Rational chemical design of Triarylmethyl-based devices and 2D materials

[eng] In this PhD thesis I have studied through state-of-the-art quantum simulations (mainly within the density functional theory approach, DFT) triarylmethyl (TAM) based systems with potential for future nano-devices and designed a series of TAM-based 2D covalent organic frameworks (from now on TAM 2D-COFs). TAMs are organic radicals (i.e. open-shell molecules) which have been used for numerous applications during the last 20 years. In the first part of this PhD thesis I have studied a series of TAM-based systems in collaboration with the experimental groups led by Profs. Jaume Veciana and Concepció Rovira and Dr. Marta Mas-Torrent, respectively, both from the Institute of Materials Science of Barcelona (ICMAB). In such collaborative studies we have evaluated the potential of TAMs for different potential applications. In the first two works we assess the possibility of using closed-shell quinoidal TAMs which, upon being chemisorbed in metal substrates give rise to an open-shell (i.e. radical) monolayer. This is demonstrated by means of on-surface techniques such as X-ray photo-electron spectroscopy (XPS) and angle-resolved ultra-violet photo-electron spectroscopy (ARUPS) and our periodic density functional theory calculations. Complementing to this work I also present a second where a similar radical SAM is formed using, in this case, a TAM-based bi-radical compound. In a third collaborative study we study the E – Z isomerisation in a hydrogenated closed-shell TAM (the so-called H-PTM) bonded with an ethylene unit. An irreversible E to Z transformation is experimentally measured with no evident explanation. Based in DFT and ab initio molecular dynamics simulations (AIMD), I was able to provide a sensible hypothesis for such results based in a sterical blocking effect in the Z conformer. The last two chapters of this PhD thesis collect the computational works focused in making theoretical predictions of yet un-synthesized systems. In Chapter 4, I present a work were we studied how to control the unpaired electron in TAMs, finding out that in these molecules there exists a linear correlation between the aryl ring twist angles and the localization of their unpaired electron. Based on this study we then looked for TAM-based systems where the aryl rings’ twist angles could be externally manipulated. In such direction, I present TAM 2D-COFs (see above) as the only possible platform where aryl ring twist angles may be externally manipulated. As reported in the second publication of Chapter 4, uniaxially stretching the structure of our designed TAM 2D-COFs allows for a fine and reversible (i.e. elastic) twisting of all aryl rings within the 2D material. This allows controlling the localization of all unpaired electron in the network, as well as the band of the material and magnetic interactions. In the last work of this chapter we assess the possibility of having chemical persistence of TAM monomers and structural flexibility through a screening procedure based in force-field calculations. In the last chapter of this PhD thesis I present two studies where it is demonstrated that TAMs, upon being covalently bonded in para- one respect each other, present electrical conductive characteristics. In the first work, in collaboration with the experimental groups from ICMAB, this is demonstrated for a PTM dimer where one of the PTM units is reduced to the anion. The resulting negative charge is found to conduct between both PTM units at room temperature. Finally, in the last predictive work of this thesis, I present a work where we demonstrate based in hybrid DFT calculations that para-connected TAM 2D-COFs behave as semimetals with energetically close-lying semiconductor solutions.[spa] En esta tesis doctoral presentada por artículos he estudiado mediante cálculos DFT (density functional theory, del inglés) sistemas basados en moléculas triaril-metil (TAM) para potenciales aplicaciones futuras. Las moléculas TAM son compuestos orgánicos radicales (es decir, con un electrón desapareado) que se han utilizado para construir diversos materiales durante los últimos 20 años. En la primera parte de la tesis presento los estudios llevados a cabo en colaboración con grupos experimentales del Instituto de Ciencia de los Materiales de Barcelona (ICMAB) expertos en la síntesis de tales compuestos. En los primeros estudios de esta parte se ha llevado a cabo la formación de una mono-capa auto-ensamblada de TAMs en diferentes superficies metálicas. Mediante técnicas de superficie i cálculos DFT periódicos hemos demostrado que utilizando moléculas TAM de capa cerrada (es decir, diamagnéticas) se puede generar una mono-capa radical, o de capa abierta (es decir, paramagnética). En un tercer estudio en colaboración con los mismos grupos experimentales estudiamos la isomerización E – Z (o cis- trans-) irreversible en un sistema TAM-etileno (de capa cerrada). Los cálculos computacionales han sido claves en este estudio para entender el bloqueo cinético que se da en el isómero Z (cis-), lo cual impide su isomerización al isómero E (trans-), a pesar de ser éste último más estable termodinámicamente. En la segunda parte de esta tesis doctoral presento una serie de estudios en los cuales hemos diseñado materiales 2D basados en moléculas TAM, aún no preparados en el laboratorio. En estos estudios se demuestra el gran potencial de dichas redes basadas en moléculas TAM y su gran versatilidad electrónica a la nano-escala. Nuestros resultados demuestran que dichos materiales 2D se puede comportar tanto como aislantes eléctricos, como semiconductores o como semimetales (tales como el grafeno) según su diseño molecular. Además, en dicho materiales es posible controlar sus propiedades electrónicas mediante la manipulación del ángulo de giro de los anillos aril en cada unidad TAM

Tailoring giant quantum transport anisotropy in disordered nanoporous graphenes

During the last 15 years bottom-up on-surface synthesis has been demonstrated as an efficient way to synthesize carbon nanostructures with atomic precision, opening the door to unprecedented electronic control at the nanoscale. Nanoporous graphenes (NPGs) fabricated as two-dimensional arrays of graphene nanoribbons (GNRs) represent one of the key recent breakthroughs in the field. NPGs interestingly display in-plane transport anisotropy of charge carriers, and such anisotropy was shown to be tunable by modulating quantum interference. Herein, using large-scale quantum transport simulations, we show that electrical anisotropy in NPGs is not only resilient to disorder but can further be massively enhanced by its presence. This outcome paves the way to systematic engineering of quantum transport in NPGs as a novel concept for efficient quantum devices and architectures.Comment: 16 pages, 3 figure

Mechanistic Insights into Electronic Current Flow through Quinone Devices

Molecular switches based on functionalized graphene nanoribbons (GNRs) are of great interest in the development of nanoelectronics. In experiment, it was found that a significant difference in the conductance of an anthraquinone derivative can be achieved by altering the pH value of the environment. Building on this, in this work we investigate the underlying mechanism behind this effect and propose a general design principle for a pH based GNR-based switch. The electronic structure of the investigated systems is calculated using density functional theory and the transport properties at the quasi-stationary limit are described using nonequilibrium Green’s function and the Landauer formalism. This approach enables the examination of the local and the global transport through the system. The electrons are shown to flow along the edges of the GNRs. The central carbonyl groups allow for tunable transport through control of the oxidation state via the pH environment. Finally, we also test different types of GNRs (zigzag vs. armchair) to determine which platform provides the best transport switchability

A surface confined yttrium(III) bis-phthalocyaninato complex: a colourful switch controlled by electrons

AMs of a Y(III) double-decker complex on ITO have been prepared and their electrical and optical properties explored, exhibiting three accessible stable redox states with characteristic absorption bands in the visible spectra, corresponding to three complementary colors (i.e., green, blue and red). These absorption bands are exploited as output signals of this robust ternary electrochemical switch, behaving hence as an electrochromic molecular-based device.This work was funded by ERC StG 2012-306826 e-GAMES, the EU project ITN iSwitch (GA no. 642196), the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), DGI (Spain) BE-WELL CTQ2013-40480-R, and Generalitat de Catalunya 2014-SGR-17. I. A. acknowledges CIBER-BBN for a grant.Peer reviewe

Unveiling the multiradical character of the biphenylene network and its anisotropic charge transport

Recent progress in the on-surface synthesis and characterization of nanomaterials is facilitating the realization of new carbon allotropes, such as nanoporous graphenes, graphynes, and 2D π-conjugated polymers. One of the latest examples is the biphenylene network (BPN), which was recently fabricated on gold and characterized with atomic precision. This gapless 2D organic material presents uncommon metallic conduction, which could help develop innovative carbon-based electronics. Here, using first principles calculations and quantum transport simulations, we provide new insights into some fundamental properties of BPN, which are key for its further technological exploitation. We predict that BPN hosts an unprecedented spin-polarized multiradical ground state, which has important implications for the chemical reactivity of the 2D material under practical use conditions. The associated electronic band gap is highly sensitive to perturbations, as seen in finite temperature (300 K) molecular dynamics simulations, but the multiradical character remains stable. Furthermore, BPN is found to host in-plane anisotropic (spin-polarized) electrical transport, rooted in its intrinsic structural features, which suggests potential device functionality of interest for both nanoelectronics and spintronics

Controlling pairing of π-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. -conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials. Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies but the external dynamical control of these states still awaits experimental realization. Here, via quantum chemical calculations, the authors demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units induces controlled pairing of pi -conjugated electrons in a reversible way

Controlling pairing of pi-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials

2D Hexagonal covalent organic radical frameworks as tunable correlated electron systems

Quantum materials hold huge technological promise but challenge the fundamental understanding of complex electronic interactions in solids. The Mott metal-insulator transition on half‐filled lattices is an archetypal demonstration of how quantum states can be driven by electronic correlation. Twisted bilayers of 2D materials provide an experimentally accessible means to probe such transitions, but these seemingly simple systems belie high complexity due to the myriad of possible interactions. Herein, it is shown that electron correlation can be simply tuned in experimentally viable 2D hexagonally ordered covalent organic radical frameworks (2D hex‐CORFs) based on single layers of half‐filled stable radical nodes. The presented carefully procured theoretical analysis predicts that 2D hex‐CORFs can be varied between a correlated antiferromagnetic Mott insulator state and a semimetallic state by modest out‐of‐plane compressive pressure. This work establishes 2D hex‐CORFs as a class of versatile single‐layer quantum materials to advance the understanding of low dimensional correlated electronic systems

Neutral organic radical formation by chemisorption on metal surfaces

Organic radical monolayers (r-MLs) bonded to metal surfaces are potential materials for the development of molecular (spin)electronics. Typically, stable radicals bearing surface anchoring groups are used to generate r-MLs. Following a recent theoretical proposal based on a model system, we report the first experimental realization of a metal surface-induced r-ML, where a rationally chosen closed-shell precursor 3,5-dichloro-4-[bis(2,4,6-trichlorophenyl)methylen]cyclohexa-2,5-dien-1-one (1) transforms into a stable neutral open-shell species () via chemisorption on the Ag(111) surface. X-ray photoelectron spectroscopy reveals that the >C=O group of 1 reacts with the surface, forming a C-O-Ag linkage that induces an electronic rearrangement that transforms 1 to . We further show that surface reactivity is an important factor in this process whereby Au(111) is inert towards 1, whereas the Cu(111) surface leads to dehalogenation reactions. The radical nature of the Ag(111)-bound monolayer was further confirmed by angle-resolved photoelectron spectroscopy and electronic structure calculations, which provide evidence of the emergence of the singly occupied molecular orbital (SOMO) of 1

Existence of multi-radical and closed-shell semiconducting states in post-graphene organic Dirac materials

Post-graphene organic Dirac (PGOD) materials are ordered two-dimensional networks of triply bonded sp2 carbon nodes spaced by π-conjugated linkers. PGOD materials are natural chemical extensions of graphene that promise to have an enhanced range of properties and applications. Experimentally realised molecules based on two PGOD nodes exhibit a bi-stable closed-shell/multi-radical character that can be understood through competing Lewis resonance forms. Here, following the same rationale, we predict that similar states should be accessible in PGOD materials, which we confirm using accurate density functional theory calculations. Although for graphene the semimetallic state is always dominant, for PGOD materials this state becomes marginally meta-stable relative to open-shell multiradical and/or closed-shell states that are stabilised through symmetry breaking, in line with analogous molecular systems. These latter states are semiconducting, increasing the potential use of PGOD materials as highly tuneable platforms for future organic nanoelectronics and spintronics