Toward ultralow-bandgap liquid crystalline semiconductors: Use of triply fused metalloporphyrin trimer-pentamer as extra-large π-extended mesogenic motifs

Article abstract of DOI:10.1002/chem.201201101

In contrast with their dimeric homologue, triply fused zinc porphyrin trimer-pentamer, as extra-large π-extended mesogens, assemble into columnar liquid crystals (LCs) when combined with 3,4,5-tri(dodecyloxy)phenyl side groups (3 P_Zn-5 P_Zn, Figure 1). Their LC mesophases develop over a wide temperature range, namely, 41-280 °C (on heating) for 5 P _Zn, and all adopt an oblique columnar geometry, typically seen in columnar LC materials involving strong mesogenic interactions. These LC materials are characterized by their wide light-absorption windows from the entire visible region up to a near infrared (NIR) region. Such ultralow-bandgap LC materials are chemically stable and serve as hole transporters, in which 5 P_Zn gives the largest charge carrier mobility (2.4×10 ^-2 cm V^-1 s^-1) among the series. Despite a big dimensional difference, they coassemble without phase separation, in which the resultant LC materials display essentially no deterioration of the intrinsic conducting properties. Low bandgap: Triply fused zinc porphyrin trimer-pentamers serve as extra-large π-extended mesogens for the formation of ultralow bandgap columnar liquid-crystalline (LC) semiconductors with very wide LC mesophase temperature ranges and light-absorption windows (see figure). The mobility of pentramer is roughly one order of magnitude greater than that of dimeric reference. Copyright

Full text of DOI:10.1002/chem.201201101

FULL PAPER
DOI: 10.1002/chem.201201101
Toward Ultralow-Bandgap Liquid Crystalline Semiconductors: Use of Triply
Fused Metalloporphyrin Trimer–Pentamer as Extra-large p-Extended
Mesogenic Motifs
Seiya Tanaka,[a] Tsuneaki Sakurai,[a] Yoshihito Honsho,[b] Akinori Saeki,[b] Shu Seki,[b]
Kenichi Kato,[c] Masaki Takata,[c] Atsuhiro Osuka,[d] and Takuzo Aida*[a]
Abstract: In contrast with their dimeric
homologue, triply fused zinc porphyrin
trimer–pentamer, as extra-large p-ex-
tended mesogens, assemble into colum-
nar liquid crystals (LCs) when com-
bined with 3,4,5-tri(dodecyloxy)phenyl
side groups (3PZn–5PZn, Figure 1).
Their LC mesophases develop over
a wide temperature range, namely, 41–
2808C (on heating) for 5PZn, and all
adopt an oblique columnar geometry,
typically seen in columnar LC materi-
als involving strong mesogenic interac-
tions. These LC materials are charac-
terized by their wide light-absorption
windows from the entire visible region
up to a near infrared (NIR) region.
Such ultralow-bandgap LC materials
are chemically stable and serve as hole
transporters, in which 5PZn gives the
largest charge carrier mobility (2.4ꢀ
10À2 cmVÀ1 sÀ1) among the series. De-
spite a big dimensional difference, they
coassemble without phase separation,
in which the resultant LC materials dis-
play essentially no deterioration of the
intrinsic conducting properties.
Keywords: liquid crystals · porphyr-
ins · self-assembly · semiconductors ·
organic electronics
Introduction
ticular attention, because they are beneficial for better over-
lapping of p-clouds, leading to better carrier-transport prop-
erties.[3] Of further interest, such LC materials can be elabo-
rated into low-bandgap soft semiconductors, which are of
potential interest for flexible ambipolar transistors[4] and
photovoltaic devices.[5] Hence, extensive efforts have been
devoted to the synthesis of large p-extended aromatic
motifs,[6–9] such as acene,[6] benzocoronene,[7] and rylenedii-
mide[8] derivatives. However, mesogens affording low-
bandgap LC semiconductors are still limited to certain p-ex-
tended motifs.[10–12] Meanwhile, Osuka and co-workers devel-
oped a series of triply fused metalloporphyrin oligomers,[13]
HOMO–LUMO gaps of which become progressively small-
er, as the number of the constituent metalloporphyrin units
becomes larger.[13d] Accordingly, their absorption ranges can
eventually be extended to near-infrared (NIR) and even IR
regions.[13b,d] Thus, triply fused metalloporphyrin oligomers
are considered as an ultimate class of p-extended aromatic
motifs for organic electronics.
Liquid crystalline (LC) organic semiconductors are potent
candidates for the development of large-area processable
soft-electronic devices. In general, discotic LC semiconduc-
tors carry p-conjugated mesogenic cores that stack up on
top of each other to form a long-range molecular order that
allows charge-carrier transport.[1] Because LC molecules can
potentially align unidirectionally under external forces,[2]
carrier-transport pathways may be tailored. For designing
LC semiconductors, large p-extended mesogens attract par-
[a] S. Tanaka, T. Sakurai, Prof. Dr. T. Aida
Department of Chemistry and Biotechnology
School of Engineering, the University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax : (+81)3-5841-7310
E-mail: aida@macro.t.u-tokyo.ac.jp
[b] Y. Honsho, Dr. A. Saeki, Prof. Dr. S. Seki
Division of Applied Chemistry
Herein, we report that trimeric, tetrameric, and pentame-
ric triply fused zinc porphyrins 3PZn–5PZn containing dode-
cyloxy side chains assemble into columnar LC mesophases
(Figure 1). Noteworthy, the core part of pentameric 5PZn
(roughly 7ꢀ40 ꢁ) is the largest among those ever utilized as
mesogens for LC materials. A few years ago, we reported
that the copper complexes of their dimeric homologues with
two immiscible side chains form columnar LC assemblies
with a rectangular geometry.[14] In contrast, when appended
with only dodecyloxy side chains, not only 2PCu, as reported
previously,[14a] but also newly prepared zinc complex 2PZn, is
amorphous in the solid state with no sign of p-stacking
Graduate School of Engineering
Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
[c] Dr. K. Kato, Prof. Dr. M. Takata
RIKEN SPring-8 Center
1-1-1 Kouto, Sayo-cho
Sayo-gun, Hyogo 679-5148 (Japan)
[d] Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science, Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201101.
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Figure 1. Molecular structures and phase profiles of triply fused metalloporphyrin oligomers 3PZn, 4PZn, 5PZn, as well as those of 2PM (M=Cu, Zn),
2P’Zn, and 2P’’Zn. Numeric values in phase profiles represent phase-transition temperatures (8C), whereas symbols G, Colh, Colr, Colob, and Iso denote
glassy, hexagonal columnar, rectangular columnar, oblique columnar, and isotropic phases, respectively.
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Use of Triply Fused Metalloporphyrin Trimer–Pentamer
FULL PAPER
(Figure 1). However, the columnar LC mesophases of 3PZn–
5PZn develop over a wide temperature range and commonly
adopt an oblique geometry typical of columnar LC materials
sembly (Figure 1). This LC material was shearable, but
poorly fluidic, as expected from the exceptionally large p-
extended core, and displayed phase properties that were not
observed for its lower homologues. Thus, in DSC, 5PZn
upon heating showed a single detectable mesophase at 41–
1828C (Figure S7 in the Supporting Information). However,
even upon further heating above 1828C, its POM micro-
graph still remained birefringent. In relation to this observa-
tion, 5PZn at 2008C showed XRD peaks that were again in-
dexed to an oblique columnar geometry with extremely
large lattice dimensions (a=84.7, b=68.4 ꢁ; g=1058; Fig-
ure 3h). At around 2808C, such characteristic diffraction
peaks completely disappeared, affording a broad diffraction
at 2–48 along with an alkyl halo centered at 168, and the ma-
terial turned fluidic. Thus, as shown in Figure 1, one more
LC mesophase exists at 182–2808C (240–1668C upon cool-
ing) in the phase diagram. Noteworthy, the liquid phase,
formed upon phase transition at 2808C, was not optically
isotropic, but weakly birefringent up to 3208C, reflecting
a strong stacking nature of the exceptionally large p-extend-
ed core of 5PZn.
with
strong
mesogenic
(p-stacking)
interactions
(Figure 2).[15] Herein, we also highlight that the LC assembly
of 5PZn gives the largest charge-carrier mobility among the
series.
The “oblique” geometry, commonly observed for colum-
narly assembled 3PZn–5PZn, has been typically seen in col-
umnar LCs formed by strong mesogenic interactions.[15] In
fact, 3PZn–5PZn in noncoordinating solvents displayed com-
plicated NMR spectra due to their strong aggregation. Even
upon high dilution in CHCl3, 4PZn (red) and 5PZn (blue)
still self-assemble, so that their absorption spectral features
resemble those in the LC state (Figure 4a and b). Needless
to say, their strong p-stacking natures contribute to the high
thermal stabilities of the resulting LC materials. Then, the
question is why 2PZn does not assemble into a LC meso-
phase. In relation to this contrast, we found that the absorp-
tion profile of 3PZn (green) in CHCl3, unlike those of 4PZn
and 5PZn, changes with the concentration of 3PZn, in which
the Q-band absorption observed at l=1300 nm in a dilute
CHCl3 solution, in particular, showed a notable blueshift
with broadening in the LC state (Figure 4a and b). This con-
centration-dependent spectral feature indicates that 3PZn
has a weaker p-stacking nature than 4PZn and 5PZn. Never-
theless, the LC clearing temperature of 3PZn, once assem-
bled columnarly, is comparably high to that of 4PZn
(Figure 1). With these results in mind, newly prepared refer-
ence compounds 2P’Zn and 2P’’n carrying smaller numbers
of the dodecyloxy side chains were prepared, and their as-
sembling behaviors in the solid state were investigated
(Figure 1). Interestingly, 2P’’n, in sharp contrast with 2PZn,
formed rectangular columnar LC mesophases (Figures 2 and
3e) exhibiting a high LC clearing temperature comparable
to the case of 4PZn (Figure 1), whereas 2P’Zn just afforded
amorphous solid (Figures S6 and S7 in the Supporting Infor-
mation). Therefore, it is now clear that the peripheral aryl
groups of 2PZn and 2P’Zn, both carrying dodecyloxy side
chains at the 3 and 5 positions, are sterically too demanding
for the mesogenic core to p-stack.
Figure 2. Schematic illustrations (core parts) of columnar liquid-crystal-
line mesophases of a) rectangular and b) oblique geometries.
Results and Discussion
According to a literature method,[13c] triply fused zinc por-
phyrin oligomers 3PZn–5PZn (Figure 1) were synthesized
from the corresponding zinc-porphyrin precursors by the
Suzuki–Miyaura cross-coupling reaction, followed by oxida-
tive cyclization of the resultant meso–meso coupled zinc-
porphyrin oligomers with 2,3-dichloro-5,6-dicyanobenzoqui-
none (DDQ)/ScACHTUNGTRENNUNG(OTf)3 (see the Supporting Information).
Dimeric 2PZn, together with its modified versions 2P’Zn and
2P’’Zn, was prepared according to a method analogous to
that reported previously.[14] These fused porphyrin oligomers
were unambiguously characterized by 1H NMR spectroscopy
and MALDI-TOF mass spectrometry (see the Supporting
Information). When trimeric 3PZn was allowed to cool from
its isotropic melt, its differential scanning calorimetry (DSC)
profile showed two LC mesophases at 150–42 and 42–128C
(Figure S7 in the Supporting Information). Both mesophases
in polarized optical microscopy (POM) showed a birefrin-
gent dendritic texture typical of columnar LCs (Figure 3b).
By means of X-ray diffraction (XRD) analysis by using
a synchrotron radiation facility,[16] these LC mesophases
were found to adopt an oblique columnar geometry
(Figure 2) with lattice parameters a=48.6 ꢁ, b=45.6 ꢁ, and
g=948 at 1508C; and a=51.8 ꢁ, b=41.0 ꢁ, and g=968 at
308C (Figure 3 f). Tetramer 4PZn upon cooling showed
a single oblique columnar mesophase over a wide tempera-
ture range at 166–298C (Figures 1 and S7 in the Supporting
Information), lattice parameters of which (a=65.4 ꢁ, b=
61.3 ꢁ; g=918; Figure 3g) are obviously larger than those
of 3PZn. Analogous to 3PZn and 4PZn, pentameric 5PZn, the
largest homologue of the series, formed a columnar LC as-
Although columnarly assembled 2P’’n–5PZn are composed
of such ultralow-bandgap p-extended mesogens, they are
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T. Aida et al.
Figure 3. Crossed-polarized optical micrographs of a) 2P’’Zn at 1868C, b) 3PZn at 1658C, c) 4PZn at 1608C, d) 5PZn at 2358C, and equimolar mixtures of
i) 2P’’Zn/3PZn at 1208C, j) 2P’’Zn/4PZn at 1208C, k) 3PZn/4PZn at 1008C, and l) 2P’’Zn/3PZn/4PZn at 1208C. X-ray diffraction patterns of e) 2P’’Zn at 1608C,
f) 3PZn at 1508C, g) 4PZn at 1808C, h) 5PZn at 2008C, and equimolar mixtures of m) 2P’’Zn/3PZn at 908C, n) 2P’’Zn/4PZn at 1108C, o) 3PZn/4PZn at 908C,
and p) 2P’’Zn/3PZn/4PZn at 1008C by using an incident X-ray beam of 1.08 ꢁ in wavelength.
chemically stable. We investigated their semiconducting
properties by means of a flash-photolysis time-resolved mi-
crowave conductivity (FP-TRMC) technique.[17] Together
with transient absorption spectroscopy (TAS) to quantify
the number of charge carriers, FP-TRMC allows for evaluat-
ing “intrinsic” charge carrier mobilities of materials without
electrodes. Upon exposure to a laser pulse of l=355 nm at
258C, LC films of 2P’’Zn–5PZn on a quartz substrate all
showed a prompt rise of a TRMC signal and its slow decay
(Figure S13 in the Supporting Information), in which the
maximum value of the TRMC signal was obviously greater,
as the size of the aromatic core was larger (Figure 5).[18] In
the case of, for example, 2P’’Zn, TAS showed a characteristic
absorption band due to a transient species at around l=
585 nm (Figure 6b). Because the p-extended mesogenic core
was larger, this characteristic absorption band was more red-
shifted to l=757 (3PZn, Figure 6e), 813 (4PZn, Figure 6 h),
and 840 nm (5PZn, Figure 6k). Because the transient spectral
features, thus observed for 2P’’Zn–5PZn, resembled the ab-
sorption spectral profiles of their chemically generated radi-
cal cations (Figure 6c, 6f, 6i, and 6l), 2P’’Zn–5PZn are hole
transporters. From the maximum values of the TRMC sig-
nals, together with TAS-based quantum efficiencies of the
photochemically generated radical cations, one-dimensional
hole mobilities mh were evaluated. As shown in Figure 5, the
mh values of 2P’’Zn–5PZn are all in a range of 10À3–
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Use of Triply Fused Metalloporphyrin Trimer–Pentamer
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in the Supporting Information). However, the observed mes-
ophase temperature ranges (124–114 and 114–28C upon
cooling) were substantially different from those of the
parent LCs (Figures 1 and S7 in the Supporting Informa-
tion). Furthermore, these LC mesophases both adopted an
oblique columnar geometry just like the case of 3PZn, in
which the lattice parameters (a=65.4, b=47.8 ꢁ, and g=
1038 at 1208C; and a=64.7, b=39.0 ꢁ, and g=978 at 908C;
Figure 3m and Table S2 in the Supporting Information),
however, were different from those of 3PZn (see before).
Therefore, it is clear that 2P’’Zn and 3PZn upon mixing coas-
semble uniformly without phase separation. The same is
true for a binary mixture of 2P’’Zn and 4PZn (1:1), which
showed a single LC mesophase with an oblique columnar
geometry (Figure 3n). We also found that 3PZn and 4PZn
upon mixing (1:1) gives an oblique columnar mesophase. In
this case, 4PZn likely dominates in determining the meso-
phase character, because the XRD pattern of this mixture
(a=61.0, b=57.0 ꢁ, and g=928 at 908C; Figure 3o and
Table S2 in the Supporting Information) well resembles that
of 4PZn alone (a=64.5, b=61.3 ꢁ, and g=918 at 908C). Fi-
nally, we investigated if a ternary mixture of 2P’’Zn, 3PZn,
and 4PZn forms a uniform LC mesophase. As expected,
a 1:1:1 mixture of these three LC molecules showed a broad
absorption profile covering from the entire visible region up
to a NIR region below l=2100 nm (Figure S11 in the Sup-
porting Information). Although the XRD profile (Fig-
ure 3p) was too poor to determine the mesophase geometry
(only provisionally, oblique geometry), the mixture in DSC
showed birefringent LC mesophases (8–90 and 90–1188C on
heating; Figures 5 and S8 in the Supporting Information). If
the coassembly is not accompanied by deterioration of the
electroconducting profiles, such a wide-range tunability of
light-absorbing window for sunlight[20] is beneficial for high-
efficacy photon-energy-conversion systems (Figure S10 in
Figure 4. UV/Vis-NIR absorption spectra at 258C of 3PZn (green), 4PZn
(red), and 5PZn (blue) in a) CHCl3 and b) the liquid-crystalline state.
10À2 cm2 VÀ1 sÀ1, in which pentameric 5PZn featuring the larg-
est p-extended mesogen gives the highest mobility; roughly
one order of magnitude greater than that of dimeric 2P’’Zn.
Unless particularly attractive interactions, such as charge-
transfer complexation, operate,[19] discotic LC molecules do
not, in general, coassemble despite their mesogenic motifs
being similar. However, we found that 2P’’Zn–4PZn coassem-
ble without phase separation. As a typical example, 2P’’Zn
and 3PZn were mixed together in CH2Cl2 at a 1:1 molar
ratio, and the residue obtained by evaporation to dryness
was heated to 2008C and then allowed to cool to room tem-
perature. Over a wide temperature range, this binary mix-
ture displayed a birefringent texture characteristic of colum-
nar LCs (Figure 3i). Just as 2P’’Zn and 3PZn alone, their mix-
ture in DSC showed two LC mesophases (Figures 5 and S8
Figure 5. Phase profiles of 2P’’Zn, 3PZn, 4PZn, 5PZn, and their equimolar mixtures. Maximum values of FP-TRMC signals and hole mobilities observed
for their liquid-crystalline mesophases at 258C.
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T. Aida et al.
Conclusion
By using trimeric, tetrameric,
and pentameric triply fused
zinc porphyrins as extra-large
tape-like p-extended mesogens,
we successfully developed ultra-
low-bandgap columnar LC sem-
iconductors 3PZn–5PZn with
very wide LC mesophase tem-
perature ranges and light-ab-
sorption windows. They com-
monly adopt an oblique colum-
nar geometry typical of colum-
nar LC materials involving
strong
mesogenic
interac-
tions.[15] The intrinsic hole mo-
bility of pentameric 5PZn is
nearly one order of magnitude
greater than that of dimeric
2P’’Zn. Despite a large dimen-
sional difference in the meso-
gens involved, 2P’’Zn–4PZn,
unlike most other discotic LC
molecules, coassemble without
phase separation, thereby al-
lowing for tuning the light-ab-
sorption window with substan-
tially no deterioration of the
conducting features. Elabora-
tion of fused metalloporphyrin
oligomers with optimal HOMO
and LUMO levels for the
photon-energy conversion[5] is
one of the subjects worthy of
further investigation.
Experimental Section
General: Unless otherwise noted, all
commercial reagents were used as re-
ceived. Recycling preparative high-
performance liquid chromatography
(HPLC) was performed at RT by
using a JAIGEL SIL SH043–10 silica-
gel column on a Japan Analytical In-
dustry (JAI) Model LC-918 HPLC
Figure 6. Transient absorption spectra (two-dimensional images) at 258C of a) 2P’’Zn, d) 3PZn, g) 4PZn, and
j) 5PZn on a quartz plate upon photoexcitation at l=355 nm. Bright and dark-colored dots indicate higher and
lower optical densities, respectively. Transient absorption spectra at 258C of b) 2P’’Zn, e) 3PZn, h) 4PZn, and
k) 5PZn on a quartz plate upon photoexcitation at l=355 nm. Differential absorption spectra in CH2Cl2 upon
+
+
+
+
C
C
C
C
generation of c) 2P’’Zn from 2P’’Zn, f) 3PZn from 3PZn, i) 4PZn from 4PZn, and l) 5PZn from 5PZn. Radi-
+
+
+
cal cation species 2P’’Zn +, 3PZn 4PZn +, and 5PZn were generated by the treatment with (4-BrC6H4)3N
C
C
C
C
C
À
SbCl6 in CH2Cl2.
system, equipped with
a variable-
wavelength UV/Vis detector. Recy-
cling preparative size-exclusion chro-
matography (SEC) was performed by
the Supporting Information). Thus, we examined by FP-
using JAIGEL 2H and 2.5H polystyrene-gel columns on a JAI model
LC-908 recycling preparative HPLC equipped with a JASCO model MD-
2010 Plus multiwavelength detector. Electronic absorption spectra were
recorded on a JASCO model V-670 spectrometer. 1H NMR spectra were
recorded in CDCl3 or [D8]THF on a JEOL Type GSX-270 spectrometer
operating at 270 MHz or a NM-Excalibur 500 spectrometer operating at
500 MHz, in which chemical shifts were determined with respect to non-
deuterated solvent residues as internal references. Matrix-assisted laser
TRMC when the binary and ternary mixtures are “electroni-
cally” uniform or defective. As shown in Figure 5, the carri-
er-transport properties of 2P’’Zn/3PZn, 2P’’Zn/4PZn, 3PZn/
4PZn, and 2P’’Zn/3PZn/4PZn are all in the same range and
also comparable to those of their parent LCs.
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Use of Triply Fused Metalloporphyrin Trimer–Pentamer
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desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry
was performed on an Applied Biosystems BioSpectrometry Worksta-
tionTM model Voyager-DETM STR spectrometer using dithranol as
a matrix. POMs were recorded on a Nikon model OPTIPHOT polarized
optical microscope equipped with a Mettler FP-82HT hot stage. DSC
measurements were performed on a Mettler model DSC 822e differential
scanning calorimeter. Cooling and heating profiles were recorded and an-
alyzed with a Mettler model STARe system. X-ray diffraction experi-
ments were carried out by using a synchrotron radiation X-ray beam
with a wavelength of l=1.08 ꢁ on BL44B2 at the Super Photon Ring
(SPring-8, Hyogo, Japan). A large Debye–Scherrer camera was used in
conjunction with an imaging plate as a detector, and all diffraction pat-
terns were obtained with a 0.018 step in 2q. During the measurements, all
samples put into a 0.5 mm thick glass capillary were rotated to obtain ho-
mogeneous diffraction patterns. The exposure time to the X-ray beam
was 2 min.
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temporal profiles of light intensity.
Acknowledgements
T.S. thanks the Japan Society for the Promotion of Science for a Young
Scientist Fellowship. The synchrotron radiation experiments were per-
formed at BL44B2 in SPring-8 with the approval of RIKEN (Proposal
No. 20110026).
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Received: March 31, 2012
Published online: && &&, 0000
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Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Use of Triply Fused Metalloporphyrin Trimer–Pentamer
FULL PAPER
Liquid Crystals
S. Tanaka, T. Sakurai, Y. Honsho,
A. Saeki, S. Seki, K. Kato, M. Takata,
A. Osuka, T. Aida* .......... &&&& — &&&&
Toward Ultralow-Bandgap Liquid
Crystalline Semiconductors: Use of
Triply Fused Metalloporphyrin
Trimer–Pentamer as Extra-large p-
Extended Mesogenic Motifs
Low bandgap: Triply fused zinc por-
phyrin trimer–pentamers serve as
extra-large p-extended mesogens for
the formation of ultralow bandgap col-
umnar liquid-crystalline (LC) semicon-
ductors 3PZn–5PZn with very wide LC
mesophase temperature ranges and
light-absorption windows. The mobility
of 5PZn is roughly one order of magni-
tude greater than that of dimeric refer-
ence 2P’’Zn (see figure).
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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ÞÞ
These are not the final page numbers!