Dehydro[12]- and [18]annulene-Fused Ball-Shaped Ruthenium Complex Oligomers: Synthesis, Aromatic/Antiaromatic Effect, and Symmetry for Near-Infrared Optical Properties

Abstract

The appropriate arrangement of near-infrared (NIR) chromophores allows for the modification of the peak wavelength in the NIR region and efficient use of NIR light. However, the preparation of novel NIR chromophores using simple procedures remains a formidable challenge. Herein, we report the synthesis of ball-shaped ruthenium complex oligomers. The metal complexes can be synthesized in a single step and interact strongly with NIR light. Alkyne-substituted low-symmetry ball-shaped ruthenium complexes were synthesized and subjected to Eglinton coupling to obtain dehydro[12] and [18]annulene-fused dimers and trimers. Fine-tuning of the reaction conditions led to the selective synthesis of the target oligomers. NMR spectroscopy confirmed that the 18π-aromatic and 12π-antiaromatic properties of the annulene influenced the ruthenium complex chromophore, and magnetic circular dichroism spectroscopy showed changes in the electronic structure of their excited state owing to molecular-symmetry differences. The absorption coefficient in the NIR region of the absorption spectra of the oligomers increased significantly, supporting the efficient use of light by oligomerization. The formation of oligomers using ball-shaped metal complexes is a simple and effective strategy for controlling NIR optical properties.

Keywords: Annulene; Aromaticity; Ball-shaped metal complex; NIR dyes; Symmetry.