Abstract
Four molecular fluorescence parameters describe the behaviour of a fluorescent molecule in very dilute (~ 10(-6) M) solution: the fluorescence spectrum FM(v¯) ;the fluorescence polarization P(M) ;the radiative transition probability k(FM) ; andthe radiationless transition probability k(IM) .These parameters and their temperature and solvent dependence are those of primary interest to the photophysicist and photochemist. FM(v¯) and P(M) can be determined directly, but k(FM) and k(IM) can only be found indirectly from measurements of the secondary parameters,the fluorescence lifetime τ(M) , andthe fluorescence quantum efficiency q(FM) ,where k(FM) =q(FM)/τ(M) and k(IM) =(1-q(FM) ) τ(M). The real fluorescence parameters F(v¯) , τ and ϕ(F) of more concentrated (c > 10(-5) M) solutions usually differ from the molecular parameters FM(v¯) , 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 FD(v¯) in addition to FM(v¯) , so that F(v¯) has two components. The excimer fluorescence parameters FD(v¯) , 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 FT(v¯) , τ(T) and ϕFT in more concentrated solutions usually differ from the real parameters F(v¯) , 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 FT(v¯) and τ(T) . The usual method of determining ϕFT 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 / = 2/2 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.