Giant Enhancement of Carrier Mobility in Bimetallic Coordination Polymers

Electrically conductive metal–organic coordination polymers (CPs) are promising candidates for a variety of technological applications. However, poor energetic and spatial overlap between the <i>sp</i>-electrons of organic ligands and the <i>d</i>-electrons of metal ion often blocks an effective charge transport (mobility) across CPs. Herein, we present a bimetallic design principle for enhancing carrier mobility in CPs. Bimetallic CPs of Fe­(III) and Cr­(III) ions coordinated to 1,3,5-benzenetricarboxylic acid (BTC) ligand (Fe–BTC–Cr) exhibited remarkably high carrier mobility at the matching mole ratio (1:1) with enhancement factors of 10² and 10⁴ in comparison to those of monometallic parents, Fe–BTC and Cr–BTC, respectively. The observation was substantiated by lowering of the band gap between the valence band and the conduction band upon the formation of a hybrid <i>p</i>–<i>n</i>-type structure in the bimetallic CPs. The direct current conductivity values of the CPs measured by four-probe technique were in good agreement with the alternating current conductivity values obtained from the electrochemical impedance spectroscopy. Our flexible approach of picking and choosing the appropriate combination of metal ions from the periodic table is expected to generate various CPs with desirable semiconducting properties

Diamagnetic Molecules Exhibiting Room-Temperature Ferromagnetism in Supramolecular Aggregates

Molecule-based materials exhibiting room-temperature ferromagnetism and semiconducting property are promising for molecular spintronic applications. Chemically tunable electronic and magnetic properties of metallo-phthalocyanine (MPc) molecules make them potential candidates in the frame. Here, we show room-temperature ferromagnetism in supramolecular aggregates of two diamagnetic MPcs, nickel­(II) phthalocyanine (NiPc; <i>S</i> = 0) and zinc­(II) hexadecafluorophthalocyanine (ZnFPc; <i>S</i> = 0). In the magnetization versus applied field (M–H) plot, recorded at room temperature, the supramolecular NiPc···ZnFPc aggregate revealed a clear hysteresis loop with coercive field (H_c) of ∼180 Oe. The H_c values were further increased with decreasing the temperature down to 95 K. The direct current (DC) electrical conductivity value of the supramolecular NiPc···ZnFPc system was observed to be significantly higher than that of a mechanical mixture of NiPc+ZnFPc. An optical band gap of ∼1.25 eV for the supramolecular solid was estimated from the Tauc plot, and no appreciable charge-transfer interaction between NiPc and ZnFPc was detected. The origin of such unusual ferromagnetism is understood with the help of Goodenough–Kanamori–Anderson (GKA) empirical rules and the Zener model of <i>sp–d</i> exchange interaction

Enhancing Intermolecular Interaction by Cyano Substitution in Copper Phthalocyanine

On-surface molecular self-assembly is one of the key paradigms for understanding intermolecular interactions and molecule–substrate interactions at the atomic scale. Phthalocyanines are planar π-conjugated systems capable of self-assembly and can act as versatile, robust, and tunable templates for surface functionalization. One of the ways to tailor the properties of phthalocyanines is by pendant group substitution. How such a scheme brings about changes in the properties of the phthalocyanines at the nanoscale has not been greatly explored. Here we present an atomic-scale picture of the self-assembly of copper phthalocyanine, CuPc, and compare it with its cyano analogue, CuPc­(CN)_8,on Au(111) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in ultrahigh vacuum (UHV) at 77 K. STM imaging reveals a tetramer unit cell to be the hallmark of each assembly. The periodicity of herringbone reconstruction of Au(111) is unchanged upon CuPc­(CN)₈ adsorption, whereas for CuPc adsorption this periodicity changes. STS measurements show an increment in the highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gap from CuPc to CuPc­(CN)₈. Extensive ab initio calculations within density functional theory (DFT) match well with the experimental observations. STM imaging shows adsorption-induced organizational chirality for both assemblies. For CuPc­(CN)₈ at LUMO energy, the individual molecule exhibits an orbital-energy-dependent chirality on top of the existing organizational chirality. It remains achiral at HOMO energy and within the HOMO–LUMO gap. No such peculiarity is seen in the CuPc assembly. This energy-selective chiral picture of CuPc­(CN)₈ is ascribed to the cyano groups that participate in antiparallel dipolar coupling, thereby enhancing intermolecular interaction in the CuPc­(CN)₈ assembly. Thus, our atomically resolved topographic and spectroscopic studies, supplemented by DFT calculations, demonstrate that pendant group substitution is an effective strategy for tweaking intermolecular interactions and for surface functionalization

Selective Sensing of Metal Ions and Nitro Explosives by Efficient Switching of Excimer-to-Monomer Emission of an Amphiphilic Pyrene Derivative

An amphiphilic pyrene derivative exhibiting unusually stable excimer emission due to strong aggregation is presented. The aggregated system served as an intelligent sensor for metal ions and nitro explosives in aqueous media. The excimer displayed excellent selectivity toward Cu²⁺ among the tested cations. The observation was interpreted on the basis of chelation of metal ions involving the hydroxyl and amino groups of two molecules, leading to the ligand-to-metal charge-transfer (CT) process. The excimer was further applied for the cell imaging of Cu²⁺ ions. Also, while treating the excimer with various nitro explosives, it displayed efficient 2,4,6-trinitrophenol sensing, corroborating mainly the CT process from pyrene to the analyte due to intercalation of the analyte within pyrene

Selective Sensing of Metal Ions and Nitro Explosives by Efficient Switching of Excimer-to-Monomer Emission of an Amphiphilic Pyrene Derivative

An amphiphilic pyrene derivative exhibiting unusually stable excimer emission due to strong aggregation is presented. The aggregated system served as an intelligent sensor for metal ions and nitro explosives in aqueous media. The excimer displayed excellent selectivity toward Cu²⁺ among the tested cations. The observation was interpreted on the basis of chelation of metal ions involving the hydroxyl and amino groups of two molecules, leading to the ligand-to-metal charge-transfer (CT) process. The excimer was further applied for the cell imaging of Cu²⁺ ions. Also, while treating the excimer with various nitro explosives, it displayed efficient 2,4,6-trinitrophenol sensing, corroborating mainly the CT process from pyrene to the analyte due to intercalation of the analyte within pyrene

Redox-Induced Photoluminescence of Metal–Organic Coordination Polymer Gel

In this study, we have prepared a redox-active and intrinsically nonphotoluminescent metal–organic gel comprised of iron ions and terephthalic acid. In the presence of small reactive organic molecules like pyrrole, aniline, and bithiophene, the gelation process was unaffected and at the same time it led to the formation of highly photoluminescent hybrid materials. The photoluminescence turn-on response was primarily attributed to the redox reaction between iron ions and the small organic molecules generating oxidized oligomers in the porous gel matrix. A remarkable morphological transformation of the metal–organic gel from spindle-like to nanofibers, induced by the redox reaction, was detected. The adopted synthetic approach is very simple and the ease of tailor-making photoluminescence in the materials by varying the combinations of redox system and reactive small organic molecule will open up new perspective in the field of coordination polymers, specifically, for sensors, photonics, and photovoltaic applications

Possible Room-Temperature Ferromagnetism in Self-Assembled Ensembles of Paramagnetic and Diamagnetic Molecular Semiconductors

Owing to long spin-relaxation time and chemically customizable physical properties, molecule-based semiconductor materials like metal-phthalocyanines offer promising alternatives to conventional dilute magnetic semiconductors/oxides (DMSs/DMOs) to achieve room-temperature (RT) ferromagnetism. However, air-stable molecule-based materials exhibiting both semiconductivity and magnetic-order at RT have so far remained elusive. We present here the concept of supramolecular arrangement to accomplish possibly RT ferromagnetism. Specifically, we observe a clear hysteresis-loop (<i>H</i>_c ≈ 120 Oe) at 300 K in the magnetization versus field (M–H) plot of the self-assembled ensembles of diamagnetic Zn-phthalocyanine having peripheral F atoms (ZnFPc; <i>S</i> = 0) and paramagnetic Fe-phthalocyanine having peripehral H atoms (FePc; <i>S</i> = 1). Tauc plot of the self-assembled FePc···ZnFPc ensembles showed an optical band gap of ∼1.05 eV and temperature-dependent current–voltage (I–V) studies suggest semiconducting characteristics in the material. Using DFT+U quantum-chemical calculations, we reveal the origin of such unusual ferromagnetic exchange-interaction in the supramolecular FePc···ZnFPc system

Increase in Electrical Conductivity of MOF to Billion-Fold upon Filling the Nanochannels with Conducting Polymer

Redox-active pyrrole (Py) monomers were intercalated into 1D nanochannels of [Cd­(NDC)_0.5(PCA)]·G_<i>x</i> (H₂NDC = 2,6-napthalenedicarboxylic acid, HPCA = 4-pyridinecarboxylic acid, G = guest molecules) (<b>1</b>) – a fluorescent 3D MOF (λ_em = 385 nm). Subsequent activation of <b>1⊃Py</b> upon immersing into iodine (I₂) solution resulted in an increment of the bulk electrical conductivity by ∼9 orders of magnitude. The unusual increase in conductivity was attributed to the formation of highly oriented and conducting polypyrrole (PPy) chains inside 1D nanochannels and specific host–guest interaction in <b>1⊃PPy</b> thereof. The Hall-effect measurements suggested <b>1⊃PPy</b> to be an n-type semiconductor material with remarkably high-carrier density (η) of ∼1.5 × 10¹⁷ cm^–3 and mobility (μ) of ∼8.15 cm² V^–1 s^–1. The fluorescence property of <b>1</b> was almost retained in <b>1⊃PPy</b> with concomitant exciplex-type emission at higher wavelength (λ_em = 520 nm). The here-presented results on [MOF⊃Conducting Polymer] systems in general will serve as a prototype experiment toward rational design for the development of highly conductive yet fluorescent MOF-based materials for various optoelectronic applications

Data Supporting Delayed Fluorescence from Inverted Singlet and Triplet Excited States.xlsx

Data Supporting: Delayed Fluorescence from Inverted Singlet and Triplet Excited States</p