Azulene-Based π-Functional Materials: Design, Synthesis, and Applications

Abstract

Conspectus Azulene, an isomer of naphthalene, is a molecule of historical interest for its unusual photophysical properties, including a beautiful blue color derived from the narrow HOMO–LUMO energy gap and anti-Kasha fluorescence from S2 to S0. More recently, it has attracted increasing attention for its novel electronic structure, including an electron-rich five-membered ring and an electron-deficient seven-membered ring with a dipole moment of 1.08 D resulting from resonance delocalization, its different reactivities at odd and even positions, and its stimuli-responsive behavior. As a key building block, azulene has been used in various fields because of its unique physicochemical properties. Recent studies have demonstrated the great potential of azulene for constructing advanced organic materials. However, exploring azulene-based materials has long been hindered by challenges in molecular design and synthesis. Most of the reported azulene-based materials have the azulene unit incorporated through the five-membered ring or seven-membered ring. Creating azulene-based novel building blocks for optoelectronics and using 2,6-connected azulene units to construct conjugated polymers that can adequately utilize the “donor–acceptor” structure of azulene remained underexplored before our contributions. Besides, for most azulene-fused polycyclic aromatic hydrocarbons (PAHs) and heteroaromatics, the azulene substructures were created during later synthesis stages, and the use of azulene derivatives as starting materials to design and synthesize PAHs and heteroaromatics intelligently is still limited. In this Account, we summarize our efforts on the design, synthesis, and applications of azulene-based π-functional materials. Our studies start with the creation of novel π-conjugated structures based on azulene. The design strategy, synthesis, and optoelectronic performance of the first class of azulene-based aromatic diimides, 2,2′-biazulene-1,1′,3,3′-tetracarboxylic diimide (BAzDI) and its π-extended and π-bridged derivatives, are presented. Notably, antiparallel stacking between adjacent azulene units derived from azulene’s dipole was observed in single crystals of BAzDI and its derivatives. Besides, we developed an azulene-fused isoindigo analogue, azulenoisoindigo, which combines the merits of both isoindigo and azulene, including reversible redox behavior and reversible proton responsiveness. Then we discuss our contributions to the design and synthesis of 2,6-azulene-based conjugated polymers. By incorporation of 2,6-connected azulene units into the polymeric backbone, two conjugated polymers with high organic field-effect transistor (OFET) performance were developed. Two 2,6-azulene-based polymers with proton responsiveness and high electrical conductivity upon protonation were also provided. We also discuss our recent studies on azulene-based heteroaromatics. Two azulene-fused BN-heteroaromatics were designed and synthesized, and they exhibited a selective response to fluoride ion and unexpected deboronization upo...

Efficient Access to Arylated Aza‐ullazines by Regioselective Functionalization of their Pyridine Ring by H−Li Exchange and Electrophilic Substitution

“…Product 10 a was obtained as a yellow solid (39 mg, 78 %) with a melting point up to 260°C. FT-IR ν (cm 5,8-diphenyl-1,3-bis(phenylethynyl)indolizino [6,5,4,3ija] [1,6] naphthyridine (10 b). In a dry 10 mL microwave vial, diethylamine (0.6 mL) and DMF (1.4 mL) were introduced.…”

“…The expected compound 1 was obtained as a yellow solid (21.5 g, 89 %). 1 H-NMR (400 MHz, CDCl 3 ) δ = 8.31 (s, 2H), 5.07 (s, 2H). (2).…”

“…The residue was then purified on silica gel chromatography (cyclohexane/EtOAc, 80/20) and product 2 was obtained as a yellow solid (1.00 g, 83 %). 1 H-NMR (400 MHz, CDCl 3 ) δ = 8.77 (s, 2H), 6.73 (t, J = 2.2 Hz, 2H), 6.42 (t, J = 2.2 Hz, 2H).…”

“…The residue was then purified on silica gel chromatography (cyclohexane/EtOAc, 95/5) and product 3 was obtained as a yellow oil which solidified upon standing (1.70 g; 99 %). 1 H-NMR (400 MHz, CDCl 3 ) δ = 8.75 (s, 2H), 7.43-7.48 (m, 4H), 7.34-7.38 (m, 6H), 7.32 (t, J = 2 Hz, 2H), 6.44 (t, J = 2 Hz, 2H). 13 [6,5,4,3-ija] [1,6]naphthyridine (4).…”

“…Ullazine is a unique planar nitrogen-containing polycyclic aromatic hydrocarbon (PAH) with 16 π-electron system, which holds great promise in organic functional materials such as polycyclic heteroaromatic arenes, [1,2] dye-sensitized solar cells [3][4][5][6][7] and organic field-effect transistors (OFET). [8][9][10] Ullazine has been first synthesized by Balli and Zeller in 1983.…”