spiropyran

Spiropyrans are a class of photochromic organic compounds in which two heterocyclic rings are joined by a single spiro carbon, creating a rigid, nonplanar closed form. In the closed spiropyran form, the two rings are orthogonal and the molecule is typically colorless and inert under ordinary lighting.

Illumination with ultraviolet light induces ring opening: cleavage of the C–O bond at the spiro centre converts the molecule to a highly conjugated merocyanine form (the open form). This merocyanine species absorbs strongly in the visible region, producing a vivid color. The open form is usually non-spiro and more planar, leading to enhanced conjugation and color. The ring opening is reversible; the merocyanine thermally relaxes back to the spiropyran, and exposure to visible light can also accelerate back-switching in some systems.

Substituents on either ring, the solvent, and temperature influence the absorption characteristics, kinetics of photoswitching, and the stability of the merocyanine form. Many spiropyrans feature electron-donating or withdrawing groups that tune the color and switching speed.

Commonly used spiropyran structures have a benzopyran (chromene) moiety spiro-fused to an indoline or similar nitrogen-containing heterocycle. Variants include amino-, nitro-, and alkoxy-substituted derivatives.

Applications rely on reversible photochromism and, in some cases, mechanochromism: spiropyrans serve as molecular switches for data storage and logic, components in smart coatings and optical filters, stimuli-responsive polymers and hydrogels, and mechanophores in polymer mechanochemistry.

Limitations include fading of the merocyanine color over time in ambient conditions, fatigue on repeated cycling, and sensitivity to solvents and oxygen in some derivatives.