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

Area of Science:

• Organic Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Azahelicenes are a class of helical organic molecules with unique photophysical properties.
• Developing new synthetic routes to functionalized azahelicenes is crucial for exploring their potential applications.

Purpose of the Study:

• To synthesize novel indolo[2,3-c]carbazole (ICz), 9H-cinnolino[3,4-c]carbazole (CnCz), and indolo[2,3-k]- and -[3,2-a]phenanthridine (IP) derivatives.
• To investigate the photophysical properties, including fluorescence, acidochromism, and aggregation-induced emission (AIE) of the synthesized compounds.
• To elucidate the helical structure and electronic properties using experimental and computational methods.

Main Methods:

• Ortho fusion strategy employing Suzuki cross couplings, intramolecular nitrene insertions, diazo couplings, and Morgan-Walls cyclizations.
• Transformation of IPs into organoboranes and helicene conjugates with tetraphenylethylene derivatives.
• X-ray crystallographic analysis and quantum chemical calculations for structural and electronic characterization.

Main Results:

• Successful synthesis of ICz, CnCz, and various substituted IPs.
• Compounds exhibit fluorescence with large Stokes shifts, strong acidochromism, and good to excellent AIE.
• Helical structures confirmed; HOMO-LUMO gaps of 3.96-4.06 eV calculated.
• CnCz displays a small singlet-triplet inversion; relative pK_a values range from 6.65-9.55.

Conclusions:

• The developed ortho fusion strategy provides access to a diverse range of functionalized azahelicenes.
• The synthesized azahelicenes possess desirable photophysical properties for potential applications in optoelectronics and sensing.
• Further studies on structure-property relationships can guide the design of advanced helical materials.