*3.1. Crystal Structure*

The trans-TTF-py2-based molecular conductor, **TTF-py2\_PF6**, was obtained through electrochemical crystallization in a CHCl3/EtOH (7:3, *v*/*v*) solution dissolving TTF-py2 and TBAPF6 [47]. The similar conditions with TBAClO4, TBABF4, or TBAX (X = Cl and Br) did not give any crystals. Single-crystal X-ray structure analysis shows two hexafluorophosphates for every three trans-TTF-py2 molecules, indicating that the average charge of trans-TTF-py2 is +0.66 (Figure 1). The cis isomer was not incorporated into the crystal. Two crystallographically independent trans-TTF-py2 (**A** and **B**) molecules exist in the crystal structure (Figure 1a), repeating the **A**–**B**–**B** order to form a one-dimensional columnar structure by π–π stacking. The closest intermolecular S···S distances between adjacent trans-TTF-py2 molecules along the π-stack axis are summarized in Table 1. This table indicates that the S···S distances between **A**–**B** are shorter than the sum of van der Waals radius of the sulfur atoms (3.60 Å) [48], whereas those between **B**–**B** are longer than that value. In fact, the side view of the π-stacking structure (Figure 1c) clearly shows that the shift of π-stacking between **B** and **B** is larger than that between **A** and **B**. Focusing on the environment of π-stack columnar structures (Figure 1d,e), one π-stack column is surrounded by four other columns. Figure 1d,e highlight the interactions of **A** and **B** with neighboring molecules, respectively. It is noteworthy that there are short contacts between the nitrogen atoms of the pyridyl groups and the hydrogen atoms of TTF cores, emphasized with orange broken lines in Figure 1d and e. **A** is surrounded by four **B** molecules and has short contacts with two of them. **B**, on the other hand, is surrounded by two each of **A** and **B**, and it interacts with all but one **A**. The intermolecular N···H distances are 2.365 Å (between **A** and **B**) and 2.626 Å (between **B** and **B**), which are shorter than the sum of the van der Waals radii of hydrogen and nitrogen atoms [48], suggesting the construction of C–H···N hydrogen bondings. To obtain a deeper insight into the hydrogen bondings connecting trans-TTF-py2 molecules, a DFT calculation was applied for estimating stabilization energies of the hydrogen bondings between **A** and **B** and **B** and **B** depicted in Figure 1d,e. The calculation indicated that the stabilization energies are –0.74 kcal/mol for the hydrogen bonding

between **A** and **B** and –1.72 kcal/mol for that between **B** and **B**. Hence, the sum of the stabilization energies of the hydrogen bondings around **A** is –1.48 kcal/mol (= 2 × (–0.74) kcal/mol) and that around **B** is –4.18 kcal/mol (= 2 × (–1.72) –0.74 kcal/mol). Calculated effective transfer integrals (*V*eff) between neighboring trans-TTF-py2 molecules along the columnar structure are *V*eff(A–B) = 390.8 meV for **A**–**B** and *V*eff(B–B) = 222.9 meV for **B**–**B** (Figure 1c), reflecting the difference in the stacking feature. On the other hand, *V*eff of trans-TTF-py2 molecules between adjacent columns are less than 10 meV in all combinations. This much difference of *V*eff obviously demonstrates the character of one-dimensional electron conductor in **TTF-py2\_PF6**.

**Figure 1.** Crystal structure of **TTF-py2\_PF6**. (**a**) Thermal ellipsoid plot of **TTF-py2\_PF6**. Counteranion (PF6 –) and crystal solvent (EtOH) were pictured in capped sticks, and hydrogen atoms are omitted for clarity. (**b**,**c**) π-stacked TTF-py2 columns viewed from different sides. Blue broken lines are written to represent the shift of π-stacks. Counteranion and crystal solvent were omitted for clarity in (**c**). (**d**,**e**) Perspective views of **TTF-py2\_PF6** along the *a* axis with placing **A** or **B** at the center, respectively. Short contacts with neighboring columns are represented as broken orange lines. (**f**,**g**) View of the angle of the hydrogen bonding site between **A** and **B** (**f**) and **B** and **B** (**g**). Hydrogen atoms of pyridyl groups were omitted for clarity. Yellow, S; Orange, P; Light green, F; Red, O; Blue, N; Gray, C; White, H.

**Table 1.** The distances between the nearest sulfur atoms along the π-stack axis.

