Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments

Journal name:
Nature Chemistry
Volume:
3,
Pages:
197–204
Year published:
DOI:
doi:10.1038/nchem.985
Published online

Abstract

Graphene, a two-dimensional layer of sp2-hybridized carbon atoms, can be viewed as a sheet of benzene rings fused together. Three benzene rings can be combined in three different ways, to yield linear anthracene and angular phenanthrene, where the rings share two C–C bonds, and the phenalenyl structure where three C–C bonds are shared between the rings. This third structure contains an uneven number of carbon atoms and, hence, in its neutral state, an uneven number of electrons — that is, it is a radical. All three structures may be viewed as being sections of graphene. Extension of this concept leads to an entire family of phenalenyl derivatives — 'open-shell graphene fragments' — that are of substantial interest from the standpoint of fundamental science as well as in view of their potential applications in materials chemistry, in particular quantum electronic devices. Here we discuss current trends and challenges in this field.

At a glance

Figures

  1. Spin density distributions of phenalenyl-based neutral radicals.
    Figure 1: Spin density distributions of phenalenyl-based neutral radicals.

    a, Triangular motifs 1 and 2 and π-extended phenalenyl radicals, shown with their spin densities, can be interpreted as triangular fragments of graphene. These systems are termed open-shell graphene fragments. The spins on triangulene (2) and more π-extended systems possess intramolecular ferromagnetic interactions owing to the π-spin topological networks. The spin density distributions are calculated using the program GAUSSIAN 03 with the UB3LYP/6-31G* level of theory. The red and blue colours show positive and negative spin densities, respectively. Each spin density is delocalized over the entire molecular skeleton, but is mainly located on the edges of the molecular framework with a largely polarized nature. b, Molecular structures of tri-t-butyl-phenalenyl19 (3), tri-t-butyl-1,3-diazaphenalenyl20 (4), 1,9-dithiophenalenyl21, 22 (5) and tri-t-butyl-6-oxophenalenoxyl24 (6), and their spin density distributions calculated with the UB3LYP/6-31G** level of theory. ESR experiments are in good agreement with the calculated results. t-Bu, t-butyl.

  2. Crystal structure and electronic structure of the [pi]-dimer of 3.
    Figure 2: Crystal structure and electronic structure of the π-dimer of 3.

    a, Crystal structure of the π-dimer. In the top view, the light-coloured molecular framework is located behind the darker one. Solution-state NMR spectroscopy showed that, at low temperatures, the radical forms the same highly symmetrical structure as the crystal structure28. b, Highest-occupied (left) and lowest-unoccupied (right) molecular orbitals of the π-dimer, optimized and calculated using GAUSSIAN 03 at the B3LYP/6-31G** level of theory. The intradimer interaction is explained by a linear combination of SOMOs. c, Out-of-plane NICS distributions at the ring centre of the monomer and the π-dimer. The red circles indicate the magnitudes of negative NICS values, shown at intervals of 0.6 Å. d, Electrostatic potential of the π-dimer. The electron density is mostly negative (red) in the internal region despite neutrality of the whole. e, A σ-dimer consisting of two anti-aromatic annulenes, each with 12 π-electrons and 12 carbon atoms. Parts a and c reproduced with permission from ref. 28, © 2006 ACS.

  3. Structural change of intra-[pi]-dimer interaction.
    Figure 3: Structural change of intra-π-dimer interaction.

    a, Proposed mechanism of the phase transition of zwitterionic bisphenalenyl boron complexes 7 (R = ethyl) and 8 (R = butyl) (ref. 31). The spins reside on outer phenalenyl moieties at high temperatures and on the π-dimer moiety at low temperatures. The phase transition is accompanied by simultaneous changes in magnetic behaviour, conductivity and infrared transmittance. b, Thermal equilibrium between the σ-dimer (left) and the π-dimer (right) of 4 in the crystalline state, and the locations of their highest-occupied molecular orbitals according to quantum chemical calculations. The bonding in the σ-dimer is only through the σ-bond, whereas multiple bonding overlaps are seen in the π-dimer34. c, The colour change for specific positions of the single crystal of 4 depends on the temperature34. d, Experimentally determined three-state energy diagram for 4 in the crystalline state34. Parts b and c reproduced with permission from ref. 34, © 2008 NPG.

  4. ESR spectra, pictures of a rechargeable battery made using a crystalline neutral radical, high-spin magnetic triangulenes, [pi]-extended phenalenyl derivatives, a [pi]-stacked radical polymer, and curved and twisted phenalenyl systems.
    Figure 4: ESR spectra, pictures of a rechargeable battery made using a crystalline neutral radical, high-spin magnetic triangulenes, π-extended phenalenyl derivatives, a π-stacked radical polymer, and curved and twisted phenalenyl systems.

    a, Spin transfer system in 9a and 9b (ref. 35). The colours of the radical and the ESR spectra change drastically with the temperature of a mixed solvent system. The spin is located on the oxophenalenoxyl moiety, represented by 9a (orange), at higher temperatures and on the TTF moiety, represented by 9b (green), at lower temperatures. b, Coin cell of a rechargeable battery containing 6-oxophenaenoxyl (6) as an electrode-active material; the battery can be used to power a propeller9, 38. c, High-spin magnetic triangulenes composed of a very high-spin polymeric neutral π-radical system. d, A π-extended phenalenyl derivative with holes. e, A possible model of a π-stacked radical polymer composed of triangulene (2), representing one of the SOMO–SOMO interactions in the π-stacking chain. f, The chemical structures and spin density distributions of a phenalenyl-fused corannulene (12) and a helicene (13). These radicals intrinsically give the chiral molecular systems a spin-delocalized nature. Part b reproduced with permission from ref. 9, © 2010 Wiley.

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Affiliations

  1. Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.

    • Yasushi Morita
  2. Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan.

    • Shuichi Suzuki,
    • Kazunobu Sato &
    • Takeji Takui

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