Robust Spin Liquidity in 2D Metal-Organic Framework Cu3 (HHTP)2 with S=1 /2 Kagome Lattice.

On one hand electron or hole doping of quantum spin liquid (QSL) may unlock high-temperature superconductivity and on the other hand it can disrupt the spin liquidity, giving rise to a magnetically ordered ground state. Recently, a 2D MOF, Cu3 (HHTP)2 (HHTP - 2,3,6,7,10,11-hexahydroxytriphenylene), containing Cu(II) S= 1 / 2 ${{ 1/2 }}$ frustrated spins in the Kagome lattice is emerging as a promising QSL candidate. Herein, we present an elegant in situ redox-chemistry strategy of anchoring Cu3 (HHTP)2 crystallites onto diamagnetic reduced graphene oxide (rGO) sheets, resulting in the formation of electron-doped Cu3 (HHTP)2 -rGO composite which exhibited a characteristic semiconducting behavior (5 K to 300 K) with high electrical conductivity of 70 S ⋅ m-1 and a carrier density of ~1.1×1018  cm-3 at 300 K. Remarkably, no magnetic transition in the Cu3 (HHTP)2 -rGO composite was observed down to 1.5 K endorsing the robust spin liquidity of the 2D MOF Cu3 (HHTP)2 . Specific heat capacity measurements led to the estimation of the residual entropy values of 28 % and 34 % of the theoretically expected value for the pristine Cu3 (HHTP)2 and Cu3 (HHTP)2 -rGO composite, establishing the presence of strong quantum fluctuations down to 1.5 K (two times smaller than the value of the exchange interaction J).

Intertwining of Localized (d) and Delocalized (π) Spins in Magnetically Frustrated Two-Dimensional Metal-Organic Frameworks.

Two-dimensional metal-organic frameworks (2D MOFs) are emerging as a new class of multifunctional materials for diversified applications, although magnetic properties have not been widely explored. The metal ions and organic ligands in some of the 2D MOFs are arranged in the well-known Kagome lattice, leading to geometric spin frustration. Hence, such systems could be the potential candidates to exhibit an exotic quantum spin liquid (QSL) state, as was observed in Cu3(HHTP)2 (HHTP = hexahydroxytriphenylene), with no magnetic transition down to 38 mK. Hereto, we have investigated the spin intertwining in a bimetallic 2D MOF system, M3(HHTP)2 (M = Cu/Zn), arising from the localized (d-electron) and delocalized (π-electron) S = 1/2 spins from the Cu(II) ions and the HHTP radicals, respectively. The origin of the spin frustration (down to 5K) was critically examined by varying the metal composition in bimetallic systems, CuxZn3-x(HHTP)2 (x = 1, 1.5, 2), containing both S = 1/2 and S = 0 spins. Additionally, to gain a deeper understanding, we studied the spin interaction in the pristine Zn3(HHTP)2 system containing only S = 0 Zn(II) ions. In view of the quantitative estimate of the localized and delocalized spins, the d-π spin correlation appears essential in understanding the unusual magnetic and/or other physical properties of such hybrid organic-inorganic 2D crystalline solids.

Ag Nanoparticles-Induced Metallic Conductivity in Thin Films of 2D Metal-Organic Framework Cu3(HHTP)2.

Two-dimensional (2D) metal-organic frameworks (MOFs) are usually associated with higher electrical conductivity and charge carrier mobility when compared with 3D MOFs. However, attaining metallic conduction in such systems through synthetic or postsynthetic modifications is extremely challenging. Herein, we present the fabrication of thin films of a 2D MOF, Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), decorated with silver nanoparticles (AgNPs) exhibiting significant conductivity enhancement at room temperature. Variable-temperature electrical transport measurements across the low-temperature (200 K) to high-temperature (373 K) regime evidenced metallic conduction. Interestingly, thin films of a 3D MOF, CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane), upon decoration with AgNPs, disclosed a converse trend. The origin of such distinctive observations on AgNPs@Cu3(HHTP)2 and AgNPs@CuTCNQ systems was comprehended by using first-principles density functional theory (DFT) calculations and attributed to an interfacial electronic effect. Our work sheds new light on rationally designing synthetic modifications in thin films of MOFs to tune the electrical transport property.

Thin Films of MOF-on-Guest@MOF: A Simple Strategy of Designing Electronic Heterostructures.

Depositing thin films of pristine metal-organic framework (MOF) on top of a lattice-matched and molecularly doped MOF could provide a new path for generating electronic heterostructures of MOFs with well-defined interfaces. Herein, the Cu3BTC2 (top-layer)/TCNQ@Cu3BTC2 (bottom-layer) system is fabricated by sequential deposition on a functionalized Au substrate, and clear-cut rectification of electrical current across the thin film was observed at room-temperature. Interestingly, the electrical current rectification ratio (RR) was found to be significantly influenced by the effect of temperature (400 K), resulting in a remarkable figure in the domain of MOFs.

Chemically Integrating a 2D Metal-Organic Framework with 2D Functionalized Graphene.

Two-dimensional metal-organic frameworks (2D MOFs) are the next-generation 2D crystalline solids. Integrating 2D MOFs with conventional 2D materials like graphene is promising for a variety of applications, including energy or gas storage, catalysis, and sensing. However, unraveling the importance of chemical interaction over an additive effect is essential. Here, we present an unconventional chemistry to integrate a Cu-based 2D MOF, Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), with 2D functionalized graphene, reduced graphene oxide (rGO), by an in situ oxidation-reduction reaction. Combined Raman spectroscopy, electron spin resonance (ESR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements along with structural analysis evidenced the chemical interaction between Cu-HHTP and rGO, which was subsequently assigned to be the key for the manifestation of significantly modified physical properties. Of particular mention is the conversion of an n-type crystalline solid to a p-type crystalline solid upon the chemical integration of Cu-HHTP with rGO, as revealed by Seebeck coefficient. More importantly, the thermoelectric power factor exhibited an increasing trend with increasing temperature, unlike an opposite trend observed due to an additive effect. The results anticipate the ability of a redox reaction to chemically integrate other 2D MOFs with rGO and show how an in situ synthesis can trigger chemical interaction between two distinctive 2D materials.

Exploring Magnetic XY Behavior in a Quasi-2D Anisotropic Triangular Lattice of Cu(II) by Functionalized Graphene.

Study on magnetism in two-dimensional (2D) spin-lattices is advancing rapidly. In this work, phase-pure botallackite (Bo) (Cu2(OH)3Br), a quasi-2D S = 1/2 anisotropic triangular spin-lattice is stabilized over 2D reduced graphene oxide (rGO) nanosheets via simple oxidation-reduction reaction chemistry. In comparison to polycrystalline Bo, such an anchoring resulted in the oriented growth of Bo crystallites in the Bo-rGO system. The Bo-rGO nanocomposite was found to be magnetically active with a Néel transition at ∼8.9 K, crossing over to possible XY anisotropy at ∼5 K-as revealed by complementary dc and ac susceptibility measurements-an unprecedented observation in the field assigned to an interfacial effect. This work demonstrates the potential usage of nonmagnetic 2D functionalized graphene to significantly modulate the magnetic properties of 2D spin-lattices.

Imparting Multifunctionality by Utilizing Biporosity in a Zirconium-Based Metal-Organic Framework.

In this work, we have synthesized nanocomposites made up of a metal-organic framework (MOF) and conducting polymers by polymerization of specialty monomers such as pyrrole (Py) and 3,4-ethylenedioxythiophene (EDOT) in the voids of a stable and biporous Zr-based MOF (UiO-66). FTIR and Raman data confirmed the presence of polypyrrole (PPy) and poly3,4-ethylenedioxythiophene (PEDOT) in UiO-66-PPy and UiO-66-PEDOT nanocomposites, respectively, and PXRD data revealed successful retention of the structure of the MOF. HRTEM images showed successful incorporation of polymer fibers inside the voids of the framework. Owing to the intrinsic biporosity of UiO-66, polymer chains were observed to selectively occupy only one of the voids. This resulted in a remarkable enhancement (million-fold) of the electrical conductivity while the nanocomposites retain 60-70 % of the porosity of the original MOF. These semiconducting yet significantly porous MOF nanocomposite systems exhibited ultralow thermal conductivity. Enhanced electrical conductivity with lowered thermal conductivity could qualify such MOF nanocomposites for thermoelectric applications.

Perspective on the Interfacial Reduction Reaction.

Chemical reactions involving oxidation and reduction processes at interfaces may vary from those in conventional liquid-phase or solid-phase reactions and could influence the overall outcome. This article primarily features a study on metal-ligand coordination at the solid-liquid interface. Of particular mention is the spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface without the need of any extraneous reducing agent, unlike in the liquid-phase reaction whereby no reduction of Cu(II) to Cu(I) took place. As a consequence of the interfacial reduction reaction (IRR), thin films of Cu-TCNQ (tetracyanoquinodimethane) and Cu-HCF (hexacyanoferrate) were successfully deposited onto a thiol-functionalized Au substrate via a layer-by-layer (LbL) method. IRR is anticipated to be useful in generating new functional and stimuli-responsive materials, which are otherwise difficult to achieve via conventional liquid-phase reactions.

Facile Synthesis of Concave Cuboid Au NCs with Precisely Tunable Dimensions and Mechanistic Insight.

Concave cuboid (CCB) nanostructure is a member of the high-index facet (HIF) nanocrystals (NCs) family, geometrically derived from regular cuboid-excavation of each face. CCB NCs hold some additional characteristics such as surface cavity and sharp edges and corners as compared to its convex counterpart that makes it relatively more active in applications like electrochemical catalysis, surface enhanced Raman spectroscopy (SERS), and plasmonics. To date, there are only few reports available on the synthesis of CCB Au NCs where Br- containing surfactants have been used as a shape directing and stabilizing agent. However, none of them led to decent yield and size tunability. Herein, we report a robust seed mediated growth strategy where cetyltrimethylammonium chloride (CTAC) and tannic acid (TA) have been used as shape-directing/stabilizing and mild reducing agents, respectively. Our method not only allows the high yield fabrication of CCB Au NCs with uniform shape and size but also precise control over dimensions and degree of surface concavity. Moreover, the investigation of growth mechanism revealed that the evolution of CCB Au NCs from cylindrical nanorods (NRs) take place via arrow-headed nanorods and truncated CCB nanostructures. Furthermore, it has been observed that the presence of excess of Cl- is indeed playing a decisive role despite the headgroup of counter cationic part of surfactant. We anticipate that our findings may pave the path to design new synthetic strategies and understand the evolution of new nanostructures.

Metamagnetism in Nanosheets of CoII-MOF with TN at 26 K and a Giant Hysteretic Effect at 5 K.

Herein, we have synthesized at room-temperature two-dimensional nanosheets of a MOF comprised of cobalt(II) ion with benzenedicarboxylic acid  ligand, which exhibited unusual magnetic properties. Direct-current magnetic susceptibility revealed an antiferromagnetic (AFM) transition at 26 K (Néel temperature,  TN) followed by a canting of the spin moments along with the concomitant appearance of a sigmoidal-shaped magnetization versus field ( M- H) curve at 15 K. Such a canted AFM ordering led to nonzero remnant magnetization with a remarkably high coercive field of ∼10 kOe at 5 K. Metamagnetism was further substantiated by the alternating-current magnetic susceptibility measurements.

Controlling the dimensionality of on-surface coordination polymers via endo- or exoligation.

The formation of on-surface coordination polymers is controlled by the interplay of chemical reactivity and structure of the building blocks, as well as by the orientating role of the substrate registry. Beyond the predetermined patterns of structural assembly, the chemical reactivity of the reactants involved may provide alternative pathways in their aggregation. Organic molecules, which are transformed in a surface reaction, may be subsequently trapped via coordination of homo- or heterometal adatoms, which may also play a role in the molecular transformation. The amino-functionalized perylene derivative, 4,9-diaminoperylene quinone-3,10-diimine (DPDI), undergoes specific levels of dehydrogenation (-1 H2 or -3 H2) depending on the nature of the present adatoms (Fe, Co, Ni or Cu). In this way, the molecule is converted to an endo- or an exoligand, possessing a concave or convex arrangement of ligating atoms, which is decisive for the formation of either 1D or 2D coordination polymers.

An anticancer drug to probe non-specific protein-DNA interactions.

A visible fluorescence switch of an eminent anti-carcinogen, ellipticine has been used to probe non-specific protein-DNA interaction. The unique pattern of protein-DNA complexation is depicted for the first time through field emission scanning electron microscopy (FE-SEM) images and spectroscopic techniques.

Excitonic cuprophilic interactions in one-dimensional hybrid organic-inorganic crystals.

The everlasting pursuit of hybrid organic-inorganic lead-free semiconductors has directed the focus towards eco-friendly copper-based systems, perhaps because of the diversity in chemistry, controlling the structure-property relationship. In this work, we report single crystals of a Cu(i) halide-based perovskite-like organic-inorganic hybrid, (TMA)Cu2Br3, (TMA = tetramethylammonium), consisting of unusual one-dimensional inorganic anionic chains of -(Cu2Br3)-, electrostatically stabilized by organic cations, and the Cu(i)-Cu(i) distance of 2.775 Å indicates the possibility of cuprophilic interactions. X-ray photoelectron spectroscopy measurements further confirmed the presence of exclusive Cu(i) in (TMA)Cu2Br3 and electronic structure calculations based on density functional theory suggested a direct bandgap value of 2.50 eV. The crystal device demonstrated an impressive bulk photovoltaic effect due to the emergence of excitonic Cu(i)-Cu(i) interactions, as was clearly visualized in the charge-density plot as well as in the Raman spectroscopic analysis. The single crystals of a silver analogue, (TMA)Ag2Br3, have also been synthesized revealing a Ag(i)-Ag(i) distance of 3.048 Å (signature of an argentophilic interaction). Unlike (TMA)Cu2Br3, where more density of states from Cu compared to Br near the Fermi level was observed, (TMA)Ag2Br3 exhibited the opposite trend, possibly due to variation in the ionic potential influencing the overall bonding scenario.

On-surface coordination chemistry: direct imaging of the conformational freedom of an axial ligand at room temperature.

The on-surface ligation of nitric oxide (NO) with Co-tetraphenylporphyrin (CoTPP) sublimed onto oxygen-reconstructed Ni(001) is studied using room-temperature scanning tunneling microscopy (STM) and complementary photoemission spectroscopies. On the oxygen-reconstructed substrates, the porphyrins are observed to form well-ordered, self-assembled layers. STM directly images the NO ligand as a characteristic feature in the center of the molecule. Under certain STM imaging conditions the dynamicity of this feature can be related to the temperature-activated conformational flexibility of the NO ligand. This provides an indirect confirmation of the bending of the Co-NO bond, as predicted from classical coordination chemistry.

Ammonia coordination introducing a magnetic moment in an on-surface low-spin porphyrin.

Amazing ammonia: The molecular spin state of Ni(II) porphyrin, supported on a ferromagnetic Co surface, can be reversibly switched between spin-off (S = 0) and spin-on (S = 1) states upon coordination and decoordination of the gaseous ligand NH3, respectively (see picture). This finding clearly indicates the possible use of the system as a single-molecule-based magnetochemical sensor and in spintronics.

Insulator-to-metal-like transition in thin films of a biological metal-organic framework.

Temperature-induced insulator-to-metal transitions (IMTs) where the electrical resistivity can be altered by over tens of orders of magnitude are most often accompanied by structural phase transition in the system. Here, we demonstrate an insulator-to-metal-like transition (IMLT) at 333 K in thin films of a biological metal-organic framework (bio-MOF) which was generated upon an extended coordination of the cystine (dimer of amino acid cysteine) ligand with cupric ion (spin-1/2 system) - without appreciable change in the structure. Bio-MOFs are crystalline porous solids and a subclass of conventional MOFs where physiological functionalities of bio-molecular ligands along with the structural diversity can primarily be utilized for various biomedical applications. MOFs are usually electrical insulators (so as our expectation with bio-MOFs) and can be bestowed with reasonable electrical conductivity by the design. This discovery of electronically driven IMLT opens new opportunities for bio-MOFs, to emerge as strongly correlated reticular materials with thin film device functionalities.

Unusual enhancement in efficiency of DSSCs upon modifying photoanodes with reduced graphene oxide.

Reduced graphene oxide (rGO) has emerged as an excellent interfacial material for improvising the performance of dye-sensitized solar cells (DSSC). Herein, we have applied rGO as interfacial layers between a fluorine doped tin oxide (FTO) coated glass substrate and semiconducting material TiO2 in a photoanode of a DSSC which showed an unusual enhancement in generating a photocurrent in comparison to the control (without rGO layers). An electrochemical impedance spectroscopy (EIS) study was performed to gain the mechanistic insights into such a remarkable enhancement of photoelectric conversion efficiency (PCE) which revealed improved charge transfer and suppressed charge recombination due to high-electrical conductivity and probably more negative work function of our rGO material compared to the bare TiO2 photoanode.

Charge-transfer interface of insulating metal-organic frameworks with metallic conduction.

Downsizing materials into hetero-structured thin film configurations is an important avenue to capture various interfacial phenomena. Metallic conduction at the interfaces of insulating transition metal oxides and organic molecules are notable examples, though, it remained elusive in the domain of coordination polymers including metal-organic frameworks (MOFs). MOFs are comprised of metal centers connected to organic linkers with an extended coordination geometry and potential void space. Poor orbitals overlap often makes these crystalline solids electrical insulators. Herein, we have fabricated hetero-structured thin film of a Mott and a band insulating MOFs via layer-by-layer method. Electrical transport measurements across the thin film evidenced an interfacial metallic conduction. The origin of such an unusual observation was understood by the first-principles density functional theory calculations; specifically, Bader charge analysis revealed significant accumulation and percolation of charge across the interface. We anticipate similar interfacial effects in other rationally designed hetero-structured thin films of MOFs.

Adjustment of the bioresistivity by electron irradiation: self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols with embedded cleavable group.

The bioresistivity of protein-repelling films, such as e.g. oligoethyleneglycol (OEG) bearing self-assembled monolayers (SAMs), can be adjusted by electron irradiation. We have studied the effect of an embedded irradiation-sensitive functional group (so called "weak link") on the irradiation sensitivity of such films. As test systems, we used two OEG-substituted alkanethiolate SAMs with different lengths of the OEG chain and a sulfone group serving as a "weak link" moiety; this group was embedded between the OEG and alkyl chains of the target molecules. The expected cleavage of the molecular chains at the predetermined "weak link" position was found to be accompanied by direct damage of the OEG matrix, with a dominance of the latter process in the case of a long OEG chain. At the same time, in the case of short OEG chain, the insertion of sulfone group resulted in a noticeable gain in the irradiation sensitivity of the respective SAMs and, consequently, in enhanced tunability of their protein-adhesive properties. These films, along with other OEG-substituted SAMs and polymer layers with and without an embedded weak link moiety, can be used as primary matrices for the fabrication of different protein patterns by electron beam lithography.

Self-assembled monolayers of perfluoroterphenyl-substituted alkanethiols: specific characteristics and odd-even effects.

Self-assembled monolayers (SAMs) formed by perfluoroterphenyl-substituted alkanethiols (C(6)F(5)-C(6)F(4)-C(6)F(4)-(CH(2))(n)-SH, FTPn) with variable length of the aliphatic linker (n = 2 and 3) were prepared on (111) Au and Ag and characterized by a combination of several complementary spectroscopic and microscopic techniques. A specific feature of these systems is the helical conformation of the FTP moieties, which, along with the high electronegativity of fluorine, distinguishes them from the analogous non-fluorinated systems and makes them attractive for different applications. The SAMs were found to be well-defined, highly ordered, and densely packed, which suggests a perfect correlation between the orientations and, in particular, twists of the FTP helices in the adjacent molecules. Significantly, the SAM exhibited pronounced odd-even effects, i.e. a dependence of the molecular orientation and packing density on the length of the aliphatic linker in the target molecules, with parity of n being the decisive parameter and the direction of the effects on Au opposite to that on Ag. The presence of the odd-even effects in the FTPn system brings new aspects into the discussion about the origin and mechanism of these phenomena. Specifically, the helical conformation of the FTP moieties in the dense phase excludes a variation of the intramolecular torsion and molecular twist as the mechanism behind the odd-even effects.