π-Expansion-Directed Modulation of Spin-Fluorescence Coupling in Spin-Crossover Hofmann-Type Frameworks

Area of Science:

• Materials Science
• Coordination Chemistry
• Supramolecular Chemistry

Background:

• Spin crossover (SCO) materials offer optical readout of spin-state switching, crucial for molecular sensors and memory devices.
• The integration of SCO with luminescence (SCO-FL coupling) is key for opto-spintronic applications.
• Designing extended π-conjugated ligands is vital for modulating SCO properties and SCO-FL coupling.

Purpose of the Study:

• To synthesize and characterize two-dimensional Hofmann-type coordination polymers using extended π-conjugated ligands.
• To investigate the spin crossover (SCO) behavior and light-induced spin-state switching in these new materials.
• To explore the relationship between ligand structure (π-expansion) and the occurrence of SCO-luminescence (SCO-FL) coupling.

Main Methods:

• Synthesis of two-dimensional Hofmann-type coordination polymers: {Fe2(PYNA)4[Ag(CN)2]4} (1) and {Fe3(PYAN)6[Ag(CN)2]6}·o-DCB (2).
• Characterization using temperature-dependent magnetic susceptibility, structural analyses, and differential scanning calorimetry.
• Variable-temperature fluorescence spectroscopy and theoretical calculations to study SCO-FL coupling and energy transfer pathways.

Main Results:

• Both compounds exhibited divergent spin crossover behaviors and light-induced excited spin-state trapping effects with reversible photoswitching.
• A contrast in SCO-FL coupling was observed: operative in compound 1 (PYNA ligand) but absent in compound 2 (PYAN ligand).
• Theoretical calculations confirmed that distinct energy transfer pathways in compounds 1 and 2 rationalize the observed SCO-FL coupling differences.

Conclusions:

• Ligand engineering, specifically π-expansion, is a powerful strategy to precisely control spin-optical coupling properties in coordination polymers.
• The findings provide a foundation for designing advanced molecular materials with tailored opto-spintronic functionalities.
• This work highlights the importance of understanding energy transfer mechanisms in achieving efficient SCO-FL coupling.