Stable 7,14-disubstituted-5,12-dithiapentacenes with quinoidal conjugation

Article abstract of DOI:10.1021/ol5017756

Two 7,14-disubstituted-5,12-dithiapentacenes (1 and 2) with quinoidal conjugation were synthesized. Their ground-state quinoidal structures were proven by X-ray crystallographic analysis. They showed very different electronic and optical properties from those of the corresponding pentacene derivatives with diene conjugation, and their stability was significantly improved. Organic field effect transistors based on solution processed thin films of 1 and 2 exhibited a hole mobility of up to 0.032 cm^-2 V^-1 s ^-1.

Full text of DOI:10.1021/ol5017756

Letter
pubs.acs.org/OrgLett
Stable 7,14-Disubstituted-5,12-Dithiapentacenes with Quinoidal
Conjugation
Qun Ye,† Jingjing Chang,† Xueliang Shi,† Gaole Dai,† Wenhua Zhang,‡ Kuo-Wei Huang,§
and Chunyan Chi*,†
†Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore
‡Institute of Materials Research and Engineering, A*STAR, 3 Research Link, Singapore, 117602, Singapore
§Division of Physical Science and Engineering and KAUST Catalysis Center, King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia
S
* Supporting Information
ABSTRACT: Two 7,14-disubstituted-5,12-dithiapentacenes (1 and
2) with quinoidal conjugation were synthesized. Their ground-state
quinoidal structures were proven by X-ray crystallographic analysis.
They showed very different electronic and optical properties from
those of the corresponding pentacene derivatives with diene
conjugation, and their stability was significantly improved. Organic
field effect transistors based on solution processed thin films of 1 and
2 exhibited a hole mobility of up to 0.032 cm−2 V−1 s−1.
cene-based molecular materials have been demonstrated
to be useful semiconductors and chromophores.1
were successfully synthesized (Scheme 1). Their ground-state
geometric structures, optical properties, and electronic proper-
ties were investigated in detail and compared with their
pentacene counterparts, the bis(triisopropylsilylethynyl)penta-
cene (TIPS-pentacene) 8 and the 6,13-diphenylpentacene
(DPP) 9 (Scheme 1). Their potential as charge transporting
materials was also evaluated in organic field effect transistors
(OFETs).
A
However, one drawback which hampers their general
applications is their intrinsic instability.2 For all acene
molecules, only one aromatic sextet ring (highlighted in
green color in Scheme 1) can be drawn, and thus the diene
character increases with the extension of molecular length,
which results in high reactivity for acenes longer than
anthracene. Typical reactions of acenes related to the diene
conjugation include addition with singlet oxygen to form
endoperoxide and dimerization under light irradiation. Several
strategies have been employed to increase the robustness of
acenes, such as substitution by trialkylsilylethynyl groups,3
electron-withdrawing fluorine atoms,4 cyano groups5 and
carboximide groups.6 Another strategy is to disturb the diene-
type conjugation by fusion of five-membered rings onto the
zigzag edges of acenes.7
Herein, we propose to develop stable heteroacenes by
incorporation of a quinoidal moiety into the acene framework.
Taking pentacene as an example, a p-quinodimethane (p-
QDM) bridged heteropentacenes containing two sulfur,
nitrogen, or oxygen atoms can be drawn with two aromatic
sextet rings (Scheme 1), and thus they could show improved
stability. In addition, the pro-aromatic p-QDM unit can
dramatically change their electronic and optical properties. In
fact, quinoidal π-conjugated systems have drawn increasing
attention recently due to their small energy gap and intrinsic
biradical character,1e,8 which lead to promising applications as
nonlinear optical chromophores9 and ambipolar/n-type semi-
conductors.10 To date, the heteroacenes and their isoelectronic
structures with quinoidal conjugation have been rarely
investigated.11 In this work, we focused on the 5,12-
dithiapentacene framework and two stable derivatives 1 and 2
Synthesis of the 5,12-dithiapentacenes 1 and 2 was outlined
in Scheme 1. Diethyl 2,5-dibromoterephthalate12 underwent a
nucleophilic attack reaction by tert-butylthiophenol to give
compound 4 in 25% yield. The diester 4 was then hydrolyzed
by sodium hydroxide to quantitatively afford the diacid
intermediate 5, which was then converted to the diacyl chloride
6 in the presence of thionyl chloride. The intramolecular
Friedel−Crafts acylation reaction of 6 was carried out with
TiCl4 to generate the key intermediate dithioquinacridone 7.
The overall yield from 4 to 7 was 90%. The attachment of two
tert-butyl groups was necessary to afford sufficient solubility for
7 and also for the subsequent synthesis. The triisopropylsilyl-
ethynyl (TIPSE) and the phenyl substitution groups were
introduced to the 7,14-positions by nucleophilic addition of 7
with the corresponding lithium reagents. Subsequent reduction
of the generated diols with tin(II) chloride afforded the target
compounds 1 and 2 in good yields. Compounds 1 and 2
showed good stability and solubility in normal organic solvents
(e.g., hexane, chloroform), and their structures were confirmed
by NMR, mass spectrometry, elemental analysis (EA), and X-
ray crystallographic analysis (vide infra).
Received: June 19, 2014
Published: July 14, 2014
© 2014 American Chemical Society
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Letter
perpendicular to the central dithiapentacene plane, which
limited the effective π-extension along the phenyl groups (vide
infra). For comparison, the absorption maxima of the TIPS-
pentacene (8) and DPP (9) are located at 644 and 603 nm,
respectively.3b,13 It is worthy to note that, for both compounds
1 and 2, they have superior stability in solution compared with
the pentacene derivatives 8 and 9. The chloroform solution of 1
was stored at ambient conditions for over 2 months, and no
obvious decomposition was observed. Under the same
conditions, the TIPS-pentacene 8 gradually decomposed within
1 day. The substantially increased stability of these quinoidal
heteroacenes would most likely originate from the disruption of
the normal diene conjugation of acenes and hence interrupting
the decomposition pathways although they have two more
electrons than that of 8 and 9. Compounds 1 and 2 showed
weak emission with the maximum at 692 and 648 nm,
respectively. For comparison, compounds 8 and 9 showed
modest fluorescence in chloroform with the maximum at 648
and 611 nm, respectively. Thus, the quinoidal heteropentacenes
1 and 2 showed much larger Stokes shifts compared to 8 and 9.
The optical energy gaps were determined as 1.88 and 2.09 eV
for 1 and 2, respectively, from the longest absorption
wavelength onset.
Scheme 1. Schematic Representation of Diene Conjugation
in Pentacene and Quinoidal Conjugation in
Heteropentacenes, and Synthetic Route of Compounds 1
and 2
The electrochemical properties of compounds 1 and 2 were
measured by cyclic voltammetry and differential pulse
voltammetry in dry CH2Cl2 (Figure 2 and Table S1 in the
The UV−vis absorption and photoluminescence (PL)
spectra of compounds 1 and 2 were recorded in diluted
chloroform solutions (Figure 1 and Table S1 in the Supporting
Figure 2. Cyclic voltammograms and differential pulse voltammo-
grams of 1 (a) and 2 (b) measured in dry CH2Cl2.
SI). Compound 1 showed two reversible oxidation waves and
one reversible reduction wave, with the half-wave potential
(E1/2) at −0.02, 0.49, and −1.83 V (vs Fc/Fc+, Fc: ferrocene),
respectively. Compound 2 showed two reversible oxidation
waves with E1/2 at −0.24 and 0.23 V, and one irreversible
reduction wave. The HOMO energy levels were determined to
be −4.67 and −4.52 eV for 1 and 2 according to the following
equation: HOMO = −(4.8 + Eoxonset),14 respectively, where
onset
Eox
is the onset potential of the first oxidation wave. For
comparison, the HOMO energy levels of 8 and 9 are −5.16 and
−5.04 eV respectively,13,15 which are 0.5 eV lower than that of
the corresponding quinoidal heteropentacene analogues 1 and
2. The electron-donating nature of the thioether linkage in the
backbone would account for the increased HOMO energy
levels for the 5,12-dithiapentacenes 1 and 2 which have two
more electrons than their counterparts 8 and 9.
Figure 1. UV−vis and PL spectra of compounds 1 and 2 in
chloroform solution (10−5 M for UV−vis absorption measurements
and 10−6 M for PL measurements).
Single crystals suitable for X-ray crystallographic analysis
were obtained for both 1 and 2 by slow diffusion of methanol
to the chloroform solutions (Figure 3). For both compounds,
the 5,12-dithiapentacene backbone is essentially planar. For 2,
the substituted phenyl rings are tilted to the central plane by
about 80° and this explains the observed shorter wavelength of
absorption maximum compared with 1. Similar to pentacene
and heavily substituted heptacene,16 both 1 and 2 pack in a
Information (SI)). The 7,14-bisTIPSE substituted compound 1
mainly absorbed in the region 500−650 nm with the maximum
at 625 nm. For the 7,14-diphenyl-substituted compound 2, the
absorption maximum was hypsochromically shifted to 558 nm.
This could be rationally explained by more extended
conjugation along the TIPSE substitution for compound 1.
For compound 2, the phenyl groups were found to be almost
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Letter
HOMO → LUMO transition at 638.5 nm (oscillator strength f
= 0.8551) for 1 and 546.7 nm (f = 0.8915) for 2 (Figures S2−
S3 in the SI), which is consistent with the experimental data.
To probe the charge transport properties of compounds 1
and 2, thin-film field effect transistors were fabricated by a
solution processing method. Both compounds showed p-type
behavior under negative gate voltage bias in the device, and the
typical transfer and output curves are shown in Figure S4 in the
SI. The devices revealed an average hole mobility of 0.032 cm2
V−1 s−1 and 0.013 cm2 V−1 s−1 in the saturation region for
compounds 1 and 2, respectively, in nitrogen. The current on/
off ratio of 1 and 2 is about 105−106, and the threshold voltage
is around −7 and 0 V, respectively. When operated in air, the
hole mobility declined to 0.01 cm2 V−1 s−1 for 1 and 0.002 cm2
V−1 s−1 for 2 with a large negative threshold voltage shift and
decreased on/off ratio of 103−104. This is due to the low
ionization energy for these two compounds, and the semi-
conductor layer might be easily doped by molecular oxygen
from an ambient environment.18
Atomic force microscope (AFM) and X-ray diffraction
(XRD) techniques were employed to investigate the thin film
morphology, and the results are shown in Figures S5−S6 in the
SI. Compound 1 exhibited well-resolved terrace-like layer-by-
layer morphology in the thin film. The thickness of one layer
was estimated to be 1.8 nm based on the AFM measurement,
which is correlated to the length of 1 along the dithiapentacene
backbone. Compound 2 exhibited similar terrace-like morphol-
ogy with a smaller crystal size. Homogeneity and continuity of
the layers are also envisaged, which are beneficial for charge
transport in the thin film. The XRD diffraction peaks
correspond well to a layer-like lamellar packing motif with a
d spacing value of 1.67 and 1.36 nm for thin films of 1 and 2,
respectively, which was approximately the thickness of one
layer of the terrace. The strong crystallinity of the thin film was
also reflected by the observation of the long-range order of
Bragg diffraction peaks. All these factors made compound 1 and
2 favorable as hole charge transporting materials in the
transistor device.
Figure 3. X-ray crystallographic structures and molecular packing of 1
(a) and 2 (b).
herringbone motif. There is no π−π stacking between the 5,12-
dithiapentacene backbone in both cases, but close contact
between the outmost benzene rings (in 1) or the diphenyl rings
(in 2) and the tert-butyl group were observed (Figure S1 in the
SI). For comparison, TIPS-pentacene 8 packs in a two-
dimensional slipped face-to-face packing motif3a and the DPP 9
packs in columns of cage-like dimers.17 By analyzing the bond
lengths of 1 and 2, both compounds had typical quinoidal type
conjugation (Figure 4). For compound 1, the bond length
alternation is smaller than that of 2, which would be due to the
more extended π-conjugation to the attached ethynyl moieties.
In summary, two 7,14-disubstituted-5,12-dithiapentacenes 1
and 2 were successfully prepared and their ground-state
geometry and physical properties were investigated in detail.
Both compounds have a typical quinoidal structure and packed
in a herringbone motif in the crystal. They showed much
improved stability compared with the corresponding diene-type
conjugated pentacenes although they have much higher lying
HOMO energy levels. They can be reversibly oxidized into the
radical cation and dication forms. Although there is no typical
π−π stacking in the crystalline form, the thin films of 1 and 2
showed moderate field effect hole mobilities due to their
ordered packing and the existence of intermolecular close
contacts. Synthesis of quinoidal heteroacenes with more
extended conjugation is underway in our laboratory.
Figure 4. Selected bond lengths, and calculated HOMO/LUMO
profiles and energy levels of 1 and 2.
Density functional theory (DFT, B3LYP/6-31G**) calcu-
lations were conducted to better understand the electronic and
optical properties of the quinoidal 5,12-dithiapentacenes 1 and
2. For compound 1, the HOMO/LUMO coefficients are
distributed along the whole conjugated framework including
the ethynyl units, while the phenyl rings in 2 are nearly
nonconjugated to the dithiapentacene backbone (Figure 4).
The largest molecular orbital coefficient is located at the p-
QDM unit. The HOMO/LUMO energy levels were calculated
to be −4.44/−2.60 eV for 1 and −4.30/−2.06 eV for 2. Time-
dependent DFT calculations predicted the longest wavelength
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures and characterization data for all new
compounds, DFT calculation details, X-ray crystallographic
data, AFM and XRD data, and more data on device fabrication
and characterization. This material is available free of charge via
the Internet at http://pubs.acs.org.
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C.; Nuckolls, C.; Haley, M. M. J. Am. Chem. Soc. 2012, 134, 10349.
AUTHOR INFORMATION
■
Corresponding Author
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*E-mail: chmcc@nus.edu.sg.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
C.C. acknowledges financial support from the NUS Start-Up
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143-000-510-112 and R-143-000-573-112). K.H. acknowledges
financial support from KAUST.
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