Effects of Intervalence Charge Transfer Interaction between π-Stacked Mixed Valent Tetrathiafulvalene Ligands on the Electrical Conductivity of 3D Metal-Organic Frameworks
Shiyu Zhang, Dillip K. Panda, Ashok Yadav, Wei Zhou, Sourav Saha
Achieving molecular-level understanding of how the structures and compositions of metal–organic frameworks (MOFs) influence their charge carrier concentration and charge transport mechanism—the key parameters that dictate their electronic band gaps and conductivity—is essential for successful development of electrically conducting MOFs that have recently emerged as one of the most coveted functional materials due to their diverse potential applications in myriad advanced electronics and energy production and storage devices. Herein, we have constructed four new 3D frameworks M4TTFTC (M = Na, K, Rb, and Cs) containing continuous π-stacks of electron rich tetrathiafulvalene tetracarboxylate (TTFTC) ligands but having different π–π and S···S distances (dπ–π and dS···S) and different amounts of aerobically oxidized TTFTC•+ radical cations. Density functional theory (DFT) calculations and diffuse reflectance spectroscopy (DRS) demonstrated that depending on the π–π-distances and TTFTC•+ population, these MOFs enjoyed varied degree of TTFTC/TTFTC•+ intervalence charge transfer (IVCT) interactions, which commensurately affected their electronic and optical band gaps (Eel and Eopt) as well as the electrical conductivity (σ). Having the shortest d^π–π^ (3.39 Å), maximum π-surface overlap, and the largest initial TTFTC•+ population (23%), the optimally oxidized Na-MOF 1'-ox displayed the narrowest band gaps (Eel = 1.67 eV and Eopt = 1.33 eV), the highest room temperature electrical conductivity (3.4 (±0.1) x 10–5 S/cm), and smallest thermal activation energy (Ea = 0.06 eV) among all four MOFs, whereas Cs-MOF 4', which had the longest dπ–π (3.68 Å) and the lowest TTFTC•+ population (0.3%), exhibited the widest band gaps (Eel = 2.15 eV and Eopt = 2.11 eV) and the lowest room temperature electrical conductivity (1.3 (±0.3) x 10–7 S/cm). Consistent with their dπ–π (3.67 Å) and TTFTC•+ population (≤ 16%), the freshly prepared but not optimally oxidized K-MOF 2' and Rb-MOF 3' initially displayed intermediate band gaps (1.9 and 2.1 eV, respectively), but upon prolonged aerobic oxidation, which elevated their TTFTC•+ population to respective saturation levels (ca. 25 % and 10 %, respectively), their band gaps shrunk and electrical conductivity surged to 1.7 (±0.3) ´ 10–5 and 1.5 (±0.2) ´ 10–5 S/cm, respectively, surpassing that of 1'-ox. The experimental results displayed the combined effects of π–π-interactions and TTFTC•+ population, which together dictated the efficacy of through-space charge movement via TTFTC/TTFTC•+ IVCT interaction, whereas the computational studies indicated that charge movement through these MOFs occurred predominantly through the π-stacked TTFTC ligands. These comprehensive studies demonstrated that IVCT interactions between the mixed-valent redox-active ligands can promote through-space charge movement, delivering an emerging design strategy for the development of electrically conducting 3D MOFs.
porous materials, electrical conductivity, charge transfer
, Panda, D.
, Yadav, A.
, Zhou, W.
and Saha, S.
Effects of Intervalence Charge Transfer Interaction between π-Stacked Mixed Valent Tetrathiafulvalene Ligands on the Electrical Conductivity of 3D Metal-Organic Frameworks, Chemical Science
(Accessed December 4, 2023)