Three‐Dimensional Intramolecular Exchange Interaction in a Curved and Nonalternant π‐Conjugated System: Corannulene with Two Phenoxyl Radicals

Abstract

In recent years, curved p-conjugated molecules such as fullerenes and carbon nanotubes have attracted much attention not only in chemistry but also in materials science. [1] Their intra/intermolecular interactions within/between threedimensional (3D) curved p-electron networks play intrinsically vital roles in their unique properties and functionalities. Among them, intramolecular magnetic interaction between electronic spins on a curved p surface was extensively studied for ionic species of C 60 such as C 60 2À and C 60 3À .[2] Their electronic structures are greatly influenced by not only the dynamic spin polarization of electrons but also the negative charges on the spherical p-conjugated system and the countercation.Thus, we have focused on neutral diradical systems, which are known to be the most useful probes for studying intramolecular magnetic interactions in organic molecules. [3] While many neutral diradical derivatives relevant to planar pconjugated systems have so far been investigated, studies on curved p-conjugated neutral diradicals are limited to a single C 60 -based system in which [60]fullerene is linked to two nitroxide radicals.[4] However, their intramolecular exchange interaction J through the C 60 skeleton was very weak (j Jk À1 B j < 0.1 K)[4b] because of the small spin delocalization onto the C 60 p network from the nitroxide radicals with spin-localized nature on the NO moieties. Therefore, in order to evaluate an intramolecular exchange interaction in a curved p-conjugated system in a quantitative manner, synthesis and isolation of a stable neutral diradical derivative with extensively spindelocalized nature on its curved p-conjugated system have been the focus of current attention in molecular magnetism and open-shell chemistry. [2,5] Recently, we studied corannulene [6] -based stable neutral monoradical systems, [7] such as a phenoxyl radical derivative 1[7d] with highly spin-delocalized nature on the intrinsically 3D bowl-shaped and nonalternant p-conjugated network. These studies inspired us to propose a 3D intramolecular exchange interaction in this class of curved p radicals: in terms of geometry, they are intermediate between a planar p radical such as a phenalenyl system [8] and a tetrahedral s radical. We have now, for the first time, synthesized and isolated a corannulene-based neutral diradical, namely, 2, which has two phenoxyl radical moieties, as air-stable crystals. Due to its highly spin-delocalized nature, 2 shows strong intramolecular exchange interaction (Jk À1 B = À405 AE 2 K) through the 3D corannulene p-electron network. Bond-length analyses and DFT calculations showed that 2 has contributions from diradical canonical forms a-g and closed KekulØ structure d (Scheme 1). Furthermore, in the crystalline state the presence of three crystallographically independent diradical molecules with different curvature enabled us to study the relationship between the curvature of the corannulene p skeleton and the

Electron-Poor Bowl-Shaped Polycyclic Aromatic Dicarboximides: Synthesis, Crystal Structures, and Optical and Redox Properties

“…The discovery of fullerenes and carbon nanotubes has motivated the pursuit of nonplanar polycyclic aromatic hydrocarbons (PAHs). − In particular, bowl-shaped PAHs are attractive synthetic targets. Interest in curved aromatic scaffolds continues to increase because such systems offer high prospects for packing control as desired in organic materials and supramolecular chemistry (e.g., in bulk heterojunction solar cells, , liquid crystals, , supramolecular polymerization, and host–guest complexation). − Being commercially available and mass-producible, corannulene has become the most utilized precursor for bowl-shaped PAHs and has been functionalized with various moieties. , Simple groups are introduced by nucleophilic substitution, , metal-catalyzed C–C bond formation, − or Rieche formylation . As well, substituents may be installed prior to constructing the corannulene scaffold. ,, Extension of the corannulene π-scaffold is achieved by introduction of electron-rich aromatic substituents followed by direct arylation or oxidative condensation. − However, it is challenging to expand the corannulene π-scaffold with electron-deficient substituents by the above-mentioned synthetic methods .…”

Normal & reversed spin mobility in a diradical by electron-vibration coupling

“…Diradicals have always attracted the attention of chemists owing to their unique role in understanding the fundamental nature of chemical bonds and of reactivity 4 – 8 . In the last decade, the class of diradicals based on open-shell π-conjugated structures have become of interest in various organic electronic and magnetic applications 9 – 11 .…”

(Pyrrole-2,5-Diyl)-Bis(Nitronyl Nitroxide) and-Bis(Iminonitroxide): Specific Features of the Synthesis, Structure, and Magnetic Properties

“…Against the background of actively investigated diradicals linked by alternant hydrocarbons, there are only a few reports about diradicals that are linked by nonalternant systems [24][25][26][27][28]. Nonbonding molecular orbitals (MOs) of such a diradical are denoted as disjoint diradicals (the two SOMOs are confined to distinct spatial regions), indicating that the triplet and singlet states are degenerate on the Hückel MO level, whereas the degeneracy tends to be disrupted via the inclusion of a configuration interaction to obtain singlet ground states [19].…”