A mononuclear iron(ii) complex constructed using a complementary ligand pair exhibits intrinsic luminescence–spin-crossover coupling†

Molecular materials that exhibit synergistic coupling between luminescence and spin-crossover (SCO) behaviors hold significant promise for applications in molecular sensors and memory devices. However, the rational design and underlying coupling mechanisms remain substantial challenges in this field. In this study, we utilized a luminescent complementary ligand pair as an intramolecular luminophore to construct a new Fe-based SCO complex, namely [FeL₁L₂](BF₄)₂·H₂O (1-Fe, L₁ is a 2,2′:6′,2′′-terpyridine (TPY) derivative ligand and L₂ is 2,6-di-1H-pyrazol-1-yl-4-pyridinecarboxylic acid), and two isomorphic analogs (2-Co, [CoL₁L₂](BF₄)₂·H₂O and 3-Zn, [ZnL₁L₂](BF₄)₂·H₂O). Magnetic studies reveal that 1-Fe exhibits thermally induced SCO within the temperature range of 150–350 K. Variable-temperature fluorescence emission spectral analysis of the three complexes confirmed the occurrence of SCO–luminescence coupling in 1-Fe. Furthermore, variable-temperature UV-vis absorption spectra and time-dependent density functional theory (TD-DFT) calculations elucidate the intramolecular luminescence emission behavior, highlighting the critical role of charge transfer processes between the L₁ ligand and Fe^II ions with different spin states. Our research presents a novel construction strategy for synthesizing synergistic SCO–luminescent materials and contributes to the understanding of the mechanisms underlying SCO–luminescence coupling.