From thia- to selenadiazoles: Changing interaction priority

Article abstract of DOI:10.1021/ol303490b

The synthesis, optical, and electrochemical properties as well as solid-state structures of a series of alkynylated, benzannulated selenadiazoles are reported. This set of compounds is compared to the lighter homologue series, the thiadiazoles. The selena

Full text of DOI:10.1021/ol303490b

ORGANIC
LETTERS
2013
Vol. 15, No. 3
666–669
From Thia- to Selenadiazoles:
Changing Interaction Priority
Benjamin D. Lindner,† Benjamin A. Coombs,† Manuel Schaffroth,† Jens U. Engelhart,†
Olena Tverskoy,† Frank Rominger,† Manuel Hamburger,*,†,‡ and Uwe H. F. Bunz*,†
€
Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, 69120 Heidelberg,
Germany, and innovationLab GmbH, Speyerer Str. 4, 69115 Heidelberg, Germany
uwe.bunz@oci.uni-heidelberg.de; manuel.hamburger@innovationlab.de
Received December 21, 2012
ABSTRACT
The synthesis, optical, and electrochemical properties as well as solid-state structures of a series of alkynylated, benzannulated selenadiazoles
are reported. This set of compounds is compared to the lighter homologue series, the thiadiazoles. The selenadiazoles show head-to-head
dimerization in the solid state, while packing of the thiadiazoles was dominated by the steric bulk of the side groups. The SeꢀN interaction is a
supramolecular motif that should drive the effective self-assembly and modulate charge transport when these compounds are used as thin films in
devices.
There is a general interest in stabilized acenes as poten-
tially useful charge transporting materials,1,2 particularly
thosewithelectron poor aromaticcores.3 Trialkylsilylethy-
nylation (e.g., TIPSA: triisopropylsilylacetylene) dramati-
cally stabilizes acenes and heteroacenes and at the same
time renders their solid state structure predictable.4,5
Mutagenesis of the molecular structure of electron-
poor acenes is critical for rational development of novel
materials with desirable molecular and solid-state
properties.
on both the bulk of the trialkylsilyl side chains and the size
of the heteroaromatic core. Moreover, some acenothiadia-
zole cores dimerize pairwise head-to-head in the solid state
enabled by NꢀS dipole interactions.6,7 This leads to
increased intermolecular overlap in the crystal and might
be favorable for charge-carrier mobility. One way to
further exploit this phenomenon, while keeping the general
structure intact, is the introduction of a more electroposi-
tive chalcogen atom, i.e. selenium or tellurium.8,9 This
should lead to an increase of dipoleꢀdipole interactions
and foster increased head-to-head dimerization.10 Herein
we report the synthesis and the solid-state properties of
Se-containing (aza)diazoles. We demonstrate that SeꢀN
interactions enforce compulsory head-to-head packing in
Extended aceno[2,1,3]thiadiazoles6 as potential repre-
sentatives show different packing arrangements depending
†
€
Universitat Heidelberg.
‡ innovationLab GmbH.
(1) Coombs, B. A.; Rutter, S. R.; Goeta, A. E.; Sparkes, H. A.;
Batsanov, A. S.; Beeby, A. RSC Advances 2012, 2, 1870–1876.
(2) Lindner, B. D.; Engelhart, J. U.; Tverskoy, O.; Appleton, A. J.;
Rominger, F.; Peters, A.; Himmel, H.-J.; Bunz, U. H. F. Angew. Chem.,
Int. Ed. 2011, 50, 8588.
(7) Boudebous, A.; Constable, E. C.; Housecroft, C. E.; Neuburger,
M.; Schaffner, S. Aust. J. Chem. 2008, 61, 755.
(8) Coombs, B. A.; Lindner, B. D.; Edkins, R. M.; Rominger, F.;
(3) Dallos, T.; Hamburger, M.; Baumgarten, M. Org. Lett. 2011, 13,
1936.
Beeby, A.; Bunz, U. H. F. New J. Chem. 2012, 36, 550.
€
(9) (a) Bleiholder, C.; Werz, D. B.; Koppel, H.; Gleiter, R. J. Am.
(4) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15.
(5) Anthony, J. E. Chem. Rev. 2006, 106, 5028.
(6) Appleton, A. L.; Miao, S.; Bromosz, S. M.; Berger, S. J.; Barlow,
S.; Marder, S. R.; Lawrence, B. M.; Hardcastle, K. I.; Bunz, U. H. F.
Org. Lett. 2009, 11, 5222.
Chem. Soc. 2006, 128, 2666. (b) Werz, D. B.; Gleiter, R.; Rominger, F.
J. Am. Chem. Soc. 2002, 124, 10638.
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khani, A. H. J. Am. Chem. Soc. 2005, 127, 3184. (b) Cozzolino, A. F.;
Vargas-Baca, I. Crys. Growth Des. 2011, 11, 668.
r
10.1021/ol303490b
Published on Web 01/15/2013
2013 American Chemical Society

selenadiazoles, while TIPS-alkyne groups in these compounds
lead to a brick wall motif packing of the aromatic cores.
Information (SI)). This is consistent with the calculated
frontier molecular orbital (FMO) energies,16 which also
explain the blue-shifted absorption of 4c compared to 3c
(see SI).
Scheme 1. General Scheme Describing Preparation of
Trialkylsilyl-Substituted Materials 1ꢀ4
Figure 1. Absorption (solid) and emission (dashed) spectra of
2cꢀ4c in hexane (top); thin film absorption spectra (bottom) of
TIPS- (2cꢀ4c; solid) and TES-substituted 2b and 3b (dashed).
Vertical lines illustrate the red shift of 2cꢀ4c in the solid state.
Spectra of 1 omitted for clarity (see SI).
The synthesis of 1ꢀ4 was carried out from the respective
known aromatic diamines.2,6,8,11ꢀ13 By condensation with
SeO2 based on a procedure from Miyashi et al.,14 com-
pounds 1ꢀ4 could be obtained in yields up to 95%
(Scheme 1). Attempts to apply this procedure to TeO2 or
TeCl4 failed due to the apparent instability of the hetero-
cyclic products. As an increasing number of electronega-
tive heteroatoms within the aromatic core should stabilize
the corresponding radical anion against oxidation,15 we
also prepared and included phenazino-selenadiazole 4c in
this study.
The UVꢀvis absorption and fluorescence spectra of2ꢀ4
are displayed in Figure 1. As expected, benzannulation
leads to a bathochromically shifted absorption and emis-
sion with negligible influence of the alkyl substituent in the
silylalkynyl group. The spectral features of the selenium
compounds are significantly shifted to longer wavelengths
relative to those of the sulfur congeners, up to 80 nm, depend-
ing on the chalcogenadiazole core size (see Supporting
The thin film absorption spectra (Figure 1, bottom)
show a pronounced red shift of the absorption edge and
broadened fine structure, which points to coplanar πꢀπ
aggregation in the solid state. While there is little difference
between 2b and 2c, the stronger red shift of 3c compared to
TES-alkynylated 3b points to optimized packing for the
bulkier side chain as has been discussed for pentacenes.4
Emission profiles are narrow with shoulders on the red-
edge of the larger compounds’ main emission peaks. Shifts
in the emission maxima mirror the pattern observed in the
absorption profiles with small Stokes shifts, due to the
rigidity of aromatic systems.
Fluorescence quantum yields were measured for materi-
als 1c and 2c, with the larger system displaying a higher
value (Table 1). The emission spectra of the anthraceno-
and phenazino-systems (3c and 4c) display significant
noise, suggesting these materials would exhibit low quan-
tum yields, but consistent data could not be recorded.
Cyclic voltammetry measurements were compared to
quantum chemical calculations to examine the electronic
properties of 1cꢀ4c. The first reduction potentials corre-
latetothe calculatedLUMOlevelsand can be comparedto
a correlation established for azaacenes (see SI).2 Absolute
€
(11) Lindner, B. D.; Engelhart, J. U.; Marken, M.; Tverskoy, O.;
Appleton, A. L.; Rominger, F.; Hardcastle, K. I.; Enders, M.; Bunz,
U. H. F. Chem.;Eur. J. 2012, 18, 4627.
(12) Miao, S.; Brombosz, S. M.; Schleyer, P. v. R.; Wu, J. I.; Barlow,
S.; Marder, S. R.; Hardcastle, K. I.; Bunz, U. H. F. J. Am. Chem. Soc.
2008, 130, 7339.
(13) 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.
(14) Tsubata, Y.; Suzuki, T.; Miyashi, T. J. Org. Chem. 1992, 57,
6749.
(15) Winkler, M.; Houk, K. N. J. Am. Chem. Soc. 2007, 129, 1805.
(16) FMOs were calculated using Spartan’10 (B3LYP, 6-311þG**).
Org. Lett., Vol. 15, No. 3, 2013
667

Table 1. Photophysical (recorded in hexanes) and Electrochemical (in DCM) Properties Recorded for Compounds 1ꢀ4
observed
S1rS0
cmꢀ1
calculated
red
abs λmax
/
emsn λmax
/
ε/
Stokes shift/
cmꢀ1
PLQY
S1rS0
cmꢀ1a
E1/2
Vb
/
compd
nm
nm
molꢀ1 dm3
Φf ( 0.1
1c
2c
3c
4c
412
586
734
711
480
597
739
724
19 900
21 400
30 700
9 780
3440
310
90
0.04
0.13
ꢀ
24 270
17 060
13 620
14 060
24 650
17 800
13 620
14 100
ꢀ1.58
ꢀ1.26
ꢀ1.12
250
ꢀ
ꢀ0.77/ꢀ1.33
a Spartan10/TDDFT/B3LYP/6-311þG**. b Half-wave reduction potential from cyclic voltammetry experiments (vs Fc/Fcþ). For details, see SI.
˚
values however show an ∼0.4 eV offset between theoreti-
cally (gas phase) and experimentally (DCM solution)
determined LUMO levels.17
shows an intermolecular distance of 3.38 and 4.57 A
(a shorter πꢀπ distance and larger offset).
These solid state π-interactions are consistent with the
above-mentioned red-shifted thin film absorption spectra
for 3 and 4. For the anthracenothiadiazoles, the TMS-
alkynylated compound exhibited a packing structure with
no head-to-head dimerization and coplanar packing only,
while TES-alkynylation led to weak head-to-head dimer-
ization with a large offset.6 This indicates competing
interaction effects arising from the bulky silylated side
chains, the stacking of the aromatic cores, and the inter-
molecular Nꢀchalcogen interactions. Comparison of
TIPSA-anthracenothiadiazolewith3cshowsthatthelatter
effect became much more pronounced with the introduction
of selenium. This is in line with recently published data on
D-A-copolymers featuring chalcogendiazole homologues.19
The SeꢀN interaction was further investigated by gas
phase quantum chemical calculations of the monomer com-
pared to a planar head-to-head dimer (Figure 3). A geometry
optimization starting from a linear coplanar arrangement of
the thiadiazole dimers (B3LYP, 6-311þG**) yielded
structures with increasing angles for the larger aromatic
cores, while the selenadiazoles (1a, 2a, 3a) formed strictly
coplanar dimers (Figure 3). In agreement with the crystal
structure, the calculated intermolecular SeꢀN distances
are shorter than the SꢀN distances (see SI).
Almost all of the prepared alkynylated selenadiazoles
show a strong tendency to crystallize,18 and single crystals
suitable for X-ray analysis were obtained for 1a and 2b, as
well as for the TIPSA compound series 1cꢀ4c (3c and 4c
shown in Figure 2). As envisaged, all crystal structures
showed pronounced head-to-head dimerization in the
solid state with remarkably short intermolecular SeꢀN
˚
distances of 2.87 (1c), 2.87 (2c), 2.82 (3c), and 2.95 A (4c),
respectively, compared to the sum of van der Waals radii
˚
(SeꢀN, 3.45 A). It is worth noting that 3c shows an even
shorter chalcogenꢀN distance than the consanguine an-
˚
thracenothiadiazole (SꢀN, 2.96 A).
The selenadiazoles 1ꢀ2 form planar head-to-head di-
mers. In the case of 3c the molecules π-stack in the solid
˚
state withanoffset of 0.06A withinthedimer and distances
˚
of 3.36 and 3.42 A between the different layers. The dimers
˚
of 4c show an offset of 1.20 A (Figure 2, bottom, left),
apparently a solid state effect of the π-stacking, which
Figure 3. Side view of the calculated dimers of 3a (bottom) and
the corresponding thiadiazole (top).
(17) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan,
G. C. Adv. Mater. 2011, 23, 2367.
Figure 2. Crystal structure of the brickwall motif of 3c (top) and
coplanar NꢀSe and NꢀN short contacts of 3c dimer (bottom,
left). Planar offset for the dimers of 4c (bottom, right). For
clarity hydrogens and some side chains were omitted.
(18) For compound 2b, a ca. 1 ꢁ 1 ꢁ 0.5 cm3 single crystal was
obtained; see SI for a photograph.
(19) Gibson, G. L.; McCormick, T. M.; Seferos, D. S. J. Am. Chem.
Soc. 2012, 134, 539.
668
Org. Lett., Vol. 15, No. 3, 2013

As a measure for electronic coupling, the FMO splitting
was analyzed (Figure 4).20 For the 3a dimer, the HOMO
splitting is neglibly small (ΔEHOMO/HOMO‑1 = 0.02 eV) while
the LUMO and LUMOþ1 differ by ΔELUMO/LUMOþ1 =
0.17 eV (cf. SI). This will translate into large transfer
integrals for electron transport. The splitting for the
anthracenothiadiazole was lower (ΔELUMO/LUMOþ1
=
0.09 eV) which is a consequence of the increased distance
and the tilted arrangement.
To quantify the stabilization ofthe dimers the zero-point
energies (ZPE) were calculated based on their ground state
energies. A stabilization (SCF þ ZPE) of 4.6, 4.8, and 5.2
kcal/mol compared to twice the monomer energy was
obtained for 1a, 2a, and 3a, respectively. For the thiadia-
zoles, the dimer stabilization was only 2ꢀ3 kcal/mol. The
dimer of 2a was 2.3 kcal/mol more stabilized than its
thiadiazole congener, pointing to strong electrostatic
dipoleꢀdipole interactions as a driving force for the observed
solid-state structure.
Figure 4. Graphic representation of the frontier orbitals of
monomer and dimer of 3a compared to the thiadiazole.
The development of new semiconducting materials for
organic electronics raises a need for advanced design
principles for (molecular) electronic properties and a con-
comitant predictability in the short-range ordering of thin
films. The strong tendency of the analyzed selenadiazoles
toaggregate in a head-to-head fashion highlightsa rational
method using coplanar alignment for the molecular en-
gineering of small molecule semiconductors. In fact, the
dimers of 3c may be regarded as and compared to an
octacene in the solid state. The offset of the 4c dimer raises
the question which modifications to the structure will lead to
planar dimerization or might also increase the step height.
The well-understood influence of selenium on FMOs
and the ease of preparation of these compounds
further rationalizes the use of these materials as
the active layer in organic thin film transistors, and
work along these lines is currently being done in our
laboratories.
Acknowledgment. Funding by the BMBF (M.H., grant
number 13N11701), DFG (U.H.F.B., B.L., Grant No.
DFGBu771-1), and the Deutsche Telekom-Stiftung (J.E.)
is gratefully acknowledged.
Supporting Information Available. Detailed synthetic
procedures, spectra, crystal data, and calculation results.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Coropceanu, V.; Cornil, J.; da Silva, D. A.; Olivier, Y.; Silbey,
R.; Bredas, J. L. Chem. Rev. 2007, 107, 926–952.
The authors declare no competing financial interest.
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