However, the energy level diagrams of these isomers become quite different in the crystal phase (Figure ). Particularly, the energy splitting between the S1 and S2 (ΔES1-S2) is large of up to 1.07 eV in o-ClTPA. According to the energy-gap law, the large ΔES1-S2 would lead to comparatively slow IC from the S2 to S1 due to small horizontal Franck-Condon factors, and multiple ISC channels (S2↔Tn and S1↔Tn) are available to facilitate the population of triplet states in o-ClTPA. Likewise, the energy splitting between the T1 and T2 (ΔET1-T2) is large of 0.70 eV which would largely restrict the IC from the T2 to T1. Under the cooperation of large ΔES1-S2 and ΔET1-T2 values, as well as relatively large SOC constant between the S1 and T2 states (0.85 cm -1 ), the reverse ISC (RISC) from the higher T2 to S1 could successfully proceed by competing with the restrained IC from the T2 to T1. On the other hand, the small energy gap between the T2 and T1 (0.24 eV) is also favorable for the efficient RISC from the T1 to S1. Finally, we also simulate the absorption and emission spectra of the three isomers in the THF solution and crystal phases, respectively. As shown in Figure , the three isomers display similar absorption spectra on which the ICT band is also gradually redshifted from o-ClTPA to m-ClTPA and then to p-ClTPA, agreeing well with experimental results obtained in dilute THF solutions. Likewise, similar structureless emission bands appear on their simulated fluorescence spectra (Figure ), and the relative position of these structureless emission bands is consistent with experimental results. However, the simulated phosphorescence spectra of these isomers in the THF phase vary greatly from experimental results (Figure ). This is because the lowtemperature effect is not involved in the simulation process due to the calculation complexity and poor accuracy. Satisfyingly, the fluorescence and phosphorescence spectra simulated for the three isomers in the crystal phase are in good accordance with experimental results.