Guest-Responsive Elastic Frustration "On-Off" Switching in Flexible, Two-Dimensional Spin Crossover Frameworks

Area of Science:

• Materials Science
• Chemistry
• Solid-State Physics

Background:

• Porous coordination polymers (PCPs) with integrated spin crossover (SCO) properties exhibit multistable spin-state switching.
• The Hofmann-type framework [Fe(thtrz)₂Pd(CN)₄]·EtOH,H₂O (1·EtOH,H₂O) shows a two-step SCO profile due to elastic frustration.

Purpose of the Study:

• To investigate the manipulation of SCO multistability by controlling elastic frustration through guest removal.
• To reveal the mechanism of reversible elastic frustration release via stepwise guest evacuation.

Main Methods:

• Stepwise guest removal from the parent PCP phase (1·EtOH,H₂O → 1·H₂O → 1·Ø).
• Variable temperature structural and magnetic susceptibility measurements.
• Light-induced excited spin state trapping (LIESST) experiments.

Main Results:

• Stepwise guest removal reversibly releases elastic frustration, switching SCO behavior from two-step to one-step.
• Desolvated phases (1·H₂O and 1·Ø) exhibit abrupt, hysteretic one-step SCO with distinct transition temperatures.
• LIESST properties change from nonquantitative to quantitative upon guest removal, demonstrating 'on-off' elastic frustration switching.

Conclusions:

• Lattice flexibility in PCPs is crucial for inducing, stabilizing, and quantifying elastic frustration in SCO materials.
• Guest interactions (direct and indirect) play distinct roles in modulating SCO behavior.
• Controlled guest removal offers a pathway to tune SCO multistability and photoresponsive properties.