Pure Organic Persistent Room-Temperature Phosphorescence at both Crystalline and Amorphous States

Persistent room-temperature phosphorescence (p-RTP) of pure organic materials is attracting increasing attention. The design of efficient phosphors and understanding the origin of p-RTP, however, remain challenging. Herein, to gain further insights into pure organic p-RTP, we prepared a group of carbazole (CZ) and methyl benzoate (MBA) conjugates with a methyl ester unit at the para (4-MBACZ), meta (3-MBACZ), and ortho (2-MBACZ) sites. These isomers merely produce prompt fluorescence in solutions, but generate blue prompt/delayed fluorescence (DF) and orange p-RTP with lifetimes up to 865.2 ms in the crystalline state. Lifetimes of p-RTP are in the order of 2-MBACZ>3-MBACZ>4-MBACZ, which might be mainly ascribed to the combined effect of packing density, intermolecular interactions and steric hindrance. Meanwhile, upon mechanical grinding, while the emission color and profile of the luminogens do not significantly change, no (4-MBACZ) or shortened (3-MBACZ, 2-MBACZ) p-RTP is observed, accompanying the change from crystalline to amorphous states. Such p-RTP at amorphous states without external hosts is rarely reported, which demonstrates important implications for the molecular design and mechanism understanding towards p-RTP. Furthermore, their p-RTP attribute and different emission colors before and after ceasing the UV irradiation endow them promising applications in encryption and anticounterfeiting fields.