Question

Explain why the fluorescence of Tokyo Green changes in acidic and basic solutions

Answer 1

The fluorescein cyclizes to form a lactone in acidic medium as shown in the image. Since the structures of fluorescein are different in acidic and basic medium, the absorption is observed at different wavelengths. Thus, fluorescence changes when going from basic to acidic medium or vice versa.

In the basic medium, there is an extended - Conjugation in basic medium as seen in the image. With the increase in the conjugation , the gap between the HOMO (Highest occupied molecular orbital) and LUMO ( Lowest unoccupied molecular orbital)energy level decreases. Thus, absorption will take place at lower frequency and higher wavelength. Absorption and emission spectra will be different of acidic and basic medium.

With the decrease in pH, Intensity of fluorescence also decreases.Hope, This helps.