Prakash Chandra Mondal

Chiral Conductive Polymers as Spin Filters

DOI: 10.1002/adma.201405249 electrode. By controlling the direction of the magnetization of nickel, it is possible to inject electrons that have mainly one spin orientation and to verify their transport through the PCT by monitoring the cyclic voltammetry (CV) curve in the electrochemical cell in which the redox couple is not chiral. The CV curves refl ect a steady state in which a double layer is formed near the working electrode. Another method to monitor the spin selectivity is by chronoamperometry, in which one monitors time-dependent current at a fi xed potential. At the beginning, when the potential is turned on, the current is high, but it is reduced with time due to the formation of the double layer. [ 18,19 ]

Spin-Dependent Transport through Chiral Molecules Studied by Spin-Dependent Electrochemistry.

Conspectus Molecular spintronics (spin + electronics), which aims to exploit both the spin degree of freedom and the electron charge in molecular devices, has recently received massive attention. Our recent experiments on molecular spintronics employ chiral molecules which have the unexpected property of acting as spin filters, by way of an effect we call “chiral-induced spin selectivity” (CISS). In this Account, we discuss new types of spin-dependent electrochemistry measurements and their use to probe the spin-dependent charge transport properties of nonmagnetic chiral conductive polymers and biomolecules, such as oligopeptides, L/D cysteine, cytochrome c, bacteriorhodopsin (bR), and oligopeptide-CdSe nanoparticles (NPs) hybrid structures. Spin-dependent electrochemical measurements were carried out by employing ferromagnetic electrodes modified with chiral molecules used as the working electrode. Redox probes were used either in solution or when directly attached to the ferromagnetic electrodes. During the electrochemical measurements, the ferromagnetic electrode was magnetized either with its magnetic moment pointing “UP” or “DOWN” using a permanent magnet (H = 0.5 T), placed underneath the chemically modified ferromagnetic electrodes. The spin polarization of the current was found to be in the range of 5–30%, even in the case of small chiral molecules. Chiral films of the l- and d-cysteine tethered with a redox-active dye, toludin blue O, show spin polarizarion that depends on the chirality. Because the nickel electrodes are susceptible to corrosion, we explored the effect of coating them with a thin gold overlayer. The effect of the gold layer on the spin polarization of the electrons ejected from the electrode was investigated. In addition, the role of the structure of the protein on the spin selective transport was also studied as a function of bias voltage and the effect of protein denaturation was revealed. In addition to “dark” measurements, we also describe photoelectrochemical measurements in which light is used to affect the spin selective electron transport through the chiral molecules. We describe how the excitation of a chromophore (such as CdSe nanoparticles), which is attached to a chiral working electrode, can flip the preferred spin orientation of the photocurrent, when measured under the identical conditions. Thus, chirality-induced spin polarization, when combined with light and magnetic field effects, opens new avenues for the study of the spin transport properties of chiral molecules and biomolecules and for creating new types of spintronic devices in which light and molecular chirality provide new functions and properties.

Non-magnetic organic/inorganic spin injector at room temperature

Spin injection into solid-state devices is commonly performed by use of ferromagnetic metal electrodes. Here, we present a spin injector design without permanent magnet; rather, the spin selectivity is determined by a chiral tunneling barrier. The chiral tunneling barrier is composed of an ultrathin Al2O3 layer that is deposited on top of a chiral self-assembled monolayer (SAM), which consists of cysteine or oligopeptide molecules. The experimentally observed magnetoresistance can be up to 20% at room temperature, and it displays an uncommon asymmetric curve as a function of the applied magnetic field. These findings show that the spin injector transmits only one spin orientation, independent of external magnetic field. The sign of the magnetoresistance depends on the handedness of the molecules in the SAM, which act as a spin filter, and the magnitude of the magnetoresistance depends only weakly on temperature.

Photospintronics: Magnetic Field-Controlled Photoemission and Light-Controlled Spin Transport in Hybrid Chiral Oligopeptide-Nanoparticle Structures.

The combination of photonics and spintronics opens new ways to transfer and process information. It is shown here that in systems in which organic molecules and semiconductor nanoparticles are combined, matching these technologies results in interesting new phenomena. We report on light induced and spin-dependent charge transfer process through helical oligopeptide–CdSe nanoparticles’ (NPs) architectures deposited on ferromagnetic substrates with small coercive force (∼100–200 Oe). The spin control is achieved by the application of the chirality-induced spin-dependent electron transfer effect and is probed by two different methods: spin-controlled electrochemichemistry and photoluminescence (PL) at room temperature. The injected spin could be controlled by excitation of the nanoparticles. By switching the direction of the magnetic field of the substrate, the PL intensity could be alternated.

Surface-Confined Heterometallic Triads on the Basis of Terpyridyl Complexes and Design of Molecular Logic Gates

Surface-confined heterometallic molecular triads (SURHMTs) were fabricated on SiOx-based solid substrates using optically rich and redox-active Fe-, Os-, and Ru-based terpyridyl complexes as metalloligands and Cu(2+) ions as linkers. Optical and electrochemical studies reveal efficient electronic intramolecular communication in these assemblies. The UV-vis spectra of the triads exhibit a superposition of the metal-to-ligand charge-transfer bands of individual complexes, providing a significant enlargement of the optical window, useful for application. Similarly, cyclic voltammograms of SURHMT layers show a variety of redox peaks corresponding to individual complexes as well as multi-redox states at a low potential. Interaction of a representative SURHMT assembly with redox-active NOBF4 was investigated and used as a basis for configuring molecular logic gates.

Fe-terpyridyl complex based multiple switches for application in molecular logic gates and circuits

Molecular logic gates and circuits are constructed using the optical and electrochemical addressable-reversible-multiple switching event of the Fe(II)-4′-pyridyl terpyridyl complex (1) using multiple analytes. The process involves oxidation–reduction of Fe2+ as well as successive quaternization–dequaternization of the free pendant pyridyl group monitored optically. The whole switching process could also be visualized by the naked eye as colour changes upon switching are quite apparent and instant.

Optical and electrochemical properties of covalent assembled bis(4′-carboxylic phenyl terpyridyl) Ru(II)-monolayer

Homoleptic monomolecular layers of optically-rich and redox-active Ru(II)-4′carboxylic-phenyl-2,2′:6′,2′′-terpyridyl (1) have been fabricated on SiOx-based substrates. The optical and redox properties of the films have been studied in detail. The molecular films showed high electrochemical and thermal stability. The combination of thermal and electrochemical stability of the surface-confined assemblies could be useful for potential applications in materials science.

Electrochemistry of Metallo-Proteins Attached through Functional Self-Assembled Monolayers on Gold and Ferromagnetic Electrodes

We report the experimental results of a study of the electron-transfer processes of redox-active metalloproteins bound to mixed self-assembled monolayers (SAMs) on magnetic (nickel or ultrathin gold-coated nickel) or nonmagnetic (gold) electrodes. Metalloproteins, such as hemoglobin (Hb), Cytochrome C (Cyt C), and Cyt C oxidase, are attached through electrostatic interactions to the free carboxylate or imidazole groups present in the mixed SAMs. The formation of both mixed SAMs and SAM/metalloprotein heterostructures were confirmed by using advanced surface analysis techniques, such as polarization modulation infrared reflection absorption spectroscopy and aqueous contact angle measurements. Electrochemical measurements indicated a stronger electronic coupling between Hb and Cyt C oxidase and the mixed-SAM-coated gold or gold-coated-nickel electrodes, whereas a weaker coupling was found between the protein and the pure nickel electrode. Surface coverage and the electron-transfer rate constant were estimated from the cyclic voltammetry data.

Covalent Assembled Monolayers of Homo- and Heteroleptic Fe(II)-Terpyridyl Complexes on SiOx and ITO-Coated Glass Substrates: An Experimental and Theoretical Study

Well-defined FeII -terpyridyl monolayers were fabricated on SiOx and conductive ITO-coated glass substrates through covalent-bond formation between the metallo-organic complexes and a preassembled coupling layer. Three different homo- and heteroleptic complexes with terminal pyridyl, amine, and phenyl groups were tested. All the films were found to be densely packed and homogeneous, and consist of molecules standing upright. They exhibited high thermal (up to ≈220 °C) and temporal (up to 5 h at 100 °C) stability. The UV/Vis spectra of the monolayers showed pronounced metal-to-ligand charge-transfer bands with a significant redshift compared with the solution spectra of the metallo-ligands with a pendant pyridyl group quaternized with the coupling layer, whereas the shift was significantly smaller when the coupling layer was bonded to the primary amine (-NH2 ) group of the complex. Cyclic voltammograms of the monolayers showed reversible, one-electron redox behavior and suggested strong electronic coupling between the confined molecules and the underlying substrate. Analysis of the electrochemistry data allowed us to estimate the charge-transfer rate constant between the metal center and the substrate. Additionally, detailed quantum-chemical calculations were performed to support and rationalize the experimentally observed photophysical properties of the FeII -terpyridyl complexes both in the solution state and when bound to a SiOx -based substrate.

Synthesis of heteroleptic terpyridyl complexes of Fe(II) and Ru(II): optical and electrochemical studies

We report the synthesis and characterization of heteroleptic terpyridyl complexes of d6 transition metal ions with Fe2+ and Ru2+ (1–3). Furthermore, we study the effect of substitution of either an electron donating group (–NH2) or electron withdrawing group (–NO2) at the 4′-position in the ligands by means of UV-vis, cyclic voltammetry, and differential pulse voltammetry measurements. The experimentally observed photophysical characteristics of the transition-metal based terpyridyl complexes are explained and supported by detailed quantum chemical calculations.