Synthesis, structure, and opto-electronic properties of regioisomeric pyrene-thienoacenes

Article abstract of DOI:10.1021/ol402971n

Three regioisomeric sulfur-bridged pyrene-thienoacenes (PTAs) have been synthesized. The crystal structures and optoelectronic properties of these ring-fused PTAs and their ring-opened precursors have been fully investigated. Among these isomers, the [3,4]-extended compound (4-S-PTA) shows the most pronounced spectral red-shift and highest quantum yield as well as large transistor mobility.

Full text of DOI:10.1021/ol402971n

Letter
pubs.acs.org/OrgLett
Synthesis, Structure, and Opto-electronic Properties of
Regioisomeric Pyrene−Thienoacenes
Shiqian Zhang,† Xiaolan Qiao,‡ Ying Chen,† Yuanyuan Wang,†,‡ Robert M. Edkins,§ Zhiqiang Liu,*,†
Hongxiang Li,*,‡ and Qi Fang†
†State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
‡Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
§Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: Three regioisomeric sulfur-bridged pyrene−thienoacenes
(PTAs) have been synthesized. The crystal structures and optoelec-
tronic properties of these ring-fused PTAs and their ring-opened
precursors have been fully investigated. Among these isomers, the [3,4]-
extended compound (4-S-PTA) shows the most pronounced spectral
red-shift and highest quantum yield as well as large transistor mobility.
rom the viewpoint of both organic synthesis and
As illustrated in Scheme 1, the PTAs were synthesized via
different routes with good yields. The acid-induced cyclization
of aromatic methyl sulfoxides, which has been developed by
F
optoelectronic properties, the chemistry of pyrene is
strongly position-dependent.1 The 1-, 3-, 6-, and 8-positions
of pyrene can be regarded as “common sites”, which are known
as the most active sites for electrophilic aromatic substitution
(SEAr) reactions, and most pyrene derivatives are extended
from these positions.1,2 Among the other “un-common sites”,
the 2- and 7-positions of pyrene are known as the molecular
node sites and are extremely difficult to derivatize by ordinary
methods, except where steric hindrance precludes reactions at
the common sites (e.g., Friedel−Crafts tert-butylation3). The
finding of Marder et al. that Ir-catalyzed direct borylation of
pyrene is highly selective for the 2- and 7-positions has opened
up a new way to functionalize pyrene.4,5 At the 4-, 5-, 9-, and
10-positions, which are also defined as the K-region of pyrene,
oxidation can take place,6 and the reaction has recently been
Mullen et al.15 and extended by others to synthesize
̈
thienoacenes,12,16 is the key step of this process. For the
synthesis of precursors S-X-S (X = 1, 2, or 4), which also serve
as open-ring analogues of PTAs for comparison studies, the
Suzuki−Miyaura cross-coupling between appropriate boronates
and aryl halides were adopted. The coupled product was
oxidized using H2O2 to generate the sulfoxide intermediates,
which can be used directly in the next step. Finally, the target
PTAs were obtained by treating the sulfoxide intermediates
with trifluoromethanesulfonic acid and subsequent demethyla-
tion in water/pyridine mixture.
Because the stability of aromatic thiophene is much better
than the sulfur-bridged 6-membered rings, most of the reported
products of this acid-induced cyclization are sulfur-bridging 5-
membered-ring-fused thienoacenes. In this work, 1-S-PTA was
prepared as expected with good yield from compound 2 using
the approach outlined above. Thus, compound 4 might be
expected to bring about 2-S-PTA. However, X-ray crystallo-
graphic analysis of the cyclization product indicated that the
sulfur atom is bonded to the 4-position rather than 2-position
of pyrene; i.e., it is 4-S-PTA. This may be due to the fact that
the 2-position lies in the nodal plane of both the HOMO and
LUMO of pyrene,4 making it the most inert site.
To synthesize 2-S-PTA for comparison studies, first ortho-
lithiated 1-methoxypyrene (5) was reacted with dimethyl
disulfide (Me2S2) to give 1-methoxy-2-methylthiopyrene (6) in
nearly quantitative yield. After selective demethylation of the
methoxy groups and subsequent triflation to give compound 7,
the Suzuki−Miyaura coupling with phenylboronic acid afford S-
extended by Mullen et al. prior to unsymmetrical bromination
̈
at the 9- and 10-positions.7 Itami and co-workers have also
reported that Pd-catalyzed oxidative direct arylation can
asymmetrically functionalize pyrene at the 4-position.8
Although much work has focused on functionalizing pyrene
derivatives at the “common” or “un-common” sites, regiose-
lective synthetic strategies and their opto-electronic properties
need to be considered carefully.9,10 To our knowledge, although
many sulfur-containing heteroaromatics,11 such as naphtho-
thiophenes and anthrathiophenes, show excellent semiconduct-
ing performance,12,13 few pyrene−thienoacenes (PTA) have
been reported.7,14 Considering the nature of both extensive
intramolecular π-conjugation and strong intermolecular π-
stacking of pyrene itself,1a well-designed pyrene-based
thienoacenes should be able to fit the structural requirments
of organic semiconductors.
Herein, we present the syntheses, crystal structures, and
photophysical properties of three isomeric PTAs and their
precursors, which are greatly dependent on the position of
substitution on the pyrene skeleton.
Received: October 15, 2013
© XXXX American Chemical Society
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Scheme 1. Synthetic Approach to Isomers of PTAs
2-S. Oxidation and then acid-induced cyclization of 8 worked
smoothly to afford 2-S-PTA as expected.
pyrenyl moieties, the bond length of PTAs are reduced more or
less than those of S-X-S, for example, 1.493 vs 1.459 Å (S-1-S
vs 1-S-PTA), 1.488 vs 1.476 Å (S-2-S vs 2-S-PTA), and 1.506
vs 1.492 Å (S-4-S vs 4-S-PTA), which indicate the relative less
π−π conjugation of S-X-S than corresponding ring-fused PTAs.
1-S-PTA and 2-S-PTA crystallized in the monoclinic space
groups P21/c and P21, respectively, and 4-S-PTA crystallized in
orthorhombic space group P212121. The PTAs adopt face-to-
face column and herringbone-like structures in the crystals, and
strong intermolecular π−π and C−H···π interactions are
observed. 4-S-PTA possesses the shortest π−π distance of
3.455 Å, and the π−π distance is 3.55 Å for 1-S-PTA and 3.54
Å for 2-S-PTA. In addition, the π−π overlap area of 2-S-PTA is
close to that of 4-S-PTA but larger than that of 1-S-PTA.
The most interesting point is that the photophysical
properties of PTAs and S-X-S clearly indicate that the π−π
conjugation in these pyrene−benzene systems greatly depends
on the position of substitution on the pyrene skeleton.
Generally, the absorption spectra of S-X-S are quite similar to
those of pyrene (Figure S-3, Supporting Information)
maintaining the well-resolved vibronic structures, albeit with
small red-shift (Figure 2). However, the absorption of PTAs is
It should be noted that when 4 or 8 were treated with neat
CF3SO3H as in a reported procedure,16 only low yields of the
desired cyclization products were obtained. This may be due to
the lower activity of position 4 and 2- of pyrene than that of
position 1 and also the possibility of competitive intermolecu-
lar17and intramolecular15 coupling reactions of aromatic methyl
sulfoxide with activated aromatic building blocks to produce
high-molecular weight polymers or cyclization products. To
synthesize 2-S-PTA and 4-S-PTA, predissolving 8 and 4,
respectively, in a solvent such as DCM or chloroform was
necessary.
1
All of the isomers were characterized fully by H NMR,
13C{H} NMR, and high-resolution MS and their structures
were obtained by single-crystal X-ray diffraction. All com-
pounds exhibit good thermal and oxidative stability in air. The
melting points of these PTAs are generally 30−50 °C higher
than those of the corresponding S-X-S, presumably due to the
improved molecular rigidity and enhanced π−π interaction.
Single crystals of the PTAs and their precursor S-X-S were
grown by slow evaporation from solutions. All of the S-X-S
compounds display significantly twisted structures with large
torsion angles of ca. 80° between the pyrene and phenyl rings
(76.40° for S-1-S, 82.58° for S-2-S, and 82.67° for S-4-S).
However, all of the PTAs display nearly planar structures (see
the side view in Figure 1), the torsion angle between the fused
pyrene and phenyl rings is only 1.34° for 1-S-PTA and 4.0° for
2-S-PTA. The 6-membered ring-fused 4-S-PTA has the largest
torsion angle, which is only 6.21°. In addition, comparing the
bond length of the C−C bond connecting the phenyl and
Figure 2. Normalized absorption (a) and fluorescence emission (b)
spectra of PTAs in hexane.
greatly red-shifted compared to S-X-S; i.e., there is a decrease of
the optical band gap as result of the ring-fusion. All of the
absorption spectra of the PTAs have a peak at 334 nm with a
pronounced vibronic progression, which is similar to the S0→S2
transition of pyrene. However, the absorption bands in 350−
500 nm are greatly extended with different structures. Because
both the HOMO and LUMO of unsubstituted pyrene have
nodal planes passing through the 2- and 7-positions, there tends
to be less interaction of these orbitals with substitutents at these
positions in the pyrene derivatives. However, substitution at
positions 1- (or equivalently 3-) and 4- of pyrene allow stronger
interactions with both the HOMO and LUMO. 4-S-PTA has
Figure 1. Molecular structures of 1-S-PTA (a), 2-S-PTA (b), and 4-S-
PTA (c) and packing diagram of 4-S-PTA (d).
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substituents at both C3 and C4, so the absorption band of the
3,4-extended 4-S-PTA is even greatly red-shifted to 480 nm.
This also indicates the band gap of 4-S-PTA is much narrow
than those of other isomers.
crystals by drop-casting of DCM solutions (1.0 mg/mL) on
octadecyltrichlorosilane (OTS)-treated SiO2/Si substrates. The
crystals were several micrometers in width and several tens to
hundreds of millimers in length, as investigated in the optical
microscopic images (Figure S-14, Supporting Information).
Strong and sharp diffraction peaks appeared in the XRD results
(Figure S-15, Supporting Information) were indexed by the
corresponding single crystals, indicating that the molecular
packing of all the ribbon-like crystals is consistent with the
single-crystal structures. In order to inspect the charge
transport property of PTAs, the crystalline ribbons were
employed to prepare bottom-gate, top-contact field-effect
transistors. The cleaning of the SiO2/Si wafers and modification
by OTS were performed according to reported processes.18
The Au source and drain electrodes were fabricated through a
“Au stripe mask”. The measurement of the device was carried
out under ambient conditions. Preliminary experiments show
that single, ribbon-like crystals of 1-S-PTA, 2-S-PTA, and 4-S-
PTA have mobilities of 1.0 × 10−4, 4.0 × 10−3, and 2.1 × 10−3
cm2/V s, respectively. Although the performance of 1-S-PTA is
moderate, the values of 2-S-PTA and 4-S-PTA are comparable
with that of reported fused 4,5-dithienylpyrene.7a
The performance of organic micro-/nanosized ribbon
transistors is strongly affected by the molecular packing in
the ribbons (such as the molecular packing mode, π−π
overlap), the quality of the micro-/nanosized ribbons, and
other factors (such as metal-semiconductor contacts, etc). The
longest π−π stacking distance and the smallest π−π overlap is
likely responsible for the lowest performance of 1-S-PTA
among these PTA isomers. The mobilities of 2-S-PTA and 4-S-
PTA are in the same order of magnitude with a little variation,
which might be ascribed to the difference in the quality of the
ribbons (Figure S-14, Supporting Information). Improved
device performance is expected by tuning the crystal growth
condition and optimizing the updated device design.
TD-DFT-calculated singlet excitations (B3LYP and CAM-
B3LYP/6-311G(d), vacuum) were used to rationalize the
absorption spectra (Figures S-37−S-39 and Tables S-4−S-9,
Supporting Information). Both functionals predict the same
order of the lowest excited states with similar orbital
descriptions but different energies and oscillator strengths.
The red-shift of 4-S-PTA relative to 1- and 2-S-PTA is due to
destabilization (ca. 0.3 eV) of the HOMO by the donor S atom
at the 4-position. 1-S-PTA has a higher lying LUMO (by ca. 0.2
eV) than the other isomers, due to reduced conjugation
between the pyrene and phenyl rings bonded through the
pyrene 2-position, shifting its absorption to higher energy.
The fluorescence emission of the PTAs depends on the
excitation wavelength (Figures S-8−S-13 and Table S-3,
Supporting Information). When they are excited at 334 nm,
they exhibit emission similar to that of pyrene; however, when
excited at longer wavelengths, their emission spectra are shown
red-shifted with a clear, pronounced vibronic structure. The
fluorescence lifetimes also indicated there are two different
emissive excited states involved in the 334 nm excitation.
Correspondingly, the quantum yields are also excitation-
dependent. 4-S-PTA possesses the highest quantum yields as
high as 0.60.
As determined by cyclic voltammetry (CV), all these PTAs
show a single reversible oxidation wave at about 0.80 eV vs Ag/
AgCl (Table 1 and Figure 3a).
a
Table 1. Opto-electronic Properties of S-X-S and PTAs.
b
c
f
λabs (nm) (log
λem
Eox
d
e
ε)
(nm)
Φ
τ (ns)
(eV)
S-1-S
338 (4.75)
344 (4.60)
343 (4.67)
365 (4.52)
384 (4.67)
433 (4.16)
373
400
382
406
405
480
0.25
0.20
0.08
0.13
0.10
0.60
234 (78%)
In summary, three approaches to the synthesis of new sulfur-
bridged multisubstituted pyrene derivative pyrene−thienoa-
cenes were successfully developed. Extensive structural and
photoelectronic measurements indicated that cyclization
resulted in greatly enhanced intramolecular π-conjugation and
intermolecular π-stacking, as well as transistor mobility. Among
these regioisomeric PTAs, a [3,4]-extended isomer showed
much more remarkable properties than [1,2]- and [2,1]-
extended isomers. We believe this fundamental research will
provide new insight for further work on sulfur-substituted fused
pyrene derivatives for OFET application. Studies on multi-
substituted and multisulfur containing pyrene−thienoacenes
are underway in our group.
S-2-S
15 (58%)
S-4-S
292 (93%)
1-S-PTA
2-S-PTA
4-S-PTA
6
3
9
0.76
0.80
0.87
a
Photophysical measurements in hexane (5.0 × 10−6 mol·L−1); CV
b
measurements in DCM (1.0 × 10−3 mol·L−1). λabs is the strong
absorption peak appearing at the longest wavelength. λem is the
c
fluorescence peak appearing at the shortest wavelength, with λex = λabs
.
d
e
Determined using pyrene and coumarin 307 as references. Decay
f
curves were recorded at λem with λex = λabs. Values recorded vs Ag/
AgCl.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details, characterization data, DFT calcu-
lations, and CIF files for compounds S-X-S and PTAs.This
material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 3. (a) Cyclic voltammetry spectrum of PTAs in CH2Cl2
solution (1.0 × 10−3 mol·L−1) and (b) typical transfer characteristics of
the transistor of 4-S-PTA.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zqliu@sdu.edu.cn.
*E-mail: lhx@sioc.ac.cn.
Notes
The properties of crystal OFETs of these PTAs were
preliminarily examined. Typical transistor characteristics are
shown in Figure 3b. The PTAs form large-scale ribbon-like
The authors declare no competing financial interest.
C
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ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (20802026, 50803033, and 20972089)
and the Independent Innovation Foundation of Shandong
University is gratefully acknowledged.
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