$^{13}$C nuclear magnetic resonance measurements were performed for a single-component molecular material Zn(tmdt)$_{2}$, in which tmdts form an arrangement similar to the so-called ${kappa}$-type molecular packing in quasi-two-dimensional Mott insulators and superconductors. Detailed analysis of the powder spectra uncovered local spin susceptibility in the tmdt ${pi}$ orbitals. The obtained shift and relaxation rate revealed the singlet-triplet excitations of the ${pi}$ spins, indicating that Zn(tmdt)$_{2}$ is a spin-gapped Mott insulator with exceptionally large electron correlations compared to conventional molecular Mott systems.
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