Fluorescence Quantum Yield Measurements.

Abstract
Four molecular fluorescence parameters describe the behaviour of a fluorescent molecule in very dilute (~ 10-6M) solution: the fluorescence spectrum F M ( v ¯ ) ;the fluorescence polarization PM ;the radiative transition probability kFM ; andthe radiationless transition probability kIM .These parameters and their temperature and solvent dependence are those of primary interest to the photophysicist and photochemist. F M ( v ¯ ) and PM can be determined directly, but kFM and kIM can only be found indirectly from measurements of the secondary parameters,the fluorescence lifetime τM , andthe fluorescence quantum efficiency qFM ,where kFM =qFM/τM and kIM =(1-qFM ) τM. The real fluorescence parameters F ( v ¯ ) , τ and ϕF of more concentrated (c > 10-5M) solutions usually differ from the molecular parameters F M ( v ¯ ) , τM and qFM due to concentration (self) quenching, so that τ > τM and ϕF < qFM. The concentration quenching is due to excimer formation and dissociation (rates kDMc and kMD , respectively) and it is often accompanied by the appearance of an excimer fluorescence spectrum F D ( v ¯ ) in addition to F M ( v ¯ ) , so that F ( v ¯ ) has two components. The excimer fluorescence parameters F D ( v ¯ ) , PD , kFD and kID together with kDM and kMD , and their solvent and temperature dependence, are also of primary scientific interest. The observed (technical) fluorescence parameters F T ( v ¯ ) , τT and ϕ F T in more concentrated solutions usually differ from the real parameters F ( v ¯ ) , τ and ϕF , due to the effects of self-absorption and secondary fluorescence. The technical parameters also depend on the optical geometry and the excitation wavelength. The problems of determining the real parameters from the observed, and the molecular parameters from the real, will be discussed. Methods are available for the accurate determination of F T ( v ¯ ) and τT . The usual method of determining ϕ F T involves comparison with a reference solution R, although a few calorimetric and other absolute determinations have been made. For two solutions excited under identical conditions and observed at normal incidence ϕ F T ϕ F R T = n 2 ∫ F T ( v ¯ ) d v ¯ n R 2 ∫ F R T ( v ¯ ) d v ¯ where n is the solvent refractive index. Two reference solution standards have been proposed, quinine sulphate in N H2SO4 which has no self-absorption, and 9,10-diphenylanthracene in cyclohexane which has no self-quenching. The relative merits of these solutions will be discussed, and possible candidates for an "ideal" fluorescence standard with no self-absorption and no self-quenching will be considered.

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