Solid-state photochromism of chromenes: enhanced photocoloration and observation of unstable colored species at low temperatures.

Solid-state photochromism of benzopyrans and naphthopyrans (chromenes) was investigated in the temperature range between 300 and 80 K. Variable-temperature diffuse reflectance spectroscopy of microcrystalline powders showed that the extent of photocoloration was greatly enhanced at low temperatures. All the chromenes examined exhibited solid-state photochromism at low temperatures, even when they showed little or no photocoloration at room temperature.

Photochromism of spiropyrans and spirooxazines in the solid state: low temperature enhances photocoloration.

In this study we have revealed that spiropyrans and spirooxazines, which have long been believed to be non-photochromic in the solid state, are generally photochromic in the solid state at least at low temperatures and that their photoreactivity increases greatly as the temperature is lowered.