Expanded Helicenes: A General Synthetic Strategy and Remarkable Supramolecular and Solid-State Behavior

Area of Science:

• Organic Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Helicenes are polycyclic aromatic hydrocarbons with a unique helical structure.
• Exploring new helicene derivatives can lead to novel materials with tailored electronic and optical properties.
• Understanding aggregation behavior is crucial for designing self-assembling systems.

Purpose of the Study:

• To synthesize a new class of helicenes, termed "expanded helicenes", featuring alternating linearly and angularly fused rings.
• To investigate the aggregation behavior of these expanded helicenes in solution and solid states.
• To explore the influence of heteroatom annulation (selenophene) on solid-state packing.

Main Methods:

• A divergent synthetic strategy employing a three-fold, partially intermolecular [2+2+n] cycloaddition reaction.
• Dynamic proton nuclear magnetic resonance (1H NMR) spectroscopy to study racemization barriers.
• Solid-state and solution-state characterization techniques to investigate aggregation phenomena.

Main Results:

• Successful synthesis of several expanded helicene derivatives.
• Observation of an unusual homochiral, π-stacked dimer formation in one compound, with slow equilibrium kinetics.
• A selenophene-annulated expanded helicene demonstrated long-range π-stacking in the solid state, unlike its benzannulated counterpart.
• Dynamic 1H NMR confirmed low racemization barriers for the newly synthesized helicenes.

Conclusions:

• The developed synthetic strategy provides versatile access to novel expanded helicenes.
• Expanded helicenes exhibit unique aggregation properties and potential for self-assembly.
• Heteroatom incorporation can significantly influence solid-state packing and material properties.
• These compounds are stable under ambient conditions due to low racemization barriers, suitable for further applications.