Electron- or hole-transporting nature selected by side-chain-directed π-stacking geometry: Liquid crystalline fused metalloporphyrin dimers

Article abstract of DOI:10.1021/ja201272t

Novel liquid crystalline (LC) semiconductors were prepared from the copper complex of a fused porphyrin dimer as the electroactive core by attaching to its periphery dodecyl and semifluoroalkyl side chains site-specifically (P=P _hetero) and sem

Full text of DOI:10.1021/ja201272t

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Electron- or Hole-Transporting Nature Selected by Side-Chain-
Directed π-Stacking Geometry: Liquid Crystalline Fused
Metalloporphyrin Dimers
Tsuneaki Sakurai,† Kentaro Tashiro,‡ Yoshihito Honsho,§ Akinori Saeki,§ Shu Seki,§ Atsuhiro Osuka,||
Atsuya Muranaka,# Masanobu Uchiyama,#,^ Jungeun Kim,r Sunyeo Ha,X Kenichi Kato,z
Masaki Takata,r,X,z and Takuzo Aida*,†
†School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
‡National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
§Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
#Institute of Physical and Chemical Research, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
^Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
rJapan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
XGraduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
zRIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
S Supporting Information
b
first LC version of this family, which carries dodecyl side chains
ABSTRACT: Novel liquid crystalline (LC) semiconduc-
tors were prepared from the copper complex of a fused
porphyrin dimer as the electroactive core by attaching to its
periphery dodecyl and semifluoroalkyl side chains site-
specifically (PtPhetero) and semifluoroalkyl side chains
alone (PtPhomo). The former and latter formed rectangu-
lar columnar and orthorhombic LC mesophases, respec-
tively, where the stacking geometries of the π-conjugated
core are quite different from one another. Although the π-
electronic properties of the core units in PtPhetero and
PtPhomo in solution are substantially identical to one
another, transient photocurrent profiles of their LC states
under time-of-flight conditions clearly showed that PtPhetero
behaves as an n-type semiconductor, whereas PtPhomo, in
contrast, behaves as a p-type semiconductor.
on one Cu porphyrin unit and triethylene glycol (TEG) side
chains on the other.4 This particular heterotropic design was
essential for the liquid crystal formation, since a prototype of this
compound carrying only dodecyl side chains did not form a LC
assembly but rather an amorphous solid. Although most por-
phyrin-based materials have been reported to show a hole-
transporting property,5 the above LC material, in contrast,
behaved as an n-type semiconductor.4 In the present work, this
unexpected observation prompted us to investigate if the elec-
tron-transporting nature of this material is intrinsic to the core
unit of the LC molecule or its particular peripheral design. In
order to address this issue,we newly synthesized heterotropically
fused PtPhetero along with homotropic PtPhomo (Figure 1) by
using semifluoroalkyl side chains instead of the TEG chains,6
since fluorinated hydrocarbons are immiscible, just like TEG,
with hydrocarbons and also have a very strong self-assembling
nature.7 Fortunately, both compounds formed a LC mesophase
with a π-stacked columnar structure. However, to our surprise,
their preferences for carrier species were totally opposite to one
another.
n general, whether organic semiconductors transport electrons
or holes has been considered to be determined mostly by the
I
intrinsic redox properties of their constituent electroactive
molecules.1 Therefore, diversification of the range of n- and
p-type organic semiconductors totally relies on the development
of new π-conjugated structural motifs.2 Toward this general
understanding, we report here that, by tailoring assembly-directing
side chains, both n- and p-type liquid crystalline (LC) semicon-
ductors can be obtained from a single π-conjugated motif.
The electroactive core unit employed in the present work is
the Cu complex of a triply fused porphyrin dimer adopting a
characteristic oblong disk shape (Figure 1). Triply fused metal-
loporphyrin oligomers are interesting π-conjugated motifs, par-
ticularly for photovoltaic applications, because of their extraordinary
high absorptivities toward sunlight, covering from ultraviolet up
to near-infrared wavelength regions.3 In 2008, we reported the
Differential scanning calorimetry (DSC; Figure S4) of PtPhetero
,
on its second heating, displayed a LC mesophase ranging
from ꢀ26 to 163 ꢀC (Figure 1). This LC temperature range is
much wider than that of its TEG version (ꢀ17 to 99 ꢀC) previously
reported.4 Likewise, PtPhomo formed a LC mesophase in a
temperature range from ꢀ32 to 199 ꢀC (Figures 1 and S4).
Polarized optical microscopy (POM) of both PtPhetero (Figure 2a)
and PtPhomo (Figure 2b) showed a clear dendritic texture
typical of columnar LCs. In X-ray diffraction (XRD) analysis
upon synchrotron radiation, PtPhetero at 30 ꢀC displayed an
Received: February 10, 2011
Published: April 08, 2011
r
2011 American Chemical Society
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Figure 1. Molecular structures and phase transition temperatures (ꢀC)
of copper complexes of fused porphyrin dimers PtPhetero and PtPhomo
.
Transition enthalpies (kJ molꢀ1) are given in parentheses.
intense diffraction peak at 2θ = 1.05ꢀ (d = 59.1 Å) along
with several minor peaks in a wider-angle region (Figure 2c
and Table S1). As in the case of the heterotropic TEG version,4
these diffractions were successfully indexed to a 2D rectangular
lattice with a symmetry group of p2mg,8 where lattice parameters
a and b were 59.1 and 32.5 Å, respectively (Figure S3). In sharp
contrast, PtPhomo carrying semifluoroalkyl side chains alone at
170 ꢀC showed a crystal-like diffraction pattern (Figure 2d and
Table S1), which was indexed to a 3D orthorhombic lattice9 with
lattice parameters a, b, and c of 70.9, 28.4, and 20.1 Å, respectively.
Due to their LC characters, both PtPhetero and PtPhomo in ther
mesophases are shearable.9b
By means of variable-temperature absorption spectroscopy in
the film state, we found that the π-stacking geometries of the
fused Cu porphyrin core in these LC mesophases are different
from each other. Porphyrin derivatives are known to change their
absorption spectral profiles upon π-stacking.10 An isotropic melt
of PtPhetero at 150 ꢀC showed two characteristic Soret absorp-
tion bands at 407 and 561 nm (Figure 2e, red).3 When the hot
melt of PtPhetero was cooled to allow the isotropic-to-LC phase
transition, a color change from dark purple to reddish brown
resulted. Accordingly, the longer-wavelength Soret band, on
stepwise cooling from 150 to 125 ꢀC, showed a 10-nm blue shift
hypochromically. This spectral change took place abruptly
around the phase transition temperature (Figure 2e). All these
Figure 2. Characterization and properties of PtPhetero and PtPhomo
in their liquid crystalline (LC) mesophases. Polarized optical micro-
graphs of (a) PtPhetero at 150 ꢀC and (b) PtPhomo at 185 ꢀC. X-ray
diffraction patterns of (c) PtPhetero at 30 ꢀC and (d) PtPhomo at
170 ꢀC. The wavelength of incident X-ray was 1.08 Å. Changes in
absorption spectra of (e) PtPhetero on cooling from 150 to 125 ꢀC and
(f) PtPhomo on cooling from 205 to 185 ꢀC. Typical transient
photocurrent profiles at 25 ꢀC of (g) PtPhetero (sample thickness,
3.3 μm; electric field, 0.94 ꢁ 104 V cmꢀ1) and (h) PtPhomo (sample
thickness, 19 μm; electric field, 6.0 ꢁ 104 V cmꢀ1) for electron (blue)
and hole (red).
features are analogous to those observed for the TEG version of
4
PtPhetero
.
Although the absorption spectral profile of an
isotropic melt of PtPhomo at 205 ꢀC (Figure 2f, red), displaying
two Soret bands at 407 and 564 nm, was just like that of
PtPhetero (vide ante), the spectra of these compounds in the
LC state were quite different from one another. When PtPhomo
was cooled to 185 ꢀC to allow the isotropic-to-LC phase transition,
its Soret bands both became less intense (Figure 2f). Further-
more, in sharp contrast with the case of PtPhetero, the longer-
wavelength Soret band exhibited an 8-nm red shift, with a new
shoulder around 660 nm. Hence, it is clear that the π-stacking
geometry of PtPhomo in the LC state differs from that of
column (a) is formed by offset stacking of the π-conjugated core
with a “slipped geometry”, while column (b) is formed by offset
stacking of the π-conjugated core with a “twisted geometry”. As
discussed already for the TEG version of PtPhetero,4 which
forms a rectangular columnar mesophase just as PtPhetero
,
column (a) seems to be the only possible structure that can
cope with both stacking of the π-conjugated core and segregation
of its two incompatible side chains. On the other hand, the
twisted geometry in column (b) is a likely candidate for homo-
tropically substituted PtPhomo, since it allows for the formation
of a continuous array of the highly self-associating semifluoroalkyl
side chains around the column. In contrast, this geometry is
unlikely for PtPhetero, because segregation of the two incompa-
tible side chains is not allowed. Based on spectral simulation,6
a dimer model for column (a) with a slipped geometry suggests
PtPhetero
.
π-Conjugated molecules usually stack with an offset geometry
in order to minimize repulsion of π-clouds.11 Considering the
heterotropic and homotropic substitution patterns of these LC
molecules, along with reported examples with monomeric por-
phyrins,12 we suggest columnar structures (a) and (b) in Figure 3
for PtPhetero and PtPhomo in the LC state, respectively, where
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from single π-conjugated motifs. In relation to this possibility,
Brꢀedas and co-workers reported in 2002 a theoretical predic-
tion15a that assembled π-conjugated motifs possibly switch their
preference for carrier species by changing the π-stacking
geometry.15 However, clear experimental supports have not been
provided so far.
In conclusion, we demonstrated that the copper complex of a
triply fused porphyrin dimer, when differently decorated at its
periphery with appropriate side chains, can afford both n- and
p-type liquid crystalline semiconductors (PtPhetero and PtPhomo
,
respectively), where the side chains employed hardly change the
intrinsic π-electronic properties but change the stacking geome-
try of the core. Although effects of assembly-directing side chains
on charge-carrier mobilities have been recognized for several LC
semiconductors,16 nearly perfect switching of carrier species, as
demonstrated for PtPhetero and PtPhomo, is unprecedented.
This finding is quite encouraging for diversification of organic
semiconductors from limited available structural motifs.
Figure 3. Schematic illustrations of the columnar structures (π-stacked
core parts) proposed for the (a) rectangular and (b) orthorhombic
columnar LC mesophases of PtPhetero and PtPhomo, respectively.
Judging from the XRD patterns (Figure 2c,d), the LC columns of
PtPhomo and PtPhetero are likely disordered along the columnar axis.
a blue shift of the Soret band, as observed for assembled
PtPhetero, when the translational operation is made along the
shorter axes of the π-conjugated units (Figure S5). If this opera-
tion is made along the longer axes, a blue shift can be expected
only when the displacement is marginal (Figure S6). On the other
hand, a π-stacked dimer model for column (b) with a twisted
geometry suggests a spectral red shift, as observed for assembled
PtPhomo, when the dihedral angle lies in a range of 60ꢀ90ꢀ
(Figure S7).4
’ ASSOCIATED CONTENT
S
Supporting Information. Details of synthesis and char-
b
acterization of fused metalloporphyrin derivatives, MALDIꢀ
TOF mass and NMR spectra, X-ray diffraction patterns, electronic
absorption spectra, time-of-flight data, and spectral simulation.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Wemeasured photocurrentprofiles of PtPhetero and PtPhomo
in their LC states (25 ꢀC) using a time-of-flight (TOF) technique.
As shown in Figure 2g (blue), PtPhetero displayed an explicit
n-type semiconducting character, where the electron mobility
under an applied electric field of 0.94 ꢁ 104 V cmꢀ1 was evaluated
as 1.3 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 (Figure S8).6 Although the TOF
profile also showed a small p-type signature, the photocurrent was
too small to evaluate.13 To our surprise, PtPhomo, in sharp
contrast with PtPhetero, behaved as a p-type semiconductor. In
the TOF profile of PtPhomo in its LC mesophase, a photocurrent
appeared only when the applied electrical bias was positive
(Figure 2h, red). In accordance with this observation, transient
absorption spectroscopy (TAS)14 of PtPhomo (Figure S12)
showed an absorption band around 690 nm, which can be assigned
toradical cation speciesPtPhomo•þ by reference to the differential
’ AUTHOR INFORMATION
Corresponding Author
aida@macro.t.u-tokyo.ac.jp
’ ACKNOWLEDGMENT
The synchrotron radiation experiments were performed at
BL02B2 and BL44B217 in SPring-8 with the approval of JASRI
(Proposal No. 20090021, the Budding Researchers Support
Proposal) and RIKEN (Proposal No. 2009A1651, the Priority
Nanotechnology Support Program), respectively. T.S. thanks the
Japan Society for the Promotion of Science for a Young Scientist
Fellowship.
•þ
spectral profile between chemically generated PtPhomo and
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