Synthesis and properties of isomerically pure anthrabisbenzothiophenes

Article abstract of DOI:10.1021/ol202843x

The synthesis of three heptacyclic heteroacenes is described, namely anthra[2,3-b:7,6-b′]bis[1]benzothiophenes (ABBTs). A stepwise sequence of aldol reactions provides regiochemical control, affording only the syn-isomer. The ABBTs are characterized by X-ray crystallography, UV-vis absorption, and emission spectroscopy, as well as cyclic voltammetry. Field effect transistors based on solution-cast thin films of ABBT derivatives exhibit charge-carrier mobilities of as high as 0.013 cm²/(V s).

Full text of DOI:10.1021/ol202843x

ORGANIC
LETTERS
2012
Vol. 14, No. 1
62–65
Synthesis and Properties of Isomerically
Pure Anthrabisbenzothiophenes
Dan Lehnherr,† Rawad Hallani,‡ Robert McDonald,†,§ John E. Anthony,*,‡, and
Rik R. Tykwinski*,^,#
Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2, Canada,
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United
€
States, and Institut fu€r Organische Chemie, Friedrich-Alexander-Universitat, Erlangen-
Nu€rnberg, Henkestrasse 42, 91054 Erlangen, Germany
rik.tykwinski@chemie.uni-erlangen.de; anthony@uky.edu
Received October 21, 2011
ABSTRACT
The synthesis of three heptacyclic heteroacenes is described, namely anthra[2,3-b:7,6-b0]bis[1]benzothiophenes (ABBTs). A stepwise sequence
of aldol reactions provides regiochemical control, affording only the syn-isomer. The ABBTs are characterized by X-ray crystallography, UVꢀvis
absorption, and emission spectroscopy, as well as cyclic voltammetry. Field effect transistors based on solution-cast thin films of ABBT
derivatives exhibit charge-carrier mobilities of as high as 0.013 cm2/(V s).
Over the past decade or so, there have been significant
advances in the development of acene derivatives for
semiconductor applications.1 Studies of these new materi-
als demonstrate remarkable potential for their use in, for
example, organic field effect transistors and photovol-
taics.1 The formation of heteroacenes, through the formal
incorporation of heteroatoms into a hydrocarbon acene
skeleton, allows for tuning of electronic properties, as well
as the realization of new chromophore structures.2
† University of Alberta.
‡ University of Kentucky.
§ X-ray Crystallography Laboratory, Department of Chemistry.
Author to whom correspondence for device fabrication and characteriza-
tion should be addressed.
^
€
Friedrich-Alexander-Universitat.
# Author to whom correspondence for molecule synthesis and character-
ization should be addressed.
(1) (a) Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048. (b) Anthony,
J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483. (c) Bendikov, M.; Wudl,
F.; Perepichka, D. F. Chem. Rev. 2004, 104, 4891–4945. (d) Lehnherr,
D.; Tykwinski, R. R. Aust. J. Chem. 2011, 64, 919–929.
(2) (a) Hinsberg, O. Liebigs Ann. Chem 1901, 319, 257–286. (b) Miao,
Q.; Nguyen, T. Q.; Someya, T.; Blanchet, G. B.; Nuckolls, C. J. Am.
Chem. Soc. 2003, 125, 10284–10287. (c) Payne, M. M.; Delcamp, J. H.;
Parkin, S. R.; Anthony, J. E. Org. Lett. 2004, 6, 1609–1612. (d) Payne,
M. M.; Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2004, 6,
3325–3328. (e) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.;
Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986–4987. (f) Chen, J.;
Kampf, J. W.; Ashe, A. J., III. Organometallics 2008, 27, 3639–3641.
(g) Appleton, A. L.; Brombosz, S. M.; Barlow, S.; Sears, J. S.; Bredas,
J.-L.; Marder, S. R.; Bunz, U. H. F. Nat. Commun. 2010, 1, 91. (h) Niimi,
K.; Shinamura, S.; Osaka, I.; Miyazaki, E.; Takimiya, K. J. Am. Chem.
Soc. 2011, 133, 8732–8739.
Figure 1. Structures of syn-andanti-ADT (1and 2, respectively) and
benzannulated relatives syn- and anti-ABBT (3 and 4, respectively).
Anthradithiophenes (ADTs, 1ꢀ2, Figure 1) are a subclass
of heteroacenes that often show promising semiconducting
properties similar to that of pentacene.3a Furthermore,
r
10.1021/ol202843x
Published on Web 11/29/2011
2011 American Chemical Society

when appropriately functionalized, ADTs normally show
improved stability relative to pentacenes.2e,3b,4 To date,
however, ADT and its derivatives have typically been syn-
thesized and studied as a mixture syn- and anti-isomers
(1 and 2, respectively),3 although the study of isomerically
pure ADTs has recently been achieved in two cases.5,6
Intrigued by the performance of ADTs in semiconduct-
ing devices and challenged by the issue of obtaining iso-
merically pure chromophores that might offer decreased
disorder compared to the isomeric ADT mixtures, we
targeted the synthesis of a new class of ADT analogues,
anthrabis[1]benzothiophenes (ABBTs, Figure 1). The ad-
ditional aromatic ring at each benzothiophene moiety of
the ABBTs results in a slightly bent or curved structure. It
was thus expected that disordered solid-state packing ob-
served for ADTs should be avoided for ABBTs via more
discriminate interactions in the solid, based on both steric
and electronic demands. Similar to ADTs, however, there
are two possible isomers for ABBTs, the syn- andanti-isomers
(3 and 4, respectively). In the present study, we report the sele-
ctive synthesis and properties of syn-ABBTs 5aꢀc(Scheme1).
The synthesis of ABBTs 5aꢀc began with commercially
available thianaphthene-3-carboxaldehyde (6). Protection
of the aldehyde as an acetal using 1,2-ethylene glycol
afforded 7. Lithiation of 7 followed by reaction with ethyl
formate provided the masked dialdehyde 8 in 70% yield. A
2-foldaldol condensation of9with8providedintermediate
10 in 94% yield.7 Global deprotection of the three acetal
groups of 10 was accomplished with catalytic In(OTf)3 (12
mol %) under reflux in acetone. These same conditions
effectedthesubsequent intramolecularaldol condensation,
leading to the one pot formation of quinone 11 in 67%
yield from 10.8 Isolated as a brown solid, quinone 11 is
sparingly soluble in common organic solvents. Neverthe-
less, reaction of 11 with various lithium acetylides could be
accomplished in THF at low temperature and afforded
diols 12aꢀc in moderate yields. Aromatization was accom-
Scheme 1. Synthesis of syn-ABBTs 5aꢀc
plished in the usual way with SnCl2 2H2O in the presence
3
of 10% aq H2SO4, providing ABBTs 5aꢀc in good yield.
(3) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem.
Soc. 1998, 120, 664–672. (b) Subramanian, S.; Park, S. K.; Parkin, S. R.;
Podzorov, V.; Jackson, T. N.; Anthony, J. E. J. Am. Chem. Soc. 2008,
130, 2706–2707. (c) Wang, J.; Liu, K.; Liu, Y.-Y.; Song, C.-L.; Shi, Z.-F.;
Peng, J.-B.; Zhang, H.-L.; Cao, X.-P. Org. Lett. 2009, 11, 2563–2566.
(d) Kim, C.; Huang, P.-Y.; Jhuang, J.-W.; Chen, M.-C.; Ho, J.-C.; Hu,
T.-S.; Yan, J.-Y.; Chen, L.-H.; Lee, G.-H.; Facchetti, A.; Marks, T. J.
Org. Electron. 2010, 11, 1363–1375. (e) Balandier, J.-Y.; Quist, F.; Stas,
S.; Tylleman, B.; Ragoen, C.; Mayence, A.; Bouzakraoui, S.; Cornil, J.;
Geerts, Y. H. Org. Lett. 2011, 13, 548–551. (f) Goetz, K. P.; Li, Z.; Ward,
J. W.; Bougher, C.; Rivnay, J.; Smith, J.; Conrad, B. R.; Parkin, S. R.;
Anthopoulos, T. D.; Salleo, A.; Anthony, J. E.; Jurchescu, O. D. Adv.
Mater. 2011, 23, 3698–3703.
Table 1. Thermal Properties of ABBTs 5aꢀc
DSC decompositiona
compd
TGA Td/ꢀCa
DSC mp/ꢀCa
onset/ꢀC
peak/ꢀC
5a
5b
5c
415
360
380
414
319
242
414
347
345
415
360
355
a Measured under nitrogen atmosphere.
(4) (a) Kaur, I.; Jia, W.; Kopreski, R. P.; Selvarasah, S.; Dokmeci,
M. R.; Pramanik, C.; McGruer, N. E.; Miller, G. P. J. Am. Chem. Soc.
2008, 130, 16274–16286. (b) Northrop, B. H.; Houk, K. N.; Maliakal, A.
Photochem. Photobiol. Sci. 2008, 7, 1463–1468. (c) Li, Y.; Wu, Y.; Liu, P.;
Prostran, Z.; Gardner, S.; Ong, B. S. Chem. Mater. 2007, 19, 418–423. (d)
Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T.
Chem. Mater. 2004, 16, 4980–4986. (e) Ono, K.; Totani, H.; Hiei, T.;
Yoshino, A.; Saito, K.; Eguchi, K.; Tomura, M.; Nishida, J.-I.; Yamashita,
Y. Tetrahedron 2007, 63, 9699–9704.
(5) For examples of the synthesis of isomerically pure ADTs, see:
(a) Li, Z.; Lim, Y.-F.; Kim, J. B.; Parkin, S. R.; Loo, Y.-L.; Malliaras,
G. G.; Anthony, J. E. Chem. Commun. 2011, 47, 7617–7619. (b) Tylleman,
B.; Vande Velde, C. M. L.; Balandier, J.-Y.; Stas, S.; Sergeyev, S.;
Geerts, Y. H. Org. Lett. 2011, 13, 5208–5211.
ABBT 5a has limited solubility in common solvents,
while derivatives 5b,c show much improved solubility
because of the larger trialkylsilyl groups and can be easily
dissolved in solvents such as THF, CH2Cl2, and CHCl3.
The constitution of the trialkylsilyl group also affects the
thermal properties as assessed by differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA,
Table 1). i-Pr3Si-substituted ABBT 5a has the highest
(6) For examples of the synthesis of isomerically pure naphthodithio-
phenes, see: (a) Shinamura, S.; Osaka, I.; Miyazaki, E.; Nakao, A.;
Yamagishi, M.; Takeya, J.; Takimiya, K. J. Am. Chem. Soc. 2011, 133,
5024–5035. (b) Osaka, I.; Abe, T.; Shinamura, S.; Takimiya, K. J. Am.
Chem. Soc. 2011, 133, 6852–6860. (c) Loser, S.; Bruns, C. J.; Miyauchi,
H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.; Marks, T. J. J. Am. Chem.
Soc. 2011, 133, 8142–8145.
(7) While the stereochemistry about the olefins formed from the aldol
condensation has not been assigned, NMR spectroscopic data suggest
only one isomer.
(8) Quinone 11 has been previously synthesized as a mixture of syn-
and anti-isomers using a significantly different approach; see: Reid, W.;
Bender, H. Chem. Ber. 1956, 89, 1574–1577.
Org. Lett., Vol. 14, No. 1, 2012
63

melting point (414 ꢀC), which is followed immediately by
decomposition. i-Bu3Si- and n-Bu3Si-substituted ABBTs
5b and 5c have progressively lower melting points, while
their decomposition points are essentially identical. Finally,
there is good agreement between the decomposition tem-
peratures as measured by TGA and DSC analysis, indicat-
ing that thermal decomposition is accompanied by weight
loss.
UVꢀvis absorption and emission spectra of 5aꢀc were
acquired in CH2Cl2 (Table 2), and maxima are identical
within (1 nm (representative spectra for 5c are shown
in Figure 3).11 Briefly, ABBTs have a λmax = 542 nm,
which is blue-shifted by 13 nm compared to the analogous
TIPS-ADT derivative, measured as a mixture of syn- and
anti-isomers.2c Two weak mid-energy absorption bands
are observed at λ = 442 and 417 nm, while in the UV
region the most significant absorption band is found at λ =
350 nm. The emission spectra of 5aꢀc show λmax,em at
548ꢀ549 nm. In each case, the Stokes shift is very small
(6ꢀ7 nm, 202ꢀ235 cmꢀ1).
The λmax values for 5aꢀc taken from UVꢀvis spectra of
thin films cast from CH2Cl2 all show a red shift versus the
solution-state values. The bathochromic shift is largest for
5a (36 nm), while smaller shifts of 15 nm and 6 nm are
observed for 5c and 5b, respectively. The large red shift
observed for 5a is consistent with the solid-state packing
from X-ray crystallographic analysis in which 5a has
improved π-stacking overlap compared to 5b and 5c.11
Cyclic voltammetry for 5aꢀc establish the redox beha-
vior and electrochemical gap, Egelectro (Table 2).11 Within
the electrochemical window scanned (1.0 to ꢀ1.8 V vs
Ag/AgNO3 electrode), only one oxidation (ca. 0.55 V vs
Fc/Fcþ) and one reduction (ca. ꢀ1.68 V vs Fc/Fcþ) event
are observed for each ABBT derivative, and all are rever-
sible, except forthe signals of 5awhichare quasi-reversible.
The gap calculated from this data for 5aꢀc (2.22ꢀ2.23 eV)
is in good agreement with the optical HOMOꢀLUMO
gaps of 2.21ꢀ2.22 eV estimated from solution-state
UVꢀvis spectroscopy (Table 2).
Field effect transistors (FETs) based on thin films of 5b
and 5c drop-cast from solution (1 wt % from toluene and
chlorobenzene, respectively) provide for an evaluation of
(10) Crystallographic data for 5a (CCDC 849435): C48H54S2Si2,
Mr = 751.21; crystal dimensions (mm) 0.45 ꢁ 0.12 ꢁ 0.11; triclinic
Figure 2. Solid-state packing of (a) 5a, (b) 5b, and (c) 5c; carbon =
gray, silicon = red, sulfur = yellow.
˚
˚
space group P1 (No. 2);a= 11.317(2) A, b= 12.747(3) A, c= 15.369(3) A;
˚
3
˚
R = 75.186(3)ꢀ, β = 78.547(3)ꢀ, γ = 84.000(3)ꢀ; V = 2097.3(8) A ; Z =
2; Fcalcd = 1.190 g cmꢀ3; μ = 0.216 mmꢀ1; λ = 0.71073 A; T = ꢀ100 ꢀC;
˚
2θmax = 51.44ꢀ; total data collected = 15556; R1 = 0.0540 [5019
2
observed reflections with Fo g 2σ(Fo2)]; wR2 = 0.1530 for 469 vari-
ables, 7943 unique reflections, and five restraints; residual electron
Not surprisingly, the shape and size of the pendent
trialkylsilyl groups also impact the solid-state packing of
ABBTs 5aꢀc (Figure 2).9 Single crystals for each com-
pound have been obtained from CH2Cl2 solutions layered
with acetone that were allowed to evaporate at 4 ꢀC.10 In
the solid state, ABBTs 5a and 5c arrange as centrosym-
metric dimeric pairs, with an interplanar spacing between
adjacent molecules of the pair that is nearly identical for
density = 0.415 and ꢀ0.362 e Aꢀ3. The C46AꢀC47A, C46AꢀC48A,
˚
C46BꢀC47B, and C46BꢀC48B distances within the disordered isopro-
˚
pyl group were constrained to be equal (within 0.03 A) to a common
refined value. The C47A C48A and C47B C48B distances were
3 3 3
3 3 3
˚
constrained to be equal (within 0.03 A) during refinement. Crystal-
lographic data for 5b (CCDC 849436): C54H66S2Si2, Mr = 835.37; crys-
tal dimensions (mm) 0.56 ꢁ 0.13 ꢁ 0.10; triclinic space group P1 (No. 2);
˚
˚
˚
a = 11.1453(11) A, b = 16.4944(16) A, c = 26.181(3) A; R =
3
˚
88.4584(13)ꢀ, β = 85.4059(14)ꢀ, γ = 89.7663(ꢀ113)ꢀ; V = 4795.7(8) A ;
Z = 4; Fcalcd = 1.157 g cmꢀ3; μ = 0.196 mm ; λ = 0.71073 A; T =
˚
˚
both structures (3.35ꢀ3.36 A). The spacing that separates
ꢀ100 ꢀC; 2θmax = 51.80ꢀ; total data collected = 18532; R1 = 0.0599
˚
one dimeric pair from its neighbor is 3.40 A for 5a and 3.38
[12672 observed reflections with Fo g 2σ (Fo2)]; wR2 = 0.1527 for
1295 variables, 18532 unique reflections, and 51 restraints; residual
2
˚
A for 5c. For both 5a and 5c, the dimeric pairs then form a
electron density = 0.598 and ꢀ0.602 e Aꢀ3. Distances involving the
˚
1-dimensional (1-D) slipped π-stacking arrangement. The
π-stacked arrays of 5b, on the other hand, afford 1-D
channels with very limited acene overlap at an interplanar
disordered triisobutylsilyl groups were restrained during refinement; see
the Supporting Information. Crystallographic data for 5c (CCDC
849437): C54H66S2Si2, Mr = 835.37; crystal dimensions (mm) 0.49 ꢁ
˚
0.17 ꢁ 0.11; triclinic space group P1 (No. 2); a = 8.9292(4) A, b =
˚
spacing of 3.44 A. This is due to edge-to-face interactions
by neighboring π-stacked channels, similar to the “sandwich
herringbone” motif seen in pentacene semiconductors.
˚
˚
15.2194(6) A, c = 18.4144(8) A; R = 83.0261(5)ꢀ, β = 76.3552(5)ꢀ, γ =
88.7465(5)ꢀ; V = 2413.78(18) A ; Z = 2; Fcalcd = 1.149 g cmꢀ3; μ =
3
˚
0.194 mmꢀ1; λ = 0.71073 A; T = ꢀ100 ꢀC; 2θmax = 52.80ꢀ; total data
˚
collected = 19417; R1 = 0.0451 [7756 observed reflections with Fo2 g 2σ
(Fo2)]; wR2 = 0.1395 for 560 variables and 9850 unique reflections;
ꢀ3
˚
residual electron density = 0.619 and ꢀ0.376 e A
(11) See the Supporting Information for details.
.
(9) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4,
15–18.
64
Org. Lett., Vol. 14, No. 1, 2012

Table 2. Optoelectronic and Electrochemical Properties of ABBT 5aꢀc
λmax λmax λonset λmax,em Stokes
opt
electro
Eg
Eg
Eox
Ered
compd (in CH2Cl2)/nm (film)/nma (film) /nma (in CH2Cl2)/nmb shift/nm (in CH2Cl2)/eVc (in CH2Cl2)/eVd (in CH2Cl2)/Vd (in CH2Cl2)/Vd
5a
5b
5c
542
542
542
578
548
557
613
624
613
548
549
548
6
7
6
2.22
2.21
2.22
2.23
2.23
2.22
0.55 e
0.54 f
0.55 f
ꢀ1.68 e
ꢀ1.69 f
ꢀ1.67 f
a Film from CH2Cl2. b Measured using λexc 504 nm. c The value used as the absorption edge corresponds to the lowest-energy absorption wavelength
with a molar absorption coefficient ε g 1000 L molꢀ1 cmꢀ1 d Cyclic voltammetry was performed in CH2Cl2 solutions containing 0.1 M n-Bu4NPF6 as
.
3
3
supporting electrolyte. The potential values (E) were calculated using the following equation: E = (Epc þ Epa)/2, where Epc and Epa correspond to the
cathodic and anodic peak potentials, respectively. Potentials are referenced to the ferrocene/ferrocenium (Fc/Fcþ) couple used as an internal standard.
All potentials represent a one-electron reduction or oxidation event. e Measured at a scan rate of 200 mV sꢀ1. f Measured at a scan rate of 150 mV sꢀ1
.
3
3
lower, with a maximum of 2.2 ꢁ 10ꢀ5 cm2/(V s). The
different behavior of 5b and 5c roughly correlates with the
lesser degree of electronic interaction suggested by the solid-
state UV-absorption spectrum (i.e., a smaller red shift vs
solution-state data for 5b relative to 5c).11
In summary, a new class of heteroacene chromophores
has been synthesized in isomerically pure form, namely
the anthra[2,3-b:7,6-b’]bis[1]benzothiophenes (syn-ABBTs.
The choice of the ethynyl groups laterally appended to
acene influences the thermal properties, aswell as the solid-
state packing. Finally, FETs fabricated from 5b and 5c
affordmoderate mobilities in the case of5c, consistent with
better π-stacking suggested for this compound through the
UVꢀvis analyses.
Acknowledgment. This work was supported by FAU
€
Figure 3. UVꢀvis spectrum of 5c in CH2Cl2 (235ꢀ585 nm).
Inset: expansion of low energy region of UVꢀvis absorption
spectra (385ꢀ585 nm) and normalized emission spectra of 5c in
CH2Cl2 (510ꢀ710 nm) using λexc = 504 nm.
Erlangen-Nurnberg and the Natural Sciences and Engineer-
ing Research Council of Canada (NSERC). D.L. thanks
NSERC (PGS-D), the Alberta Ingenuity Fund, the Uni-
versity of Alberta, Alberta Heritage, and the Killam Trusts
for scholarship support. J.E.A. and R.H. acknowledge the
Office of Naval Research for support.
the charge transport properties. Because of low solubility
and noncontinuous films, 5a was not tested in FETs. For
ABBTs 5b and 5c, both derivatives formed needle-like
structures with poor coverage of the channel region of
the device as expected from their 1-D π-stacked nature.
ABBT 5c exhibits reasonable hole transport considering
the film morphology, with charge-carrier mobilities ashigh
as 1.3 ꢁ 10ꢀ2 cm2/(V s). Hole mobilities for films of 5b were
Supporting Information Available. Experimental pro-
1
cedures, spectroscopic data, H and 13C NMR spectra
for all new compounds, and CIF files (5aꢀc). This mater-
ial is available free of charge via the Internet at http://
pubs.acs.org.
Org. Lett., Vol. 14, No. 1, 2012
65