Local gate control of Mott metal-insulator transition in a 2D metal-organic framework.

Electron-electron interactions in materials lead to exotic many-body quantum phenomena, including Mott metal-insulator transitions (MITs), magnetism, quantum spin liquids, and superconductivity. These phases depend on electronic band occupation and can be controlled via the chemical potential. Flat bands in two-dimensional (2D) and layered materials with a kagome lattice enhance electronic correlations. Although theoretically predicted, correlated-electron Mott insulating phases in monolayer 2D metal-organic frameworks (MOFs) with a kagome structure have not yet been realised experimentally. Here, we synthesise a 2D kagome MOF on a 2D insulator. Scanning tunnelling microscopy (STM) and spectroscopy reveal a MOF electronic energy gap of ∼200 meV, consistent with dynamical mean-field theory predictions of a Mott insulator. Combining template-induced (via work function variations of the substrate) and STM probe-induced gating, we locally tune the electron population of the MOF kagome bands and induce Mott MITs. These findings enable technologies based on electrostatic control of many-body quantum phases in 2D MOFs.

Fast, accurate enthalpy differences in spin crossover crystals from DFT+U.

Spin crossover materials are bi-stable systems with potential applications as molecular scale electronic switches, actuators, thermometers, barometers, and displays. However, calculating the enthalpy difference, ΔH, between the high spin and low spin states has been plagued with difficulties. For example, many common density functional theory (DFT) methods fail to even predict the correct sign of ΔH, which determines the low temperature state. Here, we study a collection of Fe(II) and Fe(III) materials, where ΔH has been measured, which has previously been used to benchmark density functionals. The best performing hybrid functional, TPSSh, achieves a mean absolute error compared to experiment of 11 kJ mol-1 for this set of materials. However, hybrid functionals scale badly in the solid state; therefore, local functionals are preferable for studying crystalline materials, where the most interesting spin crossover phenomena occur. We show that both the Liechtenstein and Dudarev DFT+U methods are a little more accurate than TPSSh. The Dudarev method yields a mean absolute error of 8 kJ mol-1 for Ueff = 1.6 eV. However, the mean absolute error for both TPSSh and DFT+U is dominated by a single material, for which the two theoretical methods predict similar enthalpy differences-if this is excluded from the set, then DFT+U achieves chemical accuracy. Thus, DFT+U is an attractive option for calculating the properties of spin crossover crystals, as its accuracy is comparable to that of meta-hybrid functionals, but at a much lower computational cost.

Mechanomagnetics in Elastic Crystals: Insights from [Cu(acac)2 ].

We predict that the magnetic properties of [Cu(acac)2 ], an elastically flexible crystal, change drastically when the crystal is bent. It is found that unbent [Cu(acac)2 ] is an almost perfect Tomonaga-Luttinger liquid. Broken-symmetry density-functional calculations reveal that the magnetic exchange interactions along the chains are an order of magnitude larger than the interchain exchange. The geometrically frustrated interchain interactions cannot magnetically order the material at any experimentally accessible temperature. The ordering temperature (TN ), calculated from the chain-random-phase approximation, increases by 24 orders of magnitude when the material is bent. We demonstrate that geometric frustration both suppresses TN and enhances the sensitivity of TN to bending. In [Cu(acac)2 ], TN is extremely sensitive to bending but remains too low for practical applications, even when bent. Partially frustrated materials could achieve the balance of high TN and good sensitivity to bending required for practical applications of mechanomagnetic elastic crystals.

Spin-orbit coupling in phosphorescent iridium(III) complexes.

We study the excited states of two iridium(III) complexes with potential applications in organic light-emitting diodes: fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] and fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) [Ir(ptz)(3)]. Herein we report calculations of the excited states of these complexes from time-dependent density functional theory (TDDFT) with the zeroth-order regular approximation (ZORA). We show that results from the one-component formulation of ZORA, with spin-orbit coupling included perturbatively, accurately reproduce both the results of the two-component calculations and previously published experimental absorption spectra of the complexes. We are able to trace the effects of both scalar relativistic correction and spin-orbit coupling on the low-energy excitations and radiative lifetimes of these complexes. In particular, we show that there is an indirect relativistic stabilisation of the metal-to-ligand charge transfer (MLCT) states. This is important because it means that indirect relativistic effects increase the degree to which SOC can hybridise singlet and triplet states and hence plays an important role in determining the optical properties of these complexes. We find that these two compounds are remarkably similar in these respects, despite Ir(ppy)(3) and Ir(ptz)(3) emitting green and blue light respectively. However, we predict that these two complexes will show marked differences in their magnetic circular dichroism (MCD) spectra.

Gaseous adsorption in melanins: hydrophilic biomacromolecules with high electrical conductivities.

The melanins are an important class of multifunctional biomacromolecules that possess a number of intriguing physical and chemical properties including electrical and photoconductivity. Unusually for a conducting organic material, eumelanin is hydrophilic and its electrical properties are strongly dependent on its hydration state. We have therefore measured adsorption isotherms for two polar adsorbates, water and ethanol, in the pressed powder pellets of synthetic eumelanin typically used in electrical studies. We show that a simple kinetic monolayer Langmuir model describes the adsorption and find that there are strong adsorbate-eumelanin interactions in both cases. These isotherms allow the proper scaling of electrical conductivity data and in doing so make progress toward a better understanding of eumelanin electrical properties, which is a critical prerequisite to the design of new eumelanin-like bioelectronic materials.

Effect of irradiation-induced disorder on the conductivity and critical temperature of the organic superconductor kappa-(BEDT-TTF)2Cu(SCN)2.

We have introduced defects into clean samples of the organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2) in order to determine their effect on the temperature dependence of the interlayer conductivity and the critical temperature T(c). We find a violation of Matthiessen's rule that can be explained by a model of involving a defect-assisted interlayer channel which acts in parallel with the bandlike conductivity. We observe an unusual dependence of T(c) on residual resistivity, inconsistent with the generalized Abrikosov-Gor'kov theory for an order parameter with a single component, providing an important constraint on models of the superconductivity in this material.

Chemical and structural disorder in eumelanins: a possible explanation for broadband absorbance.

We report the results of an experimental and theoretical study of the electronic and structural properties of a key eumelanin precursor-5,6,-dihydroxyindole-2-carboxylic acid (DHICA)-and its dimeric forms. We have used optical spectroscopy to follow the oxidative polymerization of DHICA to eumelanin and observe red shifting and broadening of the absorption spectrum as the reaction proceeds. First principles density functional theory calculations indicate that DHICA oligomers (possible reaction products of oxidative polymerization) have the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital red-shifted gaps with respect to the monomer. Furthermore, different bonding configurations (leading to oligomers with different structures) produce a range of gaps. These experimental and theoretical results lend support to the chemical disorder model where the broadband monotonic absorption characteristic of all melanins is a consequence of the superposition of a large number of nonhomogeneously broadened Gaussian transitions associated with each of the components of a melanin ensemble. These results suggest that the traditional model of eumelanin as an amorphous organic semiconductor is not required to explain its optical properties and should be thoroughly reexamined. These results have significant implications for our understanding of the physics, chemistry, and biological function of these important biological macromolecules. Indeed, one may speculate that the robust functionality of melanins in vitro is a direct consequence of its heterogeneity, i.e., chemical disorder is a "low cost" natural resource in these systems.

Towards structure-property-function relationships for eumelanin.

We discuss recent progress towards the establishment of important structure-property-function relationships in eumelanins-key functional bio-macromolecular systems responsible for photo-protection and immune response in humans, and implicated in the development of melanoma skin cancer. We focus on the link between eumelanin's secondary structure and optical properties such as broad band UV-visible absorption and strong non-radiative relaxation; both key features of the photo-protective function. We emphasise the insights gained through a holistic approach combining optical spectroscopy with first principles quantum chemical calculations, and advance the hypothesis that the robust functionality characteristic of eumelanin is related to extreme chemical and structural disorder at the secondary level. This inherent disorder is a low cost natural resource, and it is interesting to speculate as to whether it may play a role in other functional bio-macromolecular systems.