Combining π-conjugated groups by flexible alkyl chains for ultralong organic phosphorescence by photo-activation

Purely organic luminescent materials with ultralong room temperature phosphorescence by photo-activation have attracted great interest owing to their advantages of clean, safe, easy-to-use and control. Herein, we designed and synthesized a series of ultralong room temperature phosphorescence carbazole derivatives using flexible alkyl with different lengths to adjust the distance between two conjugated carbazole groups. Only, CzC3Cz crystal has the dynamic ultralong organic phosphorescence with increasing the excitation time of 365 nm UV light. By prolonging the UV excitation time (from 5 s to 3 min), room temperature phosphorescence intensity was improved about 10 times and phosphorescence lifetime increased from 48 ms to 898 ms. Additionally, it exhibited favorable stability and repeatability. Comparing the single crystal structure before and after photo-activation, intermolecular interaction of π···π and C–H···π bond strengthens indicating that metastable state appears after long-time excitation of UV light. Considering the photo-activation characteristic, encryption pattern was prepared using the sifted compounds with similar phosphorescence performance for the application of anti-counterfeiting.