Skeletal rearrangement of [4]helicenes under acid conditions; dynamic chirality and improved properties by subsequent peripheral editing

Abstract

Cationic diaza[4]helicenes are attractive fluorescent scaffolds, yet access to unsymmetrical substitution patterns remains limited. Here, we report that chiral 1,13-dimethoxyquinacridinium (DMQA) derivatives undergo an acid-mediated skeletal rearrangement after hydride or methyl addition, enabling the sequential migration of methoxy groups and providing unprecedented 1,11-and 3,11-substituted cationic [4]helicene regioisomers. Following aerobic photooxidation, the rearranged helicenes were isolated and characterized by spectroscopy, electrochemistry, crystallography, and computation.Relative to the parent 1,13-DMQA scaffold, these core-edited systems display progressive planarization, altered redox behavior, and marked hypsochromic shifts in both absorption and emission, together with improved fluorescence efficiencies. The mono-rearranged platform further enables selective peripheral editing through SNAr substitution and regioselective demethylation/refunctionalization, giving access to a broad family of O-and N-substituted dyes with finely tunable optical properties. Importantly, relocation of one methoxy group away from the helical groove lowers strongly the configurational barrier, affording configurationally-labile helicenes with enantiomerization barriers of about 19 kcal•mol⁻¹.This dynamic chirality allows asymmetric induction studies through covalently bound chiral appendages and ion pairing with enantiopure TRISPHAT anion, leading to moderate diastereomeric enrichment and measurable ECD responses.