Single-molecule conductance of dibenzopentalenes: antiaromaticity and quantum interference

Article abstract of DOI:10.1039/d0cc06810a

The effects of antiaromaticity and destructive quantum interference (DQI) are investigated on the charge transport through dibenzo-[a,e]pentalene (DBP). 5,10-Connectivity gives high single-molecule conductance whereas 2,7 gives low conductance due to DQI. Comparison of the 5,10-DBP with phenyl and anthracene analogues yields the trend GDBP ≈ GAnth > GPh, despite the aromatic anthracene having a larger HOMO-LUMO gap than 5,10-DBP. This is explained by unfavourable level alignment for 5,10-DBP. This journal is

Full text of DOI:10.1039/d0cc06810a

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Single-molecule conductance of
dibenzopentalenes: antiaromaticity and quantum
Cite this:DOI: 10.1039/d0cc06810a
interference†
Maximilian Schmidt, ‡a Daniel Wassy, ‡a Mathias Hermann,
Received 12th October 2020,
Accepted 6th December 2020
a
b
bc
afg
M. Teresa Gonzalez,
Nicolas Agrait,
Linda A. Zotti, ‡*de Birgit Esser
*
´
´
¨
and Edmund Leary ‡*b
DOI: 10.1039/d0cc06810a
rsc.li/chemcomm
The effects of antiaromaticity and destructive quantum interference with six p electrons. Breslow extended the concept by describing
(DQI) are investigated on the charge transport through dibenzo- cyclic, planar compounds with 4n p electrons as antiaromatic,2
[a,e]pentalene (DBP). 5,10-Connectivity gives high single-molecule which are chemically less stable than their olefinic counterparts
conductance whereas 2,7 gives low conductance due to DQI. Com- and which sustain paratropic ring currents.
parison of the 5,10-DBP with phenyl and anthracene analogues yields
One of the simplest compounds considered antiaromatic is
the trend GDBP E GAnth 4 GPh, despite the aromatic anthracene having cyclobutadiene (1 in Fig. 1), which contains two double bonds
a larger HOMO–LUMO gap than 5,10-DBP. This is explained by connected via two single bonds in a cyclic structure having four
unfavourable level alignment for 5,10-DBP.
p electrons (n = 1).3 This compound is highly unstable under
ambient conditions, where the use of bulky substituents is
The concept of aromaticity has captivated chemists for many essential to make (relatively) air stable examples.4 Pentalene,
years. It is characterised by the additional stability and lower n = 2, (3 in Fig. 1) is also antiaromatic5 and again highly
reactivity of planar, cyclic hydrocarbons possessing 4n + 2 unstable, only isolable employing bulky substituents.6 Both
delocalised p-electrons (where n is any integer except zero) over cyclobutadiene and pentalene can be stabilised through annu-
olefinic analogues, as codified by Hu¨ckel.1 Aromatic com- lation yielding biphenylene (2) and dibenzo[a,e]pentalene
pounds display large anisotropic diamagnetic susceptibilities (DBP, 4), shown in Fig. 1. The presence of the outer rings
due to their ability to sustain ring currents under applied diminishes, however, the antiaromatic character of the inner
magnetic fields. Benzene is the archetypal aromatic molecule rings.7
Antiaromatic compounds are predicted to give higher con-
ductance in molecular junctions compared to corresponding
aromatic or non-aromatic conjugated compounds. Breslow
a Institute for Organic Chemistry, University of Freiburg, Albertstraße 21,
79104 Freiburg, Germany. E-mail: besser@oc.uni-freiburg.de;
suggested that the relative conductance can be predicted based
on the ease to which the p-system is perturbed due to a shift of
electron density between anchor groups.8 Aromatic molecules
are predicted to be less conductive as they will resist this charge
redistribution (which disrupts aromatic bonding). Conversely,
antiaromatic compounds favour a redistribution which dis-
rupts cyclic conjugation, leading to higher conductance.
Furthermore, antiaromatic molecules tend to have smaller
HOMO–LUMO gaps compared to aromatic and non-aromatic
molecules.9 When connected to (gold) electrodes, the frontier
Web: https://www.esser-lab.uni-freiburg.de
b
˜
Instituto Madrileno de Estudios Avanzados (IMDEA), Campus Universitario de
Cantoblanco, Calle Faraday 9, 28049 Madrid, Spain.
E-mail: edmund.leary@imdea.org
c
´
´
Departamento de Fısica de la Materia Condensada, IFIMAC and Instituto ‘‘Nicolas
´
Cabrera’’, Universidad Autonoma de Madrid, E-28049, Madrid, Spain
d
e
´
´
Departamento de Fısica Aplicada I, Escuela Politecnica Superior,
Universidad de Sevilla, Seville, E-41011, Spain. E-mail: lzotti@us.es
´
´
Departamento de Fısica Teorica de la Materia Condensada and IFIMAC,
´
Universidad Autonoma de Madrid, E-28049 Madrid, Spain
f Freiburg Materials Research Center, University of Freiburg, Stefan-Meier-Str. 21,
79104 Freiburg, Germany
g Cluster of Excellence livMatS @ FIT – Freiburg Center for Interactive Materials
¨
and Bioinspired Technologies, University of Freiburg, Georges-Kohler-Allee 105,
79110 Freiburg, Germany
† Electronic supplementary information (ESI) available: Synthesis of 1–4, break-
junction methodology and theory details. CCDC 1977533, 1982996 and 1983044.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0cc06810a
Fig. 1 Chemical structures of cyclobutadiene (1), biphenylene (2), penta-
lene (3) and dibenzo[a,e]pentalene (4).
‡ These authors contributed equally to this work.
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molecular orbitals of antiaromatic molecules might be expected
to lie closer to the Fermi level, also contributing to higher
conductance. Both hypotheses consider, fundamentally, isolated
molecules. It is unclear, therefore, if this will translate to
molecular junctions, where the alignment of the molecule-
based orbitals with the electrode Fermi level is critical.10 Despite
such fundamental interest in the behaviour of antiaromatic
compounds, there are only a handful of examples of single
molecule transport studies. Moreover, there are even fewer
theoretical electron-transport calculations to substantiate these
claims.
Of distinct relevance is the effect of modulating the degree of
aromaticity within a particular structure. There are, however,
conflicting reports in the literature. Chen et al. concluded that
the conductance of a family of amine-terminated wires was high-
est for the least aromatic compound.11 The 2,5-disubstituted
Fig. 2 DBP derivatives 5 and 6, anthracene derivative 7 and compound
thiophene (most aromatic) was the least conductive, followed by 814 investigated in this study. 5 and 7 have almost the same number of
p bonds in total and thus will constitute the best test of the role of anti-
aromaticity towards electron transport.
the respective furan and finally the cyclopentadiene (least aro-
matic). Gantenbein et al. found a similar dependence studying
pyridyl-terminated compounds with tricyclic cores,12 but the least
aromatic fluorene-containing compound did not have the highest
conductance. Yang et al. studied a family of 2,5-disubstituted
Here, we report the synthesis, single-molecule conductance
5-membered heterocycles (similar to Chen et al. but with pyridyl and electronic transport calculations of two new compounds, 5
anchor groups).13 They did not find any relation between con- and 6, containing an antiaromatic DBP core with methyl
ductance and aromaticity. The authors also found no relation sulphide (SMe)-terminated phenylacetylene anchor groups in
when measuring the analogous 2,4-disubstituted compounds, either the 5,10- or the 2,7-positions respectively (Fig. 2). In order
which were instead dominated by DQI effects. Miguel et al. to ascertain the role of antiaromaticity precisely, we have also
studied a series of oligo(phenyleneethynylene)-type (OPE) com- investigated aromatic analogues of 5, anthracene 7 and phenyl
pounds14 (terminated with –SMe) in which the central ring was 8 (Fig. 2). Compounds 5, 7 and 8 have the same number of
either phenyl (most aromatic), pyridyl or pyrimidyl (least aro- p bonds connecting the anchor groups via the shortest pathway
matic). No difference in the conductance could be identified.
Regarding single molecule conductance measurements on
and practically identical S–S distance (Table 1).
SMe anchor groups were chosen for 5, 6 and 7 to facilitate
antiaromatic compounds, there are few precedents. Fujii et al. the synthetic route and avoid known issues with thiol anchor
compared an antiaromatic norcorrole nickel complex with an groups when used on gold.17 The mesityl substituents in the
aromatic nickel-porphyrin analogue. They found the former to respective 2,7- or 5,10-positions of the DBP core were attached
be approximately one order of magnitude more conductive to increase the solubility of 5 and 6. 5, 6 and 7 were synthesized
than the latter.15 Gantenbein et al. on the other hand investi- according to previously described procedures.18–22 Full details
gated a group of OPE-based compounds containing a bipheny- are shown in the ESI† along with X-ray crystal structures and
lene core.16 They could not, however, find a correlation between NICS calculations.
the presence of an antiaromatic core and a higher conductance.
To form molecular junctions, we used the statistical break-
These studies may suggest that a certain degree of change in junction method23 as described previously with a home-built
aromaticity/antiaromaticity is required to influence the con- scanning tunnelling microscope (STM).24–26 For details of
ductance significantly. The question is how much?
sample preparation and methodology, see Section 9 in the ESI.†
Table 1 Single molecule conductance and length data from STM-BJ experiments and theory plus UV-vis spectroscopy and electrochemical data
Measured
low-bias
Mean length
Theoretical (95th
Theoretical
Au–Au distance
(nm)
Theoretical
S–S distance
(nm)
Eg (UV/
vis)c
(eV)
EHOMO
(E-chem)d
(eV)
ELUMO
(E-chem)d
(eV)
Egap
(E-chem)
(eV)
conductance conductance percentile
Molecule (log(G/G0))a (log(G/G0)) (L95))b
5
6
7
8
À4.3 (0.9)
À6.2 (0.8)
À4.4 (0.9)
À4.6 (0.8)
À3.66
À5.19
À3.74
À3.87
1.58 (2.04)
1.87 (2.84)
1.37 (1.94)
1.43 (1.97)
2.29
2.84
2.28
2.28
2.03
2.58
2.02
2.02
1.73
2.06
2.48
—
À5.45
À5.51
À5.31
À3.46
À3.15
À3.02
—
1.99
2.36
2.29
—
a
b
c
Values in parentheses are the FWHM of the peak. Values have 0.4 nm added to account for the gold snap-back. Optical gap from the onsets of
d
the longest wavelength absorption band. Obtained from the onsets of the first reduction/oxidation peaks, assuming an ionization energy of
4.8 eV for ferrocene.30
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The main results from the BJ experiments are summarised in
Table 1. The 5,10-isomer 5 is clearly more conductive than the
2,7-isomer 6, which is not surprising given that 6 is 0.55 nm
longer. The fact that 6 is about 0.01 times as conductive is
surprising, however, given the typical conductance decay
(b-value) through p-systems (usually between 1–4 nmÀ1).27 Using
5,10ÀL2,7
the relation G2,7/G5,10 = e(Àb(L
)) the relative conductance of
6 would be between 0.5 and 0.1 times that of 5 if it behaved just
as a longer molecule. As even the lower estimate is a factor ten
larger than observed, we conclude that this attenuation arises via
another mechanism. Considering the conjugation paths through
the backbones, it is clear that the anchor groups in 5 are directly
conjugated whereas in 6 they are not. This suggests that DQI may
play a role as for a meta-anchored phenyl ring.28,29
Fig. 4 Transmission as a function of energy for the junctions incorporating
compounds 5–8 as show in Fig. 2.
Next, we compare the results of 5 with purely aromatic 7
(anthracene) and 8 (phenyl). The difference in S–S distance undercoordinated last gold atoms of pyramidal electrodes in a
across compounds is within 0.1 Å, and thus negligible. Fig. 3a top orientation on each side (Fig. 4a).17,34 In Fig. 4b, we show
shows the 1D histograms for each where 8 has the lowest the zero-bias transmission curves for 5–8 (we also studied the
conductance peak situated at log(G/G0) = À4.6. 7 gives the next junction formed by replacing the DBP in 5 with pentalene (5c),
highest peak centred at log(G/G0) = À4.4 followed by 5 at see Section 10.2 in the ESI†). The electronic transport generally
log(G/G0) = À4.3. Both 5 and 7 are thus notably more conduc- takes place through the tail of the HOMO, although in the case
tive than 8. There is significant overlap in the histograms of 5 of 5 the Fermi level falls almost in the middle of the HOMO–
and 7, but there is, nonetheless, a small shift to higher values LUMO gap. The conductance for each is given in Table 1. The
for 5. We also studied the conductance versus voltage (G–V) theoretical values match the trend found experimentally
behaviour of 5–8, but we found little difference in their whereby G6 { G8 o G7 o G5. The theoretical conductance
response (see ESI,† Section 9.3 for details).
differences between each are very similar to those found
To shed light on how changing the central unit affects the experimentally. Between 7 and 8, the theoretical and experi-
conductance, we performed theoretical calculations based on a mental conductance differences (log(G/G0) = 0.13 and 0.2
combination of density functional theory (DFT) and Green’s respectively) agree well with previous experimental results on
function techniques31 as described in Section 10 in the ESI.† thiol analogues.35
The HOMO–LUMO gap was corrected by the DFT+S
Firstly, the most striking difference between the shape of the
technique.32,33 We built metal–molecule–metal junctions incor- transmission curves of 5 and 6 is the presence of a strong dip
porating 5–8, in which the terminal S atoms are bound to the within the HOMO–LUMO gap of 6, which is a clear signature of
DQI.36–38 This fits the ‘‘curly-arrow’’ depiction of conjugation as
previously mentioned. Turning to the relation between 5, 7 and
8, the alignment of the frontier molecular orbitals clearly
affects the final relative theoretical conductance. We find that
the effect of switching the phenyl ring of 8 with an anthracene
(7) reduces the HOMO–LUMO gap, shifting both frontier mole-
cular orbitals closer to the Fermi level (EF), which results in a
slightly higher conductance, as observed previously.39 Substi-
tuting with the DBP unit of 5 reduces the HOMO–LUMO gap
further, resulting in the LUMO-derived peak sitting signifi-
cantly closer to EF. Narrowing of the HOMO–LUMO gap in
antiaromatic systems is well known and discussed elsewhere,40
and in the ESI.† The HOMO-peak conversely sits further down
in energy compared to that of 7. This behaviour is reflected in
the electrochemical oxidation and reduction values (Table 1). It
is worth noting that the conductance trend and the shape of the
transmission curves can be well reproduced using a simple
tight-binding model (see ESI,† Section 10.3).
We now turn to the question of how much the antiaromatic
nature of 5 contributes to the conductance. Both theory and
Fig. 3 (a) 1D conductance histograms for 5, 6, 7 and 8. Njunc = 1074 (9%)
experiment show that 5 is more conductive than 8, in agree-
(5), 831 (28%) (6), 925 (21%) (7), 1210 (57%) (8). (b–e) log(G/G0) À z 2D
ment with the initial hypothesis. 5, however, has greater overall
p conjugation, which likely contributes to the conductance
histograms generated from all plateau-containing traces for 1–4 respectively.
Note, the length scale is uncalibrated. The tip–sample bias was 0.2 V.
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´
¨
10 E. Leary, A. La Rosa, M. T. Gonzalez, G. Rubio-Bollinger, N. Agraıt
increase. Comparing 5 with 7, which have similar sized p
systems, we detect a very small difference, both experimentally
and in the theoretical transmission at the Fermi level, with 5
being slightly more conductive than 7. This is also the case for
the absolute transmission minima within the HOMO–LUMO
gap. Higher conductance ratios are obtained, however, at
higher energy as a consequence of the lower LUMO energy of
5. A similar picture is found comparing (theoretically) 8 with 5c
(ESI,† Section 10.2). Our results suggest, therefore, that in order
to take full advantage of the antiaromatic nature of
dibenzo[a,e]pentalene in a molecule junction configuration,
chemical or electric field gating, or the use of appropriate
anchor groups which promote alignment of the LUMO with
the Fermi level, are needed. We envisage most small antiaro-
matic compounds will require a degree of gating. We anticipate
LUMO-aligning anchor groups to help for non-alternant com-
pounds while HOMO-aligning groups may be required in other
systems. Exploitation of this will be pursued in later work. We
also note our work suggests that as the antiaromatic core grows,
the effect on conductance becomes more pronounced. For
comparable cores, biphenylene is slightly less conductive than
fluorene, DBP is slightly more conductive than anthracene and
norcorrole is significantly more conductive than porphyrin.
This insight will be useful in predicting the behaviour of other
systems, but more work is needed to build a complete picture.
IMDEA Nanociencia acknowledges the ‘Severo Ochoa’
Programme for Centres of Excellence in R&D (MINECO, Grant
´
and N. Martın, Chem. Soc. Rev., 2015, 44, 920–942.
11 W. Chen, H. Li, J. R. Widawsky, C. Appayee, L. Venkataraman and
R. Breslow, J. Am. Chem. Soc., 2014, 136, 918–920.
12 M. Gantenbein, L. Wang, A. A. Al-jobory, A. K. Ismael, C. J. Lambert,
W. Hong and M. R. Bryce, Sci. Rep., 2017, 7, 1794.
13 Y. Yang, M. Gantenbein, A. Alqorashi, J. Wei, S. Sangtarash, D. Hu,
H. Sadeghi, R. Zhang, J. Pi, L. Chen, X. Huang, R. Li, J. Liu, J. Shi,
W. Hong, C. J. Lambert and M. R. Bryce, J. Phys. Chem. C, 2018, 122,
14965–14970.
14 D. Miguel, L. Alvarez de Cienfuegos, A. Martın-Lasanta, S. P.
Morcillo, L. A. Zotti, E. Leary, M. Bu¨rkle, Y. Asai, R. Jurado,
D. J. Cardenas, G. Rubio-Bollinger, N. Agraıt, J. M. Cuerva and
M. T. Gonzalez, J. Am. Chem. Soc., 2015, 137, 13818–13826.
15 S. Fujii, S. Marques-Gonzalez, J.-Y. Shin, H. Shinokubo, T. Masuda,
´
´
´
¨
´
´
´
´
T. Nishino, N. P. Arasu, H. Vazquez and M. Kiguchi, Nat. Commun.,
2017, 8, 15984.
16 M. Gantenbein, X. Li, S. Sangtarash, J. Bai, G. Olsen, A. Alqorashi,
W. Hong, C. J. Lambert and M. R. Bryce, Nanoscale, 2019, 11,
20659–20666.
´
17 E. Leary, L. A. Zotti, D. Miguel, I. R. Marquez, L. Palomino-Ruiz,
´
J. M. Cuerva, G. Rubio-Bollinger, M. T. Gonzalez and N. Agrait,
J. Phys. Chem. C, 2018, 122, 3211–3218.
18 J. Wilbuer, D. C. Grenz, G. Schnakenburg and B. Esser, Org. Chem.
Front., 2017, 4, 658–663.
19 D. C. Grenz, M. Schmidt, D. Kratzert and B. Esser, J. Org. Chem.,
2018, 83, 656–663.
20 M. Hermann, D. Wassy, D. Kratzert and B. Esser, Chem. – Eur. J.,
2018, 24, 7374–7387.
21 M. Hermann, R. Wu, D. C. Grenz, D. Kratzert, H. Li and B. Esser,
J. Mater. Chem. C, 2018, 6, 5420–5426.
22 D. Wassy, M. Pfeifer and B. Esser, J. Org. Chem., 2020, 85, 34–43.
23 B. Xu and N. J. Tao, Science, 2003, 301, 1221.
24 M. T. Gonzalez, E. Leary, R. Garcıa, P. Verma, M. A. Herranz,
´
´
´
´
¨
G. Rubio-Bollinger, N. Martın and N. Agraıt, J. Phys. Chem. C,
2011, 115, 17973–17978.
25 R. Garcıa, M. A. Herranz, E. Leary, M. T. Gonzalez, G. R. Bollinger,
´
SEV-2016-0686). We thank the Comunidad de Madrid Atraccion
´
´
´
¨
M. Bu¨rkle, L. A. Zotti, Y. Asai, F. Pauly, J. C. Cuevas, N. Agraıt and
de Talento grant 2019-T1/IND-16384 (EL), NanoMagCOST (CM
´
N. Martın, Beilstein J. Org. Chem., 2015, 11, 1068–1078.
´
S2018/NMT-4321), MINECO project MAT2017-88693-R, Marıa
´
26 M. T. Gonzalez, X. Zhao, D. Z. Manrique, D. Miguel, E. Leary,
de Maeztu Programme for Units of Excellence in R&D
(CEX2018-000805-M), the University of Seville through the VI
PPIT-US program (LAZ), the Deutsche Forschungsgemeinschaft
M. Gulcur, A. S. Batsanov, G. Rubio-Bollinger, C. J. Lambert,
M. R. Bryce and N. Agraıt, J. Phys. Chem. C, 2014, 118, 21655–21662.
27 R. L. McCreery and A. J. Bergren, Adv. Mater., 2009, 21,
4303–4322.
28 T. Hansen, G. C. Solomon, D. Q. Andrews and M. A. Ratner, J. Chem.
Phys., 2009, 131, 194704.
¨
(DFG, German Research Foundation)
– project numbers
230408635, 398214985 and INST 40/467-1 FUGG – and the state
of Baden-Wu¨rttemberg through bwHPC.
29 Y. Tsuji, A. Staykov and K. Yoshizawa, J. Am. Chem. Soc., 2011, 133,
5955–5965.
30 B. W. D’Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov
and M. E. Thompson, Org. Electron., 2005, 6, 11–20.
Conflicts of interest
¨
31 F. Pauly, J. K. Viljas, U. Huniar, M. Hafner, S. Wohlthat, M. Bu¨rkle,
¨
J. C. Cuevas and G. Schon, New J. Phys., 2008, 10, 125019.
There are no conflicts to declare.
32 L. A. Zotti, M. Bu¨rkle, F. Pauly, W. Lee, K. Kim, W. Jeong, Y. Asai,
P. Reddy and J. C. Cuevas, New J. Phys., 2014, 16, 015004.
33 S. Y. Quek, L. Venkataraman, H. J. Choi, S. G. Louie, M. S. Hybertsen
and J. B. Neaton, Nano Lett., 2007, 7, 3477–3482.
Notes and references
34 M. Kamenetska, M. Koentopp, A. C. Whalley, Y. S. Park,
M. L. Steigerwald, C. Nuckolls, M. S. Hybertsen and
L. Venkataraman, Phys. Rev. Lett., 2009, 102, 126803.
35 R. Frisenda, M. L. Perrin, H. Valkenier, J. C. Hummelen and
H. S. J. van der Zant, Phys. Status Solidi B, 2013, 250, 2431–2436.
36 C. R. Arroyo, R. Frisenda, K. Moth-Poulsen, J. S. Seldenthuis,
T. Bjørnholm and H. S. van der Zant, Nanoscale Res. Lett., 2013,
8, 234.
˜
¨
1 T. M. Krygowski, M. K. Cyranski, Z. Czarnocki, G. Hafelinger and
A. R. Katritzky, Tetrahedron, 2000, 56, 1783–1796.
2 R. Breslow, Angew. Chem., Int. Ed. Engl., 1968, 7, 565–570.
3 D. J. Cram, M. E. Tanner and R. Thomas, Angew. Chem., Int. Ed.
Engl., 1991, 30, 1024–1027.
4 A. Sekiguchi, M. Tanaka, T. Matsuo and H. Watanabe, Angew. Chem.,
Int. Ed., 2001, 40, 1675–1677.
5 T. Bally, S. Chai, M. Neuenschwander and Z. Zhu, J. Am. Chem. Soc.,
1997, 119, 1869–1875.
6 K. Hafner and H. U. Su¨ss, Angew. Chem., Int. Ed. Engl., 1973, 12,
575–577.
37 X. Liu, S. Sangtarash, D. Reber, D. Zhang, H. Sadeghi, J. Shi,
Z.-Y. Xiao, W. Hong, C. J. Lambert and S.-X. Liu, Angew. Chem.,
Int. Ed., 2017, 129, 179–182.
38 L. A. Zotti and E. Leary, Phys. Chem. Chem. Phys., 2020, 22,
5638–5646.
7 C. K. Frederickson, L. N. Zakharov and M. M. Haley, J. Am. Chem.
Soc., 2016, 138, 16827–16838.
8 R. Breslow and F. W. Foss Jr, J. Phys.: Condens. Matter, 2008, 39 J. R. Quinn, F. W. Foss, L. Venkataraman and R. Breslow, J. Am.
20, 374104.
Chem. Soc., 2007, 129, 12376–12377.
40 Z. Wen and J. I.-C. Wu, Chem. Commun., 2020, 56, 2008–2011.
9 A. A. Deniz, K. S. Peters and G. J. Snyder, Science, 1999, 286, 1119.
Chem. Commun.
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