Solvent-Triggered Aggregation-Induced Reversal and Enhancement of Circularly Polarized Luminescence in Chiral Salen Metalla-Macrocycles

Self-assembled metalla-macrocycles can serve as versatile platforms to prepare functional materials. Combined with a predictable structural design, they allow for the embedding of a broad range of properties. A series of dinuclear rings M₂R (M = Zn(II), Co(II/III), R = bis-salen macrocycle) is reported, which combine chirality with aggregation-induced emission (AIE) and chiroptical sign reversal. The modular system forms from three building blocks: i) tetraphenylethylene (TPE) backbones, ii) chiral salen coordination environments, and iii) chelated transition metal ions. The chiroptical properties are modulated by the choice of metal ion, solvent, and the degree of aggregation, with growing intermolecular stacking leading to an increase of the emission intensity. Aggregation of the macrocycles leads to intensification and inversion of the circular dichroism (CD) signal, and, for Zn₂R, of the circularly polarized luminescence (CPL), with |g_lum| rising by one order of magnitude. The metalla-macrocycles are characterized by NMR, FT-IR, and ESI-MS methods and three single-crystal X-ray structures. Dynamic light scattering (DLS), scanning electron microscopy (SEM), and computations are employed to examine the aggregates, showing helically twisted fibers whose handedness is controlled by the chiral component. Gaining stimuli-responsive control over chiroptical properties contributes to new opportunities for the development of smart optical materials and sensors.