H-Bonded Donor-Acceptor Units Segregated in Coaxial Columnar Assemblies: Toward High Mobility Ambipolar Organic Semiconductors

Article abstract of DOI:10.1021/jacs.6b06792

A novel approach to ambipolar semiconductors based on hydrogen-bonded complexes between a star-shaped tris(triazolyl)triazine and triphenylene-containing benzoic acids is described. The formation of 1:3 supramolecular complexes was evidenced by different

Full text of DOI:10.1021/jacs.6b06792

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Article
H-bonded donor-acceptor units segregated in coaxial columnar
assemblies: towards high mobility ambipolar organic semiconductors
Beatriz Feringán, Pilar Romero, José Luis Serrano, Cesar L. Folcia, Jesus Etxebarria,
Josu Ortega, Roberto Termine, Attilio Golemme, Raquel Giménez, and Teresa Sierra
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06792 • Publication Date (Web): 31 Aug 2016
Downloaded from http://pubs.acs.org on August 31, 2016
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H-bonded donor-acceptor units segregated in coaxial colum-
nar assemblies: towards high mobility ambipolar organic
semiconductors.
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Beatriz Feringán,† Pilar Romero,† José Luis Serrano,‡ César L. Folcia,§ Jesús Etxebarria,§ Josu Ortega,⊥
Roberto Termine,║ Attilio Golemme,║ Raquel Giménez*† and Teresa Sierra*†
†Dpto. Química Orgánica, Instituto de Ciencia de Materiales de Aragón (ICMA), Facultad de Ciencias, Universidad de Zaraꢀ
gozaꢀCSIC, 50009 Zaragoza, Spain.
‡Dpto. Química Orgánica, Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, 50018 Zaragoza, Spain.
§Dpto. de Física de la Materia Condensada, Facultad de Ciencia y Tecnología, UPV/EHU, 48080 Bilbao, Spain.
⊥Dpto. Física Aplicada II; Facultad de Ciencia y Tecnología, UPV/EHU, 48080 Bilbao, Spain.
║LASCAMM CRꢀINSTM, CNRꢀNANOTEC Lab LiCryL, Dipartimento di Fisica, Università della Calabria, 87036 Rende
Italy.
KEYWORDS liquid crystals, triazines, triphenylenes, selfꢀassembly, Xꢀray, organic semiconductors.
ABSTRACT: A novel approach to ambipolar semiconductors based on hydrogenꢀbonded complexes between a starꢀshaped
tris(triazolyl)triazine and triphenyleneꢀcontaining benzoic acids is described. The formation of 1:3 supramolecular complexes was
evidenced by different techniques. Mesogenic driving forces played a decisive role in the formation of the hydrogenꢀbonded comꢀ
plexes in the bulk. All of the complexes formed by nonꢀmesogenic components gave rise to hexagonal columnar (Colh) liquid crysꢀ
tal phases, which are stable at room temperature. In all cases, Xꢀray diffraction experiments supported by electron density distribuꢀ
tion maps confirmed triphenylene/tris(triazolyl)triazine segregation into hexagonal sublattices and lattices, respectively, as well as
remarkable intracolumnar order. These highly ordered nanostructures, obtained by the combined supramolecular Hꢀbond/columnar
liquid crystal approach, yielded donor/acceptor coaxial organization that is promising for the formation of ambipolar organic semiꢀ
conductors with high mobilities, as indicated by charge transport measurements.
azaheterocycles such as perylenebisimide, hexaazatriꢀ
phenylene and hexaazatrinaphthylene, to name a few, have
been reported to show electron mobility.31ꢀ36
INTRODUCTION
The properties of functional materials are frequently enꢀ
hanced when they have a wellꢀorganized internal structure
and, as a consequence, molecularly organized supramolecular
materials are promising candidates for applications in organic
(opto)electronics.1ꢀ4 In this respect, it is worth exploiting the
ability of liquid crystals to selfꢀorganize in mesophases in
order to obtain functional architectures.5ꢀ8 In particular, discotꢀ
ic liquid crystals (DLC) are able to stack spontaneously into
columns9ꢀ11 to give oneꢀdimensional structures of πꢀconjugated
organic molecules. This organization facilitates longꢀrange πꢀ
orbital overlap that allows, after appropriate charge injection,
intracolumnar charge carrier mobilities with values close to or
comparable to those of amorphous silicon. These features
make columnar liquid crystals very attractive materials for use
as organic semiconductors.9ꢀ14
One approach to obtain ambipolar transport in a balanced
way is to create a coaxial p/n heterojunction by using a doꢀ
nor/acceptor (D/A) system. Recent efforts rely on D/A covaꢀ
lent amphiphilic dyads37,38 or supramolecular D/A dyads based
on hydrogen bonding39 or ionic interactions,40 which form p/n
heterojunctions in nanostructures by solventꢀassisted methods
to provide relevant photoconduction or ambipolar transport.
The columnar mesophase approach has also been directed
towards this end. In contrast to solventꢀassisted methods, this
approach allows selfꢀassembly in the bulk due to mesogenic
forces that, with suitable molecular design, yield D/A segregaꢀ
tion in the column. In this respect, the strategy is based on the
covalent attachment of donor and acceptor units in dyads,41ꢀ48
tryads,49ꢀ51 or starꢀshaped52ꢀ60 systems, with different degrees
of order reported for the columnar mesophase. A few of them
show ambipolar transport.45,47,51,54 In addition to covalent sysꢀ
tems, ambipolar transport has also been demonstrated for
columnar mesophases formed by mixed donor and acceptor
taperedꢀdendrons assembled by dipolar interactions.61
It has been postulated that charge carrier transport in DLCs
is intrinsically ambipolar,15 i.e., both hole and electron
transport are possible,16 but ambipolar behavior has only been
found for a few DLCs.15,17ꢀ22 The majority of DLCs contain
electronꢀdonor aromatic cores such as triphenylenes, phthaloꢀ
cyanines and hexabenzocoronenes and hole transport has been
measured.14 Triphenyleneꢀbased mesogens are amongst the
most widely studied columnar liquid crystals23 and they show
high hole mobility values in the columnar mesophase.24ꢀ30 On
the other hand, some electronꢀacceptor DLCs derived from
We recently described
a new starꢀshaped molecule,
tris(triazolyl)triazine, which can be readily prepared by means
of click chemistry and has electronꢀacceptor character and
gives rise to columnar liquid crystals.62,63 Functionalization of
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this starꢀshaped core at the periphery can be achieved by covaꢀ terminal alkoxy chains of the triphenylene unit, i.e., decyloxy
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lent synthesis. In this way, the electronꢀacceptor
tris(triazolyl)triazine was covalently linked through a flexible
hexamethylene spacer to three peripheral electronꢀdonor penꢀ
taalkoxytriphenylenes and the resulting materials had columꢀ
nar organization with a D/A coaxial segregation in which the
columns were formed by a single type of discotic unit.64
Moreover, the tris(triazolyl)triazine can be functionalized by a
supramolecular approach. The presence of triazine and triazole
rings means that the tris(triazolyl)triazine is able to establish
hydrogen bonding interactions with benzoic acids, thus leadꢀ
ing to 1:3 complexes with hexagonal columnar mesomorꢀ
phism.65
chains (AꢀTPC10), hexyloxy chains (AꢀTPC6) and butyloxy
chains (AꢀTPC4), with the aim of studying the influence that
these have on the organization in bulk. In all compounds the
triphenylene unit is linked to the benzoic acid by means of a
hexamethylene spacer. Mixtures of the T3C4 core and the
aforementioned benzoic acids in a 1:3 ratio, respectively, were
prepared to provide the target supramolecular complexes
shown in Figure 1.
The formation of the supramolecular complexes was conꢀ
firmed by spectroscopic techniques. All of the complexes
show hexagonal columnar mesomorphism at room temperaꢀ
ture with intracolumnar order and D/A segregation, as eviꢀ
denced by Xꢀray diffraction and electron density maps. In
addition, charge carrier mobilities were measured and these
highly ordered materials exhibit ambipolar charge transport
properties.
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The results obtained in the previous study led us to envisage
that the organization of donor and acceptors by supramolecuꢀ
lar interactions, such as hydrogen bonding, in synergy with the
mesogenic driving forces in columnar mesophases should be a
favorable and versatile approach to achieve a high level of
control over D/A segregation and hence ambipolar transport.
RESULTS AND DISCUSSION
Synthesis
Accordingly, we report here a new combined supramolecuꢀ
lar/columnar approach for the formation of starꢀshaped hydroꢀ
genꢀbonded complexes between a tris(triazolyl)triazine core
and triphenyleneꢀcontaining benzoic acids. This approach was
developed with the aim of obtaining columnar liquid crystals
with segregated columns of electron donors (triphenylene
units) and electron acceptors (tris(triazolyl)triazine) in a more
straightforward way than for covalently grafted compounds.64
OR
OR
RO
HO
OR
OR
RO
1a R = C10H21
RO
OR
R = C H
1b
1c
6
13
9
R = C H
OR
4
O
OH
OC6H12Br
BrC6H12Br
K2CO3
KI
DMF
OR
K2CO3
Butanone
MeOOC
MeOOC
2
OC4H9
O
OR
RO
O
OR
OR
RO
N
H
OR
O
N
O
O
N
H
RO
O
N
N
N
H
N
N
N
O
O
H
RO
O
H
O
OR
H
O
N
N
OC4H9
MeO
N
3a R = C10H21
3b R = C6H13
C4H9O
O
OR
OR
R = C4H9
3c
O
1,4-dioxane/H2O
KOH
T3C4-TPC10
T3C4-TPC6:
T3C4-TPC4:
:
R = C10H21
R = C6H13
R = C4H9
O
OR
OR
RO
OR
OR
RO
O
O
RO
OR
HO
A-TPC10 R = C10H21
A-TPC6 R = C6H13
Figure 1. Molecular structure of the triphenyleneꢀcontaining
supramolecular complexes.
O
OR
A-TPC4
R = C4H9
The tris(triazolyl)triazine used as a core, T3C4, has a paraꢀ
butoxyphenyl substituent in each triazole ring. These butoxy
terminal chains favor solubility in organic solvents, which is
important for the formation of the supramolecular complexes
by dissolution of a mixture of the components. Three different
triphenyleneꢀbased benzoic acids (AꢀTPC10, AꢀTPC6 and Aꢀ
TPC4) were designed for the preparation of the supramolecular
complexes. The three benzoic acids differ in the length of the
Scheme 1. Synthetic procedure for the preparation of the triꢀ
phenyleneꢀcontaining benzoic acids.
The tris(triazolyl)triazine core (T3C4) was synthesized by a
previously reported procedure.65 The triphenyleneꢀbased benꢀ
zoic acids AꢀTPC10, AꢀTPC6 and AꢀTPC4 were obtained acꢀ
cording to the synthetic pathway outlined in Scheme 1. The
synthetic procedures and characterization data are gathered in
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the supporting information. The required monohydroxyꢀ It is noteworthy that, although none of the pure compounds
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pentaalkoxytriphenylenes (1a–c) were prepared in one step
from orthoꢀdialkoxybenzenes by a previously described methꢀ
od.64 The methyl benzoates 3a–c were obtained by Williamson
etherification of the monohydroxypentaalkoxytriphenylenes
(1a–c) with methyl 4ꢀ(6ꢀbromohexyloxy)benzoate (2), which
in turn was prepared by Williamson etherification between
methyl 4ꢀhydroxybenzoate and 1,6ꢀdibromohexane. The alkaꢀ
line ester hydrolysis of 3a–c with potassium hydroxide in 1,4ꢀ
dioxane/water yielded the triphenyleneꢀbased benzoic acids Aꢀ
TPC10, AꢀTPC6 and AꢀTPC4.
are liquid crystals, mesomorphic properties are obtained from
nonꢀmesogenic components, a fact that provides strong eviꢀ
dence for the formation of the 1:3 complexes. Furthermore,
the enthalpies obtained from the DSC thermograms are unusuꢀ
ally high (Table 1), which suggests that the mesophases of
these complexes are highly ordered.
Table 1. Thermal properties of the supramolecular com-
plexes and structural parameters measured by X-ray dif-
fraction.
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Complex
Thermal properties
(T oC, [ꢁH kJ/mol])a
Lattice parameters
The hydrogenꢀbonded complexes were prepared by dissolvꢀ
ing a mixture of the tris(triazolyl)triazine core and the correꢀ
sponding benzoic acid in a 1:3 ratio in dichloromethane. After
evaporation of the solvent by continuous stirring at room
temperature, the resulting mixtures were subjected to a therꢀ
mal treatment in which the samples were heated to the isoꢀ
tropic liquid state and then cooled to room temperature.
T3C4ꢀTPC10
I 80 [25.3] Colh
Colh 86 [27.9] I
I 129 [79.0] Colh
Colh 139 [89.7] I
I 17 g
g 29 Ib
Colh 73 [24.0] Ic
a =48.0 Å
c =3.4 Å
a =42.4 Å
c =3.4 Å
T3C4ꢀTPC6
T3C4ꢀTPC4
Thermal properties and mesomorphic behavior
The thermal properties of both the pure compounds and the
Hꢀbonded complexes were studied by polarizing optical miꢀ
croscopy (POM) and differential scanning calorimetry (DSC).
a =39.6Åc
c =3.4 Åc
The pure compounds used for the preparation of the comꢀ
plexes are not liquid crystals. As described previously, T3C4
does not have mesomorphic properties.65 The triphenyleneꢀ
based benzoic acids are not mesomorphic despite the fact that
they contain the triphenylene unit, which favors columnar
mesomorphism. Instead, they are crystalline materials that, on
cooling from the isotropic state, display different crystal forms
(see ESI).
aThermal data are given from the first cooling process and the
second heating process at a rate of 10°C min–1.
bAs obtained complex.
cAfter annealing above the glass transition for 90 h.
Xꢀray diffraction (XRD) experiments allowed us to confirm
that all complexes show hexagonal columnar mesomorphism
at room temperature. XRD experiments were carried out on
asꢀobtained samples for complexes T3C4ꢀTPC10 and T3C4ꢀ
TPC6, and after annealing above the glass transition in the case
of complex T3C4ꢀTPC4. Indeed, the Xꢀray pattern of T3C4ꢀ
TPC4 immediately after the preparation of the complex (Figure
S20) confirms that it is a disordered material. The diffractoꢀ
grams of the complexes at room temperature contain a large
number of smallꢀangle reflections indicating spacings with a
ratio of d, d/√3, d/√4, d/√7, d/√9, d/√12, and d/√13. This is
characteristic of a hexagonal lattice and corresponds to reflecꢀ
tions (100), (110), (200), (210), (300), (220), and (310), reꢀ
spectively. At high angles, all diffractograms show a diffuse
halo at 4.5 Å and this is characteristic of the liquid crystal
state. In addition, an outer sharp reflection at 3.4 Å is observed
and this indicates a wellꢀdefined periodicity of the structure
along the direction of the columnar axis. The diffraction data
are summarized in Table S1 and the lattice parameters are
listed in Table 1.
All of the complexes appear as homogeneous materials by
POM. Moreover, both complex T3C4ꢀTPC10 and complex
T3C4ꢀTPC6 both show liquid crystalline behavior, with texꢀ
tures characteristic of hexagonal columnar mesophases (Figure
2a and 2b). Furthermore, as deduced from the DSC cooling
scans, both complexes retain the mesophase at room temperaꢀ
ture (Table 1 and Figures S13 and S14) and crystallization was
not observed.
In contrast, the freshly prepared complex T3C4ꢀTPC4 did not
show any texture by POM, even on cooling below room temꢀ
perature. Moreover, the DSC thermogram for the freshly preꢀ
pared complex only shows a glass transition at 29 °C on heatꢀ
ing, and this transition appears in immediately subsequent
heatingꢀcooling cycles (Figure S15). However, after annealing
the sample above this glass transition, a texture indicative of
mesomorphic behaviour was observed (Figure 2c). The coꢀ
lumnar mesophase also developed slowly at room temperature.
The heating cycle performed after annealing gave an endoꢀ
thermic peak at 73 °C (Table 1 and Figure S16) and this is
consistent with POM observations.
Figure 2. Microphotographs of the textures observed by POM for (a) T3C4ꢀTPC10, Colh, 85°C (cooling), (b) T3C4ꢀTPC6, Colh, 117°C
(cooling) and (c) T3C4ꢀTPC4, Colh, room temperature (after annealing above the glass transition).
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gram of a partially aligned sample of T3C4ꢀTPC10 (Figure 3a),
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which was oriented by mechanical treatment, the (001) reflecꢀ
tion (due to the periodic stacking distance) is reinforced along
the alignment direction, while the (100), (110), (200) and
(210) reflections (lowꢀangle reflections) are reinforced in the
perpendicular direction. This pattern is consistent with a coꢀ
lumnar mesophase in which columns are oriented along the
rubbing direction.
The number of molecules that contain the unit cell can be
estimated from the experimental data in Table 1 and assuming
a reasonable density value for these complexes close to 1 g
cm–3. The relationship between the density (ρ) of the complexꢀ
es and the measured lattice parameters is given by the equation
ρ = (M·Z)/(NA·V), where M is the molar mass of the 1:3 comꢀ
plex, Z is the number of molecules in the unit cell, NA is Avoꢀ
gadro’s number, and V is the unit cell volume (V =
a·a√3/2·c·10–24). For all three complexes a value of Z = 1 is
obtained. These results indicate that there is one complex per
unit cell and this value is consistent with the formation of 1:3
complexes.
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Figure 3. Xꢀray diffraction patterns recorded at room temperature
of (a) a partially aligned sample of T3C4ꢀTPC10, (b) T3C4ꢀTPC10
(magnification of the lowꢀangle region), (c) T3C4ꢀTPC6 and (d)
T3C4ꢀTPC4 (after annealing above the glass transition).
In an effort to seek further support for the proposed model,
electron density distributions (Fourier maps) were calculated.
These maps are constructed using the (hk0) reflections and,
therefore, they represent the projection of the structure on the
(a, b) plane, as shown in the scheme in Figure 4(b). The techꢀ
nical details of the procedure can be found in ref. 66 (see also
the supporting information).66 The resulting maps (Figure 5)
clearly indicate the segregation between columns of
tris(triazolyl)triazine and columns of triphenylene, as indicated
in Figure 4. It can be observed that triphenylene units (oval
shape) are organized around the core T3C4 (circular shape)
with hexagonal symmetry.
Interestingly, in all cases the (100) reflection is weaker than
the (200) reflection, which is the most intense (Figure 3 and
S17ꢀS19). The model proposed to account for this effect on
the basis of a structure with hexagonal symmetry is shown in
Figure 4. According to this model tris(triazolyl)triazine (blue
circles) and triphenylene (orange circles) units segregate into
two different columns. This segregation takes place because
the orientation of the links between blue and orange circles has
two possibilities (0º, 120º, 240º) or (60º, 180º, 300º) [see green
and magenta schemes in Fig. 5(a)]. Both orientations must
occur with the same probability in the structure. However, the
2D positional order is easily explained if only one of the orienꢀ
tations takes place in a given 2D stratum. In this way, each
layer is densely filled. On the other hand, both orientations can
alternate randomly in successive strata as indicated in Figure
4(a). Consequently, the projection of the structure along the
columnar axis is that represented in Figure 4(b), where blue
and orange circles are centered on sixꢀ and twoꢀfold symmetry
axes, respectively. The figure shows the family of planes that
correspond to the (100) point of the reciprocal lattice. They
contain blue and orange circles and are separated by a distance
d. However, as can be seen, planes that contain only orange
circles are intercalated in the middle between the former and
this causes an interference effect that contributes to reduce the
intensity of the (100) reflection [and also of the (h00) reflecꢀ
tions for h odd]. In contrast, the (h00) reflections with even h
values are reinforced as they receive the contribution of both
kinds of planes (separated d/2). Furthermore, in the diffractoꢀ
Figure 4. Scheme of the model proposed for the structure of the
materials under study. Tris(triazolyl)triazine and triphenylene
units are represented by blue and orange circles, respectively. (a)
The orientation of the complexes alternates randomly in succesꢀ
sive strata (separated by a distance c = 3.4 Å) in such a way that
blue and orange columns segregate. (b) The projection of the
structure on the (a, b) plane shows hexagonal symmetry. The
family of (100) planes, separated a distance d, is indicated.
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Figure 5. Electron density maps calculated for (a) T3C4ꢀTPC10, (b) T3C4ꢀTPC6, and (c) T3C4ꢀTPC4 from the diffraction patterns at room
temperature. The green and magenta schemes in (a) represent the two possible orientations of the complexes in different strata. For a given
stratum only one orientation is present in order to give rise to a densely filled layer. The scales are in units of the cell parameter a.
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The 13C cross polarization magicꢀangle spinning (13C
CPMAS) NMR spectra of the core T3C4, the acid AꢀTPC6 and
the corresponding complex, T3C4ꢀTPC6, were recorded at
room temperature (Figure 7). According to previous results,65
the most significant evidence for the formation of the complex
in a 1:3 stoichiometry is the shift of the signal corresponding
to the carbon atom of the carbonyl group. In the spectrum of
the acid AꢀTPC6 a signal at 171.6 ppm, which corresponds to
the dimeric form of the acid, is observed. In the spectrum of
T3C4ꢀTPC6 this signal is not observed but a signal appears at
166.2 ppm, which is assigned to the carbonylic carbon and
indicates that all of the acid forms the complex in a 1:3 stoiꢀ
chiometry. The signal corresponding to the carbon atom of the
triazine ring must be also shifted to lower ppm,65 together with
other signals, but it is difficult to assign these shifts due to
signal overlap.
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Study of the formation of the complexes in the
mesophase
Additional support for the formation of 1:3 complexes in the
mesophase was obtained by infrared spectroscopy and solid
state nuclear magnetic resonance (NMR) experiments.
The infrared spectra of both the complexes and the pure
compounds were recorded at room temperature. For complexꢀ
es T3C4ꢀTPC10 (Figure S21) and T3C4ꢀTPC6 (Figure 6a), the
C=O stretching band is observed at around 1709 cm–1 and this
indicates complex formation between T3C4 and benzoic acꢀ
ids.65 This band appears at a different position from the C=O
stretching band of the triphenyleneꢀcontaining benzoic acids
AꢀTPCn. For these acids, the C=O stretching bands appear
between 1683 and 1695 cm–1, depending on the crystalline
form, and corresponds to the hydrogen bonded dimers (see
ESI). Moreover, the band corresponding to the heterocyclic
rings is shifted slightly to higher wavenumber (from 1566 to
1577 cm–1) when the complex is formed.
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60
C4H9O
O
O
a)
1566
j
H
k
e
f
H
N
N
N
c
h
N
b
i
d
g
N
a
N
O
T3C4
N
H
N
N
H
O
O
H
1577
H
O
O
n
1709
m
N
l
T3C4-TPC6
o
N
N
p
O
q
1683
OC6H13
OC6H13
r
C4H9O
A-TPC6
O
v
s
t
x
u
w
1800
1700
1600
1500
3400 3200 3000 2800 2600 2400 2200 2000 1800 1600
wavenumber (cm-1
O
wavenumber (cm-1
)
)
OC6H13
C6H13O
b)
T3C4-TPC4
1707
After annealing
chains
A-TPC6
q, r, w
s, v
u
t
1708+1685
Justprepared
l
p
n
m
o
3400 3200 3000 2800 2600 2400 2200 2000 1800 1600
wavenumber (cm-1
1800
1700
1600
1500
wavenumber (cm-1
)
)
T3C4-TPC6
Figure 6. Infrared spectra recorded on KBr pellets at room temꢀ
perature of (a) T3C4, AꢀTPC6 and complex T3C4ꢀTPC6 and (b)
T3C4ꢀTPC4 as obtained (bottom) and after annealing above the
glass transition (top). On the right, the 1850–1400 cm–1 region is
shown.
j
k
i
h
T3C4
g
d
f
c e
b
a
As for XRD experiments, the infrared spectra of complex
T3C4ꢀTPC4 were recorded both immediately after preparation
of the complex and after annealing above the glass transition.
The C=O stretching bands observed in the spectra are different
(Figure 6b). The spectrum recorded just after the preparation
of the complex shows two C=O stretching bands at 1708 and
1685 cm–1, which correspond to the complex and the dimer of
the benzoic acid AꢀTPC4, respectively. The presence of these
two C=O stretching bands suggests that the complex is only
partially formed. However, after annealing, a single C=O
stretching band is observed at 1707 cm–1, which is similar to
that in the other complexes and confirms the formation of the
1:3 complex T3C4ꢀTPC4.
160
140
120
100
80
60
40
20
ppm
Figure 7. 13C CPMAS NMR spectra of T3C4, AꢀTPC6 and the
corresponding 1:3 complex, T3C4ꢀTPC6, at room temperature.
The most representative signals that shift upon complexation are
indicated.
Electrochemical and charge transport properties
Cyclic voltammetry experiments were performed in order to
study the electrochemical properties of the complexes (Figures
S25–S27). The cyclic voltammograms were recorded on dropꢀ
cast films of the complexes from solution in CH2Cl2 and the
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Journal of the American Chemical Society
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measurements were performed in an acetonitrile solution of electrodes, through the first few molecular layers over the
1
2
3
4
5
6
7
8
tetrabutylammoniumhexafluorophosphate (0.1 M). One reducꢀ
tion wave, corresponding to the tris(triazolyl)triazine core, and
oxidation waves corresponding to the triphenylene units were
observed and these are consistent with the electron acceptor or
electron donor character of these units, respectively (Table 2).
The HOMO and LUMO energy values, calculated from the
oxidation and reduction waves, are approximately –5.4 eV and
–2.7 eV, respectively, for all three complexes (Table 2).
insulator: the measured value of mobility is determined by the
orientational order at the sample/insulator interface, that can
be controlled only to some extent by using specific surface
treatments. Mobility can also be obtained from measurements
of TimeꢀResolvedꢀMicrowaveꢀConductivity (TRMC), often
following charge generation from an electron pulse. In this
case, given the very short diffusion distance covered by chargꢀ
es during the measurement time, the measured mobility is not
sensitive to orientational order. However, the main drawback
of such an approach is that hole and electron mobilities cannot
be distinguished, and this is a major problem in ambipolar
materials.
The charge transport properties were measured by the space
charge limited current (SCLC) method, as described in the SI.
With this technique, mobility can be extracted from curꢀ
rent/voltage characteristic curves if enough charges are injectꢀ
ed form the electrodes and the SCLC regime is reached. Since
charges have to travel between two electrodes, in anisotropic
materials the measured mobility reflects the macroscopic
degree of order and the orientation of the phase symmetry axes
with respect to the electrodes. It is possible to measure charge
mobility following several other experimental methods.
Among the most common ones, is the soꢀcalled Time of Flight
(TOF) technique, where a short light pulse is used to generate
charges close to the surface of a sample: the resulting transient
current generated by an applied electric field is measured and
mobility is obtained if the thickness of the sample is known. In
anisotropic materials, TOF mobility depends strongly on the
size and on the orientation of different orientational domains,
exactly like in the case of SCLC. Orientational issues are also
important when considering mobility measurements extracted
from Field Effect Transistors (FET). In this case, what is
measured is the current flowing, between the source and drain
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10
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17
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60
One of the requirements of SCLC method is to have an
ohmic contact between the injecting electrodes and the HOMO
or LUMO levels to measure hole mobility and electron mobiliꢀ
ty, respectively. For hole mobility measurements, the cells
were prepared with a glass covered by indium tin oxide (ITO)
and another one covered by gold. For measurement of electron
mobility, given the values of the LUMO energies, Ca injecting
electrodes were selected. However, when such electrodes
came into contact with the compounds the metal was oxidized
immediately. As a consequence, two other kinds of electron
injecting electrodes were tried: Al and ZnO. Al did not show
any electron injecting behavior, but ZnO did, despite the enerꢀ
gy mismatch between its conduction band and the LUMO
level of the studied compounds. Electron mobility measureꢀ
ments were then carried out in cells with two identical ZnO
electrodes.
Table 2. Electrochemical data for complexes T3C4-TPC10, T3C4-TPC6 and T3C4-TPC4
Complex
Ered vs Ag/AgCl
Ered vs FOC
Eox vs Ag/AgCl
Eox vs FOC
HOMO
(eV)b
LUMO
(eV)c
(V)
(V)a
(V)
(V)a
T3C4ꢀTPC10
T3C4ꢀTPC6
T3C4ꢀTPC4
–1.70
–1.60
–1.58
ꢀ2.13
ꢀ2.03
ꢀ2.01
1.05
1.07
1.05
0.62
0.64
0.62
–5.42
–5.44
–5.42
–2.67
–2.77
–2.79
aE1/2 = 0.43 V vs Ag/AgCl.
bEHOMO = –e[Eox vs FOC + 4.8 V].
cELUMO = –e[Ered vs FOC + 4.8 V].
All such cells, both for electron and for hole mobility measꢀ
urements, were filled by capillarity by heating the material
above the isotropic liquid transition and, after sample filling,
by cooling the cell to room temperature. The details of sample
preparation and mobility measurement are included in the SI.
after sample preparation (cooling) fell in the low mobility
range, while the same measurement carried out a few days or
weeks later fell within the higher mobility range. This was
particularly evident in the case of T3C4ꢀTPC4, which forms a
glassy phase upon cooling, followed by slow formation of the
columnar mesophase. However, similar behavior was also
observed with the other two compounds, albeit less frequently,
and this suggests a tendency towards selfꢀhealing of defects in
this class of materials. A decrease in mobility with time was
not observed in any sample. In the case of T3C4ꢀTPC10, mobilꢀ
ities in the higher mobility range were not measured.
Several thermal treatments were performed in an effort to
obtain long range, uniform alignment of the columns over
large areas of the samples. However, such attempts were not
successful and, as shown by optical microscopy, all measureꢀ
ments were performed on polydomain samples. The conseꢀ
quence of such nonꢀuniform alignment is that the measured
mobility was different in different positions of the same samꢀ
ple. In particular, for both holes and electrons, all measureꢀ
ments fell within two different ranges separated by 3–4 orders
of magnitude: no mobility measurement gave results within
the ‘empty’ range. In addition, in the same sample area deꢀ
fined by the electrode, mobility had a tendency to increase
with time: often, a first measurement performed immediately
The results, taken as average values from different positions
and different samples, are listed in Table 3. The mobility valꢀ
ues in the low range are typical of amorphous or highly disorꢀ
dered phases. The higher mobility values can be considered as
arising from areas of the sample with a more uniform director
orientation and/or a director orientation closer to the normal to
the electrodes. As always with SCLC measurements, the valꢀ
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Journal of the American Chemical Society
ues can be considered as an estimate of the lower limit for the and confirmed by electron density distribution maps. The
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mobility in the columnar phase.
supramolecular complexes exhibit ambipolar charge transport
properties, with electron mobilities roughly one order of magꢀ
nitude higher than hole mobilities.
Table 3. Charge mobility values for complexes T3C4-
TPC10, T3C4-TPC6 and T3C4-TPC4
This novel supramolecular/columnar approach has proven to
be a versatile way to obtain ordered functional liquid crystals
with a coaxial nanosegregation of donors and acceptors. The
highly ordered triphenyleneꢀcontaining structures obtained in
this work are very promising for applications as organic semiꢀ
conductors.
Complex
Mobility†
(cm2 s–1 V–1)
Hole
T3C4ꢀTPC10
T3C4ꢀTPC6
T3C4ꢀTPC4
9
(8±6)10–4
(9± 4) 10–3
[(4±1)10–9]
(1±0.3) 10–2
[(8±1)10–9]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
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57
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59
60
ASSOCIATED CONTENT
[(1.2±0.1)10–8]
Experimental procedures (synthesis and characterization) and
characterization of the supramolecular complexes (DSC thermoꢀ
grams, IR spectra, cyclic voltammograms and charge mobility
measurements) are included in the Supporting Information. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(7 ± 5) 10–2
[(2±0.8)10–6]
(6 ± 3) 10–3
[(9±2)10–7]
Electron
[(2±0.6)10–8]
†In the upper row, the average mobility in the higher range is
shown. The value in square brackets is the average mobility in the
lower range.
AUTHOR INFORMATION
Corresponding Authors
The lack of director uniformity in the samples is reflected
by the large standard deviations for the measured mobilities.
In addition, different compounds showed different tendencies
to form larger domains regardless of their orientation. For
these reasons, it does not seem reasonable to compare the
mobilities of the three studied compounds to establish a strucꢀ
ture/property relationship.
* rgimenez@unizar.es
* tsierra@unizar.es
Author Contributions
The manuscript was written through contributions of all authors. /
All authors have given approval to the final version of the manuꢀ
script.
In contrast, data should be discussed in terms of mobility for
the whole class of materials. Measurements show that columꢀ
nar phases formed by T3C4ꢀTPCn exhibit ambipolar charge
transport, with hole mobilities in the 10–3–10–2 cm2 V–1 s–1
range and electron mobilities in the 10–2–10–1 cm2 V–1 s–1
range. Such mobility values for holes and electrons are in the
mediumꢀhigh range when compared to the mobilities observed
in other, nonꢀambipolar columnar phases. The same can be
said when considering the mobilities measured in other ambiꢀ
polar smectic phases67 or phases obtained by using donorꢀ
acceptor dyads,68 polymers69ꢀ72 or even solid solutions.73 Parꢀ
ticularly for triphenylene materials, hole mobility values have
been reported in the range of 10ꢀ4 to 10ꢀ1 cm2 Vꢀ1 sꢀ1.15,24ꢀ30 In a
few cases electron mobilities have been measured, obtaining
values as fast as for hole mobility, in the range of 10ꢀ3 to 10ꢀ1
cm2 Vꢀ1 sꢀ1 by the TOF method.15 If we take into account amꢀ
bipolar columnar structures with segregated stacks of donors
and acceptors, as in our case, both the electron and the hole
mobilities exhibited by T3C4ꢀTPCn are about 1–2 orders of
magnitude higher than those reported in the literature for simiꢀ
lar systems.45,51
ACKNOWLEDGMENTS
This work was financially supported by the MINECOꢀFEDER
funds (projects MAT2015ꢀ66208ꢀC3ꢀ1ꢀP, CTQ2015ꢀ70174ꢀP,
MAT2012ꢀ38538ꢀCO3ꢀ02), and the Gobierno de AragónꢀFSE
(E04 research group and EPIF grant to B. F.). AG acknowledges
support from the ELIOTROPO project (PON03PE_00092_2). RT
was supported by MIUR under the PRIN 2012JHFYMC project.
The authors would like to acknowledge the use of ‘Servicios
CientíficoꢀTécnicos’ of CEQMA (UZ–CSIC).
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