T₁/₂ tuning in a synergistic BODIPY-tetrazole Fe(II) spin crossover-photoluminescence system via counterion variation

Abstract

Multifunctional bistable magnetic materials, particularly spin crossover (SCO)-photoluminescence (PL) systems, are of significant interest for molecular sensor applications. However, achieving predictable SCO tuning while simultaneously combining these properties remains a challenge. In this context, we synthesized a series of heteroleptic Fe(II) coordination compounds incorporating a fluorescence-active BODIPY-based 1H-tetrazole ligand (4,4-difluoro-1,3,5,7-tetramethyl-8-[(1H-tetrazol-1-yl)methyl]-4-bora-3a,4a-diaza-s-indacene), with the compounds differing by counteranion size (BF₄⁻ < ClO₄⁻ ≪ PF₆⁻ < CF₃SO₃⁻ < SbF₆⁻). Remarkably, CF₃SO₃⁻ coordinates to Fe(II), while all other anions form noncoordinating isostructural complexes. The coordination of CF₃SO₃⁻ can be reversed through a topochemical exchange with H₂O. Magnetic studies reveal that for the isostructural coordination compounds with noncoordinating anions, increasing anion size leads to incomplete and/or less abrupt spin transitions. The complex incorporating noncoordinating BF₄⁻ exhibits the most abrupt and complete SCO, demonstrating a weak yet synergistic interplay between SCO and PL signal modulation upon the spin transition, an effect attributed to electronic coupling. While not featuring SCO, the compound with a coordinating CF₃SO₃⁻ anion is crystallographically interesting as it features a phase transition, forming a subtle 4-fold superstructure below 180 K. Our study provides a versatile platform for independent tuning of SCO and PL properties, paving the way for application in multifunctional devices.