π-Extended 9b-Boraphenalenes: Synthesis, Structure, and Physical Properties

Area of Science:

• Organic Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Boraphenalenes, phenalenyl analogs with a boron atom, are of interest for their electronic and solid-state properties.
• Synthesizing complex boraphenalene structures is challenging due to a lack of efficient methodologies.

Purpose of the Study:

• To develop a novel synthetic route for constructing fully fused 9b-boraphenalene skeletons.
• To investigate the electronic properties and solid-state behavior of newly synthesized boraphenalenes.

Main Methods:

• Tandem borylative cyclization of C3-symmetric dehydrobenzo[12]annulenes.
• Electrochemical analysis to study redox properties.
• Single-crystal X-ray diffraction and thermal annealing to observe phase transitions.
• Computational calculations for intermolecular electron coupling.

Main Results:

• A new class of fully fused boron-atom-embedded polycyclic hydrocarbons with 9b-boraphenalene skeletons was synthesized.
• The compounds exhibit strong electron-accepting characteristics and reversible two-step redox behavior.
• A benzo[b]fluorene-fused derivative displayed a stepwise single-crystal-to-single-crystal (SCSC) phase transition upon thermal annealing.
• SCSC transition improved charge transport properties, and observed photoconductivity in single crystals.

Conclusions:

• The tandem borylative cyclization offers a viable method for constructing 9b-boraphenalene frameworks.
• These boron-containing polycyclic hydrocarbons possess tunable electronic properties and exhibit unique solid-state transformations.
• The observed SCSC transition and improved charge transport highlight potential applications in organic electronics.