Bis-subporphyrin-like boron(iii) complexes derived from indolo[2,3-a]carbazole-based cyclic bis-BODIPYs exhibiting persistent helical chirality

Abstract

Herein, we report the synthesis of 11,12-dihydroindolo[2,3-a]carbazole (InC) 1 incorporating expanded porphyrinoid 8, and its boron(III) complexes (8·BF₂ and 8·BF). These macrocycles exhibit a ‘figure-eight’ conformation that exists as a pair of helical enantiomers (P,P and M,M), confirmed by X-ray crystallographic analysis. The ‘figure-eight’ structures are stabilized by intramolecular hydrogen bonds such as NH⋯N and NH⋯F interactions. The synthesized macrocycles demonstrate excellent stability under ambient conditions. Further, the helically-locked structure 8·BF exhibited closely held InC units with a distance of 6.17 Å and enabled facile optical resolution using chiral HPLC. Circular dichroism spectroscopy of enantiopure 8·BF shows the Cotton effect at 692 nm extending to the near-IR region with Δε up to 150 M⁻¹ cm⁻¹. The chiral induction of racemic macrocycle 8 was employed using enantiopure mandelic acids.