Cavity-Directed Chromism of Phthalein Dyes

Phthalein dyes in the quinone dianion
form (pseudo-D3, colored) are transformed
into the lactone dianion form (pseudo-Td, colorless) through encapsulation in a Td-symmetric host even under basic conditions
(pH ∼10). The compatibility in size and symmetry between the
lactone and the cavity is essential to the transformation. Upon addition
of a guest that strongly binds to the cavity, the encapsulated phenolphthalein
is expelled, the color of the basic solution is regained, and the
host–guest complexation is thus visualized.