Quantum yields absolute

Lee, J., and Seliger, H. H. (1965). Absolute spectral sensitivity of phototubes and the application to the measurement of the absolute quantum yields of chemiluminescence and bioluminescence. Photochem. Photobiol. 4 1015-1048. 

Quantum Yields of Fluorescence. Table III lists the relative quantum yields of fluorescence of 24 3-substituted 2(lH)-pyridones. Pyridone I has the highest yield measured, which is set at 1.00. An attempt was made to measure the absolute quantum yield of I relative to rhodamine B using ferrloxalate actinometry. A Vpj value of 0.98 0.02 was obtained. However, the determination of absolute ... 

In practice it is much simpler to determine the relative quantum yield of fluorescence than the absolute quantum yield (see Table 2.1). This is done by comparing the fluorescence intensity of a given sample to that of a compound whose fluorescence quantum yield is known. For this one must... 

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]. 

Using the molecular weight change and the number of photons required for the change, the absolute quantum yield at 2537 A of PMMA was obtained where the value of 0.9 was used as the density of PMMA film. Once the absolute quantum yield at 2537 A was obtained, the absolute quantum yields over all the spectral range can be calculated from the relative data of the spectral sensiti-... 

Spectral Dependence of the Absolute Quantum Yield of the Main Chain Cleavage Reaction... 

Using this data the spectral dependence of the absolute quantum yield of the main chain fission of PMMA was calculated follow-... 

Often all three approaches ((1) to (3)) are simultaneously active, e.g., in the dual fluorescence of the classical HCT compounds like DMABN where both the energy of the long-wavelength emission (factor (1)) as well as the ratio between the two bands (factors (2) and (3) above) varies. The simultaneous presence of two bands allows very accurate ratio measuring, and small changes in this ratio can be detected more accurately as if only changes in absolute quantum yields have to be observed. These probes with dual fluorescence will be called type-3. ... 

Methods for the measurement of absolute quantum yields have been reviewed. 6 In most steady-state fluorescence experiments relative fluorescence quantum yields are determined by comparison with appropriate standards (see Section 7.7.1.2.1). 

Photochemical Experiments. The general experimental procedure was that previously described (15). The bolometer was used as a relative intensity indicator since collimation in the cylindrical geometry of an AH6 lamp is poor, and allowance for scattering is difficult. Calibration at each wave length was by means of KCr(NH3)2(NCS)4 solutions for which we now have accurate absolute quantum yields for wave lengths from 450 m/x to 750 m/x (16). For the runs at >680 m/x, a 500 watt tungsten lamp was used since the output of the AH6 mercury lamp was uncomfortably low at this wave length. 

An important measure of the luminescence is the quantum yield. In effect, this is the probability that a photon will be emitted by the lanthanide given that one photon has been absorbed by the antenna ligand. Since measurement of absolute quantum yields is particularly difficult, the overall quantum yield ( ) is normally measured with reference to certain standards (26) these are routinely [Ru(bpy)3]2+ in water or SulfoRhodamine 101 in methanol for Eu3 +, and quinoline sulfate in 0.1 M HC1 or fluorescein in 1 N NaOH for Tb3+ (27,28). A method has been developed that measures energy transfer from the lanthanide complex to an acceptor of known quantum yield (28). 

The absolute quantum yields tend to increase with the photon energy within an electronic transition. In addition, these reactions can also take place in the dark, but their ratio is different and there is a large difference in the activation barriers the effect of temperature on the reaction rate constants shows that this activation energy is very small in the photochemical reactions, of the order of 1-2 kcalmol-1. However, the dark reaction proceeds with a large activation barrier of some 25 kcalmol-1. 

This ratio gives the relative quantum yield, or the absolute quantum yield so long as that of the standard is accurate. There are some essential conditions to be met. 

Fig. 1 The pH profile for absolute quantum yields (Oabs) for the photohydration reaction of o-hydroxystyrene (open circles) and p-hydroxystyrene (closed circles). Reprinted with permission of the American Chemical Society from Ref.25...

Due to the modifications of the electronic cloud induced by complexation, the quantum yield and the excitation spectrum are also modified. As the direct determination of the absolute quantum yield is very difficult to achieve, one usually finds in the literature quantum yield values determined by comparison to well-known standards, such as quinine sulfate. For example, some values can be found in Georges (1993) or in Klink et al. (2000) for some europium complexes but may be found also in many other papers on lanthanide luminescence. Studies on the correlations between the photophysical properties of a given type of europium complexes and the energy levels can be found in Latva et al. (1997), Klink et al. (2000). A correlation has been found between the excitation properties and the stoichiometry of various Eu(III) complexes (Choppin and Wang, 1997). Note that the changes in the excitation maximum induced by complexation usually amount to a few tenths of nanometers, which requires high resolution for detection. In the case of Eu(III), a correlation has been found between the frequency... 

Both the absolute quantum yield (determined with respect to zinc tctraphcny 1 porphvrin in ethanol) and the product of the molar absorption coefficient at the excitation wavelength with the quantum yield, e Q, which represents the overall luminescence efficiency follow the... 

Absolute quantum yields measured upon ligand excitation (relative error 10%), lifetimes (standard deviation in parentheses), and associated hydration numbers of the [Ln(H290)]3 podates, 6 x 10 3 M in aqueous solutions at... 

Values reported are fractions of total yield and not absolute quantum yields. Values in parentheses are wavelengths used. 

Wegner EE, Adamson AW. Photochemistry of complex ions. III. Absolute quantum yields for the photolysis of some aqueous chromium (III) complexes. Chemical actinometry in the long wavelength visible region. J Am Chem Soc 1966 88 394-404. 

The emission lifetimes of the bipy and phen complexes of ruthenlum(II) at 77°K are generally in the range t = 0.5-10 ps. (Table 7). Since these values are intermediate to those generally observed for the fluorescence and phosphorescence of organic compounds, the radiative transition in the ruthenium complexes was suggested to be a heavy-atom perturbed spin-forbidden process (168,169). From a determination of the absolute quantum yields as well as lifetimes of a series of ruthenium(II) and osmium(II) complexes, the associated radiative lifetimes were calculated (170). The variations in these inherent lifetimes within the series could be rationalized with a semi-emipirical spin-orbit coupling model thus affording further evidence that the radiative transitions are formally spin forbidden in these systems. 

Wrighton et al, (210) studied the temperature dependence of the emission quantum yield as well as of the lifetime. The relative quantum yield was found to vary in the same manner as the lifetime. This indicates that the ratio of the absolute quantum yield to the lifetime, /t, and thus the radiative decay rate of ruthenocene is essentially independent of temperature. The temperature effects on the lifetime and quantum yields, therefore, arise mainly from changes in the nonradiative decay rates. 

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