Understanding the AIE phenomenon of nonconjugated rhodamine derivatives through a new viewpoint of symmetry breaking

Solid-state emissive materials are of great importance due to their wide applications in the field of bioimaging and optoelectronic devices. The concept of aggregation-induced emission (AIE) provides a reliable strategy to afford solid-state emissive luminogens with the guidance of restriction of intramolecular motion (RIM) mechanism. Traditional AIE luminogens (AIEgens) exhibit enhanced emission intensity under increased viscosity or decreased temperature, attributed to the RIM mechanism. However, some luminogens were found without the classic viscosity or temperature effect, but still display AIE feature. In this work, we synthesized a series of cross-shaped and closed-form rhodamine-based unorthodox AIEgens. Their abnormal emission behavior in increased viscosity and cooling experiments suggested that their strong solid-state luminescence (up to 72.7%) was not ascribed to the conventional RIM working mechanism. Through a systematical investigation including single-crystal structural analysis, optical property study and theoretical calculations, we proposed that aggregation-induced symmetry breaking (AISB) may induce intramolecular charge transfer and act as a general principle to dominate the unique aggregate-state emission behavior of the nonconjugated rhodamine derivatives. More importantly, AISB not only provides a feasible approach for the design of unique rhodamine-based AIEgens, but also helps us deeply understand the change of luminescent behavior of materials upon aggregation with a wide perspective.