Synthesis and chiral optical activity of a quadruple heterohelicene based on 1,4-dihydropyrrolo[3,2-b]pyrrole–picene hybrid

Abstract

The synthesis and structural elucidation of a diaza-quadruple helicene, comprising a double [6]helicene and two [5]helicene subunits fused around a 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) core with appended picene units, is reported. The target compound was accessed via a straightforward protocol involving a multicomponent reaction to form a tetraarylpyrrolo[3,2-b]pyrrole (TAPP) intermediate, followed by regioselective bromination and a twofold intramolecular direct arylation. X-ray crystallography confirmed the highly twisted, quadruple helical architecture, while separation of the enantiomers of the double anti-S₂-[6]helicene fragment by chiral HPLC enabled comprehensive chiroptical analysis via electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectroscopy. Density functional theory (DFT) calculations support the experimental findings by revealing pronounced π-expansion, a distinct charge-transfer character in the excited state, and enhanced alignment of electric and magnetic transition dipole moments, and allow assigning a PP helicity to the central helicenes for the enantiomer whose ECD spectrum has a negative Cotton effect at low energy. Consequently, the structural and chiroptical properties of the diaza-quadruple helicene are thoroughly characterized.