Neutral tetrathia[22]annulene[2.1.2.1] based field-effect transistors: Improved on/off ratio defies ring puckering

Article abstract of DOI:10.1039/c2cc37004b

New, neutral, slightly puckered aromatic meso-substituted tetrathia[22]annulene[2.1.2.1] macrocyclic architectures display p-type semiconductor behaviour and constitute molecular field-effect transistors with high on/off ratios (8.67 × 10⁶) and

Full text of DOI:10.1039/c2cc37004b

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COMMUNICATION
Neutral tetrathia[22]annulene[2.1.2.1] based field-effect transistors:
improved on/off ratio defies ring puckeringw
Kamaljit Singh,*a Tarunpreet Singh Virk,a Jing Zhang,b Wei Xub and Daoben Zhu*b
Received 27th September 2012, Accepted 1st November 2012
DOI: 10.1039/c2cc37004b
New, neutral, slightly puckered aromatic meso-substituted
tetrathia[22]annulene[2.1.2.1] macrocyclic architectures display
p-type semiconductor behaviour and constitute molecular field-
effect transistors with high on/off ratios (8.67 Â 106) and high
mobility (0.23 cm2 VÀ1 sÀ1) in thin films deposited on octadecyl-
trichlorosilane (OTS) modified SiO2.
of 2 and tetracyanoquinodimethane (TCNQ) in a 1 : 1 ratio.
Devices based on such highly ordered structure exhibited
ambipolar behaviour with a stable balance hole mobility of
0.04 cm2 VÀ1 sÀ1 and an electron mobility of 0.03 cm2 VÀ1 sÀ1
in an ambient atmosphere. This ‘‘molecular level heterojunction’’
provided a novel strategy for the construction of ambipolar
devices with great potential for light-emitting transistors, low-
cost devices and complementary metal oxide semiconductor
(CMOS)-like circuits.5
Organic field-effect transistors (OFETs), the critical components
of future organic electronics, attract much interest because of their
attractive applications, such as radio frequency identification tags,
flexible displays, and large-area sensors.1 Considerable effort has
been invested in molecular design to synthesize planar, electron
rich, stable molecules capable of being easily processed into devices
with high hole mobility and switchability. Neutral heteroannulenes
are porphyrin like aromatic macrocycles, which display p-type
transport properties in vacuum deposited thin films. Among these
materials, the meso-p-tolyl substituted tetrathia[22]annulene[2.1.2.1]
1 constituted an efficacious molecular field-effect transistor
and displayed reproducible bulk-like hole mobility (as high as
A characteristic feature of 1–2 (Fig. 1) is the planarity of these
aromatic systems as well as their propensity to lose two electrons to
form dicationic antiaromatic species 6 and 7. The devices based on
1 on OTS modified SiO2 films showed a favourable threshold
voltage (VT = À7.47 to À12.9 V, Tsub 25 1C) but low on/off
(Ion/Ioff) ratios (102 to 103) while the thin film devices of 3 showed a
more positive threshold voltage (ca. 20 V) with low on/off ratios
similar to 1. Thus, given the high mobility values, neutral hetero-
annulenes based on 1–3 could provide alternative and potentially
competitive materials for the next generation of low-cost and
flexible portable electronic devices where traditional semi-
conductors cannot be used. However, the transistor’s electrical
performance was not impressive owing to the low current
on/off ratios which is not suitable for switching applications.
Compared with meso-unsubstituted tetrathia[22]annulene,
the meso-substituents (Ph, p-MePh) in 1 and 2 altered the
molecular stacking pattern and significantly improved the
transport properties of the materials based on 1 and 2 in
the thin films. We wondered if the presence of strongly
electron withdrawing groups on the meso-aryl groups of such
materials would improve the charge transport properties
especially the current on/off ratios of the thin film devices.
In this communication, we report the synthesis as well as OFET
characteristics of new neutral tetrathia[22]annulene[2.1.2.1]s
bearing electronegative groups on the meso-phenyl substituents.
0.63 cm2 VÀ1 À1) in highly crystalline thin films deposited on
s
OTS modified SiO2, while the meso-phenyl substituted one 2
was a moderate electron donor with a HOMO energy level of
À4.88 eV and depicted a hole mobility of 0.29 cm2 VÀ1 sÀ1 in thin
films.2 The meso-substituted oxygen bridged tetraoxa[22]-
annulene[2.1.2.1] 3 depicted both stability as well as a competitive
hole mobility of 0.4 cm2 VÀ1 sÀ1 in thin films.3 Incidentally, these
charge mobility values are among the highest of thin film OFETs.
Further, these values are much higher than the related and most
studied a-oligofurans 4 (mobility: 0.05–0.066 cm2 VÀ1 sÀ1) and
a-oligothiophenes 5 (mobility: 0.09 cm2 VÀ1 À1).4 The superior
s
charge transport of 1–3 is believed to be benefited from the
extended p-conjugation, which lowers the reorganization energy
and enhances intermolecular p–p overlapping. Recently, we
demonstrated5 the feasibility of growing crystals of a self-assembled
mixed-stack neutral donor–acceptor p–p complex from a solution
a Organic Synthesis Laboratory, Department of Applied Chemical Sciences
and Technology, Guru Nanak Dev University, Amritsar 143005, India.
E-mail: kamaljit19in@yahoo.co.in; Fax: +91 2258819/20
b Beijing National Laboratory for Molecular Science, CAS Key
Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100080, P. R. China. E-mail: wxu@iccas.ac.cn
w Electronic supplementary information (ESI) available: Details of
experimental procedures and full characterization. CCDC 900288. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc37004b
Fig. 1 p-Conjugated OFET materials 1–5 and dications 6 and 7.
c
12174 Chem. Commun., 2012, 48, 12174–12176
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distances are marginally shorter than twice the van der Waals
radius of sulfur (3.60 A). The distances between the two sets of
the opposite sulphur atoms are 4.317 and 4.373 A. Like most
of the aromatic porphyrinoids, 11a is purple but is not as
planar as the other meso-aryltetrathia[22]annulene[2.1.2.1]s.
This is also indicated by the torsion angles between all sulphur
atoms (Table S5, ESIw). Electron delocalization was expected
and is attested by the carbon–carbon distances in the
thiophene rings as well as the macrocycle. The thiophene units
show the bond length relation Ca–Cb > Cb–Cb in concurrence
with electron delocalized systems such as porphyrins. It is worth
mentioning that the centrosymmetric tetrathiaporphyrin dication
with four thiophene rings tilted up and down from the mean
molecular plane by 22.81 and 3.71, respectively, showing6 the above
bond length relationship very clearly. Further, we have found that
the m-substituted meso-substituents around the annulene core
induced slight ring puckering of the molecule but did not affect
aromaticity. The molecules of 11a stacked in a shifted face-to-face
pattern into a layer structure along the bc plane (Fig. 2). The
intermolecular p–p interactions could be observed as the inter-
planar distance between the adjacent molecules in one stack is
in the range of 3.4–3.6 A. Such p–p overlapping will provide the
path for electron transport. As can be seen the individual
molecules in the stacking figure are somewhat curved rather
than fully planar, still showing perfect transistor behaviour.
As a quantitative probe of aromaticity, we have calculated
Nucleus-Independent Chemical Shift, NICS(1), values using
ab initio quantum mechanical DFT calculations at the
B3LYP/6-311G(d) level using the Gauge Independent Atomic
Orbitals (GIAO) method.7 The large negative NICS(1) values of
11a (À13.33 ppm) and 11b (À13.32 ppm), calculated at centre of
the molecules, indicate a marginally greater degree of aromaticity
compared to similarly substituted oxa annulenes (ca. À13.17 ppm),
reflecting the enhanced ring current effects. The shapes of the plots
of the chemical shifts as a function of distance of the NICS probe
(bq) from the molecular plane gave a clear indication of occurrence
of diamagnetic ring currents (Fig. S30 and S33, ESIw).
Scheme 1 Synthesis of 11a,b: (a) n-BuLi (2.24 mmol)/DMF
(2.2 mmol); (b) Zn (21 mmol)/TiCl4 (11.0 mmol)/pyridine
(20 mmol)/THF, reflux; (c) DDQ (3.0 mmol)/hydrazine; (d) conc.
H2SO4.
Fig. 2 X-ray crystal structure and stacking pattern of 11a (view along
the a-axis).
The bis(thien-2yl)methane derivatives 8a–b could be con-
veniently di-formylated following a dilithiation–formylation
sequence (Scheme 1). The reductive McMurry coupling of the
dialdehydes 9a–b gave the corresponding dihydrotetrathia-
annulene derivatives 10a–b in high yield. The dehydrogenation
of 10a–b by DDQ occurred immediately in toluene solution to
give insoluble black complexes which were readily reduced by
hydrazine to obtain the crystalline tetrathia-[22]annulene[2.1.2.1]
derivatives 11a–b (S2–S5, ESIw).
Thermogravimetric analysis (TGA) indicated good thermal
stability (Fig. S42 and S43, ESIw), with sublimation occurring
prior to decomposition (>355 1C, 11a and >380 1C, 11b).
Further, 11a and 11b showed exceptional stability in air and
did not require any inert atmosphere conditions for handling.
Crystals of 11a suitable for X-ray structure determination were
obtained from a methylene chloride–toluene solution of the
purified product. The compound crystallizes in an orthorhombic
unit cell and belongs to the Fdd2 space group with unit cell
parameters a = 60.299, b = 19.5214, c = 9.5973 A, a = b =
g = 901. Unlike, meso-phenyltetrathia[22]annulene[2.1.2.1],2 the
four sulfur atoms of the macrocycle are not in the same plane
(Fig. 2), as are the thiophene rings, like the [18]annulene analogue
in which the three thiophene units are totally out of plane.
Two sulphur atoms linked through the meso-bridge are below
the plane of the twisted macrocyclic ring, while the other set
intercepted by a four-carbon bridge is well above the mean
plane of the macrocyclic ring. Out of the four sulfur atoms, the
ones intercepted by four-carbon atoms are 3.092 and 3.118 A
apart, while the neighbouring sulfur atoms separated by a
three-carbon bridge are 3.038 and 3.044 A apart. These
The aromaticity of 11a was further evidenced by its 1H NMR
l
and UV-visible absorbance spectra. In the H NMR (Fig. S23,
ESIw) spectrum, a singlet is observed at 11.09 ppm, corresponding
to the protons on the ethene carbons, and an AB system
corresponding to the thiophene protons at 10.40 and 9.99 ppm.
The aromatic protons appeared at 8.46 (2H), 8.35 (2H) and
7.95 (4H) ppm. In contrast, no protons of the dihydro compound
10a appeared above 7.25 ppm (for NMR data of 11b, see
Fig. S26, ESIw). The LCMS spectrum of 11a showed a strong
molecular ion peak and minimal fragmentation.
UV-visible absorption spectra recorded in DCM showed a
sharp and strong absorption maximum for 11a at 430 nm (e =
271 514 dm3 molÀ1 cmÀ1), compared to 10a (Fig. 3a), and
several weaker absorptions at wavelengths [384, 470, 513, 552,
598 and 774 nm (e = 36 585, 3571, 4285, 15 428, 70 942,
2157 dm3 molÀ1 cmÀ1, respectively)]. The unsplit Soret band
at 430 nm of the 11a is analogous to the split Soret band at
415 and 433 nm of the low (C2) symmetry 3 while the Q-type
bands appeared to be slightly red shifted than porphycenes.8
In analogy with 3,3 the 22p annulenes 11a and 11b were found to
dissolve in sulfuric acid to give a reddish-orange solution, indicative
of formation of the dication 12a and 12b, respectively
Fig. 3 (a) UV-vis spectra of 10a (DCM), 11a (DCM) and 12a
(H2SO4); (b) CV for 11a.
c
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Table 1 Detailed performance of OFETs based on 11a, 11b thin films
Compound
Ts/1C
m/cmÀ2 VÀ1 sÀ1
Ion/Ioff
VT/V
11a
20
60
100
20
60
100
2.73 Â 10À2
7.17 Â 10À2
0.20 (0.23À)a4
1.42 Â 10
2.82 Â 105
8.67 Â 106
5.27 Â 105
1.83 Â 105
1.28 Â 105
2.42 Â 105
À17.5
À23.4
À17.7
À21.3
À42.6
À56.1
11b
1.83 Â 10À3
9.60 Â 10À3
(0.012)a
Fig. 4 AFM images (2 mm  2 mm) of 50 nm thick films of 11a: (a)
deposited on the OTS/SiO2/Si substrate, Ts = 20 1C; (b) deposited on
the OTS/SiO2/Si substrate, Ts = 60 1C; (c) deposited on the OTS/
SiO2/Si substrate, Ts = 100 1C; AFM images (5 mm  5 mm) of 50 nm
thick films of 11b: (d) deposited on the OTS/SiO2/Si substrate, Ts = 20 1C;
(e) deposited on the OTS/SiO2/Si substrate, Ts = 60 1C; (f) deposited
on the OTS/SiO2/Si substrate, Ts = 100 1C.
a
The maximum values of mobility are presented in the brackets
besides the average values.
still rare. Since, the extended p-conjugation contributes to
efficient intermolecular coupling in these solids lacking mole-
cular structural ordering, further modification of the structure
may lead to high performance solution processable materials.
The performance of the OFETs is summarized in Table 1. The
mobilities (m) were calculated in the saturation regime by the
equation: ID = mCi(W/2L)(VG À VT)2, where ID is the drain
current, m is the field-effect mobility, Ci is the gate dielectric
capacitance, W and L are the channel width and length, respec-
tively, VT is the threshold voltage. 11a deposited at 100 1C
(disappearance of sharp Soret bands and appearance of
broadened absorption bands). Also 12b depicted a consider-
ably positive (10.65 ppm) NICS(1) value [Fig. S36, ESIw]. The
UV-vis data of thin films (Fig. S40a and b, ESIw) showed
good correlation with the solution absorption spectra. Cyclic
voltammogram (CV) of 11a (Fig. 3b) in CH2Cl2 shows two
reversible oxidation peaks at E0ox = 642 and 1097 mV vs. SCE
(calibrated using Fc/Fc+ = 0.40 V vs. SCE) indicating the
formation of 20p dicationic species, while 11b showed oxida-
tion peaks at E0ox = 581 and 983 mV vs. SCE (Fig. S41, ESIw).
This data corresponds to HOMO energies of À5.04 eV
(À4.4 À E0ox vs. SCE) for 11a and À4.98 eV for 11b. Compared
with that of 2, the reduced electron donating ability of 11a and
11b is due to the presence of electron withdrawing Cl and F
atoms on the meso-phenyl group.
exhibited the best OFET performance (0.23 cm2 VÀ1 À1) with a
s
much improved on/off ratio of 5 Â 105. The FET characteristics of
11a and 11b are shown in Fig. S44, ESI.w Further, mobility of
these devices did not change significantly, upon exposure to air for
ten days, although the on/off ratio decreased by one order of
magnitude.
In summary, a facile, high yielding synthesis of new meso
substituted tetrathia[22]annulene[2.1.2.1]s has been described.
Compared to the previously reported neutral annulenes these
materials depicted much improved on/off ratios. Their charge
mobility is much superior to analogous a-oligothiophenes. This
difference serves to highlight the role that neutral tetrathiaannul-
enes can play in the development of potentially useful OFETs.
KS and TSV thank DST, New Delhi, for the Research grant
SR/S1/OC-27/2009 and UGC, New Delhi, for the SAP grant
and the Sophisticated Instruments Centre at IIT Indore for
X-ray structure determination.
The band gap of 11a and 11b was estimated to be 1.57 eV
from the absorption onsets. The HOMO levels of 11a and 11b
were estimated to be À5.04 and À4.98 eV, respectively, by CV
measurements which are comparable to the HOMO level of
gold (À4.9 eV) suggesting an effective hole mobility between
the electrode and the semiconductor leading to improved
device performance. Visualization of the HOMOs and LUMOs
of 11a and 11b revealed p-delocalization over the annulene rings of
both 11a and 11b supporting the observed aromatic character and
greater contribution of sulphur atoms to HOMOs compared to
LUMOs (Fig. S38 and S39, ESIw).
Notes and references
Morphologies of the thin (50 nm) film of the heteroannulenes
11a and 11b vacuum deposited on OTS treated SiO2/Si wafers
were studied by atomic force microscopy (AFM) (Fig. 4). The
organic semiconductors (top contact bottom gate) were deposited
on the substrate by thermal evaporation under a pressure of 8 Â
10À4 Pa at a deposition rate gradually increased from 0.1 A sÀ1 to
0.4 A sÀ1 up to the first 20 nm and then maintained 0.5 A sÀ1
until the thickness of the film was 50 nm. When the substrate was
at ambient temperature (20 1C), the film deposited on the
OTS-treated SiO2/Si substrate essentially consisted of small
grains. At elevated temperature (60 1C), the grains became
bigger in size and resulted in more ordered film on the OTS/
SiO2/Si substrate. When the substrate temperature was further
increased to 100 1C, the film cracked and lamellar crystalline
grains were observed. However, no obvious diffraction peaks
could be observed in the XRD patterns, which indicated the
amorphous character of the thin films. In fact, small molecule
based amorphous thin films with a relatively high mobility are
1 (a) C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater.,
2002, 14, 99–117; (b) S. R. Forrest, Nature, 2004, 428,
911–918; (c) H. Meng, F. Sun, M. B. Goldfinger, F. Gao,
D. J. Londono, W. J. Marshal, G. S. Blackman, K. D.
Dobbs, E. Dalen and D. E. Keys, J. Am. Chem. Soc., 2006, 128,
9304–9305; (d) D. Cahen and G. Hodes, Adv. Mater., 2002, 14(11),
789–798.
2 (a) K. Singh, A. Sharma, J. Zhang, W. Xu and D. Zhu, Chem.
Commun., 2011, 47, 905–907; (b) T. Zhao, Z. Wei, Y. Song, W. Xu,
W. Hu and D. Zhu, J. Mater. Chem., 2007, 17, 4377–4381.
3 K. Singh, T. S. Virk, J. Zhang, W. Xu and D. Zhu, Chem. Commun.,
2012, 48, 121–123.
4 O. Gidron, A. Dadvand, Y. Sheynin, M. Bendikov and
D. F. Perepichka, Chem. Commun., 2011, 47, 1976–1978.
5 J. Zhang, H. Geng, T. S. Virk, Y. Zhao, J. Tan, C.-a. Di, W. Xu,
K. Singh, W. Hu, Z. Shuai, Y. Liu and D. Zhu, Adv. Mater., 2012,
24, 2603–2607.
6 E. Vogel, P. Rohrig, M. Sicken, B. Knipp, A. Herrmann, M. Pohl,
H. Schmickler and J. Lex, Angew. Chem., Int. Ed. Engl., 1989, 28,
1651–1655.
7 A. Stanger, J. Org. Chem., 2006, 71, 883–893.
8 D. S. Garcia and J. L. Sessler, Chem. Soc. Rev., 2008, 37, 215–232.
c
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This journal is The Royal Society of Chemistry 2012