Spiropyrans

Spiropyrans are a class of photochromic organic compounds characterized by a spiro linkage between a heterocyclic ring and a benzopyran moiety. In their ground state they adopt a closed, nonconjugated spiropyran (SP) form, which is typically colorless or weakly colored. Upon stimulation by light, heat, or chemical stimuli, they undergo a reversible ring-opening reaction to form an open, highly conjugated merocyanine (MC) form that is usually colored. The spiro carbon junction prevents planarity and extended conjugation in the SP form, while opening of the central ring creates a linear, highly conjugated chromophore in the MC form.

The switching between SP and MC involves cleavage of the spiro C–O bond and the formation of an extended π-system that shifts absorption into the visible region. The MC form often exhibits intramolecular charge distribution that can be zwitterionic or neutral depending on substituents and environment. The SP ⇌ MC equilibrium is influenced by wavelength, temperature, solvent polarity, substituents on the rings, and pH. Thermal back reaction typically returns the molecule to the SP form, though the rate can vary widely among derivatives, giving a range of lifetimes for the colored state.

Applications of spiropyrans exploit their reversible photochromism and tunable kinetics. They are used in smart windows, dyes and optical displays, molecular switches for data storage, sensors for light and chemical stimuli, and as components in stimuli-responsive materials and hydrogels. Ongoing research seeks to optimize fatigue resistance, switching speeds, and color contrasts across different environments.