Near-Infrared-Absorbing Aminoxyl Radical-Boron Complexes with Reduced Visible Coloration

We report the synthesis and optical properties of new aminoxyl radical-boron complexes bearing dimethylamino and methoxy substituents as electron-donating groups. The target cationic radical complexes were obtained by oxidation of the corresponding neutral aminoxide-boron precursors. Electron spin resonance spectroscopy and density functional theory calculations revealed that the nature of the electron-donating substituent influences the spin density distribution, promoting delocalization of the unpaired electron over the conjugated framework. The radical complexes exhibit intense absorption in the near-infrared (NIR) region, with absorption maxima at 1210 nm for the dimethylamino-substituted complex and 993 and 1080 nm for the methoxy-substituted complex. Compared with previously reported analogues bearing a diarylamino-type electron-donating group, the present complexes show reduced visible coloration owing to hypsochromic shifts of the secondary absorption bands in the visible region, resulting in improved visible transparency. These results demonstrate that substituent tuning provides an effective strategy for balancing strong NIR absorption with reduced visible coloration in aminoxyl radical-boron complexes.