Triskelion-shaped AIEgens consisting of benzo[f], [g], and [h]coumarin units (2F, 2G, and 2H) were successfully synthesized through the intramolecular Ullmann coupling. These compounds adopt unique π-twisted helical geometries because of the fusion mode of the benzocoumarins, and the structural isomers have different molecular thicknesses and planarities. These compounds demonstrate remarkable AIEE behaviour in the aggregated states because molecular inversion is suppressed.

Abstract

Developing aggregation-induced emission luminogens (AIEgens) is a unique strategy for alleviating concentration quenching of π-conjugated organic compounds. This AIE behaviour can be induced not only in organic dyes with rotors but also in invertible π-conjugated systems. Herein, intramolecular Ullmann coupling was used to synthesize π-twisted triskelion-shaped AIEgens (2F, 2G, and 2H), in which benzo[f], [g], and [h] coumarins were fused into the benzene core to form a helical propeller shape. These structural isomers had unique molecular thicknesses and planarities depending on the fusion mode of the benzocoumarin. The racemic crystals, PPP and MMM, had different overlapping molecular configurations because of their twisted molecular frameworks. Compounds 2F, 2G and 2H demonstrated weak fluorescence emission in diluted solutions resulting from molecular motions caused by the isomerization process during molecular inversion. By comparison, aggregates exhibited enhanced emission intensity. These π-twisted triskelions exhibited the characteristics of AIEgens.