Radical-Pair Formation in Organic π-Stacked Architectures

Abstract

Interest in organic molecule-based materials has extraordinarily grown in both the electronic and data storage industries. Specifically, research in the development of new plastic devices for technological proposes has emerged as they may provide lower manufacturing costs, more versatile synthetic processes and better mechanical properties (i.e. transparency, flexibility) than standard inorganic materials. However, in order to compete with these inorganic-based materials in terms of effectiveness, their conductive, magnetic and switchable characteristics must be improved, such that they can be applied as high-performance molecular conductors, magnets and memory devices. The physical properties of these organic materials not only depend on the nature of its constituent molecules (magnetic, biocompatible) but on how they organize in the solids (polymeric, amorphous, crystalline). Therefore, predicting and achieving a particular final behaviour is challenging and requires investigations on the structure-property relationships as well as on the nature of the chemical species. Over the last years, continuous effort has been done aimed at developing new purely organic-based materials with technologically relevant properties. Since Gomberg's synthesis of the triphenylmethyl neutral radical, a large variety of stable organic radicals have been reported, such as nitroxides, verdazyl-, phenoxyl-, phenalenyl-, dithiazolyl- and triazinyl-based compounds, as well as several functionalized radical ions based on tetracyanoethylene (TCNE), and tetrathiafulvalene (TTF+.) among others. These particular families of organic radicals can be used as versatile templates for the synthesis of new derivatives with improved characteristics by the introduction of different substituents into their basic skeleton. However, further progress in novel materials must be achieved through a rational design of new chemical structures. In this context, the available computational tools can substantially contribute to obtain the necessary knowledge of the factors controlling the final target physical properties, namely conductivity, magnetism and bistability, which are essentially dominated by the electronic structure of these materials. The work presented in this thesis is devoted to study different purely organic derivatives that have been shown to be promising for the development of new functional materials. In particular, the attention has been focused on three different families of organic radicals that present interesting physical properties as a consequence of a Tr-stacked radical-pair formation in the condensed phase, that is: (1) radical ions, (2) phenalenyl- and (3) triazinyl-based compounds. Overall, the present PhD thesis aims at contributing to the field of computational chemistry as well as to the understanding of the electronic properties that govern the Tr-dimerization of these systems, which, ultimately, determine their magnetic and switchable behaviour.

En els últims anys, s'ha fet un esforç continu per desenvolupar materials nous purament orgànics amb propietats tecnològicament rellevants. Des de la síntesi de Gomberg del radical neutre trifenilmetil, s'ha reportat una gran varietat de radicals orgànics estables, com ara els nitròxids, verdazils, fenoxils, fenalenils, ditiazolils i compostos basats en triazinil, així com diversos ions radicals funcionalitzats basats en tetracianoetilè (TCNE-•), i tetratiafulvalè (TTF1-.), entre d'altres. Aquestes famílies de radicals orgànics s'utilitzen per a la síntesi de nous derivats amb característiques millorades a partir de la introducció de diferents substituents en el seu esquelet bàsic. No obstant això, els avenços en nous materials s'han d'aconseguir a través d'un disseny racional de les noves estructures químiques. En aquest context, les eines computacionals disponibles poden contribuir substancialment a obtenir els coneixements necessaris sobre els factors que controlen les propietats físiques desitjades, a saber, la conductivitat, el magnetisme i la biestabilitat, dominades fonamentalment per l'estructura electrònica d'aquests materials. El treball presentat en aquesta tesi està dedicat a estudiar diferents derivats purament orgànics que han demostrat ser prometedors per al desenvolupament de nous materials funcionals. En particular, l'atenció s'ha centrat en tres famílies de radicals orgànics que presenten propietats físiques interessants com a conseqüència de la formació de parells de radical Tr, aquests són: (1) els ions radicals, (2) els fenalenils i (3) els compostos basats en triazinil. En global, aquesta tesi doctoral té com a objectiu contribuir al camp de la química computacional, així com a la comprensió de les propietats electròniques que regeixen la dimerització Tr d'aquests sistemes, que, en última instància, determina el seu comportament magnètic i commutable.