Photochromic and Room-Temperature Phosphorescent D–A Hybrid Crystals Induced by Anion−π Interactions

The donor–acceptor (D–A) hybrid crystals are an emerging class of crystalline hybrid complexes composed of semiconductive organic and inorganic components at the molecular level. These unique crystals not only possess intrinsic electron-transfer photochromic properties upon the high-energy irradiations but possibly exhibit unusual room-temperature phosphorescence (RTP) emissions when suitable charge-transfer states formed with the low-energy irradiations. Herein, we reported that an interesting class of D–A hybrid crystals indeed exhibited two such properties in which two D–A components, polyoxometalates (POMs) and naphthalene diimides (NDIs), are intercontacted through uncommon anion−π interactions. To go deep into their structure–property relationships, five D–A hybrid crystals with different POMs (PW12O403–, SiW12O404– or Mo6O192–) and protonated (or zinc-coordinated) pyrazolyl-substituted NDIs have been synthesized. Their light-induced color changes and RTP properties have been systematically investigated, which revealed that the matching electron energy levels of POMs and NDIs are the first factor to decide their RTP emissions while the heavy atoms in both POMs and coordination metal cations are mainly beneficial to their quantum yields. Notably, the RTP quantum yields for these interesting photochromic D–A hybrid crystals are up to 5.9%.