A strategy for the molecular design of aggregation-induced emission units further modified by substituents

Aggregation-induced emission (AIE) molecules with strong luminescence in aggregated states have attracted persistent attention in recent years. The development of new structures of AIE units and their further modification with functional groups to satisfy more specialized applications are important research fields. However, studies on the molecular design associated with the functional modification of AIE units have not been reported to date. Herein, we designed and synthesized 13 aryl-substituted pyrrolo[3,2-b]pyrrole derivatives. Among these compounds, DPP-1CN, DPP-1MF, DPP-1MF-2Me, and DPP-1MF-2IP with electron-withdrawing groups on the phenyl groups at the 1,4-positions and electron-donating groups on the phenyl groups at the 2,5-positions of pyrrolo[3,2-b]pyrrole core showed AIE characteristics, whereas others showed aggregation-caused quenching (ACQ) characteristics. The absorption and photoluminescence (PL) emission spectra indicated that the AIE compounds exhibited weak intramolecular charge transfer (ICT) absorption and possessed large Stokes shifts, whereas the ACQ derivatives showed obvious ICT absorption. Density functional theory (DFT) calculation results suggested that the HOMOs and LUMOs of the four AIE compounds were spatially isolated that weakened the twisted intramolecular charge transfer (TICT) effect and minimized fluorescence reabsorption in the aggregated states. Single-crystal analysis also confirmed that AIE properties could be realized by the suppression of both the TICT effect and the close p p interactions in the aggregated state. These results are beneficial for understanding the relationship between molecular structure and AIE properties. The resulting structural information provides the basis for the future rationalization of functional modification of the AIE materials.