We investigate the low-temperature magnetic properties of the organic Mott insulator $\kappa \text{−}{\left(\mathrm{BEST}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ using ${}^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), where BEST denotes bis(ethylenediseleno)tetrathiafulvalene. The NMR Knight shift and EPR integrated intensity together reveal a pronounced drop in spin susceptibility below $T^{*}\approx$ 10 K, accompanied by a broad hump in the nuclear spin-lattice relaxation rate and the growth of a Lorentzian EPR component. These features closely mirror the “6 K anomaly” previously reported for the isostructural $\kappa \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$, where ET is bis(ethylenedithio)tetrathiafulvalene. Although the two salts share nearly identical electronic correlations and qualitatively exhibit the same magnetic behavior, the combined NMR and EPR results indicate that substituting the heavier BEST for ET enhances the transition temperature to the spin-singlet state. We propose that the increase in molecular mass may induce lattice softening, thereby stabilizing the lattice distortion required for valence-bond-solid order. Our results suggest that lattice dynamics play an important role in the valence-bond-solid transition in $\kappa \text{−}\mathrm{type}$ organics.

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• Frustrated magnetism
• Quantum spin liquid
• Valence bond solid
• Charge-transfer insulators
• Molecular solids
• Organic compounds
• Electron paramagnetic resonance
• Nuclear magnetic resonance