Construction of radical halogen-bonded organic frameworks with enhanced magnetism and conductivity

Abstract

The development of organic frameworks with radical skeletons is desired. In this study, we report the development of a novel two-dimensional radical halogen-bonded organic framework (XOF). The radical monomer, benzimidazole triphenylmethyl (BTTM), was synthesized through the coupling of TTM radicals with benzimidazole. Initially, the benzimidazole units were coordinated with Ag⁺ ions to create a [N···Ag···N]⁺ framework. Subsequently, the addition of iodine led to the in situ replacement of Ag⁺ with I⁺ ions, forming [N···I···N]⁺ linkers and resulting in the creation of the XOF structure. The resulting XOF-HBTTM and XOF-BTTM structures demonstrated good-crystallinity, confirmed by PXRD, HR-TEM, SEAD, and SAXS analyses. EPR measurements confirmed the preservation of radical characteristics within the XOF framework. Furthermore, SQUID measurements indicated that XOF-BTTM exhibits spin moments of S = 1/2 at 2 K, with a saturated magnetization strength peaking at 4.10 emu/g, a notable enhancement compared to 1.87 emu/g for the BTTM monomer. This improvement in magnetism is attributed to the extended spin density distribution and the presence of [N···I···N]⁺ interactions, as suggested by DFT calculations. Additionally, the radical XOF-BTTM exhibited significantly enhanced electrical conductivity, reaching up to 1.30 × 10⁻⁴ S/cm, which is two orders of magnitude higher than that of XOF-HBTTM. This increased conductivity is linked to a reduced HOMO-LUMO gap, higher carrier density, and the incorporation of triphenylmethyl radicals within the framework. This research highlights the potential of benzimidazolyl motifs in constructing functional XOFs and advances our understanding of radical organic frameworks.