Fusion at the non-K-region of pyrene: An alternative strategy to extend the π-conjugated plane of pyrene

Article abstract of DOI:10.1021/ol401904n

A large fused pyrene derivative TTTP was facilely developed through fusion at the non-K-region of pyrene, which represents the first example of extending such a π-conjugated plane at its non-K-region. The investigation of its photophysical properties and other characterizations indicated that TTTP exhibited strong aggregation behaviors and self-assembled into highly ordered one-dimensional nanowires due to its large π-conjugated plane.

Full text of DOI:10.1021/ol401904n

ORGANIC
LETTERS
2013
Vol. 15, No. 17
4378–4381
Fusion at the Non-K-Region of Pyrene:
An Alternative Strategy To Extend
the π‑Conjugated Plane of Pyrene
Lin Zou, Xiao-Ye Wang, Ke Shi, Jie-Yu Wang,* and Jian Pei*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry
and Molecular Engineering, Peking University, Beijing 100871, China
jieyuwang@pku.edu.cn; jianpei@pku.edu.cn
Received July 7, 2013
ABSTRACT
A large fused pyrene derivative TTTP was facilely developed through fusion at the non-K-region of pyrene, which represents the first example of
extending such a π-conjugated plane at its non-K-region. The investigation of its photophysical properties and other characterizations indicated that
TTTPexhibitedstrongaggregationbehaviors and self-assembledintohighlyordered one-dimensionalnanowiresduetoits largeπ-conjugatedplane.
Developing fused polycyclic aromatics with large
π-conjugated plane has been of great research interest
due to their strong intermolecular πÀπ stacking, which is
beneficial to charge carrier transport in optoelectronic
applications.1 These π-extended molecules usually exhibit
significant self-assembly properties, such as forming one-
dimensional (1D) micro- and nanowires,2 which show
great potential in electronic devices, such as organic field-
effect transistors,3 photodetectors,4 and gas sensors.5
Pyrene, which hasbeenwidely investigatedfor its unique
properties,6 is a promising building block to develop new
extended π-systems for organic optoelectronics. Among
the reported pyrene derivatives, most of them were devel-
oped through connecting pyrene with other aromatic units
(as shown in Figure 1b).7 Recently, a few pyrene deriva-
tives fused at the K-region were developed (as shown in
Figure 1c), and they exhibited interesting properties.8 How-
ever, extending the π-conjugated plane of pyrene by fusing
at the non-K-region (as shown in Figure 1d) is still an
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r
10.1021/ol401904n
Published on Web 08/27/2013
2013 American Chemical Society

Scheme 1. Synthetic Approach to TTTP
Figure 1. (a) K-region and non-K-region of pyrene; (b) function-
alized at the active positions (1, 3, 6, 8-positions) by single
bond connection; (c) fused at the K-region; (d) fused at the non-
K-region, our work.
unexplored area, which is a challenge for developing new
types of functional materials with unique properties due to
the different conjugation modes.
(2)11 gave compound 3 in 64% yield. The reaction
between compound 3 and trifluoromethanesulfonic anhy-
dride in the presence of pyridine gave compound 4 in
74% yield. A palladium-catalyzed Stille cross-coupling reac-
tion of 4 with tributyl(5-dodecylthiophene-2-yl)stannane
provided compound 5 in 73% isolated yield. A cyclo-
dehydrogenation of compound 5 was performed by oxida-
tive cyclization with FeCl3 to afford the target compound
TTTP, constructing four CÀC single bonds in one step.
After purification by column chromatography and recrys-
tallized from methanol, TTTP was obtained in 43% isolated
yield as a dark-red powder. The overall yield of TTTP from
compound 1 is around 15%. The six peripheral alkyl chains
in TTTP guarantee the solubility of the large π-plane in
common organic solvents, such as CHCl3 and chloroben-
zene, and facilitate the solution processability. The structure
and the purify of TTTP were fully characterized by 1H and
13C NMR spectroscopy and high resolution mass spectro-
metry (see the Supporting Information). To our knowledge,
this is the first work to extend the π-conjugated plane of
pyrene by fusing at the non-K-region.
The thermal properties of TTTP were examined by ther-
mogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). TTTP shows good thermal stability with
a high decomposition temperature of 375 °C under nitrogen
atmosphere. Only a pair of phase transitions ascribed to the
disturbance of alkyl chains (21 °C for cooling and 80 °C for
heating) was observed before the decomposition temperature
in DSC trace. The electrochemical property of TTTP in solu-
tion was investigated by cyclic voltammetry (CV) in acetoni-
trile containing 0.1 M n-Bu4NPF6 as supporting electrolyte
(see the Supporting Information). The HOMO and LUMO
energy levels of TTTP are estimated from the onset of the
oxidative peak and the optical energy gap (Egopt) to be À5.21
and À3.25 eV, respectively.
Herein, we develop a new strategy to expand the
π-conjugated plane of pyrene through fusion at the non-
K-region, and a thiophene-fused “superpyrene” tetrathie-
noteropyrene (TTTP) has been achieved in high yield.
TTTP represents the first example of non-K-region fused
pyrene derivatives. Thiophene units are incorporated to
further extend the π-conjugated plane and facilitate the
final cyclization reaction. In addition, the incorporation
of thiophenes is expected to improve the stability of the
desired compound and provide intermolecular SÀS inter-
actions to promote ordered organization of molecules.9
TTTP exhibits a strong self-assembly trend due to the
enlarged π-conjugated plane, and highly ordered 1D nano-
wires are obtained through the solution process.
The synthetic route to TTTP is shown in Scheme 1.
Compound 1was obtained by an iodination of 3,5-dihydroxy-
benzoic acid,10 followed by alkylation with bromohexane. A
palladium-catalyzed Suzuki coupling reaction between 1 and
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene
(8) (a) Gao, B.; Wang, M.; Cheng, Y.; Wang, L.; Jing, X.; Wang, F.
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Enkelmann, V.; Chercka, D.; Rieger, R.; Mullen, K. Chem. Commun.
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€
2011, 47, 6960. (c) Zophel, L.; Enkelmann, V.; Rieger, R.; Mullen, K.
Org. Lett. 2011, 13, 4506.
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Wang, J.-Y.; Yuan, S.-C.; Pei, J. Angew. Chem., Int. Ed. 2013, 52, 3117.
(b) Jiang, W.; Zhou, Y.; Geng, H.; Jiang, S.; Yan, S.; Hu, W.; Wang, Z.;
Shuai, Z.; Pei, J. J. Am. Chem. Soc. 2010, 133, 1. (c) Li, R.; Dong, H.;
Zhan, X.; He, Y.; Li, H.; Hu, W. J. Mater. Chem. 2010, 20, 6014.
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Eleonora, M.; Luc, G.; Alessandro, M. US6821980 B1, 2004.
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J. A. K.; Marder, T. B.; Perutz, R. N. Chem. Commun. 2005, 16, 2172. (b)
Crawford, A. G.; Dwyer, A. D.; Liu, Z.; Steffen, A.; Beeby, A.; Palsson,
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Org. Lett., Vol. 15, No. 17, 2013
4379

Figure 3. Concentration-dependent 1H NMR spectra of TTTP
(400 MHz, 298 K, CDCl3).
Figure 2. HOMO and LUMO distributions of pyrene (a, b) and
TTTP (c, d) calculated at the B3LYP/6-311G(d,p) level. The
alkyl chains were replaced by methyl groups for computational
simplicity.
signals are a result of shielding from the ring current of
neighboring aromatic molecules by cofacial stacking.3b,12
Thus, we suppose that the molecules may stack along its
short axis of the conjugated plane, so that the methylene
protons of dodecyl locate above or below the conjuga-
tion plane, while the methylene protons of ester hexyl leave
away from the large π-system. The overall results from the
concentration-dependent 1H NMR behaviors indicate that
the extended conjugated TTTP has a strong aggregation
tendency.
Concentration-dependent absorption and photolumi-
nescence spectra of TTTP were measured in CHCl3 to
further investigate the intermolecular πÀπ interaction. As
shown in Figure 4, two absorption bands with well-
resolved vibronic structures are observed in solution
at different concentrations. The absorption maxium
To deeply understand the influence of the π-extension at
the non-K-region on the geometry and electronic structure
of pyrene, density functional theory (DFT) calculations
were carried out at the B3LYP/6-311G(d,p) level. The long
alkyl chains were replaced by methyl groups for simplicity.
As shown in Figure 2, the optimized geometry of TTTP
displaysanalmostplanarπ-skeleton, and boththe HOMO
and LUMO of TTTP are mainly delocalized over the
entire backbone. Interestingly, nodal planes were observed
in the HOMO and LUMO of TTTP as shown in Figure 2.
The electron density distributes symmetrically at the two
sides of the nodal plane both in HOMO and LUMO, which
is quite similar to that in the molecular orbitals of pyrene.
The nodal planes of pyrene’s HOMO and LUMO lie
perpendicular to the molecule skeleton and pass through
the 2- and 7-positions.11b Therefore, the non-K-region fused
pyrene derivative TTTP shows a similar electronic structure
to the parent pyrene molecule and can be regarded as a
thiophene-fused “superpyrene”. The electron density distri-
bution of TTTP is quite different from the ones extended
at the K-region of pyrene,8 in which no such a nodal plane
exists, and this brings TTTP with new photophysical and
electronic properties.
The self-assembly behaviors of TTTP with an extended
π-plane were first examined by concentration-dependent
1H NMR characterization in CDCl3. All three signals of
the aromatic protons shifted upfield, and the distribu-
tion of resonances became wider as the concentration
increased from 0.5 to 10 mM at room temperature, as
shown in Figure 3. For instance, the signal at chemical shift
δ = 8.39 ppm, which is assigned to Ha, shifted upfield
to δ = 7.91 ppm as the concentration increased from 0.5 to
10 mM. Moreover, the signal at chemical shift δ= 3.30 ppm,
which corresponds to the methylene group of dodecyl (Hc),
moves to δ = 3.09 ppm as the concentration increased.
Interestingly, no shift was observed for the signal at chemi-
cal shift δ = 4.63 ppm when the concentration varied,
which is assigned to a methylene group (Hb) of hexyl chain
connected with the ester group. The upfield shifts of the
λmax peaked at 565, 523, and 488 nm, which are attributed
to the 0À0, 0À1, and 0À2 transitions, respectively. As
the concentration increased from 10À3 to10À1 mmol/L, the
intensity of 0À0 vibrational absorption at 565 nm de-
creased and those of 0À1 and 0À2 vibrational absorptions
at 523 and 488 nm increased. No shift of the absorp-
tion maxium λmax was observed, which indicates an
H-aggregation of TTTP in the high concentrated solution.13
The photoluminescence features of TTTP in dilute solution
(10À3 mmol/L) behave as a mirror-structure to its absorp-
tion spectrum, with two major vibrational emission lo-
cated at 580 and 622 nm. As the concentration increased,
significant redshift was observed (see the Supporting
Information), also indicating an aggregation trend of
TTTP in solution. As a comparison, the precursor 5 shows
absorptionfeaturessimilar tothose of 2-substituted pyrene
derivatives11b in CHCl3, peaking at 351 and 332 nm and
without concentration-dependent phenomenon. After cy-
clization, the maximum absorption of the fused TTTP
with a larger conjugated plane red-shifts to 565 nm.
The redshift is much larger than those of similar pyrene
€
(12) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mullen, K.
J. Am. Chem. Soc. 2005, 127, 4286.
(13) Spano, F. C. Acc. Chem. Res. 2010, 43, 429.
4380
Org. Lett., Vol. 15, No. 17, 2013

devrivatives extended at the K-region.8b,c Potentially,
extension of the π-plane at the non-K-region of pyrene
is more efficient to tune the bandgap of fused pyrene
derivatives.
Figure 5. SEM (a, b, scale bar: 20 μm), TEM (c, d, scale bar:
0.2 μm), and SAED (insets in c and d) analysis of the nanowires
grown from 1,4-dioxane (a, c) and cyclohexane (b, d).
Figure 4. Concentration-dependent UVÀvis absorption spectra
of TTTP in CHCl3 and UVÀvis absorption spectrum of com-
pound 5 in 10À2 mmol/L (normalized at 350 nm).
through oxidation with FeCl3 was performed success-
fully on a pyrene backbone to construct four CÀC single
bonds in one step, leading to the large conjugated
molecule TTTP. TTTP exhibits electronic structures
similar to the parent pyrene molecule, thus can be
regarded as a thiophene-fused “superpyrene”. TTTP
prefers H-aggregation in concentrated solution, which
is beneficial to charge transport. Ordered 1D nanowires
with high aspect ratios are also obtained, which are
promising for the application in organic nanoelectronic
devices.
1D nanowires were obtained through the self-assembly
of TTTP in solution as shown in Figure 5a,b. First,
a suspension of TTTP in 1,4-dioxane or cyclohexane
was heated to form a homogeneous solution. Second, the
solution was kept at a certain temperature and then cooled
toroom temperature slowly, allowing the molecules toself-
assemble through πÀπ interaction. Thus, 1D nanowires
precipitate gradually in the solution. The nanowires ob-
tained in 1,4-dioxane are about 500À800 nm in width and
hundredsof micrometers inlength, whereas those obtained
in cyclohexane are much thinner and shorter, with a width
around 300À400 nm. Deep analyses of these nanowires
were performed by transmission electron microscope
(TEM) and selected-area electron diffraction (SAED)
as shown in Figure 5c,d. Both two kinds of nanowires
exhibitthe samediffraction patterns asanalyzedby SAED,
indicating the same ordered packing mode within the
nanowires.
Acknowledgment. This work was supported by the
Major State Basic Research Development Program (Nos.
2009CB623601 and 2013CB933501) from the Ministry of
Science and Technology, and National Natural Science
Foundation of China.
Supporting Information Available. Experimental de-
tails and 1H and 13C NMR spectra. This material
is available free of charge via the Internet at http://
pubs.acs.org.
In summary, we have developed a facile strategy to extend
the conjugated plane of pyrene by fusing at its non-K-region,
which provides a new type of extended pyrene derivatives
with unique properties. The cyclodehydrogenation reaction
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 17, 2013
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