Time-resolved color-changing long-afterglow for security systems based on metal–organic hybrids

Abstract

Organic solid-state materials with p-RTP (persistent room-temperature phosphorescence) properties have made infusive achievements. However, research on metal–organic hybrids with p-RTP is limited, of which dual phosphorescent ones are relatively rare. Herein, three light-emitting compounds Himpc·HNO₃ (Himpc-HNO₃), Cd(impc)₂(H₂O)₂ (Cd-impc) and [Zn(impc)₂(H₂O)₄]·2H₂O·(Zn-impc) were deliberately synthesized, based on Himpc [5-(1H-1,2,4-imidazol-1-yl)nicotinic acid] phosphor via a solvothermal method, and all displayed different room-temperature phosphorescence performances. It is noteworthy to mention that time-resolved color-changing afterglow phenomenon, which arises from different triplet states, was evidently observed in both Cd-impc and Zn-impc. Particularly, Zn-impc is a rare example of a metal–organic hybrid material showing novel color-changing afterglow performance [yellow light (∼580 nm, 84 ms) and green light (∼504 nm, 405 ms)], which can be utilized in special security systems. By means of single-crystal structural comparison, we revealed that a well-organized molecular arrangement is responsible for the enhancement of emission. Moreover, strong π⋯π interactions in H-aggregated molecules prominently increase the phosphorescence lifetime.