High charge mobility in discotic liquid-crystalline triindoles: Just a core business?

Article abstract of DOI:10.1002/anie.201005820

The stacking order and distance in columnar phases of triindole-based liquid crystals is controlled by an ethynyl linker (see picture; orange rod) between the central triindole core (green) and the peripheral flexible chains of a discotic mesogen. Hole (h

Full text of DOI:10.1002/anie.201005820

DOI: 10.1002/anie.201005820
Liquid Crystals
High Charge Mobility in Discotic Liquid-Crystalline Triindoles: Just a
Core Business?**
Eva M. Garcꢀa-Frutos, Upendra K. Pandey, Roberto Termine, Ana Omenat, Joaquꢀn Barberꢁ,
Josꢂ L. Serrano, Attilio Golemme,* and Berta Gꢃmez-Lor*
A key parameter in the performance of organic electronics
devices is the mobility of charges. On the macroscopic level, it
has been demonstrated that the highest charge carrier
mobilities are obtained in highly ordered single-crystalline
materials.[1] However, the inherent fragility of single crystals
poses serious technological problems, thus considerably
limiting their practical applications. An interesting alternative
is offered by discotic mesogens, which are typically composed
of a central aromatic core substituted with flexible alkylic
chains.[2] Cores tend to form columnar stacks, maximizing p-
orbital overlap between adjacent molecules and thus favoring
a one-dimensional migration of charge carriers.[3] Further-
more, the inherent fluidity of liquid crystals induces advanta-
geous properties, such as the ability to self-heal structural
defects and easier alignment and processing from the
isotropic phase. However, fluidity is also associated with
intrastack dynamism of the functional units that reduces
carrier mobility in the bulk. Therefore, the preferred strategy
for improving mobility in discotic mesophases has been the
enhancement of the intermolecular order within the stacks.
To achieve this goal, different approaches have been
explored: 1) linking the cores to peripheral alkylic chains
through bulky moieties;[4] 2) introducing functional groups
providing directional interactions[5] (e.g. hydrogen bonds); or
even 3) inducing helical columnar arrangements that provide
a higher degree of order.[4a,6,7] Herein, we present an
experimental study on new triindole mesogens, one of them
exhibiting very high hole mobility (m ꢀ 1.4 cm2 Vꢁ1 sꢁ1). We
show how carrier mobility in such compounds does not
depend only on the degree of intracolumnar order along the
columns by itself, but also, as in most p-conjugated organic
semiconductors, on the stacking distance between molecules,
which usually decreases with increasing order.[8] Moreover,
results show how intracolumnar molecular distance can be
controlled by a suitable choice of the spacers between the
aromatic core and the peripheral chains, underlining the
promising role of ethynyl moieties as linkers in high-mobility
columnar phases.
Heptacyclic 10,15-dihydro-5H-diindolo[3,2-a:3’,2’-c]carb-
azole (triindole) was recently introduced as a new core for
discotic mesogens.[9] Attachment of six decyl chains (com-
pound 1 in Scheme 1) resulted in columnar hexagonal
[*] Dr. E. M. Garcꢀa-Frutos,[+] Dr. B. Gꢁmez-Lor
Instituto de Ciencia de Materiales de Madrid
Cantoblanco, 20849-Madrid (Spain)
E-mail: bgl@icmm.csic.es
Scheme 1. Structures of compounds 1–3 used in this study.
mesophases, although no stacking periodicity was observed.
Despite the intracolumnar disorder, 1 has a high hole mobility
m = 0.02 cm2 Vꢁ1 sꢁ1 in the mesophase. In an attempt to raise
the mobility values by increasing intracolumnar order, we
have investigated the effect of sterically demanding phenyl
(compound 2) and rigid alkyne (compound 3) spacers
between the peripheral alkyl chains and the central triindole
core.
The synthesis of 2 and 3 is described in the Supporting
Information. Both compounds are obtained as crystalline
solids. On heating, they show mesomorphic behavior between
room temperature and about 1508C, which is a significantly
broader temperature range when compared with compound 1
(see Table 1). In particular, distancing the disordered alkylic
chains from the central core with phenyl (compound 2) or
alkyne linkers (compound 3) does not significantly influence
the melting points but drastically increases the clearing point
of these materials. Both 2 and 3 exhibit a hysteresis
phenomenon in the crystallization process upon cooling,
U. K. Pandey, Dr. R. Termine, Prof. A. Golemme
Centro di Eccellenza CEMIF.CAL, LASCAMM CR-INSTM, CNR-IPCF
UOS CS, Dipartimento di Chimica, Universitꢂ della Calabria
87036 Rende (Italy)
E-mail: a.golemme@unical.it
Dr. A. Omenat, Dr. J. Barberꢃ, Prof. J. L. Serrano
Departamento de Quꢀmica Orgꢃnica, Instituto de Ciencia de
Materiales de Aragꢁn, Universidad de Zaragoza-CSIC
50009-Zaragoza (Spain)
[+] Present address: Departamento de Quꢀmica Orgꢃnica, Instituto de
Ciencia de Materiales de Aragꢁn, Universidad de Zaragoza-CSIC,
50009-Zaragoza (Spain)
[**] This work has been supported by CTQ2007-65683/BQU, (CTQ2009-
09030), S2009/MAT-1756/CAM, MiUR through the PRIN 2007
(2007WJMF2W) project, Gobierno de Aragꢁn, 200960I086 PIE.
Supporting information for this article (including detailed synthetic
procedures and characterization of compounds 2 and 3,
a description of sample preparation for mobility measurements,
and experimental details of mesomorphic properties) is available on
the WWW under http://dx.doi.org/10.1002/anie.201005820.
Angew. Chem. Int. Ed. 2011, 50, 7399 –7402
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1: Phase transition temperatures [8C] and corresponding enthalpy
connecting groups induces large stacking periodicities (c =
4.4 ꢀ). The alternative strategy of distancing the alkylic
chains from the central core by rigid ethynyl groups results
instead in closer adjacent disks (c = 3.9 ꢀ) for 3 (Table 1). It
should be noted that while phenyl[4a] or phenylethynyl
moieties[11] have often been used as linkers between the
flexible chains and the aromatic core, the direct attachment of
the pendant side chains through an alkyne spacer has been
rarely described.[12,13]
Charge carrier (hole) mobility of 2 and 3 was measured by
the steady-state space charge limited current (SCLC) tech-
nique,[14] which has been widely used to obtain values of
mobility in organic compounds. The current–voltage charac-
teristics typically exhibit a linear region at low applied
voltages, where the behavior is ohmic. However, at higher
voltages, where the current becomes space-charge-limited, an
approximately quadratic dependence can be observed. In this
regime the current depends only on the carrier mobility,
assuming contacts to be ohmic and the material to be trap-
free, and the Mott–Gurney[15] equation applies:
values (in brackets, [kJmolꢁ1]), of compounds 1, 2, and 3, and lattice
constants [ꢄ] of the hexagonal columnar mesophase determined by XRD
and charge carrier mobility m.
Compd.
Thermal properties[a]
Lattice
m [cm2 Vꢁ1 sꢁ1 [b]
]
constants[b]
1
2
3
C 38.9 (51.2) Colh
67.3 (0.5) I
C 39.3 (39.7) Colho
153–160 I
C 43.5 (51.2) Colho
150–165 I
a=27.7
0.02ꢂ0.005
6ꢅ10ꢁ4 ꢂ0.0004
1.4ꢂ0.3
a=29.4
c=4.4
a=29.0
c=3.9
[a] Corresponding to the first heating scan (DSC). The temperature range
for the transition Colho-I of 2 and 3 has been determined by POM
(polarizing optical microscopy). C crystal, Colh hexagonal columnar
mesophase, Colho ordered hexagonal columnar. [b] 1 (458C), 2 (258C), 3
(258C).
showing a supercooled state that maintains the structural
features of the mesophases (Supporting Information, Fig-
ure S2). This behavior favors the structural characterization
of the compounds at room temperature.[10]
The mesophases of 2 and 3 were identified as ordered
hexagonal columnar (Colho) on the basis of the typical pseudo
focal conic fan-shaped textures observed by POM (polarizing
optical microscopy) (see Figure 1) and of their X-ray dif-
9
8
V2
d3
ð1Þ
J ¼ e0erm
where J is the measured current density, m is the charge
mobility, e0 is the free-space permittivity, er is the dielectric
constant of the material, V is the applied voltage, and d is the
thickness of the device. As all of the other parameters in the
equation are measurable, charge mobility can be easily
obtained. Figure 2 shows a typical result of current–voltage
measurements for compound 3.
The measured values of mobility were highly dependent
on the alignment of each particular sample, and even on the
alignment of different areas within the same sample (see the
Supporting Information). Well-aligned samples of 2 could be
obtained, but for 3 alignment was poor or, in the best samples,
incomplete. In the best cases, the measured hole mobilities at
room temperature were m ꢀ 6 ꢁ 10ꢁ4 cm2 Vꢁ1 sꢁ1 for 2 and m
ꢀ 1.4 cm2 Vꢁ1 sꢁ1 for 3 (Table 1). Such values correlated well
with the degree of alignment estimated by optical microscopy
and decreased even by one order of magnitude when the
Figure 1. Polarizing optical photomicrographs of 2 (left) and 3 (right)
obtained at 1058C and 1178C, respectively, on cooling from the
isotropic liquid.
fractograms (Table 1). The X-ray diffraction patterns
obtained for these compounds show a diffuse halo at 4.5 ꢀ
that is typical of the liquid-like order of the aliphatic chains.
The diffraction patterns of 2 and 3, in contrast to that
observed for compound 1, present an additional diffuse
maximum with intensity reinforcement in the meridian region
in the oriented pattern of 2 (an oriented pattern of 3 could not
be obtained). This maximum corresponds to the periodic
stacking of the cores, indicating the higher order achieved by
distancing the disordered alkylic chains from the central core.
It is interesting to note that this scattering peak is broader for
2 than for 3 and this gives evidence of a larger correlation
length for the last compound. From the peak width it can be
concluded that, whereas the p–p stacking involves a few
molecules in the case of 2, it extends to several nanometers in
the case of 3.
Bulky phenyl linkers have been previously shown to
efficiently interlock molecules within the columns.[4a] How-
ever, in the case of 2, the high steric demand of these
Figure 2. Typical current–voltage curve obtained for compound 3 at
room temperature. The two straight lines, with slopes of 1 and 2,
represent ideal ohmic and SCLC behavior, respectively.
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alignment was particularly poor. Moreover, as the SCLC
technique is sensitive not only to orientational order defects
but also to the charge injection effectiveness at the electrodes,
this should be considered as a lower limit for the hole mobility
of 3. To our knowledge, the hole mobility of 3 is the highest
ever reported for discotic phases.[16–18] This result is partic-
ularly relevant as the columnar phases with the highest hole
mobility generally exhibit shorter stacking distances (around
3.4 ꢀ) and considerably more extended p systems. Further-
more, from what is known on similar model compounds,[19] the
core of 1–3 is expected to be non-planar, although examples
of columnar phases with high mobility obtained from
mesogens with non-planar cores are known.[20]
In the compounds under investigation, the correlation
length of the intracolumnar order increases in the sequence
1 < 2 < 3. However, hole mobility increases in steps of two
orders of magnitude in the sequence 2 < 1 < 3, underlining the
minor role of intracolumnar order in determining charge
mobility. On the other hand, compound 2, where the value of
the stacking distance (c = 4.4 ꢀ) is higher than in 3 (c =
3.9 ꢀ), shows the lowest charge mobility, underlining the
importance of the stacking distance.
The role of ethynyl linkers is then twofold: increasing the
order within the column and at the same time reducing the
stacking distances. However, the possible contributions to the
charge transport of a larger p-orbital overlap and/or of a
variation of the relative orientation of adjacent molecules
induced by the alkyne spacers could also be important. A
computational study aimed at clarifying this point is in
progress.
In conclusion, the strategy of introducing alkyne spacers
has been shown to efficiently increase the supramolecular
order in the discotic mesophases, thus simultaneously reduc-
ing the distance between molecules in the stack, resulting in
the discotic liquid crystalline material with the highest hole
mobility reported to date. The fact that the stacking distance
and the extent of p conjugation are far from ideal shows the
importance of triindole as a core for high-mobility columnar
phases. We have also shown that even though higher order has
been often claimed as a fundamental factor for improving
mobility in discotic liquid crystals, at least in this particular
case the stacking distance between molecules seems to be
even more important. This result can be of general relevance
for many columnar phases, as it points out the specific role of
chain linkers, within molecular periphery, in determining
stacking distances.[21] The recognition of such an important
structure–function relationship can certainly guide future
work aimed at obtaining columnar phases with high charge
mobility.
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Received: September 16, 2011
Revised: April 4, 2011
Published online: June 24, 2011
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Keywords: alkynes · liquid crystals · mesogens · semiconductors
.
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