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

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.