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Abstract: Fused benzene rings to antiaromatic compounds generally improve their stability but attenuate their antiaromaticity. The opposite case is now reported. NiII benzonorcorroles were synthesized and the effect of benzo‐fusion on the antiaromaticity was elucidated. The benzo‐fusion resulted in significant decrease of the HOMO–LUMO gaps and enhancement of the paratropic ring current effect. Furthermore, the introduction of the benzo groups induced singlet diradical character in the antiaromatic porphyrinoid. [ABSTRACT FROM AUTHOR]

Open-shell singlet nature, aromaticity, and second hyperpolarizabilities of two kinds of curved π-conjugated fragments of C60 fullerene, called C60 flakes, together with their corresponding planar analogues are theoretically investigated. It is found that one curved system, A-1, exhibits open-shell singlet nature, while the other structure, B-1, and the two corresponding planar systems possess a closed-shell ground state. The variation of the open-shell singlet nature is explained by the number of Clar's sextets in the resonance structures, which is associated with the local aromaticity, as quantified by the NICS0 values. The calculated orientationally averaged second hyperpolarizabilities show that the open-shell singlet system A-1 exhibits the best performance, and that the other curved system B-1 is the worst among the four C60 flakes, whereas all systems exhibit better performance than the original spherical C60 system. This tendency suggests that a lower dimensionality in the π-conjugated structure is beneficial to the construction of efficient nonlinear optical molecules. The present results not only show that the open-shell singlet system A-1 is a performant nonlinear optical molecule, but they also contribute to a deeper understanding of the electronic structure of curved π-conjugated molecules. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

To create a design guideline for efficient third-order nonlinear optical (NLO) molecules, the chain-length ( n) dependences of the diradical character y and the longitudinal second hyperpolarizability γ of quinoidal oligothiophenes (QTs), from monomers to octamers, involving thiophene- S, S-dioxide rings are investigated by using the density functional theory method. It turns out that the diradical character of the modified QTs is reduced as compared to those of the pristine QTs. By introducing an appropriate number of oxidized rings into the QT framework, intermediate y values can be achieved even in the systems with large values of n, in which the pristine QTs are predicted to have pure diradical character. Such intermediate diradical oligomers are shown to exhibit enhanced γ values as compared to the pristine QTs with the same value for n. From the calculation results, the introduction of the optimal number of thiophene-S,S-dioxide rings is predicted to be an efficient chemical modification for optimizing the third-order NLO properties of open-shell QTs through tuning the diradical characters. [ABSTRACT FROM AUTHOR]

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., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Fused benzene rings to antiaromatic compounds generally improve their stability but attenuate their antiaromaticity. The opposite case is now reported. Ni II benzonorcorroles were synthesized and the effect of benzo-fusion on the antiaromaticity was elucidated. The benzo-fusion resulted in significant decrease of the HOMO-LUMO gaps and enhancement of the paratropic ring current effect. Furthermore, the introduction of the benzo groups induced singlet diradical character in the antiaromatic porphyrinoid., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

To create a design guideline for efficient third-order nonlinear optical (NLO) molecules, the chain-length (n) dependences of the diradical character y and the longitudinal second hyperpolarizability γ of quinoidal oligothiophenes (QTs), from monomers to octamers, involving thiophene-S,S-dioxide rings are investigated by using the density functional theory method. It turns out that the diradical character of the modified QTs is reduced as compared to those of the pristine QTs. By introducing an appropriate number of oxidized rings into the QT framework, intermediate y values can be achieved even in the systems with large values of n, in which the pristine QTs are predicted to have pure diradical character. Such intermediate diradical oligomers are shown to exhibit enhanced γ values as compared to the pristine QTs with the same value for n. From the calculation results, the introduction of the optimal number of thiophene-S,S-dioxide rings is predicted to be an efficient chemical modification for optimizing the third-order NLO properties of open-shell QTs through tuning the diradical characters., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)