Fluorescence lifetime studies on various ionic species of sodium fluorescein (uranine)

Abstract

The radiative lifetimes τ₀ for the fluorescein cation, double cation and dianions have been calculated from the absorption measurements. The quantum yields (QY) for these species have been estimated from the measured lifetime values. The cation and anion show a larger QY than do the dications. A large Stokes shift and smaller QY for the dication seem to be related to a geometry change on excitation, causing accelerated non-radiative processes. The effect of temperature on the cation lifetime indicates that, while emission originates from the protonated species (FH⁺) at low temperatures, a new type of solvated cation species F⁺ (H₃O⁺) is formed beyond 240 K.