Purely organic room temperature phosphorescence (ORTP), due to its wide Stokes shift, low fabrication cost and unique long after glow emission, has attracted extensive attention in many applications such as data encryption, anti-counterfeiting, organic light emitting diodes and cell imaging. At present, designing organic materials with high phosphorescence and extremely long luminescent time is still a great challenge. Based on the heavy atom effect, a purely ORTP molecule was designed and synthesized in this work. This compound was white powder under ambient conditions, and can emit a bright yellow phosphorescence when the UV lamp was turned on. The maximum excitation at 366 nm, the corresponding maximum emission at 544 nm, and two shoulder peaks at 590 and 640 nm, respectively, were observed. The lifetime was 103.55 ms, and the afterglow was close to 2 s. To explore the influence of heavy atom introduction on phosphorescence, a theoretical simulation was performed by (TD) DFT. The band gap of HOMO/LUMO was only 0.02 eV, which indicated that the molecule was easily excited. Compared with similar halohaline-free compounds, It is proved that the introduction of heavy atoms is helpful in increasing the rate of spin-orbit coupling (SOC) and intersystem crossing (ISC) between singlet and triplet states. XRD spectroscopy was performed to further explore the origin of BFCzB ultralong phosphorescence and investigate the molecular packing model and the presence of interactions. InBFCzB molecules, three types of intramolecular interactions, including C-Br…π (3.373 1 ) halogen bond, C-Br…N (3.170 5 ) halogen bond and C-F…H-C (2.587 7 ) hydrogen bond were observed, which effectively limited the rotation and vibration of the molecules, thereby reducing the non-radiative energy attenuation. In addition, some intermolecular interactions between halogen atoms and adjacent molecules were found in BFCzB. C-F formed major interactions with the carbazole rings of neighboring molecules, C-F…H-C (2.527 1 ) hydrogen bond and C-F…π (2.933 5 and 3.049 4 ) halogen bond. C-Br…H-C (2.846 6 ) hydrogen bond and C-Br…π (3.531 4 ) halogen bond were also observed between Br and its neighbors. π…π stacking (3.399 2 ) was found in the carbazole group of the neighboring molecule. These intramolecular and intermolecular interactions worked together to inhibit molecular motion, further reducing the nonradiative attenuation of triplet excitons and achieving ultra-long phosphorescence. Moreover, in this work, the production of singlet oxygen (1O2) during phosphorescence of BFCzB molecular quenching in water was verified by the TMB colorimetric method. Therefore, the photodynamic antibacterial were carried out. This study can provide some reference for the design, synthesis and application of purely ORTP molecules.