Computational modelling of magnetic and conductive properties of multifunctional molecular materials

Abstract

Purely organic radical-based materials constitute a promising approach for the miniaturization of devices due to their interesting optical, electronic and magnetic properties, and are good candidates to substitute scarce and/or environmentally harmful transition metal and lanthanide compounds. However, the inclusion of any type of paramagnetic molecular entity faces critical issues related to the stability of the materials and temperature range applicability. Nevertheless, advances in the last 30 years in producing stable persistent paramagnetic organic molecules provided stable enough organic radicals and polyradicals that can be used to produce molecular materials with a wide range of properties. The use of organic radicals in the development of organic (semi)conductors gives rise naturally to multifunctional materials because the spin moment associated with the unpaired electron of the radicals endows the (semi)conductor with magnetic properties. The multifunctional character of this type of material can become even more relevant when charge transport and magnetism are not considered separately. Indeed, the magnetoresistance featured by some organic conducting magnets holds great promise for the development of spintronic devices.

In organic-radical based semiconductor materials, both charge and spin of the unpaired electron of the spin-carrying units play a key role. Indeed, the unpaired electrons of radicals are not only charge carriers but also the source of magnetic moments. The correct modelling of the electronic structure and the properties of these systems is thus a challenge due to the competition between charge localization or charge transport. Typically, organic molecular materials present low charge mobility, and hence, hopping model usually dominates the charge transport process. Computational studies based on hopping charge transfer models have been successfully applied in the analysis of closed-shell organic materials. However, in the literature, there are few studies that deal with the applicability of these

hopping models to open-shell systems, and as far as we know, none of them tackles the calculation of electric conductivity.

Bisdithiazolyl (bisDTA) radicals have furnished in recent years multiple examples of molecular materials with promising conductive and magnetic properties. This large and well-characterized family of compounds have been constructed in the quest to find a radical-based single component conductor, following different synthetic strategies that aim to enhance their electric properties whereas preventing dimerization. As a result, the bisDTA family of compounds display a wide range of conductive properties (going from insulator to metallic materials), as well as a wide range of magnetic properties (going from materials with no long-range magnetic order to materials that order antiferromagnetically or ferromagnetically). Our computational work focuses on the study of four isostructural pyridine- bridged bisDTA-multifunctional materials triggered by their magnetic and conducting properties being strongly dependent on the different S/Se ratio in the neutral radical skeleton.

The electronic structure of the four bisDTA-derived materials has been characterized by means of periodic unrestricted hybrid DFT calculations. The analyses of their band structure and density of states have served as a crucial starting point for the rationalization of their electric properties and the relevant intermolecular contacts involved in the description of the charge transport process. Furthermore, the study of their open-shell ground state and the spin delocalization has proved that the molecular building blocks act as localized spin-carrying units, allowing the rationalisation of their magnetic interactions using a HDVV Hamiltonian.

A systematic bottom-up strategy has been used in order to thoroughly compute all the relevant microscopic parameters that govern both: magnetism (magnetic exchange couplings) and charge transport (electronic coupling and reorganization energy). Microscopic interactions have been put

to test by computing the macroscopic properties (Magnetic susceptibility, Critical Temperature and Electric Conductivity) and comparing them with the available experimental data.

Furthermore, structural-property correlations analysis (ie. Magneto structural correlation maps and distance analysis) has proved that properties in these materials are sensitive to small geometry distortions.

Els materials basats en radicals orgànics constitueixen una prometedora proposta per a la miniaturització de dispositius tecnològics gràcies a les seves propietats òptiques, elèctriques i magnètiques, i són bons candidats per a substituir els escassos i/o mediambientalment nocius metalls de transició i compostos derivats dels lantànids. L'ús de radicals orgànics en el desenvolupament de (semi)conductors dona lloc de forma natural a materials multifuncionals, degut a que el moment d'espín associat a l'electró desaparellat dota addicionalment al (semi)conductor amb propietats magnètiques.

En aquesta tesi, ens centrarem en l'estudi de materials moleculars constituïts per radicals orgànics, en els quals, l'electró desaparellat del radical servirà a la vegada com a portador de càrrega i com a font de moment magnètic. La correcta modelització de l'estructura electrònica i les propietats en aquests sistemes es un repte pels mètodes actuals, degut a la competició entre localització de càrrega i el seu transport. Típicament, els estudis computacionals dedicats a la conductivitat elèctrica en materials moleculars orgànics fan servir el que s'anomena model de hopping, degut als baixos valors de conductivitat que presenten generalment aquests materials. Malgrat els avenços reportats amb aquests models en l'estudi de sistemes de capa tancada, hi ha pocs estudis dedicats a l'anàlisi de materials de capa oberta i cap en què es calculi la conductivitat elèctrica a partir de models de hopping.

S'han seleccionat quatre materials isoestructurals d'una gran família de derivats del bisditiazolil, degut a l'ampli espectre de propietats magnètiques i elèctriques que presenten, per tal de testejar una estratègia bottom-up sistemàtica per a l'estudi dels paràmetres microscòpics rellevants en la descripció del magnetisme i la conductivitat elèctrica en materials moleculars de capa oberta.

Els resultats obtinguts corroboren que l'anàlisi, mitjançant càlculs periòdics, de l'estructura electrònica, la densitat d'estats i l'estructura de bandes estableixen un punt de partida clau per a la descripció de les propietats en aquests sistemes. Mitjançant un estudi sistemàtic bottom-up s'han pogut caracteritzar tots els paràmetres microscòpics rellevants que governen tant el magnetisme com la conductivitat en aquests sistemes. Els resultats s'han pogut corroborar comparant-los amb les dades experimentals reportades. Finalment, mitjançant una anàlisi completa de les distàncies i mapes de correlació estructura-propietat s'ha evidenciat que les propietats en aquests sistemes són sensibles als subtils canvis que pot patir l'estructura cristal·lina.