Owing to their diverse structures, compositions, and potentials to advance modern electronics and energy conversion and storage technologies, electrically conductive metal-organic frameworks (MOFs) have become one of the most sought functional materials over the past decade. In order to exhibit useful electrical conductivity, the MOFs must be equipped with adequate charge-carrier density and efficient charge transport pathways. Understanding the correlations between their structures, compositions, charge transport mechanism, and electronic properties is the key to developing appropriate conducting frameworks suitable for specific applications. To this end, herein, we have constructed four 3D frameworks (1-4) having a general formula of M4TTFTC (M = Na, K, Rb, and Cs) based on electron-rich tetrathiafulvalene tetracarboxylate (TTFTC) ligand and demonstrated how the π-π distances between the overlapping TTF cores and the population of TTFTC●+ radical cations influenced the electronic band gaps and electrical conductivity of these frameworks. The crystal structure showed that Na-MOF 1 has the shortest π-π distance (ca. 3.39 Å) with 33.9 % π-surface overlap between the π-stacked TTF cores, while the electron paramagnetic resonance (EPR) revealed that it contained the population (ca. 23.3%) of aerobically oxidized TTFTC●+ radical cations in freshly prepared samples. On the other hand, K-MOF 2 and Rb-MOF 3 showed wider dπ-π (ca. 3.67 Å) and contained fewer TTFTC●+ radical cations than 1 at the beginning, but the number of the TTFTC●+ radical cations increased gradually, reaching an optimum population of 25.4% and 10.3%, respectively after 7 days. In contrast, Cs-MOF 4 had the longest π-π distance (ca. 3.68 Å) but the least π-surface overlap (ca. 0.7%). Having the shortest π-π-distance and most effective overlap between the π-stacked TTFTC ligands as well as the highest population of aerobically oxidized TTFTC●+ radical cations among all four freshly prepared MOFs, Na-MOF 1 displayed the highest electrical conductivity (3.6 x 10-5 S/cm at 293 K). In comparison, freshly prepared K-MOF 2 and Rb-MOF 3 initially displayed lower conductivity than 1, but their conductivity increased after 7 days via aerobic oxidation. Cs-MOF 4, which had the longest π-π distance and practically no detectable TTFTC●+ radical cations, displayed the lowest conductivity. These comprehensive studies revealed the structure-composition-property relationships in a family of TTFTC-based MOFs.

Electrically conductive 3D metal-organic frameworks containing π-stacked mixed-valent tetrathiafulvalene ligands

ACS Spring Meeting 2021

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