Vapoluminescence hysteresis in a platinum(II) salt-based humidity sensor: Mapping the vapochromic response to water vapor

Abstract

In an effort to address the scarcity of chemical sensors that can record precise, reproducible, and sensitive changes in humidity, this work reports a reversible system based on a coordinatively unsaturated, square-planar platinum(II) salt, [Pt(tpy)Cl]ClO4 (tpy = 2,2′6′,2″-terpyridine). The sensing methodology relies on humidity induced vapochromic/vapoluminescent behavior of the salt; the anhydrous form is yellow in color and demonstrates orange-yellow luminescence indicating weak intermolecular Pt•••Pt interactions. Exposure to water vapor changes the salt color to dark red and the luminescence to red; this is triggered by incorporation of water molecules in the crystal lattice. This reorganizes the crystal packing, and extends Pt•••Pt interactions, as verified by crystal structure. The conversion between the fully hydrated and the fully dehydrated end products are spectroscopically reversible, demonstrating recyclability across cycles. However, the vapoluminescence response trajectory of the dehydrated form to water vapor sorption does not exactly reverse trace the desorption profile of the hydrated form but shows a hysteresis effect, demonstrating the vapochromic journey to be equally important as the destination. This methodology serves as the basis of humidity sensing for both powder as well as crystalline samples. The proposed sensor demonstrates a large linear operational range of humidity sensing (10–80% for crystals) and a limit of detection of 3.3%. The crystals also demonstrate an ability to sense water vapor in the presence of interfering organic vapors. The work presents a new simple, economic and scalable method for humidity sensing.