Clar theory for molecular benzenoids

The Journey for the Synthesis of Large Acenes

Acenes, the group of polycyclic aromatic hydrocarbons (PAHs) with linearly fused benzene rings, possess distinctive electronic properties with potential applicability in material science. Hexacene was the largest acene obtained and characterized in the last century, followed by heptacene in 2006. Since then, a race for obtaining the largest acene resulted in the development of several members of this family as well as diverse innovative synthetic strategies, from solid-state chemistry to the promising on-surface chemistry. This last technique allows the obtention of large acenes, up to tridecacene, the largest acene so far. This review presents the different methodologies employed for the synthesis of acenes, highlighting the newest studies, to provide a much more thorough understanding of the essence of the electronic structure of this captivating group of organic compounds.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

Hydrogen bonding interactions can decrease clar sextet character in acridone pigments

Computed nucleus-independent chemical shifts (NICS), contour plots of isotropic magnetic shielding (IMS), and gauge-including magnetically induced current (GIMIC) plots suggest that polarization of the π-system of acridones may perturb the numbers and positions of Clar sextet rings. Decreasing numbers of Clar sextets are connected to experimental observations of a narrowing HOMO-LUMO gap and increased charge mobility in solid-state assemblies of quinacridone and epindolidione.

Pushing the Limits of Acene Chemistry: The Recent Surge of Large Acenes

Acenes, consisting of linearly fused benzene rings, are an important fundamental class of organic compounds with various applications. Hexacene is the largest acene that was synthesized and isolated in the 20th century. The next largest member of the acene family, heptacene, was observed in 2007 and since then significant progress in preparing acenes has been reported. Significantly larger acenes, up to undecacene, could be studied by means of low-temperature matrix isolation spectroscopy with in situ photolytic generation, and up to dodecacene by means of on-surface synthesis employing innovative precursors and highly defined crystalline metal surfaces under ultrahigh vacuum conditions. The review summarizes recent experimental and theoretical advances in the area of acenes that give a significantly deeper insight into the fundamental properties and nature of the electronic structure of this fascinating class of organic compounds.

On exo-cyclic aromaticity

The domain of aromaticity spans a wide range of molecules, from polycyclic aromatic hydrocarbons, heterocycles to all-metal systems. Here, in silico we demonstrate the aromaticity in C₂B₂F₄, extending beyond the limit of conventional aromatic molecules. This molecule gains the magic number of six π-electrons through an unusual electronic contribution from exo-cyclic atoms. The stability of the molecule is established through density functional theory, ab initio calculations as well as molecular dynamics simulation.

Role of Kekulé and Non-Kekulé Structures in the Radical Character of Alternant Polycyclic Aromatic Hydrocarbons: A TAO-DFT Study

We investigate the role of Kekulé and non-Kekulé structures in the radical character of alternant polycyclic aromatic hydrocarbons (PAHs) using thermally-assisted-occupation density functional theory (TAO-DFT), an efficient electronic structure method for the study of large ground-state systems with strong static correlation effects. Our results reveal that the studies of Kekulé and non-Kekulé structures qualitatively describe the radical character of alternant PAHs, which could be useful when electronic structure calculations are infeasible due to the expensive computational cost. In addition, our results support previous findings on the increase in radical character with increasing system size. For alternant PAHs with the same number of aromatic rings, the geometrical arrangements of aromatic rings are responsible for their radical character.

The Clar Structure in Inorganic BN Analogues of Polybenzenoid Hydrocarbons: Does it Exist or Not?

The Clar structure of polybenzenoid hydrocarbons (PBHs) have attracted considerable interest of both theoretical and experimental chemists since it was proposed in the 1950s. However, it remains unclear whether the Clar structure could exist in inorganic PBHs, the boron nitride (BN) analogues where the alternate boron and nitrogen atoms are used to replace the carbon atoms of PBHs. Here, we carry out thorough density functional theory (DFT) calculations to probe the possibility of Clar structures in BN analogues of PBHs. A strong correlation (r(2) =0.975) between the ring number (n=3-10) of BN analogues of [n]acenes and energy differences between the most and least stable isomers is identified, suggesting the existence of Clar structures in inorganic PBHs. In addition, the slightly weaker correlations in comparison to that (r(2) =0.989) of the organic PBHs is rationalized by the reduced aromaticity, which is revealed by two aromatic indices: ELFπ and SCI.

Role of the coupler to design organic magnetic molecules: LUMO plays an important role in magnetic exchange

The occupation number and spatial position of the LUMO play an important role in magnetic exchange in diradicals.

Clar theory for radical benzenoids

Eric Clar's ideas concerning "aromatic sextets" are extended from closed-shell benzenoids to the case of radical benzenoids, particularly those where the unpaired electrons are largely localized on sites of one "type" (starred or unstarred). A quantitative format in terms of a new Clar polynomial is introduced to make quantitative correlations with a selection of numerical data, including delocalization energies, spin densities, and energy gaps between states of different spin multiplicities. The correlations are generally quite good, thereby further validating Clar's ideas and our extension and quantification of them.