Synthesis, Stability, and X-Ray Crystallographic Structure Analysis of Spiro[1H-azulenium-1,1′-cycloalkane] Ions

Catalyst-free assembly of giant tris(heteroaryl)methanes: synthesis of novel pharmacophoric triads and model sterically crowded tris(heteroaryl/aryl)methyl cation salts

A series of giant tris(heteroaryl)methanes are easily assembled by one-pot three-component synthesis by simple reflux in ethanol without catalyst or additives. Diversely substituted indoles (Ar1) react with quinoline aldehydes, quinolone aldehydes, chromone aldehydes, and fluorene aldehydes (Ar2CHO) and coumarins (Ar3) in 1:1:1 ratio to form the corresponding tris(heteroaryl)methanes (Ar1Ar2Ar3)CH along with (Ar1Ar1Ar2)CH triads. A series of new 2:1 triads were also synthesized by coupling substituted indoles with Ar2CHO. The coupling reactions could also be carried out in water (at circa 80 °C) but with chemoselectivity favoring (Ar1Ar1Ar2)CH over (Ar1Ar2Ar3)CH. The molecular structure of a representative (Ar1Ar2Ar3)CH triad was confirmed by X-ray analysis. Model tris(heteroaryl/aryl)methylium salts were generated by reaction with DDQ/HPF6 and studied by NMR and by DFT and GIAO-DFT.

Construction of Polycyclic π-Conjugated Systems Incorporating an Azulene Unit Following the Oxidation of 1,8-Diphenyl-9,10-bis(phenylethynyl)phenanthrene

Polycyclic aromatic hydrocarbons (PAHs) that incorporate either heptagons or pentagons consist of non-planar molecular structures with unusual optoelectronic properties, and the design of a relatively simple and efficient method to construct these highly fused π-conjugated systems with odd-membered rings is in high demand. This work describes the use of silver(I) cations to promote the efficient synthesis of azulene-embedded PAH 2, which is a structural isomer of tribenzo[fg,ij,rst]pentaphene 3, via tandem oxidative transannulation between the phenyl and arylethynyl moieties. This method involves a carbophilic interaction of the silver(I) cation with the acetylene units, which facilitates an electron transfer in the initial step. The synthesized PAH 2 and the protonated cation 2 H⋅BF₄ were fully characterized by X-ray crystallographic analysis, electronic absorption, electrochemical measurement, and quantum chemical calculation. The azulene-embedded PAH 2 exhibited a low-energy absorption band and amphoteric redox events, which were characterized as non-alternant characteristics originating from the azulene unit.

Synthesis and properties of azulene-functionalized BODIPYs

A small series of BODIPY derivatives incorporating azulene were synthesized via Suzuki–Miyaura cross-coupling reaction. There has better conjugation through 2-position as building block.

Synthesis of substituted azulenes via Pt(II)-catalyzed ring-expanding cycloisomerization

Substituted azulenes, valuable structures for electronic devices and pharmaceuticals, have been synthesized by the platinum(II)-catalyzed intramolecular ring-expanding cycloisomerization of 1-en-3-yne with ortho-disubstituted benzene. This novel method provides an alternative route for the efficient synthesis of substituted azulenes. The reaction mechanism of selected catalytic transformations was explored using density functional calculations.

Conjugated oligomers incorporating azulene building blocks – seven- vs. five-membered ring connectivity

The properties of isomeric azulene derivatives based on 7- versus 5-membered ring substitution were examined by the synthesis and characterization of well-defined electroactive oligomers.

Stimuli-responsive azulene-based conjugated oligomers with polyaniline-like properties

Novel azulene building blocks, prepared via the cycloaddition of thiophene-S,S-dioxides and fulvenes, allow for incorporation of the seven-membered ring of the azulene nucleus directly into the backbone of conjugated materials. This unique mode of incorporation gives remarkably stable, stimuli-responsive materials upon exposure to acid. This simple doping/dedoping strategy provides for effective optical band gap control and on/off fluorescence switching, reminiscent of polyaniline.