Star-Shaped Conjugated Molecules with Oxa- or Thiadiazole Bithiophene Side Arms

Article abstract of DOI:10.1002/chem.201600984

Star-shaped conjugated molecules, consisting of a benzene central unit symmetrically trisubstituted with either oxa- or thiadiazole bithiophene groups, were synthesized as promising molecules and building blocks for application in (opto)electronics and electrochromic devices. Their optical (E_g(opt)) as well as electrochemical (E_g(electro)) band gaps depended on the type of the side arm and the number of solubilizing alkyl substituents. Oxadiazole derivatives showed E_g(opt) slightly below 3 eV and by 0.2 eV larger than those determined for thiadiazole-based compounds. The presence of alkyl substituents in the arms additionally lowered the band gap. The obtained compounds were efficient electroluminophores in guest/host-type light-emitting diodes. They also showed a strong tendency to self-organize in monolayers deposited on graphite, as evidenced by scanning tunneling microscopy. The structural studies by X-ray scattering revealed the formation of supramolecular columnar stacks in which the molecules were organized. Differences in macroscopic alignment in the specimen indicated variations in the self-assembly mechanism between the molecules. The compounds as trifunctional monomers were electrochemically polymerized to yield the corresponding polymer network. As shown by UV/Vis-NIR spectroelectrochemical studies, these networks exhibited reversible electrochromic behavior both in the oxidation and in the reduction modes.

Full text of DOI:10.1002/chem.201600984

DOI: 10.1002/chem.201600984
Full Paper
&
Host–Guest Systems |Hot Paper|
Star-Shaped Conjugated Molecules with Oxa- or Thiadiazole
Bithiophene Side Arms
Kamil Kotwica,[a] Anastasia S. Kostyuchenko,[b, c] Przemyslaw Data,[d] Tomasz Marszalek,[e]
Lukasz Skorka,[a] Tomasz Jaroch,[f] Sylwia Kacka,[a] Malgorzata Zagorska,[a]
Robert Nowakowski,[f] Andrew P. Monkman,[d] Alexander S. Fisyuk,*[b, c] Wojciech Pisula,*[e, g]
and Adam Pron*[a]
Abstract: Star-shaped conjugated molecules, consisting of
a benzene central unit symmetrically trisubstituted with
either oxa- or thiadiazole bithiophene groups, were synthe-
sized as promising molecules and building blocks for appli-
cation in (opto)electronics and electrochromic devices. Their
optical (Eg(opt)) as well as electrochemical (Eg(electro)) band
gaps depended on the type of the side arm and the number
of solubilizing alkyl substituents. Oxadiazole derivatives
showed Eg(opt) slightly below 3 eV and by 0.2 eV larger than
those determined for thiadiazole-based compounds. The
presence of alkyl substituents in the arms additionally low-
ered the band gap. The obtained compounds were efficient
electroluminophores in guest/host-type light-emitting
diodes. They also showed a strong tendency to self-organize
in monolayers deposited on graphite, as evidenced by scan-
ning tunneling microscopy. The structural studies by X-ray
scattering revealed the formation of supramolecular colum-
nar stacks in which the molecules were organized. Differen-
ces in macroscopic alignment in the specimen indicated var-
iations in the self-assembly mechanism between the mole-
cules. The compounds as trifunctional monomers were elec-
trochemically polymerized to yield the corresponding poly-
mer
network.
As
shown
by
UV/Vis-NIR
spectroelectrochemical studies, these networks exhibited re-
versible electrochromic behavior both in the oxidation and
in the reduction modes.
icists nearly 20 years ago[1] due to distinctly different properties
as compared to their linear analogues (see, for example,
ref. [2]). This involved, among others, improved solution proc-
essability, interesting electrochemistry, and promising electrical
transport properties. These molecules usually consist of an aro-
matic, heteroaromatic or triphenylamine core to which oligo-
thiophene or donor/acceptor-type groups with thiophene
rings are radially attached. Benzene is probably the most pop-
ular aromatic core,[1,3–7] however, more extended cores should
also be mentioned such as truxene.[8,9] Star-shaped molecules
with heterocyclic cores such as triazine are also reported.[1,11,12]
Larger cores containing heterocycles are frequently built from
fused aromatic and heterocyclic rings such as benzodithio-
phene,[13] benzotrithiophene,[14–16] or phenylquinoxaline.[17] Star-
shaped compounds with triphenylamine-type cores, inducing
non- planarity of the molecule, were also synthesized.[18–25] It
should be mentioned that the triazine core can play the role
of an electron accepting unit to which arms with electro-do-
nating groups are attached, whereas, inversely, arms contain-
ing electro-accepting units can be attached to an electro-do-
nating core.[20,22]
Introduction
Star-shaped conjugated molecules containing oligothiophenes
in their side arms began to draw interest of chemists and phys-
[a] K. Kotwica, L. Skorka, S. Kacka, Prof. M. Zagorska, Prof. A. Pron
Faculty of Chemistry, Warsaw University of Technology
Noakowskiego 3, 00664, Warsaw (Poland)
E-mail: apron@ch.pw.edu.pl
[b] A. S. Kostyuchenko, Prof. A. S. Fisyuk
Department of Organic Chemistry, Omsk F. M. Dostoevsky State University
Mira av. 55A, Omsk (Russian Federation)
E-mail: fisyuk@chemomsu.ru
[c] A. S. Kostyuchenko, Prof. A. S. Fisyuk
Laboratory of New Organic Materials, Omsk State Technical University
Mira av. 11, Omsk , 644050 (Russian Federation)
[d] Dr. P. Data, Prof. A. P. Monkman
Physics Department, University of Durham
South Road, Durham , DH1 3LE (United Kingdom)
[e] Dr. T. Marszalek, Prof. W. Pisula
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
E-mail: pisula@mpip-mainz.mpg.de
[f] Dr. T. Jaroch, Prof. R. Nowakowski
Institute of Physical Chemistry, Polish Academy of Sciences
Kasprzaka 44/52, 01224 Warsaw (Poland)
Star-shaped molecules with thienylene (or thienyl) groups
frequently show interesting optical properties including photo-
luminescence of high quantum yield,[19] solvatochromism,[12]
and electrochromism.[26] They are electrochemically active[1,7,11]
and can form a 3D network through electrochemical polymeri-
zation.[9,15,26] Due to good electrical transport properties they
[g] Prof. W. Pisula
Department of Molecular Physics, Faculty of Chemistry
Lodz University of Technology, Zeromskiego 116, 90-924 Lodz (Poland)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201600984.
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can also serve as components of active layers in organic field-
effect transistors.[3,4,8,23] However, the largest effort was direct-
ed towards the application of these semiconductors (especially
those of donor/acceptor character) in bulk heterojunction-type
organic photovoltaic cells.[5,16,17,22,25,27]
number and the position of the solubilizing alkyl substituents.
Capability of self-assembling into highly ordered 2D monolay-
ers is another interesting feature of these new compounds as
well as their molecular topology-dependent 3D supramolecular
organization in extruded fibers. We also show that they can be
applied as electroluminophores in guest/host-type light-emit-
ting diodes (LEDs). The use of star-shaped donor/acceptor mol-
ecules as emitters in LEDs is rare, although it was reported for
star molecules containing oxadiazole and carbazole units in
their arms[28] and in carbazole dendrimers with a fluorinated
phenylene oxadiazole core.[29] Finally, we prove that the new
In this paper we report on new, solution-processable star-
shaped molecules with benzene central core to which donor/
acceptor-type side arms are attached consisting of either oxa-
or thiadiazole bithiophene groups. We demonstrate that the
redox properties of these molecules can be tuned not only by
the chemical nature of the acceptor unit but also by the
Scheme 1. Synthetic route to star-shaped molecules containing either thiadiazole- or oxadiazolebithiophene side arms.
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molecules can serve as trifunctional monomers yielding poly-
mer networks upon their electrochemical polymerization.
These networks show reversible electrochromism both in the
oxidation and in the reduction modes.
Results and Discussion
Synthesis
The star-shaped molecules were prepared from ethyl ester of
alkyl-substituted 2,2’-bithiophene-5-carboxylic acid in three
steps (Scheme 1). In the preparation of the compounds the
procedure described by Kostyuchenko et al.[30] (based on
a modification of the Fiesselmann reaction) was followed. The
ester was converted into the corresponding hydrazide deriva-
tive (1a,b) by heating hydrazine monohydrate at reflux in alco-
hol. Treatment of 1a,b with benzene-1,3,5-tricarboxylic (trimes-
ic) acid chloride yielded alkyl-substituted N’1,N’3,N’5-tris[(3-
decyl-2,2’-bithien-5-yl)carbonyl]benzene-1,3,5-tricarbohydra-
zides (2a,b). These compounds 2a,b were converted into the
final products of the oxadiazole bithiophene series, that is,
2,2’,2’’-benzene-1,3,5-triyltris[5-(2,2’-bithien-5-yl)-1,3,4-oxadia-
zoles], termed in the subsequent text as S*OBT1 and S*OBT2,
in the reaction with phosphorus oxychloride.[31] Treatment of
2a,b with Lawesson’s reagent resulted in their conversion into
2,2’,2’’-benzene-1,3,5-triyltris[5-(2,2’-bithien-5-yl)-1,3,4-thiadia-
zoles], that is, compounds of the thiadiazole bithiophene
series (S*TBT1 and S*TBT2) in good yields. The de-
Figure 1. a) Solution (CH2Cl2 solvent) absorption spectra of S*TBT1, S*TBT2,
S*OBT1 and S*OBT2; b) solution (CH2Cl2 solvent) emission spectra of
S*TBT1, S*TBT2, S*OBT1 and S*OBT2 (lexc =lmax abs).
tailed description of the synthesis of all compounds
described in this paper, together with their spectro-
scopic characterization (NMR, IR) and elemental anal-
ysis can be found in the Supporting Information.
Table 1. UV/Vis and photoluminescence spectroscopic parameters of the studied star-
shaped molecules.
lmax
lmax
lmax
Stokes Quantum Eg(opt)
(absorption,
CH2Cl2 solution) thin film)
[nm]
(absorption, (emission,
shift
CH2Cl2 solution)[a] [nm]
[nm]
yield
[%]
[eV]
Absorption and emission spectroscopy
[nm]
S*OBT1 363
S*OBT2 375
S*TBT1 390
S*TBT2 402
381
376
398
404
453
434(shoulder),449
494
90
59
104
71
33
23
15
14
2.98
2.88
2.77
2.68
Absorption spectra of all four compounds are pre-
sented in Figure 1a whereas their principal optical
parameters are collected in Table 1. All spectra are
characterized by a dominant, unresolved band as-
cribed to the p–p* transition in the conjugated
system. Note that the p–p* bands of the thiadiazole
derivatives (S*TBT1 and S*TBT2) are bathochromical-
473(shoulder), 488
[a] lexc =lmax abs.
ly shifted by 27 nm as compared to the same bands in the cor-
responding oxadiazole compounds, that is, S*OBT1 and
S*OBT2. This is associated with stronger aromaticity of thiadia-
zole as compared to oxadiazole and improved conjugation in
the compounds of the S*TBT series. Aromatic stabilization
energy of thiadiazole is nearly twice higher than that of oxa-
diazole.[32,33] Its stronger aromaticity is also manifested in
higher values of the harmonic oscillator measure of aromaticity
(HOMA) indices[34] calculated for the thiadiazole derivatives as
compared to the corresponding oxadiazole ones.[33,35] One
should additionally note that the position of the p–p* band is
also dependent on the number of alkyl solubilizing groups,
being bathochromically shifted by 12 nm for the derivatives
with two alkyl substituents in each arm. The same effect of
alkyl substituents on molecular planarization (as evidenced
from B3LYP/6-31G* calculations) and on their increase in aro-
maticity (determined by the bond length alternation (BLA) pa-
rameter) was reported for penta-ring linear compounds consti-
tuted of a central tetrazine,[36] thiadiazole[37] or oxadiazole[38,39]
electron accepting group symmetrically disubstituted with
alkyl- or dialkylbithiophenes. As a result, weak positive induc-
tive effect of alkyl substituents leads to lowering of the optical
gap.
Although trisubstituted benzene as the central unit of the
studied star-shaped molecules should assure their good pla-
narity and conjugation, the planarity-sensitive p–p* band in
the spectra of these molecules is hypsochromically shifted by
13 to 16 nm, as compared to the case of the corresponding
linear oxa- or thiadiazoles disubstituted with either alkylbithio-
phene or dialkylbithiophene.[37,39,40]
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The p–p* band in the thin film spectra of molecules contain-
ing one alkyl solubilizing group in each arm (S*TBT1 and
S*OBT1) is bathochromically shifted with respect to the posi-
tion of this band in the solution spectra (see Table 1). This
means that lower planarity of these molecules in solution has
its origin in the interaction with the solvent molecules and in
the solid state this planarity is improved. This effect is not ob-
served in the case of molecules containing two alkyl groups
per arm (S*TBT2 and S*OBT2) whose solution and solid state
spectra are very similar. Evidently, their better planarity in the
solution is not being further improved in the solid state. Solid
state spectra of all four molecules studied are presented in the
Figure S1 of the Supporting Information.
Photoluminescence spectra of S*TBT1 and S*OBT1, that is,
molecules with one solubilizing alkyl substituent in each arm
are featureless similarly as their absorption spectra (see Fig-
ure 1b). Vibrational structure, although not very highly pro-
nounced, is observed in the emission spectra of molecules
with two alkyl substituents (S*TBT2 and S*OBT2), which may
be interpreted in terms of some rigidification of the molecular
structure in the excited state, possibly through benzoid-qui-
noid type of transformation occurring upon excitation.[41] Rela-
tively large Stokes shifts are measured for S*TBT1 and S*OBT1,
90 and 104 nm, respectively. They are larger than the shifts re-
ported for the corresponding linear compounds containing ox-
adiazole or thiadiazole units[37–40] and additionally corroborate
significant, excitation-induced changes in their molecular struc-
ture. Smaller Stokes shifts are found for S*TBT2 and S*OBT2
(59 and 71 nm) comparable to those noted for the linear com-
pounds.
Electrochemistry
Figure 2 shows cyclic voltammograms of all four molecules. It
is known that thiophene-capped conjugated molecules are ca-
pable of electropolymerizing by CaꢀCa oxidative coupling. Al-
though the presence of an electro-accepting group such as ox-
adiazole or thiadiazole in the monomer shifts the oxidative
polymerization potential to higher values, the electropolymeri-
zation is still possible, provided that the terminal thiophene
ring is separated from the acceptor unit by an additional thie-
nylene spacer.[43,44] S*OBT1 and S*TBT1 whose Ca position in
the terminal thienyl ring is not blocked by an alkyl substituent,
also electropolymerize. However, since both compounds are
trifunctional monomers a polymer network is formed rather
than a linear chain. At the vertex potential the formed polymer
is in its oxidized (polycationic) state and its reduction to the
neutral state is manifested by a cathodic peak appearing in
the cyclic voltammogram upon the reverse scan (Figure 2a,b).
Note that no cathodic peaks are present in the voltammo-
grams of S*OBT2 and S*TBT2 (Figure 2c,d) whose polymeri-
zation is impeded by blocking the Ca positions, with alkyl
groups. Thus, in these cases the polymerization through CaꢀCa
coupling is impossible and other types of couplings are highly
improbable, due to the steric reasons. For both compounds
the radical cations formed at relatively high potentials are very
reactive and undergo consecutive reactions of chemical
Figure 2. Cyclic voltammograms of the studied star-shaped molecules (con-
centration 5ꢁ10ꢀ4 m in 0.1m Bu4NPF6/CH2Cl2; scan rate: 50 mVsꢀ1).
nature, leading to oxidative degradation of the molecules,
which is manifested in a partial or total loss of their electro-
chemical activity. The same behavior was previously observed
in the case of linear monomers consisting of oxiadiazole or
thiadiazole disubstituted with alkylbithiophene groups.[38,43]
The reduction process of all four molecules is irreversible, simi-
larly as linear oxa- and thiadiazoles disubstituted with alkyl- or
dialkylbithiophenes (Figure 2).[37,39,40] Molecules containing thia-
diazole electron withdrawing units are easier to reduce and
easier to oxidize as compared to their oxadiazole analogues
(see Figure 2 and data presented in Table 2) yielding lower ion-
ization potential (IP) and higher electron affinity (EA) values. As
a result, the electrochemical gaps (Eg(electro)) of S*TBT1 and
S*TBT2 are smaller than those of S*OBT1 and S*OBT2. The
trend is similar as in the case of linear donor–acceptor com-
pounds consisting of the same electron-withdrawing and -do-
nating groups, however, in the star-shaped molecules the elec-
trochemical band gaps are smaller by 120 to 170 meV.[39,40]
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Table 2. Electrochemical parameters of the studied star-shaped mole-
cules derived from cyclic voltammograms.
Table 3. Molecular orbital energies along with ionization potential (IP)
and electron affinity (EA) calculated in solution at B3LYP/6-31G(d,p) level
of theory.
Compound
Eox,onset
[V]
Ered,onset
[V]
IP[a]
[eV]
EA[b]
[eV]
Eg (electro)
[eV]
Compound
HOMO
[eV]
LUMO
[eV]
IP
[eV]
EA
[eV]
S*OBT1
S*OBT2
S*TBT1
S*TBT2
0.900
0.820
0.770
0.715
ꢀ2.010
ꢀ1.990
ꢀ1.875
ꢀ1.960
5.700
5.620
5.570
5.515
ꢀ2.790
ꢀ2.810
ꢀ2.925
ꢀ2.840
2.91
2.81
2.65
2.68
S*OBT1
S*OBT2
S*TBT1
S*TBT2
ꢀ5.65
ꢀ5.47
ꢀ5.57
ꢀ5.40
ꢀ2.14
ꢀ2.05
ꢀ2.30
ꢀ2.21
5.56
5.42
5.76
5.36
ꢀ2.41
ꢀ2.38
ꢀ2.60
ꢀ2.52
[a] Calculated according to equation: IP (eV)= jej(Eox, onset +4.8), [b] Calcu-
lated according to equation EA (eV)=ꢀjej(Ered, onset +4.8)
LUMO ones are mainly spread over the central benzene ring
and the adjacent oxadiazole or thiadiazole units. The calculat-
ed HOMO and LUMO energies are listed in Table 3.
DFT calculations
DFT calculations were carried out with the goal to confront
theoretical predictions with the experimental findings. In par-
ticular, they covered molecular orbital localization versus spin
density distribution, ionization potential (IP) and electron affini-
ty (EA) values as well as vertical excitations. This is illustrated in
Figure 3a,b where the frontier orbitals of S*OBT1 and S*TBT1
are presented. The frontier orbitals of S*OBT2 and S*TBT2 are
depicted in Figure S2 of the Supporting Information. In both
cases the HOMO orbitals are localized on the bithiophene moi-
eties, pushed towards the edges of the molecule, whereas the
Figure 4a,b show the unpaired spin density distribution in
the radical anion and radical cation forms of S*OBT1 and
S*TBT1. The obtained spin density contours can be considered
Figure 4. Spin density contours of radical cation and radical anion at B3LYP/
6-31G(d,p) level of theory (isosurface value=0.0004); a) S*OBT1; b) S*TBT1
as a superposition of the corresponding HOMO or LUMO con-
tours and prove that the unpaired electron is evenly delocal-
ized over the molecular backbone. For S*OBT2 and S*TBT2
very similar data are obtained (see Figure S3 in the Supporting
Information).
Vertical excitations contributing to the electronic transitions
were analyzed using the TD-DFT approach. For the most signif-
icant transitions (osc. strength >0.1) Natural Transition Orbital
(NTO) analysis was performed to better understand their
origin. In all cases, due to symmetry constrains, the ground-to-
first-excited-state transition was highly privileged (respective
NTOs are presented in Tables S2 to S5 in the Supporting Infor-
mation). For the oxadiazole derivatives (S*OBT1 and S*OBT2)
this transition was a mixture of charge transfer from end-
capped bithiophenes to the central benzene ring with some
small contribution of oxadiazole. In the case of thiadiazole-sub-
Figure 3. Molecular orbital contours obtained at B3LYP/6-31G(d,p) level of
theory (isosurface value=0.03); a) S*OBT1; b) S*TBT1.
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stituted derivatives the first transition was mainly a charge
transfer from the bithiophene moiety to the thiadiazole one as
revealed by NTO eigenvalues. The same trend was observed in
the experimentally measured and theoretically calculated elec-
tronic transitions, the latter were consistently less energetic by
0.27 eV. IP values determined experimentally from the cyclic
voltammetry measurements are close to those calculated using
DFT at B3LYP/6-31G(d,p) level of theory. Higher differences be-
tween the measured and calculated values are found for EA.
However, both trends are very similar (compare data collected
in Tables 2 and 3).
Figure 5. Device structure and energy diagram of the studied guest/host
light-emitting diodes. PEDOT=poly(3,4-ethylenedioxythiophene);
PVK=poly(9-vinylcarbazole); mCP=1,3-bis(N-carbazolyl)benzene;
TPBi=2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole).
Electroluminescence and light-emitting diodes
The investigated derivatives combine solution processability
with luminescent properties. We were therefore tempted to
verify whether it is possible to fabricate light-emitting diodes
(LEDs) through solution processing. We chose the guest–host
configuration for the emissive layer.[45] In this type of diode,
“guest” luminescent molecules (usually few percent) are dis-
persed in a host matrix, which consists of one or two organic
semiconductors of a relatively wide energy gap. Singlet exci-
tons formed in the host matrix as a result of electron and hole
injection from the electrodes are then transferred to the guest
molecule through the Fçrster energy transfer. Then they re-
combine emitting a photon. Fçrster energy transfer requires
partial overlap of the host emission spectrum and the guest
absorption spectrum.
An alternative mechanism of the electroluminescence, in-
volving charge carrier trapping can also be envisioned in this
type of emissive layers. If the LUMO level of the guest mole-
cule is lower than that of the host and, in addition, the HOMO
level of the guest is higher than that of the host, the guest
molecules constitute traps for both types of charge carriers in-
jected to the matrix from the electrodes and the excitons can
be formed directly on the guest molecule without the necessi-
ty of the charge transfer from the host matrix to the guest lu-
minophore.[46]
Devices of the following structure were fabricated: ITO/PE-
DOT:PSS/PVK/S*TBT2 (or S*OBT2)-mCP/TPBi/LiF/Al. The energy
levels alignment of all diode components is schematically pre-
sented in Figure 5. The electroluminescence bands registered
for emissive layers containing 1 wt% of S*TBT2 or S*OBT2 are
featureless and show clear maxima at 475 nm and 520 nm, re-
spectively. Note that the electroluminescence bands are batho-
chromically shifted by about 30 nm with respect to the corre-
sponding bands in the solution photoluminescence spectra
(see Table 1). Both compounds exhibit diode-like behavior with
the turn on voltage of ꢁ7 V as evidenced by their current and
luminance versus voltage characteristics shown in Figure 6a,b.
The luminance values exceeding 1800 cdmꢀ2 and luminous ef-
ficiencies approaching 3.5 cdAꢀ1 were measured for diodes
with 1 wt% S*TBT2 emitter. The highest external quantum effi-
ciency (EQE) of 1.35% was measured for the luminance of
about 500 cdmꢀ2 but it never dropped below 0.8% for the
whole range of higher luminances (see Figure 6c). The external
Figure 6. a) Current versus voltage and luminance versus voltage character-
istics for diode with active layers containing with 1 wt% of S*OBT2; b) cur-
rent versus voltage and luminance versus voltage characteristics for diode
with active layers containing with 1 wt% of S*TBT2; c) external quantum ef-
ficiency versus luminance for diode with active layers containing with 1 wt%
of S*OBT2; d) external quantum efficiency versus luminance for diode with
active layers containing with 1 wt% of S*TBT2.
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Table 4. Electroluminescence data for diodes containing S*TBT2 and
S*OBT2 dispersed in 1,3-bis(N-carbazolyl)benzene (mCP) matrix (1–
15 wt%).
S*TBT2
S*OBT2
]
[a]
[b]
[cdmꢀ2
]
[cdAꢀ1
]
[cdmꢀ2
[cdAꢀ1
]
1 wt%
3 wt%
5 wt%
10 wt%
15 wt%
1803
1847
–
1028
508
3.43
2.85
–
2.14
2.02
1428
1438
1229
434
1.67
1.74
1.76
0.94
1.01
531
[a] Luminance; [b] Luminous efficiency.
quantum and luminous efficiencies of diodes containing
1 wt% of S*OBT2 were 30% lower (see Figure 6d and Table 4).
We have tested the effect of the electroluminophore content
in the mCP matrix on the electroluminescence parameters of
the fabricated diodes. The results are collected in Table 4. For
the emitter contents of 1% and 3% the diode parameters are
comparable, whereas for its higher contents they worsen. This
effect can be considered as a manifestation of nanoaggrega-
tion of the electroluminophore at its higher contents that
favors the non-radiative quenching.
Figure 7. a) STM image and b) the proposed adsorption geometry of a mon-
olayer of S*TBT1 on HOPG. Scanning area and the measurement parame-
ters: a) 46ꢁ46 nm2 (zoom image 11ꢁ11 nm2); Vbias (tip)=1.1 V, It =0.3 nA.
Self-assembly and supramolecular organization in monolay-
ers and in bulk
Low molecular mass organic semiconductors containing alkyl
substituents can exhibit a strong tendency to self-organize in
monolayers deposited on appropriate substrates. This is also
the case of linear compounds of thiadiazole or tetrazine disub-
stituted with alkyl- or dialkylbithiophenes.[37,47] Intrinsic three-
fold symmetry of star-shaped thia- or oxadiazole bithiophene
molecules described here, together with the same type of sym-
metry of the graphene layer, should facilitate the formation of
ordered 2D supramolecular structures on highly oriented pyro-
lytic graphite (HOPG) substrates. Our attempts to obtain or-
dered monomolecular layers showed that star-shaped mole-
cules containing thiadiazole units have a stronger tendency to
self-organize on HOPG than their oxadiazole analogues. Mono-
layers of S*TBT1 and S*TBT2 form 2D polycrystals in which
large, ordered domains of the size of few hundreds nanome-
ters, differing in their orientation, can be distinguished. STM
images of S*OBT1 and S*OBT2 reveal smaller isolated ordered
domains embedded in an amorphous matrix. Figure 7a shows
a representative STM image of a particular 2D domain of self-
assembled S*TBT1 molecules.
The image of one domain is characterized by a hexagonal
network of black spots, attributable to regularly spaced molec-
ular cavities. The cavities are separated from each other in
three directions by the same distance of (2.8ꢂ0.2) nm. Based
on the geometrical features of the observed STM image, a plau-
sible model of the molecules adsorption geometry can be ob-
tained using the HyperChem software package (see Figure 7b).
According to this model, hexagonally packed S*TBT1 mole-
cules constitute a molecular network of triangles in which
every second triangle is empty forming a molecular cavity
(marked in Figure 7b by black dots). The remaining triangles
are “filled” with three arms of three neighboring molecules.
The empty cavities are visible in the STM image as regularly
spaced black spots distant by 2.8 nm, that is, by the distance
corresponding to the intermolecular separation.
Besides the monolayer assembly, the bulk organization of
the compounds was also studied. All four compounds are crys-
talline at ambient temperatures exhibiting one phase transition
in the differential scanning calorimetry (DSC) scans attributed
to the melting point. The melting temperature varies with the
number of alkyl side chains from 147.68C and 132.68C for
S*OBT1 and S*TBT1, respectively, to 99.98C and 110.28C for
S*OBT2 and S*TBT2. As expected, the higher the number of
substituents is attached, the lower the phase transition tem-
perature is found. The supramolecular organization in bulk was
investigated by two-dimensional wide-angle X-ray scattering
(2D-WAXS) on macroscopically aligned fiber samples. These
samples were prepared by mechanical extrusion, which in-
duced macroscopic orientation of the molecules. For the meas-
urements, which were performed at 308C, the samples were
mounted vertically in front of the 2D detector.
Figure 8 shows the 2D patterns of the four compounds con-
firming by the high number of distinct reflections their crystal-
line nature. Although the unit cells could not be clearly identi-
fied due to this large number of scattering intensities, distinct
differences in the supramolecular organization between the
compounds is evident from the patterns. For this reason, the
analysis was focused on the influence of the molecular design
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mers by the density of the alkyl substituents.[59] The shape of
the oligomer was modified from rod- to disc-like by the
number of attached substituents. Since the molecular confor-
mation of S*OBT1 is less rigid and more flexible, the molecules
might adapt a more rod-like shape under the mechanical
shearing during fiber extrusion and thus orient with the aro-
matic plane along the alignment direction thereby switching
the columnar orientation. Surprisingly, S*TBT1 and S*TBT2 do
not exhibit such difference in orientation probably due to
a high flexibility of the molecular structure of both systems.
Polymer networks through electrochemical polymerization
of S*OBT1 and S*TBT1
Figure 8. 2DWAXS patterns of a) S*OBT1, b) S*OBT2, c) S*TBT1 and
d) S*TBT2 recorded at 308C. The fibers were placed vertically in front of the
detector. Arrows indicated the p-stacking reflections.
As already stated, S*OBT1 and S*TBT1 can electropolymerize
through oxidative coupling of CaꢀCa carbons in terminal thien-
yl rings to yield a polymeric network. Polymerization of
S*OBT1 through cyclic voltammetry is presented in Figure 9
on the organization in the fiber samples rather than on the
crystallographic parameters. All four compounds self-assemble
in one-dimensional columnar stacks similar to other star-
shaped molecules.[48–54] The pattern of S*OBT2 displays a typical
distribution of reflections of crystalline columnar superstruc-
tures (Figure 8b).[55] The scattering intensities on the equatorial
plane of the pattern are related to the formation of the colum-
nar stacks that are aligned along the fiber direction. Reflections
in the wide-angle scattering range located on the meridional
and off-meridional are attributed to the intracolumnar packing
of the molecules. Thereby, the molecules are titled with re-
spect to the columnar axis what is characteristic for the crystal-
line phase of such systems. From the position of the off-meri-
dional reflections, a p-stacking distance of 0.37 nm is deter-
mined. The pattern of S*OBT1 also indicates the formation of
columns, but interestingly, the p-stacking reflections appear on
the equatorial plane instead of the meridional one (Figure 8a).
This change is attributed to a switch of the macroscopic align-
ment of the columnar superstructures in the sample. The loca-
tion of the p-stacking peak indicates a perpendicular orienta-
tion of the S*OBT1 stacks to the fiber axis. In these structures,
the S*OBT1 molecules are also tilted with a p-stacking of
0.35 nm, which is slightly smaller than that determined for
S*OBT2 due to less sterically demanding side chains. Com-
pounds S*TBT1 and S*TBT2 also revealed a perpendicular ori-
entation of the columns with respect to the alignment direc-
tion of the fiber. Compared with S*OBT1 and S*OBT2, S*TBT1
and S*TBT2 are not tilted within the stacks revealing a p-stack-
ing of 0.36 nm.
Figure 9. Electrochemical polymerization of S*OBT1 in the cyclic voltamme-
try mode. Concentration of the monomer: 5ꢁ10ꢀ4 m in 0.1m Bu4NPF6/
CH2Cl2; scan rate: 50 mVsꢀ1
.
whereas analogues scans registered for S*TBT1 can be found
in Figure S4 in the Supporting Information. Uniform, good
quality films can be grown either on platinum or ITO electro-
des, in the latter case suitable for UV/Vis-NIR spectroelectro-
chemical investigations.
In the oxidation mode both poly(S*OBT1) and poly(S*TBT1)
yield voltammograms, which are characteristic of p-type
doping of conjugated polymers, that is, their oxidation to the
polycationic state (see Figure 10). In both cases the appearing
redox peaks are superimposed on a capacitive current associat-
ed with the formation of a double layer, inherent of the p-
doping process.[59,60] Clear anodic peaks with maxima at E=
0.805 V and E=0.863 V versus Fc/Fc+ for poly(S*TBT1) and
poly(S*OBT1), respectively, can be attributed to the transfor-
mation of neutral macromolecules into polycations. Upon re-
verse scan the corresponding cathodic peaks appear at E=
0.740 V and E=0.803 V. Easier oxidation of poly(S*TBT1), as
compared to poly(S*OBT1), indicates lower IP of the former.
The same trend is observed in linear alternating copolymers
consisting of oxa- or thiadiazole and oligothiophene.[40] Electro-
chemical data of poly(S*TBT1) and poly(S*OBT1) are collected
in Table 5.
A reversed orientation of columnar structures of star- or
disc-shaped molecules in mechanically oriented fiber samples
was so far triggered either by additional non-covalent forces,
such as dipole–dipole interactions, or by an increased molecu-
lar aspect ratio.[56,57] Comparing S*OBT1 and S*OBT2, the
outer side chains play a major role for the alignment. The addi-
tional hexyl side chains change the molecular character of
S*OBT2 into a more disc-like shape that typically orient in
their columnar structures along the fiber direction. This behav-
ior is in agreement with the control of the molecular shape
and the self-assembly of phenylene-thienylene-based oligo-
The polymerization process strongly influences the IP values
of the resulting products, which are lower by about 250 meV
for poly(S*OBT1) and poly(S*TBT1) with respect to the corre-
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spectral changes induced by electrochemical reduction or oxi-
dation of conjugated polymers. Typically, these processes in-
volve bleaching of the p–p* band, characteristic of the neutral
state, with concomitant growth of new bands ascribed to poly-
anionic or polycationic states, which appear at higher wave-
length. At the potential of the end of the reduction (or oxida-
tion) peak in the cyclic voltammogram, the p–p* band is usual-
ly completely bleached. This is the case of linear, alternating
copolymers of oxa- or thiadiazole and oligothiophenes.[40,61]
Poly(S*OBT1) and poly(S*TBT1) behave, however, different-
ly. In the case of poly(S*OBT1) the first spectral changes, in-
duced by its oxidation to a polycationic state, appear at E=
0.65 V, in good agreement in the onset of the anodic peak in
the cyclic voltammogram of this polymer. With increasing
working electrode potential two bands, ascribed to the poly-
cationic state appear at 1223 nm and 733 nm. Note that the
p–p* band shows high intensity even at potentials where its
bleaching should be complete, for example, at E=0.90 V.
Figure 10. Cyclic voltammograms of electrochemical polymerization prod-
ucts registered in monomer-free 0.1m Bu4NPF6/CH3CN; scan rate: 50 mVsꢀ1
a) poly(S*OBT1); b) poly(S*TBT1).
:
Upon reduction of poly(S*OBT1) two bands attributable to
the polyanion state appear at 1290 nm and 762 nm; the
bleaching of the p–p* band is also incomplete, similarly as in
the case of the oxidation process. Clear isosbestic points ob-
served in Figure 11a and b unequivocally show that the elec-
trochemical oxidation or reduction of poly(S*OBT1) result in
mutual interconversion of the neutral and polyionic states.
Spectroelectrochemical behavior of poly(S*TBT1) is qualitative-
ly the same (see Figure 11c,d). These rather unusual spectral
responses of poly(S*OBT1) and poly(S*TBT1) to the working
electrode potential change indicate that at a certain concentra-
tion of charge (positive or negative) imposed on the polymer
network, its further electrochemical oxidation (or reduction) is
impeded. Moreover, this imposed charge renders unoxidized
segments of the network less planar as manifested by the hyp-
sochromic shift of the non-bleached p–p* band. Evidently,
complete oxidation (reduction) of a conjugated polymer star-
shaped network is more difficult than a linear polymer chain.
Table 5. Electrochemical parameters of poly(S*OBT1) and poly(S*TBT1)
derived from cyclic voltammograms. Optical gap is given for comparison.
Compound
Eox,onset Ered,onset IP[a]
EA[b]
[eV]
Eg(electro) Eg(opt)
[V]
[V]
[eV]
[eV]
[eV]
poly(S*OBT1)
poly(S*TBT1)
0.65
0.51
ꢀ1.93
ꢀ1.84
5.45 ꢀ2.87 2.58
5.31 ꢀ2.96 2.35
2.28
2.13
[a] Calculated according to equation IP (eV)= jej(Eox,onset +4.8). [b] Calcu-
lated according to equation EA (eV)=ꢀjej(Ered,onset +4.8).
sponding monomers. The reduction process is also affected by
the electrochemical polymerization, however, to
a lesser
extent, lowering the EA values of poly(S*OBT1) and pol-
y(S*TBT1) by 80 and 35 meV, respectively, as compared to the
corresponding monomers (compare Tables 2 and 5). Reduction
to a radical anion, which is totally irreversible in the monomers
(S*OBT1 and S*TBT1), becomes quasi-reversible in the poly-
merization product (compare Figures 2 and 10). For pol-
y(S*TBT1) one reduction peak with a clear maximum at
ꢀ2.048 V versus Fc/Fc+ is registered with the corresponding
oxidation peak at ꢀ1.980 V. Similarly, in the cyclic voltammo-
gram of poly(S*OBT1) one reduction peak is present at E=
ꢀ2.094 V, which, however has two anodic counterparts at
ꢀ2.023 V and ꢀ1.786 V upon reverse oxidation. The double-
step oxidation of the nascent radical anions, especially in view
of large separation of both cathodic peaks, may indicate a com-
plexity of this process involving electrochemical–chemical acts.
Finally, it should be stated that electrochemical band gaps of
poly(S*OBT1) and poly(S*TBT1) are significantly larger than
the band gaps of linear alternating copolymers consisting of
oxa- or thiadiazole and oligothiophenes.[40,61] This may indicate
that the effect of the band gap increase has its origin in the
complex topology of the formed polymeric network, which re-
duces the planarity and by consequence makes the conjuga-
tion more difficult to achieve.
Conclusion
We have elaborated a convenient and efficient method of the
preparation of star-shaped donor–acceptor molecules consist-
ing of a benzene central unit and three symmetrically-attached
arms of thia- or oxadiazole substituted with alkyl- or dialkylbi-
thiophene. All studied molecules show blue photolumines-
cence in solution; they are also electroluminescent and in the
guest–host light-emitting diode configuration, with 1,3-bis(N-
carbazolyl)benzene as a host, they emit either bluish (oxadia-
zole derivatives) or green (thiadiazole derivatives) light. Due to
their three-fold symmetry, compatible with that of the gra-
phene layer, they readily self-assemble in monolayers deposit-
ed on highly oriented pyrolytic graphite, as evidenced by scan-
ning tunneling microscopy (STM). The derivatives are also
processable and can be extruded in a form of fibers exhibiting
highly ordered supramolecular organization. These derivatives,
whose Ca position in the terminal thiophene rings is not
blocked by an alkyl substituent, can electrochemically polymer-
ize to yield a polymer network. The obtained networks are am-
Before discussing specific cases of poly(S*OBT1) and pol-
y(S*TBT1), it is instructive to describe general features of the
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Experimental Section
Instrumentation and materials
The IR spectra were recorded on an Infralum FT-801 spectrometer
1
in KBr pellets for solid or in thin films for liquid compounds. The H
and 13C NMR spectra were obtained in CDCl3 or [D6]DMSO with
TMS as an internal standard, using a Bruker DRX 400 spectrometer
(400 and 100 MHz, respectively). The elemental analyses were car-
ried out on a Carlo Erba 1106 CHN analyzer. The melting points
were determined on a Kofler bench. The reaction course and
purity of the products were checked by thin-layer chromatography
on Sorbfil UV-254 plates. The plates were visualized with UV light.
All chemicals were of analytical grade and purchased from Sigma–
Aldrich UV/Vis-NIR.
Solution and thin films UV/Vis-NIR spectra were recorded in chloro-
form on a Cary 5000 (Varian) spectrometer whereas the emission
spectra were measured using an Edinburgh FS 900 CDT fluorome-
ter (Edinburgh Analytical Instruments). Photoluminescence quan-
tum yields were determined using quinine sulfate in 0.05 moldmꢀ3
H2SO4(ffl =0.51) as a standard.[62]
Electrochemical studies and electropolymerization
Electrochemical properties of all four molecules were investigated
using cyclic voltammetry (CV). All cyclic voltammograms were re-
corded in a dry argon atmosphere on an Autolab potentiostat (Eco
Chimie) using a platinum working electrode of the surface area of
3 mm2, a platinum wire counter electrode and an Ag/0.1m AgNO3/
CH3CN reference electrode. The electrolytic medium consisted of
the compound studied (c=5·10ꢀ4 m) dissolved in a 0.1m Bu4NPF6/
dichloromethane electrolyte. The potential of the reference elec-
trode with respect to the ferrocene redox couple was always mea-
sured after each experiment.
The electropolymerization was performed in the same electrolytic
medium using either platinum disk or ITO glass as working electro-
des. The best quality polymer films were obtained in the cyclic vol-
tammetry-type process. The potential was cycled (scan rate=
50 mVsꢀ1) between the neutral state of the growing polymer
(lower vertex potential equal to 0 V vs. Fe/Fe+) and the higher
vertex potential corresponding to the onset of the monomer irre-
versible oxidation peak (as shown in Figure 9). In addition, the best
uniformity of the layers deposited on the ITO plate was achieved,
when the working and counter electrode were placed in the “face
to face” geometry.
Figure 11. UV/Vis-NIR spectra of poly(S*OBT1) and poly(S*TBT1) films de-
posited on an ITO electrode. a) Spectra of poly(S*OBT1) registered at step-
wise increasing working electrode potentials (p-doping mode); b) spectra
poly(S*OBT1) registered at step-wise decreasing working electrode poten-
tials (n-doping mode); c) spectra of poly(S*TBT1) registered at step-wise in-
creasing working electrode potentials (p-doping mode); d) Spectra pol-
y(S*TBT1) registered at step-wise decreasing working electrode potentials
(n-doping mode), where EOC =open circuit potential. Electrolyte: 0.1m
Bu4NPF6/CH3CN; E versus Fc/Fc+.
STM experiments
Monomolecular layers were prepared by drop-casting from a solu-
tion of the investigated compound in hexane (ꢁ2 mgLꢀ1) on
a freshly cleaved surface of highly oriented pyrolytic graphite
(HOPG, SPI Supplies, USA). The layers dried under ambient condi-
tions were then imaged in air at room temperature by means of
an STM system (University of Bonn, Germany).[63] All images were
recorded in a constant current mode using mechanically cut Pt/Ir
(80:20%) tips. The proposed real-space models of the monomolec-
ular layers were obtained by the correlation of the layer structure
deduced from the STM images and the molecular model of the in-
vestigated adsorbate determined using the HyperChem software
package.
bipolar in character and can be oxidatively (p-type) and reduc-
tively (n-type) doped, as evidenced by their cyclic voltammo-
grams. UV/Vis-NIR spectroelectrochemical investigations indi-
cate that the network topology of the resulting macromolecu-
lar compounds strongly impedes their p- and n-type doping
and their large segments remain undoped at potentials corre-
sponding to the end of the doping process.
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Diodes fabrication and characterization
Guest/host-type electroluminescent diodes were fabricated by mo-
lecular dispersion of either S*OBT1 or S*TBT1 (1 to 15 wt%) in an
one-component matrix consisting of 1,3-bis(N-carbazolyl)benzene
(mCP). Poly(9-vinylcarbazole) (PVK) and 2,2’,2’’-(1,3,5-benzinetriyl)-
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porting layer and hole-blocking layers, respectively. The active
layer of ca. 50 nm was deposited on top of an ITO electrode pre-
coated with a PEDOT:PSS layer of about 50 nm thickness and with
a PVK layer of about 10 nm thickness. In the subsequent step
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connected to a Source Meter Unit (and calibrated with NIST calibra-
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support from National Centre of Science in Poland (NCN, Grant
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Received: March 1, 2016
Published online on && &&, 2016
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FULL PAPER
Bright stars: Star-shaped conjugated
molecules with oxa- or thiadiazole bi-
thiophene side arms were investigated
as promising molecules and building
blocks for application in (opto)electron-
ics and electrochromic devices (see
figure).
&
Host–Guest Systems
K. Kotwica, A. S. Kostyuchenko, P. Data,
T. Marszalek, L. Skorka, T. Jaroch,
S. Kacka, M. Zagorska, R. Nowakowski,
A. P. Monkman, A. S. Fisyuk,* W. Pisula,*
A. Pron*
&& – &&
Star-Shaped Conjugated Molecules
with Oxa- or Thiadiazole Bithiophene
Side Arms
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
13
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ