N-Annulated [5]Helicenes: Syntheses, (Anti)Aromaticity and Properties

Area of Science:

• Organic Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Helicenes are chiral polycyclic aromatic hydrocarbons with unique three-dimensional structures.
• Exploring modifications to helicene frameworks can lead to novel electronic and optical properties.
• Nitrogen annulation offers a pathway to tune the electronic characteristics of π-conjugated systems.

Purpose of the Study:

• To synthesize and characterize nitrogen-annulated [5]helicene (NH5) and its derivatives.
• To investigate the impact of nitrogen incorporation on the electronic structure and properties of [5]helicene.
• To evaluate the charge transport capabilities of NH5 derivatives for potential electronic applications.

Main Methods:

• Synthesis of NH5 via cycloisomerization reactions.
• Preparation of dibenzo-extended derivatives using Scholl-type cyclodehydrogenation.
• Spectroscopic (absorption, fluorescence) and electrochemical characterization.
• Measurement of charge carrier mobility using space-charge-limited current (SCLC) techniques.

Main Results:

• Successful synthesis of saddle-like NH5 with an antiaromatic azepine core and enhanced N-C π-orbital overlap.
• NH5 exhibits a less stabilized HOMO, smaller HOMO-LUMO gap, red-shifted absorption/fluorescence, and paratropic ring current.
• NH5 derivatives demonstrate hole mobility up to 10⁻³ cm² V⁻¹ s⁻¹, exceeding that of N-annulated PAHs like perylene.

Conclusions:

• Nitrogen annulation effectively modifies the electronic structure and optical properties of [5]helicene.
• The antiaromatic azepine core in NH5 contributes to tunable aromaticity and enhanced electronic communication.
• NH5 derivatives show promising hole transport properties, suggesting potential in organic electronics.