When aggregates light up or fade out

Abstract

The aggregation phenomenon in various solvent systems is addressed herein using a model unsymmetrical A′–π–D–(π–A)2 chromophore based on a central triphenylamine donor (D) and one/two NO2(A′)/diphenyltriazine(A) peripheral acceptors. The nonplanar arrangement of the nitrophenyl moiety accounts for a diminished ICT in the solid state, while substantial electron redistribution upon excitation and formation of a charge-separated excited state is seen in solution. Low-energy linear HOMO → LUMO or high-energy V-shaped HOMO → LUMO+1 transitions from the central triphenylamino donor to the nitrophenyl and diphenyltriazine acceptors can be selectively employed based on the environment. However, when excluding the nitro group, non-emissive large aggregates are formed, which contrasts with differently shaped and emissive aggregates of the nitro-substituted system observed in various good/poor solvent pairs such as THF/water, DMF/water, THF/hexane, benzene/hexane, and DCM/hexane. Based on the solvent pairs, both AIE and ACQ phenomena can be observed, including transitional fluorescence in the DCM/hexane system. The presence of the nitro group and the interplay of the solvent polarity, miscibility, excited-state dipolarity, and restriction of intramolecular motion determine whether the aggregates emit or quench.