Chiral Bay-Alkynylated Tetraazaperylenes: Photophysics and Chiroptical Properties

Synthesis of Alternatively-Twisted Nanographenes by Semi-Deprotection-Induced Cyclization

Twisted nanographenes (NGs) are currently attracting a lot of attention owing to their geometrical and electronic structures that differ substantively from conventional planar and nonplanar NGs, while the strategic synthesis of twisted NGs is still a topic of interest because the products are often interconvertible among unidirectionally, alternatively, or randomly twisted geometries and otherwise obtained as a mixture of them. Herein, we report the conformationally specific synthesis of twisted NGs where the geometry was reinforced by introducing 1,4-dioxane rings at a K-region of a central pyrene core that bears a large contortion. The 1,4-dioxane rings were generated by semi-deprotection, of tetraoxa[4.4.4]-propellanes in precursor molecules, which were confirmed to be engaged in forming C-C bonds via a Friedel-Crafts type mechanism. The large contortion within the pyrene core causes a narrowed HOMO-LUMO gap on account of unusual p z -lobe overlap between +z and -z sides, giving rise to red emission with a high quantum yield of 94% as well as stable redox processes of 2e- uptake/release.

Chemical oxidation of a double-twisted nanographene

Chemical oxidation of a double-twisted nanographene led to quantitative generation of corresponding dicationic species, which exhibited an intense absorption band tailing to 1500 nm wherein two orthogonally arranged π-skeletons contributed to transitions and a high dissymmetry factor of -0.0209 was recorded at 1443 nm in the NIR-II region.

Twisted Nanographenes with Robust Conformational Stability

Owing to a lack of methodology for rationally and selectively synthesizing twisted nanographenes, it is usually inevitable that we obtain nanographenes as a mixture with various geometries, such as unidirectionally twisted, alternatively twisted, randomly twisted, and even wavy structures, reflecting the high activation barriers among them. Otherwise, they are interconvertible if the barriers are low enough such that only averaged properties can be observed under a thermal equilibrium. Recently, we reported on a double-twisted nanographene containing four [6]helicene units within the skeleton. In this paper, we discuss the robust conformational stability of the nanographene, both experimentally and computationally. The results indicate that the nanographene could only be racemized at temperatures exceeding 200 °C, and the first flip of one of the four [6]helicene units is the rate-degerming step.