Pyrenes, Peropyrenes, and Teropyrenes: Synthesis, Structures, and Photophysical Properties

Article abstract of DOI:10.1002/anie.201604741

The design of a relatively simple and efficient method to extend the π-conjugation of readily available aromatics in one-dimension is of significant value. In this paper, pyrenes, peropyrenes, and teropyrenes were synthesized through a double or quadruple benzannulation reaction of alkynes promoted by Br?nsted acid. This novel method does not involve cyclodehydrogenation (oxidative aryl–aryl coupling) to arrive at the newly incorporated large arene moieties. All of the target compounds were synthesized in moderate to good yields and were fully characterized with the structures unambiguously confirmed by X-ray crystallography. As expected, photophysical characterization clearly shows increasing red-shifts as a function of extended conjugation within the fused ring systems.

Full text of DOI:10.1002/anie.201604741

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201604741
German Edition: DOI: 10.1002/ange.201604741
Annulations
Pyrenes, Peropyrenes, and Teropyrenes: Synthesis, Structures, and
Photophysical Properties
Wenlong Yang, Jorge H. S. K. Monteiro, Ana de Bettencourt-Dias, Vincent J. Catalano, and
Wesley A. Chalifoux*
Abstract: The design of a relatively simple and efficient
method to extend the p-conjugation of readily available
aromatics in one-dimension is of significant value. In this
paper, pyrenes, peropyrenes, and teropyrenes were synthesized
through a double or quadruple benzannulation reaction of
alkynes promoted by Brønsted acid. This novel method does
Figure 1. Homologues of pyrene (“pyrenacenes”) superimposed on
a 5-AGNR.
not involve cyclodehydrogenation (oxidative aryl–aryl cou-
pling) to arrive at the newly incorporated large arene moieties.
All of the target compounds were synthesized in moderate to
good yields and were fully characterized with the structures
unambiguously confirmed by X-ray crystallography. As
expected, photophysical characterization clearly shows
increasing red-shifts as a function of extended conjugation
within the fused ring systems.
For example, Bardeen and co-workers assessed its potential
use as a singlet fission material, thus pointing out that
including substituents on peropyrene should improve pero-
pyreneꢀs singlet fission properties.[6] However, a good method
to introduce substituents onto peropyrene to improve its
physical properties is still lacking. As the next longer
analogue, teropyrene is expected to exhibit a bathochromic
shift in both absorption and emission bands with respect to
both pyrene and peropyrene, as well as a lower band gap
owing to its extended p system. In 1975, Misumi and co-
workers reported the first teropyrene synthesis from multi-
layered metacyclophanes by a transannular reaction/dehy-
drogenation sequence, and it was characterized only by its
absorption spectrum because of poor solubility.[7] Bodwell
and co-workers reported the synthesis of a series of bent
teropyrenes (teropyrenophanes) and fully characterized them
spectroscopically with the structures unambiguously con-
firmed by X-ray crystallography.[8] Cyclodehydrogenation/
oxidation was the key step for the synthesis of all of these
compounds. The difficult preparation, poor solubility, and
difficulty associated with the functionalization of peropyrene
and teropyrene have seriously hindered the scientific com-
munityꢀs ability to probe the properties of these materials and
to explore their use in electronic and optoelectronic applica-
tions. Therefore, the development of new methods which are
suitable for the synthesis of functionalized as well as soluble
pyrenacenes such as pyrene and more importantly peropyr-
ene and teropyrene derivatives is highly desirable.
Alkyne benzannulation reactions have been shown to
proceed using a variety of p-Lewis acids such as gold(III),[9]
platinum(II),[10] ruthenium(II),[10b,c] indium(III),[10b,11] and
antimony(V).[12] Brønsted acids and some other electrophiles,
such as ICl, I2, and NBS have also been used for alkyne
benzannulation reactions.[13] 4,10-Disubstituted pyrenes were
successfully synthesized through a gold(I)- or platinum(II)-
catalyzed twofold benzannulation reaction of 2,6-dialkynyl-
biphenyls.[14] However, neither peropyrenes nor teropyrenes
have been prepared through alkyne benzannulation reactions.
Herein, we report an efficient approach to generate soluble
pyrene, peropyrene, and teropyrene derivatives by a double
W
ell-defined, nanosized polycyclic aromatic hydrocarbons
(PAHs) have attracted significant interest because of their
wide use in electronic and optoelectronic devices, including
organic light-emitting diodes (OLEDs), organic field-effect
transistors (OFETs), and in solar cell applications.[1] In
particular, the bottom-up synthesis of PAHs which can be
recognized as nanographenes or graphene nanoribbon
(GNR)-type segments has sparked considerable interest in
many fields because of their interesting optical and electronic
properties.[2] The most common approach used to prepare
these PAHs is an oxidative aryl–aryl bond-forming reaction
[3]
À
(the Scholl reaction) as the key C C bond-forming step.
However, this reaction does not work well for making
functionalized PAHs because of undesired rearrangements
which occur under these harsh reaction conditions.[4] The
development of a synthetic protocol to arrive at functional-
ized and soluble pyrenacenes (homologues of pyrene) such as
pyrene, peropyrene, and teropyrene derivatives, which can be
viewed as fundmental 5-armchair edge GNR (5-AGNR)
oligomers (Figure 1), would therefore be of significant value.
Pyrene itself has been widely investigated because of its
commercial availability and easy modifiability. Therefore,
plenty of p-extended derivatives can be synthesized using
pyrene as a building block.[5] Peropyrene has attracted recent
attention because of its fascinating photophysical properties.
[*] Dr. W. Yang, Dr. J. H. S. K. Monteiro, Prof. A. de Bettencourt-Dias,
Prof. V. J. Catalano, Prof. W. A. Chalifoux
Department of Chemistry, University of Nevada, Reno
1664 N. Virginia St., Reno, NV 89557 (USA)
E-mail: wchalifoux@unr.edu
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201604741.
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

Angewandte
Communications
Chemie
or quadruple benzannulation reaction of alkynes promoted
by a Brønsted acid. With a relatively simple and efficient
protocol to access peropyrenes and teropyrenes we can now
begin to study these materials in more depth as well as assess
their suitability in a host of applications.
We first considered a quick and easy preparation of the
corresponding precursors for pyrene, peropyrene, and tero-
pyrene. We developed a new method to prepare the 2,6-
dialkynylbiphenyls 1 (see the Supporting Information), and
this method also made it convenient to explore the synthesis
of larger p-system compounds. We first treated 1a with TFA
in anhydrous CH2Cl2 at room temperature (Scheme 1). After
Scheme 2. Synthesis of peropyrene derivatives 6a–c. Reagents and
conditions: a) TFA, CH2Cl2, RT; b) TfOH, CH2Cl2, 08C.
TfOH at 08C, the peropyrene derivatives 6a–c were obtained
in moderate to good yield.
Scheme 1. Synthesis of the pyrene derivatives 3. Reagents and con-
ditions: a) TFA, CH2Cl2, RT; b) TfOH, CH2Cl2, 08C. TFA=trifluoroacetic
acid, Tf=trifluorormethanesulfonyl.
With proof-of-concept that we could promote a double
alkyne benzannulation to provide pyrene and peropyrene
derivatives, we wondered whether a fourfold cyclization
would be feasible. Upon treating the compounds 7a and 7b
with excess TFA at room temperature, we observe the same
phenomenon where only one of the two alkynes on each
phenyl substituent successfully cyclizes (Scheme 3). This
cyclization leads to a mixture of doubly cyclized intermediates
8 and 9 in a 1:2 ratio of isomers (identified by 1H NMR
spectroscopy and not separated; see the Supporting Informa-
tion for details). The mixture of 8 and 9 was further reacted
with TfOH to afford the corresponding target compounds 10a
and 10b in 61 and 67% yield, respectively. It should be noted
that this yield constitutes roughly an 88–90% yield per alkyne
cyclization. What is more remarkable is that these large PAHs
show good solubility in many common organic solvents.
Single crystals of 3a, 6b, and 10a suitable for X-ray
crystallographic analysis were obtained. The structure of 3a is
relatively flat with the aryl substituents twisted out of
conjugation, as expected (Figure 2a). The peropyrene 6b,
on the other hand, shows slight twisting of the PAH core
resulting from steric repulsion of the aryl groups and the
hydrogen substituents at the bay positions (Figure 2b). This
twist results in the peropyrene being axially chiral with an
end-to-end twist angle of 188. The teropyrene 10a also shows
slight twisting of the PAH core resulting from repulsion in the
bay region (Figure 2c). In this case the ends are coplanar and
the center of the PAH is twisted 158 out of planarity with
respect to the ends. In all three structures, the twisting of the
aryl substituents out of planarity can explain the absence of
p-stacking interactions between neighboring PAHs.
2 hours, only the monocyclized product, phenanthrene deriv-
ative 2a, was formed and was fully characterized by NMR and
IR spectroscopy, as well as HRMS. The 1H NMR spectrum of
2a showed a doublet at d = 10.3 ppm, which is assigned to the
proton in the “bay” position of the newly formed phenan-
threne (Ha; Scheme 1). This extreme deshielding is attributed
to the planarized phenanthryl geometry formed in 2a after
the first cyclization, which places Ha directly in the deshield-
ing zone of the remaining alkyne.[15] Only a trace amount of
the pyrene derivative 3a was formed with the addition of
excess TFA (100 equivalents) after 24 hours at room temper-
ature. When 1a was refluxed in dichloroethane in the
presence of excess TFA (100 equivalents) for 2 days, 3a was
formed in higher yield but 2a was still detected in an
appreciable amount. We found that by adding a stronger acid,
such as TfOH, rapid cyclization of the remaining alkyne
occurred.[13d] When cyclizing 1a with only TfOH we observed
side products and lower yields. Our optimal reaction con-
ditions consisted of using TFA first to cleanly provide 2a and
subsequent addition of TfOH at 08C to cyclize the remaining
alkyne. This method provided pyrene derivatives 3a–c in
moderate yields.
With optimized reaction conditions in hand we looked at
applying our double alkyne cyclization method to target
larger PAH fragments such as peropyrene. Compounds 4a–c
(Scheme 2) were synthesized in an analogous manner as 1a–c
(see the Supporting Information). Upon treatment of 4a–c
with TFA in anhydrous CH2Cl2 at room temperature, the
monocyclized products 5a–c were formed cleanly. The proton
The absorption spectra of 3a, 6b, and 10a are shown in
Figure S1 (see the Supporting Information). They absorb in
the UV and visible region, and the absorption bands show
a characteristic vibrational progression. The increase in
1
Hb in the bay position was diagnostic by H NMR spectros-
copy as it was deshielded to d ꢀ 11.1 ppm. Upon addition of
2
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
These are not the final page numbers!

Angewandte
Communications
Chemie
Scheme 3. Synthesis of teropyrene derivatives 10a and 10b. Reagents
and conditions: a) TFA, CH2Cl2, RT; b) TfOH, CH2Cl2, 08C.
conjugation from 3a, to 6b to 10a is mirrored in the
bathochromic shift of the absorption bands and the observed
increase in molar absorptivity [e; see Figures S4(a) and
S4(b)]. The determined e values for 3a (2.2 ꢁ 104 mÀ1 cmÀ1 at
352 nm), 6b (5.6 ꢁ 104 mÀ1 cmÀ1 at 467 nm), and 10a (2.8 ꢁ
105 mÀ1 cmÀ1 at 572 nm) are on the same order of magnitude
as for similar reported compounds.[16] The excitation and
emission spectra are shown in Figure 3. The emission maxima
are deep blue at l = 392 nm for 3a, green at l = 486 nm for 6b,
and red at l = 590 nm for 10a, and the emission bands for the
latter show vibrational fine structure, analogous to the
absorption and excitation spectra. The increased conjugation
is reflected in the observed bathochromic shift of emission
[Figure 3; see Figure S4(c)] as well as the decrease in the
Stokes shift (Table 1). The observed emission spectra are
mirror images of the excitation and absorption spectra, and
indicates that the same states are involved in absorption and
emission.
Figure 2. Thermal ellipsoid plots (ellipsoids at 50% probability, hydro-
gen atoms omitted for clarity) of a) the pyrene 3a, b) the peropyrene
6b showing an 188 chiral twist of the PAH core, and c) the teropyrene
10a showing the center of the PAH twisting about 158 out of planarity
with respect to the ends.
Table 1: Stokes shift, emission lifetimes (t), radiative rates (krad), and
emission efficiencies (F) obtained for the compounds in toluene.
Compound
Stokes shift [cmÀ1
]
t [ns]
krad [nsÀ1
]
F [%]
Pyrene 3a
Pyrene 3b
Peropyrene 6b 790
Peropyrene 6c 800
Teropyrene 10a 590
Teropyrene 10b 560
2540
2540
17.4Æ0.1 0.0044
16.0Æ0.1 0.0053
2.5Æ0.1 0.2468
2.6Æ0.1 0.2081
2.5Æ0.2 0.2444
2.6Æ0.1 0.2335
7.7Æ0.9
8.4Æ0.7
61.7Æ6.4
54.1Æ2.6
61.1Æ4.2
60.7Æ5.0
The emission lifetimes and efficiencies for all three
compounds, summarized in Table 1, were measured in
toluene. The emission lifetimes are 17.4 Æ 0.1 ns (3a), 2.5 Æ
0.1 ns (6b), and 2.5 Æ 0.2 ns (10a), which correspond to
emissive rates of 0.0044, 0.2468, and 0.2444 nsÀ1, respectively.
All decay curves were fit to a mono exponential (see
Figures S6, S8, and S10), thus indicating that emission
occurs from one excited state. The emission efficiencies are
7.7 Æ 0.9% (3a), 61.7 Æ 6.4% (6b), and 61.1 Æ 4.2% (10a).
The lowest efficiency found for 3a is a combination of the
inefficient absorption and losses resulting from the rotation of
the phenyl rings on the side chain. In the case of 6b and 10a,
the increase in the absorption coefficient (see Figure S1) and
the decrease in the lifetime contribute to an increase in the
emission efficiencies. As shown in Table 1 and Figures S2, S3,
S7, S9, and S11, changes in the size of the side chains do not
lead to significant change in the spectroscopic properties of
compounds with the same PAH core.
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

Angewandte
Communications
Chemie
[1] a) J. Wu, W. Pisula, K. Mꢄllen, Chem. Rev. 2007, 107, 718 – 747;
b) S. Sergeyev, W. Pisula, Y. H. Geerts, Chem. Soc. Rev. 2007, 36,
1902 – 1929; c) S. Laschat, A. Baro, N. Steinke, F. Giesselmann,
C. Hꢅgele, G. Scalia, R. Judele, E. Kapatsina, S. Sauer, A.
Schreivogel, M. Tosoni, Angew. Chem. Int. Ed. 2007, 46, 4832 –
4887; Angew. Chem. 2007, 119, 4916 – 4973; d) W. Pisula, X.
Feng, K. Mꢄllen, Adv. Mater. 2010, 22, 3634 – 3649; e) J. Mei, Y.
Diao, A. L. Appleton, L. Fang, Z. Bao, J. Am. Chem. Soc. 2013,
135, 6724 – 6746; f) M. Ball, Y. Zhong, Y. Wu, C. Schenck, F. Ng,
M. Steigerwald, S. Xiao, C. Nuckolls, Acc. Chem. Res. 2015, 48,
267 – 276; g) J. T. Markiewicz, F. Wudl, ACS Appl. Mater.
Interfaces 2015, 7, 28063 – 28085; h) T. Wçhrle, I. Wurzbach, J.
Kirres, A. Kostidou, N. Kapernaum, J. Litterscheidt, J. C.
Haenle, P. Staffeld, A. Baro, F. Giesselmann, S. Laschat,
Chem. Rev. 2016, 116, 1139 – 1241.
[2] A. Narita, X.-Y. Wang, X. Feng, K. Mullen, Chem. Soc. Rev.
2015, 44, 6616 – 6643.
[3] M. Grzybowski, K. Skonieczny, H. Butenschçn, D. T. Gryko,
Angew. Chem. Int. Ed. 2013, 52, 9900 – 9930; Angew. Chem.
2013, 125, 10084 – 10115.
[4] T. J. Sisto, L. N. Zakharov, B. M. White, R. Jasti, Chem. Sci. 2016,
7, 3681 – 3688.
Figure 3. Excitation (left) and emission (right) spectra of a) 3a (solid
line; lexc =353 nm and lem =389 nm), b) 6b (dashed line;
lexc =465 nm and lem =487 nm), and c) 10a (dotted line;
lexc =570 nm and lem =591 nm) in toluene at 298 K.
[5] T. M. Figueira-Duarte, K. Mꢄllen, Chem. Rev. 2011, 111, 7260 –
7314.
In summary, we have developed a facile and efficient
method for building pyrene, peropyrene, and teropyrene
skeletons through a Brønsted acid promoted alkyne benzan-
nulation strategy. Notably, no cyclodehydrogenation was
involved to make the PAHs, and all the pyrenes, peropyrenes,
and teropyrenes display good solubility in many common
organic solvents. The UV/Vis absorption and fluorescence
emission spectra show a gradual bathochromic shift as
a function of increasing size of the p system. The successful
synthesis of pyrenes, peropyrenes, and teropyrenes exempli-
fies the application of our method in the rapid construction of
PAHs and suggest its potential use to synthesize larger fused
ring systems with defined width, such as graphene nano-
ribbons. The ease of access of these new molecules will enable
their synthesis and more importantly extensive character-
ization, as they are of potential interest for electronic and
optoelectronic materials.
[6] V. M. Nichols, M. T. Rodriguez, G. B. Piland, F. Tham, V. N.
Nesterov, W. J. Youngblood, C. J. Bardeen, J. Phys. Chem. C
2013, 117, 16802 – 16810.
[7] T. Umemoto, T. Kawashima, Y. Sakata, S. Misumi, Tetrahedron
Lett. 1975, 16, 1005 – 1006.
[8] a) B. L. Merner, L. N. Dawe, G. J. Bodwell, Angew. Chem. Int.
Ed. 2009, 48, 5487 – 5491; Angew. Chem. 2009, 121, 5595 – 5599;
b) B. L. Merner, K. S. Unikela, L. N. Dawe, D. W. Thompson,
G. J. Bodwell, Chem. Commun. 2013, 49, 5930 – 5932.
[9] N. Asao, T. Nogami, S. Lee, Y. Yamamoto, J. Am. Chem. Soc.
2003, 125, 10921 – 10925.
[10] a) A. Fꢄrstner, V. Mamane, J. Org. Chem. 2002, 67, 6264 – 6267;
b) V. Mamane, P. Hannen, A. Fꢄrstner, Chem. Eur. J. 2004, 10,
4556 – 4575; c) T. A. Chen, T. J. Lee, M. Y. Lin, S. M. Sohel,
E. W. Diau, S. F. Lush, R. S. Liu, Chem. Eur. J. 2010, 16, 1826 –
1833.
[11] a) A. Fꢄrstner, V. Mamane, Chem. Commun. 2003, 2112 – 2113;
b) R. Ste˛z˙ycki, M. Grzybowski, G. Clermont, M. Blanchard-
Desce, D. T. Gryko, Chem. Eur. J. 2016, 22, 5198 – 5203.
[12] S. Yamaguchi, T. M. Swager, J. Am. Chem. Soc. 2001, 123,
12087 – 12088.
[13] a) M. B. Goldfinger, T. M. Swager, J. Am. Chem. Soc. 1994, 116,
7895 – 7896; b) M. B. Goldfinger, K. B. Crawford, T. M. Swager,
J. Am. Chem. Soc. 1997, 119, 4578 – 4593; c) T. Yao, M. A.
Campo, R. C. Larock, J. Org. Chem. 2005, 70, 3511 – 3517; d) A.
Mukherjee, K. Pati, R.-S. Liu, J. Org. Chem. 2009, 74, 6311 –
6314.
[14] a) D. B. Walker, J. Howgego, A. P. Davis, Synthesis 2010, 3686 –
3692; b) T. Matsuda, T. Moriya, T. Goya, M. Murakami, Chem.
Lett. 2011, 40, 40 – 41.
[15] For another example of this deshielding effect, see: M. Kastler, J.
Schmidt, W. Pisula, D. Sebastiani, K. Mꢄllen, J. Am. Chem. Soc.
2006, 128, 9526 – 9534.
[16] a) S. C. Martens, T. Riehm, S. Geib, H. Wadepohl, L. H. Gade, J.
Org. Chem. 2011, 76, 609 – 617; b) S. Chen, F. S. Raad, M.
Ahmida, B. R. Kaafarani, S. H. Eichhorn, Org. Lett. 2013, 15,
558 – 561.
Acknowledgements
W.A.C. acknowledges the Donors of the American Chemical
Society Petroleum Research Fund for support of this research
(PRF 53543-DNI1). W.A.C. also thanks the University of
Nevada, Reno for startup funds and the National Science
Foundation for financial support through a CAREER Award
(CHE-1555218). A.d.B.-D. acknowledges financial support
through the NSF (CHE-1363325), and J.H.S.K.M. thanks
CNPq (Conselho Nacional de Desenvolvimento Cientꢂfico
e Tecnolꢃgico—Brazil) for a Postdoctotal Fellowship (grant
232574/2014-6).
Keywords: alkynes · annulations · arenes · Brønsted acid ·
macrocycles
Received: May 15, 2016
Published online: && &&, &&&&
4
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Annulations
W. Yang, J. H. S. K. Monteiro,
A. de Bettencourt-Dias, V. J. Catalano,
W. A. Chalifoux*
&&&& — &&&&
Pyrenes, Peropyrenes, and Teropyrenes:
Synthesis, Structures, and Photophysical
Properties
Pyrenes with a twist: The synthesis of
soluble pyrenes, peropyrenes, and tero-
pyrenes using a two- and fourfold alkyne
benzannulation reaction promoted by
Brønsted acid is reported. The com-
pounds show an expected red shift in
absorbance and emission as a function of
conjugation length with quantum yields
of up to 62%. X-ray crystallographic
analysis of these polycyclic aromatic
hydrocarbons shows significant twisting
of the aromatic core, thus demonstrating
how flexible nanographenes can be.
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!