Organic solid photochromism by photoreduction mechanism: Viologen embedded in solid polar aprotic polymer matrix

Reversible photocolor developments of viologens embedded in poly( N ‐vinyl‐2‐pyrrolidone) films, a typical polar aprotic solid matrix, were found to be affected by the kinds of viologen cation as well as the paired anion. The color developments in the corresponding low‐molecular‐weight solvents are connected closely to the solubility of viologens in these solvents; viologens are highly sensitive in the polar aprotic solvents in which they have poor solubilities, such as N ‐methyl‐2‐pyrrolidone and hexamethyl phosphoric triamide. These facts confirm the color‐development mechanism consistings of electron transfer to the photoexcited viologen cation from the paired anion in polar aprotic solid matrices such as poly( N ‐vinyl‐2‐pyrrolidone).