Segregated and alternately stacked donor/acceptor nanodomains in tubular morphology tailored with zinc porphyrin-C60 amphiphilic dyads: Clear geometrical effects on photoconduction

Article abstract of DOI:10.1021/ja211334k

Amphiphilic zinc porphyrin (P_Zn; electron donor, D)-fullerene (C₆₀; electron acceptor, A) dyads 2 and 3, bearing an identical hydrophilic wedge with triethylene glycol chains but different linkers between the P_Zn and C

Full text of DOI:10.1021/ja211334k

Communication
pubs.acs.org/JACS
Segregated and Alternately Stacked Donor/Acceptor Nanodomains
in Tubular Morphology Tailored with Zinc Porphyrin−C60
Amphiphilic Dyads: Clear Geometrical Effects on Photoconduction
Richard Charvet,*,†,○ Yohei Yamamoto,*,†,‡ Takayuki Sasaki,‡ Jungeun Kim,§ Kenichi Kato,#,∥
Masaki Takata,§,∥ Akinori Saeki,# Shu Seki,# and Takuzo Aida*,†,⊥
†ERATO−SORST Nanospace Project, Japan Science and Technology Agency (JST), 2-3-6 Aomi, Koto-ku, Tokyo 135-0064, Japan
‡Division of Materials Science and Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Faculty of Pure and
Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
§Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
∥RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
#Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
⊥Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-8656, Japan
S
* Supporting Information
heterojunctions.3 In fact, we succeeded in tailoring elaborate
D−A heterojunctions in fibrous4 and tubular5 morphologies
ABSTRACT: Amphiphilic zinc porphyrin (PZn; electron
and also in liquid crystals6 using strategically designed D−A
donor, D)−fullerene (C60; electron acceptor, A) dyads 2
and 3, bearing an identical hydrophilic wedge with
triethylene glycol chains but different linkers between
the PZn and C60 units, self-assemble into nanotubes with
essentially different dimensional and geometrical features
from one another. The nanotube from dyad 2 with an
ester linker consists of a bilayer wall formed with coaxially
segregated D and A nanodomains along the tube axis
(coaxial D−A heterojunction), thereby displaying explicit
photoconductivity with ambipolar carrier transport proper-
ties. In contrast, the nanotube from dyad 3 with a rigid
arylacetylene linker consists of a monolayer wall with an
alternate geometry of D/A stacking, resulting in poor
photoconducting outputs. Such a geometrical difference
also significantly affects the photovoltaic properties.
dyads. Successful examples so far reported by our group feature
hexabenzocoronene and oligothiophene as D components,
which are combined with trinitrofluorenone, fullerene, and
perylenediimide as electron acceptors.4−6 Some of the resultant
assemblies indeed displayed remarkable photoconductive and
even photovoltaic outputs.5a−c,6 However, their absorption
coefficients for visible light are not satisfactorily large.
Therefore, we newly focused on porphyrin derivatives as the
donor components and designed amphiphilic porphyrin−
fullerene (C60) dyad 1 (Figure 1). As reported previously,
this compound self-assembles into photoconducting nanofibers
with a core−shell A−D heterojunction.7 However, such an ideal
one-dimensional (1D) nanostructure develops only when dyad
1 is enantiomerically pure with respect to the chiral fullerene
unit. In contrast, its racemate self-assembles into spheres.
Obviously, the molecular design involving optical resolution is
tedious and not practical in view of device applications.
Here we report newly designed zinc porphyrin (PZn)−C60
dyads 2 and 3 (Figure 1), which self-assemble into discrete 1D
nanostructures. Although dyads 1−3 differ from one another in
their linker parts for connecting the porphyrin and C60 units,
the nanostructures from dyads 2 and 3 are both tubules (Figure
4) rather than solid fibers. As highlighted in this communica-
tion, these nanotubes possess completely different D/A
geometries from each other. The nanotube from 2 consists of
a bilayer wall featuring coaxially segregated D and A domains
along the tube axis (Figure 4a). In sharp contrast, the nanotube
from 3 is composed of a monolayer wall with an alternate
geometry of D/A stacking (Figure 4b). Noteworthy, the former
nanotube displays much better photoconduction profiles and
photovoltaic outputs than the latter.
ailoring electron donor (D)−acceptor (A) heterojunc-
Ttions is one of the most essential subjects in the design of
optoelectronic materials for photon−energy conversion.1 In
particular, for the development of high-performance solution-
processable photovoltaic cells, rational molecular assembling
strategies are needed to construct segregated D and A domains
for transporting holes and electrons, respectively. Furthermore,
such segregated D and A domains should be connected
together at a wider interface, preferably with nanoscale
precision, for a better photoinduced charge-separation
efficiency.2 Although utilization of so-called ‘bulk D−A
heterojunctions’ may be a practical solution for device
fabrication, such structures are hard to elaborate because they
are formed just coincidentally by phase separation of D and A
components. Moreover, D and A tend to form a charge-transfer
(CT) complex, which often hampers photovoltaic outputs. In
this context, the use of D−A dyads, if properly designed, may
provide a rational synthetic strategy for nanoscale D−A
Received: December 4, 2011
Published: January 25, 2012
© 2012 American Chemical Society
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Figure 1. Molecular structures of dyads 1−3 and resultant
nanostructures upon self-assembly. d indicates nanotube diameters.
Figure 2. (a, c) SEM and (b, d) TEM micrographs of air-dried
suspensions of tubularly assembled 2 (a, b) and 3 (c, d).
Dyads 2 and 3 were synthesized and unambiguously
characterized as described in the Supporting Information.8
Electronic absorption spectroscopy of a toluene solution of 2
displayed a Soret band and Q-band of the PZn unit at 414 and
544 nm, respectively (Figure S1a, black).8 Differential pulse
voltammetry (DPV) of its CH2Cl2 solution showed two
reduction and multiple oxidation peaks (Figure S2a).8 From
the offsets of the redox peaks and absorption edges of the PZn
and C60 units, the energy levels of each chromophore in dyad 2
were determined as shown in Figure S2b.8 Apparently, the PZn-
to-C60 photoinduced electron transfer (PET) is energetically
allowed. In fact, most (85−90%) of the fluorescence of 2 from
its PZn unit was quenched (Figure S3).8 The same holds true
for dyad 3 (Figure S3).8
Upon addition of MeOH, a toluene solution of 2 displayed a
marked color change from pink to pale orange when the
MeOH content in the medium reached 65% in volume ([2] = 4
× 10−5 M, Figure S1a, inset).8 When allowed to stand overnight
at 25 °C, the resulting solution turned into an orange-red-
colored suspension (Figure S1a, inset).8 Of interest, in ∼4 h
after the addition of MeOH, the Soret absorption band at 414
nm fell off abruptly, affording a new absorption band at 441 nm
with a shoulder at 400 nm (Figure S1a and S1b).8 The spectral
change thus observed is typical of J-aggregated zinc
porphyrins.9 Scanning (SEM; Figure 2a) and transmission
(TEM; Figure 2b) electron micrographs of an air-dried
suspension allowed us to confirm the formation of nanotubes5
with a uniform diameter and wall thickness of 32 and 5.5 nm,
respectively. Considering the edge-to-edge distance between
the PZn and C60 units in 2 (∼3.2 nm when extended), the
nanotubular wall most likely adopts a bilayer configuration. The
nanotube displayed electron diffractions (EDs) with a
periodicity of 0.51 nm (Figure S4),8 which is assignable to
the stacking distance of the J-aggregated PZn units.
Furthermore, X-ray diffraction (XRD) analysis (Figure 3, red)
showed a set of three distinct peaks with 2θ = 3.45°, 6.62°, and
12.6° (d-spacings: 1.79, 0.935, and 0.493 nm, respectively),
along with broad halos (2θ = 7° and 14°) by scattering of the
TEG domain. Since such sharp diffractions are likely caused by
Figure 3. XRD patterns of tubularly assembled 2 (red) and 3 (blue).
Numerical values indicate d-spacings (nm).
Zn2+, the PZn units in the nanotubular wall are placed
periodically. Therefore, we conclude that the nanotube adopts
a coaxial D/A configuration, where an inner tubular domain,
composed of clustered C60 units (A), is laminated on both of its
sides by a molecular layer of J-aggregated PZn units (Figure 4a).
Consequently, the hydrophilic TEG chains cover both the inner
and outer surfaces of the nanotube. In fact, a water droplet on a
cast film of the nanotubes showed a contact angle of 32°
(Figure S5a),8 which is far smaller than that on a film sample
cast from a CH2Cl2 solution of nonassembled 2 (64°, Figure
S5b).8
As described previously, dyad 3, when allowed to assemble in
toluene/MeOH, likewise afforded nanotubules. However,
judging from the SEM (Figure 2c) and TEM (Figure 2d)
micrographs, the tube diameter (d = 7−8 nm) and wall
thickness (t = 1.7−1.8 nm) are much smaller than those of
nanotubularly assembled 2 (vide ante, d = 32 nm, t = 5.5 nm).
Note that the wall thickness thus estimated is even smaller than
the edge-to-edge distance between the PZn and C60 units in 3
(∼3.2 nm), suggesting that the nanotubular wall is a monolayer,
where the dyad most likely adopts a tilted orientation relative to
the radial direction of the nanotube (Figure 4b). Suppose that
the TEG chains of 3 prefer to be exposed to the polar medium
(toluene/MeOH) as in the case of the assembly of 2 (vide
ante), the PZn and C60 units in the nanotube from 3 likely stack
with an alternate geometry (Figure 4b). Accordingly, XRD
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Figure 4. Schematic representations of tubularly assembled 2 (a) and
3 (b). Blue-, orange-, and yellow-colored parts represent PZn, C60, and
linker units, respectively, while chains drawn with red and white
spheres denote triethylene glycol (TEG) chains. d and t indicate
nanotube diameters and wall thicknesses.
Figure 5. (a) I−V profiles at 25 °C of cast films of nanotubularly
assembled 2 (red) and 3 (blue) on Au electrodes upon photo-
irradiation (λ = 300−650 nm). Inset: A change in electric current at 25
°C of a cast film of tubularly assembled 2 upon turning on and off the
light. (b) TRMC profiles at 25 °C of cast films of tubularly assembled
2 (red) and 3 (blue) upon irradiation by a pulsed laser at 355 nm.
Inset shows TRMC profiles acquired in a longer time scale. (c)
Photocurrent action spectrum (red) and electronic absorption
spectrum (black) at 25 °C of a cast film of tubularly assembled 2.
(d) Time-of-flight (TOF) transient photocurrent profiles at 25 °C of a
cast film of tubularly assembled 2 for hole (green) and electron (pink)
under E-fields of −1.27 × 105 and +1.27 × 105 V cm−1, respectively,
upon irradiation by a pulsed laser at 355 nm. Inset shows hole (green)
and electron (pink) mobilities as a function of applied E-field. (e, f) I−
V (e) and log I−V (f) profiles at 25 °C of cast films of tubularly
assembled 2 (red) and 3 (blue) in PV devices in the dark (open
circles) and upon photoirradiation (closed circles, light power density;
46 mW cm−2).
analysis of tubularly assembled 3 displayed a characteristic
diffraction with 2θ = 4.95° (d-spacing: 1.25 nm; Figure 3, blue),
which is assignable to the distance between the PZn units that
sandwich C60.10 In relation to this observation, tubularly
assembled 3 in toluene/MeOH showed a weak and broad
absorption tail centered at 700 nm (Figure S6a, inset),8 which is
assignable to a CT absorption band typically observed for zinc
porphyrins upon complexation with fullerenes.10,11 Of interest,
two X-ray diffraction peaks in a small-angle region at 2θ = 1.46°
and 2.95° (d-spacings: 4.24 and 2.10 nm, respectively; Figure 3,
blue) suggest that the PZn−C60 units upon alternate stacking in
the nanotube form a helically twisted array (Figure 4b).
Due to the coexistence of the electron donor (PZn) and
acceptor (C60) units, the nanotubes from 2 and 3 were both
photoconductive. However, their performances obviously
differed from one another. Figure 5a shows current−voltage
(I−V) profiles of cast films of tubularly assembled 2 (red) and
3 (blue) upon photoirradiation (λ = 300−650 nm), where the
former nanotube, featuring the coaxially segregated PZn−C60
heterojunction, displayed a much larger photocurrent than the
latter adopting the alternate geometry of PZn/C60 stacking. As
also shown in Figure 5a (inset), the photocurrent switching of
tubularly assembled 2 was prompt and repeatable by turning on
and off the light. When this coaxial D−A heterojunction
nanotube was exposed to a pulsed laser at 355 nm, charge
carriers were generated, whose time-resolved microwave
conductivity12 (TRMC, ϕΣμmax = 1.02 × 10−3 cm2 V−1 s−1)
was 6.5 times greater than that of the nanotube from 3 (ϕΣμmax
= 0.16 × 10−3 cm2 V−1 s−1) (Figure 5b). The photocurrent
action spectrum of the nanotube from 2 was in good agreement
with its absorption spectral profile (Figure 5c), indicating that
the photocurrent, as also expected from the PZn fluorescence
quenching (vide ante, Figure S3),8 is generated mainly by an
electron transfer from the photoexcited PZn species to the C60
unit. Of particular interest, time-of-flight (TOF) photo-
conductivity measurements indicated that tubularly assembled
2 is ambipolar and capable of transporting both holes and
electrons (Figure 5d). When the magnitude of an applied
electric field (E-field) was changed, the hole (μh) and electron
(μe) mobilities were varied (Figure 5d, inset), but they
remained rather comparable to each other over a wide E-field
range. For example, at an E-field of 0.36 × 105 V m−1, μh and μe
became identical to one another (∼1.8 × 10−3 cm2 V−1 s−1).
Such an ambipolar charge transport character with well-
balanced μh and μe is important for photovoltaic (PV)
outputs.5c In fact, tubularly assembled 2 shows a photovoltaic
response. For this study, a conventional top-contact device
configuration was not applicable because the nanotube upon
drop casting did not form a pinhole-free homogeneous film.
Instead, we employed a particular device configuration (Figure
S7)5c,8 analogous to field-effect transistors, where film samples
were allowed to bridge two different electrodes horizontally.
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Thus, a nanotube suspension was cast onto a fluoroalkyl-coated
glass substrate, prepatterned with PEDOT:PSS/Au/Ti
[PEDOT, poly(3,4-ethylenedioxythiophene); PSS, poly-
(styrenesulfonate)] and TiOx/Ti/Al electrodes with an 8−15
μm separation.8 Upon exposure to light with a power density of
46 mW cm−2 from the back side of the glass substrate, the film
sample displayed a PV response with an open-circuit voltage
(VOC) and a short-circuit current (ISC) of 0.66 V and 16 pA,
respectively (Figure 5e and 5f, red). ISC was switched promptly
and repeatedly upon turning on and off the light with an on/off
current ratio greater than 103 (Figure S8a, red).8 In sharp
contrast, a cast film of the nanotube of 3, likewise integrated
into the same device configuration, exhibited a very poor PV
response with VOC and ISC values of only 0.16 V and 0.9 pA,
respectively (Figure 5e and 5f, blue). ISC and VOC were
enhanced by increasing the light intensity (Figure S8b and S8c,
respectively)8 but much less so than the case of 2 having a
segregated D−A stacking. Although the device configuration
employed here is not appropriate for evaluating the power
conversion efficiencies, it is now clear that the nanotube of 2 is
superior to that of 3 for PV applications.
In conclusion, we developed PZn−C60 nanotubes with
segregated and alternately stacked donor (D; PZn)/acceptor
(A; C60) configurations and demonstrated clear geometrical
effects on optoelectronic outputs. The nanotube from 2 with a
coaxial D−A heterojunction along the tube axis (Figure 4a)
displays much better photoconducting properties than that
from 3 with an alternate D/A stacking geometry (Figure 4b).
Together with its explicit ambipolar carrier transport character,
the PV output of the former nanotube is superior to the latter.
The results presented here contribute to the progress of
bottom-up nanoscale organic electronics.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Details of synthesis, self-assembly, MALDI-TOF mass spectra,
NMR spectra, electronic absorption spectra, DPV, contact
angles, fluorescence spectra, SEM, TEM, ED, and PV data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
racharvet@yahoo.com; yamamoto@ims.tsukuba.ac.jp; aida@
macro.t.u-tokyo.ac.jp
Present Address
○ADOCIA, 115 avenue Lacassagne, 69003 Lyon, France.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The synchrotron radiation experiments were performed at
BL02B2 and BL44B2 in SPring-8 with the approval of JASRI
(Proposal Nos. 2008A1644 and 2008B1777, the Priority
Nanotechnology Support Program) and RIKEN (Proposal
No. 20100024), respectively. This work was partly supported
by a Grant-in-Aid for Young Scientist B (23750144) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan, and Kato Science Foundation (H23_2006).
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