Aliphatic Chains and External Pressure as Tools for Fine-Tuning Spin Transition Temperature and Cooperativity

In switchable spin crossover (SCO) molecular materials, control of their critical temperature and cooperativeness is essential for practical applications. This requires systematic fine-tuning of the molecular packing in the crystal. Therefore, here we report on the synthesis and characterization of a series of Fe^II SCO complexes, {Fe^IIL}(ClO₄)₂, derived from a tripodal tren-based imino-methyl-pyridine hexadentate-type ligand L with alkoxy chains of varying lengths (C3 to C22). The complexes crystallize in the monoclinic P2₁/c space group, displaying an identical coordination core. The crystal packing, modulated by the length of the alkoxy chains, is governed by the amphiphilic nature of the complexes, where the hydrophilic pseudo-octahedral cationic heads and ClO₄^- anions define double-layers separated by the growing hydrophobic layer generated by the alkoxy chains. All compounds undergo thermal- and photoinduced SCO behavior featuring transition temperatures and cooperativeness which depend, for most of them, on the odd-even parity of the chains. The pressure dependence of the SCO for selected members (C3, C8, C15, and C16) has been investigated through magnetic, UV-vis, and IR measurements. Both internal pressure generated by the growth of the aliphatic layer and the applied external pressure effects are compared and discussed in the context of fine-tuning modulation of SCO properties.