Fluorescence Quantum Yield Measurements.

Four describe the behaviour of a fluorescent molecule in very dilute (~ 10 ) solution: the fluorescence spectrum ;the fluorescence polarization ;the radiative transition probability ; andthe radiationless transition probability .These parameters and their temperature and solvent dependence are those of primary interest to the photophysicist and photochemist. and can be determined directly, but and can only be found indirectly from measurements of the secondary parameters,the fluorescence lifetime , andthe fluorescence quantum efficiency ,where = and =(1- ) The , and of more concentrated ( > 10 ) solutions usually differ from the molecular parameters , and due to concentration (self) quenching, so that > and < The concentration quenching is due to excimer formation and dissociation (rates and , respectively) and it is often accompanied by the appearance of an excimer fluorescence spectrum in addition to , so that has two components. The , , and together with and , and their solvent and temperature dependence, are also of primary scientific interest. The (technical) , and in more concentrated solutions usually differ from the real parameters , and , 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 and . The usual method of determining involves comparison with a reference solution , although a few calorimetric and other absolute determinations have been made. For two solutions excited under identical conditions and observed at normal incidence where is the solvent refractive index. Two reference solution standards have been proposed, quinine sulphate in HSO 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.