Dibenzoanthradiquinone Building Blocks for the Synthesis of Nitrogenated Polycyclic Aromatic Hydrocarbons

Article abstract of DOI:10.1021/acs.orglett.0c01536

A straightforward method for the synthesis of two dibenzo[a,h]anthracene-5,6,12,13-diquinone building blocks is reported. To showcase their usefulness, a series of dibenzo[a,h]anthracene nitrogenated derivatives have been synthesized that show different optoelectronic, redox, and charge transport properties, illustrating their potential as organic semiconductors.

Full text of DOI:10.1021/acs.orglett.0c01536

pubs.acs.org/OrgLett
Letter
Dibenzoanthradiquinone Building Blocks for the Synthesis of
Nitrogenated Polycyclic Aromatic Hydrocarbons
Jose I. Martínez, Juan P. Mora-Fuentes, Marco Carini, Akinori Saeki, Manuel Melle-Franco,
and Aurelio Mateo-Alonso*
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ABSTRACT: A straightforward method for the synthesis of two
dibenzo[a,h]anthracene-5,6,12,13-diquinone building blocks is
reported. To showcase their usefulness, a series of dibenzo[a,h]-
anthracene nitrogenated derivatives have been synthesized that
show different optoelectronic, redox, and charge transport
properties, illustrating their potential as organic semiconductors.
he synthesis of increasingly large polycyclic aromatic
1
Thydrocarbons (PAHs)
is attracting growing attention
due to their potential applications in electronics, photonics,
energy storage and conversion, sensing, and information
storage, among others. PAHs can be considered small cut
outs of graphene that show discrete HOMO−LUMO gaps and
energy levels that can be tuned with the number of fused rings,
their arrangement, and the number and position of embedded
heteroatoms, providing an infinite number of possibilities to
prepare organic semiconductors with different properties.
Cyclocondesation reactions between o-quinone and o-
diamine precursors have been widely used for the construction
of nitrogen-doped PAHs (N-PAHs).1b−f On one hand this
type of condensation fuses together the π-systems of both
precursors through the formation of a pyrazine ring, while, on
the other hand, it introduces simultaneously two electron-
withdrawing nitrogen atoms in the framework. Our ability to
increase structural complexity by using such cyclocondensation
reactions depends on the development of efficient methods to
synthesize PAHs with sets of o-quinones and o-diamines in
different positions. To date there is a number o-quinones that
have been used in the synthesis of complex N-PAHs, some of
which include o-quinones of anthracene,2 phenanthrene,3
phenantroline,3b pyrene,1f,4 coronene,5 hexabenzocoronene,6
and graphene quantum dots.7 However, the methods for the
introduction o-quinones in PAHs are not always straightfor-
ward or general. Not surprisingly, their use depends directly on
their synthetic availability. For example, although, the synthesis
of pyrene o-quinones was reported in 1937,4a it was not until
20054b when they became accessible through a one-step
method from pyrene derivatives, which resulted in a substantial
increase in their interest.
Figure 1. Dibenzo[a,h]anthracenes (1a and 1b) and dibenzo[a,h]-
anthracene-di-o-quinones (2a and 2b). Library of N-PAHs 3a, 3b, 4,
and 5.
Clar sextets, one on each ring of the tri-p-phenylene residue,
and to two K-regions with a double bond character. The
Received: May 5, 2020
Dibenzo[a,h]anthracene (1a) is constituted by five fused
rings and can be structurally described as a planarized tri-p-
phenylene group fused to two additional rings in opposite bays
(Figure 1). Electronically, this arrangement gives rise to three
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oxidation of the K-regions of dibenzo[a,h]anthracene to the di-
o-quinone (2a) and its subsequent derivatization with o-
phenylenediamine to produce the corresponding diquinoxaline
have been described by Stephenson in 1949.8 However,
dibenzo[a,h]anthracene-di-o-quinone 2a was obtained as a
byproduct of the synthesis of the mono-o-quinone and its
isolation was only partially achieved by fractional sublimation.
Since then, to the best of our knowledge, there has been an
additional report describing diquinone 2a, but just as an
undesired oxidation byproduct in 1958.9 Therefore, alternative
methods need to be sought.
Herein, we make available a straightforward method for the
synthesis of dibenzo[a,h]anthracene-5,6,12,13-diquinones 2a
and 2b from their respective dibenzo[a,h]anthracenes 1a and
1b. In addition, we describe a synthetic methodology to
prepare dibenzo[a,h]anthracenes 1a and 1b in a gram scale
that does not involve any chromatographic purification. To
showcase the usefulness of di-o-quinones 2a and 2b as building
blocks, a series of N-PAHs (3a, 3b, 4, and 5) have been
synthesized by cyclocondensation with different o-diamines
leading to a small library of derivatives that show different
electronic absorption and fluorescence properties that cover
the visible and the NIR, different redox properties, and
pseudoconductivy values that illustrate their potential as
organic semiconductors.
reddish crude mixture was obtained, which we suspected was a
mixture of different oxidation intermediates of dibenzo[a,h]-
anthracene diquinone. When we subjected this crude mixture
to a subsequent oxidation process using K2Cr2O7, the yields
after chromatography improved substantially to 14%. Diqui-
none 2a showed very limited solubility and could only be
characterized by IR. Diquinone 2a was then allowed to react
with o-phenylenediamine 1110 in a 3:1 mixture of CHCl3/
AcOH yielding 3a in 59% as a yellow solid (Scheme 2). It was
Scheme 2. Synthesis of N-PAHs 3a, 3b, 4, and 5 by
Condensation of Dibenzo[a,h]anthracene-5,6,12,13-
diquinones 2a and 2b with the Corresponding o-Diamines
11, 12, and 13
The synthesis of dibenzo[a,h]anthracene (Scheme 1) started
with the iodination of the commercially available 1,4-
Scheme 1. Synthetic Route for the Preparation of the
Dibenzo[a,h]anthracene-5,6,12,13-diquinones
found that 3a was scarcely soluble in common organic solvents
such as CHCl3 or toluene, but still thanks to the four triiso-
propylsilyl (TIPS) groups, it was possible to establish its
structure by NMR (1H and 13C) and MS, which confirmed not
only the structure of 3a but also that of diquinone 2a.
Given the low solubility of 2a and 3a, we decided introduce
tert-butyl groups in positions 3 and 10 of the dibenzo[a,h]-
anthracene core. To do so, a tert-butyl substituted phenyl-
boronic acid was employed in the Suzuki reaction step that
generated 9b (87%) with two tert-butyl groups in the tri-p-
phenylene core. Then, the same synthetic route was used for
2a. Tri-p-phenylene 9b was subjected to desilylation into 10b
(>99%) and subsequently to PtCl2 catalyzed benzanulation
(63%) to provide 3,10-di-tert-butyl-dibenzo[a,h]anthracene 1b
in a gram scale without any chromatographic purification.
Then oxidation of 1b using NaIO4/RuCl3 and subsequently
K2Cr2O7 yielded the desired 3,10-di-tert-butyldibenzo[a,h]-
anthracene-5,6,12,13-diquinone 2b (17%). Diquinone 2b was
highly soluble in comparison to 2a, and it could be
characterized by NMR (1H and 13C) and HRMS.
At this stage, we assessed the reactivity of diquinone 2b with
different diamines with an increasingly long aromatic core.
Diquinone 1b was allowed to react with 1,2-phenylenediamine
11, 5,6-diaminobenzothiadiazole 12,11 and 2,3-diamino-
phenazine 13,12 in a 3:1 mixture of CHCl3/AcOH (Scheme
2). N-Doped PAHs 3a (76%), 4 (22%), and 5 (37%) were
isolated, respectively, by flash chromatography as yellow, dark
red, and purple solids, respectively, and were highly soluble in
halogenated or aromatic solvents at room temperature, which
allowed their characterization by NMR (1H and 13C) and
HRMS.
dibromobenzene 6 into diiodide 7. Next, (trimethylsilyl)-
ethynyl groups were introduced by Sonogashira coupling on
the iodide positions to generate dibromide 8, which was
subsequently subjected to Suzuki coupling with phenylboronic
acid to yield 9a. Desilylation of 9a into 10a took place
quantitively. Benzannulation of 10a using PtCl2 yielded
dibenzo[a,h]anthracene 1a in a 57% yield. All these synthetic
steps required no chromatographic purification, and therefore
1a could be prepared in a straightforward manner. Chemo-
selective oxidation of the K-regions was first attempted using
NaIO4 and RuCl3, but only small amounts of the desired
diquinone 2b (>1%) were obtained after chromatographic
purification. Longer reaction times or reoxidation of the crude
under the same conditions did not alter the outcome of this
reaction. Notably, after workup a large amount of a dark
B
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Needle-shaped crystals suitable for X-ray diffraction were
obtained for 3a, 3b, 4, and 5, which further confirmed their
structure (Figure 2). The X-ray structures show that the
Figure 3. (a) Molar absorptivity coefficients in CHCl3. (b)
Normalized fluorescence spectra and quantum yields in CHCl3. (c)
CV curves in 0.1 M solution of nBu4NPF6 in CH2Cl2. Potentials
versus Fc/Fc+. (d) FP-TRMC (λ = 355 nm, I0 = 9.1 × 1015 photons
cm−2) of products 3a, 3b, 4, and 5.
Figure 2. Front view (left) and packing (right) of (a) 3a, (b) 3b, (c)
4, and (d) 5 observed on the crystal structures.
cases to the HOMO−LUMO transition. The optical HOMO−
LUMO gaps were estimated from the onset of the longest
wavelength absorption (2.63, 2.49, 1.94, and 1.82 eV,
respectively, for 3a, 3b, 4, and 5) and show the same trends
as the calculated ones (Table S2).
The solutions of 3 and 4 under UV light show a green and
red fluorescence, while no fluorescence could be observed for 5
with the naked eye (Figure S2). The fluorescence spectrum of
3b (λmax = 502 nm; Φ = 0.09) appears bathochromically
shifted in comparison to that of 3a (λmax = 470 nm; Φ = 0.08),
in agreement with the electronic absorption spectra (Figure 3b
and Table S1). The fluorescence spectra of 4 (λmax = 632 nm;
Φ = 0.36) and 5 (λmax = 670 nm; Φ = 0.10) extend between
the red and the NIR regions.
The cyclic voltammograms of 3a, 3b, 4, and 5 were recorded
in CH2Cl2 using nBu4NPF6 as the electrolyte and ferrocene
(Fc/Fc+) as the reference (Figure 3c). In products 3a, 3b, and
4 two different reduction processes were observed. In the case
of 3a a small cathodic current at low potentials was attributed
to aggregation as noted for similar compounds.13 The redox
potentials (Table S3) were similar for 3a and 3b (−1.52 and
−1.72 V; −1.54 and −1.71 V, respectively), while they were
less cathodic than in the case of 4 (−0.93 and −1.04 V). The
voltammograms of 5 showed the same set of two reduction
processes but at even lower cathodic potentials and with an
additional reduction wave (−0.69, −0.81, and −1.23 V). None
of these products shows any redox process in the oxidative
scan. The electrochemical LUMO levels (or electron affinities)
were estimated from the onset of the first reduction are −3.59,
−3.60, −3.92, and −4.17 eV for 3a, 3b, 4, and 5, respectively
(Table S4). These values are in a reasonable agreement with
the ones calculated at the B3LYP-CH2Cl2-6-311+g(2d,p)/
B3LYP-6-31g(d,p) level of theory (Table S5).
aromatic cores are virtually flat in all cases, but in the case of
3b, one of the quinoxaline arms is out-of-plane. Conversely,
the packing is quite different. In the case of 3a the molecules
are π−π stacked (3.49 Å), while in the case of 3b with the tert-
butyl group the packing is herringbone. The crystal structure of
compound 4 combined π−π (3.55 Å) and herringbone
stacking. The structure of 5 showed a clear π−π stacking
(3.41 Å) between molecules.
The nuclear independent chemical shift (NICS) values were
calculated for 3a, 3b, 4, and 5 (Figure S1). The NICS(0)
values of the tri-p-phenylene residue of the dibenzo[a,h]-
anthracene core oscillate between −3.9 and −6.0 ppm, while
NICS(0) values of the K-region rings oscillate between +2.9
and +5.1 ppm, which is consistent with the aromatic and
antiaromatic character, respectively, predicted by Clar rules.
The NICS(0) values of the additional rings oscillate between
−3.4 and −14.9 ppm, which is consistent with the quinoxaline,
diazanaphthothiadiazole, and tetraazatetracene character of 3,
4, and 5, respectively.
Solutions of 3, 4, and 5 show the same bright colors as the
corresponding solids. Compounds 3a and 3b show a similar
bright yellow color, while the solutions of 4 and 5 are red and
dark red, respectively (Figure S2). The UV−vis absorption of
3a and 3b (Figure 3a and Table S1) show similar absorption
features with a small bathochromic shift of the band for 3b
around 450 nm. The absorption spectra of 4 and 5 show the
same set of bands observed for 3a and 3b, but bathochromi-
cally shifted. The shifts increase with the number of rings fused
to the dibenzoanthracene core. TD-DFT calculations carried
out at the B3LYP-CH2Cl2-6-311+g(2d,p)/B3LYP-6-31g(d,p)
level showed the same trend in the absorption maxima (Figure
S3 and Table S2). The lowest energy band corresponds in all
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The charge transporting properties of 3a, 3b, 4, and 5 were
evaluated by flash-photolysis time-resolved microwave con-
ductivity (FP-TRMC)14 directly on the powder solids (Figures
3d and S4). This contactless technique allows measuring
directly the pseudoconductivity (φΣμmax) that can be
considered the minimum/intrinsic mobility of the material.
φΣμmax values approaching 10−4 cm2 V−1 s−1 were obtained in
all cases. Compounds 3a and 3b (0.69 × 10−4 and 0.65 × 10−4
cm2 V−1 s−1, respectively) showed similar φΣμmax values, which
were slightly lower for 4 and 5 (0.54 × 10−4 and 0.44 × 10−4
cm2 V−1 s−1, respectively). These values are in the same range
as those observed on molecular organic semiconductors.14,15
The frontier orbitals calculated at the B3LYP-6-31g(d,p)
level show that the electron densities in both the HOMO and
the LUMO spread throughout the whole aromatic core,
indicating that these are highly conjugated systems (Figure 4).
Figures S1−S4, Table S1−S5, general methods and full
synthesis and characterization details of 3a, 3b, 4, and 5
(PDF)
Accession Codes
CCDC 1916388−1916391 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Aurelio Mateo-Alonso − POLYMAT, University of the Basque
Country UPV/EHU, 20018 Donostia-San Sebastian, Spain;
Ikerbasque, Basque Foundation for Science, Bilbao, Spain;
orcid.org/0000-0002-5316-2594; Email: amateo@
polymat.eu
Authors
Jose I. Martínez − POLYMAT, University of the Basque
Country UPV/EHU, 20018 Donostia-San Sebastian, Spain;
orcid.org/0000-0003-4436-7333
Juan P. Mora-Fuentes − POLYMAT, University of the Basque
Country UPV/EHU, 20018 Donostia-San Sebastian, Spain
Marco Carini − POLYMAT, University of the Basque Country
UPV/EHU, 20018 Donostia-San Sebastian, Spain
Akinori Saeki − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; PRESTO, Japan Science and Technology Agency
(JST), Kawaguchi, Saitama 332-0012, Japan; orcid.org/
0000-0001-7429-2200
Figure 4. Frontier orbitals at the B3LYP-6-31g(d,p) level for 3a, 3b,
4, and 5.
In the case of the HOMO, the densities over the external rings
of the tri-p-phenylene residue decrease as the number of rings
increase along the K-regions. In the case of the LUMOs, the
electron densities are mostly localized over the extended K-
regions and appear to be conjugated through the central ring of
the tri-p-phenylene residue.
Manuel Melle-Franco − CICECOAveiro Institute of
Materials, University of Aveiro, 3810-193 Aveiro, Portugal;
orcid.org/0000-0003-1929-0477
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01536
In summary, we have reported a methodology to prepare
and isolate two inaccessible and valuable building blocks for
the synthesis of N-PAHs, namely dibenzo[a,h]anthracene di-o-
quinones 2a and 2b, including a gram scale synthesis of the
corresponding dibenzo[a,h]anthracene precursors that does
not involve any chromatographic purification. To showcase,
the usefulness of di-o-quinones 2a and 2b as building blocks, a
series of N-PAHs (3a, 3b, 4, and 5) have been synthesized by
cyclocondensation with different o-diamines leading to a small
library of derivatives. Optoelectronic, redox, and theoretical
studies on 3a, 3b, 4, and 5 confirm that dibenzo[a,h]-
anthracene di-o-quinones are a highly tunable platform to
prepare N-PAHs with a broad range of absorbing and emitting
properties that extend from the visible into the NIR, which,
combined with their low LUMO levels and φΣμmax values
approaching 10−4 cm2 V−1 s−1, illustrate their potential as
organic semiconductors. Overall this work makes accessible a
family of derivatives that have received virtually no attention
since 1949, expanding the building block toolbox for the
synthesis of N-PAHs.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Basque Science Foundation for Science
(Ikerbasque), POLYMAT, the University of the Basque
́
Country (Grupo de Investigacion GIU17/054), Gobierno de
́
Espana (Ministerio de Economia y Competitividad CTQ2016-
̃
77970-R), Gobierno Vasco (PIBA 2019-09 and BERC
programme) and acknowledge technical and human support
provided by SGIker of UPV/EHU and European funding
(ERDF and ESF). This project has received funding from the
European Union’s Horizon 2020 research and innovation
programme under Grant Agreement No. 664878. This project
has received funding from the European Research Council
(ERC) under the European Union’s Horizon 2020 research
and innovation programme (Grant Agreement No. 722951).
Support through the project IF/00894/2015 and within the
scope of the project CICECO-Aveiro Institute of Materials,
UIDB/50011/2020 and UIDP/50011/2020, financed by
national funds through the Portuguese Foundation for Science
and Technology/MCTES is gratefully acknowledged.
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
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(8) Stephenson, E. F. M. 551. The Schmidt reaction on 1:2−5:6-
dibenzanthra-3:4-quinone. J. Chem. Soc. 1949, 0, 2620−2623.
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