Fluorescence Emission Modulation of Alkynyl-Functionalized Iron(II) Spin-Crossover Complexes

Combining spin crossover (SCO) and fluorescence properties could respond to multiple physical stimulus and is meaningful for the development of switches and imaging tools. In this work, we present a new family of neutral mononuclear iron(II) complexes, denoted as Fe( L^1–3)₂ and Fe( L³)₂·CH₂Cl₂ with N₄O₂ donor sets. These complexes were synthesized employing a series of tridentate Schiff base ligands, which contain an ethynyl group (H L^1–3 = 3-ethynyl-N′-((4-methylpyridin-2-yl)methylene)benzohydrazide ( 4MePAH-3EyBHZ, H L¹), 3-ethynyl-N′-((4-methoxylpyridin-2-yl)methylene)benzohydrazide ( 4OMePAH-3EyBHZ, H L²), 3-ethynyl-N′-((5-methylpyridin-2-yl)methylene)benzohydrazide ( 5MePAH-3EyBHZ, H L³)). Variable-temperature magnetic susceptibility measurements revealed that Fe( L¹)₂ exhibits an abrupt and complete SCO with characteristic temperature T_1/2↓ = 176 K and T_1/2↑ = 178 K; Fe( L²)₂ exhibits an abrupt and complete SCO with T_1/2↓ = 214 K and T_1/2↑ = 219 K; Fe( L³)₂ exhibits an abrupt and complete SCO with T_1/2↓ = 218 K and T_1/2↑ = 226 K. The partial conformational inversion is observed between the high spin and low spin state of Fe( L²)₂ evidenced by the variable-temperature single-crystal X-ray diffraction analysis. An unusual increase of emission intensity was observed in the fluorescence spectra of Fe( L¹)₂ and Fe( L²)₂ during the heating SCO process. These findings are both valuable in the study of spin-fluorescence interaction and synthesis of new multifunctional SCO complexes.