Isomeric Azulene-Based Carbon-Centered Radicals Derived from N-Heterocyclic Carbenes

Azulene (isomer with naphthalene), a representative nonalternant hydrocarbon, has attracted significant attention as a building block for π-extended molecules owing to its distinctive electronic structure and physicochemical properties that differ from those of conventional alternant hydrocarbons. Nevertheless, the development of stable carbon-centered radicals utilizing an azulene moiety remains relatively scarce. Herein, we report the electronic structures and optical properties of azulene-based carbon-centered radical isomers, 1 and 2, which were designed and synthesized by attaching N-heterocyclic carbenes (NHCs) to the 6-position of the seven-membered ring or 2-position of the five-membered ring of azulene, respectively. Density functional theory calculations reveal that the spin density in 1 is primarily localized on the five-membered ring of the azulene, indicating it as π-extended cyclopentadienyl radicals, whereas the spin density is predominantly distributed in the seven-membered ring of azulene for 2, serving as an example of π-extended cycloheptatrienyl radicals. Furthermore, although both radicals 1 and 2 exhibit anti-Kasha emission, the emission wavelength of 2 (λ_em ∼ 495 nm) is significantly red-shifted compared to that of 1 (λ_em ∼ 396 nm).