Insights into Molecular Packing Effect on the Emission Properties of Fluorenone-based Molecules in the Aggregate State

Aggregation-induced emission (AIE) has attracted huge attention during the past two decades, and preventing strong π-π interactions in the aggregate state has been widely accepted as a strategy to bring AIE effect. However, the relationship among molecular structure, molecular packing, and emission properties for AIE-active molecules remains unclear. Herein, we have systematically investigated the emission properties of four fluorenone derivatives, and found that changing the substituents and substitution positions results in significantly different emission characteristics. 27-DPA exhibits nearly quenched emission in both solution state and aggregated state, while its regioisomer, 36-DPA, is a typical AIE molecule. In addition, two-color polymorph-dependent emission can be found for the 36-TPA. X-ray single-crystal analysis was then conducted to understand these distinct emission properties. Our results emphasize the importance of molecular packing modes on the AIE effect, in particularly for molecules containing molecular rotors, and further show that formation of three-dimensional molecular packing network is an effective way to suppress the molecular motions to achieve AIE properties