Tetraaza[7]-[15]helicenes Synthesized by Two-Step Strategy: Length-Controlled Chiral π-Systems Exhibiting Amplified...

Helicenes are chiral π-conjugated molecules whose properties strongly depend on their lengths. Systematic studies of these compounds have been limited by synthetic challenges. Here we report a concise two-step strategy (defined as the helicene-forming sequence from aminohelicene precursors) to access a homologous series of tetraaza[7]-[15]helicenes. Optical spectra converge beyond [11]H, defining a conjugation ceiling, while chiroptical responses amplify sharply, yielding |g_lum| up to 0.028. Fluorescence quantum yields show a nonmonotonic dependence, with [7]H and [15]H maintaining both high Φ_F (0.39 and 0.36) and large |g_lum|, resulting in outstanding, albeit semi-quantitative, CPL performance, with figures of merit reaching 0.010 and CPL brightness values of approximately 490. TD-DFT calculations attribute this amplification to the delayed alignment of electric and magnetic transition dipoles, while ¹H NMR shifts of inner protons provide an independent probe of structural reorganization within the helical cavity. Additionally, experiment and theory have consistently identified [11]H as the critical helicene length at which the framework undergoes a qualitative transition. Notably, the [15]helicene constitutes the longest helicene ever resolved into its enantiomers, underscoring the synthetic power of this modular approach. Importantly, our synthetic route is effective for constructing higher-order helicenes, offering a generalizable platform for length-controlled, heteroatom-containing helicenes. These findings establish long tetraazahelicenes as a rare platform where through-bond conjugation and through-space orbital coupling act cooperatively to govern photophysical and chiroptical properties.