Publications

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.

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.

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. 

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. 

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. 

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.