Enhancement of the Lowering Effect on Energy Levels of LUMO by the Formation of B-N Dative Bond for Near-infrared Light Absorption Properties Based on 1,3,4,6,8,9 b-Hexaazaphenalene

Abstract

We illustrate the development of 1, 3, 4, 6, 8, 9b-hexaazaphenalene (6AP) derivatives which exhibit excellent near-infrared (NIR) light absorption properties and deep LUMO levels. The electron-withdrawing character of the 8-nitrogen atom in the 6AP scaffold was enhanced by the introduction of a B−N dative bond. This formation of a boron-nitrogen Lewis acid-base pair succeeded in strengthening the electron-withdrawing ability of the 8-nitrogen atom, which induced the low-lying LUMO levels of the resultant boron complexes. The data from UV−vis−NIR absorption spectroscopy and cyclic voltammetry suggested that these deep LUMO levels were the origins of the narrow HOMO−LUMO gap caused by selectively lowering the LUMO energy level. Furthermore, the enhancement of the molar absorption coefficients was observed in the boron complex compared to other azaphenalene derivatives in the previous reports. From theoretical calculations, the electron-withdrawing character of the 8-nitrogen atom induced the weak but substantial delocalization of the LUMO into the 7, 9-positions, where the LUMOs of other azaphenalene derivatives hardly exist. This electronic effect could influence the intrinsic symmetry forbidden character of electronic transition between the HOMO and the LUMO, resulting in the larger molar absorption coefficients by partially transforming to the allow character compared to those of other azaphenalene derivatives. In summary, the introduction of the boron-nitrogen Lewis acid-base pair into the π-conjugated system of azaphenalene afforded the NIR light absorption properties with the ϵ reaching ∼10⁴ M⁻¹cm⁻¹ and the deep LUMO levels around −4.0 eV (estimated by cyclic voltammetry).