34 | December 2021 www.facs.website levels of the two orbitals occupied by two electrons, diradicals can be created with a singlet ground state. Indeed, Borden et al. found that diradical 3 with an electron-withdrawing fluorine atom at the C2 position (X = F, singlet-triplet energy difference ΔEST = –9.7 kcal mol –1)14 is a singlet-state molecule. We focused on the oxygen functional group, for which an increased number of derivatives are possible, and examined the most stable spin multiplicity of 4 (Table 1). As a result, we found that 4a (X = OH) with an oxygen functional group is indeed a singlet ground-state molecule that is a candidate for the experimental verification of the π-single bond (vide infra). Interestingly, 5 (X = SiH3) with an electron-donating substituent, SiH3, was also found to have a singlet ground state.15 These substituent effects can be explained by the pseudo-π orbital interaction at the C2-position of the 1,3-diradical and the spiro-conjugation effect (Figure 2). In the case of the oxygen functional group, the interaction between ψS and the pseudo-π orbital σCO* at the C2 position stabilizes the ψS orbital (Figure 2a, left), resulting in a singlet ground state with π-single bonding wherein two electrons selectively occupy the ψS,OR orbital. In the case of 4a with X = OH, a CASSCF(2,2) calculation shows that the two electrons are accommodated in the ψS,OR orbital with 81% probability, and the bond order between C1 and C3 is 0.62. The electronic absorption spectrum was predicted by TD-DFT calculations, and 4a was found to be a species exhibiting strong absorption at 420 nm (oscillator intensity (f) = 0.32); furthermore, the absorption at 420 nm was found to be due to the π π* transition of the π single bond.16 In addition to the pseudo π interaction, spiroconjugation plays a role in increasing the energy gap between ψS,OR and ψA,OR, thereby increasing it to the energy spacing ΔEST of 4b (Table 1, Figure 2a). In the case of 5 with the introduction of electron-donating silicon functional groups, the singlet was estimated to be the most stable spin multiplicity (Table 1), and CASSCF calculations revealed that an electron configuration in which the electrons occupied the antibonding orbital (π*) ΨA,Si with 71% probability was favorable (Figure 2b). This substituent effect was attributed to the strong interaction of the electron-donating σCSi with the ψS orbital, resulting in ψA,Si as the HOMO and ψS,Si as the LUMO. This indicates that the singlet state of 5 exists without π-single bonding in the frontier orbitals, in contrast to the case of 4a,b, where π-single bonding is introduced by the electron-withdrawing substituents. Novel π-Single Bond (C-π-C)17–19 The present author’s research group has been promoting research with the aim of not only understanding the process of bond homolysis, but also developing new chemistry based on the unexpected discoveries made in such research. In this regard, an unexpected finding in our research was that the localized singlet diradical exhibited strong absorption in the visible region (around 600 nm in the case of diradical 6), whereas the corresponding triplet diradical 7 showed absorption in the ultraviolet region (around 350 nm). (Figure 3). To investigate the origin of the remarkable spin multiplicity effect on the absorption wavelength of the diradical species, we computed the orbitals corresponding to the electronic transitions using quantum chemical calculations (Figure 3). The absorption in the visible region around 600 nm is computed to be mainly due to the electronic transition from the π-bonding orbital (π) to the antibonding orbital (π*) between C1 and C3 based on a highlevel ab initio method (CAS(2,2)+DDCI3) (Figure 3). Thus, the singlet diradical 6 entails π-bonding between C1 and C3, although an open-shell singlet structure exists. In other words, the localized singlet 1,3-diradical with an electron-withdrawing substituent at the C2-position has a novel bonding system with a π-single bond (C–π–C) between C1 and C3. This new group of compounds has a planar four-coordinated carbon atom, which is different from the generally known molecular structure. Very recently, a new bonding system has been reported for heterocyclic systems.20–28 In addition, despite a small π-electron system, they exhibit strong absorption in the visible region and are expected to provide new photoantenna sites that absorb abundant solar energy. Fur thermore, as recently proposed by Nakano and co-workers,29 the open-shell π-bond is expected to have a high two-photon absorption capacity because of the third-order nonlinear effect; once π-single bond (C–π–C) compounds are isolated, they may function as next-generation optical materials. Kinetic Stabilization of π-SingleBonded Compounds based on Substituents and Solvent Viscosity The singlet diradical 6 is stable for at least a day in the low-temperature matrix state (77 K) but has a lifetime of approximately 200 ns at room temperature.13 Therefore, it is difficult to handle the chemical species at room temperature. To extend the lifetimes of π-single-bonded compounds, we investigated the substituent effects of the alkoxy and aryl groups, which can be relatively easily modified (Figure 4). Although diradical 8, which has a planar cyclic acetal skeleton at the C2 position, X X X,X EST* in kcal mol–1 F,F 2 3 4a H,H OH,OH O O 4b 5 +0.9 –9.7 –6.7 –12.2 * EST = ES – ET. ES : singlet energy; ET : triplet energy SiH3,SiH3 –5.2 Table 1 Substituent effect on ground-state spin multiplicity ground state singlet triplet singlet singlet singlet H H H S,H A,H H H S A CH A S Figure 1 Orbital interaction of S with CH nO RO OR O O A,OR S,OR O O O O H Si Si S,Si A,Si H3Si SiH3 S A CO* CSi A S OR OR OR OR SiH3 SiH3 SiH3 SiH3 * * (a) (b) pseudo interaction spiroconjugation singlet state with -single bonding singlet state with no -single bonding pseudo interaction Figure 2. (a) alkoxy group effect and (b) silyl group effect on the singlet ground state of 1,3-direadicals
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