www.asiachem.news December 2021 | 41 (SA-CASSCF) method (Figure 13). Although the SA-CASSCF method entails a large computational cost, it is able to obtain the electronic states and energies of the multiconfigurational singlet diradicals with an open-shell nature more precisely. As shown in Figure 13, the activation energy of the adiabatic bond-homolysis process to give the puckered-type intermediate 33* from the electronically excited state trans-32* was found to be very small, approximately 2 kcal mol–1, and the two fluorescence peaks observed in the emission spectrum of trans-32 were from trans-32* and 33*. The activation energy from 33* to 6* is as high as 10 kcal mol–1, which is consistent with the experimental observation that the emission from 6* (~650 nm) was not observed. A similar adiabatic excited-state cleavage reaction was performed for trans38*, and the energy barrier for the bond homolytic cleavage process was as high as approximately 6 kcal mol–1, consistent with the fact that only phenyl-group-derived fluorescence was observed from trans-38*. Thus, the existence of a third minimal-energy structure in the bond homolysis process of trans-32 even in the electronically excited state was revealed. Conclusion In this article, we elucidate the kinetic stabilization of localized singlet diradicals entailed in the bond homolysis process to extend their lifetime and investigate their chemical properties. By thoroughly studying the bond homolysis process, which is a fundamental chemical reaction, we propose novel bonding styles and new reaction intermediates. We hope that more research will be conducted using these new chemical concepts in the future. Finally, since we are often asked about the difference between “diradical” and “biradical,” we will explain the definition of “diradical” used in this paper. According to the IUPAC Gold Book, a diradical is a chemical species with two strongly interacting radicals in a molecule and two spin multiplicities, a singlet (↑↓) and a triplet (↑↑); by contrast, when two radicals are far apart in a molecule, the spin interaction is small, and they are judged to be two doublet species (2 × ↑), they are called biradicals. We would like to express our sincere gratitude to the students who have graduated from the Graduate School of Engineering, Osaka University, and the Graduate School of Science, Hiroshima University, who had devoted themselves to the research summarized herein. 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