Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section A(iii) / 5 / iii / Solution

The left-hand open quinoid dianion is pink/purple; the right-hand closed lactone is colorless. In the colored form, a longer conjugated electronic system extends through the quinoid framework. The resulting lower-energy electronic transitions can absorb visible light, producing the observed complementary color.

The closed lactone contains a tetrahedral central carbon that interrupts conjugation between the aromatic fragments. Its remaining electronic transitions are mainly at higher energy, so the molecule does not strongly absorb visible wavelengths. It still absorbs ultraviolet light: colorless does not mean absence of electronic transitions.

The explanation concerns the two drawn forms of phenolphthalein. It does not assert that a single colored form remains dominant at arbitrarily high pH; additional equilibria can alter the spectrum outside the usual indicator range.