Fluorescein derivatives have been used widely in chemical biology, material sciences, and other scientific disciplines owing to their brightness, high quantum yields, and biocom- patibility.1,2) Fluorescein exists as a complex organic solid that can be categorized into three forms: (I) the lactonoid form, which is a colorless solid; (II) the quinoid form, a red solid; and (III) the zwitterionic form, a yellow solid.3—7) In aqueous solutions, the behavior of ...