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the direction perpendicular to the rows (y direction, highlight-
ed by arrow), various domains succeed and long-range order
no longer exists. In each domain, several rows associate with
close contacts between PI segments. Close contacts break and
rows stagger at boundaries between domains.
the molecules. The mesomorphic character also allows the
spontaneous healing of self-organization defects and the de-
velopment of long-range correlated segregated arrangements
after thermal annealing and slow cooling of the samples.
Statistical results about the distribution of different packing
patterns were obtained. The ratio of groups with two PI-TP2
molecules (43.80%) is similar to that with three PI-TP2 mole-
cules (50.90%). These two packing patterns account for most
of the regions. The STM Figure nevertheless displayed a region
C with eight grouped rows of PI-TP2 molecules (borders
marked with gray lines, Figure 8). However, the case was rare
and may not be stable. There was also some kind of disor-
dered zones at both ends (surrounded by the dark gray rectan-
gles). It could be speculated that groups with less than four
molecules were more stable under the current conditions.
Since PI-TP2 is a LC material, this kind of disorder might en-
hance the liquid character, which is of significant importance
in LC semiconductors.
Electron and hole mobility measured by using the time-of-
flight technique
As we have just seen, the discotic-based triads display ordered
LamColobl mesophases, with differentiated columnar stacks in
the bulk state, and PI-TP2 shows ordered self-assembly on
HOPG/liquid interface. High charge carrier mobilities are there-
fore expected from the presence of the polycyclic p-conjugat-
ed aromatic cores and from their regular face-to-face stacking
in the mesophases. TP discogens are indeed known as ambi-
polar mesomorphous semiconductors giving rise to one-di-
mensional columns, along which both electrons and holes are
hopping,[21,22] whereas PI and CI derivatives have shown elec-
tron-transporting properties suitable for n-type semiconduc-
tors.[36,40]
According to the above discussion, PI-TP2 molecules could
form stable 2D nanostructures on the HOPG substrate. High-
resolution STM images could be obtained easily. However, we
could not obtain satisfying STM data in terms of the assem-
bling structures for the other three triad molecules. According
to previous STM studies, stable adsorption of coronene deriva-
tives like CI-TP2 was often hard to achieve, unless they were
immobilized in host nano-networks.[69] The decrease or ab-
sence of ethynyl groups in BI-TP2 and CI-TP2, respectively,
moreover increases the molecular flexibility and conformations,
and allows molecules to easily twist or cluster. The shorter
spacer between the rigid donor and acceptor of the molecules
then makes STM imaging more difficult.
Charge mobility is the most important parameter for organic
semiconductors, as it determines the performance of the mate-
rials in electronic devices. There are mainly three techniques to
measure the conductivity of LC semiconductors: Time-of-flight
(TOF), pulse-radiolysis time-resolved microwave conductivity
(PR-TRMC), and organic field-effect transistor (OFET). TOF and
OFET techniques require that the molecules self-assemble ac-
cording to different orientation patterns, and that the electron-
ic charges hop along the column over a minimum distance of
10 mm. The PR-TRMC technique does not need well-ordered
supramolecular organizations because it just measures the
local charge transport over a few molecules only. Because it is
difficult to control the alignment of discotic mesogens, huge
differences occasionally occur between measured mobilities,
making comparisons often difficult. One of the advantages of
the TOF method is that the hole and the electron mobility can
be obtained directly and independently and that both can be
faced accurately.
The 2D self-assembly structure of PI-TP2 is formed through
van der Waals forces. STM figures revealed that molecules reg-
ularly align in parallel rows, but with irregular shifts between
rows along the perpendicular packing axis. Meanwhile, the
other three triads could not be observed by STM under these
conditions because of the low affinity of these diimide cores
with HOPG, the increasing molecular conformational disorder,
and the shortening of the linkers between donor and acceptor
groups. The molecular arrangement in the interfacial region is
remarkably comparable to the model proposed for the colum-
nar sub-lattice of the 3D bulk structure in the LamColobl meso-
phase. The segregation pattern and the alternating rows struc-
ture is for instance the same, except that both in-plane pack-
ing directions are long-range correlated in the 3D structure
and that lattice parameters are somewhat different. The
domain A lattice thus shows almost the same angle (114
against 1198), a shrunken a-parameter (4.4 against 6.94 nm)
and an expanded b-parameter (1.9 against 1.48 nm). These dis-
crepancies reflect that PI-TP2 molecules involved in the solid/
liquid interface arrangement interact with the graphite sub-
strate, whereas interactions between stacked triads themselves
determine the organization in the bulk. Furthermore, the triad
molecules on the graphite surface form a sort of 2D crystal
structure, whereas the 3D bulk organization is a fluid meso-
phase and involves dynamics with rotations and translations of
Considering the similarity of the chemical structures of the
three triads and their supramolecular organization, the charge
mobility of one representative triad has been investigated. In
particular, the most thermally stable compound, CI-TP2, was
selected for the measurements, to gain reliable results; the
other two compounds showing, as described above, some
structural modifications upon heating. The charge carrier mobi-
lity of CI-TP2 sample in LC cell has been measured by the TOF
technique in standard conditions.
A nitrogen gas laser
(337 nm, with pulse width of 600 ps), which can be absorbed
by CI[39] and TP[58] units, induced photocurrent decay curves of
the positive and negative charge carriers shown in Figure 9
(2008C, double logarithmic plot), for various external electric
field strengths. Because the transient times are short, the
charge carriers are of the electronic type (i.e., holes and elec-
trons); ionic carriers can safely be excluded as they drift much
slower in the highly viscous discotic LC mesophases.[3] It is ob-
vious that both the positive and negative charge carrier cur-
rent decay curves are non-dispersive and that the drift times
Chem. Eur. J. 2015, 21, 10379 – 10390
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