Electron transfer across a spiro link: extreme solvatofluorochromism of a compact spiro-bridged N,N-dimethylaniline-phthalide dyad.

Intramolecular electron transfer over the sp3-C-link in dimethyl-aniline-spiro-phthalide leads to extreme solvatofluorochromism (11 600 cm-1) and a fluorescence maximum shift from 357 (hexane) to 595 nm (acetonitrile). The spiro link enhances π-σ* negative hyperconjugation leading to C-O bond elongation and electron transfer over a longer distance than in a non-spirocyclic analogue.