The crystalline nanoparticles can be assembled by 3-bromo- and 3-iodo-carbazole (3-BrC and 3-IC) based on the halogen–halogen interaction in suspension aqueous solutions. As the colloid-like suspension was dropped onto film the particles further aggregate as rod-like structures with size of 3μm in length and 1μm in width. The halogen–halogen interaction are well proved by single crystal X-ray data, and the data reveal that each bromine atom interacts with the neighboring two, and each iodine atom interacts with the neighboring five and I–I interaction is stronger than that of Br–Br. Both 3-BrC and 3-IC can emit novel self-protective room temperature phosphorescence (RTP) in the range of 480 to near 800nm at the excitation of 338nm, and 3-BrC shows additionally the delayed fluorescence emission from 350 to 480nm, both possessing the charge-transfer character caused by halogenations. RTP decay possesses the bi-exponential property and RTP lifetimes are 3.37, 31.16ms (with ethanol) and 1.52, 30.83ms (with THF) for 3-BrC or 3.53, 14.95ms (with ethanol) and 1.68ms, 13.74ms (with THF) for 3-IC, showing “ heavier atom ”, I, makes intersystem crossing rate k ISC from both S 1 to T 1 and T 1 to S 0 faster. For the results, the detection limits of 3-BrC and 3-IC can reach 2.4×10 −7 and 9.0×10 −8 molL −1 , respectively, with wider linear range and higher precision compared with other systems.