A three-dimensional Hofmann-type metal–organic framework with switchable spin crossover and fluorescence properties

Abstract

The design and construction of functional materials combined with spin crossover (SCO) and fluorescence have become a hot topic in magnetic research fields owing to their potential applications in fluorescent molecular switches and fluorescent thermometers. In this study, a three-dimensional (3D) Fe^II-based Hofmann-type metal–organic framework (MOF), namely, {Fe(BPPE)[Ag(CN)₂]₂}·DMF·2EtOH (1), was synthesized by using a luminescent ligand (E)-1,2-diphenyl-1,2-bis(4-(pyridin-4-yl)phenyl)ethene (BPPE). The magnetic results revealed a thermally induced one-step spin crossover (SCO) process with a transition temperature of T_1/2 = 220 K for 1, with desolvated 1 (1-de) exhibiting incomplete SCO behavior with a thermal hysteresis of 30 K. Fluorescence measurements confirmed the coexistence of spin crossover and fluorescence properties in 1, featuring decreased fluorescence intensity during the spin-state crossing from low spin to high spin states. However, a remarkable synergistic coupling between SCO and fluorescence is observed in 1-de, manifested by an enhancement of fluorescence intensity during the SCO process. This work provides an approach to explore the interplay between magnetic and fluorescence properties in SCO-MOFs and is significant for the design and construction of magnetic and fluorescent bi-functional materials.