Supramolecular switchable multifunctional materials

Abstract

This work has explored the design, synthesis, and supramolecular assembly of Fe(II) coordination compounds with the aim of developing multifunctional materials combining spin-crossover (SCO) and photoactive properties. A sequential approach was employed, consisting of isolating first a SCO Fe(II) crystalline precursor and further adding a photoactive coligand. Three ligands were explored to synthesize the iron(II) complexes: a tridentate ligand H4L1 and two bidentate ligands H2L2 and H2L3. The ligand H4L1 was synthesised and fully characterised prior to its coordination to Fe(II). All isolated SCO precursor Fe(II) complexes, regardless of the ligand employed, exhibit spin-crossover behaviour. Two Fe(II) supramolecular materials based on H4L1 were successfully crystallised and structurally characterised in the presence of two photoactive coligands: 4,4’-azopyridine and 1,2-bis(4-pyridyl)ethylene. However, upon formation of the corresponding cocrystals, these systems were found to stabilise exclusively the high-spin (HS) state, reflecting the rigid coordination environment imposed by the tridentate ligand and the supramolecular packing. Additional photoactive coligands, including1,2-bis(2′-methyl-5′-(pyrid-4″-yl)thien-3′-yl)perfluorocyclopentene, 4-(phenylazo)benzoic acid and 2-(4-hydroxyphenylazo)benzoic acid, were also explored, although they did not form crystalline supramolecular materials suitable for structural characterisation. In parallel, Fe(II) complexes incorporating the bidentate ligands H2L2 and H2L3 were also explored. In these systems, three ligands are required to complete the octahedral coordination sphere, leading to a more flexible coordination environment that is expected to better accommodate SCO after the supramolecular organisation. These complexes, previously studied within the research group, were investigated for cocrystallization with photoactive ligands, despite the synthetic challenges encountered. Finally, an alternative strategy based on the incorporation of the oxalate anion was explored in order to introduce an inorganic component with intrinsic magnetic properties, potentially acting as a single-molecule magnet.