Postsynthetic Bromination of a Dinuclear Aluminum Helicate and Its Chiroptical Properties.

Dinuclear aluminum-based triple-stranded helicates were functionalized through postsynthetic bromination with N-bromosuccinimide, enabling direct modification of a preassembled helical framework. By controlling the reagent equivalents, mono- and octadecabrominated derivatives were selectively obtained, while the helical structure was preserved. Increasing the degree of bromination induced red shifts in both the absorption and emission wavelengths, reflecting systematic modulation of the electronic structures. Transient absorption measurements revealed that the octadecabrominated derivative exhibited a triplet excited-state absorption (ESA) amplitude approximately five times greater than that of the other compounds. In addition, the complexes exhibit large Stokes shifts, and optical resolution of these complexes yields enantiomers exhibiting mirror-image circular dichroism and circularly polarized luminescence.