It is understandable because when the size of aggregates decreases, there will be more free molecules on the surface of the aggregates. They will undergo active molecular motions because of the lack of spacial restriction, resulting in an enhanced non-radiative decay and a relatively lower QY. Similarly, both powders and crystals of TPE-BBT demonstrated higher absolute QYs than its PLNPs and aggregates because of the higher degree of aggregation (Table ). The QYs of TPE-BBT in all states are higher than those of TPA-BBT (Table ). To gain deeper insights into the photophysical properties of TPE-BBT and TPA-BBT, their single-crystal structures were obtained and analyzed. Both TPE-BBT and TPA-BBT had a layer-by-layer packing mode (Figures and). The BBT cores of both molecules were immobilized by SS, NN, and S interactions, while there were more intermolecular interactions between two closely contacted TPE-BBT within one layer (d 1 d 14 ) than those in TPA-BBT (d 1 d 6 ) (Figures and). In TPE-BBT crystals, the molecular motions of alkyl chains were strongly restricted by C-HS, C-HN, and C-H interactions (d 4 d 10 and d 12 d 14 ). In comparison, the flexible alkyl chains in TPA-BBT were less affected by these interactions, which provided a liquid-like environment for active molecular motions. In addition, the comparison of their intermolecular interactions between two closely contacted molecules in two nearby layers revealed that both molecules had 5 short-range interactions and thus had a similar degree of restriction of molecular motions (Figures and). Considering the intermolecular interactions of two closely contacted molecules in different directions, it is clear that TPE-BBT boasted stronger intermolecular interactions, while TPA-BBT would undergo more vigorous molecular motions upon photoexcitation.