${}^1\mathrm{H}$ nuclear magnetic resonance (NMR) measurements in $[{\text{YPc}}_2]{}^0$, an organic compound formed by radicals stacking along chains, are presented. The temperature dependence of the macroscopic susceptibility of the NMR shift and of the spin-lattice relaxation rate $1/T_1$ indicate that the unpaired electron spins are not delocalized but rather form a $S=1/2$ antiferromagnetic chain. The exchange couplings estimated from those measurements are all in quantitative agreement. The low-energy spin dynamics can be described in terms of diffusive processes and the temperature dependence of the corresponding diffusion constant suggests that a spin gap at $~1$ K might be present in this compound.

• Received 29 October 2010

DOI:https://doi.org/10.1103/PhysRevB.83.174419