Reagents, starting materials, and solvents were purchased from Sigma-Aldrich, Acros Organic, TCI America and EMD Chemicals and used as received. Z-DPTTF ligand was prepared following a literature protocol (Han et al., 2007).

To a solution of Z-DPTTF ligand (7.2 mg, 0.02 mmol) in DMAc (2 ml) placed in a screw-capped vial, a separately prepared solution of Zn(NO₃)₂•6H₂O (11.9 mg, 0.04 mmol) and TCPB (11.2 mg, 0.02 mmol) in 2:1 DMAc/H₂O mixture (1.5 ml) was added slowly. Once all these starting materials were fully dissolved upon gentle shaking, 1 M HNO₃ ethanolic solution (20 µl) was added to it. The resulting mixture was then heated in an oven at 80°C for 24 h. The resulting dark-red crystals (48%) were used for single-crystal x-ray diffraction (SXRD) analysis and the corresponding evacuated powder was used for electrical and optical measurements. Elemental analysis: Calc. for Zn₂C₅₂H₃₇O_10.5S₄N₂: C 55.92, H 3.34, and S 11.48%. Found: C 56.01, H 3.44, and S 11.53%

The dark-red colored evacuated sine-MOF powder was placed in a small open vial, which was then placed inside a larger screw-capped vial containing few I₂ chips. The larger vial was capped tightly and sealed with parafilm tape to keep the sine-MOF crystals exposed to iodine vapor for 3 days, which caused the sine-MOF powder to turn black. The I₂-treated sine-MOF vial was removed from the I₂ chamber, left open overnight, and finally washed thoroughly with hexane until the washing solution became colorless indicating that the material was free of any physisorbed I₂. Elemental analysis: Calc. for Zn₂C₅₅H₅₀O₁₄S₄N₂I_1.5: C 46.77, H 3.57, S 9.08%, and I 13.48. Found: C 46.97, H 2.80, S 8.24, and I 13.44%.

A solvothermal reaction between Zn₂(NO₃)₂•6H₂O, TCPB, and Z-DPTTF in a DMAc/H₂O mixture at 80°C for 24 h yielded dark-red sine-MOF crystals. SXRD analysis revealed that sine-MOF [Zn₂ (DPTTF)TCPB•3DMA]_n crystallized in an orthorhombic space group Pnma (Figures 1A,B and Supplementary Figure S1). The TCPB ligands formed Zn₂(COO)₄ paddlewheel nodes, but unlike typical pillared paddlewheel MOFs, they did not form 2D sheets of these nodes in sine-MOF. Instead, they formed a 3D framework thanks to a significant twist of TCPB ligand, which was evident from large dihedral angles (ca. 43–47°) between the central benzene ring and terminal benzoate rings. The axial sites of the Zn₂(COO)₄ paddlewheel nodes were occupied by Z-DPTTF ligands, although these dipyridyl ligands did not act as typical pillars found in pillared paddlewheel MOFs. Instead, each U-shaped Z-DPTTF ligand bridged two adjacent Zn₂ nodes formed by two 1,3-benzoate groups of the same TCPB ligand. Each Zn₂ paddlewheel node carried one U-shaped Z-DPTTF ligand at the top axial position and another at the bottom axial position, which then bridged two adjacent Zn₂ nodes from the top and bottom axial positions, respectively. Thus, the consecutive Zn₂ nodes located along the b-axis were connected by U-shaped Z-DPTTF ligands in an alternating top/bottom fashion that resembled a full sine-wave (Figure 1C), prompting us to label this new architecture as sine-MOF. The formation of this uncommon architecture was made possible by the bent geometry of Z-DPTTF ligands having an angle of 36° between the two dithiolene rings and a dihedral angle of 10° between two cis-pyridyl rings. The bent geometry and short central C=C bond length (1.34 Å) of Z-DPTTF indicated that they existed in the neutral form in pristine sine-MOF (Gao et al., 2014; Su et al., 2017). Due to an alternate up/down orientation of Z-DPTTF ligands, sine-MOF lacks intermolecular π–π and S•••S interactions between the TTF cores, but it enjoys π–π interaction between the dithiolene rings of Z-DPTTF and two benzoate rings of TCPB ligand that have a centroid-to-centroid distance of 3.66 Å (Supplementary Figure S2).

The experimental powder X-ray pattern (PXRD) of pristine sine-MOF was consistent with the simulated one obtained from the SXRD analysis, which confirmed the phase purity and crystallinity of the evacuated bulk material (Figure 2). Iodine is a mild oxidant that is known to chemically oxidize TTF and other electron rich π-systems (Su et al., 2017; Gordillo et al., 2020). Exposure of pristine sine-MOF crystalline powder to I₂-vapors afforded a black material that was washed thoroughly with hexanes until the wash solution became colorless indicating that no residual physisorbed I₂ was left in the I₂-treated sine-MOF. The PXRD pattern of the I₂-treated sine-MOF matched with that of the pristine material, confirming the retention of its structural integrity and crystallinity (Supplementary Figure S3).

Thermogravimetric analysis (TGA) was performed on vacuum-dried pristine and I₂-treated sine-MOF samples in N₂ atmosphere (Figure 3). The TGA profile of pristine sine-MOF revealed a gradual 10% weight loss until 300°C corresponding to the loss of solvent molecules, followed by a sharp 33% weight loss due to framework decomposition. The TGA profile of the I₂-doped sine-MOF displayed an initial weight loss of 14% corresponding to the loss of MeOH and water molecules, followed by another significant weight loss step above 400°C that corresponded to framework decomposition.

The diffuse reflectance spectra (DRS) of pristine and I₂-doped sine-MOFs were measured. Pristine sine-MOF displayed a broad band centered on 480 nm, which was ca. 60 nm red-shifted with respect to the UV-vis absorption spectrum (λ_max) of Z-DPTTF recorded in DMF (Figures 4A,B). From the onset of the λ_max peak of the Z-DPTTF ligand its optical bandgap of 2.3 eV was calculated. The optical bandgaps of pristine and I₂-treated sine-MOFs (E _g = 1.8 and 1.2 eV, respectively) (Figure 4B) were narrower than that of the free ligand, probably because of π–π and π-donor/acceptor interactions between the Z-DPTTF and TCPB ligands in pristine and I₂-treated wavy MOFs, respectively. The results from the corresponding Tauc plot (Figure 4C) were in good agreement with those determined from DRS and revealed ∼0.6–0.7 eV narrower bandgap for the I₂-doped sine-MOF with respect to the pristine MOF. The narrower bandgap of I₂-treated sine-MOF is likely due to a partial oxidation of the Z-DPTTF ligands to Z-DPTTF^•+ radical cations within the framework.

Solid state cyclic voltammetry (CV) (Figure 5) and square wave voltammetry (SWV) (Supplementary Figure S4) were used to investigate the redox properties of sine-MOF. TTF compounds are known to display two reversible one electron oxidation steps corresponding to TTF^•+ and TTF²⁺ formation. The CV of pristine sine-MOF displayed two quasi-reversible oxidation processes (Figure 5A) with anodic peaks at 0.68 and 0.96 V (vs Ag/AgCl, 0.1 M Bu₄NPF₆ in MeCN) corresponding to stepwise one-electron oxidation of Z-DPTTF to Z-DPTTF^•+ and Z-DPTTF²⁺. The anodic peaks of I₂-doped wavy-MOF (Figure 5B) appeared toward more positive potentials at 0.79 and 1.05 V suggesting that such I₂-mediated partially oxidized framework was more difficult to oxidize electrochemically than pristine sine-MOF.

Solid-state electron paramagnetic resonance (EPR) confirmed the presence of Z-DPTTF^•+ radical cations within the sine-MOF. A weak EPR signal (Figure 6) was present in pristine sine-MOF indicating that most of the Z-DPTTF ligands were in the neutral state and that a small percentage may have been oxidized by air as has been previously reported for other TTF-based MOFs. (Narayan et al., 2012; Park et al., 2015; Su et al., 2017; Wang et al., 2017a; Leong et al., 2018). In contrast, a strong EPR signal (g ≈ 2.006) was observed (Figure 6) for I₂-treated sine-MOF indicating that a significant population of Z-DPTTF ligands were oxidized to paramagnetic Z-DPTTF^•+ radical cations. The elemental analysis data of I₂-treated sine-MOF (vide supra) corresponded to an empirical formula of Zn₂C₅₅H₅₀O₁₄S₄N₂I_1.5. Based on the I/S ratio, we estimated that there was roughly one I₃ ⁻ anion for two DPTTF ligands (each DPTTF has four S atoms), meaning that approximately half of the DPTTF ligands were partially oxidized to DPTTF^•+ radical cations, which were accompanied by an equal number of I₃ ⁻ counterions for charge balance. Furthermore, based on the empirical formula and quantitative EPR analysis, we estimated that the pristine sine-MOF possessed 6.8 x 10¹³ spins/mg or 7.6 x 10¹⁹ spins/mol, which corresponded to only 0.01% DPTTF^•+ population (possibly produced by negligible aerobic oxidation). In contrast, the I₂-treated sine-MOF possessed 2.6 x 10¹⁶ spins/mg or 3.6 x 10²² spins/mol, which corresponded to a noticeably higher 6.1% DPTTF^•+ population. Thus, elemental analysis and quantitative EPR analysis together helped us quantify the DPTTF^•+ population in the I₂-treated partially oxidized sine-MOF.

Finally, we measured the room temperature electrical conductivity of pressed pellets of pristine and I₂-treated sine-MOFs, which provided us insights into the effect of partial oxidation of Z-DPTTF ligands in the latter. DC-sweep measurements of pressed sine-MOF-pellets sandwiched between two conductive stainless-steel electrodes coated with Ag paste were conducted. Both materials displayed linear current-voltage (I-V) responses between –1 and +1 V (Figure 7), confirming ohmic contact between the pellets and electrodes. From the slopes of the corresponding I-V curves, the electrical conductivity of pristine and I₂-treated sine-MOFs was determined to be 1 × 10⁻⁸ and 5 × 10⁻⁷ S/m, respectively. The 50-fold higher conductivity of I₂-treated sine-MOF was attributed to partial oxidation of Z-DPTTF ligands to Z-DPTTF^•+, which enhanced the charge carrier concentration. However, the increase was modest and the conductivity was still lower than other I₂-treated TTF-based MOFs possibly because sine-MOF lacked sufficient π–π or S•••S interactions between the Z-DPTTF ligands, which hindered through-space charge movement, while the Zn₂ paddlewheel nodes were not conducive to through-bond charge movement. As result, pristine and I₂-treated sine-MOFs likely relied on less effective charge hopping mechanism, which caused modest conductivities even though the latter possessed larger number of mobile charge carriers due to the presence of DPTTF^•+ radical cations.