To understand in depth the photophysical properties, we studied the transient spectroscopy (TA) spectra and dynamic trace of BTD-HeBr in tetrahydrofuran, as shown in Fig. . The positive band from 500 to 600 nm corresponds to stimulated emission (SE), while the negative band from 650 to 730 nm was attributed to the excited-state absorption (ESA). The proposed mechanism of transient component evolution is shown in Fig. and Supplementary Fig. The photophysical properties of the emitters in the aggregate state are crucial for high-sensitive X-ray detection and imaging applications. The curves of PL intensity in water/THF mixtures with different water fractions proves that the three HLCT molecules show aggregation-induced emission (AIE) characteristics (Supplementary Fig. ), which can demonstrate potential performance as X-ray scintillators. The development of scintillators with higher efficiency and faster timing properties is always the primary aim of this field. The key to achieving intrinsic efficient and fast organic scintillators is to eliminate the occupation of T1 states without any loss of the utilization efficiency of triplet excitons. As expected, the fluorescence lifetime decreases from 5.10 ns for BTD-FL to 4.48 ns for BTD-FLBr due to the increased SOC and (R)ISC processes by introducing bromine atoms. Notably, the fluorescence lifetime further decreases to 3.74 ns in BTD-HeBr, showing that through-space bromine atoms can significantly enhance the (R)ISC processes due to the numerous spatial interactions between bromine atoms and emitters in the aggregate state (Fig. and Supplementary Fig. ). In addition, due to the efficient hRISC processes between the triplet and singlet states, the BTD-FL exhibits a high PLQY of 75.3% in neat film (Supplementary Fig. ). In contrast, the direct connection of bromine lead to largely fluorescence quenching in BTD-FLBr because of the enhanced non-radiative decay rate of the triplet states. Therefore, BTD-FLBr with through-bonded bromines only exhibits a relatively low PLQY of 52.4%, which is disadvantageous to achieve highly efficient X-ray detection. However, the nonconjugated alkyl chains with through-space bromine atoms in BTD-HeBr not only enhance the (R)ISC processes but also significantly suppress the non-radiative decays of the triplet states. The detailed discussions will be given in the next part. As a result, the PLQY of BTD-HeBr is largely increased to 100%, implying a promising organic scintillation performance. To more accurately ascertain the changes in the photophysical processes, we calculated the radiative transition rate (kr), and nonradiative transition rate (knr) of the three emitters (Fig. ). According to the formulas 29 kr = Φf/τ and knr = 1/τ -kr (where Φf represents the PLQY and τ is the PL decay lifetime), the kr of 1.48×10 8 s -1 in BTD-FL slightly decreases to 1.17×10 8 s -1 in BTD-FLBr but largely increases to 2.67×10 8 s -1 in BTD-HeBr, which indicates that a short fluorescence lifetime and a high PLQY value can greatly facilitate the singlet radiative transition process. On the other hand, in comparison with the relative lower knr of BTD-FL (4.81×10 7 s -1 ), the knr of BTD-FLBr (1.06×10 8 s -1 ) is obviously higher, due to the fluorescence quenching by through-bond bromine atoms. In contrast, the even lower knr of BTD-HeBr (3.80 ×10 5 s -1 ) indicates the little effect of through-space bromines on fluorescence quenching. It is found that BTD-HeBr with through-space bromine atoms not only largely improves the radiative rate, but also dramatically suppresses the non-radiative decay rate, which are beneficial for achieving highsensitive X-ray detection.