Super‐heptazethrene

Abstract

The challenging synthesis of a laterally extended heptazethrene molecule, the super-heptazethrene derivative SHZ-CF3, is reported. This molecule was prepared using a strategy involving a multiple selective intramolecular Friedel-Crafts alkylation followed by oxidative dehydrogenation. Compound SHZ-CF3 exhibits an open-shell singlet diradical ground state with a much larger diradical character compared with the heptazethrene derivatives. An intermediate dibenzo-terrylene SHZ-2H was also obtained during the synthesis. This study provides a new synthetic method to access large-size quinoidal polycyclic hydrocarbons with unique physical properties.

Stable Organic (Bi)Radicals by Delocalization of Spin Density into the Electron‐Poor Chromophore Core of Isoindigo

“…Molecules with a large biradical character and a small singlet–triplet energy gap like OS‐2 .. are usually characterized by comparatively short lifetimes. Thus super‐heptazethrenes and phenoxyl substituted rylene bisimides and corannulenes with similar molecular properties (Δ E ST =0.041–0.082 eV, y= 0.64–0.83) readily decompose within hours under ambient conditions in solution and/or weeks in the solid state. Conversely, so far available materials with comparable stabilities like OS‐2 .. ( τ 1/2 =several weeks in solution and infinite in the solid state) exhibit considerably larger singlet–triplet energy gaps, smaller biradical characters or require substantial spin delocalization .…”

“…Thus, much efforts have been invested in the last decades to stabilize such intrinsically reactive materials by kinetic blocking of the radical centers and/or thermodynamic stabilization by introduction of electron withdrawing substituents and π‐electron delocalization . Based on this strategy, reasonably stable open‐shell PAHs have been synthesized using quinodimethanes, bisphenalenyles, zethrenes or diindeno fused π‐scaffolds with significant open‐shell biradicaloid character . Only recently, we and others have introduced 2,6‐di‐ tert ‐butylphenoxyl substituents into electron deficient rylene bisimides or PAHs to generate singlet open‐shell biradicals with record biradical characters of up to y= 0.83 which, however decomposed very fast .…”

Modern zethrene chemistry

“…Very recently, our group successfully synthesized the first super-heptazethrene derivative 48 from the tetraaldehyde intermediate 49 via an intramolecular FriedelCrafts alkylation, followed by oxidative dehydrogenation strategy. 27 Compound 48 is relatively stable due to thermodynamic and kinetic stabilization. Broadened signals in the NMR spectrum were detected for 48 at room temperature, and the signals became sharper as the temperature decreased.…”

A Thermally Populated, Perpendicularly Twisted Alkene Triplet Diradical

“…[13] Furthermore,l arge,p lanar, polycyclic hydrocarbons with singlettriplet gaps of 1-4 kcal mol À1 were recently reported. [14,15] No further experimental investigations of 1 have been reported, but Lenoir et al recently published calculations at the UB3LYP/6-311 + G(2d,p) level of theory and obtained as inglet-triplet splitting of 3.4 kcal mol À1 (DH ST in the gas phase) and torsional angles of 538 8 in the ground-state singlet 1(S 0 )a nd 908 8 in the triplet 1(T 1 ). [3] Thec orresponding calculated values for bifluorenylidene 2 at the same level of theory are 19.5 kcal mol À1 ,348 8 (S 0 ), and 908 8 (T 1 ).…”