The highest occupied molecular orbitals (HOMO) of both molecules were delocalized along the conjugated backbone, while the lowest unoccupied molecular orbitals (LUMO) were located mainly on the BBT core (Figures and). The energy gap between HOMO and LUMO in TPE-BBT was slightly larger than that of TPA-BBT, because the electron-donating ability of TPE moiety is weaker than that of TPA moiety. The weaker D-A interaction in TPE-BBT can intrinsically reduce the dark TICT effect and ensure a high QY. Besides, we also quantitatively analyzed the intramolecular motions of TPE-BBT (Figures and) and TPA-BBT (Figures and) in both the gas phase and crystalline phase by reorganization energy calculation. Normal modes in the low-frequency region are contributed by the changes of dihedral angels and are closely related to non-radiative decay. For both molecules, the contribution from dihedral angles largely decreased in the crystalline, compared with that in the gas phase.