Precise control of π-electron magnetism in metal-free porphyrins

Article abstract of DOI:10.1021/jacs.0c07791

The porphyrin macrocycle can stabilize a set of magnetic metal ions, thus introducing localized net spins near the center. However, it remains elusive but most desirable to introduce delocalized spins in porphyrins with wide implications, for example, for

Full text of DOI:10.1021/jacs.0c07791

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Article
Precise Control of Pi-electron Magnetism in Metal-free Porphyrins
Yan Zhao, Kaiyue Jiang, Can Li, Yufeng Liu, Chengyang Xu, Wenna Zheng, Dandan Guan, Yaoyi
Li, Hao Zheng, Canhua Liu, Weidong Luo, Jinfeng Jia, Xiaodong Zhuang, and Shiyong Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07791 • Publication Date (Web): 22 Sep 2020
Downloaded from pubs.acs.org on September 22, 2020
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Precise Control of -electron Magnetism in Metal-free
Porphyrins
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Yan Zhao1, Kaiyue Jiang2, Can Li1, Yufeng Liu1, Chengyang Xu1, Wenna Zheng1, Dandan Guan1,3,4, Yaoyi
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Li1,3,4, Hao Zheng1,3,4, Canhua Liu1,3,4, Weidong Luo1,5, Jinfeng Jia1,3,4, Xiaodong Zhuang2*, Shiyong Wang1,3,4
*
1Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Shenyang National
Laboratory for Materials Science, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai
200240, China
2The meso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
3Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai, 200240, China.
4Shanghai Research Center for Quantum Sciences, Shanghai 201315, China
5Institute of Natural Sciences, Shanghai Jiao Tong University, Shanghai, 200240, China.
These authors contributed equally to this work.
*Corresponding Authors: shiyong.wang@sjtu.edu.cn, zhuang@sjtu.edu.cn
ABSTRACT
The porphyrin macrocycle can stabilize a set of magnetic metal ions, thus introducing localized net spins near the
center. However, it remains elusive but most desirable to introduce delocalized spins in porphyrins with wide
implications, for example, for building correlated quantum spins. Here, we demonstrate that metal-free porphyrins
host delocalized -electron magnetism as revealed by scanning probe microscopy and different level of theory
calculations. Our results demonstrate that engineering of -electron topologies introduces spin polarized singlet
state and delocalized net spins in metal-free porphyrins. In addition, the -electron magnetism can be switched
on/off via STM manipulation by tunning the interfacial charge transfer. Our results provide an effective way to
precisely control the -electron magnetism in metal-free porphyrins, which can be further extended to design new
magnetic functionalities of porphyrin-based architectures.
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INTRODUCTION
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Porphyrin molecules have a wide spectrum of physicochemical and biological properties with diverse applications
in sensors, molecular electronics/spintronics and medical applications1–3. The rich functionalities of porphyrin
complexes originate from the center macrocycle, which can stabilize a variety of elements inside and thus gives
rise to distinct structural, electronic, magnetic, optoelectronic, and transport properties4–13. Magnetic
metalloporphyrins have been extensively studied by various techniques, exhibiting kondo and spin excitation
effects, Yu-Shiba bound states, and spin-state switching13–27. Recently, hybridized graphene and metalloporphyrin
nanostructures have been achieved, hosting combined intriguing electronic and magnetic properties28–33. The
magnetism of metalloporphyrin architectures originates from the stabilized metal ions, and thus localizes at the
center of the molecules. It remains elusive but most desirable to develop strategies to introduce delocalized spins
in porphyrins for designer new magnetic functionalities in porphyrin architectures.
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Recent advances of on-surface synthesis allow for tailoring -electron topologies at the single chemical bond level,
and thus provide the ability to precisely engineer -electron magnetism in low-dimensional graphene
nanostructures by introducing sublattice imbalance, topological frustration and topological defects34–50. Examples
like triangulenes41,43,48,51, bowtie-shape nanographenes47, chiral graphene nanoribbons45, polymers52 and
nanographenes with topological defects46,49 have been recently synthesized and characterized on surfaces. Similar
as nanographenes, the porphyrin macrocycle is aromatic and can be interpreted as a multiple-bridged aromatic
diaza[18]annulene system53,54. Thorough tailoring peripheral -electron topology, it is in principle able to
introduce tunable -electron magnetism in metal-free porphyrin architectures, thus greatly enriching magnetic
functionalities of porphyrin-based systems. Although substituted open-shell porphyrins have been realized using
conventional solution synthesis methods, on-surface synthesis of unsubstituted porphyrins with -electron
magnetism remains to be explored55,56
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Here, we demonstrate such ability to precisely control delocalized magnetism in metal-free porphyrins through
tailoring their peripheral -electron topologies. Using combined solution and on-surface synthesis, three metal-
free porphyrins with different -electron topologies have been synthesized. Their chemical structures have been
resolved by using high resolution non-contact atomic force microscopy, permitting us to clearly distinguish their
-electron topologies at the single chemical bond level. Using low-temperature scanning tunneling spectroscopy,
their -electron magnetism has been confirmed by Kondo effects. The experimental observations can be nicely
elucidated by clar empirical rule, mean-field Hubbard model and spin-polarized density functional theory
calculations. In addition, the -electron magnetism can be reversibly switched on/off via STM manipulation by
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tuning the charge transfer from the underneath metal substrate. The realization of -electron magnetism in metal-
free porphyrin may be further extended to build correlated spins in porphyrin-based nanostructures, with
implications for quantum information and spintronics applications.
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RESULTS AND DISCUSSION
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Using combined in-solution and on-surface synthesis, metal-free porphyrins with different -electron topologies
have been synthesized. As shown in Fig. 1a, the precursor 5,10,15,20-tetrakis(2,6-dimethylphenyl)porphyrin is
synthesized in solution by using freshly distilled pyrrole and 2,6-dimethylbenzaldehyde (cf. detailed synthesis
method in supplementary fig. 1). After in-solution synthesis, the precursor is thermally deposited on a Au(111)
o
substrate held at room temperature following a subsequent annealing to 295 C for 10 minutes. At elevated
temperature, thermal-induced cyclodehydrogenation of methyl units occurs, giving rise to fully aromatic
porphyrins39,57. Except for the main cyclodehydrogenation, two common side reactions take place occasionally on
Au(111): i) few methyl units detach from the precursors forming five-membered carbon rings after subsequent
cyclodehydrogenation, and ii) partial dehydrogenation of methyl unit (or post passivation of radical carbon sites
by additional hydrogens) sometimes occurs and introduces sp3 hybridized carbon sites at edges. As shown in Fig.
1b, three products have been frequently observed and characterized. The Product #1 contains two pentagon carbon-
rings at opposite sides, and the Product #3 has a sp3 hybridized carbon site passivated by two hydrogen atoms. In
addition, we also observed some molecules are metalated by Au adatoms, forming Au-Porphyrin complexes. Their
chemical structures have been clearly resolved by high resolution nc-AFM imaging using a CO-functionalized tip
(cf. Fig. 1c-e). The number of unpaired -electrons (radicals) in the resulted porphyrins has been determined by
using Clar’s empirical rule (cf. all the possible -electron networks in supplementary Fig. 4 and 5)58,59. As shown
in Fig. 1, the Product #1 is a closed-shell molecule, while the Product #2 and #3 have two and one unpaired -
electron(s), respectively (as marked by red dots in Fig. 1). The presence of radicals suggests an open-shell
configuration in the Product #2 and #3, thus giving rise to delocalized -electron magnetism.
The energy spectra and frontier molecular orbitals of the resulted porphyrins have been calculated by using mean-
field Hubbard model (MFHM) and spin- polarized density functional theory (SP-DFT), which have been widely
used to determine the magnetic structure of nanographenes and show nice agreements with experiments37. As
shown in Fig. 2a-c, the Product #1 has a closed-shell ground state as confirmed by combined Clar empirical rule,
MFHM and SP-DFT calculations. Experimentally, differential conductance (dI/dV) spectroscopy has been used
to detect the electronic structure of achieved porphyrins. High resolution dI/dV mappings are able to resolve spatial
distribution of molecular orbitals. As shown in Fig. 2d, dI/dV spectra taken above the center of the Product #1
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reveal two resonances at -1.4 V and 0.9 V, which can be assigned to the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO), respectively. dI/dV mappings at these two energies resolve the
spatial distribution of these two molecular states, which agree well with HOMO and LUMO orbital shapes as
calculated by MFHM and SP-DFT. In addition, we did not observe the kondo resonance near the fermi level. These
results confirm that the Product #1 is a closed-shell molecule without any unpaired -electrons.
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Despite its slightly different -electron topology from that of the Product #1, the Product #2 has drastically
different magnetic ground state. The Product #2 has in total 58 - electrons, with 2 -electrons more than those of
the Product #1. Although both of them host even number of -electrons, the Product #2 has an open-shell electronic
structure as opposed to the closed-shell configuration of the Product #1 (cf. Fig. 3a). The reason is due to
topological frustration, that is, it is impossible to concomitantly pair all pz orbitals to form  bonds in the Product
#2. This picture can be verified by both MFHM and SP-DFT calculations as shown in figure 3b-e. The calculated
energy levels in Fig. 3b-c show that there are two degenerated states below fermi level, which are spatially
separated with enhanced orbital density at opposite sides (cf. Fig. 3d and e), and singly occupied by two electrons
with oppositely oriented spins. In other words, the ground electronic configuration breaks spin-spatial symmetry
and exhibits antiferromagnetic ordering, which can be nicely seen in the calculated spin density maps in Fig. 3f
and g. The mechanism behind is very similar to Clar Goblet, where topological frustration induces spin polarized
singlet state in the bowtie-shape nanographene36,47
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However, experimental dI/dV measurements reveal that the Product #2 hosts a net spin of S=1/2 by showing a
sharp Kondo resonance instead of the expected singlet ground state with a net spin of S=0 (cf. Fig. 4g). As
illustrated in Fig. 4a, we attribute this discrepancy to the interfacial charge transfer, where one electron is
transferred from molecule to gold substrate making it positively charged. We can exclude the possibility of
negative charging through comparing spin density distributions differences (cf. supplementary Fig. 11). Similar
charging effects have been previously observed in several graphene nanostructures, such as the zigzag termini of
N=7 armchair graphene nanoribbons60,61, and N=6 zigzag graphene nanoribbon39 (N denotes the width of graphene
nanoribbons in terms of the number of carbon atoms). As shown in Fig. 4b-c, the electronic structure of the
positively charged Product #2 is calculated by MFHM and SP-DFT, revealing a singly occupied molecular state
below fermi level. This means the charged molecule is with a net magnetic moment of S=1/2. On metal substrate,
this magnetic moment will be screened by Au(111) electron reservoir, which can be understood by single impurity
Anderson Model17,62. As shown in Fig. 4g, dI/dV spectra reveal such screening effect by showing a sharp Kondo
resonance at Fermi level. In addition, the observed spin density distribution nicely agrees with calculated results,
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further supporting that the observed zero-bias peak originates from Kondo resonance (cf. Fig. 4d-f). As per
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훤 =
Anderson model, the Kondo peak width varies with temperature with full width at half maximum (FWHM) of
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(훼푘 푇)2 + (2푘 푇 )2
푇
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훼
is the Kondo temperature, and is the slope of linear
, where T is the temperature,
퐵 퐾
퐵
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퐾. The experimental obtained temperature dependent FWHM can be well fitted by
growth of the width at
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the previous equation, giving a Kondo temperature of 22 K (cf. Fig 4h-i and supplementary Fig.6). We can exclude
that the sharp Kondo resonance originates from a high-spin state of S=1 by measuring its response to magnetic
field (cf. supplementary Fig. 7).
Using similar techniques, the magnetic properties of the Product #3 has been determined. Since one carbon site is
passivated by two hydrogen atoms, the Product #3 has one -electron less than the Product #2 with an odd number
of 57 -electrons in total. It is impossible to concomitantly pair odd number of pz orbitals to form  bonds, thus
generating one  radical as marked by a red dot in Fig. 5a. MFHM and SP-DFT calculations confirm the presence
of a singly occupied state below fermi level in the Product #2, that is, a net spin of S=1/2. As shown in Fig. 5d, the
passivation of -radical locally modifies its spin density distribution, which gets significantly quenched near the
CH2 site (cf. Clar non-kekulé structures in Supplementary Fig. 4). The emergence of net spins in porphyrins with
odd number of  electrons is very similar to nanographenes with sublattice imbalance as proposed by E. Lieb in
198934. As shown in Fig. 5g, dI/dV spectroscopy measurements confirm the emergence of a net spin of S=1/2 in
the Product #3. The net spin couples with underneath electron reservoir, generating Kondo resonances by showing
a sharp Fano-shape peak at fermi level in dI/dV spectra with peak intensity proportional to spin density magnitude.
In addition, the resolved spin density distribution fits very well with the calculated result (cf. Fig. 5e and f). The
nice agreements between experiments and calculations in gas phase indicate that the Product #3 is neutral on
Au(111).
Although the Product #3 shares very similar adsorption geometries with that of the Product #2, the former is neutral
on Au(111) while the latter is positively charged. By carefully checking the AFM images in Fig. 1c-e, one can see
that the presence of sp3carbon (CH2) site slightly lifts the corner near the CH2 site away from the surface, which is
imaged as a local bright protrusion in nc-AFM imaging due to a stronger Pauli repulsion. This slight adsorption
difference leads to a completely different charging behavior. Previous DFT calculations by using a set of van der
Waals correction schemes reveal that despite very similar adsorption heights of most graphene nanostructures on
Au(111), the magnetism (charge transfer) is very sensitive to small perturbations (adsorption height variations),
supporting our observations63. To gain more evidences of the charging effect, we performed STM manipulation to
locally modify the adsorption configurations. After each manipulation, we performed nc-AFM imaging, constant-
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height current imaging and dI/dV spectroscopy to trace the charging transfer. As shown in fig. 6a, the porphyrin
molecule is initially neutral by showing a Kondo resonance at Fermi level. After manipulation, the molecule rotates
an angle of 185o around the out-of-plane axis (cf. Fig. 6b). Although nc-AFM and constant-height current image
indicate a similar adsorption geometry after rotation, dI/dV spectroscopy reveals that the molecule is positively
charged by showing a broad peak above fermi level instead of exhibiting a sharp Kondo resonance at fermi level
(cf. the No. 2 spectrum in Fig. 6d). In other words, the electron previously occupied the singly occupied state
transfers to gold substrate, thus making the molecule positively charged and leading all orbitals doubly occupied.
Further manipulation experiment was used to rotate the molecule again. As shown in Fig. 6c, the Kondo resonance
appears after a successive rotation of 125o, suggesting the molecule becomes neutral again. In total, we performed
successive 9 times STM manipulations, and found that the charging effect is extremely sensitively to adsorption
variations (cf. supplementary Fig.10). Similar manipulation experiments have been performed on the Product #1
and #2 without obtaining this switching effect. These results indicate that slightly decoupling the molecule from
surface can effectively quench the charging effect, such as by lifting the molecule slight away from the substrate
by functional groups. Other possibilities may also change the coupling between molecule and substrate, such as
different adsorption sites, Au(111) surface reconstruction (with dangling bond at herringbone corners), etc.
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We notice that the Kondo peak positions slightly shift away from fermi level. Even for the same molecule, the
Kondo peak position as well as peak width varies a little after STM manipulations (cf. Supplementary Fig. 10). As
revealed by previous second-order perturbative model, the kondo shape and peak position depend sensitively on
hybridization with substrate as well as charge transfer from substrate18. Charge transfer from substrate will affect
the occupation number, and hence shift the peak center away from fermi level. Hybridization with substrate will
change the peak shape. For the strong hybridization case, the Kondo resonance exhibits a dip in dI/dV spectroscopy.
For the case without hybridization, the Kondo resonance behaves as a symmetric peak centered at fermi level. In
our case, the peak shape is slightly asymmetric and centered a little away from fermi level, indicating a weak
hybridization with substrate.
CONCLUSION
In conclusion, we demonstrate an effective approach to engineer -electron magnetism in metal-free porphyrins
by using combined non-contact atomic force microscopy, scanning tunneling microscopy/spectroscopy as well as
theory calculations at different levels. Three types of porphyrins with different -electron topologies have been
successively synthesized and characterized down to the single chemical bond level. The -electron network in
porphyrins with even number of -electron can either host a spin polarized singlet ground state or closed-shell
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configuration depending whether the network gets frustrated or not. Porphyrins with odd number of  electrons
host nets spin similar as sublattice imbalance induced spins in nanographenes. In addition, the magnetism can be
switched on/off by tuning charge transfer from underneath Au(111). Our method reported herein provides ample
opportunities for further designer correlated spins in porphyrin-based nanostructures to study their many-body
effects.
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ASSOCIATED CONTENT
Supporting Information. Detailed descriptions of experimental and theoretical procedures, Clar non-kekulé
structures of the Product #2 and 3, Magnetic field and temperature dependence of Kondo resonance, dI/dV spectra
and mappings, and simulated local density of state maps. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Yan Zhao and Kaiyue Jiang contributed equally to this work.
ACKNOWLEDGEMENT
S. W. acknowledges the financial support from the National Natural Science Foundation of China (No. 11874258),
the Shanghai Municipal Science and Technology Qi Ming Xing Project (No. 20QA1405100) and Fok Ying Tung
Foundation for young researchers. This work is also supported by the Ministry of Science and Technology of
China (Grants No. 2016YFA0301003, No. 2016YFA0300403), the National Natural Science Foundation of China
(Grants No. 11521404, No. 11634009, No. 11574202, No. 11874256, No. 11790313, No. 11674226, No.
U1632102, No. 11674222, No. U1632272 and No. 11861161003), the Strategic Priority Research Program of
Chinese Academy of Sciences (Grant No. XDB28000000), and the Shanghai Municipal Science and Technology
Major Project (Grant No.2019SHZDZX01). Y.L. thanks ECNU Multifunctional Platform for Innovation (001) of
East China Normal University for providing computing resources.
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Figure 1. Synthesis of porphyrins with different -electron topologies. (a), In-solution synthesis of molecular
precursor 5,10,15,20-tetrakis(2,6-dimethylphenyl) porphyrin. (b), Chemical structures of three On-surface
synthesized porphyrins with red dots indicating the unpaired -electrons. (c-e), Nc-AFM frequency shift images
(Resonant frequency: 26 KHz, Oscillation amplitude: 80 pm, Scale bars: 0.4 nm) of the porphyrins in (b).
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Figure 2. Electronic structure of a closed -shell porphyrin with two embedded pentagon rings. (a), Chemical
structure of the product #1. (b-c), Mean-field Hubbard model and spin-polarized density functional theory
calculated energy spectrum of the porphyrin in (a). (d), dI/dV spectra taken at the two locations marked on the
inset constant-height current image (Bias: 10 mV; scale bar: 0.25 nm). The resonance below fermi level is assigned
to the superposition of HOMO-1 and HOMO, and the resonance above fermi level to LUMO. (e), Up: spatial
resolved local density of states maps taken at the biases marked by dashed lines in (d); Down: DFT simulated
density of states maps. Scale bars: 0.5 nm.
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Figure 3. Spin polarized singlet ground state in the Product #2. (a), Chemical structure of the Product #2 with two
unpaired  electrons as marked by red dots. (b-c), Mean-field Hubbard model and spin-polarized density functional
theory calculated energy spectrum of the porphyrin in (a). (d-e), DFT calculated frontier orbitals of the two singly
occupied states as marked in (b) and (c). (f-g), Mean-field Hubbard model and spin-polarized density functional
theory calculated spin density distributions. Red/green: spin up, blue/yellow: spin down.
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Figure 4. Electronic structure of the positively charged Product #2. (a), Illustrative showing charge transfer from
the Product #2 to underneath Au(111), thus making it positively charged. (b-c), Mean-field Hubbard model and
spin-polarized density functional theory calculated energy spectrum of the charged Product #2. (d), Spin-polarized
DFT calculated spin density distributions. Green: spin up, Yellow: spin down. (e-f), Constant-height current image
(Bias voltage: 1 mV, Scale bars: 0.5 nm), and DFT simulated SOMO LDOS map of the charged Product #2. (g),
dI/dV spectra taken on the locations marked in (e). (h), Temperature dependent dI/dV spectra showing the
broadening of Kondo resonance peak as increase the temperature. The dashed lines are fitted curves using a Frota
function. The spectra are taken at the position marked by a cross in (e). (i), The full-width at half maximum of
Kondo resonance peak as a function of temperature. The solid red line is fitted curve giving a Kondo temperature
of 22K.
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Figure 5. Electronic structure of the Product #3. (a), Chemical structure of the Product #3 with one unpaired 
electron as marked by a red dot. (b-c), Mean-field Hubbard model and spin-polarized density functional theory
calculated energy spectrum of the Product #3. (d), Spin-polarized density functional theory calculated spin density
distributions. Green: spin up, Yellow: spin down. (e-f), Constant-height current image (Bias voltage: 1 mV, Scale
bars: 0.5 nm), and DFT simulated SOMO LDOS map of the Product #3. (g), dI/dV spectra taken at the locations
marked in (e).
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Figure 6. Switching magnetic state by STM manipulation. (a-c), From left to right: Chemical structure, nc-AFM
image (Resonant frequency: 26 KHz, Oscillation amplitude: 80 pm, Scale bars: 0.2 nm), Constant-height current
image (Bias voltage: 1 mV, Scale bars: 0.25 nm) of the Product #3 after two manipulation steps. (d), dI/dV spectra
taken at the marked positions in (a-c).
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