Reversible photochromic properties of 4,5,6-triaryl-4H-pyran derivatives in a solid state

Three 2-amino-4,5,6-triaryl-4H-pyran (NPR) derivatives and two 4,5,6-triaryl-4H-pyran (PR) derivatives with different substituents at the 6-position of the 4H-pyran unit, such as benzofuran (NPR-Bf and PR-Bf), benzothiophene (NPR-Bt and PR-Bt), and a phenyl ring (NPR-Ph), are designed and synthesized. NPR-Bf, NPR-Bt, PR-Bf, and PR-Bt all exhibit an eye-detectable reversible photochromic phenomenon in solid-state appearance color with high-contrast, short response time, and excellent fatigue resistance, whereas NPR-Ph is non-photochromic. UV-light irradiation also leads to a solid-state fluorescence color change for NPR-Bf and NPR-Bt. The reversible solid-state photochromic activities of these compounds are confirmed to be ascribed to the intramolecular ring-closing and ring-opening reactions upon UV-light and white-light irradiation by the experiments and theoretical calculations. Furthermore, the strong π-conjugation effect of the benzofuran/benzothiophene unit is determined to be beneficial for the generation of the ring-closing products and thus photochromic activities. These photochromic compounds can be developed as rewritable optical recording media because of good reversibility between the photochromic and bleaching processes. This work provides useful information for developing novel 4H-pyran-based photochromic materials in the solid state.