Construction of spin crossover-fluorescence bifunctional iron(ii) complexes with modified bis(pyrazole)pyridine ligands

Abstract

The design and construction of bifunctional molecular materials combined with spin crossover (SCO) and fluorescence have become a hot topic in magnetic materials research. However, achieving the synergistic effect between SCO and fluorescence in one system is complicated. Here, we designed two modified 2,6-bis(1-pyrazolyl)pyridine (bpp) ligands 4-(naphthalen-1-yl)-2,6-bis(1-pyrazolyl)pyridine (L₁) and (E)-2,6-bis(1-pyrazolyl)-4-(2-(pyren-1-yl)vinyl)pyridine (L₂), and synthesized two mononuclear Fe^II complexes [Fe(L₁)₂](ClO₄)₂ (1) and [Fe(L₂)₂](ClO₄)₂·2.63CH₂Cl₂ (2). The magnetic results showed that both 1 and 2 exhibit a complete SCO behavior, with 2 displaying a thermal hysteresis of about 7 K around 220 K. In addition, the transition temperature is 366 K for 1 and 215/222 K (T_1/2↓/T_1/2↑) for 2. Fluorescence measurements confirmed the synergistic effect between fluorescence and SCO in 1, featuring increased photoluminescence (PL) intensity during the spin-state crossing from low spin to high spin. However, 2 showed the coexistence of SCO and fluorescence without a synergistic effect. Complex 1 indicates that introducing fluorescent groups into SCO molecules provides a reasonable strategy for constructing magnetic and fluorescent bi-functional materials.