Nonacethrene Unchained: A Cascade to Chiral Contorted Conjugated Hydrocarbon with Two sp3-Defects

We demonstrate that structurally complex carbon nanostructures can be achieved via a synthetic approach that capitalizes on a π-radical reaction cascade. The cascade is triggered by oxidation of a dihydro precursor of helical diradicaloid nonacethrene to give a chiral contorted polycyclic aromatic hydrocarbon named hypercethrene. In this ten-electron oxidation process, four σ-bonds, one π-bond, and three six-membered rings are formed in a sequence of up to nine steps to yield a 72-carbon-atom warped framework, comprising two configurationally locked [7]helicene units, a fluorescent peropyrene unit, and two precisely installed sp3-defects. The key intermediate in this cascade is a closed nonacethrene derivative with one quaternary sp3-center, presumably formed via an electrocyclic ring closure of nonacethrene, which, when activated by oxidation, undergoes a reaction cascade analogous to the oxidative dimerization of phenalenyl to peropyrene. By controlling the amount of oxidant used, two intermediates and one side product could be isolated and fully characterized, including single-crystal X-ray diffraction analysis, and two intermediates were detected by electron paramagnetic resonance spectroscopy. In concert with density functional theory calculations, these intermediates support the proposed reaction mechanism. Compared to peropyrene, the absorption and emission of hypercethrene are slightly red-shifted on account of extended π-conjugation and the fluorescence quantum yield of 0.45 is decreased by a factor of ∼2. Enantiomerically enriched hypercethrene displays circularly polarized luminescence with a brightness value of 8.3 M-1 cm-1. Our results show that reactions of graphene-based π-radicals-typically considered an "undefined decomposition" of non-zero-spin materials-can be well-defined and selective, and have potential to be transformed into a step-economic synthetic method toward complex carbon nanostructures

Nonacethrene Unchained: A Cascade to Chiral Contorted Conjugated Hydrocarbon with Two sp³-Defects

We demonstrate that structurally complex carbon nanostructures can be achieved via a synthetic approach that capitalizes on a π-radical reaction cascade. The cascade is triggered by oxidation of a dihydro precursor of helical diradicaloid nonacethrene to give a chiral contorted polycyclic aromatic hydrocarbon named hypercethrene. In this ten-electron oxidation process, four σ-bonds, one π-bond, and three six-membered rings are formed in a sequence of up to nine steps to yield a 72-carbon-atom warped framework, comprising two configurationally locked [7]helicene units, a fluorescent peropyrene unit, and two precisely installed sp; 3; -defects. The key intermediate in this cascade is a closed nonacethrene derivative with one quaternary sp; 3; -center, presumably formed via an electrocyclic ring closure of nonacethrene, which, when activated by oxidation, undergoes a reaction cascade analogous to the oxidative dimerization of phenalenyl to peropyrene. By controlling the amount of oxidant used, two intermediates and one side product could be isolated and fully characterized, including single-crystal X-ray diffraction analysis, and two intermediates were detected by electron paramagnetic resonance spectroscopy. In concert with density functional theory calculations, these intermediates support the proposed reaction mechanism. Compared to peropyrene, the absorption and emission of hypercethrene are slightly red-shifted on account of extended π-conjugation and the fluorescence quantum yield of 0.45 is decreased by a factor of ∼2. Enantiomerically enriched hypercethrene displays circularly polarized luminescence with a brightness value of 8.3 M; -1; cm; -1; . Our results show that reactions of graphene-based π-radicals-typically considered an "undefined decomposition" of non-zero-spin materials-can be well-defined and selective, and have potential to be transformed into a step-economic synthetic method toward complex carbon nanostructures

Molecular Magnetic Switches

The design and synthesis of molecular switches is of growing importance considering the steep increase in the production of consumer electronics in the recent years. The function of these devices is based on binary electronic circuits and can be achieved by use of bistable magnetic materials. This article reviews four types of molecular systems, which can be switched between two spin states in response to an external stimulus, illustrating working principles that can motivate the design of systems for practical applications. As an outlook, organic diradicaloid molecules are introduced as potential molecular magnetic switches that do not rely on the use of metals

Dimethylnonacethrene – en route to a magnetic switch

Dimethylnonacethrene is the first derivative of the cethrene family that is energetically more stable than the product of its electrocyclic ring closure. Compared to the shorter homologue dimethylcethrene, the new system is EPR-active, because of a significantly lowered singlet–triplet gap, and displays remarkable stability. Our results suggest that adjustment of the steric bulk in the fjord region can enable realisation of diradicaloid-based magnetic photoswitches

Dimethylnonacethrene – En Route to a Magnetic Switch

We present the first derivative of the cethrene family that displays features previously unobserved in this class of helical diradicaloids. In contrast to all other reported cethrenes, dimethylnonacethrene is energetically more stable than the product of its 6π electrocyclization, because of the methyl substituents installed in the fjord region. The result is a stable and isolable open form, which could not be detected previously for the unsubstituted parent compound on account of high reactivity. Compared to the shorter homolog dimethylcethrene, π-extension results in lowering of the singlet–triplet energy gap (ΔEST) of dimethylnonacethrene and appearance of an EPR signal at room temperature. In comparison with planar isomer nonazethrene and linear analog nonacene, featuring the same number of benzenoid rings, helical dimethylnonacethrene exhibits significantly lower ΔEST and remarkable stability under ambient conditions. Our results suggest that further adjustment of the steric bulk in the fjord region can enable realization of a hydrocarbon-based magnetic photoswitch

π-Radical Cascade to a Chiral Saddle-Shaped Peropyrene

Reactions of open-shell molecular graphene fragments are typically thought of as undesired decomposition processes because they lead to the loss of desired features like π-magnetism. Oxidative dimerization of phenalenyl to peropyrene shows, however, that these transformations hold promise as a synthetic tool for making complex structures via formation of multiple bonds and rings in a single step. Here, we explore the feasibility of using this “undesired” reaction of phenalenyl to build up strain and provide access to non-planar polycyclic aromatic hydrocarbons. To this end, we designed and synthesized a diradical system with two phenalenyl units linked via a biphenylene backbone. The design facilitates an intramolecular cascade reaction to a helically twisted saddle-shaped product, where the key transformations—ring-closure and ring-fusion—occur within one reaction. The negative curvature of the final peropyrene product, resulting from the presence of an eight-membered ring, was confirmed by single-crystal X-ray diffraction analysis and the helical twist was validated via resolution of the product’s enantiomers that display circularly polarized luminescence

Pi Radical Cascade to a Chiral Saddle Shaped Peropyrene

Author Keywords: phenalenyl · biradicals · peropyrene · negative curvature · circularly polarized luminescenceReactions of open-shell molecular graphene fragments are typically thought of as undesired decomposition processes because they lead to the loss of desired features like π-magnetism. Oxidative dimerization of phenalenyl to peropyrene shows, however, that these transformations hold promise as a synthetic tool for making complex structures via formation of multiple bonds and rings in a single step. Here, we explore the feasibility of using this “undesired” reaction of phenalenyl to build up strain and provide access to non-planar polycyclic aromatic hydrocarbons. To this end, we designed and synthesized a biradical system with two phenalenyl units linked via a biphenylene backbone. The design facilitates an intramolecular cascade reaction to a helically twisted saddle-shaped product, where the key transformations—ring-closure and ring-fusion—occur within one reaction. The negative curvature of the final peropyrene product, induced by the formed eightmembered ring, was confirmed by single-crystal X-ray diffraction analysis and the helical twist was validated via resolution of the product’s enantiomers that display circularly polarized luminescence and high configurational stability.H2020 European Research Council. Grant Number: ERC StG 716139 / INSPIRALSchweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung. Grant Numbers: CRSK-2_190365, PP00P2_170534, PP00P2_198900, TMCG-2_213829 / CASCADERUniversität Zürich. Grant Number: FK-21-131Deutsche Forschungsgemeinschaft. Grant Number: 417643975Open Access funding provided by Universität Zürich