  Cross-correlation between magnetism and dielectric is expected to offer novel
emergent phenomena. Here, magnetic excitations in the organic donor-acceptor
spin-chain system, TTF-BA, with a ferroelectric ground state is investigated by
$^1$H-NMR spectroscopy. A nonmagnetic transition with a ferroelectric order is
marked by sharp drops in NMR shift and nuclear spin relaxation rate $T_1^{-1}$
at 53 K. Remarkably, the analyses of the NMR shift and $T_1^{-1}$ dictate that
the paramagnetic spin susceptibility in TTF-BA is substantially suppressed from
that expected for the 1D Heisenberg spins. We propose that the spin-lattice
coupling and the ferroelectric instability cooperate to promote precursory
polar singlet formation in the ionic spin system with a nonmagnetic
ferroelectric instability.
