Turning 3D Covalent Organic Frameworks into Luminescent Ratiometric Temperature Sensors

In this study, we report hybrid crystalline lanthanide-containing 3D covalent organic framework (Ln@3D COF) materials that are suitable for temperature sensing applications. Different routes to obtain these hybrid materials were tested and compared for material quality and thermometric properties. In the first approach, a bipyridine-containing 3D COF (Bipy COF) was grafted with a range of visible emitting lanthanide (Eu³⁺, Tb³⁺, Dy³⁺, and Eu³⁺/Tb³⁺) β-diketonate complexes. In the second approach, a novel nanocomposite material was prepared by embedding NaYF₄:Er,Yb nanoparticles on the surface of a nonfunctionalized 3D COF (COF-300). To the best of our knowledge, the luminescent materials developed here are the first 3D COFs to be tested as ratiometric temperature sensors. In fact, for the Bipy COF, two different types of thermometers were tested (the Eu³⁺/Tb³⁺ system and a rare Dy³⁺ system), with both showing excellent temperature sensing properties. The reported NaYF₄:Er,Yb/COF-300 nanocomposite material combines upconverting nanoparticles with 3D COFs, similar to previously reported metal organic framework (MOF) nanocomposite materials; however, this type of hybrid material has not yet been explored for COFs. As such, our findings open a new pathway toward potential multifunctional materials that can combine thermometry with other modalities, such as catalysis or drug delivery, in just one nanocomposite material.