Topology-Controlled Chirality and Spin Selectivity in Two-Dimensional Covalent Organic Frameworks
Chao Jiang, Chuanyu Jin, Yao Lv, Xing Han, Z. Li, Zeyi Liu, Qiang Yan, Y Liu, Sheng Dai, D. Li, Yong Cui
Abstract
Controlling electron spin is fundamental to spintronics, where the chirality-induced spin selectivity (CISS) effect enables magnet-free spin-polarized transport. However, achieving robust and tunable spin polarization in solid-state materials remains a critical challenge. Here, we show that topological isomerism offers a direct strategy to modulate chirality and spin selectivity in two-dimensional (2D) covalent organic frameworks (COFs). Two Zn(salen)-based chiral COFs, TPE-KGM and TPE-SQL, were synthesized from identical precursors yet crystallized into distinct kgm and sql networks. Although compositionally identical, the kgm COF exhibits only weak local geometrical chirality, whereas the sql develops long-range structural chirality that markedly enhances spin selectivity. Circular dichroism (CD) and circularly polarized luminescence (CPL) reveal stronger chiral amplification in the sql topology, while magnetic conductive atomic force microscopy (mc-AFM) confirms enhanced spin polarization of 83% for TPE-SQL versus 53% for TPE-KGM, in sharp contrast to inorganic kgm crystals, where spin frustration and electronic chirality in kgm lattices typically yield higher spin polarization. Furthermore, both frameworks can be exfoliated into nanosheets and integrated with monolayer WSe 2 to form van der Waals heterostructures, achieving spin injection with degrees of circular polarization of 8.1% (TPE-SQL) and 3.9% (TPE-KGM) at 78 K. These findings establish topology as a powerful design principle for governing chirality and spin selectivity in crystalline polymers, advancing the development of chiral optoelectronic and valleytronic materials.