Amorphous Red Phosphorus: Host-Guest Assembly with Cyclodextrin

Amorphous red phosphorus (ARP) has recently been identified as a zig-zag ladder polymer, yet its function beyond a low-cost phosphorus source remains unexplored. Here we report the first host-guest chemistry of ARP. In water, hydrophobic ARP threads through α- or β-cyclodextrin (CD) to produce water-soluble pseudo-polyrotaxanes, as evidenced by nuclear magnetic resonance (NMR) spectroscopy, atomic force microscopy (AFM) imaging and AFM-based single-molecule force spectroscopy (SMFS). β-CD, whose cavity diameter (0.60 nm) matches the 0.582 nm van-der-Waals width of ARP, assembles readily with ARP; α-CD, which has a smaller cavity, requires conformational distortion before assembly, producing only ∼30% fully threaded chains even after 10 days. Threading converts collapsed ARP globules into extended chains whose mechanical stiffness increases with CD coverage. After assembly with β-CD, the water solubility of ARP can be significantly enhanced, which builds a method to tune the hydrophilicity of an inorganic polymer chain without the need for traditional chemical modification. This work establishes ARP as an inorganic building block for supramolecular assembly, opening routes to phosphorus-based functional materials.