merocyanines

Merocyanines are a class of dyes consisting of open-chain, charge-separated forms of cyanine dyes. They typically arise as ring-opened forms of cyanines or as the colored merocyanine forms produced by photochromic transformations of spiropyran or spirooxazine systems. In many cases, the merocyanine is in equilibrium with the corresponding closed cyanine form, with the balance shifting with solvent, temperature, and light.

Structural features include an extended conjugated chain linking two heteroaromatic fragments, often with a push–pull arrangement that creates a strong intramolecular charge transfer. The result is intense absorption in the visible to near-infrared region and pronounced solvatochromism. Photophysical behavior, including fluorescence, is sensitive to substituents, counterions, and the environment.

Formation and synthesis: Merocyanines are prepared by construction of the open-chain structure or generated in situ by opening reactions of cyanine dyes or of photochromic precursors. The cyanine/merocyanine equilibrium is influenced by solvent polarity, pH, temperature, and light irradiation.

Applications: They are used as dyes and pigments with near-IR absorption, components in optical data storage and switching materials, and in nonlinear optical applications. Their sensitivity to their environment also makes them useful in chemical and biological sensing as solvatochromic probes.

See also: cyanine dyes, hemicyanines, spiropyrans, spirooxazines, photochromism.