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.
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