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To elucidate the electronic state that exists in the boundary region between the spin-singlet phase and the high-pressure side antiferromagnetic phase in the modified generalized phase diagram, we carried out x-ray diffraction, electron spin resonance, and nuclear magnetic resonance measurements of the tetramethyl-tetrathiafulvalene (TMTTF) family salt, (TMTTF) 2 I. The unit-cell volume of (TMTTF) 2 I is between that of (TMTTF) 2 Br and (TMTTF) 2 PF 6 . We found that (TMTTF) 2 I undergoes a spin-singlet phase transition at 21 K without involving paramagnetic charge-ordering phase. Therefore, the charge-ordering transition is not necessary for the spin-singlet transition of TMTTF salts as seen in other conventional spin-singlet salts. Finally, we discuss possible mechanisms of the spin-singlet phase transition.

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