Inducing Open-Shell Character in Porphyrins through Surface-Assisted Phenalenyl π-Extension

Article abstract of DOI:10.1021/jacs.0c07781

Organic open-shell compounds are extraordinarily attractive materials for their use in molecular spintronics thanks to their long spin-relaxation times and structural flexibility. Porphyrins (Pors) have widely been used as molecular platforms to craft persistent open-shell structures through solution-based redox chemistry. However, very few examples of inherently open-shell Pors have been reported, which are typically obtained through the fusion of non-Kekulé polyaromatic hydrocarbon moieties to the Por core. The inherent instability and low solubility of these radical species, however, requires the use of bulky substituents and multistep synthetic approaches. On-surface synthesis has emerged as a powerful tool to overcome such limitations, giving access to structures that cannot be obtained through classical methods. Herein, we present a simple and straightforward method for the on-surface synthesis of phenalenyl-fused Pors using readily available molecular precursors. In a systematic study, we examine the structural and electronic properties of three surface-supported Pors, bearing zero, two (PorA2), and four (PorA4) meso-fused phenalenyl moieties. Through atomically resolved real-space imaging by scanning probe microscopy and high-resolution scanning tunneling spectroscopy combined with density functional theory calculations, we unambiguously demonstrate a triplet ground state for PorA2 and a charge-transfer-induced open-shell character for the intrinsically closed-shell PorA4.

Full text of DOI:10.1021/jacs.0c07781

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Article
Inducing Open-shell Character in Porphyrins
through Surface-assisted Phenalenyl #-Extension
Qiang Sun, Luis M. Mateo, Roberto Robles, Pascal Ruffieux,
Nicolas Lorente, Giovanni Bottari, Tomás Torres, and Roman Fasel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07781 • Publication Date (Web): 28 Sep 2020
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Inducing Open-shell Character in Porphyrins through Surface-as-
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sisted Phenalenyl π-Extension
⊥
¶
† ‡ ¶
‖ ¶
⊥
‖,º
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, ,
,
Qiang Sun, , Luis M. Mateo, , Roberto Robles, Pascal Ruffieux, Nicolas Lorente,* Gio-
vanni Bottari,*
† ‡ §
, ,
† ‡ §
, , ,
⊥
, ,+
,
Tomꢀs Torres*
and Roman Fasel*
⊥
nanotech@surfaces Laboratory, Empa-Swiss Federal Laboratories for Materials Science and Technology, 8600
Dübendorf, Switzerland
†
Departamento de Química Orgꢀnica, Universidad Autꢁnoma de Madrid, 28049 Madrid, Spain
‡
IMDEA-Nanociencia, Campus de Cantoblanco, 28049 Madrid, Spain
‖
Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), Paseo de Manuel de Lardizabal 5, 20018 Donostia-San
Sebastián, Spain
º
Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain
§
Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autꢁnoma de Madrid, 28049 Ma-
drid, Spain
+Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
ABSTRACT: Organic open-shell compounds are extraordinarily attractive materials for their use in molecular spintronics
thanks to their long spin-relaxation times and structural flexibility. Porphyrins (Pors) have widely been used as molecular
platforms to craft persistent open-shell structures through solution-based redox chemistry. However, very few examples of
inherently open-shell Pors have been reported, which are typically obtained through the fusion of non-Kekulé polyaromatic
hydrocarbon moieties to the Por core. The inherent instability and low solubility of these radical species, however, requires
the use of bulky substituents and multi-step synthetic approaches. On-surface synthesis has emerged as a powerful tool to
overcome such limitations, giving access to structures that cannot be obtained through classical methods. Herein, we present
a simple and straightforward method for the on-surface synthesis of phenalenyl-fused Pors using readily available molecular
precursors. In a systematic study, we examine the structural and electronic properties of three surface-supported Pors, bear-
ing zero, two (PorA
scanning probe microscopy and high-resolution scanning tunneling spectroscopy combined with density functional theory
(DFT) calculations, we unambiguously demonstrate a triplet ground state for PorA and a charge transfer induced open-
2
) and four (PorA ) meso-fused phenalenyl moieties. Through atomically resolved real-space imaging by
4
2
shell character for the intrinsically closed-shell PorA .
4
INTRODUCTION
unoccupied molecular orbital (LUMO) gap.12 Such π-ex-
tended Pors can be obtained either by attaching other aro-
matic units to the macrocycle via conjugative linkers
through the meso,β-fusion of aromatic units via intramo-
Porphyrins (Pors), tetrapyrrolic macrocycles with a planar
9
,13-16
or
structure and an aromatic circuit of 18 π-electrons, are het-
1
erocycles showing high thermal stability, rich coordination
17-24
2
3
lecular oxidative coupling.
Using the latter synthetic
chemistry, tunable redox features, and excellent photo-
4
strategy, a wide range of π-extended Pors has been re-
ported, showing, in some cases, absorptions reaching far
into the NIR region (i.e., 1417 nm) and optical HOMO-
LUMO gaps as low as 0.61 eV. However, the vast majority
of these systems presents a closed-shell character, while
only few examples of open-shell analogues have been re-
ported.
ily attractive materials in spintronics, presenting longer
spin-relaxation times and easier tunability than inorganic
materials.
physical properties. Their unique features make these
chromophores prime candidates for a wide range of appli-
5
6
cations ranging from materials science, catalysis, photo-
25
7-10
11
voltaics, or medicine, to mention a few.
26
Remarkably, many of these properties can be precisely
tuned through chemical modifications, such as the com-
27,28
Organic open-shell structures are extraordinar-
2
plexation of metal ions or the modification of number and
4
nature of the peripheral substituents. Among such struc-
tural modifications, the π-extension of the Por core is par-
ticularly appealing, since it leads to compounds with a
strong near-infrared (NIR) absorption, stemming from a
small highest occupied molecular orbital (HOMO)-lowest
27,29-31
Along this line, Osuka and co-workers re-
ported the preparation of several inherent and persistent
Por-based radicals using rational molecular design and, in
most examples, redox chemistry. These radicals are either
28
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charged (anions/cations) or neutral with the unpaired elec- charge transfer to the underlying surface gives rise to in-
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tron poorly delocalized over the macrocycle.
duced open-shell character to the intrinsically closed-shell
PorA4, as reflected by a characteristic Kondo resonance.
Over the past years, the preparation and study of π-ex-
tended Pors with highly delocalized radicals has, therefore,
gathered significant attention. For this purpose, an inter-
esting yet poorly explored strategy consists in the fusion of
inherently radical polyaromatic hydrocarbons (PAHs) to
b)
32-34
the macrocycle.
Among these PAHs, non-Kekulé struc-
tures like phenalenyl – its smallest member – have been
widely used as a building block to generate radicaloid and
biradicaloid structures with remarkable thermodynamic
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d)
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stability.
However, the synthetic access to
phenalenyl-based structures is often hampered by multi-
step synthesis, low yields, and poorly soluble intermediates
and products. Moreover, the inherently low HOMO-LUMO
gap of phenalenyl leads to stability issues, such that the
characterization of these species needs to be carried out in
degassed solutions or through in-situ generation of the rad-
3
2
ical species.
On-surface synthesis stands as a promising tool to prepare
open-shell PAH systems, offering significant advantages
over the more classical “wet” synthesis. The employed ultra-
high vacuum (UHV) conditions and the atomically clean,
catalytic surfaces used, provide the ideal playground for
both synthesis and characterization of structures that are
f)
4
1-50
hardly accessible through solution-based methods.
In-
deed, recently, elusive PAHs or nanographenes with open-
shell characters such as Clar's goblet and triangulenes have
4
7,51-
been successfully realized through on-surface synthesis.
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4
The on-surface chemistry of Pors and related porphy-
5
5,56
rinoids has emerged in the past decades,
mainly on model systems like porphines,
focusing
tetra-
On-surface fab-
5
7,58
5
9-61
62-64
phenylPors,
and derivatives thereof.
rication of π-extended Pors by fusing the macrocycle with
5
8,65-67
PAHs has only recently been reported.
However, the
few reported examples involve hybrids of Por with nanog-
raphenes, not focusing on the possible open-shell character
of the π-system.
0 2
Figure 1. Molecular structures of a) PorA , c) PorA , and e)
Herein, we present the on-surface synthesis and character-
PorA . In c,e), turquoise hexagons identify Clar’s sextets,
4
ization of three Zn(II)Pors, namely PorA
PorA , with zero, two, and four meso,β,β triply-fused
phenalenyl moieties, respectively. While PorA was pre-
pared by “wet” synthesis, the two- and four-fold
phenalenyl-fused Por derivatives PorA and PorA were
fabricated through a surface-assisted cyclodehydrogena-
0
, PorA
2
, and
whereas in a,e), red dashed lines indicate a 18 π-electron cir-
4
cuit. HOMA and bond length analysis of b) PorA
0 2
, d) PorA ,
and f) PorA . The scale bars for bond length and HOMA apply
4
0
to b), d) and f). Zn-N bonds do not correlate with the color
scale and have been colored in orange.
2
4
Results and Discussion
68
tion reaction from meso-2,6-dimethylphenyl (dmp) sub-
stituted precursors Por(dmp) and Por(dmp) , respec-
Synthesis of PorA
4h-symmetric H
0
, and precursors of PorA
2
and PorA
4
2
4
D
2
PorA and H Por(dmp)
0
2
4
were prepared
tively. This study reveals the electronic impact of the
phenalenyl substituents fused to the Por macrocycle, lead-
by condensing pyrrole with formaldehyde or 2,6-dime-
thylbenzalehyde, respectively, under classical Lindsey con-
ing to the open-shell character for PorA
strated by DFT calculations and resonance structure analy-
sis. Further proof of the open-shell character of PorA is
provided by the coexistence of singly and doubly β-hydro-
genated Pors with the intact PorA . Furthermore, we have
explored the electronic properties of all three fabricated
molecules by STS. For PorA , an open-shell triplet ground
state is experimentally demonstrated by inelastic spin exci-
tation and Kondo scattering at low energies. For PorA
2
, which is demon-
69
ditions (Scheme 1a,c). While the condensation reaction
involving 2,6-dimethylbenzaldehyde afforded
Por(dmp) in 15% yield, the low reactivity and solubility
of formaldehyde limited the yield of H PorA (i.e., 1.5%)
with a considerable amount of polypyrrolic tar obtained as
side-product. D2h-symmetric H Por(dmp) was prepared
in 27% yield by condensing meso-H-dipyrromethane with
,6-dimethylbenzaldehyde under standard Lindsey condi-
tions (Scheme 1b). Subsequent metalation of H PorA
Por(dmp) , and H Por(dmp) with Zn(OAc) in reflux-
2
H
2
4
2
0
2
2
2
2
2
4
,
,
DFT calculations and resonance structure analysis both
predict a closed-shell electronic structure. Interestingly,
2
0
H
2
2
2
4
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ing THF afforded the corresponding zinc metalated deriva-
tives PorA , Por(dmp) , and Por(dmp) , respectively, in
excellent yields. PorA , Por(dmp) , and Por(dmp) , as
well as their precursors, were fully characterized by a wide
range of spectroscopic and spectrometric techniques (see
Supporting Information).
for PorA ), suggesting the successful formation of the tar-
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get structures. Finally, the molecular structures were unam-
biguously confirmed by bond-resolved nc-AFM imaging
(Figure 2c,f,i).
0
2
4
In a first step, the potential open-shell character of PorA
PorA , and PorA was evaluated based on their resonance
structures. PorA presents a conjugated circuit of 18 π-elec-
0
,
2
4
Scheme 1. Synthetic route to the preparation of a)
0
a
PorA
0
, b) PorA
2
, and c) PorA
4
.
trons, fulfilling Hückel's rule (Figure 1a), and its predicted
electronic character is closed-shell. However, the fusion of
two and four phenalenyls to the Por core leading to PorA
and PorA , respectively, gives rise to non-Kekulé resonance
structures (Figure 1c,e and Figures S4.1 and S4.2). In the
case of PorA , several resonance structures, all of which are
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non-Kekulé, can be drawn (Figure S4.1). Among those, the
ones which show the maximum number of Clar’s sextets
(i.e., 2) and minimum number of unpaired electrons (i.e.,
2) ‒ as the one drawn in Figure 1c (vide infra) ‒ can be ex-
pected to contribute most to the ground state electronic
structure. It is worth to notice that none of the relevant res-
onance structures preserves the inner Por 18 π-electron ar-
omatic circuit. For PorA , a closed-shell Kekulé structure
4
with four Clar's sextets on the phenalenyls and an 18 π-elec-
tron circuit on the central macrocycle can be drawn (Figure
1
e). In addition, several non-Kekulé structures can be pro-
posed (Figure S4.2), one of which minimizes the number of
radicals (i.e., 2) while maintaining the four Clar's sextets
(
Figure S4.2b). To obtain further insights, harmonic oscilla-
70,71
tor model of aromaticity (HOMA)
formed on DFT optimized PorA
analysis has been per-
and PorA in the gas
2
4
phase. These studies show excellent agreement with the
proposed resonance structures, assigning high and low de-
gree of benzenoid character to the outermost six-mem-
bered rings and the pyrrolic units, respectively (Figure 1d,f).
Furthermore, gas-phase DFT calculations at the B3LYP
level show that, for PorA
uration is 0.46 eV more stable than the closed-shell one. On
the other hand, in the case of PorA , the closed-shell elec-
2
, the open-shell electronic config-
4
a
Reagents and conditions: i) 1) BF
3
O(Et)
2
, r.t., 2) DDQ, reflux,
tronic configuration is 0.21 eV more stable than the open-
shell one by theory.
3) NEt , r.t. ii) Zn(OAc) , THF, reflux; DDQ = 2,3-dichloro-5,6-
3
2
dicyano-1,4-benzoquinone.
Particular attention was therefore paid to PorA , aimed at
2
On-surface Synthesis and Characterization of PorA
0
, PorA
2
,
confirming its predicted open-shell character. Surprisingly,
after a close inspection of the reaction products by taking a
number of high-resolution nc-AFM images, we found that
and PorA
4
.
PorA was deposited on Au(111) at room temperature and
0
there were two other species besides PorA (Figure 3, panel
2
examined by STM imaging after cool down to 4.5 K, yield-
ing samples of high purity and homogeneity (Figure 2a).
On the other hand, samples with deposited precursors
iii) which were almost indiscernible from normal topo-
graphic STM images (Figure 3, panel ii). One species fea-
tures two bright protrusions at opposite β-sites of the mac-
rocycle in nc-AFM images (Figure 3a, panel iii), while an-
other only features one protrusion at a β-site (Figure 3b,
panel iii). Since constant-height nc-AFM imaging is very
sensitive to the apparent height of the underlying struc-
Por(dmp)
2
and Por(dmp) were annealed above 300 ºC to
4
promote the surface-assisted cyclodehydrogenation be-
tween the methyl groups and the macrocycle, giving PorA
2
6
6
and PorA
4
, respectively
.
The corresponding large-scale
STM images of PorA
2
and PorA confirm the presence of
4
tures, each bright protrusion suggests a doubly hydrogen-
individual molecules, but also reveal several side-products,
which are tentatively identified as covalently-linked dimers
and trimers (Figure 2d,g). High-resolution STM images
3
ated sp hybridized carbon (CH
2
) as also observed in other
47,48
open-shell nanographene systems.
Remarkably, these
hydrogenated β positions correspond with the sites of the
unpaired electrons in the proposed resonance structure of
(
Figure 2b,e,h) reveal fully planarized structures with the
expected symmetry (i.e., D4h for PorA and PorA , and D2h
0
4
PorA (Figure 1c).
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Figure 2. a) Large-scale STM image of PorA
.5 V, I = 50 pA). b) Close-up STM (V = ‒0.6 V, I
Large-scale STM image of PorA (chemical structure in the inset) obtained upon annealing of Por(dmp)
= ‒0.2 V, I = 50 pA). e) Close-up STM (V = ‒0.06 V, I = 150 pA) and f) constant-height nc-AFM (V = ‒0.005 V) images of PorA
Large-scale STM image of PorA (chemical structure in the inset) obtained upon annealing of Por(dmp)
4
‒0.2 V, I = 50 pA). h) Close-up STM (V .
0
(chemical structure in the inset) as-deposited on Au(111) at room temperature (V
= 400 pA) and c) constant-height nc-AFM (V = ‒0.001 V) images of PorA
at 300 °C on Au(111) (V
s
= ‒
. d)
0
t
s
t
s
0
2
2
s
t
s
t
s
2
. g)
4
4
at 330 °C on Au(111) (V
= ‒0.005 V) images of PorA
s
=
t
s
= ‒0.1 V, I
t
= 150 pA) and i) constant-height nc-AFM (V
s
72
STM induced atomic manipulation can be employed to se-
resonance. Thus, dI/dV spectra were taken on PorA
2
with
3
lectively break one C-H bond and transform C(sp )H
2
into
a focus on the low bias range (Figure 3d). Interestingly, we
observed characteristic conductance steps symmetrically
located around the Fermi level, along with a zero-bias peak.
The step-like conductance increase can be associated with
the inelastic excitation between the ferromagnetic (triplet)
ground state and an antiferromagnetic (singlet) excited
state, while the zero-bias peak is usually related to the
many-body Kondo state arising from the screening of a lo-
calized spin by itinerant electrons from the metal surface.
However, unlike spin excitation spectra for organic systems
with singlet ground states,
ance steps and a zero-bias peak points toward a triplet S = 1
ground state. The low-energy dI/dV spectrum can be
nicely fit by a dynamical scattering model developed by M.
Ternes, in which we constructed a S = 1 system with two
ferromagnetically exchange-coupled spins. An exchange
coupling energy J = 19.4 meV was derived from the fit. Our
DFT calculations on the surface and in gas phase both pre-
dict a ferromagnetic ground state, with a 25 meV higher an-
tiferromagnetic state (gas phase). Note the obtained value
of J is comparable to the exchange coupling energy of open-
shell nanographenes on surface with few tens of meV.
Moreover, we found that inclusion of a non-zero Kondo
scattering parameter Jρ is crucial to reproduce both the
2
C(sp )H, as the C-H bond-dissociation energy is lower in
47,48
CH
2
than in CH.
To further elucidate the chemical struc-
tures of the hydrogen passivated species, we applied a cor-
responding STM manipulation protocol. The tip was ini-
tially positioned above a bright protrusion (CH
2
) of a di-hy-
drogenated PorA (H ) with a typical STM setpoint of V = ‒
2
2
0.1 V and I = 10 pA (Figure 3a). The tip was thereafter re-
tracted by 4 Å to limit the tunneling current at the open-
feedback loop. Then the voltage bias was gradually ramped
up to 3.4 eV until an abrupt change of the tunneling current
occurred, indicating a manipulation event. The resulting
structure was imaged by both high-resolution STM and nc-
AFM, revealing the formation of the mono-hydrogenated
73
5
1,74
the coexistence of conduct-
75
72
PorA
PorA
2
(H) species (Figure 3b). Repeating the same protocol,
(H) can be further transformed into PorA (Figure
2
2
3
c). The success rate of such atomic manipulation is as high
as 90% without disrupting the molecular structure. These
manipulation experiments suggest that PorA (H) and
PorA (H ) are derived from initially formed PorA through
its passivation with H from the cyclodehydrogenation pro-
cess. DFT calculations on the surface show that passivating
PorA is favorable by 0.79 eV per H, in agreement with the
experimental observations. The presence of PorA (H) and
PorA (H ) implies a high reactivity of PorA , indicating a
2
2
2
2
2
5
1,53
2
2
s
2
2
2
cusp above the excitation energy and the zero-bias peak in
the spectrum, with a value of ‒0.14 determined from the fit.
Here, J is the exchange coupling strength between the mag-
certain radical character, in line with the predicted open-
shell structure (Figure 1b).
netic impurity and the substrate, and ρ
density. The Kondo temperature is directly related to Jρ
∝ exp(-1/ρJ). In Figure 3e, we show the spin excitation
maps of PorA obtained by constant height STS at ± 22 mV,
s
is the free electron
An important and direct fingerprint of molecules with
open-shell character is the presence of low-energy spectral
features in the differential conductance (dI/dV) spectros-
copy revealing inelastic spin excitations and/or a Kondo
s
, as
T
K
2
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which agree with the spin density map of the ferromagnetic
triplet) ground state. The nodal pattern of the experi-
mental maps is different from the one of the simulated spin
density map because the p-wave nature of the CO-function-
alized tip was not taken into account in the simulations.
The effect of the substrate will be discussed by our detailed
DFT calculations in the following context.
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Figure 3. a) i): Chemical structure of PorA
V) images where the doubly hydrogenated sites are indicated by arrows. b) i): Chemical structure of PorA
STM (V = ‒0.06 V, I = 200 pA) and iii) nc-AFM (V = ‒0.005 V) images where the doubly hydrogenated site is indicated by an arrow.
c) i): Chemical structure of PorA ; ii): its high-resolution STM (V = ‒0.06 V, I = 200 pA) and iii) nc-AFM (V = ‒0.005 V) images.
The stepwise transformation of a) PorA (H ) to b) PorA (H) to c) PorA can be achieved by STM tip induced hydrogen abstraction.
Scale bar: 5 Å. d) dI/dV spectrum acquired over PorA at the position marked in c) (Vmod = 0.8 mV). The data is shown by blue
2 2
(H ); ii): high-resolution STM (V
s
= ‒0.04 V, I
t
= 100 pA) and iii) nc-AFM (V
s
= ‒0.005
2
(H); ii) its high-resolution
s
t
s
2
s
t
s
2
2
2
2
2
68
markers, and the solid red line is a fit based on a dynamical scattering model developed by M. Ternes. DFT calculated spin density
maps for the ferromagnetic and antiferromagnetic ground states are displayed in the inset, where red and yellow densities denote
the spin-up and spin-down contributions, respectively. e) Constant-height STS maps taken at bias voltages of ‒22 mV and 22 mV
(
Vmod = 4 mV). The spin excitation maps were acquired with a CO-functionalized tip. Scale bar: 5 Å.
The electronic properties of PorA , PorA , and PorA were computed projected density of states (PDOS) of the sur-
0
2
4
further investigated by wider bias range differential con-
ductance dI/dV spectroscopy to probe their local density of
face-supported PorA (Figure 4c) and its corresponding
simulated dI/dV maps (Figure 4d), respectively. The fusion
of two phenalenyls to the Por core in PorA significantly al-
0
states (LDOS). The dI/dV spectrum of PorA
0
reveals a wide
2
energy gap around the Fermi level which is enclosed by two
broad peaks beyond ±1V (Figure 4a). The energy gap on
Au(111), around 2.3 eV derived from the onsets of the peaks,
ters the molecular energy level structure, and the resulting
dI/dV spectrum exhibits three broad peaks in the probed
energy range (Figure 4e). Spin-polarized DFT calculations
were performed to elucidate the electronic structure of
77
is comparable to the one of tetraphenyl-Por on Au(111),
and the corresponding negative and positive ion reso-
nances can accordingly be assigned to the Por HOMO and
LUMO, respectively. dI/dV maps were acquired at the onset
energies of the two peaks (Figure 4b), showing the spatially
resolved HOMO (‒1.0 V) and LUMO (1.3 V) orbitals. DFT
PorA on Au(111) (Figure S2.2). They reveal a rather compli-
2
cated energy level renormalization for this open-shell mol-
ecule (Figure 4g). The DOS projected on the frontier mo-
lecular orbitals shows that the gas-phase HOMO of PorA
2
gets empty for the spin-up electrons upon adsorption on
Au(111), which is also reflected in the calculated charge
transfer of −0.73 electrons. We note that a direct compari-
calculations of the PorA on Au(111) were performed to give
0
further insights (Figure S2.1). Both the experimental dI/dV
spectrum and maps are in good agreement with the DFT
son between the dI/dV maps of PorA acquired at different
2
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bias voltages with the DFT computed dI/dV maps is diffi-
cult due to the underestimation of the HOMO-LUMO gap
inherent to DFT. In order to ease the comparison, we have
Turning to PorA
4
, its dI/dV spectrum shows a broad density
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of states from 0.5 V to 1.2 V and a sharp peak around 1.9 V
(Figure 4i). Our DFT calculations indicate a charge transfer
of −1.11 electrons from the molecule to the surface, which
agrees with the HOMO getting empty in the DOS projected
on molecular orbitals (Figure S2.4 and Figure 4k). We can
therefore assign the broad feature in the dI/dV spectrum to
the emptied spin-down HOMO, the doubly degenerate
LUMO, and the LUMO+1. The sharp peak at 1.9 V is as-
signed to the LUMO+2. This assignment of molecular or-
bitals is corroborated by the excellent match between ex-
perimental and theoretical maps (Figure 4j and 4l, respec-
tively), which further confirms the DFT prediction of a siz-
able electron transfer from the Por to the substrate.
used the cation of PorA (with 1 electron being removed) in
2
the gas phase as a model system of the surface supported
molecule. Calculations of the gas phase cation at the B3LYP
level of theory combined with the results on the surface ob-
tained at the PBE level (see discussions in Figure S2.2 and
S2.3) allow us to assign the peak at 0.6 V in Figure 4e to the
empty molecular spin-up HOMO, while the peak starting
from 1.3 V is assigned to the spin-down LUMO and
LUMO+1 (Figure 4f-h) and the broad peak at the negative
side from ‒0.4 V is assigned to the spin-up HOMO-1. It is
0
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important to note that, despite the charge transfer of PorA
2
to the Au(111) substrate, a ferromagnetic ground state is also
predicted by DFT (Figure S2.3).
Figure 4. a,e,i) Wide bias range STS spectrum of PorA
acquisition position of the STS spectra are shown in the inset; b,f,j) constant-current STS maps recorded at the energies of the
prominent peaks; c,g,k) DFT computed PDOS (black curve) on the surface-supported PorA , PorA and PorA , respectively, where
0 2 4
, PorA and PorA , respectively. The corresponding STM images and the
0
2
4
the colored areas indicate the PDOS on its corresponding gas-phase molecular orbitals (gas phase molecular orbitals indicated in
Figure S2); d) simulated dI/dV maps matching the experimental ones in (b); h) simulated dI/dV maps matching the experimental
ones in (f); l) simulated dI/dV maps matching the experimental ones in (j). Scale bars in b,d,f,h,j,l): 5 Å.
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a)
Journal of the American Chemical Society
Pors presenting zero (i.e., PorA
i.e., PorA ) phenalenyl moieties fused to the macrocycle
were investigated. While PorA was prepared by solution
chemistry and deposited as such on Au(111), PorA and
PorA were obtained by surface-assisted cyclodehydro-
0
), two (i.e., PorA
2
), or four
b)
d)
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(
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4
genation from appropriate meso-(2,6-dimethylphenyl)-
functionalized Por precursors.
The potential open-shell character of these three Pors was
evaluated, firstly, by consideration of their resonance
structures and by gas-phase DFT calculations. In the case
c)
Exp. data
Frota fit
HWHM from Frota fit
Fermi-liquid fit:
TKondo = 78.2 ± 1.7 K
of PorA
suggesting a closed-shell character, which is confirmed by
DFT. On the other hand, DFT calculations on PorA pre-
dict an open-shell character, which is in line with the non-
Kekulé resonance structures that can be proposed for this
0
, only Kekulé resonance structures can be drawn,
0
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17.0 K
14.3 K
2
12.8 K
10.0 K
7
.9 K
.5 K
Por. In the case of PorA , DFT calculations and resonance
4
4
structure analysis both predict a closed-shell character,
while some reasonable non-Kekulé resonance structures
can also be proposed.
The three surface-supported Pors were characterized by
high-resolution STM and atomically resolved nc-AFM im-
Figure 5. a) Low-bias STM topography (V
pA) and b) constant-height STM image (V
on Au(111). Scale bar: 5 Å. c) Temperature evolution of the STS
spectra for PorA with the experimental data fit by the Frota
s
= –0.05 V, I
t
= 450
s
= 5 mV) of PorA
4
aging. Interestingly, in the case of PorA , the presence of
2
singly and doubly β-hydrogenated Por species was also ob-
served, further supporting the hypothesis of a marked
open-shell character. Moreover, the doubly hydrogenated
Por could be converted into the singly hydrogenated spe-
4
function. The acquisition position of STS spectra is marked in
a) (Vmod = 0.8 mV). d) Extracted half width at half maximum
HWHM) of the Kondo resonance as a function of tempera-
ture, which is fit using the Fermi-liquid model to determine
the Kondo temperature.
(
cies and the latter into pristine PorA
nipulation. Unequivocal evidence for an open-shell char-
acter of PorA was obtained from low-bias STS, where we
2
by STM atomic ma-
2
The charging of a closed-shell molecule as a consequence
of molecule-to-surface charge transfer could give rise to un-
paired electrons on the molecule and induce a magnetic
moment. Indeed, we observed a pronounced zero-bias
resonance in low-energy dI/dV spectra acquired on PorA
Figure 5c). The resonance can be fit by a Frota function de-
scribing a Kondo resonance, from which the resonance
width (half width at half maximum, HWHM) can be ab-
stracted. The HWHM of the resonance exhibits a charac-
teristic temperature dependent broadening following the
Fermi-liquid model, evidencing a Kondo resonance with a
found coexistence of inelastic excitations and a zero-bias
peak, suggesting a triplet ground state. Additionally,
charge transfer to the surface gives rise to induced open-
78
shell character to the intrinsically closed-shell PorA ,
4
4
which presents a characteristic Kondo resonance. Our re-
sults evidence a facile strategy to the formation of open-
shell Por molecules, which are promising for future molec-
ular electronics and spintronics.
(
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Synthesis detail of the precursor molecules, experimental
and theoretical details, resonance structures for PorA2 and
PorA4
Kondo temperature T = 78 K extracted from the Fermi-liq-
uid fit (Figure 5d). This unambiguously evidences the pres-
ence of an unpaired electron with S=1/2 on the surface sup-
K
ported PorA
netic properties of the surface supported PorA
the DFT calculations that indicate a charge transfer of ‒1.11
electrons and a magnetic moment of 0.69 µ , compatible
4
molecule. The charge-transfer induced mag-
4
agree with
B
AUTHOR INFORMATION
Corresponding Authors
with a S=1/2 ground state. Moreover, constant-height cur-
rent maps around the Kondo peak (V = 5 mV), approximat-
ing the Kondo map of PorA on Au(111) (Figure 5b) show a
good agreement with the corresponding DFT computed
spin density map (Figure S2.5). We note that due to the
*roman.fasel@empa.ch
*nicolas.lorente@ehu.eus
*tomas.torres@uam.es
4
*giovanni.bottari@uam.es
atomic defects existing in both PorA
the yields of the defect-free PorA and PorA
and 5%, respectively.
2
and PorA
4
samples,
2
4
are around 15
%
Author Contributions
¶
During the reviewing of this manuscript, we became aware
Q.S., L.M.M. and R.R. contributed equally to this work.
79
of a related work on π-extended porphyrins.
Notes
CONCLUSION
The authors declare no competing financial interest.
We have reported an in-depth study on the effect of
phenalenyl -extension of Pors as an efficient way to in-
duce open-shell character in Pors. For this purpose, three
ACKNOWLEDGMENT
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This work was supported by the Swiss National Science Foun-
conjugated
switches, Organometallics 2014, 33, 7078-7090.
16) Hadmojo, W. T.; Lee, U.-H.; Yim, D.; Kim, H. W.; Jang, W.-
D.; Yoon, S. C.; Jung, I. H.; Jang, S.-Y., High-performance near-
infrared absorbing n-type porphyrin acceptor for organic solar
cells, ACS Appl. Mater. Interfaces 2018, 10, 41344-41349.
meso-ferrocenyl-porphyrin
fluorescent redox
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dation under Grant No. 200020_182015. Financial support from
Spanish MICINN (CTQ2017-85393-P) and ERA-NET/European
Commission/MINECO (UNIQUE, SOLAR-ERA.NET Cofund 2
N° 008/ PCI2019-111889-2) are acknowledged. IMDEA Nano-
ciencia acknowledges support from the “Severo Ochoa” Pro-
gramme for Centres of Excellence in R&D (MINECO, Grant
SEV2016-0686). R.R. and N.L. are grateful for funding from the
EU-FET Open H2020 Mechanics with Molecules project (grant
(
(
17) Lewtak, J. P.; Gryko, D. T., Synthesis of π-extended
porphyrins via intramolecular oxidative coupling, Chem.
Commun. 2012, 48, 10069-10086.
(18) Richeter, S.; Jeandon, C.; Kyritsakas, N.; Ruppert, R.; Callot,
H. J., Preparation of six isomeric bis-acylporphyrins with
chromophores reaching the near-infrared via intramolecular
friedel−crafts reaction, J. Org. Chem. 2003, 68, 9200-9208.
766864). Q.S. thanks Kristjan Eimre for helping in HOMA
analysis.
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Table of Contents (ToC)
Charge-transfer
induced open-shell
doublet character
Inherent
open-shell
triplet character
5
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PorA
4
S = 1/2
PorA
2
S = 1
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9
0
J = 19.4 mV
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