Role of molecular packing in RTP features of positional isomers: The case study of triimidazo-triazine functionalized with ethynyl pyridine moieties

Organic materials characterized by multi-component emissive behavior including RTP features are extremely desirable for various applications. Frequently, long lasting emissions of solids are originated from intermolecular interactions whose role is far from being fully understood. In this context, positional isomers with similar molecular properties but different packing arrangement can be a useful tool to get a deeper comprehension of the mechanisms involved in the solid state emissive behavior. Here, the results obtained on two derivatives of cyclic triimidazole (TT) functionalized with a pyridin-2-yl or 4-yl ethynyl group are presented and interpreted through spectroscopical, structural and computational studies. The two isomers are hardly emissive in solution but become good emitters in blended PMMA films displaying almost overlapping fluorescence and phosphorescence. In solid-state, two additional lower energy phosphorescences are activated through the establishment of either π-π stacking or synergic π-π/hydrogen bond interactions. Stronger aggregated RTP features are observed in the pyridin-2-ylethynyl derivative which displays tighter π-π stacking interactions.