Exactly Opposite Luminescence Behaviors of Aggregated Diketopyrrolopyrrole Derivatives Depending on Different Molecular Conformations

A new diketopyrrolopyrrole (DPP) derivative, DPPA-4-Caz, with a distorted molecular conformation has been synthesized, exhibiting aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties, with a solid-state fluorescence quantum yield of 0.286 and a 37 nm redshift in emission upon grinding. This compound can exhibit either AIE or aggregation-caused quenching effect, and these totally contradictory phenomena are attributed to its two polymorphic forms correspondingly. When the molecule bending is S-type, a loose intermolecular packing pattern is formed in the aggregation state, where the aromatic rings’ rotation is constrained, leading to luminescence. When bending is Z-type, the molecules are parallel-stacked and quench upon aggregation. Five other compounds with similar structures were also synthesized in order to investigate the effects of substituents and alkyl chain length on the properties of AIE and MFC. The results show that the electronic effect of the substituent plays a major role in the production of AIE, while the alkyl chain length has a greater effect on MFC properties.