Copper-bipyridine grid frameworks incorporating redox-active tetrathiafulvalene: structures and supercapacitance.

Redox active tetrathiafulvalene (TTF) and its derivatives when used as electrode additives have exhibited improved energy efficiency and sustainability in batteries. However, the structure-property relationship has not been investigated in detail until very recently. In this work, three redox-active TTF compounds were synthesized, and formulated as [Cu(HL)(bpa)] (1), [Cu(bpe)(HO)]·2n(HL)·nMeOH·nHO (2), and [Cu(bpp)(HO)]·2n(HL) (3) (L = dimethylthio-tetrathiafulvalene-bicarboxylate) for this work. The effects of conjugated state and spacer length of the linkers on structural assembly and band gap as well as the interactions of TTF-TTF/TTF-bpy are discussed. Compound 1 is a bpa and HL co-coordinated 1D Cu(ii) polymer. Compounds 2 and 3 are 2D Cu(ii)-bipyridine (4,4) MOFs incorporating HL (1) as free anion columns. The photocurrent density of 2 is larger than those of 1 and 3 due to a strong charge transfer from TTF to bpe in compound 2. The supercapacitance performances of these compounds were evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) techniques. The results revealed that the 2D MOF structures of 2 and 3 are beneficial for good specific capacitance values (C). This work revealed the structure-property relationships of TTF derivatives for use as electrode active materials in energy transfer and storage.