Controlling light emitting properties in bis(pyrenyl)-di-imines by tuning the chemical functionality of the spacer group

Aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE) are dictated by the molecular aggregation which in turn is decided by the chemical features present in a molecule. The photophysical properties of bis(pyrenyl)-di-imines were investigated in solution as well as the solid state. It was observed that the light-emitting properties are dependent on the type of spacer group present in the compound. The presence of an alkylene spacer [ethylene (L3), butylene (L4), and hexylene (L5)] in bis(pyrenyl)-di-imines showed AIE at higher concentrations as well as in the solid state whereas the presence of a phenylene (L1) spacer and the absence of any alkyl/aryl spacer (L2) in the compound have resulted in ACQ in the solid state. Crystal structure analysis of L1 has explained its non-emissive nature in the solid state, while the AIE in the compounds, L3-L5, where flexible alkylene spacers are present, provided an idea about the solid-state arrangement of the molecules.