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Computational Design of Organic Diradicals for Energy Applications

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Computational Design of Organic Diradicals for Energy Applications Computational design of organic diradicals for energy applications by Diego López Carballeira Directed by Fernando Ruipérez and Aurelio Mateo Alonso Donostia, 2018 (cc)2018 DIEGO LOPEZ CARBALLEIRA (cc by-nc-nd 4.0) Acknowledgements Agora que xa cheguei ó final do meu doutoramento en Donostia, hai que destacar o afortunado que me sinto de ter traballado co meu director de tese, Fernando Ruipérez. Estivo sempre ahí para dar apoio e consellos, traballou comigo incansablemente, e foi comprensivo cando foi necesario. Por iso, Fernando merece ser mencionado en primeiro lugar. Así mesmo, non podo pasar por alto a David Casanova, que gracias ós seus profundos coñecementos e a súa boa disposición, fixo grandes aportacións a esta tese. Tamén quero destacar a aportación que tiveron na miña formación Marcos Mandado Alonso e Ignacio Pérez Juste namentres estiven en Vigo na miña etapa de master. Tamén de Vigo é José Manuel Hermida Ramón, responsable de primeira man de que puidera estar doutorandome en Polymat a día de hoxe. Alén de toda a xente vencellada dalgunha forma na miña carreira académica, é necesario mencionar a todos aqueles que foron esenciais para que eu puidera manter a saúde psicolóxica. Por iso, sempre terei un bo recordo dos compañeiros de Trisquele, que fixeron un pouquiño máis doado que Donostia se convirtese no meu fogar. Tamén debo agradecer ós meus pais ó seu perseverante apoio e a súa preocupación. Por último, pero non menos importante, debo mencionar a Cristina, que estivo ó meu carón nos malos e nos bos momentos, e axudou a facer que sexa a persoa que son hoxe. Contents Contents Chapter 1 1 1. Radicals and diradicals 3 2. Photovoltaics 8 3. Organic batteries 13 4. Motivation and scope of the work 18 Chapter 2 21 1. Schrödinger equation 23 2. Ab initio methods 25 Chapter 3 41 1. Introduction 43 2. Performance of modern DFT functionals and G3n-RAD DFT methods in the calculation of E[T1] , IP and EA 45 3. Evaluation of TDDFT methods for the computation of low-lying excited states 69 Chapter 4 75 1. Introduction 77 2. Computational details 80 3. Diradical character in conjugated quinones and methylene derivatives 82 4. Effect of structural modifications 97 5. Other diradical structures 113 i Contents Chapter 5 155 1. Introduction 157 2. Computational details 160 3. Redox properties of conjugated quinones 162 4. Effect of structural modifications 178 5. Other diradical structures 198 Chapter 6 227 1. Introduction 229 2. Computational details 231 3. Singlet fission in conjugated quinones and methylene derivatives 232 4. Effect of structural modifications 249 5. Other diradical structures 267 Chapter 7 319 Resumen 327 References 335 ii Chapter 1. Introduction Chapter 1 Introduction 1 Chapter 1. Introduction 1. Radicals and diradicals Since 1897, when Moses Gomberg synthesized for the very first time the trityl radical,1,2,3 the chemistry of radicals has been subject of constant investigation. More than a century later, this special kind of molecules has been suggested as potential candidates for electronic devices, 4 environmentally friendly redox active materials, 5 , 6 , 7 and optoelectronic devices.8,9 Most of the known radicals show an intrinsic instability and high reactivity in processes such as dimerization or polymerization, frequently appearing as intermediates in photochemical and thermal reactions. Nevertheless, there is a non-negligible number of radicals with longer life-times and stable enough to be characterized in solution (persistent radicals), 10 or even to be generated, isolated and stored. Radical systems, also known as free radicals or monoradicals, are essentially those systems with an unpaired electron. Since the spin quantum number of an electron is S=1/2, the spin multiplicity (2S+1) for monoradicals is 2, that is, doublet. Those molecules with two unpaired and independent electrons are commonly denoted as diradicals or biradicals, depending on certain particularities. The long distance between the radical centers may produce an electron exchange interaction close to zero (J≈0), situation in which the appropriate denomination is biradical. Biradicals should be seen as two doublets inside the same moiety. In 3 Chapter 1. Introduction the other hand, those molecules with two unpaired electrons and an electron exchange interaction different to zero (J≠0) are denominated diradicals, and may lead to two different spin multiplicities: singlet and triplet. Diradical molecules Localized Delocalized Kekulé Non-Kekulé Antiaromatic Figure 1: Different types of diradical molecules. From left to right, the represented molecules are para-benzyne, para-benzoquinone, meta- benzoquinone and cyclobutadiene. Diradicals can be classified as localized and delocalized (see Figure 1). The latter can be further classified as Kekulé and non-Kekulé diradicals. Kekulé diradicals can be described as a structure with all the electrons paired (closed shell contribution) and as a structure with unpaired electrons (open shell contribution). Molecules such as meta- quinodimethane, tetramethylenemethane or oxyallyl derivatives are classified as non-Kekulé delocalized diradicals, which are not fully conjugated in their Kekulé structures. Additionally, antiaromatic molecules are also classified as delocalized systems, defined as planar cyclic molecules with 4n -electrons. Thus, cyclobutadiene can be 4 Chapter 1. Introduction represented with two rectangular isomers, where the diradical square structure plays the role of transition state. When the two unpaired electrons are located in two non-bonding molecular orbitals, the diradical may be described by six electronic configurations: two triplets, two open-shell singlets and two closed-shell singlets (see Figure 2). If the molecular orbitals are degenerate, the system is denoted as diradical; when they are quasi-degenerate, the preferred term is diradicaloid. a b T1,2 , T1,2 a b S1,2 , S1,2 S1,1 , S2,2 Figure 2: Electronic configurations describing the ground state of a molecule with two unpaired electrons. Following Hund´s rule,11 the most stable multiplicity for diradical molecules is the triplet followed by the open-shell singlet. The large electron-electron repulsion interaction (K) prevents the existence of two electrons sharing the same orbital, so that the closed-shell singlet is the least stable possibility. However, if the overlap integral between both molecular orbitals is quite small or negligible, Hund´s rule may not apply and the closed-shell singlet state might be the ground state. The electron exchange interaction (J) is proportional to the overlap integral; J<0 denotes a singlet 5 Chapter 1. Introduction ground state, while J>0 states for a triplet. When the molecular orbitals are non-degenerate (diradicaloids), an energy gap appears between them. Thus, the closed-shell state appears as a possible electronic configuration for the ground state. If this energy gap is larger than the electron- electron repulsion (K), the preferred spin multiplicity is the singlet state. This means that a diradicaloid molecule can show an electronic configuration that may be described as an intermediate situation between a diradical (degenerate orbitals) and a closed-shell molecule. One of the most important parameters for the study of DFT diradicals is the singlet-triplet energy gap (E[T1] ), which DFT can be calculated from J as: E[T1] =E[S0]–E[T1]=2J. At this point, the diradical character appears as a magnitude to measure how important is the diradical configuration on a particular system, and it can be estimated from the occupation of the lowest unoccupied molecular orbital (LUMO). Many experimental research has been performed to investigate this kind of molecules.12,13,14,15,16,17,18 For instance, usual reactions as homolytic bond-cleavage or cyclization reactions depend fundamentally on localized diradicals that play the role of intermediate compounds. While triplet diradicals have been routinely investigated due to their long lifetimes, there is a lack of experimental data available for singlet diradicals. For example, molecules as ortho- and para- quinodimethanes, as well as Thiele´s19 or Tschitschibabin´s20 6 Chapter 1. Introduction hydrocarbons are illustrative Kekulé-type delocalized diradicals. Unfortunately, Kekulé-type diradicals are often unstable or difficult to stabilize.21,22 Nowadays, there is a renewed interest in diradical and multiradical systems due to the remarkable optical and magnetic properties they exhibit, in addition to their specific chemical reactivity. 23 For example, recent theoretical and experimental studies have opened a novel field of open-shell -conjugated systems, such as polycyclic aromatic hydrocarbons and graphene, endorsing unique physico- chemical properties relevant in optoelectronics 24 and spintronics. 25 , 26 , 27 Some of these interesting properties are low-energy excited states, 28 , 29 tailorable low-spin/high-spin gaps, 30 , 31 high-yield singlet fission processes, 32 , 33 , 34 short intramolecular distances in the solid state35,36 and large two- photon absorptions,37 among others, and have been found to be originated in the singlet open-shell electronic structure of diradical character. Besides, singlet diradicals are involved in homolytic cleavage processes, and they are of great importance to investigate chemical reactivity and reaction mechanisms.38 For the sake of clarity, in this thesis, the difference between diradical and diradicaloid will be ignored and the term used to designate them will be “diradical”, despite most
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