2D-Delocalized vs Confined Diradicals

Resumen de la comunicación oral seleccionadaDiradicals are beautiful chemical objects where the more basic and intricate aspects of the chemical bonding are revealed.1 Not this being important enough, nowadays, diradical-based substrates are becoming very appealing for new organic electronic applications. We focus here in conjugated organic diradicals formed by competition between non-aromatic quinoidal structures and their canonical aromatic forms. How this quinoidal(closed-shell)-vs-aromatic(open-shell) energetic balance producing the diradical is affected by several situations has been our objective in the last few years.2 Now, we focusses on how the properties of diradicals are influenced when several diradical canonical forms are available in such a way that create a 2D (i.e., bidimensional) electron delocalization surface in which the diradical substructures are in cross-conjugation mode producing the curious effect of diradical confinement.3 Herein, the diradical molecular properties of compound 1 in Figure 1 will be discussed in connection with 2D delocalization, cross-conjugation and surface confinement. 1. Rajca, A., Chem. Rev., 1994, 94, 871; Abe, M., Chem. Rev. 2013, 113, 7011. 2. Zeng, Z.; X. Shi, L.; Chi, C.; Casado, J.; Wu, J. Chem. Soc. Rev. 2015, 44, 6578. 3. Yuan, D.; Huang, D.; Medina Rivero, S.; Carreras, A.; Zhang, C.; Zou, Y.; Jiao, X.; McNeill, C.R.; Zhu, X.; Di, C.; Zhu, D.; Casanova, D.; Casado, J. CHEM, 2019, accepted.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Diradicals and their driving forces

Several series of aromatic and quinoidal compounds, such as oligothiophenes (Scheme 1), oligophenylene-vinylenes, oligoperylenes (oligophenyls) and graphene nanoribbon derivatives, are studied in the common context of the capability to stabilize diradical structures. [1,2,3,4]. In this work, we try to clarify how several driving forces (i.e., thermodynamic and entropic) are responsible for the generation of diradical and diradicaloid structures. A combination of different types of molecular spectroscopies (i.e., electronic absorption, electronic emission, excited state absorption, vibrational Raman, vibrational infrared, etc.) as well as hybridized with thermal and pressure-dependent techniques are shown to provide important information about the origin of the formation and stabilization of diradicals. From a conceptual point of view, we analyze these properties in the context of the oligomer approach which is the study of the evolution of these spectroscopic quantities as a function of the oligomer size. References [1] P. Mayorga Burrezo, J.L. Zafra, J. Casado. Angew. Chem. Int. Ed., 2017, 56, 2250. [2] J. Casado, R. Ponce Ortiz, J. T. Lopez Navarrete, Chem. Soc. Rev. 2012, 41, 5672. [3] P. Mayorga Burrezo, X. Zhu, S. F. Zhu, Q. Yan, J. T. Lopez Navarrete, H. Tsuji, E. Nakamura, J. Casado, J. Am. Chem. Soc. 2015, 137, 3834-3843. [4] J. Casado, Para-quinodimethanes: A unified review of the quinoidal-versus-aromatic competition and its implications. Top. Curr. Chem. 2017, 375, 73.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Uncommon Polycyclic Open-Shell Structures: Cross-Conjugated Dianions and Mechano-Made Biradicals

Significant advances in any field of research often come from unexpected experimental results. There are plenty of examples where the realization of uncommon behaviors in the long run trigger new applications. Examples of these in the field of organic electronics are the discovery of conductivity in PA or the serendipitous finding of PCBM as electron acceptor in bulk heterojunction solar cells, etc. The two findings I will describe here are not obviously of comparable relevance but allow me to introduce the reader in the topic of the talk [1-3]. The two stories deal with biradical or diradicals, or pairs of unpaired electrons in singlet ground electronic state configurations which allow a series of interesting properties as small singlet-triplet gaps, thermally activated intersystem crossing, magnetic hysteresis in pure organics, etc. The first case concerns with the properties of dianions of oligothiophenes which are stabilized in cross-conjugation forms (versus typical linear conjugations). This structure allows each anion to weakly interact each other thus forming the singlet open-shell and promoting the small gap with the triplet. The second case is about neutral biradicals generated after pressure application of a closed-shell molecule which provokes the rupture of one bond and generation of trapped, kinetically persistent biradicals. Their understanding and implications are discussed

Uncommon Molecular Structures for Organic Electronics: Cross-Conjugated Biradical Dianions and Mechano-Made Biradicals

Significant advances in any field of research often come from unexpected experimental results. We all know that uncommon behaviors are destined to envisage new applications whereas collection of typical features only helps to improve existing ones. There are plenty of examples of this in the field of organic electronics such as the discovery of conductivity in PA, the behavior of PCBM as electron acceptor in bulk heterojunction solar cells or singlet fission for second generation photovoltaics. The two findings I will describe here are not obviously of comparable importance but allow me to introduce the reader in the topic of the talk.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Oligorylene nanographenes and x-shape multiradicaloid molecules: structural insights from Raman spectroscopy

Raman spectroscopy has been the technique of choice for the structural study of carbonaceous materials, including graphitic substrates, carbon nanotubes, fullerenes, conducting polymers and graphene.1 In the material science and organic chemistry communities, the most popular use of Raman spectroscopy in the case of graphene has been for the elucidation of the density of defects in relationship with the portion of intact material (benzenoid rings) through the analysis of the G and D Raman bands. Molecular versions of graphenes or nanographenes have become very interesting materials for a number of reasons.2 For instance, nanographenes allow a molecular understanding of the spectroscopic properties of the infinite 2D sheet following a bottom-up approach. With this “oligomer approach”, physical, structural and electronic properties are studied as a function of the nanographene size. In this presentation, we will show an oligomer approach based on the rylene units by studying up to octarylene.3 We will show how the Raman spectra of these oligorylene undergo a sudden transformation from typical G&D-type spectra to polyacetylene-type highlighting unexpected structural features that radically changes the “tiring” interpretation of the Raman spectra of nanographenes to a new and vivid vision.4 On the other hand, in the context of new 2D multiradicaloid molecular-based materials, we will show our latest approach to their building blocks by exploiting the concept of X-shape conjugation, pseudo-cross-conjugation or cross-conjugation. A “cocktail” of all these ingredients, “sweetened” by Raman spectroscopy, is proposed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Chameleon-like behaviour of cyclo[n]paraphenylenes in complexes with C70: on their impressive electronic and structural adaptability as probed by Raman spectroscopy

Este artículo ha sido publicado por Royal Society of ChemistryA series of four 1:1 host-guest supramolecular complexes of [n]CPPs and C70 have been analyzed by Raman spectroscopy in solid state and complemented with the analysis of their spectroscopic responses under mechanical and thermal stresses. By following the frequency behaviour of the G and RBM modes we have found that [10]CPP in the [10]CPP@C70 complex displays a more “ordered” structure. However, in [11]CPP@C70, the nanoring gets ovallized with closer contacts with the C70 poles and less conformational restriction in the flattened region. By mechanical and thermal stresses we are able to modify the lying conformation of [10]CPP@C70 towards a standing shape. [11]CPP@C70 resists pressure changes, although it tends to shift from the standing to the lying orientation by heating. As for the crystal cell, the [n]CPPs occupy the residual empty spaces while the main crystallographic positions are reserved to C70. These are new examples of the impressive adaptability of the [n]CPP molecules to different physico-chemical environments, a chameleon-like property which reveals the delicate equilibrium provided by cyclic conjugation and ring strain.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Molecular diradicals

Resumen de la comunicaciónDiradicals are defined as molecules with two unpaired electrons which define distinctive manifolds of singlet and triplet states. Diradicals have been recently shown to be fruitful candidates for applications such as enhanced non-linear optical chromophores, sensitizers for singlet exciton fission in photovoltaics, or electrically ambipolar substrates in organic field effect transistors, etc. [1]. Quinoidal-based oligothiophenes produced by double dicyanomethylene substitution through the 1,4 positions of the thiophenes have been demonstrated to be diradical precursors [2]. Solid-state structure and supramolecular interactions are key ingredient to have enhanced properties in the substrates for organic electronics. Quinoidal compounds display typical p-p stacking aggregation and, as such, are able to form p-dimers and s-dimers depending on the degree of diradical character [3]. Another intrinsic molecular property of quinoidal molecules is their inherent tendency for reductive redox processes given the stabilization by aromatization upon acceptation of electrons. [4]. In this contribution, we describe an example of tetracyano substituted oligothiophenes with the central thiophenes substituted at the beta positions of the central units with pyrrolo diones groups, from a dimer to a pentamer. In this series of quinoidal oligomers, a transition from closed-shell to open-shell (diradical) structures is described, accompanied in parallel with a change in the mode of supramolecular aggregation from herringbone stacking to cholesteric-type in the neutral state allowing the maximization of the electrical conductivity, an unprecedented stability upon n-type doping and excellent thermoelectric performance.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Oligomers of thieno[3,4-c]pyrrole-4,6-dione: Raman spectra

Oligothiophenes are π-conjugated compounds made by concatenation of thiophenes. [1] Due to the low aromaticity of thiophene, inter-ring π-electron delocalization is favored which has strong implication in their electro-optical properties and in their applications in organic electronic devices. [2] In despite of the plenty of oligothiophenes reported so far, new derivatives are welcome which would enhance these properties in regard of their exploitation in new devices. [3] In this presentation, we will show our latest development of oligothiophenes based on the thieno[3,4-c]pyrrole-4,6-dione (TPD, See Figure 1) units, and with different size, from a dimer to a hexamer. In particular, thieno[3,4-c]pyrrole-4,6-dione (TPD) motifs are of great interest due to their high power conversion efficiency (PCE) and its moderate short-circuit current (Jsc) in organic photovoltaic (OPV) devices, [4] when implemented in donor-acceptor polymers, making them excellent candidates for their application in polymer solar cells.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

[n]cycloparaphenylenes with charges

Oligophenylenes (polyphenylenes) are constituted by an array of conjugated benzenes where inter-ring electron delocalization tends to extend over the whole chain (linear conjugation) being intrinsically limited, among other factors, by terminal effects. Alternatively, cyclic conjugation is envisaged as the unlimited free-boundary versionofconjugation which will impact the structure of molecules in rather unknown ways. The cyclic version of oligophenylenes, cycloparaphenylenes ([n]CPPs with n the number of phenyl rings) were first synthesized in 2008 by Beztozzi and Jasti.1 Today the whole [n]CPP series from [5]CPP to [18]CPP has been prepared. [n]CPPs represent ideal models to investigate new insights of the electronic structure of molecules and cyclic conjugation when electrons or charges circulate in a closed circuit without boundaries. Radical cations and dications of [n]CPP from n=5 to n=12 have been prepared and studied by Raman spectroscopy.2 Small [n]CPP dications own their stability to the closed-shell electronic configuration imposed by cyclic conjugation. However, in large [n]CPP dications cyclic conjugation is minimal and these divalent species form open-shell biradicals. The Raman spectra reflect the effect of cyclic conjugation in competition with cyclic strain and biradicaloid aromatic stabilization. Cyclic conjugation provokes the existence of a turning point or V-shape behavior of the frequencies of the G bands as a function of n. In this communication we will show the vibrational spectroscopic fingerprint of this rare form of conjugation. [1] R. Jasti, J. Bhattacharjee, J. B. Neaton, C. R. Bertozzi, “Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures”, J. Am. Chem. Soc. 130 (2008), 17646–17647. [2] M. P. Alvarez, P. M. Burrezo, M. Kertesz, T. Iwamoto, S. Yamago, J. Xia, R. Jasti, J. T. L. Navarrete, M. Taravillo, V. G. Baonza, J. Casado, “Properties of Sizeable [n]CycloParaPhenylenes As Molecular Models of Single-Wall Carbon Nanotubes By Raman Spectroscopy: Structural and Electron-Transfer Responses Under Mechanical Stress”, Angew. Chem. Int. Ed. 53, (2014), 7033−7037.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech