Synthesis and Properties of Boron-Containing Heteromerous Bistricyclic Aromatic Enes: Structural Effects on Thermodynamic Stability and Photoreactivity

Bis(tricyclic) Aromatic Enes That Exhibit Efficient Fluorescence in the Solid State

We report herein that bis(tricyclic) aromatic enes (BAEs) consisting of 6-6-6-membered frameworks such as acridine, xanthene, thioxanthene, and thioxanthene-S,S-dioxide act as a new class of organic luminophores that exhibit blue-to-green fluorescence in the solid state and in polymer film with good to excellent quantum yields. The BAEs were prepared by the palladium-catalyzed double cross-coupling reaction of phenazastannines or 10,10-dimethyl-10H-phenothiastannin with 9-(dibromomethylene)xanthene, 9-(dibromomethylene)thioxanthene, or 9-(dibromomethylene)-9H-thioxanthene-10,10-dioxide. Microcrystals or powder samples of the BAEs exhibited brilliant fluorescence with good to high quantum yields (Φ = 0.45-0.88). Furthermore, more efficient emission of blue-to-green light (Φ = 0.59-0.91) was observed for the BAEs dispersed in the poly(methyl methacrylate) (PMMA) films. Density functional theory (DFT) calculations suggest that the photo-absorption of the (thio)xanthene moiety-containing BAEs proceeds via π-π* transitions, whereas the optical excitation of 10,10-dioxido-9H-thioxanthene moiety-containing BAEs involves an intramolecular charge transfer from the acridine/thioxanthene part to the electron-accepting 10,10-dioxido-9H-thioxanthene moiety.

Controlling the thermodynamic stability of conformational isomers of bistricyclic aromatic enes by introducing boron and silicon atoms

Overcrowded bistricyclic aromatic enes (BAEs) have several conformational isomers, including twisted and anti-folded conformers. These compounds change color depending on their conformation because each isomer exhibits distinct absorption bands. In this study, we introduced several heteroatoms such as boron and silicon into BAEs to control the energy difference between the twisted and anti-folded conformers, thereby achieving chromic properties. The heteroatom-containing BAEs showed absorption bands attributable to both twisted and anti-folded conformers, suggesting the coexistence of these conformers in solution. Although the solution appeared bluish, yellowish crystals were obtained by recrystallization. Single-crystal X-ray diffraction analysis confirmed that the yellowish crystals existed in the anti-folded conformation. The color of these crystals changed from yellowish to bluish upon heating and grinding, demonstrating thermochromism and mechanochromism. In addition, when cyanide ions were added to the BAE solution, the color changed from bluish to colorless, indicating the chemochromic properties of the BAEs.

Twisted diphenoquinones fused with thiophene rings: thiophene analogs of bianthrone

The twisted conformer of bistricyclic aromatic enes (BAEs) has a small HOMO-LUMO gap owing to the twisted double bond. In this study, we synthesized diphenoquinones fused with thiophene rings as a new twisted conformer-predominant BAE. They exhibited deep LUMO energy levels and apparent n-type semiconductor properties.

Comment on "Asymmetric Bistricyclic Aromatic Enes"

This Correspondence argues against the use of the adjectives "symmetrical" and "asymmetric" in the recent publication entitled "Fluorenylidene-Cyclopentadithiophene Based Asymmetric Bistricyclic Aromatic Ene Compounds: Synthesis and Substituents Effects", by Beibei Xiao, Yongrui Yang, Shengnan Chen, Ye Zou, Xing Chen, Kanglei Liu, Nan Wang, Yali Qiao, and Xiaodong Yin (Chem. Eur. J., 2023, 29, e202301055).[1].