Determination of Luminescence Quantum Yields

In order to determine whether a given species is a good luminophore, its emission quantum yields has to be measured  

The simplest method for the measurement of the luminescence quantum yield of a luminophore is based on the comparison with a standard species with known quantum yield. The luminescence quantum yield of such reference compounds are practically independent on the excitation wavelength, hence they can be utilized for the whole spectral range of their absorption. 

In practice, the quantum yield is generally determined by comparing the emission spectmm of the examined luminophore with that of a suitably selected standard, both recorded under the same instrumental conditions (excitation wavelength, excitation and emission slits, detector settings, etc.). The comparison involves the emission intensities of the two species (sample and reference) integrated over the whole wavelength range, that is, the areas underneath the respective emission spectra, measured from the baseline. 

It should be noted that modem spectrofluorimeters utilize gratings as monochromators and thus perform a linear dispersion of the radiation with respect to wavelength. Hence, the values of the area of the emission spectra to be used in the calculation of the quantum yield must be obtained on a spectmm reported as a function of X and not, e.g., frequency or wavenumber. The two luminescence spectra to be compared should exhibit similar shape and occur on the same wavelength range if these conditions cannot be fulfilled, the spectra must be corrected before the comparison (See [1, 13, 14] and Sect. 5.3.1 above). If the same excitation wavelength is used for the sample and reference solutions, the unknown luminescence quantum yield can be obtained from the formula [1, 15, 16]  

Equation 5.22 can be utilized only for solutions whose absorbance at the excitation wavelength is lower than 0.1 such a limit is also useful to minimize 

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A basic problem which can be encountered in the application of photophysics to colloidal systems are difficulties involved in the measurement of true luminescence spectra and determination of luminescence quantum yields of molecules in light scattering media. Gade and Kaden have produced a theory for this effect which can be used to take account of readsorption and re-emission effects in suspensions. 

Techniques for the measurement of emission spectra and determination of luminescence quantum yields are well established and are presented in several books and review articles. In addition, Demas s 1983 book on measurement of luminescence lifetimes still serves as an excellent resource for general approaches. ... 

Very little is known on these topics for actinides, as it seems that the determination of quantum yields is not a common feature in this case To the best of our knowledge, a single publication gives a few values of luminescence quantum yields in various media for uranyl (Katsumura et al 1989) and nothing is known for Cm. 

The luminescence data are to be frilly characterized. It means measuring the quantum yields and evaluating the efficiency of the sensitization vs the intrinsic quantum yield and the lifetime. A careful measure of the data must be performed, in particular for the determination of the quantum yields. Two kinds of measurements can be done ... 

In another study, Kondrat eva (103) made a determination of the luminescent quantum yield of the 5D4 state of the terbium ion in aqueous solution. The method used was based upon fluorescent-lifetime measurements and had previously been used by Rinck (96) and Geisler and Hellwege (96) to determine the quantum yield of rare earths in crystals. Kondrat eva made his studies on chloride and sulfate solutions, using the electronic shutter technique of Steinhaus et al. (66). 

Absolute luminescence quantum yield measurements are not made in photophysical practice and are left to specialized laboratories such as the National Physical Laboratory (UK) or the National Bureau of Standards (USA). These provide the quantum yields of a variety of primary standards that are used in practice to determine an unknown quantum yield e. First the luminescence spectrum of the primary standard is measured, and then that of the unknown sample is compared with it as the ratio of the integrated spectra. 

When heat is produced in the sample after the photolytic flash, the refractive index of the liquid changes and the probe beam is deflected. The intensity of this probe beam measured by a photomultiplier tube placed behind the pinhole decreases as the temperature of the irradiated volume increases (then its density and its refractive index decrease). The total optical signal change is a measurement of all the heat produced in the sample, i.e. the sum of non-radiative transitions, chemical reactions and solvation energies. Luminescence does not contribute to this signal (nor does scattered light) and for this reason thermal lensing can be used to determine luminescence quantum yields. 

The measured intensity is a function of the total number of excited species created by the excitation pulse and of the luminescence quantum yield, Q. In other words, for a given lifetime, the Q value determines the ease of detection of a given species. The notion of radiative vs. non radiative pathways is in fact very important as will be detailed below (sects. 3-5). 

The three Ndm, Er111, and Ybm chelates display sizeable metal-centered NIR luminescence in HBS-buffered (pH 7.4) aqueous solutions. Their photophysical characteristics are summarized in table 18. The hydration numbers calculated from eqs. (10a) and (9a) are very small, 0.31 and 0.16 for Ndm and Ybm, respectively, and compare well with the results obtained for the tetrapodal ligand H890a. The overall luminescence quantum yields in aqueous solution are comparable to those obtained for H890a, but smaller than those determined for chelates withH890b (compare tables 17 and 18). Upon deuteration of the solvent, from 3- to 10-fold increases are observed in the luminescence quantum yields. Moreover cytotoxicity studies on several cell lines have shown the Ybm chelate to be non-toxic, opening the way for applications in cell imaging (Comby et al., 2007). 

Analytical strategies based on the activation effect caused by the analyte on the QD luminescence emission also have been proposed. In a pioneering work, the addition of Zn and Mn ions to colloidal solutions of CdS or ZnS QDs resulted in an important enhancement of the luminescence quantum yield of the nanoparticles. This effect was attributed to the passivation of surface trap sites that are either being filled or energetically moved closer to the band edges.33 46 This behavior provided the basis for the optical sensing of such metal cations with QDs. Chen and Zhu47 proposed a method for the determination of trace levels of silver ions based... 

Absorption and emission spectra provide basic information (molar absorption coefficients, luminescence quantum yields), but also their changes upon association between two species can be used to determine the stoichiometry and stability constant of host-guest complexes. Moreover, evidence for the existence of photo-induced processes can be simply obtained in some cases from the fluorescence spectra. 

By applying Fermi s golden rule, Forster derived a very important relation between the critical transfer distance R0 and experimentally accessible spectral quantities (Equation 2.35),° 67,68 namely the luminescence quantum yield of the donor in the absence of acceptor A, orientation factor, k, the average refractive index of the medium in the region of spectral overlap, n, and the spectral overlap integral, J. The quantities J and k will be defined below. Equation 2.35 yields remarkably consistent values for the distance between donor and acceptor chromophores D and A, when this distance is known. FRET is, therefore, widely applied to determine the distance between markers D and A that are attached to biopolymers, for example, whose tertiary structure is not known and thus... 

In this section, we discuss the intensity of the metal luminescence obtained upon ligand excitation on the basis of the product of the metal luminescence quantum yield upon excitation at a certain wavelength and the molar absorption coefficient of the ligand in the complex at the same wavelength (Table 7). We would like to recall that the metal luminescence intensity is the photophysical property of main interest in this research dealing with the antenna effect in Eu3+ and Tb3+ complexes. Furthermore, this quantity is determining for some applications of these compounds (Section III). 

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