Origin of Weak Magnetic Coupling in a Dimanganese(II) Complex Bridged by the Tetrathiafulvalene-Tetrathiolate Radical.

Magnetic exchange coupling (J) between different spin centers plays a crucial role in molecule-based magnetic materials. Direct exchange coupling between an organic radical and a metal is frequently stronger than superexchange through diamagnetic ligands, and the strategy of using organic radicals to engender desirable magnetic properties has been an area of active investigation. Despite significant advances and exciting bulk properties, the magnitude of J for radical linkers bridging paramagnetic centers is still difficult to rationally predict. It is thus important to elucidate the features of organic radicals that govern this parameter. Here, we measure J for the tetrathiafulvalene-tetrathiolate radical (TTFtt3-•) in a dinuclear Mn(II) complex. Magnetometry studies show that the antiferromagnetic coupling in this complex is much weaker than that in related Mn(II)-radical compounds, in contrast to what might be expected for the S-based chelating donor atoms of TTFtt. Experimental and computational analyses suggest that this small J coupling may be attributed to poor overlap between Mn- and TTFtt-based magnetic orbitals coupled with insignificant spin density on the coordinating S-atoms. These factors override any expected increase in J from the comparatively strong S-donors. This work elucidates the magnetic coupling properties of the TTFtt3-• radical for the first time and also demonstrates how multiple competing factors must be considered in rationally designing organic radical ligands for molecular-based magnetic compounds.

Steric Engineering of Cyclometalated Pt(II) Complexes toward High-Contrast Monomer-Excimer-Based Mechanochromic and Vapochromic Luminescence.

Ligands play a crucial role in the supramolecular photoluminescence properties of Pt(II) square-planar complexes. To improve the luminescence color responses of N∧C∧N cyclometalated Pt(II) complexes to external stimuli such as mechanical stress and chemical vapors, we have conducted a steric engineering of the previous systems 1a-1d [Inorg. Chem. 2017, 56, 4978-4989] by introducing two tert-butyl groups to the tridentate ligand to form complexes 2a-2c. Unlike the "too low" or "too high" steric hindrance of the NCNPt core in 1a-1d, the combined steric effects of the tert-butyl groups at one side and the pentiptycene group at the other side of the NCNPt core in 2b are "just right" for generating as-prepared powders with pure monomer (green) emission or pure excimer (red) emission, depending on the rate of precipitation from solutions. The synergistic steric effects are also beneficial to the solid-state luminescence quantum efficiency (30-36%). As a result of the differences in steric interactions and thus in the relative monomer vs excimer emission intensity, each complex of 2a-2c performs a two-step luminescence mechanochromism and vapochromism with different color patterns. This work provides an intriguing example of steric engineering of Pt(II) complexes toward highly emissive molecular solids with high-contrast mechanochromic and vapochromic luminescence.