Electronic Communication across Porphyrin Hexabenzocoronene Isomers

Article abstract of DOI:10.1002/anie.201903654

Single-molecule electronic components (SMECs) are envisioned as next-generation building blocks in quantum circuit systems. However, challenges such as the reproducibility of the electrode attachment to the individual molecules hamper their fundamental in

Full text of DOI:10.1002/anie.201903654

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Electronic Communication across Porphyrin-Hexabenzocoronene
Isomers
Authors: Max M. Martin, Philipp Haines, Frank Hampel, Norbert Jux,
and Dominik Lungerich
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903654
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10.1002/anie.201903654
Angewandte Chemie International Edition
COMMUNICATION
Electronic Communication across Porphyrin-Hexabenzocoronene
Isomers
Max M. Martin,[a] Philipp Haines,[b] Frank Hampel,[a] Norbert Jux*[a] and Dominik Lungerich*[c]
Abstract: Single-molecule electronic components (SMECs) are
envisioned as next generation building blocks in quantum circuit
systems. However, challenges such as the reproducibility of the
electrode attachment to the individual molecules hamper their
fundamental investigation. For our purpose, we introduce quasi
optoelectronic electrodes (QOEs) that allow for rapid investigations of
the properties and suitability of compounds for molecular electronic
devices. In particular, we probe hexa-peri-hexabenzocoronene (HBC)
as a model system for D6h symmetrical nanographenes, with
porphyrins as QOEs attached to the periphery. We prepared
selectively bis-porphyrin functionalized HBCs with an ortho-, meta-
and para-substitution geometry and studied their communication
properties, in correlation to the geometrical alignment and size of the
system, by electrochemistry and optical spectroscopy. Further
insights into structure–property relationships were gained by DFT
calculations and X-ray diffraction analysis.
atomically precise bottom-up syntheses and functionalization,[7]
on the one hand, but also towards applications on the other hand,
such as in molecular electronic devices,[8] were conducted. Aside
from difficulties in the selective preparation of suitable compounds,
another obstacle preventing SMECs to enter the field of
commercialization, is based on the poor control over the
connection of the molecules to the electrodes.[9] Break junction
experiments, which rely on e.g., strong interactions between gold
electrodes and sulfur containing anchor groups, reveal to be quite
efficient in binding the molecules. However, they often fail in
controlling the specific coordination site at the electrodes, which
leads to fluctuations in each device performance.[2] Recently, a
variety
of
molecules,
including
e.g.,
porphyrins,[9,10]
oligothiophenes,[11] and hydrocarbons,[12] were investigated in
single-molecule conductance studies in correlation to their length
and conformational state.
Herein, our interest is focused on the relative effect of the position
of the electrodes within the nanographene and how it relates to
the size of the p-system. For that purpose, we decided not to build
actual molecular electronic devices, but rather to prepare model
compounds that enable predictions about the potential
characteristics of a SMEC in dependence of the angular
alignment of the electrodes. We use D6h symmetrical hexa-peri-
hexabenzocoronene (HBC), a hexagonal fragment of graphene,
which allows for the geometrically precise functionalization in the
periphery. We prepared selectively a logical series of bis-
functionalized HBCs, containing an ortho-, meta- and para-
substitution pattern (Figure 1). To overcome the difficulty of the
electrode attachment at an HBC-based electronic component, we
introduced peripherally attached porphyrins as quasi
Since the invention of conventional electronic components in the
1950s, the growths in technological advancement, as described
by Moore’s Law,[1] goes hand in hand with the subsequent
downsizing of the electronic elements. In that respect, the
terminus of nano-sized electronics, can be envisioned in the
fabrication of electronic circuits based on single-molecule
electronic components (SMECs).[2] Unlike conventional,
macroscopic systems, at the transition to the molecular level
(<100 nm) quantum mechanical effects, such as the Coulomb
blockade[3] and rectification,[4] start to dominate the performance
of electronic devices. Subsequently, it is crucial to gain full control
over the shape and size and therefore about the characteristics of
molecules used for SMECs. Nanographenes, which are small
fragments of the periodic 2D-material graphene, and related
structures received considerable attention amongst the physical
sciences in recent years.[5] Due to their unique (opto)electronic
properties,[6] intensive investigations towards efficient and
nanographene-based
electronics
QOEs
[a]
M. M. Martin, Dr. F. Hampel, Prof. Dr. N. Jux
Department of Chemistry and Pharmacy & Interdisciplinary Center
for Molecular Materials (ICMM), Organic Chemistry II
Friedrich-Alexander-University Erlangen-Nuernberg
Nikolaus-Fiebiger-Str. 10, 91058 Erlangen, Germany
E-mail: norbert.jux@fau.de
ortho
[b]
[c]
P. Haines
Department of Chemistry and Pharmacy, Physical Chemistry I
Friedrich-Alexander-University Erlangen-Nuernberg
Egerlandstr. 3, 91058 Erlangen, Germany
Dr. D. Lungerich
meta
Department of Chemistry & Molecular Technology Innovation
Presidential Endowed Chair
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: dominik.lungerich@gmail.com
para
Figure 1. HBC as a molecular model system for graphene-based electronics,
probed by porphyrins as quasi optoelectronic electrodes (QOEs) in ortho-,
meta- and para-geometry.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201903654
Angewandte Chemie International Edition
COMMUNICATION
optoelectronic electrodes (QOEs).[12d] This enables to study the
electronic communication between the porphyrins across the
nanographene´s p-system, probed by optical spectroscopic
methods.[13] Finally, we target to understand if fluctuations in
device performances, due to a different electrode alignment, are
size-dependent and how they can be utilized in nanographene
based integrated circuits.[14]
A general strategy to the desired ortho-, meta- and para-
conjugates 7Zn, 8Zn and 9Zn, is depicted in Scheme 1. We
followed a post-functionalization approach, relying on Suzuki-
coupling reactions of appropriately halogenated HBCs with
borylated porphyrin 10. Therefore, ortho- and para-unsubstituted
hexaphenylbenzenes (HPBs) 1 and 3 were prepared via the
common Diels-Alder strategy, reacting tetracyclones with tolans.
On the other hand, the uncommon meta-unsubstituted HPB 2 is
not available via standard techniques. Typically, tris-substituted
systems with an 1,3,5- substitution pattern, obtained from [2+2+2]
cyclotrimerization reactions of unsymmetrical tolans, are used as
a meta-equivalents.[15] However, we synthesized meta-HPB 2
selectively by the recently developed FpNA (functionalization of
para-nitroaniline) method.[16] The syntheses of 1, 2, 3, 10 and the
respective precursors are described in the supporting information.
With unsubstituted HPBs 1, 2 and 3 in hand, we carried out a
selective bis-iodination at the respective vacant peripheral
positions, utilizing [bis(trifluoroacetoxy)iodo]benzene and I2,
yielding the bis-iodo HPBs in good yields.[16,17] The oxidation
reactions to the planar bis-iodo-HBCs 4, 5 and 6 were carried out
under standard Scholl conditions, using FeCl3 as an oxidant.[16]
Finally, Suzuki coupling reactions of 4, 5 and 6 with boronic ester
porphyrin 10, Pd(PPh3)4 and Cs2CO3 were conducted, yielding
the ortho-, meta- and para- conjugates 7Zn, 8Zn, and 9Zn in
moderate yields around 50%. Typical side-products were mono-
coupled, mono-dehalogenated conjugates. However, attempts
changing the labile iodide to the more robust bromide via a post-
bromination of 1, 2 and 3, which is known for e.g., the preparation
of hexa-bromo-HPB,[18] resulted in unselective over-bromination
of the HPBs. Since the yields for isomers 7Zn, 8Zn and 9Zn were
virtually equivalent, an influence of steric repulsion on the reaction
efficiency can be ruled out.
ortho
meta
para
Scheme 1. Synthesis of ortho-, meta- and para-bis-porphyrinato-zinc(II) HBCs 7Zn, 8Zn, and 9Zn; a) bis(trifluoroacetoxy)iodo]benzene, I2, CH2Cl2; b) FeCl3,
CH3NO2, CH2Cl2; c) 10, Pd(PPh3)4, Cs2CO3, toluene, DMF; full synthetic details can be extracted from the supporting information; single-crystal X-ray structure
of meta-HPB 2 is depicted as an ORTEP model with thermal ellipsoids drawn at 50 % probability; hydrogen atoms are omitted for clarity.[24]
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10.1002/anie.201903654
Angewandte Chemie International Edition
COMMUNICATION
Alternatively,
substituted systems like 7
ortho
B-band
a)
5.0
5E5
can be prepared via a Diels-
Alder
supporting information) or
by mixed
route[19]
(see
4,5E5
4.5
44.E05
a
cyclotrimerization
approach.[20]
7Zn
8Zn
3.5
3,5E5
3E5
3.0
9Zn
The electronic interaction
between the two porphyrins
via the HBC unit was
investigated by UV/Vis
absorption and steady state
emission spectroscopy as
depicted in Figure 2 and
summarized in Table 1.
2.5
2,5E5
2.0
2E5
1.5
1,5E5
b
-band
1E5
1.0
Q-bands
50000
0.5
Depending
substitution
on
geometry,
the
0
300
350
400
450
500
550
600
650
different spectral features
are observed. The most
distinct indicator is the
shape of the porphyrins B-
band (Soret-band), which
wavelength / nm
b)
c)
d)
1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
1
1
l
l
exc. = 357
exc. = 424
lexc. = 357
lexc. = 425
lexc. = 434
l
l
l
exc. = 357
exc. = 426
exc. = 435
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
changes
clearly
with
respect to the angular
alignment of the porphyrins
to each other.[21] The B-
band is, compared to
0
0
550
600
650
700
750
550
600
650
700
750
550
600
650
700
750
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
wavelength / nm
wavelength / nm
wavelength / nm
reference
tetra(3,5-di-tert-
butylphenyl)-porphyrinato-
zinc(II) (Figure 2, black line
in insert), red-shifted and
porphyrin
Figure 2. Absorption and emission spectra of 7Zn, 8Zn, and 9Zn, measured in CH2Cl2 at rt; a) absorption spectra; inserts
show magnifications of the B-band (left) and Q-bands (right); spectrum of reference compound tetra(3,5-di-tert-butylphenyl)-
porphyrinato-zinc(II) (black, dashed) is added to the magnification of the B-bands; b), c), d) steady state fluorescence
emission spectra of zinc-porphyrin-HBCs 7Zn, 8Zn and 9Zn.
is visible. Direct excitation of the porphyrins’ B-band results in the
significantly broadened. Ortho-conjugate 7Zn shows an
absorption maximum at 424 nm with a small at 435 nm, whereas
in the case of meta-porphyrin-HBC 8Zn a rising split in the B-band
with one maximum at 425 nm and a shoulder peak at 433 nm
emerges. Even stronger, para-conjugate 9Zn shows a distinct
split of the B-band with two maxima at 426 nm and 435 nm,
respectively. The B-band shows a distinct correlation between
substitution geometry and electronic properties. The left
maximum of the B-band decreases in intensity and gets red
shifted in the order of ortho- to meta- to para-geometry. The
intensity of the right maximum, however, shows the opposite trend.
Noteworthy, HBCs functionalized with only one porphyrin show no
split in the B-band absorption.[13a,g] Therefore, the relative
distortions of the B-band can be understood as qualitative
measure for the electronic communication ability across
nanographene based SMECs. Interestingly, the HOMO-LUMO
optical transitions, reflected in the porphyrins Q-bands, remain
unaffected by the substitution pattern and appear at 550 and
590 nm for all three isomers. Also, the absorption of the HBC’s b-
band is uninfluenced by the substitution geometry and appears at
357 nm for all three isomers 7Zn, 8Zn, and 9Zn.
same fluorescence signals, though at a higher intensity. With
respect to substitution geometry, the emission intensity at 648 nm
is slightly higher than at 600 nm for the ortho-conjugate 7Zn,
whereas for meta- and para-systems 8Zn, 9Zn the opposite is
true. Spectroscopic details are listed in Table 1. The same
absorption/emission behavior can be observed for the respective
free-base conjugates 7FB, 8FB, 9FB (see supporting information
Figure S5). Electrochemical information of 7Zn, 8Zn and 9Zn
were gained by cyclic and differential pulse voltammetry in CH2Cl2
with Fc/Fc+ used as a reference and 0.1 M TBAPF6 as supporting
electrolyte. They all contain three reduction and oxidation steps.
(Table 1 and Figure S7-S9). The HOMO-LUMO gaps of 7Zn, 8Zn
and 9Zn, determined by the difference of the first reduction at -1.5
V and the first oxidation at 0.7 V, are 2.2 eV, which are in good
agreement with the optical bandgaps (Table 1). In cyclic
voltammetry, the oxidations unveil reversibility, while the
reduction steps show quasi-reversible behavior. However, the
optoelectronic details are not resolved in the electrochemcial
measurements.
Steady state fluorescence spectra (Figure 2b-d) show upon
excitation of the HBC’s b-band at 357 nm, similar to earlier
reported HBC–porphyrin conjugates,[13a,d-g] an efficient energy
transfer from the central HBC core to the porphyrins. Therefore,
only fluorescence of the porphyrins’ Q-bands at 600 and 648 nm
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10.1002/anie.201903654
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In order to estimate a size-correlated angle-dependency[22] of
nanographenes, we compared spectroscopic data on benzene-
spaced ortho, meta, para bis-porphyrin conjugates, published by
Joo, Osuka, Kim and co-workers.[21a] We noticed that the left
maximum of the B-band undergoes an angle dependent shift
(Table S3), as it is true for herein presented conjugates. The right
maximum (if existent), however, is always located at » 434 nm, no
matter if the porphyrins are linked via a benzene or HBC spacer.
Therefore, based on the position of the left maximum, we defined
a)
b)
63° 80°
an
angular
response
parameter
from
the
ratio
c)
l
left-max(o,m,p)/lleft-max(o) and plotted it against the substitution
angle. As shown in Figure 4a, a quadratic fit describes well the
angle dependency, which is significantly stronger for the small
benzene fragment (blue line) than compared to the larger
hexabenzocoronene (red line). From the second derivative of the
parabolic dependency, the angular response coefficient car is
plotted against the diameter of the nanographene (Figure 4b).[11a]
From the linear fit, we estimate a threshold diameter of » 14 Å for
D6h symmetric peri-condensed nanographenes. Thus,
considering the next larger nanographene with armchair
periphery (C114), the diameter of 19.8 Å suggests that the
substitution geometry will have only a minor influence on the
(opto)electronic characteristics.
1,024
1,019
1,014
1,009
1,004
0,999
4,0E-06
3,0E-06
2,0E-06
1,0E-06
0,0E+00
-1,0E-06
-2,0E-06
a)
b)
Figure 3. Single crystal X-ray structure of 7FB;[24] a) depicted as ORTEP model
with thermal ellipsoids drawn at 50% probability; hydrogen atoms and solvent
molecules are omitted for clarity; b) depicted as stick model; aryl moieties
omitted for clarity; c) geometry optimized structures of free-base ortho, meta,
para bis-porphyrin-HBC conjugates 7FB, 8FB and 9FB and depiction of the
highest occupied molecular orbitals (HOMOs) at the DFT B3LYP 6-31G* level
of theory; tBu groups were replaced by H-atoms; orbitals are visualized at an
iso-value = 0.015.
angular
response
0
5
10
15
20
graphene
No angular
response
diameter / Å
60
120
180
240
300
attachement angle
The porphyrins of ortho system 7Zn experience a sterically more
stressed situation compared to meta and para conjugates 8Zn
and 9Zn. However, lack in communication, due to an orthogonal
alignment of the porphyrin and HBC plane, can be excluded. As
shown in Figure 3a and b, the single crystal X-ray structure of 7FB
clearly demonstrates dihedral angles <90°.[24] In order to shed
light on the spectroscopic outcome observed for bis-porphyrin-
HBCs, we conducted DFT calculations at the B3LYP 6-31G* level
of theory (Figure 3c). We replaced the tBu groups and the central
Zn-ion by hydrogens to reduce computational cost. With an
increasing core-to-core distance of the porphyrins from 10.7 Å to
18.3 Å and 21.3 Å for the ortho-, meta- and para-conjugates,
respectively, through-space communication has only minor
impact on the spectral features. However, the situation clarifies
upon inspecting the distribution of the molecular orbitals. Although
the majority of the highest occupied molecular orbital (HOMO) is
located on the porphyrins, part of it is also based on the HBC core.
Hereby, the conjugation pathway is found to go always across the
middle ring of the HBC core. In the ortho-case, rather than
communicating directly through the biphenyl fragment, the
communication takes an angular pathway through the HBC’s
central ring.[23] Less pronounced, this indirect pathway is also true
for the meta-conjugate. On the other hand, para-structures find
their most direct communication through the middle of the HBC.
Figure 4. Angular and size dependent communication correlation: a) quadratic
fitting (y = ax2 + bx + c) of the angular response as function of the attachment
angle in benzene (blue) and HBC (red) spaced bis-porphyrin conjugates; dotted
black line corresponds to electronically isotropic graphene; b) diameter
correlation for D6h symmetrical armchair nanographenes; green rim with 14 Å
diameter.
Summarized, the successful preparation and characterization of
a
logical series of ortho-, meta- and para-bis-porphyrin
functionalized hexabenzocoronenes was achieved. Depending
on the substitution pattern a distortion of the porphyrins’ B-band,
which reflect the strength of the electronic interaction between the
two optical electrodes in the ground state, were observed. Hereby,
the porphyrins’ interaction via the HBC’s p-surface increases from
ortho-, to meta-, and para-alignment, respectively. Hence,
depending on the geometrical arrangement, specific electric
resistances can be addressed. However, extrapolation of the
herein found dependency with respective benzene-spaced
conjugates suggest that this effect becomes neglectable for
nanographenes with diameters >14 Å. In order to verify this
hypothesis, we are working on the preparation of larger bis-
porphyrin substituted nanographenes. While the observed
photophysical phenomena require further investigations, the first
experimental insights are important steps towards understanding
and engineering novel nanographene based electronics. Time-
resolved photophysical experiments as well as in-depth time-
dependent DFT calculations are currently in progress.
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10.1002/anie.201903654
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COMMUNICATION
Table 1. Spectroscopic and electrochemical data for 7Zn, 8Zn and 9Zn,
recorded at rt in CH2Cl2.
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ortho-7Zn
meta-8Zn
para-9Zn
[8]
[9]
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Abs:
l
Soret / nm
423.8 (53.1)
435 (28.3)[a]
425.4 (50.0)
433 (40.7)[a]
425.8 (45.9)
434.6 (46.5)
(
e
/ 104M-1cm-1)
Abs:
l
Q-bands / nm
550 (3.50)
589 (0.92)
551(3.60)
589 (1.00)
551(3.80)
589 (1.10)
(
e
/ 104M-1cm-1)
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FWHMSoret / nm
21.7
23.1
24.9
Fluo: lexc
:
b
-band
600 (0.25)
648 (0.27)
600 (0.28)
647 (0.27)
600 (0.30)
647 (0.29)
l
emi / nm (rel. int.)
Fluo:
l
exc: Soretmax
599 (0.99)
648 (1.00)
600 (1.00)
649 (0.95)
600 (1.00)
650 (0.92)
l
emi / nm (rel. int.)
Ered1, Ered2, Ered3 / V
Eox1, Eox2, Eox3 / V
Eg / eV
-1.8, -1.7, -1.5
0.7, 1.0, 1.3
2.2
-1.8, -1.7, -1.5
0.7, 1.0, 1.3
2.2
-1.8, -1.7, -1.5
0.7, 1.0, 1.3
2.2
[a] Shoulder.
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Acknowledgements
Funded by the Deutsche Forschungsgemeinschaft (DFG) –
Projektnummer 182849149 – SFB 953. M.M.M. thanks the
German Fond der Chemischen Industrie (FCI) and the Graduate
School Molecular Science (GSMS) for financial support. D.L.
thanks the Alexander von Humboldt foundation (AvH) and the
Japan Society for the Promotion of Science (JSPS) for a
fellowship.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Molecular electronics • Porphyrinoids •
Hydrocarbons • UV/Vis spectroscopy • Exchange interactions
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10.1002/anie.201903654
Angewandte Chemie International Edition
COMMUNICATION
[24] CCDC 1892714 (2), 1892716 (7FB) contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif.
This article is protected by copyright. All rights reserved.

10.1002/anie.201903654
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Hexabenzocoronene was utilized as
a model system for nanographenes
and probed spectroscopically by
peripherally attached porphyrins as
quasi optical electrodes. A selective
M. M. Martin, P. Haines, F. Hampel, N.
Jux,* D. Lungerich*
Page No. – Page No.
Electronic Communication across
Porphyrin-Hexabenzocoronene
Isomers
functionalization
of
the
nanographene in ortho-, meta-, and
para-position revealed alternation in
communication properties between
the porphyrins. These insights help to
understand the role of electrode
alignments in next generation single-
molecule nanoscale electronics.
This article is protected by copyright. All rights reserved.