www.asiachem.news December 2021 | 35 Ph Ph Me Me Ph Ph MeO OMe 6 max ~600 nm max ~350 nm 7 Ph Ph MeO OMe has a large singlet–triplet energy gap and absorbs light at around 500 nm, its lifetime is two orders of magnitude shorter than that of 6, which is approximately 2 ns. This indicates that the steric hindrance between the acetal skeleton and the phenyl group at the C2-position raises the energy barrier for the transformation from the π-bond to the more stable σ-bond. In the case of 9 with OC3H7, the lifetime was one order of magnitude longer than that of 6. When the aryl group was also modified with substituents that increased the steric hindrance, relatively long-lived singlet diradicaloids 10 and 11 with lifetimes of 5 and 24 μs, respectively, were obtained. Thus, the introduction of a bulky aryl group and an alkoxy group enables the elongation of the lifetime of the π-single-bonded species. Recently, the present author’s group observed a notable ef fect of viscosity on the lifetime of the singlet diradicaloid 11, which increased to 2 s in 2,4-dicyclohexyl-2-methyl pentane (DCMP, η = 38.7) at 4000 atm and 293 K.30 This phenomenon is understood as the dynamic solvent effect. That is, for reactions that require a large reaction space, the logarithm of the reaction rate is proportional to the logarithm of the reciprocal of the viscosity.31 The bond formation process of 11 should involve the movement of bulky aryl groups having a large space. This implies that the reaction requires sufficient reaction space. In fact, when the viscosity of the solvent was increased at atmospheric pressure, the lifetime of 11, which is the inverse of the reaction rate, became longer (Figure 5a). Under high pressure conditions with viscous solvents, which are known to increase in viscosity with increasing pressure, the logarithm of the lifetime of 11 was proportional to the induced pressure (Figure 5b). Ph Ph MeO OMe 6 Ph Ph C3H7O OC3H7 9 200 ns 1899 ns Ar Ar O O Ar’ Ar’ 10 Ar = OMe OMe at 293 K in benzene 5.4 μs Ar Ar EtO OEt 11 23.8 μs Ar = Ph Ph 8 O O 2 ns iPr iPr Figure 3. Electronic absorption characteristic in the singlet and triplet diradicals. Figure 4. Substituent effect on the lifetime of singlet diradicaloids with π-single bond Figure 5. (a)The solvent dependency of the lifetime of 11 at 293 K and atmospheric pressure. (b)The pressure dependency of the lifetime of 11 at 293 K in DCMP. (a) (b)
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