We have theoretically clarified the fundamental relationship between diradical character (y: a theoretical index of open-shell nature) and second hyperpolarizability (γ: the third-order nonlinear optical (NLO) property at the molecular level), i.e., open-shell molecules with intermediate y are predicted to possess significant γ values compared to those of closed-shell (y ~ 0) and pure diradical (y ~ 1) systems [1]. In order to achieve open-shell singlet molecules with intermediate y, practical molecular design guidelines for tuning open-shell electronic structures are needed. Quinoidal oligothiophenes (QTs) have attracted much attention in various fields of materials science and engineering owing to their unique open-shell electronic structures [2,3]. Takahashi and coworkers have succeeded in synthesizing QTs from monomer to hexa-oligomer (n = 6) [2,3], which enables a detailed understanding of the chemical and electronic structures of QTs. In such QTs, y values were found to increase with the increase of n, and from quantum chemical calculations, the y value of hexa-oligomer system was predicted to be close to 1 [4]. Such a rapid increase of y with the increase of n was found to be related to the rapid increase of the aromaticity of the thiophene rings in the central regions [3,4]. Thus, chemical modifications for tuning the aromatic characters are expected to lead to a moderate change of y of QTs. In this study, we investigate the effects of introducing S,S-dioxo-thiophene rings into the QT framework by using quantum chemical calculations. Aromaticity of a S,S-dioxo-thiophene ring is known to be significantly reduced from that of a thiophene ring, and thus the introduction of such units to QTs is expected to change the chain-length dependences of y and γ as compared to the regular QTs. We also discuss the effects of the positions and the number of S,S-dioxo-thiophene rings in QTs on the bond-length alternation (BLA) patterns, y, and γ. On the basis of the results, we will discuss practical molecular designs for tuning diradical characters and physico-chemical properties of open-shell one-dimensional oligomers involving heteroatoms. [1] Nakano, M. et al. J. Phys. Chem. A 2005, 109, 885; Phys. Rev. Lett. 2007, 99, 033001. Nakano, M. Excitation Energies and Properties of Open-Shell Singlet Molecules, Springer, 2014. [2] Takahashi, T. et al., J. Am. Chem. Soc. 2005, 127, 8928. [3] Ortiz, R. P. et al. Chem. Eur. J. 2010, 16, 470. Casado,
J. et al., Chem. Soc. Rev. 2012 41, 5672. [4] Kishi, R. et al., J. Phys. Chem. C 2013, 117, 21498.