# Fluorescent Carbazole‐Derived Aza[5]Helicenes: Synthesis, Functionalization, and Characterization

## TL;DR

This paper describes the synthesis and properties of fluorescent carbazole-derived helicenes, which show strong fluorescence and unique structural features.

## Contribution

The study introduces new synthetic methods and characterizes novel fluorescent helicene compounds with aggregation-induced emission.

## Key findings

- The compounds exhibit large Stokes shifts and strong acidochromism.
- They show aggregation-induced emission, especially when conjugated with tetraphenylethylene.
- Quantum chemical calculations revealed HOMO–LUMO and S1–T1 energy gaps.

## Abstract

5,8‐Dihydroindolo[2,3‐c]carbazole (ICz), 9H‐cinnolino[3,4‐c]carbazole (CnCz), and variously alkyl‐, alkenyl‐, and aryl‐substituted indolo[2,3‐k]‐ and ‐[3,2‐a]phenanthridines (IPs) were synthesized using an ortho fusion strategy with Suzuki cross couplings, intramolecular nitrene insertions, diazo couplings, and Morgan–Walls cyclizations as key reactions. The IPs were additionally transformed into organoboranes and helicene conjugates with tetraphenylethylene derivatives. The compounds fluoresce with large Stokes shifts, exhibit strong acidochromism, and show a good to excellent aggregation‐induced emission. Their helical structure was elucidated by x‐ray crystallographic analysis and by quantum chemical calculations. HOMO–LUMO gaps of 3.96−4.06 eV and S1‐T1 gaps were calculated, with CnCz showing a small singlet‐triplet inversion. Relative pK
a values of 6.65−9.55 were estimated for the different types of azahelicenes.

Variously substituted indolophenanthridines, indolocarbazoles, and cinnolinocarbazoles synthesized by ortho fusion from teraryl precursors show significant Stokes shifts, especially for the protonated states of indolophenanthridines and cinnolinocarbazole. These compounds combine helical and axial chirality and show prominent aggregation‐induced emission, which is considerably enhanced in conjugates with tetraphenylethylene.
.