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

Area of Science:

• Materials Science
• Coordination Chemistry
• Solid-State Chemistry

Background:

• Switchable spin crossover (SCO) molecular materials require precise control over critical temperature and cooperativeness for practical applications.
• Fine-tuning molecular packing in crystals is crucial for modulating SCO properties.

Purpose of the Study:

• To synthesize and characterize a series of Fe(II) SCO complexes with varying alkoxy chain lengths on a tripodal ligand.
• To investigate the influence of alkoxy chain length and parity on SCO behavior, including transition temperature and cooperativeness.
• To study the pressure dependence of SCO properties and compare internal versus external pressure effects.

Main Methods:

• Synthesis of {FeII L}(ClO4)2 complexes with L being a tren-based hexadentate ligand with C3 to C22 alkoxy chains.
• Crystallization and X-ray diffraction analysis to determine crystal structures.
• Magnetic, UV-Vis, and IR measurements to study thermal and photoinduced SCO behavior.
• Variable temperature and pressure studies (magnetic, UV-Vis, IR) on selected complexes.

Main Results:

• Identical coordination cores were observed across all synthesized complexes, crystallizing in the P21/c space group.
• Crystal packing is governed by amphiphilic nature, with hydrophilic heads and hydrophobic alkoxy chains forming layered structures.
• SCO transition temperatures and cooperativeness are modulated by alkoxy chain length and odd-even parity.
• Pressure studies revealed distinct effects of internal pressure from aliphatic layers and applied external pressure on SCO properties.

Conclusions:

• Alkoxy chain length and odd-even parity are effective parameters for fine-tuning SCO properties in Fe(II) complexes.
• The interplay between crystal packing, amphiphilicity, and pressure is key to understanding and controlling SCO behavior.
• This work provides a systematic approach to designing molecular materials with switchable properties for potential applications.