PhD Thesis defense: Exploring the boundaries of aromaticity through computational analysis of excited states and complex molecular topologies

PhD Program

Sílvia Escayola Gordils
Aula Magna de la Facultad de Ciencias de la Universita de Girona
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PhD Thesis defense: Exploring the boundaries of aromaticity through computational analysis of excited states and complex molecular topologies

Quantum Chemistry / Computational and Theoretical Chemistry.

Supervisors: Albert Poater, Miquel Solà, Eduard Matito (Ikerbasque Research Professor, DIPC).

Aromaticity is a widely used concept in the prediction, design, and understanding of key aspects related to reactivity, structure, and properties of molecules. Linked to cyclic or three-dimensional molecular systems, it involves electron delocalization within a closed circuit—either a ring or a 3D structure—resulting in enhanced thermodynamic stability, significant magnetic anisotropies, and unusual chemical shifts, among other properties. With nearly two centuries of history and despite not being a physical observable, the analysis of aromaticity and antiaromaticity, the latter leading to increased reactivity and instability, is still of great interest. For instance, aromaticity has applications explaining the behavior of photoexcited molecules, a challenge that prevents the rapid development of new materials or photochemical reactions. The growing use of (anti)aromaticity in excited state chemistry supports the idea that concepts established for the ground state are transferable to excited states. Yet, their inherent complexity demands further exploration, particularly as the applicability of specific aromaticity rules and indicators in certain excited states remains elusive. Furthermore, the discovery of new aromatic compounds extends well beyond simple annulenes, encompassing structures with complex topologies. These include large molecules with multiple delocalization pathways, multifold aromaticity (with the presence of sigma-, pi-, delta-, and/or phi-aromaticity), three-dimensional, non-planar structures, etc. Such examples necessitate a reevaluation of established boundaries of aromaticity.

In this thesis, we focus on the computational study of aromaticity as a tool for characterizing challenging molecular systems, particularly those in their low-lying excited states or with intricate molecular topologies. Thus, our investigation is divided into two main blocks: Chapter 5, which centers on characterizing pro-aromatic quinoidal systems in their excited states, and Chapter 6, dedicated to examining molecules with complex topologies. Overall, we determine whether aromatic character correlates with specific molecular properties while exploring the utility and limitations of various aromaticity indicators. Additionally, we offer guidelines for molecular design to achieve specific aromatic features. Through this work, we aim to contribute to the accurate classification of chemical systems, thereby facilitating advancements in practical applications.

Kekulé diradicaloids, with their unique electronic properties and potential applications across various fields, notably in optoelectronics, are a central theme of our investigation. These pro-aromatic molecules exhibit low-lying triplet and singlet excited states, with the open-shell structure having the potential to become the ground state. The stability of these states can be partially ascribed to the transition of the central ring from quinoidal to aromatic forms. The latter are significantly influenced by resonance structures in which conjugated rings exhibit Huckel-aromatic character. However, structural modifications can induce the central rings to adopt a mixed character between different resonance structures involving 4n + 2 and 4n pi-electrons, with hybrid Huckel-Baird-aromatic characteristics. The absence of definitive spectroscopic techniques for elucidating the electronic structure of these molecules in the excited state—key for advancing new material development—underscores the importance of thorough analyses and computational assessments. To explore the applicability of aromaticity rules in characterizing these molecules, we systematically analyzed variously substituted Kekulé diradicaloids. This approach, presented in Chapter 5, has enabled us to establish guidelines for the rational design of molecules with specific singlet-triplet energy gaps and excited state Huckel-Baird aromaticity.

The second block, corresponding to Chapter 6, is composed of four sections devoted to the study of aromaticity in (sub)phthalocyanines and related compounds, C80H30 curved nanographene, double aromatic substituted tropylium ions, and polyhedral boranes. Each case faces challenges in evaluating aromaticity due to complex system topologies that limit the applicability of traditional indicators. Yet, determining the aromatic or non-aromatic nature of these systems is crucial for comprehending their inherent properties. Notably, we establish a correlation between the aromaticity of the external conjugated pathways and the Q bands in the UV-Vis absorption spectra of (sub)phthalocyanines. We identified a 75 pi-electron conjugated pathway—not following any of the known aromaticity rules—in a grossly warped nanographene, C80H30. Besides, we proposed potential sigma- and pi-doubly aromatic tropylium ion derivatives, noting the challenge of achieving double aromaticity due to electronic and steric effects. Lastly, we examined the aromaticity in 3D boron clusters to explain their stability and gain deeper insights into their chemical bonding.