Skip to main content

Theoretical description of the multifaceted aromaticity and resonance effects in the ground- and excited-state molecular systems

Periodic Reporting for period 1 - MulArEffect (Theoretical description of the multifaceted aromaticity and resonance effects in the ground- and excited-state molecular systems)

Reporting period: 2018-10-01 to 2020-09-30

Aromatic rings in porphyrins and their naturally occurring derivatives are among the most important chemical individuals in the world: no aerobic life on this planet can do without the characteristic carbon- and nitrogen-based macrocycles, which carry oxygen in the bloodstream (as a part of hemoglobin) and allow plants to capture sunlight’s energy with their chloroplasts. Over the last decades the exceptional electron-transport and energy-harnessing capabilities of the macro- and polycyclic aromatic species have been utilized in cancer therapy, drug delivery, bio-imaging, molecular electronics, solar cells, lighter converters, bio-sensors, quantum computing, photoluminescent materials, photodetectors, and many, many others, making aromaticity one of the most commonly exploited theoretical concepts in chemistry – according to the ISI Web of Science, in 2018 there were about 45 papers published every day that contained the word aromatic (or its antithesis) in the title, keywords or abstract. On the other hand, the lack of a rigorous definition and the resulting superfluous diversity (dozens of types and rules of aromaticity) and numerous examples of the discrepancies between different aromaticity criteria proposed in the literature, have become the main reasons for this concept being perceived by some members of the chemical community as an elusive, questionable and suspicious concept. But, if rightly?

In this project we propose a profound paradigmatic change of the concept of aromaticity quantification to reveal its true colors and unearth its real predictive power. The long-term goal of this project is to understand how aromaticity and different resonance effects determine the physicochemical properties in such systems. In the first goal, we developed a novel computational method called the electron density of delocalized bonds (EDDB) that provides both a detailed description of local aromaticity of selected molecular fragments as well as the bird's-eye view on the global aromaticity of nanoscopic-size molecules and assemblies at a reasonable computational cost. The second research goal of the proposal was to use the EDDB method to gain insights into the mechanisms of the resonance-driven phenomena in the multifaceted aromatics that are instrumental in the design of new catalysts, spin-bearing materials, organic field-effect transistors, and many other.
The new method called the electron density of delocalized bond (EDDB) has been developed, fully implemented and released as a standalone program called RunEDDB with the user-friendly interface working with most of the commonly used quantum chemical software, and a special website dedicated to the EDDB method and containing installation instructions, manual, tutorials, gallery, and support, has been prepared. The new formalism connecting the chemical effect of electron delocalization (underlying chemical aromaticity) and the cascade of mathematical operations (orbitals projections) has been proposed to enable direct assessment of the energetical stabilizing/destabilizing effects associated with the electron density of electrons delocalized in aromatic rings/fragments. This make EDDB the only method available that enables (within a single theoretical paradigm) both visualization and quantification of global and local aromaticity in molecular systems regardless of their size, topology, and electronic state. Additionally, the released RunEDDB program offers a unique perspective to study aromaticity and chemical resonance in large-scale molecular systems containing the intra- and intermolecular resonance-assisted hydrogen bonds, RAHBs.

The newly proposed method and the released software was utilized to quantitatively assess the effect of d-orbital conjugation in metallacycle of different size, topology, and type of conjugation (Hückel, Craig-Möbius, hybrid, etc.). Also, the world’s first Baird aromatic all-metal clusters were discovered based on the predictions made by the EDDB method. Moreover, the sophisticated techniques of the EDDB-based partitioning of the molecular electron densities were used to better understand the origins of the so-called adaptive aromaticity in metallapentalenes. Next, we assessed and validated the newly proposed EDDB-based spin-density decomposition scheme for the M10A-type systems, which can be widely applied in π-conjugated spin-bearing materials and thus a correct understanding of their electronic structures is of high importance. Also, we used the EDDB method to deepen the understanding of how aromaticity can be utilized as a feasible design strategy to manipulate the excited state energy levels in CIBA-type systems (which are excellent candidates for singlet-fission materials). Finally, we investigated the interplay between pyrrolic and macrocyclic aromaticity in porphyrins and phthalocyanines determining their conformational flexibility and spectroscopic properties. Interestingly, the results obtained using the EDDB method revealed the close relationship between the frequency shifts observed in the AFM experiment and the pyrrolic and annulenic aromaticities/olefinicities in the macro- and polycyclic aromatic compounds.
The general goal of this project was to deepen understanding on how aromaticity and different resonance effects determine physicochemical properties in such systems. The first goal was to develop a novel computational method called the electron density of delocalized bonds (EDDB) that provides both a detailed description of local aromaticity of selected molecular fragments as well as the bird's-eye view on the global aromaticity of nanoscopic-size molecules and assemblies at a reasonable computational cost. The EDDB method and all the capabilities it provides has been developed and tested for a wide range of systems proving its robustness; within the last one year the EDDB method was cited in such top journals as Nat. Chem., JACS, Chem. Rev., Acc. Chem. Res., etc. The second research goal was to use the RunEDDB program implementing the EDDB method to gain insights into the mechanisms of the resonance-driven phenomena in the multifaceted aromatics that are instrumental in the design of new catalysts, spin-bearing materials, organic field-effect transistors, etc. The publications presenting results obtained within the project were well-received and did resonate with the chemical community (1 mention in Chemistry Word news, 1 ‘hot paper’, etc.). Also, several external collaborations have been established with theoretical and experimental chemists from all over the world, which may help the EDDB method to become a tool of reference in the field of aromaticity measurements in the nearest future.