Evaluation of Aromaticity for Open‐Shell Singlet Dicyclopenta‐Fused Acenes and Polyacenes Based on a Magnetically Induced Current

Abstract

The aromaticity of dicyclopenta-fused acenes (DPAs) and polyacenes (PAs) of increasing size has been studied by evaluation with the GIMIC method at the DFT level of the magnetically-induced currents (MICs), and by analyzing their spatial distributions. For these open-shell singlet molecules, spin-restricted and -unrestricted treatments provide very different MICs, the latter ones providing the most reliable solution. These MICs and the differences between spin-restricted and -unrestricted treatments are interpreted in terms of the bond current strengths and the current gradients, which indicate the bond aromaticity and enable the spatial distributions of the diatropic and paratropic currents to be analyzed, respectively. In particular, they allow the rationalization of the MICs in correlation with the odd-electron density distributions and their diradical characters. These calculations demonstrate that 1) in increasingly large PAs the bond current strengths get smaller and smaller than in benzene and get almost similar in the central and terminal rings, 2) for DPAs the MICs increase from dominant paratropic currents and antiaromaticity in the small compounds to diatropic currents and aromaticity in the larger ones, and 3) in the largest DPAs, the central rings are characterized by large diatropic currents and the terminal five-membered rings, for which the odd-electron densities are localized by weak ones.

First Principle Investigations of Long-range Magnetic Exchange Interactions via Polyacene Coupler

“…Diradicals 2, 4 and 6 have central naphthalene unit contribution to the active space while for 8 and 10 this contribution is from central tetracene unit. This revelation is important here, as in literature the boundary of the originating open-shell ground state for oligoacene is roughly established at heptacene 18,73,74 and the delocalized radical also has the similar qualitative appearance 75,76. The exchange coupling constant values in this even-acene series decreases in going from 2 to 4 to 6 but increases in going from 6 to 8.…”

Characterization of Benzo[a]naphtho[2,3‐f]pentalene: Interrelation between Open‐shell and Antiaromatic Characters Governed by Mode of the Quinoidal Subunit and Molecular Symmetry

“…Not only are their unique electronic structures scientifically intriguing, they have potential in applications as organic optoelectronic materials [7] such as field‐effect transistors, [8–14] nonlinear optics, [15–17] and singlet fissions [18–20] . The singlet open‐shell character, which often inhabits o ‐ or p ‐quinoidal subunits in polycyclic hydrocarbons, [6,21–24] is frequently associated with the aromatic or antiaromatic nature due to an inherently small HOMO‐LUMO gap [25–27] . Understanding the interrelation between open‐shell and antiaromatic characteristics is crucial to describe the nature of unique π‐conjugations [28–31] .…”

First Principle Investigations of Long-range Magnetic Exchange Interactions via Polyacene Coupler

“…roughly established at heptacene 18,73,74 and the delocalized radical also has the similar qualitative appearance. 75,76 The exchange coupling constant values in this even-acene series decreases in going from 2 to 4 to 6 but increases in going from 6 to 8. This sudden rise in the coupling constant could possibly the effect of the open shell diradical nature of the coupler that might come into play for such longer polyacene.…”