Synthesis of Anthraquinone Mono‐ and Diboron Complexes with Near‐Infrared Panchromatic Absorption

Anthraquinone boron complexes bearing β-iminoenolate ligand(s) exhibit strongly substituent-dependent absorption properties. Unsubstituted monoboron and diboron complexes show sharp UV–Vis–NIR absorption spectra resulting from increased molecular rigidity induced by boron coordination. In contrast, dimethylamino-substituted complexes display near-infrared (NIR) panchromatic absorption, attributed to enhanced intramolecular charge transfer and multiple electronic transitions.

Abstract

Anthraquinone monoboron complexes 3 and 4, possessing a donor–acceptor (D–A) structure, and diboron complexes 7 and 8, featuring a donor–acceptor–donor (D–A–D) structure, were readily synthesized in two steps from commercially available reagents. These boron complexes adopt a β-iminoenolate structure and their absorption properties are markedly affected by the type of the substituent groups. The unsubstituted monoboron complex 3 (fwhm = 3,950 cm⁻¹) and diboron complex 7 (fwhm = 2,510 cm⁻¹) exhibited sharp UV–Vis–NIR absorption spectra, which can be attributed to increased molecular rigidity induced by boron coordination. In contrast, the dimethylamino-substituted monoboron complex 4 (fwhm = 7,580 cm⁻¹) and diboron complex 8 (fwhm = 8,770 cm⁻¹) showed broad absorption bands due to the enhanced intramolecular charge-transfer (ICT) character. Dimethylamino-substituted monoboron complex 4 and diboron complex 8 exhibited panchromatic absorption ranging from 400 nm to 1,000 nm and 400 nm to 1,250 nm, respectively. The panchromatic absorption of the dimethylamino-substituted boron complexes 4 and 8 is attributed not only to the enhanced ICT character but also to the combination of multiple transitions.

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