Chanel F. Leong

Controlling charge separation in a novel donor–acceptor metal–organic framework via redox modulation

Charge transfer metal–organic frameworks represent a versatile class of multifunctional materials that offer an unprecedented combination of physical properties. The framework [(Zn(DMF))2(TTFTC)(DPNI)] incorporating the donor and acceptor, tetrathiafulvalenetetracarboxylate (TTFTC) and N,N′-di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNI) respectively, exhibits charge transfer by virtue of donor–acceptor interactions within its crystalline structure. This through-space interaction is manifested by the formation of ligand-based radicals in the as-synthesised material and leads to a partial degree of charge separation. Five distinct electronic states of the framework can be accessed using solid state electrochemical and spectroelectrochemical techniques, including for the first time in application to metal–organic frameworks, EPR spectroelectrochemistry (SEC). The degree of charge transfer is controllable via redox modulation and has been quantified using complementary DFT modelling of the charge transfer states.

Crystal Structures, Magnetic Properties, and Electrochemical Properties of Coordination Polymers Based on the Tetra(4-pyridyl)-tetrathiafulvalene Ligand

Seven new coordination polymers based on the redox-active tetra(4-pyridyl)-tetrathiafulvalene ligand (TTF(py)4) and different transition-metal ions, namely, {[Cu(hfac)2][TTF(py)4]·2(CH2Cl2)}n (1), {[Co(acac)2][TTF(py)4]0.5·(CHCl3)}n (2), {[Mn(hfac)2][TTF(py)4]0.5}n (3), {[Cu2(OAc)4][TTF(py)4]0.5·1.5(CHCl3)·0.5(H2O)·(CH3CN)}n (4), {[Mn(SCN)2][TTF(py)4]·6(CH2Cl2)}n (5), {[Mn(SeCN)Cl][TTF(py)4]}n (6), and {Cu2[TTF(py)4]2·(ClO4)2·2.5(CH2Cl2)·1.5(CH3CN)}n (7), were synthesized and characterized. The tetrapyridyl ligand coordinates to metal ions in a bidentate or tetradentate fashion, forming complexes 1-7 with different structures. Complex 1 exhibits a one-dimensional chain structure. Complexes 2, 3, and 4 possess similar (4,2)-connected binodal two-dimensional networks, while complexes 5 and 6 have similar (4,4)-connected binodal two-dimensional networks with two different rings. Complex 7 shows a 2-fold interpenetrated (4,4)-connected binodal PtS-type three-dimensional framework. Meanwhile, these complexes feature diverse nonclassical hydrogen bonding interactions. In addition, magnetic and solid-state electrochemical properties for typical complexes have been studied.

Synthesis, properties and surface self-assembly of a pentanuclear cluster based on the new π-conjugated TTF-triazole ligand

The new π-extended redox-active ligand with both TTF and triazole units, 6-(4,5-bis(propylthio)-1,3-dithiol-2-ylidene)-1H-[1,3]dithiolo[4′,5′:4,5]benzo [1,2-d] [1–3]triazole, has been successfully prepared. Based on the versatile ligand and Cu(tta)2 precursors (tta− = 4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-dione), a TTF-based pentanuclear CuII cluster (Cu5(tta)4(TTFN3)6) is synthesized and structurally characterized. Their absorption and electrochemical properties are investigated. Antiferromagnetic couplings are operative between metal ion centers bridged by triazoles in the complex. The self-assembled structure of the cluster complex on a highly oriented pyrolytic graphite (HOPG) surface was observed using scanning tunneling microscopy and density functional theory (DFT) calculations have been performed to provide insight into the formation mechanism. The introduction of the redox-active TTF unit into the cluster complexes with interesting magnetic properties renders them promising candidates for new multifunctional materials.

Mixed Valency as a Strategy for Achieving Charge Delocalization in Semiconducting and Conducting Framework Materials

The fundamentally important phenomenon of mixed valency has been discussed in detail over the past 50 years, predominantly in the context of dinuclear complexes, which are used as model systems for understanding electron delocalization in more complex biological and physical systems. Very recently, mixed valency has been shown to be an important mechanism for charge transfer, leading to delocalization and conductivity in two- and three-dimensional framework materials such as metal-organic frameworks and related systems including covalent organic frameworks and semicrystalline semiconducting metal-organic graphenes. This Viewpoint provides a current perspective on the field of mixed-valence frameworks, where the property is either intrinsic or generated postsynthetically via an external stimulus. Aspects of the spectroscopy and applications of these materials are also discussed, highlighting the future potential for exploiting mixed valency in extended solid-state systems.

Chapter 7:Conducting Framework Materials

Electronic conductivity in framework materials is a highly-sought after functional property, with the capacity to revolutionise a wide range of technologically- and industrially-useful fields. Examples of conducting metal-organic frameworks remain relatively limited at the present time; however enormous advances have been made over the past five years. This chapter details the latest developments in the exciting research frontier of electronically-conducting metal-organic frameworks (MOFs). Central to the discussion is a series of parameters which have now emerged for materials’ design, including redox matching, donor–acceptor, mixed-valence, and π-interactions. At both the fundamental and applied levels, enormous opportunities exist if long-range electronic conductivity can be harnessed, including intimate understandings of charge transport in 3D coordination space, and potential applications across catalysis, solid-state sensing, energy storage and conversion devices, amongst numerous others.