Biophotothermal applications boosted by intramolecular motions from aggregates

Motion is one of the basic attributes of matter. Research on molecular motion has a long history, and most studies have been conducted in the isolated state, gas phase, or solution state. Molecular motions in aggregate states are always hampered by steric effects. For example, the aggregation-induced emission phenomenon can be attributed to the idea that the motion of luminogens is more easily prohibited in the aggregate state than in the solution state. Because molecular motions may noradiatively annihilate excitons, emissions are enhanced upon aggregation owing to the enhanced suppression of such a process. However, photothermal conversion can be achieved based on the intramolecular motions in the aggregates, which is extremely desirable in the energy, seawater desalination, and nanomedicine fields. Recently, the research groups of Tang and Ding have achieved remarkable progress in aggregate-state intramolecular motions. They proposed "intramolecular motion-induced photothermy" and "intramolecular bond stretching vibration-induced photothermy" mechanism. However, robust intramolecular motions can proceed even in an aggregate state, thereby achieving efficient photothermal conversion by engineering the molecular flexibility and aggregate-state behavior. The above methods are more direct and efficient when compared with the conventional photothermal conversion approaches, including introducing photoinduced electron transfer and energy transfer between fluorogens and quenchers and inducing intermolecular p-p stacking effects. This research shows value in both theoretical research and practical application. This research has also attracted the attention of researchers belonging to other groups, resulting in more interesting studies. For example, the research group of Peng from the Dalian University of Technology observed that barrier-free rotation of trifluoromethyl group can be achieved by introducing trifluoromethyl groups into BODIPY to effectively transform the energy absorbed by a molecule into heat in the aggregate state. The photothermal conversion efficiency was recorded to be as high as 88.3%, fully proving the huge potential of realizing photothermal conversion by utilizing the intramolecular motion in the aggregate state. In this review, we will summarize the recent advances based on the intramolecular motion in the aggregate state. Such motion can be easily achieved by introducing motion factors into molecules via molecular engineering. The motions of functional groups, including the rotations of phenyl rings and trifluoromethyl groups and the twisting of double bonds, are introduced. Additionally, twisted intramolecular charge transfer can occur along with environmental changes to cause motions between the electronic donor and acceptor units; this is also a powerful phenomenon in the aggregate state. However, the additional factor of molecular gap obtained by decreasing the intermolecular interactions must be considered because such motions require a certain space. In contrast, the intramolecular bond stretching vibration occurs within a short distance. It is spontaneous, vigorous, and less sensitive to the external environmental constraints. Thus, compounds with high photothermal efficiencies can be obtained by considering the above factors in molecular design. Furthermore, designing polymers when considering these factors can improve the practical photothermal conversion because such polymers exhibit a high light-harvesting ability owing to large molecular conjugation. The biological applications in photoacoustic imaging, thermal infrared imaging, and photothermal therapy have been well studied based on the vigorous molecular motions and excellent photothermal conversion in the aggregate state. With this review, we hope that such topics will draw more attention from readers belonging to multidisciplinary fields, and more interesting research and influential applications will be stimulated in the future.