Azulene-Fused Acenes

Area of Science:

• Organic Chemistry
• Materials Science
• Physical Chemistry

Background:

• Non-alternant, non-benzenoid polycyclic hydrocarbons (PHs) possess distinct electronic properties compared to their all-benzenoid counterparts.
• Azulene-fused acenes represent a novel class of π-conjugated molecules with potentially unique characteristics.

Purpose of the Study:

• To synthesize and characterize novel azulene-fused acene molecules (1, 2, and 3).
• To investigate their electronic properties, stability, and aromaticity.
• To explore the behavior of their dicationic forms, particularly diradical character.

Main Methods:

• Synthesis of three azulene-fused acene molecules.
• X-ray crystallographic analysis for structural determination.
• Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR) spectroscopy for electronic property analysis.
• Theoretical calculations for electronic structure elucidation.

Main Results:

• Successful synthesis of azulene-fused acenes (1, 2, 3), isoelectronic to pentacene, hexacene, and heptacene.
• X-ray, NMR, and computational data indicate localized aromaticity in six- and five-membered rings, with non-aromatic seven-membered rings.
• Molecules exhibit enhanced stability over corresponding acenes.
• Chemical oxidation formed dications; dication of molecule 3 showed significant open-shell diradical character (30.2%).

Conclusions:

• Azulene-fused acenes combine properties of both azulene and acenes, offering improved stability.
• The observed diradical character in the dication of molecule 3 is attributed to the aromaticity recovery of specific structural units.
• These findings open new avenues for designing π-conjugated systems with tunable electronic and magnetic properties.