The infrared spectra and structures of the three solid forms of fluorescein and related compounds.

The three solid forms of fluorescein, yellow, red and (newly isolated) colourless, have been shown to differ distinctly in structure. By spectroscopic methods, principally infrared absorption, the colourless form has been assigned the lactone structure, the red solid the p-quinone structure, and the yellow solid a zwitterion structure with a positive charge distributed over the oxygen-bearing ring.