Optical memory effect in a dynamic gadolinium–tetracyanidoplatinate coordination polymer for sensing deviations in temperature and humidity

Abstract

Materials combining photoluminescence (PL) with sorption properties, such as those based on supramolecular assemblies or coordination polymers, are considered good candidates for the construction of optical sensors of external stimuli. We report a new dynamic coordination polymer, {[K(H₂O)₃]₈[Gd(H₂O)₆]₁₄[Pt(CN)₄]₂₅·28H₂O}_n (1) based on tetracyanidoplatinate(II) ions as well as K(I) and Gd(III) aqua-complexes. These molecular building blocks are self-assembled by employing {Pt–CN–Gd} cyanido bridging and non-innocent {Pt–Pt} metallophilic interactions. 1 exhibits visible-light PL related to metal-to-metal-to-ligand charge transfer transitions occurring within the metallophilic {[Pt^II(CN)₄]}_n²ⁿ⁻ stacks, water vapor sorption properties and reversible thermal dehydration. Thanks to these prerequisites, the PL features strongly depend on relative humidity (RH) and temperature, determining the number of interstitial water molecules. By changing RH, one can tune the emission from the strongest deep green emission at ca. 80% RH (1^H1 phase), through weaker green emission at ca. 20% RH (1^H2, denoted also as H), to yellow-orange PL at 0% RH (e.g., Ar atmosphere, 1^A1). The return from low to high RH differs due to the hysteresis in the sorption isotherm. Thus, at ca. 20% RH (1^A2, denoted also as A), the emission color is yellow; however, at ca. 80% RH, the recovery of 1^H1 with its deep green emission appears. This switching effect is enriched by thermal dehydration. Starting from green-emissive H, on heating to 150 °C, the new blue-emissive phase 1^T1 is produced. After cooling to room temperature and reaching ca. 20% RH, the new greenish blue PL is realized (1^T2, denoted also T). The recovery of initial phases is done at a RH of ca. 80% providing the route to the original 1^H1. As a result, at ca. 20% RH, three phases (A, T, and H) with different PL appear depending on the history of the material. This optical memory effect enables the usage of the reported material as an optical sensor of RH and temperature. In particular, this material can act as an alarm indicating the appearance of high temperatures or low RH.