The two-photon absorption (TPA) of a bis(acridine) dimer (8) having singlet diradical character in its ground state was found to be enhanced by more than 2 orders of magnitude as compared to its closed-shell counterpart (12), which has the same structural backbone and atom composition. The dimer, a tetracation species consisting of two connected acridinium cation moieties with high coplanarity, was obtained during our attempts to synthesize triplet carbenes by double oxidation of an allenic precursor (3b). High conjugation over the two aromatic rings connected by dimerization was revealed by X-ray analysis, and a small HOMO-LUMO gap was found in the visible-near-infrared one-photon absorption spectrum in solution and in the crystalline state, exhibiting that the ground state of 8 has singlet diradical nature. Ab initio molecular orbital calculations of the ground state also suggested that 8 has an intermediate diradical character (y) of 0.685. Interconversion between diradical tetracation dimer 8 and closed-shell dication dimer 12 was achieved by oxidation/reduction in good yields and was accompanied by formation of monoradical trication dimer 13 as an intermediate. TPA measurements at near-infrared wavelengths revealed that diradical dimer 8 has large TPA cross sections (3600 GM at 1200 nm), while closed-shell 12 has TPA cross sections of <21 GM. This result represents a straightforward comparison between the TPA activity of molecules with the same structural backbone and atom composition but with different degree of the diradical character, supporting the theoretical prediction that enhanced TPA intensity can be observed in the intermediate y region (0 < y < 1).

A long‐range corrected spin‐unrestricted density functional theory (LC‐UDFT) approach has been used to investigate the diradical character (yi) and third‐order nonlinear optical properties of linear/cyclic [N]acenes and [N]phenylenes in their ground states, where N is the number of benzene rings in each linear and cyclic configuration. It has been found that linear and cyclic acenes with N = 5–10 show singlet diradical characters, while linear and cyclic phenylenes with equivalent number of benzene rings have closed‐shell configurations. The amplitudes of third‐order nonlinear optical polarizability (γzzzz) for open‐shell linear/cyclic acenes are larger especially in intermediate range of diradical character than those of closed‐shell phenylene counterparts. For example, the γzzzz values of [5]cyclic acene (y0 = y1 = 0.320) is 4.89 × 103 a.u. which is about 6 times larger than those of 0.80 × 103 a.u. for closed‐shell [5]cyclophenylene. Similarly, γzzzz value of [5]linear acene is also about 9 times larger than its closed‐shell [5]linear phenylene counterpart. Our results show interesting insights into the relationship among the architectures, diradical characters and γzzzz values of different acenes and phenylenes. © 2012 Wiley Periodicals, Inc.

From the viewpoint of “diradical character,” referred to as “diradicalology,” we investigate the second hyperpolarizability γ—the molecular third‐order nonlinear optical (NLO) property—of one‐dimensional supermolecular systems composed of acetylene‐linked phenalenyl/pyrene rings using long‐range corrected spin‐unrestricted density functional theory. It turns out that the pyrene‐based superpolyenes (Py‐n) behave like closed‐shell systems, whereas phenalenyl‐based superpolyenes (Ph1‐n and Ph2‐n) have different diradical characters depending on the linked form, that is, Ph1‐n and Ph2‐n have intermediate and pure diradical characters, respectively. In comparison with Py‐n and Ph2‐n, the longitudinal γ of Ph1‐n is significantly larger, and it displays larger enhancement as a function of system size. Substitutions to the terminal rings by donor (NH2) and acceptor (NO2) groups further enhance γ in Ph1‐n, more than in Ph2‐n and Py‐n. These results are in agreement with the structure–property relationships derived for open‐shell NLO systems with symmetric and asymmetric charge distributions (Nakano et al., J. Chem. Phys. 2010, 133, 154302). © 2012 Wiley Periodicals, Inc.

Using the long‐range corrected spin‐unrestricted density functional theory method, the impact of antidot structure on the open‐shell character and the second hyperpolarizability (γ) of graphene nanoflakes (GNFs) has been investigated for rectangular GNFs with and without antidot structure, referred to as antidot and perfect GNFs, respectively. It is found that the two GNFs exhibit different multiradical characters; the antidot GNF shows intermediate open‐shell character in contrast to the perfect GNF presenting nearly pure tetraradical character. This antidot structure dependence of open‐shell character can be explained in terms of their natural orbital distributions. Such difference in open‐shell characters affects their γ values; the γ of the antidot GNF is about 1.7 times larger than that of the perfect GNF, the feature of which is in agreement with our previous results on the diradical character dependence of γ. © 2012 Wiley Periodicals, Inc.