Oligothiophene-functionalized benzene and tetrathienoanthracene: Effect of enhanced π-conjugation on optoelectronic properties, self-assembly and device performance

Article abstract of DOI:10.1002/ejoc.201300731

The versatility of fourfold Stille coupling to afford oligothiophene- functionalized tetrathienoanthracene (TTA and benzene derivatives is described. The influence of the larger, more rigid TTA core on the electronic and structural properties was investig

Full text of DOI:10.1002/ejoc.201300731

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FULL PAPER
Semiconductors
A. A. Leitch, K. A. Stobo, B. Hussain,
M. Ghoussoub, S. Ebrahimi-Takalloo,
P. Servati, I. Korobkov,
J. L. Brusso* ................................... 1–11
Oligothiophene-Functionalized Benzene
and Tetrathienoanthracene: Effect of En-
hanced π-Conjugation on Optoelectronic
Properties, Self-Assembly and Device Per-
formance
The tetrathienoanthracene (TTA) frame-
work is explored as a star-shaped core that
extends π-conjugation in two dimensions.
The effect of “expanding” the conjugation
by lengthening the oligothiophene side
chains is demonstrated with enhanced op-
toelectronic properties. Compared with its
more flexible benzene analogue, the TTA
derivatives show smaller band gaps and
tight π-stacked structures.
Keywords: Semiconductors / Pi interac-
tions / Substituent effects / Molecular de-
vices / Conducting materials
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J. L. Brusso et al.
FULL PAPER
DOI: 10.1002/ejoc.201300731
Oligothiophene-Functionalized Benzene and Tetrathienoanthracene: Effect of
Enhanced π-Conjugation on Optoelectronic Properties, Self-Assembly and
Device Performance
Alicea A. Leitch,[a,b] Kimberly A. Stobo,[a,b] Bashir Hussain,[a,b] Mireille Ghoussoub,[c]
Saeedeh Ebrahimi-Takalloo,[c] Peyman Servati,[c] Ilia Korobkov,[a] and Jaclyn L. Brusso*[a,b]
Keywords: Semiconductors / Pi interactions / Substituent effects / Molecular devices / Conducting materials
The versatility of fourfold Stille coupling to afford oligo-
thiophene-functionalized tetrathienoanthracene (TTA) and
benzene derivatives is described. The influence of the larger,
more rigid TTA core on the electronic and structural proper-
ties was investigated. Comparative optical and electrochemi-
cal studies demonstrate a diminishing HOMO–LUMO gap as
the length of the oligothiophene chain increases, with the
rigid TTA compounds exhibiting lower energy gaps than the
more flexible benzene analogues. X-ray crystallography
studies reveal that the thiophene-substituted TTA structure
consists of slipped π-stacks with several close intrastack π–π
interactions; no close contacts are observed for the benzene
derivative due to twisting of the thiophene ligands out of
plane. Consequently, the molecules in the benzene analogue
are isolated from their neighbours. The thiophene and bithio-
phene functionalised TTA compounds were fabricated into
OTFT devices in bottom-contact configuration and exhibited
mobilities of ca. 10–4 cm2 V–1 s–1 and on/off ratios of more than
four orders of magnitude.
With respect to polymeric materials, the majority consist
of one-dimensional (1D) chains in which delocalization oc-
curs through the polymer backbone. Although this facili-
tates efficient transport of charge carriers and excitons
along the chain, it leads to largely anisotropic charge mo-
bility. Although crystallinity in polymer films may be ob-
served in some cases [e.g., regioregular poly(3-hexylthio-
phene)], the inclusion of solubilizing groups can lead to dis-
order in the bulk, resulting in twisting of the polymer back-
bone and subsequent diminishing of the effective conjuga-
tion.[4] This, coupled with the anisotropic charge mobility
commonly observed in 1D conjugated polymers, limits the
total electronic communication. Small molecule OSCs offer
an attractive alternative to polymer-based systems because
well-defined molecular structures may lead to a greater de-
gree of order within the bulk. A particularly important
requirement for the realization of high performance thin
film optoelectronic devices for molecular materials is tight
π-stacking interactions, which provide a pathway for charge
transport. For example, field-effect transistors (FETs) based
on small molecules (e.g., pentacene)[5] generally attain
higher mobilities than polymer-based devices owing to the
crystallinity and, hence, morphology of the OSC. Unfortu-
nately, the stability of higher oligoacenes under ambient
conditions proves problematic for device operation and du-
rability and provides the impetus to develop stable alterna-
tives.[6] To this end, incorporation of heteroatoms into an
acene framework offers an opportunity to not only vary
the physical and electronic properties, but also contribute
Introduction
In recent years, organic semiconductors (OSC) have seen
significant development in part due to their potential use in
optoelectronic device applications. Such interest stems from
the ability to tune the electronic/optical properties, coupled
with the desirable characteristics of plastics (versatility of
chemical synthesis, mechanical flexibility, light weight, sim-
ple and low-cost processing) and, in the case of small mole-
cule OSCs, well-defined molecular structures. These attract-
ive features, which are observed in both polymeric and mo-
lecular materials, are the driving force behind the develop-
ment of new and modified π-conjugated materials.[1,2] How-
ever, in addition to chemical structure, organization of or-
ganic charge-transport materials is of utmost importance
because their electronic properties are critically dependent
on the extent of molecular order.[3] The ability to transport
charge is therefore vital to the implementation of molecular
materials into next-generation electronic devices.
[a] Department of Chemistry, University of Ottawa,
Ottawa, ON K1N 6N5, Canada
E-mail: jbrusso@uottawa.ca
Homepage: http://mysite.science.uottawa.ca/jbrusso/index.html
[b] Centre for Catalysis Research and Innovation, University of
Ottawa,
Ottawa, ON K1N 6N5, Canada
[c] Department of Electrical and Computer Engineering,
University of British Columbia,
Vancouver, BC V6T 1Z4, Canada
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300731.
2
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Oligothiophene Functionalized Benzene and Tetrathienoanthracene
to improved stability. This is accomplished through lower-
ing of the HOMO energy levels thus making the system less
susceptible to air oxidation under ambient conditions.[6] It
is also expected that inclusion of sulfur into an extended π-
conjugated molecular framework will enhance intermo-
lecular interactions, because it possesses a larger atomic or-
bital than carbon. This can significantly improve intermo-
lecular communication in the solid state, which is necessary
to facilitate carrier transport.[1] In that regard, many thi-
enoacene-based small molecule semiconductors have been
developed, and, in fact, have contributed significantly to the
development of high-performance organic field-effect tran-
sistors showing mobilities higher than 1.0 cm2 V–1s–1.[1,7]
To further improve the performance of thienoacene
OSCs, a number of research groups have focused on in-
creasing the dimensionality by extending the π-conjugation
in two dimensions. It is anticipated that this approach will
enhance the degree of intermolecular communication due
to the larger conjugated framework while maintaining the
Figure 1. Star-shaped core moieties that can lead to increased di-
mensionality in OSCs.
In that regard, tetrathienoanthracene [TTA (5c), Fig-
increased stability afforded by the heteroatoms.[8,9] The de- ure 2] represents an ideal structure for use as the core moi-
velopment of “star-shaped” conjugated molecules, in which ety in star-shaped conjugated materials.[9,14–16] Not only
the dimensionality is extended through a central core unit, does it possess a planar, delocalized π-electron system, it
has been shown to afford materials with physical properties has also been shown to form a highly organized 2D thin-
that can be markedly different from their simple, linearly film network and exhibits high thermal stability in air.[14]
conjugated analogues.[10] In recent years, a number of stud- Recently, we reported the preparation and characterization
ies have been carried out in which a variety of core moieties of halogenated (5d) and 5-hexyl-2-thienyl functionalized
have been implemented to increase the dimensionality of (6a) tetrathienoanthracene.[15] It was demonstrated that in-
the OSCs (Figure 1).[11–13] In general, tight π-stacking inter- creasing the effective conjugation of the thienoacene core
actions are observed when substituents are attached to a enhanced the optoelectronic properties and led to a greater
rigid core [e.g., trithienothiophene (3)[13] or dibenzothieno- number of close intermolecular contacts. Continuing with
tetrathiophene (4)[11]].
this approach to “widening” the degree of conjugation and
Figure 2. A family of oligothiophene-functionalized tetrathienoanthracene and benzene derivatives.
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J. L. Brusso et al.
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to investigate the versatility of the synthetic methodology, lated, leading to materials that could then be used for com-
we herein report the preparation and characterization of parative purposes (Scheme 2). Bright-yellow plates of 9a
bithiophene (7a) and terthiophene (8a) functionalized tetra- that were suitable for X-ray work were obtained through
thienoanthracene. The benzene derivatives 9a and 10a were recrystallization from ethanol/dichloroethane mixtures. In
also isolated to compare the effect on the optoelectronic the case of 10a, crystal growth proved difficult due to its
properties and solid-state structure between the rigid TTA limited solubility.
cores to that of uncyclized analogues (i.e., the benzene
core). Furthermore, in addition to augmenting the oligothio-
phene functionality, we also sought to investigate the effect
of increased alkyl chain length. Although extension of the
alkyl chain is expected to have little effect on the molecular
properties, it is anticipated to have an impact on the solid-
state structure as well as affording increased solubility. As
such, the octylated derivatives 5b and 6b were isolated.
Comparative studies by using cyclic voltammetry, UV/Vis
and fluorescence spectroscopy were performed to illustrate
the enhanced optoelectronic properties associated with “ex-
panding” the conjugation of the TTA core. The solid-state
structures of 6b and 9a were studied by X-ray crystallogra-
phy, which revealed an increase in the number of close π–π
contacts for 6b compared with 6a and demonstrated that
Scheme 1. Preparation of alkylated tetrathienoanthracene deriva-
tives. Reagents and conditions: (a) 1,2,4,5-tetrabromobenzene,
tight π-stacking interactions are favoured for rigid cores
over nonrigid systems (i.e., 9a). We also report preliminary
studies of the thin-film FETs of 6a and 7a.
[Pd(PPh3)2Cl2], DMF, 130 °C; (b) FeCl3, MeNO2/CH2Cl2, room
temp.
Results and Discussion
Synthesis
Our initial attempts to prepare oligothiophene-function-
alized TTA focused on fourfold Stille coupling of tetra-
bromobenzene with stannylated oligothiophenes followed
by oxidative cyclodehydrogenation with FeCl3.[14,17] As out-
lined in Scheme 1, this route was effective for alkylated thio-
phene substituted benzene (i.e., in the preparation of 5a and
5b); however, when oligothiophenes such as bithiophene or
terthiophene were used, an uncharacterizable solid was ob-
tained. This may be attributed to the electron-donating
Scheme 2. Preparation of the benzene derivatives 9a and 10a. Rea-
gents and conditions: (a) 1,2,4,5-tetrabromobenzene, [Pd(PPh3)2-
Cl2], DMF, 130 °C.
To overcome the synthetic challenges associated with oxi-
character of the alkylated oligothiophenes, which facilitates dative cyclodehydrogenation of oligothiophene-function-
side reactions of the β-thienyl positions generating a com- alized benzenes, focus shifted to the preparation of the TTA
plex mixture of materials.[17,18] Although oligothiophene- core prior to functionalization. As described previously,[15]
functionalized TTA was not accessible through this syn- 5-hexyl-2-thienyl functionalized TTA can be prepared
thetic procedure, benzene analogues 9a and 10a were iso- through fourfold Stille coupling of 2-tributylstannyl-5-hex-
Scheme 3. Preparation of oligothiophene-functionalized TTA derivatives. Reagents and conditions: (a) [Pd(PPh3)2Cl2], DMF, 130 °C.
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Oligothiophene Functionalized Benzene and Tetrathienoanthracene
ylthiophene (11a) with 5d (Scheme 3). Although byproducts octylated derivatives exhibit more cathodic potentials than
due to incomplete substitution were obtained, they can be their hexylated analogues. Interestingly, the shift in oxi-
removed through hot washes with ethyl acetate followed by dation potential observed upon further increasing the
recrystallization from dichloroethane. To demonstrate the number of thiophene moieties attached to the TTA core
versatility of this approach, fourfold Stille coupling of was much less prevalent; a cathodic shift of only 20 mV was
stannylated bithiophene (13a) and terthiophene (14a) with observed for the oxidation potential of 7a compared with
5d to afford 7a and 8a was carried out. Numerous attempts 6a (cf. ca. 200 mV shift for 5aǞ6a). Furthermore, the CV
were made to afford single crystals of 7a that were suitable of 7a reveals two closely spaced irreversible oxidation waves
for X-ray analysis; however, recrystallization only generated and, upon repeated cycling, a coppery film appeared on the
a microcrystalline solid. The challenges associated with electrode. Although hexyl chains block the reactive sites on
crystal growth may be attributed to the diminished solu- the terminal thiophene moieties, this may be attributed to
bility related to extension of the π-conjugated framework; polymerization at the β-thienyl positions on the oligo-
i.e., the hexyl chains became insufficient to overcome the thiophene arms following the formation of the radical cat-
strong π–π interactions. In the case of 8a, purification from ion species.
byproducts proved problematic; however, following Soxhlet
extractions the desired material was observed in MALDI-
TOF spectra. To address the diminished solubility associ-
ated with larger π-conjugated frameworks, octylated deriva-
tives 5b and 6b were prepared according to the routes out-
lined in Schemes 1 and 3, respectively. We have therefore
established a versatile route to alkylated oligothiophene
functionalized benzene and tetrathienoanthracene. As dem-
onstrated through the preparation of a series of compounds
with differing core moieties, various oligothiophene lengths
and augmented alkyl chains, this approach provides a facile
method with which to modify π-conjugated materials.
The benzene derivatives, 9a and 10a, exhibit similar re-
dox behaviour, with two closely spaced oxidation waves sep-
arated by 80 and 140 mV, respectively, suggesting sequential
formation of a radical cation and a dication. Because the
separation between the first and second oxidation waves re-
flects the thermodynamic stability of the radical cation
(E1/22ox – E1/21ox = –0.059 logKdispr),[19] the greater potential
difference between the first and second oxidation potentials
for 10a implies its radical cation is more thermodynamically
stable than that of the less conjugated derivative 9a. As may
be expected, the rigidity associated with the TTA core leads
to a more stable radical cation 7a·+ (Kdispr = 1.3ϫ10–8)
compared with the benzene derivatives 9a·+ (Kdispr
=
4.4ϫ10–2) and 10a·+ (Kdispr = 5.2ϫ10–3).
Electrochemical, Spectroscopic and Computational Studies
It is interesting to note that similar shifts in oxidation
To probe the influence of extended conjugation through potential were observed for the benzene analogues com-
oligothiophene-functionalization on the optoelectronic pared to the TTA derivatives with the same number of thio-
properties of tetrathienoanthracene and benzene based phene moieties. For example, 6a and 9a are both comprised
compounds, electrochemical and optical spectroscopy stud- of eight thiophene moieties, whereas 7a and 10a contain
ies were performed on solutions of 5b, 6b, 7a, 9a and 10a twelve. For both the benzene and TTA derivatives, a 20 mV
in dichloromethane; the results are summarized in Table 1. shift in oxidation potential was observed upon increasing
Cyclic voltammetry (CV) of 5b and 6b both revealed a sin- the number of thiophene moieties by four (e.g., 6aǞ7a and
gle quasireversible redox process corresponding to the for- 9aǞ10a). Although the shifts in potential are similar, the
mation of the radical cation (see the Supporting Infor- oxidation potentials for the TTA analogues are more cath-
mation). The oxidation potential of 6b is shifted in the cath- odic than the benzene derivatives. Thus, it may be inferred
odic direction by 110 mV with respect to 5b as a result of that the TTA derivatives should exhibit improved stability
the enhanced conjugation length. Similar results were ob- due to lower HOMO energy levels, making the system less
served for the hexylated derivatives 5a and 6a; however, the susceptible to air oxidation under ambient conditions.[6]
Table 1. Theoretical,[a] electrochemical[b] and photophysical[c] properties.
5a[d]
6a[d]
7a
9a
10a
5b
6b
calcd.
EHOMO
Egap
[eV]
[eV]
–5.21
3.35
1.15
–5.12
2.95
0.94
–5.08
2.75
0.92
1.39
–4.89
437
521
2.38
512
–5.11
3.08
1.07
1.15
–5.30
356
447
2.77
494
–5.03
2.79
1.05
1.19
–5.25
395
480
2.58
517
–5.21
3.35
1.02
–5.12
2.95
0.91
calcd.
Eox1 [V]
Eox2 [V]
EHOMO
expt
[eV][e]
–5.33
431
440
2.83
435
–5.11
475
493
2.52
490
–5.13
431
440
2.83
435
–4.98
475
493
2.52
490
abs
λmax [nm]
abs
λedge [nm]
Egap [eV][f]
opt
PL
λmax [nm][g]
Stokes shift [eV]
0.03
0.08
0.42
0.97
0.74
0.03
0.08
[a] DFT/B3LYP/6-31G(d,p) level of theory where R = Me. [b] In CH2Cl2, 0.1 m nBu4NPF6 as supporting electrolyte, referenced to the
Fc/Fc+ couple of ferrocene at +0.48 V vs. SCE.[20] [c] Measurements performed in CH2Cl2. [d] Data taken from ref.[15] and provided here
for comparison. [e] Derived using the following expression: EHOMO = –e(Eox – EFe/Fe+ + 4.8).[21] [f] Calculated from λedge. [g] In CH2Cl2,
upon excitation at 320 nm for 5a and 5b, 380 nm for 6a and 6b, 436 nm for 7a, 356 nm for 9a, and 396 nm for 10a.
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Optical spectroscopy provides further evidence of en- moieties attached to the benzene core. This is consistent
hanced π-conjugation associated with augmented oligo- with an increase in conjugation and is similar to the shifts
thiophene chain length. Comparison of the absorption and observed in the TTA analogues. Furthermore, the broader
emission profiles for the TTA derivatives revealed a and less defined absorption and emission profiles for the
bathochromic shift as the oligothiophene chain length in- benzene analogues compared with the TTA derivatives are
creased (i.e., 55 nm for 5aǞ6a; 28 nm for 6aǞ7a; see attributed to the less rigid structure.[17] These results are
Table 1 and Figure 3), which is indicative of enhanced elec- consistent with DFT geometry optimization calculations
tronic delocalization. Although the absorption and emis- and crystal structure elucidation.
sion profiles for 5a, 6a and 7a are similar, peak broadening
Comparison of the Stokes shifts reveals larger energies
and loss of fine structure are observed upon increasing for the benzene derivatives compared with TTA based com-
oligothiophene chain length. This may be attributed to a pounds; this may also be explained by molecular flexibility.
less rigid structure due to the flexibility of the oligothio- The values obtained for 5a, 5b, 6a and 6b are quite promis-
phene substituents. As expected, the addition of thiophene ing when compared with other fused thienoacenes (e.g.,
moieties leads to a decrease in the optical energy gap, albeit 0.28 eV for pentathienoacene),[23] because low Stokes shifts
to diminishing extents, upon elongating the oligothiophene are indicative of little structural distortion upon excitation.
arms (0.31 eV for 5aǞ6a; 0.13 eV for 6aǞ 7a). Such obser- This strongly suggests that the reorganization energy of the
vations have been noted elsewhere[22] and are consistent charge carrier (polaron or radical cation), which limits the
with the trends observed in the redox potentials and DFT intrinsic mobility of OSCs, will be low. The larger value
calculations (see below). Not surprisingly, the absorption obtained for 7a may be attributed to the instability of the
and emission spectra for 5b and 6b are identical to that excited state, as observed for the radical cation in the CV,
of 5a and 6a, respectively (see the Supporting Information, in addition to the potential flexibility afforded by the longer
Figure S4).
oligothiophene arms.
The overall bathochromic shifts upon functionalization
suggests highly delocalized states with communication be-
tween the core moieties and oligothiophene substituents. To
further explore the delocalized character, density functional
theory (DFT) calculations at the B3LYP level with the 6-
31G(d,p) basis set were performed. The HOMO contours
(see the Supporting Information, Figure S5 and S6) illus-
trate that the electron density is distributed from the core
out to the periphery along the oligothiophene substituents
in both the TTA and benzene derivatives. Furthermore, as
the number of thiophene rings increase, the calculated
HOMO energy level rises as the HOMO–LUMO gap nar-
rows, albeit to diminishing extents, upon elongating the
oligothiophene moiety. This observation is consistent with
the trends observed in the electrochemical and spectro-
scopic behaviour of these systems.
X-ray Crystallography
Because long-range molecular order and π-orbital over-
lap are of paramount importance to obtain high charge car-
rier mobilities in OSCs, the solid-state structures of 6b and
9a were studied by X-ray crystallography. It was recently
demonstrated that extending the conjugation in two dimen-
sions, through the attachment of 5-hexyl-2-thienyl moieties
to a TTA core, favoured close π-stacking interactions,[15]
and contained a greater number of close π–π interactions
Figure 3. Absorption (top) and emission (bottom) spectra of 5a
(green solid line), 6a (blue solid line) and 7a (red solid line) as well
as the benzene derivatives 9a (blue dashed line) and 10a (red dashed
line).
The optical spectra of the benzene derivatives also pro- compared with the protonated derivative 5c. In addition to
vide evidence of increased effective conjugation associated augmenting the oligothiophene functionality, we also
with elongation of the oligothiophene groups. Batho- sought to investigate the influence of increasing the alkyl
chromic shifts of ca. 50 and 23 nm are observed in the ab- chain length. As evidenced by optical spectroscopy, exten-
sorption and emission spectra, respectively, for 10a with re- sion of the alkyl chain has little effect on the molecular
spect to 9a (see Table 1 and Figure 3). Based on the onset properties; however, it was anticipated to have an impact
of UV/Vis absorption, a decrease in the optical energy gap on the solid-state structure. To this end, single crystals of
is associated with an increase in the number of thiophene 6b were analysed by X-ray diffraction. Crystals of 6b belong
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Oligothiophene Functionalized Benzene and Tetrathienoanthracene
to the triclinic space group P-1 and consist of slipped π-
stacks running along the x-direction. Two views of the crys-
tal structure, showing the unit-cell packing and the slipped
π-stacked structure, are provided in Figure 4. Similar to 6a,
these molecules are essentially planar, with the core atoms
showing minimal displacement from the mean plane of the
26-atom TTA core (0.0302 and 0.0213 Å for 6b and 6a,
respectively). Interestingly, replacement of the hexyl chains
with octyl substituents leads to twisting of the thiophene
moieties attached to the TTA core. Whereas the hexylated
derivative exhibits only a slight propeller-like distortion
with relatively similar dihedral angles [4.3(4)° and 7.8(3)°],
the octylated derivative displays two unique thiophene twist
angles from the molecular plane; one significantly twisted
[27.2(1)°] and the second relatively planar with the TTA
core [6.20(1)°]. Furthermore, the octyl substituents in 6b
form an interdigitated network between the slipped π-
stacks, similar to the hexyl chains in 6a. Consequently, the
degree of lateral communication is diminished in both ma-
terials.
Comparison of the slipped π-stacked structures of oct-
ylated versus hexylated thiophene functionalized TTA
demonstrates the influence that larger substituents play on
the packing pattern. It is interesting to note that inclusion
of octyl substituents increased the number of close contacts
within the π-stacked structure compared with the hexylated
derivative 6a.[15] This is attributed to the steeply inclined
slipped π-stacked structure; in order to accommodate the
octyl chains the degree of slippage increases [30.94(1)° cf.
34.34(2)° for 6a]. The larger degree of inclination, quanti-
fied in terms of the slippage angle (a smaller angle value
indicates a higher degree of slippage), leads to a decrease in
the interplanar separation between molecules [3.365(2) Å
for 6a; 3.361(2) Å for 6b].[15,24] Diminishing the interplanar
separation results in an increase in the number of close con-
tacts within the slipped π-stacks for 6b, which are within or
nominally larger than the van der Waals separation,[25] as
shown in Figure 4. The observation of a number of π–π
contacts in 6b may be attributed to the extended π-conju-
gated core; i.e., the potential for π–π interactions is in-
creased due to the large molecular framework. It was there-
fore anticipated that 7a should exhibit an even greater
number of π–π interactions when similar π-stacking is ob-
served; however, despite significant efforts, we were unable
to obtain crystals of 7a that were suitable for single-crystal
X-ray analysis.
Figure 4. Unit cell (top), slipped π-stack (middle) drawings, and the
crystal structure of 6b showing atom numbering and close intermo-
lecular contacts along the π-stack (bottom). In the π-stacked struc-
ture, octyl chains are omitted for clarity. C–CЈ contacts are shown
in blue; S–CЈ are in red.
To shed light on the effect that a fused versus nonfused
core has on supramolecular organization, single crystals of
9a were analysed by X-ray diffraction and found to belong
to the triclinic space group P-1. The bithiophene substitu-
ents are significantly twisted with respect to the benzene
core due to steric interactions with the benzyl protons, and
Band Structure Calculations
As the crystal structures of 6a and 6b illustrate, aug-
limited intermolecular communication is observed (see the mentation of alkyl chain length plays an important struc-
Supporting Information, Figure S7). As a result, the only tural role with respect to the degree of slippage and in-
close contacts belong to the aliphatic substituents. This terplanar separation within the slipped π-stacks. Such dif-
demonstrates the necessity for the fused thienoanthracene ferences in molecular packing will clearly have an impact
core to facilitate strong π–π interactions and overcome on the solid-state properties of these systems. To probe the
alkyl–alkyl interactions.
electronic consequences of these differences, we carried out
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extended Hückel theory (EHT) band structure calculations
The output characteristics of 6a and 7a TFTs are illus-
on the crystal structures of 6a, 6b and 9a (see the Support- trated in Figure 6, whereby the drain current is measured
ing Information, Figure S8). Such calculations have been as a function of drain voltage changing between 0 and
very useful in understanding the electronic structure of or- –36 V for gate biases of –20 to 4 V with a step of 6 V. These
ganic molecular superconductors[26] and thin-film field-ef- figures demonstrate that both devices show a clear “pinch-
fect transistors.[27] The results suggest that 6a and 6b are off” point for transition from linear to saturation regions.
indirect semiconductors, with calculated band gaps of 2.04 In the saturation region, the current slowly increases with
and 2.16 eV, respectively. These values correlate reasonably increasing negative gate voltage, as expected. To probe the
well with the experimentally obtained optical energy gap of effect of solution concentration on the device performance,
2.52 eV. Conversely, the band structure of 9a reveals a direct two different chlorobenzene solutions for both 6a and 7a
semiconductor with a calculated band gap of 2.69 eV, a with concentrations of 8.3 and 6.25 mg/mL were prepared
value that is comparable to the optical energy gap of at 80 °C. From these studies, the best performance was de-
2.77 eV obtained experimentally.
termined to occur at a concentration of 8.3 mg/mL for both
materials; however, the effect appears to be less significant
for 7a than for 6a (see the Supporting Information, Fig-
ure S9).
Device Fabrication and Testing
To investigate the charge transport properties, bottom-
contact configuration thin-film transistors (TFTs) were fab-
ricated by spin-coating chlorobenzene solutions of 6a and
7a onto (100) silicon wafer substrates coated with 100 nm
thermally grown SiO2. Drain and source electrodes were
patterned by electron beam evaporated gold in the form of
interdigitated electrodes, as shown in the inset of Figure 5.
The transfer current-voltage characteristics of 6a and 7a
TFTs are illustrated in Figure 5, with a channel width to
length ratio of 40000:50 measured at a drain source voltage
of –37 V to ensure biasing in the saturation region. As can
be seen, both devices exhibit more than four orders of mag-
nitude on/off ratios. Devices with 7a show a threshold volt-
age of around –6 V in comparison to –4 V for 6a devices.
Whereas the devices exhibit similar current levels, 7a TFTs
show slightly higher currents at high gate biases. Using the
saturation current equation, the field effect mobility of
these devices can be extracted as 5.2ϫ10–4 and
6.0ϫ10–4 cm2V–1s–1 for 6a and 7a, respectively.
Figure 6. Output characteristics of 6a (top) and 7a (bottom) TFTs
for different gate biases, showing a smooth transition from the lin-
ear to saturation regions.
Figure 5. Drain current as a function of gate voltage for 6a and 7a
TFTs with width to length ratio 40000:50, measured at drain-
source voltage of –37 V. Inset illustrates the photomicrograph of
interdigitated source and drain electrodes used in this measure-
ment. Scale bar 50 μm.
The relatively modest mobilities obtained for 6a and 7a
devices are comparable to that of drop-cast TFTs based on
5a (2.5ϫ10–4 cm2V–1s–1; ON/OFF ratio 104)[14] and may be
attributed to the packing of the molecules in thin films.
8
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Oligothiophene Functionalized Benzene and Tetrathienoanthracene
reactions were performed under an atmosphere of dry nitrogen.
Melting points are uncorrected. 1H and 13C NMR spectra were run
in either CDCl3 or C2D2Cl4 solutions at room temp. or 50 °C with
a Bruker Avance II 300 MHz spectrometer, a Bruker Avance
400 MHz spectrometer or a Bruker Avance 500 MHz Wide Bore
spectrometer. Infrared spectra were recorded with a Bomem Hart-
mann & Braun MB-Series FTIR spectrometer using NaCl optics
and a Nujol mull. UV/Vis spectra were measured with a Varian
Cary-100 spectrophotometer and the fluorescence spectra were ob-
tained with a Photon Technology International (PTI) spectrofluo-
Whereas favourable π–π interactions are observed in the
single-crystal structures, these features may not translate to
the thin film. Similar challenges associated with thin-film
morphology have been reported for tetraphenyltetracene
(rubrene), which exhibits a planar structure with strong π–
π interactions and very high mobility in single crystals,[28]
however, fails to show any appreciable mobility in thin films
due to the twisted geometry it adopts.[29] Detailed struc-
tural/morphological analysis of thin films of these com-
pounds is needed to fully elucidate these results. As demon- rimeter. Both UV/Vis and fluorescence spectra were measured on
dichloromethane solutions with a 1 cm precision quartz cuvette.
Elemental analyses were performed by MWH Laboratories, Pho-
enix, AZ, USA.
strated with 5a, these values may be improved by fabricat-
ing devices by using vacuum sublimation methods
(1.9ϫ10–2 cm2V–1s–1; ON/OFF ratio 106).[14]
Electrochemistry: Cyclic voltammetry was performed with a Prince-
ton Applied Research (PAR) VersaSTAT 3 potentiostat/galvanos-
tat/frequency response analyser and V3-Studio electrochemical
software [V 1.0.281 (c) 2008 PAR] employing a glass cell fitted with
platinum electrodes. The measurements were carried out on dichlo-
romethane solutions (dried by distillation over CaH2) containing
Conclusions
The versatility of fourfold Stille coupling to afford a
series of oligothiophene-functionalized benzene and tetra-
thienoanthracene materials has been demonstrated. The 0.1m tetrabutylammonium hexafluorophosphate (Aldrich) as sup-
porting electrolyte with a scan rate of 100 mV/s. The experiments
were referenced to the Fc/Fc+ couple of ferrocene at +0.48 V vs.
SCE.[20]
TTA framework can be used as a convenient building block
to access extended planar delocalized π-electron systems. As
may be expected, solubility becomes problematic as the π-
conjugated framework is extended for the TTA derivatives;
however, these solubility issues may be alleviated through
the incorporation of longer chain and/or swallowtail alkyl
substituents. Nonetheless, 5-hexyl-2-thienyl (6a) and 5Ј-
Crystal Growth: Crystals of 6b that were suitable for X-ray analysis
were grown by slowly cooling a saturated dichloroethane solution.
Crystals of 9a that were suitable for X-ray analysis were grown by
recrystallization from an ethanol/dichloroethane solution.
X-ray Measurements: Crystals of 6b and 9a were mounted on thin
glass fibres using paraffin oil. Data were collected at 200.15 K with
a Bruker AXS KAPPA single-crystal diffractometer equipped with
a sealed Mo tube source (wavelength 0.71073 Å) APEX II CCD
detector. Raw data collection and processing were performed with
the APEX II software package from BRUKER AXS.[33] The struc-
tures were solved by direct methods, completed with difference
Fourier synthesis, and refined with full-matrix least-squares pro-
cedures based on F2 using the SHELXTL suite of programs. Disor-
der was observed in the alkyl chains of both compounds. Further
details are provided in the Supporting Information.
hexyl-2,2Ј-bithien-5-yl
(7a)
functionalized
tetrathi-
enoanthracene, their benzene analogues 9a and 10a, as well
as octylated derivatives 5b and 6b have been characterized
by cyclic voltammetry, UV/Vis and fluorescence spec-
troscopy. These comparative studies illustrate the enhanced
optoelectronic properties associated with “expanding” the
conjugation of the aromatic core. The solid-state structures
of 6b and 9a were studied by X-ray crystallography, which
revealed an increase in the number of close π–π contacts
for 6b compared with that of 6a, and demonstrated that
tight π-stacking interactions are favoured for rigid cores
over nonrigid systems (i.e., 9a). Preliminary OTFT devices
of 6a and 7a in bottom-contact configuration showed un-
Fabrication of Thin-Film Transistors: Thin-film transistors (TFTs)
were fabricated by spin-coating solutions of 6a and 7a in chloro-
benzene on (100) silicon wafer substrates coated with 100 nm ther-
optimized mobilities of ca. 10–4 cm2V–1s–1 and on/off ratios mally grown SiO2. Moderately doped p-type wafer was used as the
gate of the transistor and was contacted by a 200 nm Al film de-
posited on the back of the wafer. Drain and source electrodes were
patterned by electron beam evaporated gold in the form of inter-
digitated electrodes having different channel length and width for
several test devices. Compounds 6a and 7a were dissolved in
chlorobenzene with a typical concentration of 6.25 mg/mL and
stirred on a hotplate at 90 °C for 20 min and 2 h, respectively, be-
fore spin coating. All the samples were spin coated at 2000 rpm for
30 s.
of more than four orders of magnitude. Although these val-
ues are moderate, this may be attributed to the disorder
associated with the spin-coating process at room tempera-
ture. Detailed structural/morphological analysis of thin
films of these compounds may provide a more in-depth un-
derstanding of these results.
Experimental Section
Preparation of 9a: 1,2,4,5-Tetrabromobenzene (0.50 g, 1.27 mmol),
PdCl2 (22 mg, 0.124 mmol), PPh3 (66 mg, 0.252 mmol) and 13a
(4.12 g, 7.64 mmol) were stirred in degassed DMF (0.2 mL) at
140 °C for 16 h. After cooling to room temp., diethyl ether was
added and the crude product (0.98 g, 0.910 mmol, 72%) was fil-
tered and washed with diethyl ether. Recrystallization from EtOAc
afforded small yellow needles; crystals that were suitable for X-ray
crystallography were obtained from ethanol/dichloroethane mix-
tures; m.p. Ͼ107 °C (dec.). 1H NMR (CDCl3, room temp.,
400 MHz): δ = 7.62 (s, 2 H), 6.96 (d, J = 3.7 Hz, 4 H), 6.94 (d, J
General Procedures and Starting Materials: The reagents 1,2,4,5-
tetrabromobenzene, palladium chloride and triphenylphosphane
were obtained commercially and used as received. Compounds 5-
(hexyl)-5Ј-(tributylstannyl)-2,2Ј-bithiophene (13a),[30] 5-(hexyl)-5ЈЈ-
(tributylstannyl)-2,2Ј:5Ј,2ЈЈ-terthiophene (14a),[31] 5d[15] and 5-(oct-
yl)-2-(tributylstannyl)thiophene (11b)[32] were prepared as outlined
in the literature. All solvents were of at least reagent grade; tetra-
hydrofuran (THF) was dried by distillation from sodium, and di-
methylformamide (DMF) was dried with 4 Å molecular sieves. All
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FULL PAPER
= 3.5 Hz, 4 H), 6.85 (d, J = 3.7 Hz, 4 H), 6.64 (d, J = 3.6 Hz, 4 solution of 12b (0.85 g, 0.01 mmol) in CH2Cl2 (40 mL). Caution!
H), 2.76 (t, J = 7.5 Hz, 8 H), 1.65 (m, J = 7.5 Hz, 8 H), 1.41–1.27 Nitromethane is an explosive liquid, which can detonate upon extreme
(m, 24 H), 0.88 (t, J = 6.9 Hz, 12 H) ppm. 13C NMR (CDCl3, heat. Its contact with amines, alkali metals, and strong reducing
room temp., 400 MHz): δ = 145.67, 139.62, 139.01, 134.74, 133.40,
132.83, 128.37, 124.92, 123.59, 123.28, 31.73, 31.72, 30.33, 28.90,
agents should be strictly avoided. After 50 min, methanol (400 mL)
was added and the reaction was stirred for 45 min. The crude prod-
uct was collected by filtration and washed with methanol to give
22.72, 14.24 ppm. IR: ν
= 3064.3 (w), 1501.3 (m), 1239.2 (w),
˜
max
1209.8 (w), 1191.9 (w), 1162.4 (w), 1057.2 (w), 1038.1 (w), 907.2 the crude product (696 mg, 0.817 mmol, 83%). Recrystallization
(w), 867.5 (m), 816.4 (s), 803.4 (s), 793.0 (s), 738.0 (w), 722.9
(w) cm–1. C62H70S8 (1071.72): calcd. C 69.48, H 6.58; found C
69.30, H 6.76.
from ethyl acetate gave 5b as a yellow microcrystalline solid
(320 mg, 38%); m.p. Ͼ167 °C (dec.). 1H NMR (CDCl3, room
temp., 400 MHz): δ = 8.60 (s, 2 H), 7.30 (s, 4 H), 3.00 (t, J = 7.5 Hz,
8 H), 1.83 (m, J = 7.56 Hz, 8 H), 1.49–1.18 (m, 40 H), 0.88 (t, J =
6.7 Hz, 12 H) ppm. 13C NMR (CDCl3, room temp., 400 MHz): δ
= 146.3, 133.3, 133.2, 125.2, 120.2, 118.1, 31.9, 31.6, 30.9, 29.4,
29.3, 29.2, 22.7, 14.2 ppm. C54H74S4 (851.42): calcd. C 76.18, H
8.76; found C 76.42, H 8.78.
Preparation
of
10a:
1,2,4,5-Tetrabromobenzene
(0.21 g,
0.533 mmol), PdCl2 (9 mg, 0.0508 mmol), PPh3 (28 mg,
0.107 mmol) and 14a (2.00 g, 3.22 mmol) were stirred in degassed
DMF (0.1 mL) at 130 °C for 16 h. After cooling to room temp.,
diethyl ether was added and the crude product (0.75 g, 0.538 mmol,
100%) was filtered and washed with diethyl ether. Recrystallization
from DCE afforded an orange microcrystalline powder (0.65 g,
0.467 mmol, 88%); m.p. 196–198 °C. 1H NMR (C2D2Cl4, 50 °C,
300 MHz): δ = 7.71 (s, 2 H), 7.14–7.08 (m, 8 H), 7.05–7.01 (m, 8
H), 6.98 (d, J = 3.8 Hz, 4 H), 6.72 (d, J = 3.6 Hz, 4 H), 2.82 (t, J
= 7.6 Hz, 8 H), 1.72 (m, J = 7.4 Hz, 8 H), 1.49–1.30 (m, 24 H), 0.93
(t, J = 6.7 Hz, 12 H) ppm. 13C NMR (C2D2Cl4, 50 °C, 300 MHz): δ
= 145.78, 139.84, 138.03, 136.89, 135.12, 134.10, 133.10, 132.48,
128.48, 124.77, 124.36, 123.72, 123.54, 123.50, 31.37, 31.31, 30.03,
Preparation of 6b: Compound 5d (0.200 g, 0.278 mmol), PdCl2
(5 mg, 0.028 mmol), PPh3 (15 mg, 0.057 mmol) and 11b (0.812 g,
1.67 mmol) were stirred in degassed DMF (0.1 mL) at 130 °C for
16 h. After cooling to room temp., hexanes were added and the
crude product (0.240 g, 0.204 mmol, 73%) was filtered off and
washed with hexanes. Recrystallization from EtOAc afforded a
bright-yellow microcrystalline solid (196 mg, 0.166 mmol); crystals
that were suitable for X-ray crystallography were obtained from
dichloroethane; m.p. 132–141 °C. 1H NMR (CDCl3, room temp.,
400 MHz): δ = 7.84 (s, 2 H), 7.14 (s, 4 H), 7.00 (s, J = 3.4 Hz, 4
H), 6.65 (d, J = 3.4 Hz, 4 H), 2.81 (t, J = 7.7 Hz, 8 H), 1.73 (m, J
= 7.4 Hz, 8 H), 1.48–1.28 (m, 24 H), 0.91 (t, J = 6.8 Hz, 12 H) ppm.
13C NMR (CDCl3, room temp., 400 MHz): δ = 145.6, 137.0, 135.2,
133.2, 133.0, 124.7, 124.4, 124.0, 117.7, 117.3, 32.0, 31.6, 30.3, 29.5,
29.4, 22.8, 14.2 ppm. C70H82S8 (1179.90): calcd. C 71.25, H 7.00;
found C 71.47, H 6.91.
28.60, 22.40, 13.95 ppm. IR: ν
= 3059.6 (w), 1595.8 (w), 1536.1
˜
max
(w), 1297.8 (w), 1235.1 (w), 1209.8 (w), 1162.5 (w), 1063.5 (w),
1044.4 (m), 1001.9 (w), 909.6 (m), 865.2 (w), 791.7 (s), 737.9 (w),
723.4 (m) cm–1.
Preparation of 7a: Compound 5d (1.00 g, 1.39 mmol), PdCl2
(0.049 g, 0.276 mmol), PPh3 (0.146 g, 0.557 mmol) and 6 (4.51 g,
8.36 mmol) were stirred in degassed DMF (2.4 mL) at 120 °C for
16 h. After cooling to room temp., hexanes were added and the
crude product (1.54 g, 1.10 mmol, 79%) was filtered and washed
with hexanes. The crude material was washed with hot EtOAc and
recrystallized from DCE to afford an orange microcrystalline pow-
der (0.72 g, 0.516 mmol, 37%); m.p. Ͼ214 °C (dec.). 1H NMR
(CDCl3, 50 °C, 500 MHz): δ = 7.11 (s, 2 H), 6.69 (d, J = 2.9 Hz, 4
H), 6.63 (d, J = 3.1 Hz, 4 H), 6.57 (s, 4 H), 6.54 (d, J = 3.2 Hz, 4
H), 6.51 (d, J = 2.7 Hz, 4 H), 2.74 (t, J = 7.9 Hz, 8 H), 1.69 (m, J
= 7.5 Hz, 8 H), 1.47–1.34 (m, 24 H), 0.96 (t, J = 6.9 Hz, 12 H) ppm.
CCDC-939234 (for 6b) and -939235 (for 9a) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Electrochemistry, absorption and emission spectroscopy, crys-
tallography, EHT band structures, DFT calculations, device fabri-
cation and testing, NMR spectra.
IR: ν
= 1547.4 (w), 1530.3 (w), 1496.7 (m), 1304.5 (w), 1181.4
˜
max
(w), 1054.3 (w), 852.37 (m), 785.9 (s), 722.9 (m) cm–1. C78H74S12:
C 67.10, H 5.34; found C 66.90, H 5.13. Given the low solubility
of this compound 13C NMR spectroscopic analysis was not pos-
Acknowledgments
The authors would like to thank Bobak Gholamkhass for assist-
ance with TFT device fabrication and testing. The authors are
grateful to Natural Sciences and Engineering Research Council of
Canada (NSERC) for funding this work and Canada Foundation
for Innovation (CFI) for the instrumentation. A. A. L. acknowl-
edges support through an NSERC postdoctoral fellowship.
1
sible. The identity of this compound was confirmed by H NMR
spectroscopy and elemental analysis.
Preparation
of
12b:
1,2,4,5-Tetrabromobenzene
(0.54 g,
1.37 mmol), PdCl2 (17 mg, 0.096 mmol), PPh3 (50 mg, 0.191 mmol)
and 11b (4.0 g, 8.23 mmol) were stirred in degassed DMF (0.7 mL)
at 140 °C for 16 h. After cooling to room temp., the crude material
was dissolved in hexanes and any remaining solids were filtered off.
After an aqueous work up, the organic phase was concentrated to
afford the crude product as an oil. Crystallization was achieved by
using EtOAc, which afforded colourless needles (0.85 g, 0.99 mmol,
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Received: May 17, 2013
Published Online: ᭿
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