Fluorescence Quantum Yield Measurements

Four molecular fluorescence parameters describe the behaviour of a fluorescent molecule in very dilute (~ 10(−6)M i. the fluorescence spectrum [Formula: see text]; ii. the fluorescence polarization P(M); iii. the radiative transition probability k(FM); and iv. the radiationless transition probability k(IM). These parameters and their temperature and solvent dependence are those of primary interest to the photophysicist and photochemist. [Formula: see text] and P(M) can be determined directly, but k(FM) and k(IM) can only be found indirectly from measurements of the secondary parameters, v. the fluorescence lifetime τ(M), and vi. the fluorescence quantum efficiency q(FM), where k(FM)=q(FM)/τ(M) and k(IM)=(1–q(FM)) τ(M). The real fluorescence parameters [Formula: see text] , τ and ϕ(F) of more concentrated (c > 10(−5) M) solutions usually differ from the molecular parameters [Formula: see text] , τ(M) and q(FM) due to concentration (self) quenching, so that τ > τ(M) and ϕ(F) < q(FM). The concentration quenching is due to excimer formation and dissociation (rates k(DM)c and k(MD), respectively) and it is often accompanied by the appearance of an excimer fluorescence spectrum [Formula: see text] in addition to [Formula: see text] , so that [Formula: see text] has two components. The excimer fluorescence parameters [Formula: see text] , P(D), k(FD) and k(ID) together with k(DM) and k(MD), and their solvent and temperature dependence, are also of primary scientific interest. The observed (technical) fluorescence parameters [Formula: see text] , τ(T) and [Formula: see text] in more concentrated solutions usually differ from the real parameters [Formula: see text] , τ and ϕ(F), 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 [Formula: see text] and τ(T). The usual method of determining [Formula: see text] 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 [Formula: see text] where n is the solvent refractive index. Two reference solution standards have been proposed, quinine sulphate in N H(2)SO(4) 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.