Electronic Quartet and Triplet States of Polyanionic C60 Fullerene and Their Anomalous Spin Relaxation As Studied by cw-ESR/2D-Electron Spin Transient Nutation Spectroscopy
“…All these experimental results demonstrate that the extensive spin delocalization onto the corannulene moiety (Figure 3, see below) results in significant electronic‐spin communication via the curved and nonalternant π‐surface of corannulene 23. Furthermore, in terms of the curved π‐conjugated structure, we believe that diradical 2 has its own place as an intermediate case between thus‐far reported planar π‐conjugated systems and completely 3D π‐conjugated species such as neutral C 60 in the triplet state, noting that spin–orbit contributions are relevant to the particular canonical resonance structures (see Figure S12 in the Supporting Information) 2d,f…”
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
“…All these experimental results demonstrate that the extensive spin delocalization onto the corannulene moiety (Figure 3, see below) results in significant electronic‐spin communication via the curved and nonalternant π‐surface of corannulene 23. Furthermore, in terms of the curved π‐conjugated structure, we believe that diradical 2 has its own place as an intermediate case between thus‐far reported planar π‐conjugated systems and completely 3D π‐conjugated species such as neutral C 60 in the triplet state, noting that spin–orbit contributions are relevant to the particular canonical resonance structures (see Figure S12 in the Supporting Information) 2d,f…”
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
“…This is an expected result since different degrees of aggregation from solute extrusion will occur upon freezing of different solvents. 8 (a) Purported observed and simulated EPR spectra of C 60 3- reproduced from ref (copyright American Chemical Society). (b) EPR spectrum of C 120 O 3-/4- .…”
Section: Resultsmentioning
confidence: 99%
“…(a) Purported observed and simulated EPR spectra of C 60 3- reproduced from ref (copyright American Chemical Society). (b) EPR spectrum of C 120 O 3-/4- .…”
Section: Resultsmentioning
confidence: 99%
“…600 cm -1 . More recently, Shohoji, Takui, and co-workers interpreted a portion of the EPR spectrum attributed to the C 60 3- ion in terms of a quartet state.…”
Aspects of the electron paramagnetic resonance (EPR) spectra of C60 n-fulleride ions (n ) 2, 3) and the EPR signal observed in solid C60 are reinterpreted. Insufficient levels of reduction and the unrecognized presence of C120O, a ubiquitous and unavoidable impurity in air-exposed C60, have compromised most previously reported spectra of fullerides. Central narrow line width signals ("spikes") are ascribed to C 120O n-(n ) odd). Signals arising from axial triplets (g ∼ 2.0015, D ) 26-29 G) in the spectrum of C60 2-are ascribed to C120O n-(n ) 2 or 4). Their D values are more realistic for C120O than C60. Less distinct signals from "powder" triplets (D ∼ 11 G) are ascribed to aggregates of C120O n-(n ) odd) arising from freezing nonglassing solvents. In highly purified samples of C60, we find no evidence for a broad ∼30 G signal previously assigned to a thermally accessible triplet of C60 2-. The C60 2-ion is EPRsilent. Signals previously ascribed to a quartet state of the C60 3-ion are ascribed to C120O 4-. Uncomplicated, authentic spectra of C60 -and C60 3-become available when fully reduced samples are prepared under strictly anaerobic conditions from freshly HPLC-purified C60. Solid off-the-shelf C60 has an EPR signal (g ∼ 2.0025, ∆Hpp ∼ 1.5 G) that is commonly ascribed to the radical cation C60•+ . This signal can be reproduced by exposing highly purified, EPR-silent C60 to oxygen in the dark. Doping C60 with an authentic C60•+ salt gives a signal with much greater line width (∆Hpp ) 6-8 G). It is suggested that the EPR signal in airexposed samples of C60 arises from a peroxide-bridged diradical, •C60-O-O-C60• or its decomposition products rather than from C60 •+ . Solid-state C60 is more sensitive to oxygen than previously appreciated such that contamination with C120O is almost impossible to avoid.The addition of electrons to C 60 is its most characteristic chemical property. As a consequence, over the past decade, a great deal of effort has been put into characterizing the electronic structure of fulleride anions, C 60 n-. 1 In this paper, we show that the electron paramagnetic resonance (EPR) characterization of these ions has been significantly compromised by imprecise levels of reduction and by the presence of an unrecognized impurity in [60]-fullerene, namely, C 120 O.In addition, we question the widely held belief that the EPR signal commonly observed in solid "pristine" C 60 is due to the radical cation C 60•+ . [2][3][4][5][6] In contrast to the easy addition of electrons, the removal of electrons from C 60 is extremely difficult. 7 Thus, the formation of air-stable C 60 •+ radical cation centers is not easy to chemically rationalize. Now that a synthetic route to C 60•+ has become available, 8 a purposeful doping experiment into C 60 can be performed.The C 60 3-ion is a particularly important fulleride ion to characterize well because it gives rise to superconductivity in A 3 C 60 fulleride phases (A ) alkali metal). The triply degenerate nature of the t 1u lowest unoccupied molecular or...
“…The use of the topological symmetry enables an unlimited number of electron spins to align parallel to each other in the ground state, which is termed the unlimitedness of topological degeneracy. On the other hand, the orbital degeneracy due to the group theoretical high symmetry is finite, e.g., the utmost quartet ( S = 3/2) state demonstrated even in a ground-state C 60 3- . The meta - or 1,3,5-connectivity-based topological degeneracy of SOMOs (π-NBMOs) has been a primary guiding principle for molecular design of organic high-spin systems. , …”
[structure: see text] Topological symmetry-based extensions of a pi-conjugation network in an odd alternant phenalenyl radical have enabled us to control the spin density distribution systematically. ESR/ENDOR and NICS studies on the topological isomers of oxophenalenoxyl have revealed that the unpaired electron tends to localize in the antiaromatic ring systems.
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