  $^{13}$C nuclear magnetic resonance measurements were performed for a
single-component molecular material Zn(tmdt)$_{2}$, in which tmdt's 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.
