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Titanium-oxo cluster (TOC)-based metal–organic frameworks (MOFs) have received considerable attention in recent years due to their ability to expand the application of TOCs to fields that require highly stable frameworks. Herein, a new cyclic TOC formulated as [Ti6O6(OiPr)8(TTFTC)(phen)2]2(1, where TTFTC = tetrathiafulvalene tetracarboxylate and phen = phenanthroline) was crystallographically characterized. TOC 1takes a rectangular ring structure with two phen-modified Ti6clusters as the width and two TTFTC ligands as the length. An intracluster ligand-to-ligand (TTF-to-phen) charge transfer in 1was found for TOCs for the first time. Compound 1undergoes topotactic conversion to generate stable TOC-MOF P1, in which the rectangular framework in 1formed by a TOC core and ligands is retained, as verified by comprehensive characterization. P1shows an efficient and rapid selective adsorption capacity for cationic dyes. The experimental adsorption capacity (qex) of P1reaches a value of up to 789.2 mg/g at 298 K for the crystal violet dye, which is the highest among those of various adsorbents. The calculated models are first used to reveal the structure–property relationship of the cyclic host to different guest dyes. The results further confirmed the host MOF structure of P1.

Metal–organic frameworks (MOFs) have attracted noticeable attention as promising candidates for electrochemical energy storage. However, the lack of electrical conductivity and the weak stability of most MOFs result in poor electrochemical performances. Here, a tetrathiafulvalene (TTF)-based complex, formulated as [(CuCN)2(TTF(py)4)] (1) (TTF-(py)4= tetra(4-pyridyl)-TTF), is assembled by in situ generation of coordinated CN–from a nontoxic source. Single-crystal X-ray diffraction analysis reveals that compound 1possesses a two-dimensional layered planar structure, which is further stacked in parallel to form a three-dimensional supramolecular framework. The planar coordination environment of 1is the first example of a TTF-based MOF. Attributed to the unique structure and redox TTF ligand, the electrical conductivity of 1is significantly increased by 5 orders of magnitude upon iodine treatment. The iodine-treated 1(1-ox) electrode displays typical battery-type behavior through electrochemical characterizations. The supercapattery based on the 1-oxpositrode and AC negatrode presents a high specific capacity of 266.5 C g–1at a specific current of 1 A g–1with a remarkable specific energy of 62.9 Wh kg–1at a specific power of 1.1 kW kg–1. The excellent electrochemical performance of 1-oxis one of the best among those reported supercapatteries, demonstrating a new strategy for developing MOF-based electrode materials.

Poor electrical conductivity and instability of metal–organic frameworks (MOFs) have limited their energy storage and conversion efficiency. In this work, we report the application of oxidatively doped tetrathiafulvalene (TTF)-based MOFs for high-performance electrodes in supercapatteries. Two isostructural MOFs, formulated as [M(py-TTF-py)(BPDC)]·2H2O (M = NiII(1), ZnII(2); py-TTF-py = 2,6-bis(4′-pyridyl)TTF; H2BPDC = biphenyl-4,4′-dicarboxylic acid), are crystallographically characterized. The structural analyses show that the two MOFs possess a three-dimensional 8-fold interpenetrating diamond-like topology, which is the first example for TTF-based dual-ligand MOFs. Upon iodine treatment, MOFs 1and 2are converted into oxidatively doped 1-oxand 2-oxwith high crystallinity. The electrical conductivity of 1-oxand 2-oxis significantly increased by six∼seven orders of magnitude. Benefiting from the unique structure and the pronounced development of electrical conductivity, the specific capacities reach 833.2 and 828.3 C g–1at a specific current of 1 A g–1for 1-oxand 2-ox, respectively. When used as a battery-type positrode to assemble a supercapattery, the AC∥1-oxand AC∥2-ox(AC = activated carbon) present an energy density of 90.3 and 83.0 Wh kg–1at a power density of 1.18 kW kg–1and great cycling stability with 82% of original capacity and 92% columbic efficiency retention after 10,000 cycles. Ex situ characterization illustrates the ligand-dominated mechanism in the charge/discharge processes. The excellent electrochemical performances of 1-oxand 2-oxare rarely reported for supercapatteries, illustrating that the construction of unique highly dense and robust structures of MOFs followed by postsynthetic oxidative doping is an effective approach to fabricate MOF-based electrode materials.

Organic–inorganic hybrid metal–polyphenols as stable structural modules have gained extensive interest due to their diverse applications. However, titanium-oxo compounds (TOCs) with large molecular polyphenols have been less explored, and they were expected to be different from small polyphenols with isolated metal ions. Herein, 4-methyl-esculetin (Mesc), a catechol derivative, was selected to construct three TOCs, namely, [Ti17O24(Mesc)4(OiPr)16] (1), [Ti12O14(OiPr)18][Ti16O14(Mesc)12(OiPr)14] (2), and [Ti3O(Mesc)2(OAc)2(OiPr)4] (3). These compounds were structurally characterized. Photocurrent responses were evaluated using the compound-sensitized TiO2electrodes. It was found that the current densities of 1–3electrodes are in the order of 1≫ 3> 2, which relates to the ligand-to-TiO core and ligand-to-ligand charge transfers (LMCT and LLCT, respectively). Density functional theory calculations showed that the lowest band gap of 1originates from its LLCT. Compound 1reacted with polyphenol tannin (TA) to form a fully transparent and robust gel (1–TA), and the gelation properties were investigated. Using the gel as a nano-TiO2fixing agent, solar cell electrodes were prepared by a low-temperature wet method. The photocurrent responsive behavior of the 1–TA/TiO2electrode was compared with that of the 1-sensitized traditional high-temperature-treated TiO2electrode. Although the current density of the former is somewhat lower than that of the traditional electrode, the low-temperature wet preparation of the 1–TA/TiO2electrode is more energy-efficient and sustainable.