Goodbye to Quinine in Sulfuric Acid Solutions as a Fluorescence Quantum Yield Standard.

Quinine is by far the most popular fluorescence quantum yield standard used nowadays. In this work, we exploit a modified version of a recently developed SAFE (simultaneous absorption and emission measurement) method for investigating the temperature dependence of fluorescence quantum yields. The accuracy of the method was verified using rhodamine 6G and rhodamine B solutions, which have well characterized quantum yield temperature dependences. Subsequently, we investigated the quantum yield temperature dependence of quinine solutions in 0.05 M sulfuric acid and 0.1 M perchloric acid. The results show a large temperature dependence of fluorescence quantum yield for quinine in 0.05 M sulfuric acid. This temperature dependence is particularly pronounced near room temperature (from 20 to 25 °C), where the quantum yield changes by -0.45% per Celsius degree. In contrast, the fluorescence quantum yield of quinine in 0.1 M perchloric acid shows no temperature dependence between 20 and 45 °C, where, in the entire range, the luminescence quantum yield value remains constant and equal to 0.60 ± 0.007. These results clearly indicate that it should be recommended to use quinine in 0.1 M perchloric acid rather than quinine in 0.05 M sulfuric acid as a standard solution for the determination of luminescence quantum yields.