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

R. Murase, T. A. Hudson, T. Aldershof, K. Nguyen, J. G. Gluschke, E. P. Kenny, X. Zhou, T. Wang, M. P. van Koeverden, B. J. Powell, A. P. Micolich, B. F. Abrahams, D. M. D'Alessandro, Journal of the American Chemical Society, 2022, Accepted June 2022.

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 (EPR) and visible-near infrared spectroelectrochemistry elucidated the mechanism for the 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 (PDMS) on silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state [Cu(BTDAT)(MeOH)], computational band structure calculations confirmed the delocalized nature of the framework. 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.

E. R. Kearns, R. Gillespie, D. M. D'Alessandro, Journal of Materials Chemistry A, 2021, DOI: 10.1039/D1TA08777K.

The world is facing a climate emergency: unchecked pollution coupled with rising CO2 levels is putting unprecedented strain on the planet’s ecosystems. Technologies for environmental remediation are thus becoming increasingly important. One very promising class of candidate materials for these applications is Metal-Organic Frameworks (MOFs). Owing to their ultrahigh surface areas and the tunability of their chemical structures, these porous materials show enormous promise in catalysis, adsorption and separations. To date, MOFs have not been well represented in industry, in part due to difficulties in shaping and handling the polycrystalline powders. 3D Printing offers a powerful and versatile approach to shape MOFs into monoliths with a broad range of uses from gas storage and separations to light generation. This review focuses on recent progress in shaping MOFs via 3D printing with the current state-of-the-art in energy and environmental applications. Several techniques are examined including Fused filament Fabrication (FFF), Digital Light Processing (DLP), Selective Laser Sintering (SLS) and Direct Ink Writing (also know as Robocasting). At present, the latter technique is most compatible with MOFs and is represented in almost every application examined herein. However, more complex techniques, such as DLP and SLS, show great promise, and as the techniques continue to develop may also become prominent methods for shaping industrially applied MOF-based technologies.

T. Wang, R. Sabatini, B. Chan, J. Hou, V. Huynh, N. Proschogo, Z. Xie, L. Gao, J. Zhang, B. Hawkett, R. Clarke, C.J. Kepert, V. Chen, G. Lakhwani, D. M. D'Alessandro, ACS Materials Letters, 2021, DOI: 10.1021/acsmaterialslett.1c00332.

Nanoconfinement offers opportunities to tune physical properties of molecular entities by altering their assembled structures. This also applies to acene-based molecules with potentially rich π-π interactions. Unlike most of the previous cases with acene-based guests directly incorporated into hosts, we take a further step by oligomerizing a fluorescent anthryl monomer, 9-vinylanthracene, inside nanochannels of a metal-organic framework, which is a pillared 3-dimensional kagome net of [Zn2(bdc)2(dabco)] (bdc2- = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). The fluorescence emission of the guest can be significantly enhanced after oligomerization, which is likely due to the suppressed molecular motion of oligomerized molecules in the nanospace. The case we have demonstrated for fluorescence enhancement via confined oligomerization provides inspiration for the design of luminescent composites and is encouraging for further exploration of molecules in nanoconfined space.

B. Ding, B. Chan, N. Proschogo, M. B. Solomon, C. J. Kepert and D. M. D'Alessandro, Chemical Science, 2021, 12, 3608-3614.

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.

D. M. D'Alessandro and P. M. Usov, Australian Journal of Chemistry, 2021, 74, 77–93. {Invited Beckwith Review}

Spectroelectrochemistry (SEC) encompasses a broad suite of electroanalytical techniques where electrochemistry is coupled with various spectroscopic methods. This powerful and versatile array of methods is characterised as in situ, where a fundamental property is measured in real time as the redox state is varied through an applied voltage. SEC has a long and rich history and has proved highly valuable for discerning mechanistic aspects of redox reactions that underpin the function of biological, chemical, and physical systems in the solid and solution states, as well as in thin films and even in single molecules. This perspective article highlights the state of the art in solid-state SEC (ultraviolet–visible–near-infrared, infrared, Raman, photoluminescence, electron paramagnetic resonance, and X-ray absorption spectroscopy) relevant to interrogating solid state materials, particularly those in the burgeoning field of metal–organic frameworks (MOFs). Emphasis is on developments in the field over the past 10 years and prospects for application of SEC techniques to probing fundamental aspects of MOFs and MOF-derived materials, along with their emerging applications in next-generation technologies for energy storage and transformation. Along with informing the already expert practitioner of SEC, this article provides some guidance for researchers interested in entering the field.

B. Ding, M. B. Solomon, C. F. Leong and D. M. D'Alessandro, Coordination Chemistry Reviews, 2021, 439, 213891, DOI: 10.1016/j.ccr.2021.213891.

Over the past three decades, the field of coordination polymers (CPs) and metal–organic frameworks (MOFs) has developed rapidly due to the potential applications of these materials in gas storage, separations, catalysis and switching. Introducing redox properties into CPs and MOFs to electrochemically modulate their properties for the development of solid state electronic devices has been an interesting strategy applied in recent years. Notably, a challenge within this area is the engineering of framework materials with desired redox properties targeted at a specific function. This review explores some of the key redox-active ligands that have been investigated within the previous eight years as redox-active components of solid state materials and examines a number of key strategies for their integration into CPs and MOFs. This review further highlights emergent redox-active ligands in the literature, which take inspiration from the rich field of organic electronics. While these ligands remain largely unexplored in the field of MOFs and CPs, they offer new opportunities for the improvement of solar cells, light induced photo switches, efficient photoelectrocatalysts and long range, rapid charge transfer in MOF materials.

D. A. Sherman, R. Murase, S. G. Duyker, Q. Gu, W. Lewis, T. Lu, Y. Liu, D. M. D'Alessandro, Nature Communications, 2020, DOI: 10.1038/s41467-020-15510-7.

Reversible structural transformations of porous coordination frameworks in response to external stimuli such as light, electrical potential, guest inclusion or pressure, amongst others, have been the subject of intense interest for applications in sensing, switching and molecular separations. Here we report a coordination framework based on an electroactive tetrathiafulvalene exhibiting a reversible single crystal-to-single crystal double [2 + 2] photocyclisation, leading to profound differences in the electrochemical, optical and mechanical properties of the material upon light irradiation. Electrochemical and in situ spectroelectrochemical measurements, in combination with in situ light-irradiated Raman spectroscopy and atomic force microscopy, revealed the variable mechanical properties of the framework that were supported using Density Functional Theory calculations. The reversible structural transformation points towards a plethora of potential applications for coordination frameworks in photo-mechanical and photoelectrochemical devices, such as light-driven actuators and photo-valves for targeted drug delivery.

A. Das and D. M. D'Alessandro, CrystEngComm, 2015, 17, 706-718. {Invited contribution; Front cover}

Metal–organic frameworks (MOFs) have been targeted as solid state sorbents for postcombustion carbon dioxide capture due, in part, to the enormous tunability of their structures through the incorporation of different functional sites. The isosteric heat of adsorption (Qst) provides one measure of the interaction of a solid sorbent with guest molecules, and has a bearing on the low pressure (<1 bar) CO2 uptake, selectivity and regenerability of a material. It is a key factor in the design of adsorbents for gas separation; however, it is sometimes overlooked in the evaluation of MOFs for CO2 capture. This highlight article draws together the impact of various functional sites on the CO2 heat of adsorption, and examines the interplay between functional sites and other factors such as competing water adsorption that influence a material's suitability for CO2 capture from industrial streams.

A. Das, M. Choucair, P. D. Southon, J. A. Mason, M. Zhao, C. J. Kepert, A. T. Harris and D. M. D'Alessandro, Microporous and Mesoporous Materials, 2013, 174, 74-80.

Post-synthetic modification of H3[(Cu4Cl)3(BTTri)8] or CuBTTri, where H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene, with piperazine (pip) has yielded the grafted framework H3[(Cu4Cl)3(BTTri)8(pip)12], pip-CuBTTri, which exhibits an improved CO2 uptake at pressures pertinent to postcombustion flue gas capture compared with the non-grafted material. In particular, the volumetric capacity of pip-CuBTTri was 2.5 times higher than that of CuBTTri at ca. 0.15 bar and 293 K. A chemisorption mechanism for CO2 adsorption was proposed on the basis of diffuse reflectance infrared spectra (DRIFTS) and the high initial isosteric heat of adsorption (−Qst, ≈96.5 kJ/mol). Application of the Ideal Adsorbed Solution Theory (IAST) to a simulated mixture of 0.15 bar CO2/0.75 bar N2 revealed a selectivity factor of 130. Both pressure and temperature swing processes were found to be suitable for facile regeneration of the material over multiple adsorption–desorption cycles.

C. F. Leong, T. B. Faust, P. Turner, P. M. Usov, C. J. Kepert, R. Babarao, A. W. Thornton and D. M. D'Alessandro, Dalton Transactions, 2013, 42, 9831-9839. {Front Cover; Top 10 most accessed articles in March 2013}

A new microporous framework, Zn(NDC)(DPMBI) (where NDC = 2,7-naphthalene dicarboxylate and DPMBI = N,N′-di-(4-pyridylmethyl)-1,2,4,5-benzenetetracarboxydiimide), containing the redox-active benzenetetracarboxydiimide (also known as pyromellitic diimide) ligand core has been crystallographically characterised and exhibits a BET surface area of 608.2 ± 0.7 m2 g−1. The crystallinity of the material is retained upon chemical reduction with sodium naphthalenide (NaNp), which generates the monoradical anion of the pyromellitic diimide ligand in the framework Zn(NDC)(DPMBI)·Nax (where x represents the molar Na+/Zn2+ ratio of 0.109, 0.233, 0.367 and 0.378 from ICP-AES), as determined by EPR, solid state Vis-NIR spectroelectrochemistry and UV-Vis-NIR spectroscopy. The CO2 uptake in the reduced materials relative to the neutral framework is enhanced up to a Na+/Zn2+ molar ratio of 0.367; however, beyond this concentration the surface area and CO2 uptake decrease due to pore obstruction. The CO2 isosteric heat of adsorption (|Qst|) and CO2/N2 selectivity (S), obtained from pure gas adsorption isotherms and Ideal Adsorbed Solution Theory (IAST) calculations, are also maximised relative to the neutral framework at this concentration of the alkali metal counter-ion. The observed enhancement in the CO2 uptake, selectivity and isoteric heat of adsorption has been attributed to stronger interactions between CO2 and both the radical DPMBI ligand backbone and the occluded Na+ ions.

A. Das, P. D. Southon, M. Zhao, C. J. Kepert, A. T. Harris and D. M. D'Alessandro, Dalton Transactions, 2012, 41, 11739-11744.

The metal–organic framework Ni2(dobdc) (CPO-27-Ni, where dobdc = 1,4-dioxido-2,5-benzenedicarboxylate) has been post-synthetically modified with piperazine (pip) – a known ‘accelerator’ to improve the kinetics of CO2 uptake in alkanolamine solvents for chemical absorption – and the impact of the modification on the CO2 uptake and selectivity over N2 has been probed. While the modified framework, Ni2(dobdc)(pip)0.5 (pip-CPO-27-Ni), exhibits a lower uptake of CO2 compared with the non-grafted material, the selectivity for CO2 over N2 at 25 °C and at pressures pertinent to post-combustion flue gas capture (0.1–0.15 bar) is enhanced. Mechanistically, the interaction between the CO2 molecules and the free amine sites in pip-CPO-27-Ni occurs via physisorption and chemisorption interactions, in which CO2 binds to the framework with an isosteric heat of adsorption (−Qst) of 40.5 kJ mol−1 at very low coverage (P = 0.033 mbar), followed by binding at a higher heat of adsorption (−Qst = 46.2 kJ mol−1 at P = 3.55 mbar). Pure water adsorption isotherms revealed a two-step mechanism for uptake in CPO-27-Ni, consistent with adsorption into the first and second hydration spheres of Ni2+ followed by subsequent uptake via physisorption into the pores. Additional steric hindrance in pip-CPO-27-Ni results in a single step only. The working capacity over multiple cycles was also investigated using a temperature swing adsorption process which revealed reversible CO2 adsorption and desorption of 10 wt% over 10 cycles.

D. M. D'Alessandro, B. Smit and J. R. Long, Angewandte Chemie-International Edition, 2010, 49, 6058-6082.

The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO2 emissions; however, currently the capture alone will increase the energy requirements of a plant by 25–40 %. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO2 emissions to the atmosphere, namely postcombustion (predominantly CO2/N2 separation), precombustion (CO2/H2) capture, and natural gas sweetening (CO2/CH4). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO2 separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal–organic frameworks.

B. Bechlars, D. M. D'Alessandro, D. M. Jenkins, A. T. Iavarone, S. D. Glover, C. P. Kubiak and J. R. Long, Nature Chemistry, 2010, 2, 362-368. {Highlighted in Nature Chem.2, 351-352}

The field of molecular magnetism has grown tremendously since the discovery of single-molecule magnets, but it remains centred around the superexchange mechanism. The possibility of instead using a double-exchange mechanism (based on electron delocalization rather than Heisenberg exchange through a non-magnetic bridge) presents a tantalizing prospect for synthesizing molecules with high-spin ground states that are well isolated in energy. We now demonstrate that magnetic double exchange can be sustained by simple imidazolate bridging ligands, known to be well suited for the construction of coordination clusters and solids. A series of mixed-valence molecules of the type [(PY5Me2)VII(µ-Lbr) VIII(PY5Me2)]4+ were synthesized and their electron delocalization probed through cyclic voltammetry and spectroelectrochemistry. Magnetic susceptibility data reveal a well-isolated S = 5/2 ground state arising from double exchange for [(PY5Me2)2V2(µ-5,6-dimethylbenzimidazolate)]4+. Combined modelling of the magnetic data and spectral analysis leads to an estimate of the double-exchange parameter of B = 220 cm−1 when vibronic coupling is taken into account.

D. M. D'Alessandro and F. R. Keene, Chemical Society Reviews, 2006, 35, 424-440.

Mixed-valence chemistry has a long and rich history which is characterised by a strong interplay of experimental, theoretical and computational studies. The intervalence charge transfer (IVCT) transitions generated in dinuclear mixed-valence species (particularly of ruthenium and osmium) have received considerable attention in this context, as they provide a powerful and sensitive probe of the factors which govern electronic delocalisation and the activation barrier to intramolecular electron transfer. This tutorial review discusses classical, semi-classical and quantum mechanical theoretical treatments which have been developed over the past 35 years for the analysis of IVCT absorption bands. Particular attention is drawn to the applicability of these models for the analysis of mixed-valence complexes which lie between the fully localised (Class II) and delocalised (Class III) limits in the “localised-to-delocalised” (Class II–III) regime. A clear understanding of the complex interplay of inter- and intramolecular factors which influence the IVCT process is crucial for the design of experimental studies to probe the localised-to-delocalised regime and in guidance of the development of appropriate theoretical models.