values of emission quantum efficiencies, and emission quantum yields, in agreement with the well known effect of luminescence quenching due to the vibrational modes of the water molecule. 

For the sake of comparison with t], values of the experimental emission quantum yields, exp (see Section IV.B.2.d), are also given. 

The influence of ligand substituents on the nature and decay properties of excited bipy and phen complexes was also studied (171,172). Ru(diphenylbipy)3 and Ru(dlphenylphen)3 have optical properties differing somewhat from those of the parent unsubstituted complexes. The MLCT absorption bands of the substituted systems are more intense ( -g values are about twice as large as those of the parent complexes), the emission quantum yields are considerably larger and the emission lifetimes at 77 K are shorter (Table 7). While the radiative rate constant is larger for the substituted complexes, the nonradiative decay constant is smaller. 

The lowest singlet excited state (iT- Tr ) of, for example, ethyl benzene, shows absorption and emission maxima at, respectively (1), n-260 and 280 nm. The temperature dependence of the emission in dilute polar organic solution has been investigated (lb), and it was found that there is a temperature dependent non-radiative rate component that follows an Arrhenius law Ae" / T with an activation energy AE v2300 cm-1 and AMO S-l, The ratio of the emission quantum yield ( e) and lifetime(T) is temperature independent, consistent with a temperature independent radiative rate constant k, and limiting low temperature values of and t, achieved by v250K, are 0.12 and 21 ns, respectively, in dichloroethane solution. 

Nonradiative Deactivation Involving a Second Excited State. A somewhat different situation is presented by the pressure effects reported for the MLCT emissions from the ruthenium(Il) complex Ru(bpy)f+. At ambient temperature, in a fluid solution this species shows little unimolecular photochemistry and relatively small emission quantum yields (ff>r < 0.1) [32]. Initial pressure studies on the luminescence of this ion in 18°C aqueous solution detected little sensitivity to pressure [60], as might be expected for a weakly coupled nonradiative mechanism owing to the low compressibility of water. However, detailed studies by Fetterolf and Offen [32,61] painted a more complex picture. These workers probed the temperature dependence of AF and confirmed the small negative value at low temperature but also demonstrated a remarkable temperature dependence for this parameter. 

Radiative rates For most transition metal complexes in solution, emission quantum yields are small, thus radiative decay is only a minor component of the overall deactivation mechanism. Limited studies show pressure effects on kr to be small, a few percent over the hydrostatic pressure ranges of principal interest here and these effects can largely be attributed to solvent perturbations [15, 16]. The relatively small values of kr for most lurainactive metal complexes in fluid solutions, suggest that such modest changes will not have much impact on interpretations of pressure effects on lifetimes or quantum yields. 

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