Research ArticleCHEMICAL PHYSICS

Survival of spin state in magnetic porphyrins contacted by graphene nanoribbons

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Science Advances  16 Feb 2018:
Vol. 4, no. 2, eaaq0582
DOI: 10.1126/sciadv.aaq0582
  • Fig. 1 Synthetic strategy to produce hybrid molecular systems composed of a magnetic FeTPP bonded to GNRs.

    (A) Structure of the monomers DBBA and Br4-FeTPP(Cl) used for the on-surface reaction. DBBA is three-dimensional, appearing with two enantiomeric forms on the substrate. (B) Structure of a hybrid polymer of covalently linked monomers created after the Ullmann coupling step. At the annealing temperatures, the Cl ligand of the porphyrin is detached, whereas the DBBA molecule maintains its three-dimensional shape. (C) Structure of a molecular hybrid after the CDH step. The CDH reaction forms the new ring shadowed red in the image and can additionally fuse the porphyrin core in a clockwise (red arrow) or anticlockwise (blue arrow) manner to the contact phenyl. The fusion renders planar structures, in contrast to the known saddle shape of pristine FeTPP fragments. Note that two mirror-symmetric bonding configurations can form, depending on the chirality of the GNR. (D) High-resolution dI/dV image measured at constant height with a CO-terminated tip [Vs = 5 mV, Vac = 2 mV root mean square (rms), and Rt ~ 1 gigaohm over pristine Au(111) regions], showing several molecular hybrids created on a Au(111) surface. Numbers quantify the GNRs connections to each of the four porphyrin centers of the image. The arrow points to an FeTPP moiety with a three-dimensional structure, whereas the other three on the image are planar.

  • Fig. 2 Imaging and spectroscopy of intact FeTPP connected to cGNRs.

    (A) Constant current STM image (with a metal tip) of an FeTPP moiety connected to three cGNRs (Vs = 0.21 V and It = 16 pA). The corresponding structure (as in Fig. 1C) is superimposed. As in the study of Rubio-Verdú et al. (16), the intact FeTPP fragment has a saddle shape, with two lobes due to two pyrrole units pointing upward (red). (B) Constant height dI/dV map of the same structure as in (A) with CO-terminated tip (Vs = 0 mV, Vac = 2 mV rms, and Rt ~ 1 gigaohm). Arrows point to new six-membered rings created after CDH step at the contact region. (C) dI/dV spectra taken on the central Fe atom, on the red pyrrole, and on the blue pyrrole (Rt ~ 50 megaohm on site and Vac = 0.4 mV rms). arb. units, arbitrary units.

  • Fig. 3 Imaging and spectroscopy of contacted FeTPP fused with C4 symmetry.

    (A to C) Constant height dI/dV maps of planar FeTPP fused to one, two, and four cGNRs, respectively, measured with a CO-functionalized tip (Vs = 0 mV, Vac = 2 mV rms, and Rt ~ 1 gigaohm). All images share the same scale bar. (D to F) Structures corresponding to the hybrids pictured in (A) to (C), respectively. Only a part of the models is shown here for clarity. The red bonds in the structures indicate the clockwise fusion of porphyrin core to the contact phenyl. The green dashed rectangular in(E) highlights the junction structure between FeTPP and cGNRs, with three rings easily recognized in the included Laplacian-filtered image of (B). (G) dI/dV spectra taken on the central Fe atoms of the structures in (A) to (C) (Rt ~ 50 megaohm on site and Vac = 0.4 mV rms). The dI/dV spectra are vertically shifted for clarity.

  • Fig. 4 Reduction of spin excitation energy with the symmetry of the fused FeTPP.

    (A and B) Constant height dI/dV maps with a CO tip of hybrid cGNR-FeTPP structures with flat porphyrin moieties, planarized following a mirror-symmetric sequence of fusion orientation or with no symmetry (sym.), respectively (Vs = 0 mV, Vac = 2 mV rms, and Rt ~ 1 gigaohm). All images share the same scale bar. (C and D) Structures of the hybrids pictured in (A) and (B), respectively. Only a part of the models is shown here for clarity. The red and blue bonds in the structures indicate the creations of five-membered rings by the clockwise and anticlockwise fusion of porphyrin core to the contact phenyl, respectively. The green dashed line in (C) indicates the mirror plane of the structure. (E) dI/dV spectra taken on the central Fe atom of the structures in (A) and (B) (Rt ~ 50 megaohm on site and Vac = 0.4 mV rms). The dI/dV spectra are vertically shifted for clarity. (F) Summary of spin excitation (Excit.) energy measured for the different contacted FeTPP structures studied here. The values represent the average extracted for a total of 20 systems studied in total (see figs. S1 and S2 for further information). asymm., asymmetric.

  • Fig. 5 Comparison of LDOS of pristine and contacted cGNRs.

    (A) dI/dV spectra on the center of a contacted porphyrin (top), over a pyrrole group (middle), and over the connected cGNR (bottom). The colored shadows mark relevant bands and resonances, as discussed in the Supplementary Materials and by Rubio-Verdú et al. (16). (B and C) Constant height dI/dV maps of a pristine cGNR measured at the onset values of valence and conduction bands, respectively (Rt ~ 5 gigaohm; the tip was functionalized with CO). (D and E) Constant height dI/dV maps of two cGNRs connected to a planar FeTPP (the structure shown in Fig. 3B) measured as in (A) and (B) at the onset of valence band (VB) and conduction band (CB), respectively. The brighter segment in the upper cGNR branch is caused by the Au(111) herringbone reconstruction underneath.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/eaaq0582/DC1

    note S1. Symmetry of fused FeTPP moieties

    note S2. Spectra on pyrrole subunits of GNR-contacted FeTPPs

    note S3. Anomalous contacts between FeTPP and GNRs

    note S4. Electronic structure of contacted FeTPPs

    fig. S1. Statistics of different types of FeTPP connected to cGNRs.

    fig. S2. dI/dV spectra taken over the pyrrole subunits of cGNR-contacted FeTPP moieties.

    fig. S3. Additional bond formation between planar FeTPP and cGNRs.

    fig. S4. Comparison of wide-range dI/dV spectra on pristine and on contacted FeTPP.

    fig. S5. Frontier orbitals of FeTPP and their localization in the macrocycle.

  • Supplementary Materials

    This PDF file includes:

    • note S1. Symmetry of fused FeTPP moieties
    • note S2. Spectra on pyrrole subunits of GNR-contacted FeTPPs
    • note S3. Anomalous contacts between FeTPP and GNRs
    • note S4. Electronic structure of contacted FeTPPs
    • fig. S1. Statistics of different types of FeTPP connected to cGNRs.
    • fig. S2. dI/dV spectra taken over the pyrrole subunits of cGNR-contacted FeTPP
      moieties.
    • fig. S3. Additional bond formation between planar FeTPP and cGNRs.
    • fig. S4. Comparison of wide-range dI/dV spectra on pristine and on contacted
      FeTPP.
    • fig. S5. Frontier orbitals of FeTPP and their localization in the macrocycle.

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