Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect

Abstract

Localized singlet diradicals have attracted much attention, not only in the field of bond-homolysis chemistry, but also in nonlinear optical materials. In this study, an extremely long lived localized singlet diradical was obtained by using a new molecular design strategy in which it is kinetically stabilized by means of a macrocycle that increases the molecular strain of the corresponding σ-bonded compound. Notably, the lifetime of this diradical (14 μs) is two orders of magnitude longer than that of a standard singlet diradical without a macrocyclic structure (≈0.2 μs) at 293 K. The species is persistent below a temperature of 100 K. In addition to the kinetic stabilization of the singlet diradical, the spontaneous oxidation of its corresponding ring-closed compound at 298 K produced oxygenated products under atmospheric conditions. Apparently, the "stretch effect" induced by the macrocyclic structure plays a crucial role in extending the lifetime of localized singlet diradicals and increasing the reactivity of their corresponding σ-bonded compounds.

Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character

“…As predicted by the computations, the singlet diradical S- DR3b was extremely long-lived with a lifetime approximately 1000-fold that of S- DR2b ( τ 293 = 209 ns), 49 and 11-fold that of S- DR3a ( τ 293 = 14.2 μs). 68 Thus, the experimental results demonstrate that the stretch effect induced by the macrocyclic structure increases the kinetic stabilisation of the singlet diradicaloid.…”

“…As predicted by the computations, the singlet diradical S-DR3b was extremely longlived with a lifetime approximately 1000-fold that of S-DR2b (s 293 ¼ 209 ns), 49 and 11-fold that of S-DR3a (s 293 ¼ 14.2 ms). 68 Thus, the experimental results demonstrate that the stretch effect induced by the macrocyclic structure increases the kinetic stabilisation of the singlet diradicaloid. Variable temperature laser ash photolysis (VT-LFP) measurements were conducted at ve temperatures in the range of 273-303 K. The activation parameters E a , log A, DH ‡ , DS ‡ , and DG ‡ 293 of the ring-closing reaction of S-DR3b to CP3b in benzene were determined by the Arrhenius and Eyring plots (Table 4, Fig.…”

“…The Diels–Alder cycloaddition of 4 and cyclopentadiene delivered azoalkane 5 , which was subjected to two sequential Sonogashira cross-coupling reactions to produce azoalkane 8 . 68 Macrocyclisation was accomplished by the one-pot consecutive inter- and intramolecular Suzuki–Miyaura coupling of azoalkane 8 and bisboronic ester 9 in the presence of Pd(OAc) 2 and SPhos ligand, 80 which delivered azo compound AZ3b in approximately 3% isolated yield with regard to 8 . The flanking alkyl substituents (–C 8 H 17 ) are required to ensure sufficient solubility.…”

“…The moderate increase in the lifetime of S- DR3a to τ 293 = 14.2 μs in benzene ( Scheme 2b ) indicated that the construction of macrocyclic structures is a useful strategy to extend the lifetime of singlet diradicaloids and enable more detailed investigations. 68 This finding prompted us to devise a new fine-tuned macrocyclic structure. In this study, singlet diradicaloid S- DR3b featuring a naphthalene-containing macrocyclic system was designed and examined by computational and experimental studies.…”

Bulky Substituent Effect on Reactivity of Localized Singlet Cyclopentane‐1,3‐diyls with π‐Single Bonding (C‐π‐C) Character

“…In the past two decades we have studied the electronic, stretch, and steric effects on the reactivity of 2,2‐dialkoxy‐1,3‐diarylcyclopentane‐1,3‐diyls (S‐ DR1) generated in the photodenitrogenation of the corresponding azoalkanes AZ1 (Scheme ). These studies have shown that the alkoxy group greatly influences the lifetime of S‐ DR1 (Table ).…”