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material to the number of photons of light that the material absorbs. Practically, QYs are
important as they allow the quantitative assessment of fluorescence from materials and the
effects of interferences on the fluorescence properties. Such measurements can be made on
solutions, powders, and thin films and apply to a wide range of applications including, for
example, fluorescent materials for whiteners and white lights, organic and inorganic light-
emitting diode materials, biology, fluorescent probes and quantum dots, laser threshold
requirements, determining suitability of wavelength shifters, and for studies of radiation-
less transitions in molecular systems. There are a variety of methods for measuring QYs,
and many of these have been described in the literature (Resch-Genger et al.,
2007
). The
measurement of absolute quantum yields is difficult owing to the range in experimen-
tal errors that need to be avoided or compensated for. Relative quantum yields are more
commonly measured by comparing an unknown sample to that of a sample with a known
QY in the same spectral region. In this case, the accuracy of the unknown QY is determined
by that of the known reference sample. As in all fluorescence measurements, care is needed
to minimize interactions occurring within the sample that could impact on the QY. Such
factors include inner filter effects and factors affecting the fluorophore's microenvironment
such as temperature, solvent, pH, presence of dissolved oxygen and other quenchers, polar-
ity, viscosity, and fluorophore binding. All of these are capable of introducing significant
errors to QY determination. Because the efficiency of the fluorescence process within a
given sample is determined by the QY, this parameter is of major importance. The QY is
essential for the calculation of quenching-rate constants, radiative and nonradiative rate
constants, and energy transfer. In essence the QY is needed to help describe or define the
samples photo-physical behavior (Fery-Forgues and Lavabre,
1999
].
Measuring the true QY of any fluorophore is complex. One approach is to use an inte-
grating sphere (IS). In this, the sphere is hollow, with entrance and exit ports, and the
interior is coated with a diffuse reflective coating. Light scattered by the interior of the
IS
is uniform and evenly distributed over all angles. As a result the flux (total power) of any
light source can be measured without errors caused by complex optical geometries and
arrangements.
5.4.21 Measuring Quantum Yields: The Three-Measurement Technique
The three-measurement technique (
Figure 5.23
) is one of several methods of measuring the
quantum yield of a sample. When used with an integrating sphere (IS) it is a method that
provides compensation for the reabsorption effects in the measurements.
Measurement 1: Excitation Power - NO sample is in the IS
.
No sample is present in the
•
IS
.
The excitation monochromator is set to the excitation wavelength.
•
•
Emission scan across spectral range including the excitation peak.
•
Determine the peak wavelength from the measurement.
•
Correct the spectrum for the integration period to give the number of photons per second.
