Prospects for electroactive and conducting framework materials

Role of Counterions in the Structural Stabilisation of Redox-Active Metal-Organic Frameworks

The crystal structures of metal-organic frameworks (MOFs) are typically determined by the strong chemical bonds formed between the organic and inorganic building units. However, the latest generation of redox-active frameworks often rely on counterions in the pores to access specific charge states of the components. Here, we model the crystal structures of three layered MOFs based on the redox-active ligand 2,5-dihydroxybenzoquinone (dhbq): Ti2 (Cl2 dhbq)3 , V2 (Cl2 dhbq)3 and Fe2 (Cl2 dhbq)3 with implicit and explicit counterions. Our full-potential first-principles calculations indicate that while the reported hexagonal structure is readily obtained for Ti and V, the Fe framework is stabilised only by the presence of explicit counterions. For high counterion concentrations, we observe the formation of an electride-like pocket in the pore center. An outlook is provided on the implications of solvent and counterion control for engineering the structures and properties of porous solids.

Multi-Redox Responsive Behavior in a Mixed-Valence Semiconducting Framework Based on Bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane

The two-dimensional (2-D) framework, [Cu(BTDAT)(MeOH)] {BTDAT = bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane}, possesses remarkable multi-step redox properties, with electrochemical studies revealing six quasi-stable redox states in the solid state. In situ electron paramagnetic resonance and visible-near infrared spectroelectrochemistry elucidated the mechanism for these multi-step redox processes, as well as the optical and electrochromic behavior of the BTDAT ligand and framework. In studying the structural, spectroscopic, and electronic properties of [Cu(BTDAT)(MeOH)], the as-synthesized framework was found to exist in a mixed-valence state with thermally-activated semiconducting behavior. In addition to pressed pellet conductivity measurements, single-crystal conductivity measurements using a pre-patterned polydimethylsiloxane layer on a silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state of [Cu(BTDAT)(MeOH)], computational band structure calculations predicted delocalized electronic transport in the framework. On the balance of probabilities, we propose that [Cu(BTDAT)(MeOH)] is a Mott insulator (i.e., electron correlations cause a metal-insulator transition). This implies that the conductivity is incoherent. However, we are unable to distinguish between activated transport due to Coulombically bound electron-hole pairs and a hopping mechanism. The combined electrochemical, electronic, and optical properties of [Cu(BTDAT)(MeOH)] shine a new light on the experimental and theoretical challenges for electroactive framework materials, which are implicated as the basis of advanced optoelectronic and electrochromic devices.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

A cofacial metal-organic framework based photocathode for carbon dioxide reduction

Innovative and robust photosensitisation materials play a cardinal role in advancing the combined effort towards efficient solar energy harvesting. Here, we demonstrate the photocathode functionality of a Metal-Organic Framework (MOF) featuring cofacial pairs of photo- and electro-active 1,4,5,8-naphthalenediimide (NDI) ligands, which was successfully applied to markedly reduce the overpotential required for CO2 reduction to CO by a well-known rhenium molecular electrocatalyst. Reduction of [Cd(DPNDI)(TDC)] n (DPNDI = N,N'-di(4-pyridyl)-1,4,5,8-naphthalenediimide, H2TDC = thiophene-2,5-dicarboxylic acid) to its mixed-valence state induces through-space Intervalence Charge Transfer (IVCT) within cofacial DPNDI units. Irradiation of the mixed-valence MOF in the visible region generates a DPNDI photoexcited radical monoanion state, which is stabilised as a persistent species by the inherent IVCT interactions and has been rationalised using Density Functional Theory (DFT). This photoexcited radical monoanion state was able to undergo charge transfer (CT) reduction of the rhenium molecular electrocatalyst to effect CO generation at a lower overpotential than that required by the discrete electrocatalyst itself. The exploitation of cofacial MOFs opens new directions for the design philosophy behind light harvesting materials.

Quantification of the mixed-valence and intervalence charge transfer properties of a cofacial metal-organic framework via single crystal electronic absorption spectroscopy

Gaining a fundamental understanding of charge transfer mechanisms in three-dimensional Metal-Organic Frameworks (MOFs) is crucial to the development of electroactive and conductive porous materials. These materials have potential in applications in porous conductors, electrocatalysts and energy storage devices; however the structure-property relationships pertaining to charge transfer and its quantification are relatively poorly understood. Here, the cofacial Cd(ii)-based MOF [Cd(BPPTzTz)(tdc)]·2DMF (where BPPTzTz = 2,5-bis(4-(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole, tdc^2- = 2,5-thiophene dicarboxylate) exhibits Intervalence Charge Transfer (IVCT) within its three-dimensional structure by virtue of the close, cofacial stacking of its redox-active BPPTzTz ligands. The mixed-valence and IVCT properties are characterised using a combined electrochemical, spectroelectrochemical and computational approach. Single crystal electronic absorption spectroscopy was employed to obtain the solid-state extinction coefficient, enabling the application of Marcus-Hush theory. The electronic coupling constant, H _ab, of 145 cm^-1 was consistent with the localised mixed-valence properties of both this framework and analogous systems that use alternative methods to obtain the H _ab parameter. This work demonstrates the first report of the successful characterisation of IVCT in a MOF material using single crystal electronic absorption spectroscopy and serves as an attractive alternative to more complex methods due to its simplicity and applicability.

Electrically Conductive Metal-Organic Frameworks

Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the “through-bond” approach, continuous chains of coordination bonds between the metal centers and ligands’ functional groups create charge transport pathways. In the “extended conjugation” approach, the metals and entire ligands form large delocalized systems. The “through-space” approach harnesses the π–π stacking interactions between organic moieties. The “guest-promoted” approach utilizes the inherent porosity of MOFs and host–guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.

Electrical conductivity and magnetic bistability in metal–organic frameworks and coordination polymers: charge transport and spin crossover at the nanoscale

This review aims to reassess the progress, issues and opportunities in the path towards integrating conductive and magnetically bistable coordination polymers and metal–organic frameworks as active components in electronic devices.

Introduction: minerals to metal-organic frameworks

Mineralogy and materials design have always been closely intertwined. Here, I review some of the earliest work in modern materials chemistry to explicitly take inspiration from mineral structures and properties, and introduce the invited contributions to this theme issue. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'.