Skeletal rearrangement of [4]helicenes under acidic 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 S_NAr 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 strongly lowers 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 an enantiopure TRISPHAT anion, leading to moderate diastereomeric enrichment and measurable ECD responses.