When a fluorophore absorbs a photon of light, an energetically excited state is formed. The fate of this species is varied, depending upon the exact nature of the fluorophore and its surroundings, but the end result is deactivation (loss of energy) and return to the ground state.
The fluorescence quantum yield is the ratio of photons absorbed to photons emitted through fluorescence. In other words the quantum yield gives the probability of the excited state being deactivated by fluorescence rather than by another, non-radiative mechanism.
The measurement of fluorescence quantum yields can often be difficult and troublesome. However, it is possible to make such measurements routinely, and following the HORIBA Jobin Yvon guidelines in the link below should make this an achievable goal.
Measurement of the absolute photoluminescence quantum yield (PLQY) of thin films is a more complex procedure than the corresponding solution measurement. This is due to the fact that films are a high refractive index medium, which in turn results in substantial waveguiding of the luminescence. To overcome the angular dependence of the emission from films, integrating spheres are generally used to collect the emitted light. The use of integrating spheres has usually required a laser as the excitation source in combination with a fibre coupled CCD camera or a calibrated photodiode as the luminescence detectors. We can however, fit an integrating sphere into the sample chamber of both the FluoroLog and FluoroMax Spectrofluorometers to measure solid state photoluminescence quantum yields of films. This approach significantly simplifies the experimental method as the need for special equipment on the excitation and detection side is relaxed.
The results are rewarding since it shows that PLQYs can be determined in an easy way with an instrument that has additional flexibility compared to the experimental set-ups that are generally used, and will be found in very many laboratories. This should make PLQY measurements accessible to a large number of groups with the minimum of cost.
We gratefully acknowledge the help of Dr L Palsson, Dr A Monkman and Dr A Beeby (University of Durham, UK) for the Fluorescence Quantum Yield information provided on this page.
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