Size-dependent Stability and Luminescence Property in Organic Aggregates

Unveiling the dependence of stability and luminescence properties on the size of organic aggregates is crucial for biomedical and optoelectronic applications. Taking the helical hexaphenylsilole (HPS) and planar 3-(2-cyano-2-phenylethenyl-Z)-NH-indole (CPEI) aggregates of different sizes as examples, their stability and luminescent properties are investigated using multiscale modeling and thermal vibration correlation function approach. The size of stable aggregates formed depends on the molecular shape, with the critical aggregate sizes of 2.62 nm (2 molecules) and 2.87 nm (10 molecules) for helical HPS and planar CPEI, respectively. Their critical sizes for luminescence are 2.99 nm (6 molecules) and 2.87 nm (10 molecules), respectively. For HPS aggregates, as the size increases the luminescence is blue-shifted and enhanced owing to denser molecular packing until the size is large enough (4.66 nm, 20 molecules) the luminescence tends to remain unchanged; and thermal annealing makes these changes more pronounced. In contrast, the luminescent properties of CPEI aggregates are insensitive to aggregate size and thermal annealing treatment. These findings provide dynamic insights into the AIE mechanism and invaluable guidance for optimizing the size of AIE-based nanoparticles in practical applications.