Discovery of and insights into one-photon and two-photon excited ACQ-to-AIE conversion via positional isomerization

Two positional isomers (regioisomers) through changing the substituted position of perylenetetracarboxylic diimide and benzanthrone moieties were designed and synthesized. These two regioisomers exhibit totally different aggregation behaviors. The meta (bay)-substituted product shows the aggregation-caused quenching feature as the fluorescence quantum yield in tetrahydrofuran-water mixtures is reduced by 80% with an increase in the water fraction of the medium from 0% to 60%. Interestingly, an aggregation-induced emission is observed for the ortho-substituted product as the fluorescence quantum yield is increased by nearly one order of magnitude with an increase in the water fraction. This fluorescence enhancement effect for the ortho-substituted product is because of the restriction of the intramolecular rotation between the perylene core and the benzanthrone substituent in the aggregated state. The ortho-substituted product also exhibits excellent two-photon absorption performance because the ortho-substitution retains the planarity of the perylene core and improves the intramolecular charge-transfer among the molecules. Moreover, the ortho-substituted product shows two-photon excited aggregation-induced emission, which has potential application in organic two-photon excited fluorescence imaging agents.