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

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

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

Kondo Effect in a Neutral and Stable All Organic Radical Single Molecule Break Junction

Organic radicals are neutral, purely organic molecules exhibiting an intrinsic magnetic moment due to the presence of an unpaired electron in the molecule in its ground state. This property, added to the low spin-orbit coupling and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar as the radical character is stable in solid state electronic devices. Here we show that the paramagnetism of the polychlorotriphenylmethyl radical molecule in the form of a Kondo anomaly is preserved in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic changes. Indeed, our results demonstrate that the Kondo anomaly is robust under electrodes displacement and changes of the electrostatic environment, pointing to a localized orbital in the radical as the source of magnetism. Strong support to this picture is provided by density functional calculations and measurements of the corresponding nonradical species. These results pave the way toward the use of all-organic neutral radical molecules in spintronics devices and open the door to further investigations into Kondo physics

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

Camouflaged Holes Assist Negative Charge Motion in Radical-Anion Molecular Wires

Charge transfer in molecular wires over varying distances is a subject of great interest in the field of molecular electronics. By increasing the distance between the electroactive centers, transport mechanisms generally accounted for on the basis of tunneling or superexchange operating over small distances, progressively gives way to hopping assisted transport. However, the underlying molecular sequential steps that likely take place during hopping and the operative mechanism occurring at intermediate distances have received much less attention given the difficulty in assessing detailed molecular-level information. We describe here the operating mechanisms for unimolecular electron transfer in the ground state of radical-anion mixed-valence derivatives occurring between their terminal perchlorotriphenylmethyl/ide groups through thiophene-vinylene oligomers that act as conjugated wires of increasing length up to 30 Å. In this sense, while in the shorter radical-anions a flickering resonance mechanism is the operative one, in the larger molecular wires, as a unique finding, the net transport of the electron is assisted by an electron-hole delocalization.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Operative Mechanism of Hole-Assisted Negative Charge Motion in Ground States of Radical-Anion Molecular Wires

Charge transfer/transport in molecular wires over varying distances is a subject of great interest. The feasible transport mechanisms have been generally accounted for on the basis of tunneling or superexchange charge transfer operating over small distances which progressively gives way to hopping transport over larger distances. The underlying molecular sequential steps that likely take place during hopping and the operative mechanism occurring at inter mediate distances have received much less attention given the difficulty in assessing detailed molecular-level information. We describe here the operating mechanisms for unimolecular electron transfer/transport in the ground state of radical-anion mixed-valence derivatives occurring between their terminal perchlorotriphenylmethyl/ide groups through thiophene−vinylene oligomers that act as conjugated wires of increasing length up to 53 Å. The unique finding here is that the net transport of the electron in the larger molecular wires is initiated by an electron− hole dissociation intermediated by hole delocalization (conformationally assisted and thermally dependent) forming transient mobile polaronic states in the bridge that terminate by an electron−hole recombination at the other wire extreme. On the contrary, for the shorter radical-anions our results suggest that a flickering resonance mechanism which is intermediate between hopping and superexchange is the operative one. We support these mechanistic interpretations by applying the pertinent biased kinetic models of the charge/spin exchange rates determined by electron paramagnetic resonance and by molecular structural level information obtained from UV−vis and Raman spectroscopies and by quantum chemical modeling

Structural control over spin localization in triarylmethyls

Triarylmethyls (TAMs) are a class of long-lived purely organic radicals discovered at the beginning of the twentieth century. The chemical versatility and high stability of TAMs have lead to their application in many fields of science and technology. All compounds of this class are composed of three aryl rings bonded to a central carbon atom, where their unpaired electron mainly resides. Due to the π-conjugated electronic nature of this molecular structure, the possibility arises of controlling the unpaired electron localization (i.e. spin localization) by the torsion angles of the three aryl rings. By using density functional theory calculations (DFT) we have carefully investigated this phenomenon for a wide range of TAMs and probed how it is influenced by other important parameters such as chemical functionalization and temperature. Our results demonstrate that a single general spin versus structure relation is followed for all of our studied TAMs confirming that having a predictable structure-dependent spin localisation is an intrinsic feature of these radicals. Considering that spin localisation in TAMs is linked to many other important properties (e.g. magnetic interactions, optical absorption bands, magnetoresistance phenomena), the fact that manipulation of aryl ring twist angles could lead to molecular level control over such features presents enormous potential for future scientific and technological applications

Direct covalent grafting of an organic radical core on gold and silver

The functionalisation of surfaces with organic radicals, such as perchlorotriphenylmethyl (PTM) radicals or tris(2,4,6-trichloro-phenyl)methyl (TTM) radicals, is appealing for the development of molecular spintronic devices. Conventionally, organic radicals are chemisorbed to metal substrates by using long alkyl or aromatic spacers resulting in a weak spin–electron coupling between the radical and the substrate. To circumvent this problem, here we have employed a new design strategy for the fabrication of radical self-assembled monolayers (r-SAMs). This newly designed radical–anchor (R–A) molecule, a TTM based radical disulfide (1), can be easily synthesized and it was here characterized by electron spin resonance (ESR), cyclic voltammetry (CV) and superconducting quantum interference device magnetometry (SQUID). We have succeeded in fabricating TTM based r-SAMs by using thiolate bonds (Au–S and Ag–S) where the TTM cores are only one-atom distance from the metal surface for the first time. The resultant robust 1/Au and 1/Ag r-SAMs were well characterized, and the electrochemical and the magnetic properties were unambiguously confirmed, proving the persistence of the molecular spin.This work was funded by ERC StG 2012-306826 e-GAMES. The authors also thank ITN iSwitch 642196 project, the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), DGI (Spain) BE-WELL CTQ2013-40480- R and FANCY CTQ2016-80030-R, and Generalitat de Catalunya 2014-SGR-17. The authors also acknowledge the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015- 0496). We thank Dr V. Lloveras for ESR spectroscopy characterization, Mr A. Bernab´e for MALDI-TOF measurements and Dr G. Sauthier from ICN2 for XPS and UPS measurements. We also thank Dr N. Crivillers for useful discussions. S. T. B. and I. A. acknowledge support from the Spanish MINECO grant CTQ2015-64618-R grant and, in part, by Generalitat de Catalunya grants 2014SGR97 and XRQTC. IA acknowledges the Spanish Ministerio de Educaci´on Cultura y Deporte for a FPU PhD scholarship. Access to supercomputer resources as provided through grants from the Red Espanola de Supercomputación is also acknowledged.Peer reviewe