Electron spin resonance spectroscopic evidence of the transition to a spin-singlet state in the monomer Mott insulator (BEDT-TTF)Cu[N(CN) 2]2

(BEDT-TTF)Cu[N(CN)₂]₂ (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene) is a monomer Mott insulator, in which the BEDT-TTF molecules form a three-dimensional (3D) distorted diamond structure; however, its physical properties resemble those of a quasi-one-dimensional (Q1D) zigzag BEDT-TTF chain structure. We investigated the magnetic ground state of this compound using electron spin resonance (ESR) spectroscopy in the X-band region (9.12 GHz) to confirm the occurrence of a transition to a spin-singlet state. The ESR spectra showed an asymmetric spectral line comprising two Lorentzian components, suggesting different correlations of the two independent 1D zigzag chains of the BEDT-TTF molecules. The spin susceptibility derived from the ESR spectra showed an opening of the spin gap below the transition temperature Tc≈ 25 K possibly due to alternating intrachain interactions in both zigzag chains with different spin correlations.