An efficient approach to the synthesis of novel pyrene-fused azaacenes

Article abstract of DOI:10.1021/ol401438a

An efficient synthetic approach for functionalization of both the active sites (1,3-) and the K-region (4,5,9,10-) of pyrene was accomplished by bromination and oxidation with considerable yield. These novel pyrene-fused azaacenes were thoroughly investig

Full text of DOI:10.1021/ol401438a

ORGANIC
LETTERS
2013
Vol. 15, No. 14
3594–3597
An Efficient Approach to the Synthesis
of Novel Pyrene-Fused Azaacenes
Xing Feng,† Fumitaka Iwanaga,† Jian-Yong Hu,*,†,‡ Hirotsugu Tomiyasu,†
Masahiro Nakano,‡ Carl Redshaw,# Mark R. J. Elsegood,§ and Takehiko Yamato*,†
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University,
Honjo-machi 1, Saga-shi, Saga, 840-8502, Japan, Emergent Molecular Function
Research Group, RIKEN Center for Emergent Matter Science (CEMS), Wako,
Saitama, 351-0198, Japan, Department of Chemistry, The University of Hull, Hull,
HU6 7RX, U. K., and Chemistry Department, Loughborough University, Leicestershire,
LE11 3TU, U. K.
yamatot@cc.saga-u.ac.jp; jian-yong.hu@riken.jp
Received May 22, 2013
ABSTRACT
An efficient synthetic approach for functionalization of both the active sites (1,3-) and the K-region (4,5,9,10-) of pyrene was accomplished by
bromination and oxidation with considerable yield. These novel pyrene-fused azaacenes were thoroughly investigated by X-ray diffraction
studies, electrochemistry, and DFT calculations.
The rational design and synthesis of organic π-conju-
gated molecules with potential applications in electronics
and optoelectronics (e.g., organic light-emitting diodes
(OLEDs), organic field-effect transistors (OFETs), organ-
ic photovoltaics (OPVs), etc.) has been the focus of much
attention.1,2 n-Acenes show excellent charge mobilities and
lower band gap with increasing numbers of fused rings.3
However, the stability and solubility of the higher acenes
(n > 5) also gradually decrease, making their applications
somewhat limited. Only a few examples of higher acenes
(5 e n e 9) have been synthesized and isolated, and in some
cases stable derivatives are known.4,5 On the other hand,
the properties of polycyclic aromatic hydrocarbons
(PAHs) depend on their molecular sizes and structures.6
An example is the system reported by Wudl et al., who
introduced a nonpropeller twisted topology and found
that strategically located phenyl substituents greatly im-
proved the stability of heptacene for the preparation of
light-emitting diodes.7 Another approach is the replace-
ment of the CH moieties by nitrogen atoms in PAHs,
which can also enhance the stability.
Azaacenes are formed by using imine nitrogen atoms
(ꢀNd) in the place of the CH group of acenes and have
been shown to possess enhanced electron affinity, high
stability, and good solubility with less negative reduction
potential (compared to acenes), not only by experiment
but alsobytheoretical investigations.2 The electron affinity
of the resultant compounds is noticeably higher than those
of analogous PAHs, which are promising candidates for
n-type organic semiconductors.8,9
† Saga University.
‡ RIKEN Center for Emergent Matter Science (CEMS).
# The University of Hull.
§ Loughborough University.
(1) Electronic Materials: The Oligomer Approach; M€ullen, K., Wegner,
G.; Eds.; Wiley VCH: Weinheim, Germany, 1998.
(2) Wang, C.-L.; Dong, H.-L.; Hu, W.-P.; Liu, Y.-Q.; Zhu, D.-B.
Chem. Rev. 2012, 112, 2208–2267.
(3) Duong, H. M.; Bendikov, M.; Steiger, D.; Zhang, Q.-C.; Sonmez,
G.; Yamada, J.; Wudl, F. Org. Lett. 2003, 5, 4433–4436.
(4) Chun, D.; Cheng, Y.; Wudl, F. Angew. Chem., Int. Ed. 2008, 47,
8380–8385.
(5) Biermann, D.; Schmidt, W. J. J. Am. Chem. Soc. 1980, 102, 3163–
3173.
(6) Gierschner, J.; Cornil, J.; Egelhaaf, H.-J. Adv. Mater. 2007, 19,
173–191.
(7) Xu, Q. F.; Duong, H. M.; Wudl, F.; Yang, Y. Appl. Phys. Lett.
2004, 85, 3357–3359.
(8) More, S.; Bhosale, R.; Choudhary, S.; Mateo-Alonso, A. Org.
Lett. 2012, 14, 4170–4173.
r
10.1021/ol401438a
Published on Web 07/03/2013
2013 American Chemical Society

Scheme 1. Synthetic Routes for the Preparation of 7, BTMABr3 = Benzyltrimethylammonium Tribromide
Pyrene is a classical member of the family of PAHs and
possesses high thermal stability, photoluminescence effi-
ciency, and enhanced hole-injection ability. Much recent
effort has been devoted to the synthesis of pyrene-based
materials for organic electronics.10 There are two strategies
to effectively functionalize the pyrene core to control
the geometry. One way is direct electrophilic substitution
of the pyrene at the active sites, namely, 1-, 3-, 6-, and
8-positions, or directly to attack at the 4-, 5-, 9-, and
10-positions by employing sterically bulky tert-butyl
groups at the 2- and 7-positions.10 Recently, our group
has reported a novel 1,3,5,9-tetrabromo-7-tert-butylpyrene;11
bromopyrenes are pivotal intermediates for functionalizing
the pyrene core by Suzuki/Sonogashira cross-coupling
reactions or nucleophilic substitution.10 The resulting sub-
stitution compounds can effectively avoid passive aggrega-
tions, which allow them to be used as emitters in OLEDs.12
Anotherstrategyistooxidize the K-regionof the pyrene by
using ruthenium chloride as a catalyst.13 This involves
expansion of the conjugation of the linear aromatic back-
bone using additional aromatic rings via condensation
reactions with diamines; in the case of N-PAHs this occurs
in surprisingly good yields.14,15 N-PAH type structures
exhibit planar conformations, which result in larger delo-
calization of electrons with higher charge carrier abilities
recently reported the first example of direct bromination
and oxidation at the K-region of the pyrene without the
employment of the sterically tert-butyl groups for utiliza-
tion in OFETs devices with high hole mobility.16
The excellent prospects of the previous examples based
on pyrene have motivated researchers to further explore
new and facile synthetic routes for the synthesis of pyrene-
fused acenes or azaacenes. Bodwell et al. reported a regio-
selective synthesis of 4,5-dialkoxy-1,8-dibromopyrenes
via an effective protective procedure.17 Recently, Mateo-
Alonso et al. reported a synthetic route for the preparation
of 1,3,6,8-tetraoctylpyrene-4,5,9,10-tetraone by an indirect
method.18 Indeed, it is seemingly impossible that both
bromination and oxidation of the pyrene would be con-
sidered a straightforward and simple method. The main
reason is that the solubility of the bromopyrene would
decrease as the number of bromine atoms increase. Re-
€
cently, Mullen et al. reported the latest results on 2,7-
dibromo- and diiodo-pyrene-4,5,8,19-tetraones;19 how-
ever, pyrene has a nodal plane passing through the C2
and C7 carbon atoms in the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO), and thus the substitutions at those
positions have less perturbation on the electronic proper-
ties of the pyrene core than the substitutions at the
active positions of 1-, 3-, 6-, and 8-positions and other
positions.20 Based on the assumption that the stable
bromopyrene can dissolve in common organic solvents
for oxidation, we explored a convenient synthetic route for
preparing new pyrene derivatives as promising organic
semiconducting materials. First, bromination at the active
and a lower energy band gap, as required in n-type organic
€
2
semiconductors. It is also noteworthy that Mullen et al.
(9) Winkler, M.; Houk, K. N. J. Am. Chem. Soc. 2007, 129, 1805–1815.
€
(10) (a) Figueira-Duarte, T. M.; Mullen, K. Chem. Rev. 2011, 111,
7260–7314. (b) Hu, J.-Y.; Pu, Y.-J.; Nakata, G.; Kawata, S.; Sasabe, H.;
Kido, J. Chem. Commun. 2012, 48, 8434–8436.
(11) Feng, X.; Hu, J.-Y.; Iwanaga, F.; Seto, N.; Redshaw, C.; Elsegood,
M. R. J.; Yamao, T. Org. Lett. 2013, 15, 1318–1321.
(12) (a) Hu, J.-Y.; Era, M.; Elsegood, M. R. J.; Yamao, T. Eur. J.
Org. Chem. 2010, 72–79. (b) Hu, J.-Y.; Feng, X.; Seto, N.; Do, J.-H.;
Zeng, X.; Tao, Z.; Yamao, T. J. Mol. Struct. 2013, 1035, 19–26.
(13) Hu, J.; Zhang, D.; Harris, F. W. J. Org. Chem. 2005, 70, 707–708.
€
(16) (a) Zophel, L.; Beckmann, D.; Enkelmann, V.; Chercka, D.;
€
€
Rieger, R.; Mullen, K. Chem. Commun. 2011, 47, 6960–6962. (b) Zophel,
€
L.; Enkelmann, V.; Mullen, K. Org. Lett. 2013, 15, 804–807.
(17) Venkataramana, G.; Dongare, P.; Dawe, L. N.; Thompson,
D. W.; Zhao, Y.; Bodwell, G. J. Org. Lett. 2011, 13, 2240–2243.
(18) Kulisic, N.; Moreab, S.; Mateo-Alonso, A. Chem. Commun.
2011, 47, 514–516.
(19) Kawano, S.-i.; Baumgarten, M.; Chercka, D.; Engkelmann, V.;
€
Mullen, K. Chem. Commun. 2013, 49, 5058–5060.
€
(14) Fogel, Y.; Zhi, L.-J.; Rouhanipour, A.; Andrienko, D.; Rader,
€
H. J.; Mullen, K. Macromolecules 2009, 42, 6878–6884.
(15) Gao, B.-X.; Wang, M.; Cheng, Y.-X.; Wang, L.-X.; Jing, X.-B.;
Wang, F.-S. J. Am. Chem. Soc. 2008, 130, 8297–8306.
Org. Lett., Vol. 15, No. 14, 2013
3595

Table 1. Physical and Electrochemical Properties of Compounds 7
a
b
c
d
e
e
λabs
nm
,
λabs
nm
,
λedge
nm
,
EHOMO(CV)
,
Egap(opt)
,
EHOMO(cal)
,
Egap(calcd)
,
Td,f
°C
eV
eV
eV
eV
7a
7b
7c
397, 419
405
409, 424
410
442
455
435
ꢀ5.86
ꢀ5.72
ꢀ6.02
2.80
2.72
2.85
ꢀ5.71
ꢀ5.33
ꢀ6.04
3.51
3.18
3.56
416
435
391
395, 418
408, 424
a Measured in CH2Cl2. b Measured in a film. c CV measured in 0.1 M Bu4NClO4/DCM with a scan rate of 100 mV sꢀ1, and the values are calculated
onset). d Calculated from λedge e DFT/B3LYP/
.
onset
using the HOMO levels of ferrocene from the empirical formula: HOMO = ꢀ(4.8 þ Eox
ꢀ Eox
(Fc)
6-31G* using Gaussian. f Obtained from TGA analysis.
1,3-positions of the pyrene afforded the 1,3-dibromo-7-
tert-butylpyrene (2), using the tert-butyl group as a pro-
tective group.21 Compound 2 was then oxidized at the
K-region using ruthenium chloride based catalysis13
(Scheme 1).
those of reported n-acence analogues (5 < n < 7);22 this is
thought to be due to the ꢀCdNꢀ in the molecular frame-
work that contributes to enhanced thermal stability.
The key intermediate, 1,3-dibromo-7-tert-butylpyrene-
4,5,9,10-tetraone 3 was easily prepared by bromination
and successive oxidation in considerable yield. Herein, the
bulky tert-butyl group in this system could play a signifi-
cant role, not only in suppressing πꢀπ stacking interac-
tions and preventing excimer formations but also in
substantially improving the solubility.12 Compound 2
was chosen because the solubility is considerable, and the
1
symmetrical structure is easy to confirm by H NMR
following oxidation. The novelty of this system is due
to the following: (1) This is a feasible example of both
bromination at the active sites of 1,3-positions and succes-
sive oxidation at the K-region of 4,5,9,10-positions in the
pyrene ring using a simple experimental procedure; (2) the
active sites at the 1,3-positions can yield C-functionalized
pyrene by Suzuki/Sonogashira cross-coupling reactions,
thereby avoiding the π-aggregations; and (3) the K-region
of 4,5,9,10-positions can offer an effective strategy to ex-
pand the π-conjugations for pyrene-fused N-PAHs via
condensation reactions. Using 3 as a starting material,
compounds 7 were synthesized by classical methods in two
steps (condensation reaction of 3 with 1,2-diamine to
afford 4 (83%) and Suzuki cross-coupling reaction of 4
with p-substituted phenylboronic acids gave 7 in good
yields, Scheme 1). As a comparison, we also followed the
Mateo-Alonso strategy;18 i.e. oxidation of 7-tert-butyl-1,3-
dimethoxyphenyl-pyrene (5) afforded the tetraketone 6
(28%), which then reacted with the 1,2-diamine in an
acetic acid solution yielding 7b (80%). All synthesized
pyrene-fused azaacenes 7 possessed good solubility in
common organic solvents and exhibited high decomposi-
tiontemperatures (Td) of up to 416 °C for 7a, 435 °C for7b,
and 391 °C for 7c (Table 1), which are much higher than
Figure 1. X-ray structure of pyrene-fused azaacene 7c: (a) top
view, (b) side view, (c) face-to-face πꢀπ stacking (top view) and
(d) side view.
A single crystal of 7c (CCDC 923414) suitable for X-ray
diffraction studies was obtained from a mixture of CH2Cl2
and hexane (1:1). The structure is shown in Figure 1.
Analysis of the packing reveals that a linear, azaacene unit
is arranged in a trapezoid-type alignment with face-to-face
˚
πꢀπ stacking (d = 3.58 A). The distance is longer than the
15
value (d = 3.48 A) reported by Wang et al. and is
˚
attributed to both the tert-butyl group at the 7-position
and the terminal formylphenyl moieties located at active
sites of 1,3-positions, which prevent the close approach of
neighboring pyrene molecules. The twist angles (57.50(4)°
and 53.27(5)°) between the formylphenyl groups and
the pyrene core are larger than our recent reported
value (43.6°).21b This is explained in terms of a steric
hindrance effect controlling the πꢀπ stacking interactions.
Clearly, the distance associated with the πꢀπ stacking
interactions would be altered through introduction of
bulky moieties at the 1,3-sites of pyrene for optical material
applications.
(20) (a) Crawford, A. G.; Dwyer, A. D.; Liu, Z.-Q.; Steffen, A.;
Beeby, A.; Palsson, L.-O.; Tozer, D. L.; Marder, T. B. J. Am. Chem. Soc.
2011, 133, 13349–13362. (b) Crawford, A. G.; Liu, Z.-Q.; Mkhalid,
I. A. I.; Thibault, M.-H.; Schwarz, N.; Alcaraz, G.; Steffen, A.; Collings,
J. C.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B. Chem.;Eur. J.
2012, 18, 5022–5035.
(21) (a) Figueira-Duarte, T. M.; Simon, S. C.; Wagner, M.; Druzhinin,
€
S. I.; Zachariasse, K. A.; Mullen, K. Angew. Chem., Int. Ed. 2008, 47,
10175–10178. (b) Feng, X.; Hu, J.-Y.; Yi, L.; Seto, N.; Tao, Z.; Redshaw,
(22) Mochida, K.; Kawasumi, K.; Segawa, Y.; Itami, K. J. Am.
Chem. Soc. 2011, 133, 10716–10719.
C.; Elsegood, M. R. J.; Yamato, T. Chem.;Asian J. 2012, 7, 2854–2863.
3596
Org. Lett., Vol. 15, No. 14, 2013

As shown in Figure 2, as for the diphenylquinoxalines,23
all pyrene-fused azaacenes 7 exhibited well-resolved spec-
tra withclearvibronicstructuresinthe near-UVand visible
region; short wavelengths 300ꢀ350 nm were consistent
with the πꢀπ* transitions, and the longer wavelengths
375ꢀ450 nm were attributed to the nꢀπ* transitions with
less intensity than that of the former. The lowest energy
bands of 7 range from 391 to 420 nm, with molar absorp-
tion coefficients of 2.00 ꢁ 10ꢀ4, 5.91 ꢁ 10ꢀ4, and 2.62 ꢁ
10ꢀ4 molꢀ1 cmꢀ1 L. The optical band gaps were estimated
from UV absorption at 2.80 eV for 7a, 2.72 eV for 7b, and
2.85 eV for 7c, respectively.
Due to rigid profiles azaacenes 7 contain the same
π-conjugated pyrene-azaacenes fragments but at the 1,3-
positions with different terminal substitutions, which
showed indistinctive absorption spectra both in CH2Cl2
solutions and in the solid state. Although the exact aggre-
gation behavior of 7 in thin film is not clear from the
absorption spectra, bathochromic shifts (e10 nm) were
observed arising from intermolecular interactions.
Figure 3. Computed molecular orbital plots (B3LYP/6-31G*) of
7, up for LUMOs and down for HOMOs.
pyrene-azaacene molecular framework in 7a, and just
to the azaacene core in 7c (Figure 3 and SI), indicating
that the electron-donating/accepting groups located at the
para-positions could significantly influence the HOMO
distributions by conjugating with the pyrene core in these
azaacene systems.
In summary, a novel synthetic route to new pyrene-fused
azaacenes 7 was explored starting from the 1,3-dibromo-7-
tert-butylpyrene-4,5,9,10-tetraone, which is the first exam-
ple of both bromination at the active sites (1,3-positions)
and oxidation at the K-region (4,5,9,10-positions) of the
pyrene. All these pyrene-fused azaacenes 7 exhibited good
solubility and high stabilities. The experimental (crystal
packing, UV/vis, and CV) and theoretical (DFT calcula-
tions) results indicated that these azaacenes might be
promising candidates for n-type organic semiconductors.
This work offers an available/facile method to functio-
nalize the pyrene core at six positions (1,3,4,5,9,10-)
and provides a strategy to prepare new pyrene-fused
azaacenes.
Figure 2. UVꢀvis absorption spectra of 7: (a) in CH2Cl2 solu-
tion and (b) in a film.
The redox behaviors of 7 were investigated in CH2Cl2
solution by cyclic voltammetry (CV) (see Supporting
Information). Electrochemical irreversible redox waves
were observed for 7; the HOMO energy levels derived
from the onset of oxidation potentials are ꢀ5.86 eV for 7a,
ꢀ5.72 eV for 7b, and ꢀ6.02 eV for 7c. It is noteworthy that
theelectron-withdrawing natureoftheꢀCHO grouphada
remarkable effect on the redox properties of these azaa-
cenes. The presence of the ꢀCHO moiety is beneficial in
terms of increasing the π-conjugation of the entire mole-
cular structure and leads to a lower HOMO level.
The geometries and electronic structures associated with
7 were evaluated by the density functional theory (DFT;
B3LYP/6-31G*); the HOMOꢀLUMO gaps (3.51 eV for
7a, 3.18 eV for 7b, 3.56 eV for 7c) are consistent with the
experimental observations. As expected, the LUMOs of 7
were mostly located on the azaacene core; however, it is
notable that the HOMOs of 7 were significantly different,
in that they spread from mainly terminal methoxyphenyl
groups and partially the pyrene core in 7b to the entire
Acknowledgment. This work was performed under the
Cooperative Research Program of “Network Joint Re-
search Center for Materials and Devices (Institute for
Materials Chemistry and Engineering, Kyushu University)”.
We would like to thank the International Collaborative
Project Fund of Guizhou province at Guizhou University
for financial support. We would also like to thank the
Royal Society ofChemistry (RSC) for the award of a travel
grant (to CR).
Supporting Information Available. Details of experi-
mental procedures, 1H/13C NMR spectra, X-ray crystal-
lographic data, photophysical data, TGA analysis, and
computational plot. This material is available free of
charge via the Internet at http://pubs.acs.org.
(23) Li, S.-R.; Lee, C.-P.; Kuo, H.-T.; Ho, K.-C.; Sun, S.-S. Chem.;
Eur. J. 2012, 18, 12085–12095.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 14, 2013
3597