New sulfur bridged neutral annulenes. Structure, physical properties and applications in organic field-effect transistors

Article abstract of DOI:10.1039/c0cc03974h

New, neutral, meso-substituted tetrathia[22]annulene[2,1,2,1] aromatic macrocyclic architectures display p-type semiconductor behaviour and constitute efficacious molecular field-effect transistors with reproducible bulk-like carrier mobility (as high as

Full text of DOI:10.1039/c0cc03974h

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New sulfur bridged neutral annulenes. Structure, physical properties
and applications in organic field-effect transistorsw
Kamaljit Singh,*a Amit Sharma,a Jing Zhang,b Wei Xub and Daoben Zhub
Received 20th September 2010, Accepted 12th October 2010
DOI: 10.1039/c0cc03974h
New, neutral, meso-substituted tetrathia[22]annulene[2,1,2,1]
aromatic macrocyclic architectures display p-type semiconductor
behaviour and constitute efficacious molecular field-effect tran-
sistors with reproducible bulk-like carrier mobility (as high as
entities have gained significance in biomedicine,4 micro-
electronic devices5 as well as for the development of advanced
materials (e.g., conducting polymers,6 organic field effect
transistors, OFETs)7 etc., and are thus challenging synthetic
targets. Appending substituents on 6 at methine carbons
(meso-positions) appeared particularly attractive to us as it
would provide experimental probes for expanding the scope to
understand features related to structure and aromaticity, and
would allow access to the hitherto under-appreciated evalua-
tion of meso-elaborated cyclic conjugated architectures 10 as
active materials for organic field-effect devices. In that light,
while the McMurry strategy introduced by Vogel8 remains the
method of choice for constructing the annulene ring 6, the
synthesis of meso-aryl substituted building block 7, however,
was conveniently achieved through the known aldehyde–
thiophene condensation route.
0.63 cm2 VÀ1 À1) on highly crystalline thin films deposited on
S
octadecyltrichlorosilane modified SiO2.
Aromaticity of cyclic conjugated chemical entities in terms of
cyclic delocalization of mobile electrons is a distinctive feature
implicated in the understanding of structural, electronic
as well as magnetic properties.1 The neutral alkene linked
thiaannulenes, 1 and 2 represent 18p- and 24p-electron
systems which deviate from planarity and are devoid of any
ring current and are thus neither aromatic nor antiaromatic.2
The conjugation in these apparently conjugated ring systems is
restricted to the thiophene rings only. On the other hand
tetrathiaporphycene 3 is found to be a distorted non-planar
20p system, which in spite of being strained, depicts remarkable
stability.3 Whereas the UV data (lmax 336, 274, 229, 210 nm)
of 3 depicts modest conjugation, the NMR data does not
show any obvious ring current. The (4n + 2)p tetrathia[22]-
annulene[2,1,2,1] 4, which ought to be a planar and aromatic
22p system, depicts NMR data similar to 3 (4np) i.e. without
an obvious ring current. Further, corroborated by the X-ray
evidence, the UV data suggests non-planarity and it shows a
tendency to get reduced electrochemically to the diatropic 22p
dianion.3 The six-thiophene system 5 possesses only partial
conjugation (UV data: lmax 431, 394, 381, 272, 222 nm). This
28p system in its cyclic voltamogram shows two reversible
oxidation peaks (0.51 and 0.78 eV), indicating the formation
The bis(thien-2yl)methane derivatives 7a–d could be con-
veniently di-formylated following a dilithiation–formylation
sequence (Scheme 1).9 The reductive McMurry coupling
of the dialdehydes 8a–c gave the corresponding dihydro-
tetrathiaannulene derivatives 9a–c in high yield. The dehydro-
genation of 9a–c 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 10a–c (see ESIw for experi-
mental details).
1
of 26p dicationic (4n + 2)p species. In its H NMR spectrum
(11.28 and 12.10 ppm), recorded in 98% D2SO4, strong ring
currents were observed and the UV data (98% H2SO4)
depicted bathochromic shifts in comparison to the original
spectrum, showing peaks at 885, 858, 521 nm. The only neutral
tetrathiaannulene possessing alternate methine and ethene
links reported thus far is 6.3 The peripheral aromaticity of 6
was evidenced by its 1H NMR [two singlets at d 12.34 (methine H),
11.36 (ethene H) ppm] and UV-Vis absorption spectra
(417, 503, 540, 579, 771 nm). These cyclic conjugated chemical
Being a 22p macrocycle, annulene 10a may be expected to
be capable of losing two electrons (Scheme 2) to form a 20p
dication. The cyclic voltammogram (Fig. 1) of 10a shows two
reversible oxidation peaks at 552.6 and 968.2 mV (vs. SCE),
respectively, indicating that a 20p dication 11 is formed.
a Organic Synthesis Laboratory, Department of Applied Chemical
Sciences & Technology, Guru Nanak Dev University,
Amritsar-143005, India. E-mail: kamaljit19in@yahoo.co.in;
Fax: (+) 91-183-2258819/20
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; Fax: +86 1062569349
Scheme 1 Synthesis of 10a–c; n-BuLi (4.1 mmol)/THF, 20 1C;
(b) DMF (9.7 mmol); (c) Zn (54 mmol)/TiCl4 (27 mmol), pyridine
(45 mmol), THF, reflux; (d) DDQ (1.05 mmol), hydrazine.
w Electronic supplementary information (ESI) available: Additional
experimental details. CCDC 747545. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc03974h
c
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Chem. Commun., 2011, 47, 905–907 905

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Scheme 2 Electrochemical oxidation of 10a to dicationic 11.
Fig. 2 Crystal structure of 10a. (a) Molecular structure of 10a
showing distances between sulfur atoms in the crystal. (b) Stacking
pattern of 10a in the crystal view along the b-axis (p–p interaction was
observed with an interplanar distance of 3.526 A (red arrow)).
interactions could be observed as the interplanar distance
between the adjacent molecules in one column is 3.526 A.
Such a column stacking and p–p overlapping will provide the
path for electrical transport.
Fig. 1 Cyclic voltammogram (CV) for 10a (DCM, electrolyte TBAPF6;
working electrode: Pt; reference electrode: Ag/AgCl; 80 mV sÀ1
(for CV of 10b and 10c, see Fig. S1 and S2, ESIw).
The aromaticity of 10a was further evidenced by its
l
1H NMR and UV-visible absorbance spectra. In the H NMR
Crystals of 10a suitable for an X-ray structure determination
were obtained from a methylene chloride–toluene solution of
the purified product. The compound crystallizes in a mono-
clinic unit cell and belongs to the C2/m space group with
unit cell parameters a = 10.8683, b = 20.9734, c = 5.9459 A,
b = 98.7021. The four sulfur atoms of the macrocycle are in
the same plane (Fig. 2), as are the thiophene rings, unlike the
[18]annulene analogue 1 in which the three thiophene units are
totally out of plane. Of the two sets of sulfur atoms, the ones
intercepted by four-carbon atoms are 3.110 A distance apart,
while the distance between the neighbouring sulfur atoms
separated by a three-carbon bridge is 3.018 A. These distances
are marginally shorter than twice the van der Waals radius of
sulfur (3.60 A). The distance between the opposite sulfur
atoms is 4.333 A. Since the compound is purple and is planar,
as also indicated by the torsion angles (01) between all sulfur
atoms, 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, very markedly showed10
the above bond length relationship. Further, we have found
that the meso-substituents around the annulene core do not
induce any ring puckering of the molecule and/or affect
aromaticity. The molecules of 10a stacked in a face-to-face
pattern along the c-axis into columns. The intermolecular p–p
spectrum, a singlet is observed at 11.08 ppm, corresponding to
the protons on the ethene carbons and an AB system corres-
ponding to the thiophene protons at 10.38 and 10.01 ppm. The
aromatic protons appeared at 8.48 (6H) and 8.00 (4H) ppm. In
contrast, no protons of the dihydro compound appeared below
7.39 ppm. The FAB mass spectrum of 10a was very simple and
showed a strong molecular ion peak (100%) and no fragmenta-
tion. In the UV-visible spectrum, recorded in DCM (Fig. 3),
there are sharp and strong absorption maxima at 429 nm
(e/dm3 molÀ1 cmÀ1 222 860), compared to 9a, and several
weaker absorptions at longer wavelength [519, 557, 600 and
777 nm (e/dm3 molÀ1 cmÀ1 2900, 9200, 57380 and 1700)]. The
first absorption (at 429 nm) is analogous to the typical Soret
band of the porphyrins and porphycenes, while the absorptions
at longer wavelengths are similar to the Q-bands, but with a
bathochromic shift with respect to porphycene. These spectra
are clearly typical of an aromatic porphyrinoid macrocycle.
Fig. 3 UV/VIS spectra of 10a (DCM and H2SO4) and 9a in DCM.
c
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Table 1 Detailed performance of OFETs based on thin films of 10a–c
on OTS-treated SiO2/Si substrates with different substrate tempera-
at the meso-position significantly alters the stacking pattern in
the solid state and improves the electrical transport properties
of these aromatic macrocylic molecules. It has been revealed
that these sulfur bridged annulenes are promising active
materials for OFETs.
tures (Tsub
)
Tsub/1C
ma/cm2 VÀ1 sÀ1
Ion/Ioff
VT/V
10a
10b
10c
25
60
25
60
25
60
0.25 (0.29)
1.58 Â 10À2
1.34 Â 103
1.48 Â 103
3 Â 102
À12.9
À5.11
À7.47
À17.6
À49.0
À50.1
K. S. thanks DST, New Delhi for the research grant
SR/S1/OC-27/2009 and National Single Crystal X-ray Diffrac-
tion Facility, IIT Bombay for X-ray analysis.
0.44 (0.63)
0.28 (0.31À) 3
1.34 Â 103
8.96 Â 105
3.03 Â 104
5.04 Â 10 (9.68 Â 10À3
)
2.45 Â 10À2 (3.16Â10À2
)
Notes and references
a
The maximum values of mobilities are given in parentheses alongside
the average values.
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cycle is less crowded, enabling it to have a planar or near-planar
structure with its p-electrons fully conjugated around the
periphery. Recording of UV-visible spectrum in H2SO4 resulted
in considerable broadening of the peaks, suggestive of forma-
tion of dicationic 20p antiaromatic species 11.
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octadecyltrichlorosilane (OTS) modified or bare SiO2. The
AFM (Fig. S4, ESIw) and XRD (Fig. S5, ESIw) characteri-
zations revealed that, in contrast to 10c, the films of both 10a
and 10b are highly crystalline. The substrate temperature also
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10a, 10b and 10c, respectively, corresponding to d-spacings of
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for 10a, 10b and 10c, Fig. S6, ESIw). Thus, 10a, 10b and 10c,
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The electrical transport properties of thin films of 10a–c
were characterized by FET devices (Fig. S7, ESIw). All of these
materials behaved as p-type organic semiconductors. Data of
device performance (for full data with a variety of substrates,
see Table S1, ESIw) are summarized in Table 1. The highest
performance with a mobility as high as 0.65 cm2 VÀ1 SÀ1 was
observed on thin films of 10b deposited on OTS-modified SiO2
at a substrate temperature of 25 1C. This value is among the
highest of thin film OFETs. Comparing with meso-unsubstituted
tetrathia[22]annulene 6, the meso-substituents (substituted and
unsubstituted phenyl group) significantly improved the
transport properties of these materials. This could presumably
be attributed to the change of molecular stacking pattern of
10a–c w.r.t the meso-unsubstituted counterpart 6,7 as specified
above, in the thin films.
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In conclusion, three new meso-elaborated neutral, aromatic
tetrathia[22]annulene[2,1,2,1] derivatives, derived solely from
thiophene, have been synthesized. Introduction of substituents
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 905–907 907