Local Aromaticity of the Lowest-Lying Singlet States of [n]Acenes (n = 6−9)

Abstract

The local aromaticities of the six-membered rings in the two lowest-lying singlet states of [n]acenes (n = 6-9) have been assessed by means of three probes of local aromaticity based on structural, magnetic, and electron delocalization properties. Important differences between the local aromaticities of the closed-shell and diradical singlet electronic states are found. Thus, while the inner rings have the largest aromatic character in the closed-shell singlet states, the outer rings become the most aromatic for the diradical singlet states.

Why 1,2-quinone derivatives are more stable than their 2,3-analogues?

“…Diradical singlet state of 2,3-anthraquinone and its larger analogues (n ≥ 3, n being the number of the rings), obtained using the unrestricted broken symmetry UB3LYP/6-311+G(d) method, is found to be more stable than the closed-shell singlet state; the energy differences amount to 5.2, 15.9, 12.1, and 10.2 kcal/mol for 2,3-anthra-, tetra-, penta-, and hexaquinones, respectively. This is similar to what is found for acenes [61], although in these latter cases, the closedshell species are more stable until n < 5 or 6. The change in the electronic ground state can also be understood using the Clar's π-sextet model [62].…”

“…It was recently shown that the lowest energy state of higher polyacenes can correspond to diradical singlet state instead of the closed-shell singlet state [59][60][61]. Similar results are also observed for some studied quinones.…”

“…It should be noted that Clar structure shown in Scheme 5b is reinforced by all applied aromaticity indices (see Table S1 in Supporting Information). Summarizing, in 2,3-quinones for n > 2, the diradical singlet situation is favored, as in the case of the acenes [25,59,61,63] and in other polycyclic aromatic hydrocarbons [26][27][28][29][30][64][65][66][67][68][69] and graphene nanoflakes [70,71]. Therefore, for 2,3-isomers, only the results of the ground states (singlet closed shell for n ≤ 2 and singlet open shell for n ≥ 3) are presented below.…”

Chemical Bonding and Electronic Properties of the Co Adatom and Dimer Interacting with Polyaromatic Hydrocarbons

“…Indeed, this hexagonal nano-flake has attracted great interest with respect to its interesting properties like electronic absorption spectra, 119 and it has been successfully applied for modeling graphene in previous theoretical work. [120][121][122][123][124] Several theoretical studies have focussed on acenes [125][126][127] and oligoacenes [128][129][130][131][132] using DFT and ab-initio methods. However, because of the polyradical character of oligoacenes, 125,126 the spin ground state of these molecules have to be checked.…”

“…[120][121][122][123][124] Several theoretical studies have focussed on acenes [125][126][127] and oligoacenes [128][129][130][131][132] using DFT and ab-initio methods. However, because of the polyradical character of oligoacenes, 125,126 the spin ground state of these molecules have to be checked. Singlet-triplet (S-T) excitation energies give valu- 137 The first S-S and S-T excitation energies were calculated to 1.94 and 1.77 eV, respectively.…”