The dissociation behavior of the asymmetric hetero‐phenalenyl dimers is theoretically investigated from the viewpoint of the open‐shell singlet third‐order nonlinear optical (NLO) systems. It is found that the second hyperpolarizability γ of all the investigated phenalenyl dimers exhibit bell‐shape behavior with maximum at the intermediate dissociation region, and that the γ value of the most effective asymmetric dimer reaches about 20 times the maximum of the original phenalenyl dimer. This behavior is clarified by the new definition of diradical/ionic characters, identifying that the significant enhancement of γ occurs in the intermediate ionic region as well as in the intermediate diradical region. The present result extends the concept of diradical character based design of NLO molecules toward ionic closed‐shell region in a realistic molecular framework, and demonstrates the intermediate ionic systems as a new target in the NLO molecules with open‐shell molecular frameworks.

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 intermediate 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 intermediate 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.

Using the spin-unrestricted density functional theory method, we investigate the interplay between the diradical character y and second hyperpolarizabilities γ (the third-order nonlinear optical (NLO) properties at the molecular scale) of corannulene derivatives with two phenoxyl radicals. This molecule in the singlet state exhibits intermediate y and thus displays a significantly larger γ value than the triplet state and the closed-shell bis-phenol analogue. We also examine the planar molecules involving a coronene moiety in place of the curved corannulene. The intermediate y and large γ values of the corannulene systems are found to originate not from their curved skeleton but from the equilibrium between benzenoid/quinoid resonance forms due to delocalization of the radical electrons of the terminal phenoxyl rings. The longitudinal γ value of the singlet state is found to be comparable to that of s-indaceno[1,2,3-cd;5,6,7-c'd']diphenalene, which is known to be one of the organic molecules with the largest two-photon absorption cross section in this size of pure hydrocarbons. The present system is thus expected to be a promising candidate for highly efficient open-shell NLO molecules.

Tunability of the open-shell character, charge asymmetry, and third-order nonlinear optical (NLO) properties of covalently linked (hetero)phenalenyl dimers are investigated by using the density functional theory method. By changing the molecular species X and substitution position (i, j) for the linker part, a variety of intermonomer distances R and relative alignments between the phenalenyl dimers can be realized from the geometry optimizations, resulting in a wide-range tuning of diradical character y and charge asymmetry. It is found that the static second hyperpolarizabilities along the stacking direction, γ , are one-order enhanced for phenalenyl dimer systems exhibiting intermediate y, a feature that is in good agreement with the "y-γ correlation". By replacing the central carbon atoms of the phenalenyl rings with a boron or a nitrogen, we have also designed covalently linked heterophenalenyl dimers. The introduction of such a charge asymmetry to the open-shell systems, which leads to closed-shell ionic ground states, is found to further enhance the γ values of the systems having longer intermonomer distance R with intermediate ionic character, that is, charge asymmetry. The present results demonstrate a promising potential of covalently linked NLO dimers with intermediate open-shell/ionic characters as a new building block of highly efficient NLO systems.