# Tetraaza[7]–[15]helicenes Synthesized by Two‐Step Strategy: Length‐Controlled Chiral π‐Systems Exhibiting Amplified Circularly Polarized Luminescence

## TL;DR

Scientists developed a new method to create chiral molecules that emit polarized light, which could be useful in advanced optical technologies.

## Contribution

A two-step synthesis strategy enables the creation of length-controlled tetraazahelicenes with amplified circularly polarized luminescence.

## Key findings

- The two-step synthesis allows for the production of tetraaza[7]–[15]helicenes with tunable photophysical and chiroptical properties.
- The [15]helicene is the longest helicene ever resolved into enantiomers, demonstrating the method's synthetic power.
- The [11]helicene marks a structural transition point, influencing photophysical behavior and chiroptical amplification.

## Abstract

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.

A modular two‐step strategy provides tetraaza[n]helicenes (n = 7–15) as a homologous series with tunable photophysical and chiroptical properties. Extension beyond n = 11 induces pronounced through‐space π–π interactions, narrowing the energy gap and amplifying circularly polarized luminescence.