Azulene-Embedded [n]Helicenes (n=5, 6 and 7)

Area of Science:

• Organic Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Azulene is a non-benzenoid aromatic molecule with distinct structural and electronic characteristics.
• Helicenes are chiral polycyclic aromatic hydrocarbons known for their unique optical and electronic properties.
• Integrating azulene into helicene structures offers a pathway to novel materials with tailored functionalities.

Purpose of the Study:

• To synthesize and characterize a series of azulene-embedded [n]helicenes ([n]AzHs) using a bottom-up approach.
• To investigate the chiroptical properties and enantiomeric behavior of these novel compounds.
• To explore the structure-property relationships influenced by the incorporation of the azulene moiety.

Main Methods:

• Bottom-up synthetic strategy to construct azulene-embedded [n]helicenes ([n]AzHs, n=5, 6, 7).
• Isolation and characterization of P- and M-enantiomers, particularly for [7]AzHs via recrystallization.
• Spectroscopic analysis to determine photophysical properties, including optical energy gap and emission characteristics.

Main Results:

• Successful synthesis of [5]-, [6]-, and [7]AzHs, featuring an azulene unit fused with benzene rings.
• Isolation of enantiomerically pure P- and M-[7]AzHs.
• Observation of unique properties such as a low-lying first electric state (S1), small optical energy gap, and anti-Kasha emission.
• [6]- and [7]AzHs demonstrated significant chiroptical responses with high absorption dissymmetry factors (|gabs| ≈ 0.02).

Conclusions:

• The synthesized azulene-embedded helicenes expand the library of non-benzenoid helicenes.
• The azulene moiety imparts unique photophysical properties, including anti-Kasha emission and strong chiroptical responses.
• This work provides valuable insights into structure-property relationships in complex aromatic systems.