Topological Band Engineering\nof One-Dimensional π‑<i>d</i> Conjugated Metal–Organic\nFrameworks

One-dimensional (1D) <i>π-d</i> conjugated\nmetal–organic\nframeworks (c-MOFs) have garnered widespread research interest in\nchemical energy storage and conversion. In this work, we introduce\na universal principle to engineer the topological bands of 1D c-MOFs.\nConnected by <i>d</i> orbitals of transition metals, two\nequivalent hidden molecular π orbitals in 1D c-MOFs can generate\na staggered hopping within and between the organic ligands, forming\nSu–Schrieffer–Heeger-shaped 1D topological bands. Guided\nby this discovery, we investigate the electronic structures of the\ntypical 1D c-MOF assembled from Ni atoms and 2HQDI (QDI = 2,5-diamino-1,4-benzoquinonediimine)\nprecursors (NiQDI) by first-principles calculations, revealing 1D\ntopological bands around the Fermi level. Due to local bonding variations\nat the QDI terminations, these two hidden molecular π orbitals\nbecome atomically bonded but electronically separated at the edge\nQDI, creating spatially localized in-gap topological edge states at\nthe end of the NiQDI chain. This definitive signature for 1D topological\nbands is identified through differential conductance spectra in scanning\ntunneling microscopy measurements. Our results provide conclusive\nexperimental evidence for topological bands in 1D c-MOFs, paving the\nway for exploring the topological physics in organic materials through\nfrontier molecular orbitals.