Communication: A dramatic transition from nonferromagnet to ferromagnet in finite fused-azulene chain

Abstract

One-dimensional fused-azulene oligomers (n = 2 − 6) are studied with the effective valence bond as well as density functional theory methods. A nonferromagnetic (closed-shell singlet) to ferromagnetic (triplet) ground state transformation is witnessed with increasing length of oligomers. The computational results are interpreted in terms of spin coupling between the unpaired electrons of two nonbonding molecular orbitals localized, respectively, on the top and bottom chains of the oligomers.The present study provides a theoretical suggestion for understanding the ferromagnetic spin polarizations that has been observed very recently in graphene nanoribbons.

In the quest for a stable triplet state in small polyaromatic hydrocarbons: an in silico tool for rational design and prediction

“…However, we notice that the rate of decay of the adiabatic ST gap for polyazulene is much lower than that of the polyacenes. Therefore, contrary to what has been observed at the model Hamiltonian 32 and UDFT level of theory, 33 accurate multireference calculations show that the polyazulenes do not form stable triplet states within 6 fused rings.…”

“…32 This observation is fundamentally different from that of the acenes and was also predicted from the UB3LYP level of theory. 33 A previous study by Malrieu and co-workers investigated the development of spin bistability in donor-acceptor complexes and they have proposed a few succinct rules to achieve the goal. 34 Much work has also been done in the understanding of the inherent aromaticity of such systems.…”

Magnetoelectric Coupling on the Fused Azulene Oligomers

“…A finite spin gap indicates that only a singlet ground state is found for this molecule, in agreement with previous results from Refs. [8,21,22]. A non vanishing singlet-triplet spin gap for the azulene molecule with U/t shows that the azulene molecule does not undergo any magnetic phase transition and remains always in a singlet S = 0 state.…”

“…These fuzed azulene configurations have been theoretically studied using different computational techniques [8,15,21,22]. Density functional theory and effective valence bond explorations show the presence of a non-ferromagnetic to ferromagnetic transition when 6 or more azulene molecules are fuzed together [21]. A subsequent study by density matrix renormalization group (DMRG) reports a ferromagnetic transition for a shorter azulene oligomer (5-fuzed azulene molecules), together with a charge polarization.…”

“…1(c) and those in Refs. [8,15,21,22] have already been suggested as possible, and unique so far, purely organic multiferroics.…”