Humidity-Triggered Reversible 0–1D Phase Transition in Hybrid Antimony Halides

Stimulus-responsive inorganic–organic hybrid metal halides (IOMHs) have shown great potential in applications such as sensing and anti-counterfeiting. IOMHs can undergo a variety of structural changes when triggered by humidity; however, relevant reports of structural dimensionality change from zero dimension (0D) to one dimension (1D) are rare. This study investigates the synthesis, structure, and properties of two antimony-based IOMHs, namely 0D-(Mp)3SbCl6·MeCN and 1D-(Mp)2SbCl5 (Mp = protonated morpholine; MeCN = acetonitrile). Photophysical characterizations show that (Mp)3SbCl6·MeCN, when being excited at 375 nm, exhibits typical self-trapped exciton triplet state broad-band emission, with a peak at 620 nm and a quantum yield as high as 75.06%. Under humid conditions, the 0D structure of (Mp)3SbCl6·MeCN undergoes a phase transition, leading to the 1D structure of (Mp)2SbCl5. This transition is accompanied by fluorescence quenching. X-ray powder diffraction, Raman spectroscopy, and thermogravimetric analysis confirm the phase transition process and its reversibility. Based on the high contrast of fluorescence before and after phase transition, (Mp)3SbCl6·MeCN is demonstrated as an ideal material for fluorescence water sensing, capable of detecting trace amounts of water in tetrahydrofuran with a detection limit of 0.2% v/v.