A guest-dependent thermochromic feature in a metal–organic framework and its thin film on different supports

Abstract

A three-dimensional NbO-type metal–organic framework (MOF) is composed of paddle-wheel-type dinuclear Cu₂ secondary units and 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylate (EBTC⁴⁻) linkers. Two types of nanometer-sized cavities are formed in this framework with ca. 8.5 Å in diameter for the small one and dimensions of ca. 8.5 × 8.5 × 21.5 Å for the larger and irregular elongated cavity. The guest molecules, H₂O, DMF and DMSO, occupy the cavities of the as-prepared MOF crystal (labeled as MOF 1). MOF 2 was obtained by the guest-exchange approach using CH₃OH, and the H₂O and CH₃OH molecules reside in the cavities of 2. Two MOFs show greenish-turquoise color at ambient temperature due to the d–d transition of Cu²⁺ ions in the framework, and the reversible thermochromic behavior owing to the change of the coordination environment of Cu²⁺ ions with varying temperatures. The films of 1 and 2 were fabricated on the α-Al₂O₃ and SiO₂ supports by the seeded growth method, displaying similar reversible thermochromic behavior to the corresponding MOFs. This study suggested the possibility of novel thermochromic materials in the rational design of MOFs.