A carbon nanohorn-porphyrin supramolecular assembly for photoinduced electron-transfer processes

Article abstract of DOI:10.1002/chem.201000299

A supramolecular assembly of zinc porphyrin-carbon nanohorns (CNHs) was constructed in a polar solvent. An ammonium cation was covalently connected to the CNH through a spacer (sp) (CNH-sp-NH₃⁺) and bound to a crown ether linked to a zinc porphyrin (Crown-ZnP). Nanohybrids CNH-sp-NH ₃⁺;Crown-ZnP and CNHsp-NH₃⁺ were characterized by several techniques, such as high-resolution transmission electron microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. The photoinduced electron-transfer processes of the nanohybrids have been confirmed by using time-resolved absorption and fluorescence measurements by combining the steady-state spectral data. Fluorescence quenching of the ZnP unit by CNH-spNH₃⁺ has been observed, therefore, photoinduced charge separation through the excited singlet state of the ZnP unit is suggested for the hybrid material, CNH-sp-NH₃⁺;Crown-ZnP. As transient absorption spectral experiments reveal the formation of the radical cation of the ZnP unit, electron generation is suggested as a counterpart of the charge-separation on the CNHs; such an electron on the CNHs is further confirmed by migrating to the hexylviologen dication (HV²⁺). Accumulation of the electron captured from HV⁺ is observed as electron pooling in solution in the presence of a hole-shifting reagent. Photovoltaic performance with moderate efficiency is confirmed for CNH-sp-NH₃⁺;CrownZnP deposited onto nanostructured SnO₂ films.

Full text of DOI:10.1002/chem.201000299

DOI: 10.1002/chem.201000299
ꢀ
A Carbon Nanohorn Porphyrin Supramolecular Assembly for Photoinduced
Electron-Transfer Processes
Marꢀa Vizuete,[a] Maria Josꢁ Gꢂmez-Escalonilla,[a] Josꢁ Luis G. Fierro,[b]
Atula S. D. Sandanayaka,[c] Taku Hasobe,*[c, d] Masako Yudasaka,*[e] Sumio Iijima,[e]
Osamu Ito,*[f] and Fernando Langa*[a]
Abstract: A supramolecular assembly
time-resolved absorption and fluores-
cence measurements by combining the
steady-state spectral data. Fluorescence
quenching of the ZnP unit by CNH-sp-
cal cation of the ZnP unit, electron
generation is suggested as a counter-
part of the charge-separation on the
CNHs; such an electron on the CNHs
is further confirmed by migrating to
the hexylviologen dication (HV2+). Ac-
ꢀ
of zinc porphyrin carbon nanohorns
(CNHs) was constructed in a polar sol-
vent. An ammonium cation was cova-
lently connected to the CNH through a
spacer (sp) (CNH-sp-NH3+) and bound
to a crown ether linked to a zinc por-
+
NH3 has been observed, therefore,
photoinduced
charge
separation
through the excited singlet state of the
cumulation of the electron captured
+
ꢀ
C
phyrin (Crown ZnP). Nanohybrids
ZnP unit is suggested for the hybrid
from HV
is observed as electron
+
+
ꢀ
ꢀ
CNH-sp-NH3 ;Crown ZnP and CNH-
material, CNH-sp-NH3 ;Crown ZnP.
pooling in solution in the presence of a
hole-shifting reagent. Photovoltaic per-
+
sp-NH3 were characterized by several
As transient absorption spectral experi-
ments reveal the formation of the radi-
techniques, such as high-resolution
transmission electron microscopy, ther-
mogravimetric analysis, X-ray photo-
electron spectroscopy, and Raman
spectroscopy. The photoinduced elec-
tron-transfer processes of the nanohy-
brids have been confirmed by using
formance with moderate efficiency is
+
ꢀ
confirmed for CNH-sp-NH3 ;Crown
ZnP deposited onto nanostructured
SnO2 films.
Keywords: carbon nanohorns
·
electron transfer photovoltaic
·
cells · porphyrinoids · supramolec-
ular chemistry
[a] M. Vizuete, Dr. M. J. Gꢀmez-Escalonilla, Prof. F. Langa
Instituto de Nanociencia, Nanotecnologꢁa
y Materiales Moleculares (INAMOL)
Universidad de Castilla-La Mancha
45071 Toledo (Spain)
[e] Dr. M. Yudasaka, Prof. S. Iijima
Nanotube Research Center
National Institute of Advanced Industrial and Technology
Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
E-mail: m-yudasaka@aist.go.jp
Fax : (+34)925268840
E-mail: Fernando.LPuente@uclm.es
[f] Prof. O. Ito
Institute of Multidisciplinary Research for Advanced Materials
Tohoku University, Katahira, Sendai, 980-8577 (Japan)
E-mail: ito@tagen.tohoku.ac.jp
[b] Prof. J. L. G. Fierro
Instituto de Catꢂlisis y Petroleoquꢁmica, CSIC
Cantoblanco, 28049, Madrid (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000299.
[c] Dr. A. S. D. Sandanayaka, Dr. T. Hasobe
School of Materials Science
Advanced Institute of Science and Technology
Asahidai, Nomi, 923-1292 (Japan)
[d] Dr. T. Hasobe
Department of Chemistry, Faculty of Science and Technology
Keio University, Yokohama 223-8522 (Japan)
PRESTO, JST, 4-1-8, Kawaguchi, Saitama, 332-0012 (Japan)
E-mail: hasobe@keio.ac.jp
10752
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FULL PAPER
Introduction
and the hole-shift reagents in the solution, which leads to
photovoltaic cells.[8] Recently, it has been confirmed that
photo-illumination of the chemically attached chromophores
on CNHs induces efficient charge separation.[10] We also re-
ported that the chemically modified CNHs are dispersible in
organic solvents, when the attached chromophores are solu-
ble in the solvents.[11–14] A few studies have reported the ap-
plication of chemically modified CNHs in photovoltaic cells,
with the porphyrins dispersible in solutions;[15] however, the
maximum efficiency was relatively low compared with other
nanocarbon structures.[8] Thus, it is necessary to modify the
type of connection by using other porphyrins to improve the
stabilization of the photoinduced charge-separated states to
the extent that they persist for appropriately long times to
mediate the electron-hole pair to the appropriate electrodes.
Finally, we have constructed photovoltaic cells to obtain
higher efficiencies.
Carbon-based nanostructured materials have been exten-
sively investigated for use in nanotechnological applica-
tions.[1] The most widely investigated all-carbon materials
are fullerenes and carbon nanotubes.[1] Recently, carbon
nanohorns (CNHs), which are single-graphene tubules with
highly strained conical ends, have been synthesized and are
intriguing materials.[2] In CNHs, horn-like structures are as-
sembled to form dahlia-like spherical aggregates with diam-
eters ranging between 50 and 100 nm.[2] One of the merits of
CNHs is the high carbon purity due to the absence of any
metal impurities, resulting from the synthetic procedure of
using laser ablation methods on highly purified graphite
without any metal catalysts.[2] Thus, CNHs have attracted a
great deal of attention for various potential applications,
such as gas storage, catalyst supports, drug carrier systems,
and optoelectronic devices.[2]
To fabricate the CNHs on substrates, it is important to
disperse CNHs in solution and thus obtain homogeneous
CNH solutions, which then allows various precise studies
with spectroscopic methods to be carried out on the solution
state.[3] Besides the dispersion improvements, covalent and
noncovalent attachments of various active compounds both
onto the graphite-like side walls and at the conical-shaped
tips, which only slightly perturb the p-electronic network of
CNHs, have improved the wide applications of the dispersed
CNHs. In this respect, Tagma-
In the present study, we first synthesized the ammonium
cation-connected CNH through a spacer (sp) (CNH-sp-
NH3+), which is dispersible homogeneously in polar sol-
+
vents; therefore, CNH-sp-NH3 has potential for various ap-
+
plications.[16] We further functionalized CNH-sp-NH3 with
[17]
ꢀ
a crown ether appended to a porphyrin (Crown ZnP) to
+
ꢀ
produce an inclusion complex, CNH-sp-NH3 ;Crown ZnP,
(Scheme 1) to investigate the light-induced charge-separa-
tion processes in polar media.
tarchis and co-workers have
succeeded in
a
number of
chemical
modifications of
CNHs.[3]
To mimic natural photosyn-
thesis, nanohybrids consisting of
carbon-based materials and por-
phyrin pigments are particularly
promising as artificial photosyn-
thetic models,[4,5] in which the
visible-light absorbing porphyr-
ins act as light-harvesting an-
tennae and high electron
donors.[6,7] On the other hand,
carbon-based materials, such as
fullerenes and carbon nano-
tubes act as strong electron ac-
ceptors. Thus, the porphyrins
form donor–acceptor nanohy-
brids with carbon nanomateri-
als, which function as photosyn-
thetic reaction centers under
light illumination.[4–7] It has also
been reported that direct exci-
tation of the nanocarbon mate-
rials with light also induces the
charge separation;[8,9] thus, gen-
erated electrons on CHNs can
+
ꢀ
be transferred to the electrodes Scheme 1. Formation of CNH-sp-NH3 ;Crown ZnP.
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10753

F. Langa, T. Hasobe, M. Yudasaka, O. Ito et al.
Results and Discussion
8.70 as one broad singlet. Finally, the MALDI-TOF mass
spectrometric analysis revealed the molecular ion peak of
ꢀ
ꢀ
ꢀ
Synthesis of Crown ZnP: Crown ether porphyrin (Crown
ZnP) was prepared according to the procedures shown in
Scheme 2. The precursor porphyrin with a methyl ester
Crown ZnP at m/z 1171.3244 (calcd: 1170.44) (see Fig-
ure S4 in the Supporting Information).
Preparation and characteriza-
+
tion of CNH-sp-NH3
:
As
shown in Scheme 3, the treat-
ment of the CNH-COOH with
tert-butyl-N-(6-aminohexyl) car-
bamate
hexane;
(N-Boc-1,6-diamino-
Boc=tert-butoxycar-
bonyl) in DMF afforded the
protected f-CNH. Finally, the
Boc protecting group of the f-
CNH was easily cleaved upon
treatment with HCl/dioxane
(4m) yielding quantitatively the
+
target CNH-sp-NH3
.
+
The target CNH-sp-NH3
was characterized by using vari-
ous analytical and spectroscopic
methods, all data being consis-
tent with the proposed struc-
ture. Figure 1 shows the Raman
+
spectrum of CNH-sp-NH3
,
which reveals the presence of
two prominent bands, one at n˜
ꢁ1350 cmꢀ1 (D-band) and one
n˜ ꢁ1590 cmꢀ1
(G-band).
Scheme 2. Reagents and conditions: a) BF3OEt2, CHCl3, RT, 1 h; then DDQ, RT, 1 h, EtN3, 16%; b) LiAlH4,
THF, RT, 12 h, 98%; c) 4’-(chlorocarbonyl)benzo[18]crown-6, CH2Cl2, pyridine, reflux, 4 days, 35%; d) Zn-
ACHTUNGTRENNUNG(OAc)2·2H2O, CHCl3, MeOH, reflux, 2 h, 49%.
at
These bands are known to be
characteristic of CNHs.[20] The
D-band is attributed to defect
sites, such as sp3 carbons, origi-
group (1) was prepared in 16%
yield by the Lindsey method[18]
and the intermediate porphyrin
with an alcohol group (2) was
obtained in 98% yield by using
a literature procedure.[19] Por-
phyrin with crown ether (3) was
obtained in moderate yield
(35%) by esterification of alco-
hol 2[18] with 4’-(chlorocarbo-
nyl)benzo[18]crown-6 in reflux-
ing dichloromethane. Finally,
the free-base porphyrin 3 was
treated with zinc acetate to
ꢀ
Scheme 3. Reagents and conditions: a) H2O2; b) NH2-(CH2)6-NH-Boc, DMF, 508C, 5 days; c) HCl–dioxane
(4m), 12 h.
obtain the desired Crown ZnP in 49% yield. All these com-
nating from a one-phonon second-order process producing
an elastic scattering[21] and is commonly used for monitoring
the process of functionalization (change of C-atom hybridi-
zation from sp2 to sp3). An increase in the intensity of the
D-band for CNH-COOH and CNH-sp-NH3+, as compared
to that of the pristine CNH, was observed (Figure 1), which
provides evidence of functionalization of these two materi-
als. Furthermore, the additional increase of the D-band
pounds were characterized by H NMR and 13C NMR spec-
1
troscopy, and MALDI-TOF mass spectrometry (see the
Supporting Information). Analysis of the H NMR spectra
1
ꢀ
of Crown ZnP revealed all the expected signals (see Fig-
ure S2 in the Supporting Information) including the
[18]crown-6 entity hydrogen signals, which appear between
d=3.5 and 4.5 ppm; the typical fingerprint of the porphyrin
appears at d=8.87 and 8.75 ppm as two doublets and at d=
+
from CNH-COOH to CNH-sp-NH3 is attributed to the fur-
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Porphyrin Carbon Nanohorn Assembly
FULL PAPER
+
NH3 by identifying and quantifying the functional groups
anchored to the surface of CNHs.[22] The high-resolution
C 1s, O 1s, N 1s, and Cl 2p core-level spectra of a sample of
+
CNH-sp-NH3 are displayed in Figure 3a–d, respectively.
Figure 1. Raman spectra of pristine CNH (solid line), CNH-COOH (dash
+
dotted line) and CNH-sp-NH3 (dotted line) obtained by excitation at
l=532 nm. The spectra are normalized with the peak intensity of the G
band at l=1590 cmꢀ1; the peak height at l=1350 cmꢀ1 increases in the
order of pristine CNH (solid line), CNH-COOH (dash dotted line), and
+
CNH-sp-NH3 (dotted line).
ther functionalization (and additional sp3 carbon atoms)
owing to the presence of the amide group, ethylene spacer,
+
and -NH3
.
+
To further characterize CNH-COOH and CNH-sp-NH3
,
thermogravimetric analysis (TGA) was employed. In
+
Figure 2, the obtained thermogram for CNH-sp-NH3 is de-
Figure 3. High-resolution core-level spectra of CNH-sp-NH3+Clꢀ: a) C 1s,
b) O 1s, c) N 1s, and d) Cl 2p.
Figure 2. TGA curves recorded on raising the temperature at a rate of
108C minꢀ1 under N2 of pristine CNH (solid line), CNH-COOH (dash
+
dotted line), and CNH-sp-NH3 (dotted line).
The C 1s peak was satisfactorily curve-resolved with four
components (Figure 3a) according to the peak assignment
used by Stankovich et al.[23] The most intense peak at
picted and compared with that of CNH-COOH and pristine
CNH. Pristine CNH is thermally stable under a N2 atmos-
phere up to 8008C; CNH-COOH shows weight loss result-
ing from decarboxylation until 7008C by about 8% com-
284.4 eV is attributed to the sp2 C C bonds of graphitic
ꢀ
carbon. The component at 286.4 eV has been often assigned
ꢀ
ꢀ
ꢀ
to C OH (together with C N and C Cl bonds) and the
components at 287.8 and 289.6 eV to C=O and -COO- spe-
cies, respectively. A weak component at around 291.0 eV
was observed and corresponds to a p!p* transition of the
carbon atoms in graphene structures.[24] Similarly, the O 1s
spectrum has been curve-resolved with two components
(Figure 3b). A main component at 532.4 eV corresponds to
+
pared with the pristine CNH. The analysis of CNH-sp-NH3
presents a higher weight loss of about 27% at 5508C, owing
to the cleavage of the amide bond connecting the alkyl am-
monium cation. These losses correspond to one functional
group per ꢁ40 carbon atoms (6–7 benzene rings) of the
CNH framework for both compounds. This result confirms
+
that the attachment of the -sp-NH3 group occurs through
O=C surface groups and a minor one at 533.9 eV is often as-
[25]
ꢀ
the monoprotected diamine (N-Boc-1,6-diaminohexane) to
CNH-COOH with the amide bonds, followed by the acid
treatment.
X-ray photoelectron spectroscopy (XPS) provided clear
additional evidence of the proposed structure of CNH-sp-
sociated with O C bonds, demonstrating the existence of
oxygen functional groups on the graphene surface. The
high-resolution N 1s spectrum (Figure 3c) displays two com-
ponents, one at a binding energy of 400.2 eV, belonging to N
atoms bonded to alkyl chains, and another at 401.8 eV of
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F. Langa, T. Hasobe, M. Yudasaka, O. Ito et al.
+
similar intensity originating from the -NH3 moiety.[25] In
blockages near the horns of CNHs, owing to small ZnP mol-
addition, the Cl 2p core-level displayed the Cl 2p3/2 compo-
ecules (Figure 4 top right).
nent at 198.3 eV (Figure 3d) associated with the Clꢀ ion,
+
which acts as counter anion for the -NH3 moiety (see
Electrochemical characterization: Cyclic voltammetric stud-
Scheme 3).[25]
ies were performed to understand the redox behavior of the
+
ꢀ
Among the atomic percentages derived from XPS (see
the Supporting Information, Table S1), the C percentage is
85.7%, the remaining contributions are: O atoms 11.5%, N
CNH-sp-NH3 ;Crown ZnP nanohybrid. Figure 5 shows the
ꢀ
oxidation of Crown ZnP with increasing amounts of CNH-
atoms 1.8%, and Cl atoms 1.0%. The Cl atom percentage,
+
which is related to its presence as a counter ion of -NH3
,
+
reveals that the -NH3 ion accounts for almost one half of
the N content, as each CNH-sp-NH3 unit includes two
+
+
kinds of N atoms (amide N and terminal NH3 in 1:1 ratio);
thus, the ratio NH3+/Clꢀ =0.9 fits fairly well with the theo-
retical value of 1, as expected from the formula in
Scheme 3. Excess O atoms may result from unreacted
-COOH and/or adsorbed O2.
+
ꢀ
Preparation of the nanohybrid CNH-sp-NH3 ;Crown ZnP:
+
ꢀ
By mixing Crown ZnP with CNH-sp-NH3 in DMF at
+
ꢀ
room temperature, the nanohybrid CNH-sp-NH3 ;Crown
ZnP was readily prepared as a stable, homogeneous, black
solution. The formation of CNH-sp-NH3 ;Crown ZnP was
substantiated by several techniques, as shown in the follow-
ing sections.
+
ꢀ
ꢀ
Figure 5. Cyclic voltammograms of Crown ZnP (blue line) upon addition
+
of increasing amounts of CNH-sp-NH3 (red to green line (final concen-
trations=Crown ZnP (4.27ꢄ10ꢀ7 m) and CNH-sp-NH3 (0.5 mg/100 mL;
+
HRTEM characterization of the nanohybrids: The nano-
ꢀ
+
ꢀ
1720 mL in 2 mL)) containing 0.1m (nBu)4NClO4 in DMF. The black line
structures of CNH-sp-NH3 ;Crown ZnP were observed by
corresponds to CNH-sp-NH3+. Scan rate: 100 mVsꢀ1
.
using high-resolution transmission electron microscopy
(HRTEM). A typical image is shown in Figure 4 top, in
which the dahlia-like structure is visible, meaning that the
chemical treatments do not destruct much of the original
+
ꢀ
sp-NH3 . The first oxidation potential of Crown ZnP was
located at E1/2 =0.37 V versus Ag/AgNO3, corresponding to
the one-electron oxidation of the ZnP moiety. Addition of
dahlia-like structure. The existence of Zn atoms was clearly
+
ꢀ
indicated in the EXD spectrum of CNH-sp-NH3 ;Crown
+
ZnP, as shown in Figure 4 bottom, which corresponds to
CNH-sp-NH3 resulted in a small cathodic shift of the oxi-
ꢀ
dation wave (ꢁ20 mV) of the Crown ZnP moiety, owing to
the interaction with CNH moieties,[17] which supports the
fact that the high electron-donor ability of the ZnP moiety
is retained even in the nanohybrids.
Steady-state absorption spectroscopic characterization: Ab-
ꢀ
sorption spectral changes of Crown ZnP on addition of
CNH-sp-NH3 in DMF solution are shown in Figure 6. The
+
sharp absorption band at l=420 nm is assigned to the Soret
band of the ZnP unit. The intensity of the l=420 nm band
decreases on addition of CNH-sp-NH3+, with featureless
bands in the UV-visible and near-IR regions, which is an ad-
ditional piece of evidence for the formation of the inclusion
[17]
ꢀ
complexes with Crown ZnP.
Also, the presence of the
isobestic points at l=420 and 433 nm implies a predominant
+
ꢀ
conversion from the Crown ZnP to the CNH-sp-NH3
;
ꢀ
Crown ZnP nanohybrid (see Figure 6b).
Steady-state fluorescence measurements: With the excita-
tion at l=433 nm (one of the isobestic points) of the
+
ꢀ
Figure 4. Top: HRTEM image of CNH-sp-NH3 ;Crown ZnP obtained
+
ꢀ
Crown ZnP moiety in the nanohybrids, the fluorescence
ꢀ
from DMF solution. Bottom: EDX spectrum of CNH-sp-NH3 ;Crown
ZnP; Cu is due to the grid used for holding specimen.
bands appeared at l=650 and 700 nm, as shown in Figure 7.
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ꢀ
Porphyrin Carbon Nanohorn Assembly
FULL PAPER
+
experiment performed with the CNH-sp-NH3 precursor (f-
CNH, see Scheme 3) shows only slight quenching of the
ꢀ
Crown ZnP fluorescence (see Figure S5 in the Supporting
Information). These results suggest that the intramolecular
character of the quenching of the fluorescence is caused by
the photophysical events in the excited singlet state of the
1
ꢀ
Crown ZnP moiety ( ZnP*) in the vicinity of CNHs, such
as a charge-separation process.[17]
Fluorescence lifetime measurements: The fluorescence time
ꢀ
profiles of the Crown ZnP moiety in the nanohybrid solu-
tion measured by using the excitation with picosecond
pulsed-laser light at l=400 nm and monitored by streak-
scope are shown in Figure 8, in which the shortening of the
Figure 8. Fluorescence–time profiles in the l=600–700 nm region in
DMF solution; lex =400 nm. i) Crown ZnP (4.27ꢄ10ꢀ4 m; 4 mL), ii and
ꢀ
+
iii) CNH-sp-NH3 ;Crown ZnP nanohybrids (4.27ꢄ10ꢀ4 m: 0.5 mg/
ꢀ
100 mL=3 mL:1 mL for (ii) and 2 mL:2 mL for (iii)).
ꢀ
Figure 6. a) Absorption spectral changes of Crown ZnP (black line;
ꢀ
fluorescence lifetime of the Crown ZnP moiety was ob-
4.27ꢄ10ꢀ7 m in DMF) on addition of increasing amounts of CNH-sp-
+
served. The fluorescence lifetime shortening is almost maxi-
NH3 (concentration 0.5 mg/100 mL; from 30 to 1720 mL in 2 mL DMF).
b) Spectra of wavelength expansion near Soret-band region, showing two
+
mum under similar concentrations of CNH-sp-NH3 to the
steady-state fluorescence quenching. From the curve-fittings
with bi-exponential functions for both time profiles ii) and
isobestic points at l=420 and 433 nm.
ꢀ
iii), the fluorescence lifetime (tf)sample of the Crown ZnP
moiety was evaluated to be 370 ps (fraction=40%) and
1200 ps (fraction=60%), respectively, which are shorter
ꢀ
than that of the unbound Crown ZnP [(tf)ref =2040 ps (frac-
tion=100%)].
These observations indicate efficient quenching of the
+
Crown-1ZnP* moiety by CNH-sp-NH3 in the nanohybrids.
The shorter fluorescence lifetime in polar DMF can be at-
tributed to the charge separation within the nanohybrids via
+
Cꢀ
the 1ZnP* moiety, generating CNH -sp-NH3 ;Crown
ꢀ
ZnP +. The rate constant (kSCS) and the quantum yield
C
(FSCS) of the charge separation through the ZnP* moiety
1
Figure 7. Fluorescence spectra of Crown ZnP (4.27ꢄ10ꢀ7 m; 2 mL)
before and after addition of CNH-sp-NH3 (0.5 mg/100 mL 1720 mL) in
DMF; lex =433 nm (isobestic point).
ꢀ
were evaluated from the shorter fluorescence component by
+
[26]
ꢀ
the difference from that of the unbound Crown ZnP. The
kS value was determined to be 2.2ꢄ109 sꢀ1 and the FS
CS
CS
value was 0.81 (a total value was given as FSCS ꢄfraction=
0.33), suggesting a moderately efficient charge-separation
process through the Crown-1ZnP*. On the other hand, for
ꢀ
The fluorescence intensities of the Crown ZnP moiety were
quenched (67%) on addition of CNH-sp-NH3 in DMF,
which provides evidence for the formation of the nanohy-
brid structure of CNH-sp-NH3 ;crown ZnP.
+
the longer fluorescence lifetime a kS value of 3.3ꢄ108 sꢀ1
CS
+
[17]
A control
was determined and the FS value was 0.40 (FSCS ꢄfrac-
ꢀ
CS
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10757

F. Langa, T. Hasobe, M. Yudasaka, O. Ito et al.
tion=0.24), which may be assigned to a slow charge-separa-
In a control experiment without CNH-sp-NH3+, such
+
tion process for the relatively distant Crown-1ZnP* moieties
HV accumulation was not observed, although direct pho-
C
from the CNH surface. The total FS evaluated from the
toinduced electron transfer from Crown-1ZnP* (or
Crown-3ZnP*) to HV2+ is possible, which indicates that
CS
fluorescence lifetimes is 0.57, which is in satisfactory agree-
ment with the steady-state fluorescence quenching of 67%,
supporting the reliability of both methods.
+
CNH-sp-NH3 is indispensable for the accumulation of
+
C
HV
.
Electron-mediating experiments: To confirm the charge sep-
aration in the nanohybrids, steady-state absorption spectral
measurements were performed by irradiation of the nanohy-
brids with light in the presence of hexyl viologen (HV2+),
Nano-second transient absorption spectra: To investigate
the transfer and migration mechanisms of electron and hole,
the transient absorption spectra were recorded upon excita-
ꢀ
tion of the Crown ZnP moiety of the nanohybrids with l=
which is a well-known electron-mediating reagent. The accu-
532 nm laser light after
a
nanosecond pulse duration
+
+
ꢀ
C
mulation of HV was confirmed by a characteristic band at
(Figure 10). In the CNH-sp-NH3 ;Crown ZnP nanohybrids,
l=620 nm (Figure 9), resulting from the electron pooling to
HV2+, when a hole shifter such as 1-benzyl-1,4-dihydronico-
tinamide (BNAH) was added.[27]
+
Figure 10. Nanosecond transient absorption spectra of CNH-sp-NH3
;
Crown ZnP (4.27ꢄ10ꢀ4
m : 0.5 mg/100 mL=2 mL:2 mL) observed by
l=532 nm (5 ns pulse width and ꢁ3 mJ/pulse) laser irradiation in Ar-sa-
ꢀ
turated DMF. Inset: Absorption–time profiles.
Figure 9. Steady-state absorption spectral changes observed after l=
+
ꢀ
532 nm laser light irradiation of CNH-sp-NH3 ;Crown ZnP (4.27ꢄ
the observed transient absorption band in the l=600–
800 nm region with a maximum at l=610–640 nm can be at-
10ꢀ4 m: 0.5 mg/100 mL=2 mL:2 mL) in the presence of HV2+
(0.1 mm)
and BNAH. Each spectrum is the maximum one observed with repeated
laser light irradiation. [BNAH]=i) 0, ii) 0.2, iii) 0.4, iv) 0.6, v) 0.8, vi) 1.0,
vii) 1.2, viii) 1.4, and ix) 1.6 mm in Ar-saturated DMF (1.0 cm optical cell
length; the l=532 nm laser light has a 5 ns pulse width and ꢁ3 mJ/pulse
power).
+
tributed to the Crown ZnP
moiety.[6] The absorption
ꢀ
C
bands in the near-IR region with a peak at l=1410 nm can
be considered as trapped electrons in the CNHs, as a coun-
+
ꢀ
C
terpart to the Crown ZnP moiety, as similar broad ab-
sorption bands were observed for the trapped electrons in
CNH (CNH ) in our previous papers.
Cꢀ
[11,12]
The time profile
Each absorption spectrum exhibits the maximum value
obtained after the repeated irradiation with l=532 nm
laser-light (usually 2–5 times of 5 ns pulsed laser light).
at l=640 nm shown in the inset of Figure 10 was curve-
fitted by an exponential function, giving a first-order rate
+
constant of 1.2ꢄ107 sꢀ1 for the Crown ZnP moiety in Ar-
ꢀ
C
From the observed absorbance at l=620 nm, the maximum
saturated DMF. This rate constant can be attributed to the
+
C
Cꢀ
concentration of HV was found to be 0.08 mm, from which
intra-nanohybrid charge recombination (kCR) within CNH -
+
sp-NH3 ;Crown ZnP +. In addition, the decay rate at l=
ꢀ
C
the conversion was evaluated to be 80% versus the feed of
HV2+ (0.1 mm) in the presence of excess BNAH (1.6 mm).
1410 nm is almost the same as that at l=640 nm, supporting
the fact that the l=1410 nm absorption is due to CNH as
a counterpart of the Crown ZnP moiety. Thus, the life-
time of the radical ion pair (RIP), tRIP, of CNH -sp-NH3
Cꢀ
On the other hand, in the absence of BNAH, no absorption
+
+
ꢀ
C
C
of HV was accumulated even after the laser-light irradia-
Cꢀ
+
tion of the nanohybrids. On increasing the BNAH concen-
;
+
+
ꢀ
C
C
tration, amounts of HV increased, which indicates that
Crown ZnP was evaluated to be 85 ns from the value of
kCR. In the inset of Figure 10, although complete decay was
observed at l=1410 nm, the slow decay part was observed
at l=640 nm after 200 ns, which may be attributed to sur-
+
C
back electron transfer from the electron-rich HV to the
+
C
electron-poor ZnP is suppressed by the hole shift (HS)
+
C
from ZnP to BNAH. After the hole shift, BNAH changes
to BNAH +, which irreversibly converts to 1-benzyl nicoti-
viving Crown ZnP after the fate of CNH -sp-NH3 due
to the encounter of an electron with defects in CNH.
+
+
C
C
Cꢀ
ꢀ
namide (BNA+), as a stable hole trap.
10758
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ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 10752 – 10763

ꢀ
Porphyrin Carbon Nanohorn Assembly
FULL PAPER
+
[11,12]
Cꢀ
tron-mediating process from CNH -sp-NH3 to HV2+
If both BNAH and HV2+ were added to the nanohybrid
solutions, the transient absorption bands in the near-IR
region almost disappeared within 100 ns (Figure 11). In-
.
The most prominent difference is the shorter lifetime of the
+
C
generated HV (less than 200 ns), suggesting that the back
electron transfer from the Crown ZnP to HV·+ is working
+
ꢀ
C
+
C
to consume HV
.
Electron transfer scheme: These observations, induced by
the illumination of nanohybrids with light, can be explained
by the processes summarized in Scheme 4, in which charge
ꢀ
Figure 11. Nanosecond transient absorption spectra of Crown ZnP with
+
1720 mL of CNH-sp-NH3 (4.27ꢄ10ꢀ4 m: 0.5 mg/100 mL=2 mL:2 mL) in
the presence of HV2+ (0.1 mm) and BNAH (1.6 mm) observed by l=
532 nm (ꢁ3 mJ/pulse) laser-light irradiation in Ar-saturated DMF. Inset:
Absorption–time profiles.
Scheme 4. Schematic representation for the photoinduced electron trans-
+
ꢀ
fer processes of the nanohybrid CNH-sp-NH3 ;Crown ZnP.
stead, the increasing absorption was observed in the l=
600–800 nm region with a peak at l=620 nm. Although the
separation initially takes place through the 1ZnP* moiety,
+
+
Cꢀ
C
ꢀ
generating CNH -sp-NH3 ;Crown ZnP in DMF solution.
absorption peak position is similar to the that of Crown
On addition of HV2+ and BNAH, the electron on CNH me-
ꢀ
ZnP +, quite different time profiles suggest that the l=
diates to HV2+, leading to an accumulation of HV +. From
C
C
620 nm band in the presence of HV2+ is ascribed to the
HV absorption band. Thus, Figure 11 indicates that the
HV absorption band appeared at l=620 nm with com-
plete annihilation of CNH -sp-NH3 at l=1410 nm, sug-
the one-electron oxidation (ꢁ0.4 V) and the lowest singlet
excited-state energy (2.0 eV) of the ZnP, the charge-separa-
tion process becomes exothermic, if the reduction potential
of CNH is less negative than ꢀ1.6 V versus Ag/AgNO3,
which is reasonably negative enough to be compatible with
the reported reduction potential of single-walled carbon
nanotubes (SWCNT) of ꢀ0.6 V.[12] The intermolecular elec-
tron-mediating (EM) and hole-shifting (HS) processes with
additives are also exothermic processes or almost isoener-
getic processes.
+
C
C
+
+
Cꢀ
Cꢀ
gesting that an electron-mediating process from CNH -sp-
NH3 to HV2+ takes place generating HV +, as an electron
+
C
pool in solution. On the other hand, the absorption band of
+
ꢀ
C
the Crown ZnP moiety was completely consumed by the
hole shift to BNAH.
The rise of HV at l=620 nm in the inset of Figure 11
+
C
may correspond to the intermolecular electron-mediating
+
Cꢀ
process from CNH -sp-NH3
to HV2+ within 200 ns
Photovoltaic cells: To construct the photovoltaic cells, the
(krisefast =5ꢄ106 sꢀ1), almost coinciding with the decay of
drop-cast method was applied to fabricate films of CNH-sp-
CNH -sp-NH3 at l=1410 nm. Taking the HV2+ concen-
NH3 ;Crown ZnP onto an OTE/SnO2 electrode. The ab-
+
+
Cꢀ
ꢀ
+
ꢀ
tration into consideration, the bimolecular rate constant for
sorption spectrum of OTE/SnO2/CNH-sp-NH3 ;Crown ZnP
(Figure 12a) exhibits the Soret band and Q-band, whereas
OTE/SnO2/CNH-sp-NH3 shows a featureless broad band
+
C
the apparent HV accumulation process was calculated to
be kEM =1010 mꢀ1 sꢀ1, which is close to the diffusion-con-
+
trolled limit (kDiff =5ꢄ109 mꢀ1 sꢀ1 in DMF).[28]
in the visible region (Figure 12b). Compared with the ab-
If BNAH is also present, the formed absorption band of
sorption spectra in Figure 6, broadening of the absorption
+
+
ꢀ
C
HV at l=620 nm persists without decay after 2–3 ms as
bands of OTE/SnO2/CNH-sp-NH3 ;Crown ZnP was ob-
ꢀ
shown in the inset of Figure 11, indicating that back electron
served, suggesting the aggregations of the Crown ZnP moi-
+
+
ꢀ
C
C
transfer from electron-rich HV
to Crown ZnP
was
eties in the films.
almost suppressed by the excess BNAH, owing to the con-
sumption of Crown ZnP supporting the hole shift from
Photoelectrochemical measurements were performed in a
three-compartment cell equipped with a potentiostat, OTE/
+
ꢀ
C
the Crown ZnP to BNAH.[28]
SnO2/CNH-sp-NH3 ;Crown ZnP or OTE/SnO2/CNH-sp-
+
+
ꢀ
ꢀ
C
On addition of only HV2+ to nanohybrids, the transient
spectra showed a decrease of the near-IR bands (see Fig-
ure S6 in the Supporting Information), accompanied by the
rapid decay of the l=1410 nm band, confirming the elec-
NH3 as a working electrode, and Pt wire as a counter elec-
+
trode.[29,30] The I–V characteristics of the OTE/SnO2/CNH-
+
ꢀ
sp-NH3 ;Crown ZnP electrode under the illumination with
white light are shown in Figure 13; the photocurrent density
Chem. Eur. J. 2010, 16, 10752 – 10763
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10759

F. Langa, T. Hasobe, M. Yudasaka, O. Ito et al.
In Figure 13a, photocurrent responses of the OTE/SnO2/
+
ꢀ
CNH-sp-NH3 ;Crown ZnP electrode (response A) and
+
OTE/SnO2/CNH-sp-NH3 electrode (response B) under the
white light illumination are performed by applying bias po-
tential (0.2 V vs. SCE). Both photocurrent responses are
prompt, steady, and reproducible during repeated on/off
cycles of the light illumination. The short circuit photocur-
rent densities (Isc) at applied bias potentials of 0.2 V versus
SCE are about 0.40 mAcmꢀ2 for the OTE/SnO2/CNH-sp-
+
ꢀ
NH3 ;Crown ZnP electrode (see response A) and
0.15 mAcmꢀ2 for the OTE/SnO2/CNH-sp-NH3 electrode
+
(see response B). This indicates that the OTE/SnO2/CNH-
Figure 12. Steady-state absorption spectra of a) OTE/SnO2/CNH-sp-
+
+
+
ꢀ
sp-NH3 ;Crown ZnP electrode is superior to the OTE/
ꢀ
NH3 ;Crown ZnP film and b) OTE/SnO2/CNH-sp-NH3 film.
+
SnO2/CNH-sp-NH3 one. The photocurrent action spectra
+
ꢀ
of the OTE/SnO2/CNH-sp-NH3 ;Crown ZnP electrode,
+
OTE/SnO2/CNH-sp-NH3
electrode, and OTE/SnO2/
ꢀ
Ccrown ZnP electrode were measured by examining the
wavelength dependence of the incident photon-to-current
conversion efficiency (IPCE). The IPCE values are calculat-
ed by normalizing the photocurrent densities for incident
light energy and intensity using of the expression given in
Equation (1),[31]
IPCE ð%Þ ¼ 100 ꢂ 1240 ꢂ i=ðWin ꢂ lexÞ
ð1Þ
in which, i is the photocurrent density (Acmꢀ2), Win is the
incident-light intensity (W cmꢀ2), and lex is the excitation
wavelength (in nm). The photocurrent action spectra were
recorded by using the standard three-compartment cell
under an applied bias potential of 0.2 V versus SCE. As
shown in Figure 14, the photocurrent action spectrum of
+
Figure 13. A) I–V characteristics of the OTE/SnO2/CNH-sp-NH3
;
ꢀ
Crown ZnP electrode under illumination with white light (red curve;
AM 1.5 condition; input power=100 mWcmꢀ2) and in the dark (black
curve). B) Photocurrent generation responses of a) OTE/SnO2/CNH-sp-
+
+
ꢀ
NH3 ;Crown ZnP and b) OTE/SnO2/CNH-sp-NH3 at an applied bias
voltage of 0.2 V versus SCE. Electrolyte: LiI 0.5m, I2 0.01m in aceto-
ACHTUNGTRENNUNG .
nitrile; input power=100 mWcmꢀ2
Figure 14. Photocurrent action spectra of IPCE; a) OTE/SnO2/CNH-sp-
increases compared with the dark current. The photocurrent
density increases with an electrochemical bias, suggesting in-
+
+
ꢀ
NH3 ;Crown ZnP electrode, b) OTE/SnO2/CNH-sp-NH3
electrode,
ꢀ
and c) OTE/SnO2/Crown ZnP electrode at an applied bias potential of
creased charge separation and the facile transport of charge
0.2 V versus SCE. Electrolyte: LiI 0.5m, I2 0.01m in acetonitrile.
+
ꢀ
carriers in the OTE/SnO2/CNH-sp-NH3 ;Crown ZnP elec-
trode under a positive bias potential. Importantly, the pho-
tocurrent generation density of the electrode can be con-
trolled by using the three-compartment cell (Figure 13), in
+
ꢀ
IPCE for OTE/SnO2/CNH-sp-NH3 ;Crown ZnP electrode
which the net photocurrent generation density of the OTE/
follows the same pattern as the absorption spectrum in Fig-
ure 12a; furthermore, the maximum value of IPCE reaches
9% at l=450 nm, which is 4.5 times larger than that of the
+
ꢀ
SnO2/CNH-sp-NH3 ;Crown ZnP electrode at +0.2 V
(ꢁ0.58 mAcmꢀ2) was higher than the case at 0 V
(ꢁ0.23 mAcmꢀ2).
+
OTE/SnO2/CNH-sp-NH3 electrode (2% in l=400–450 nm
10760
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Chem. Eur. J. 2010, 16, 10752 – 10763

ꢀ
Porphyrin Carbon Nanohorn Assembly
FULL PAPER
residue was washed several times with boiling petroleum ether. Then, the
solvent was evaporated and the resultant colorless solid was redissolved
in dry CH2Cl2 (20 mL). Then, pyridine (1 mL) was added followed by
porphyrin 2[19] (see Scheme 2, 50 mg, 0.05 mmol). The reaction mixture
was refluxed under an argon atmosphere for four days. The solvent was
evaporated, and the crude product was purified by column chromatogra-
phy (silica gel, CH2Cl2 followed by CH2Cl2/CH3OH 90:10). Evaporation
of the solvents yielded the desired free-base porphyrin crown-H2P com-
region) and 2.2 times larger than that of the OTE/SnO2/
ꢀ
Crown ZnP electrode (4% at 450 nm).
The maximum IPCE value (9%) obtained for the OTE/
+
ꢀ
SnO2/CNH-sp-NH3 ;Crown ZnP electrode, is comparable
to that for the covalently grafted porphyrin to CNHs.[15]
Even by the comparison with the covalently linked porphy-
rin to the single-walled carbon nanotubes, the obtained
1
pound 3 as a purple solid (19 mg, 35%); H NMR (400 MHz, CDCl3) d=
+
ꢀ
IPCE value for CNH-sp-NH3 ;Crown ZnP is comparable
8.81 (d, 2H, b-pyrrole H, J=4.58 Hz), 8.70 (d, 2H, b-pyrrole H, J=
4.58 Hz), 8.65 (s, 4H, b-pyrrole H), 8.23 (d, 2H, phenyl H, J=7.78 Hz),
7.89 (dd, 1H, benzocrown phenyl H, J=8.44 Hz, J=1.60 Hz), 7.81 (d,
2H, phenyl H, J=7.78 Hz), 7.74 (d, 1H, benzocrown phenyl H, J=
1.60 Hz), 7.29 (s, 6H), 6.96 (d, 1H, benzocrown phenyl H, J=8.44 Hz),
5.71 (s, 2H), 4.34–4.20 (m, 4H, crown ethylene H), 4.05–3.95 (m, 4H,
crown ethylene H), 3.85–3.66 (m, 12H, crown ethylene H), 2.64 (s, 9H),
1.86 (s, 18H), ꢀ2.54 ppm (br s, 2H, -NH); MS ( MALDI-TOF, dithra-
nol): m/z: 1109.6 [M+1].
or better.[32,33] These results clearly demonstrate that photo-
induced ET processes in the CNH-sp-NH3 ;Crown ZnP
hybrid possesses the light-energy conversion ability (vide
infra), supporting considerably high potentials for applica-
tion to photovoltaic devices.
+
ꢀ
Conclusion
ꢀ
Synthesis of Crown ZnP: Crown-H2P 3 (16 mg, 0.014 mmol) was dis-
solved in CHCl3 (10 mL), saturated zinc acetate in methanol was added,
and the resulting mixture was refluxed for 2 h. Then, the reaction mixture
was washed with water and dried over anhydrous Na2SO4. The crude
We have prepared a new supramolecular assembly of CNH
+
ꢀ
ꢀ
and
a
zinc porphyrin (CNH-sp-NH3 ;Crown ZnP) for
light-sensitive nanohybrids by relatively simple self-assembly
procedures. By the excitation of the nanohybrids, the gener-
ation of the unique charge-separated state, CNH -sp-NH3
Crown ZnP via the excited singlet state of the Crown
ZnP moiety was confirmed. As ZnP and CNH persist
longer than 100 ns, they have enough time to mediate the
electron on CNH to HV2+ to concomitantly shift to the hole
ꢀ
Crown ZnP solid was purified by washing several times with chloro-
form–pentane to give pure Crown ZnP (8 mg, 49%); 1H NMR (CDCl3,
ꢀ
400 MHz) d=8.87 (d, 2H, b-pyrrole H, J=4.64 Hz), 8.75 (d, 2H, b-pyr-
role H, J=4.64 Hz), 8.70 (br s, 4H, b-pyrrole H), 8.24 (d, 2H, phenyl H,
J=7.92 Hz), 7.88 (dd, 1H, benzocrown phenyl H, J=8.48 Hz, J=
1.60 Hz), 7.80 (d, 2H, phenyl H, J=7.92 Hz), 7.73 (d, 1H, benzocrown
phenyl H, J=1.60 Hz), 7.27 (s, 6H), 6.95 (d, 1H, benzocrown phenyl H,
J=8.48 Hz), 5.70 (s, 2H), 4.30–4.21 (m, 4H, crown ethylene H), 4.00–3.92
(m, 4H, crown ethylene H), 3.81–3.68 (m, 12H, crown ethylene H), 2.63
(s, 9H); 1.85 ppm (s, 18H); 13C NMR (CDCl3, 100 MHz,) d=166.4,
149.9, 149.8, 149.7, 142.8, 139.2, 139.0, 138.9, 137.3, 135.3, 134.5, 132.0,
131.1, 131.0, 130.6, 127.6, 126.1, 125.5, 124.2, 122.8, 119.4, 118.8, 118.5,
117.9, 114.5, 112.2, 70.9, 70.7, 69.3, 66.5, 21.7, 21.6, 21.4 ppm; UV/Vis
Cꢀ
+
;
+
ꢀ
ꢀ
C
+
C
Cꢀ
of ZnP to BNAH under appropriate concentration ranges.
+
C
Thus, accumulation of HV was observed by irradiation of
the nanohybrids in light in the presence of a sacrificial hole
shifter, BNAH, as summarized in Scheme 4, in which the
ET mechanisms are confirmed with the transient absorption
(DMF): lmax (log e): 600 (3.1), 561 (3.5), 427ACTHNUTRGNEUGN(5.0), 405 nm (3.9); MS
(MALDI-TOF, dithranol): m/z: 1171.3244 [M+1].
and time-resolved fluorescence measurements. Usually, the
+
Synthesis of CNH-sp-NH3 (see Scheme 3): CNH-COOH (50 mg) was
+
C
accumulated HV can be further used as an electron source
suspended in DMF (100 mL) and the resulting mixture was sonicated for
5 min. N-Boc-1,6-diaminohexane was added (Boc=tert-butoxycarbonyl,
150 mg, 0.6 mmol) and the mixture was stirred at 508C for five days to
obtain f-CNH (Scheme 3), which was recovered by vacuum filtration
over a Millipore membrane (PTFE, 0.1 mm), extensively washed with
DMF, CH2Cl2, and Et2O, and dried under high vacuum. To produce
CNH-sp-NH3+, the Boc protecting group was removed by adding a solu-
tion of HCl–dioxane (4m; 20 mL) in a round-bottom flask, which was
cooled by an ice-water bath under an argon atmosphere. After the mix-
ture had been stirred overnight, it was filtered on a Millipore membrane
(PTFE, 0.1 mm), followed by addition of fresh CH2Cl2 with sonication, fil-
tration through a 0.1 mm PTFE membrane and finally washing the solid
transferring to catalysts for H2 evolution in aqueous solu-
tion. Additionally, photoelectrochemical cells of the CNH-
+
ꢀ
sp-NH3 ;Crown ZnP hybrid materials were fabricated onto
+
OTE/SnO2 electrodes. The OTE/SnO2/CNH-sp-NH3
;
ꢀ
Crown ZnP electrode exhibited a maximum IPCE of 9%.
Based on theses experiments, we conclude that organization
+
of molecular assemblies achieved with the CNH-sp-NH3
;
ꢀ
Crown ZnP hybrid materials is a key factor in attaining im-
proved light-energy conversion properties.
+
material with EtOH. The collected black solid of CNH-sp-NH3 was fi-
nally dried under high vacuum (54 mg). The FT-IR spectrum of CNH-sp-
+
ꢀ
NH3 exhibits the C H stretching vibration modes of the alkyl chains
Experimental Section
around n˜ =2916 and 2850 cmꢀ1, and a broad band around n˜ =3400 cmꢀ1
corresponding to the cationic ammonium group. Additionally, the cova-
lent amide bonds are proven by the presence of carbonyl vibrations at
Chemicals: CNHs were produced by CO2 laser ablation of graphite in
the absence of any metal catalyst under an Ar atmosphere (760 Torr) at
room temperature; the purity of CNHs was as high as 90% with less
amorphous carbons. Then, the CNHs were treated with H2O2, which gen-
erated the corresponding carboxylic acid groups in CNH-COOH. All
other chemicals utilized in the synthesis and spectroscopic measurements
were purchased from Aldrich Chemicals (Milwaukee, WI) and were used
as received. Tetrabutylammonium perchloride, nBu4NClO4, used in the
electrochemical studies, was obtained from Fluka Chemicals.
n˜ =1637 cmꢀ1 (see Figure S7 in the Supporting Information).
+
Formation of CNH-sp-NH3 ;Crown ZnP: crown ZnP (4.27ꢄ10ꢀ7 m in
ꢀ
ꢀ
+
2 mL DMF) and CNH-sp-NH3 (concentration 0.5 mg/100 mL in DMF)
were stirred for 2 min at room temperature.
+
ꢀ
Preparation of OTE/SnO2/CNH-sp-NH3 ;Crown ZnP films: The desired
+
ꢀ
volume of the CNH-sp-NH3 ;Crown ZnP suspension (ꢁ1 mL) was
simply deposited onto an OTE/SnO2 electrode by using the drop-cast
method. The OTE/SnO2 electrodes (with a surface area of 0.5ꢄ0.5 cm)
Synthesis of Crown-H2P, 3 (see Scheme 2): A solution of 4-carboxyben-
zo[18]crown-6 (200 mg, 0.56 mmol), SOCl2 (408 mL, 5.6 mmol), and pyri-
dine (1 mL) in dry CH2Cl2 (30 mL) was refluxed under argon gas for 3 h.
After the mixture was cooled, the solvent was evaporated and the crude
+
ꢀ
coated with CNH-sp-NH3 ;Crown ZnP gave the OTE/SnO2/CNH-sp-
+
ꢀ
NH3 ;Crown ZnP electrode. As reference electrodes, OTE/SnO2/CNH-
+
ꢀ
sp-NH3 and OTE/SnO2/Crown ZnP were prepared similarly.
Chem. Eur. J. 2010, 16, 10752 – 10763
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10761

F. Langa, T. Hasobe, M. Yudasaka, O. Ito et al.
[6] a) H. Imahori, Y. Sakata, Adv. Mater. 1997, 9, 537–546; b) D. M.
Guldi, M. Prato, Acc. Chem. Res. 2000, 33, 695–703; c) M. E. El-
Khouly, O. Ito, P. M. Smith, F. D’Souza, J. Photochem. Photobiol. C
2004, 5, 79–104; d) H. Imahori, S. Fukuzumi, Adv. Funct. Mater.
2004, 14, 525–536; e) F. D’Souza, O. Ito, Coord. Chem. Rev. 2005,
249, 1410–1422.
[7] R. Chitta, A. S. D. Sandanayaka, A. L. Schumacher, L. D’Souza, Y.
Araki, O. Ito, F. D’Souza, J. Phys. Chem. C 2007, 111, 6947–6955.
[8] a) T. Hasobe, S. Fukuzumi, P. V. Kamat, Angew. Chem. 2006, 118,
769–773; Angew. Chem. Int. Ed. 2006, 45, 755–759; b) T. Hasobe, S.
Fukuzumi, P. V. Kamat, J. Am. Chem. Soc. 2005, 127, 11884–11885;
c) T. Hasobe, Phys. Chem. Chem. Phys. 2010, 12, 44–57.
Acknowledgements
Financial support from the Ministry of Science and Innovation of Spain,
FEDER funds (Project CTQ2007-63363/PPQ and Consolider project
HOPECSD2007-00007) and the Fund from Junta de Comunidades de
Castilla-La Mancha (Project PCI08-038) is gratefully acknowledged. The
authors appreciate the JSPS for the support of Dr. A. S. D. Sandanayaka.
We thank M. Irie, Dr. M. Nakamura, and Dr. M. Zhang for performing
the TM and XDS experiments, and Dr. T. Azami and Dr. D. Kasuya for
the preparing CNH. We also thank M. J. de la Mata for carrying out the
TGA experiments. This work was partially supported by Grant-in-Aids
for Scientific Research (No. 21710104 to T.H.) and special coordination
funds for promoting science and technology from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan.
[9] a) A. S. D. Sandanayaka, Y. Takaguchi, T. Uchida, Y. Sako, Y. Mori-
moto, Y. Araki, O. Ito, Chem. Lett. 2006, 35, 1188–1189; b) A. S. D.
Sandanayaka, H. Orisaka, T. Kyotani, Y. Araki, O. Ito, Carbon 2007,
45, 684–687.
[10] C. Cioffi, S. Campidelli, C. Sooambar, M. Marcaccio, G. Marcolon-
go, M. S. Meneghetti, D. Paolucci, F. Paolucci, C. Ehli, A. Rahman,
M. G. V. Sgobba, D. M. Guldi, M. Prato, J. Am. Chem. Soc. 2007,
129, 3938–3945.
[11] a) A. S. D. Sandanayaka, G. Pagona, N. Tagmatarchis, M. Yudasaka,
S. Iijima, Y. Araki, O. Ito, J. Mater. Chem. 2007, 17, 2540–2546;
b) G. Pagona, A. S. D. Sandanayaka, Y. Araki, J. Fan, N. Tagma-
tarchis, G. Charalambidis, P. Trohopoulos, A. G. Coutsolelos, B. Boi-
trel, M. Yudasaka, S. Iijima, O. Ito, Adv. Funct. Mater. 2007, 17,
1705–1711.
[12] For carbon nanohorns, see: a) G. Pagona, A. S. D. Sandanayaka, Y.
Araki, J. Fan, N. Tagmatarchis, M. Yudasaka, S. Iijima, O. Ito, J.
Phys. Chem. B 2006, 110, 20729–20732; b) G. Pagona, A. S. D. San-
danayaka, A. Maignꢅ, J. Fan, G. C. Papavassiliou, I. D. Petsalakis,
B. R. Steele, M. Yudasaka, S. Iijima, N. Tagmatarchis, O. Ito, Chem.
Eur. J. 2007, 13, 7600–7607; c) M. Zhang, M. Yudasaka, T. Muraka-
mi, K, Ajima, A. S. D. Sandanayaka, O. Ito, K. Tsuchita, S. Iijima,
Proc. Natl. Acad. Sci. USA 2008, 105, 14773–14778; d) A. S. D. San-
danayaka, O. Ito, M. Zhang, K. Ajima, S. Iijima, M. Yudasaka, T.
Murakami, K. Tsuchita, Adv. Mater. 2009, 21, 4366–4371.
[13] a) P. Keller, A. Moradpour, J. Am. Chem. Soc. 1980, 102, 7193–
7196; b) P. Keller, A. Moradpour, E. Amouyel, H. Kagan, J. Mol.
Catal. 1980, 7, 539–542; c) A. I. Krasna, J. Photochem. Photobiol. A
1980, 31, 75–82; d) J. M. Calvert, T. J. Manuccia, R. J. Nowak, J.
Electrochem. Soc. 1986, 133, 951–953; e) R. J. McMahon, R. K.
Force, H. H. Patterson, M. S. Wrighton, J. Am. Chem. Soc. 1988,
110, 2670–2672; f) Photosensitization of Porphryins and Phthalocya-
nines (Ed.: I. Okura), Gordon and Breach Science Publishers, Am-
sterdam, 2001.
[1] a) P. M. Ajayan, Chem. Rev. 1999, 99, 1787–1800; b) S. Prabhpreet,
S. Campidelli, S. Giordani, D. Bonifazi, A. Bianco, M. Prato, Chem.
Soc. Rev. 2009, 38, 2214–2230; c) M. S. Dresselhaus, G. Dresselhaus,
P. Avouris, Carbon Nanotubes: Synthesis Structure, Properties and
Applications, Springer, Heidelberg, 2001; d) S. Reich, C. Thomsen, J.
Maultzsch, Carbon Nanotubes: Basic Concepts and Physical Proper-
ties, Wiley-VCH, Weinheim, 2004; e) M. Meyyappan, Carbon Nano-
tubes: Science and Applications, CRC Press, New York, 2005.
[2] a) S. Iijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F.
Kokai, K. Takahashi, Chem. Phys. Lett. 1999, 309, 165–170; b) S.
Iijima, Physica B+C 2002, 23, 1–5; c) J. Fan, M. Yudasaka, Y.
Kasuya, D. Kasuya, S. Iijima, Chem. Phys. Lett. 2004, 397, 5–10;
d) K. Murata, A, Hashimoto, M. Yudasaka, D. Kasuya, K. Kaneko,
S. Iijima, Adv. Mater. 2004, 16, 1520–1522; e) H. Murakami, K.
Ajima, J. Miyawaki, M. Yudasaka, S. Iijima, K. Shiba, Mol. Pharm.
2004, 1, 399–405.
[3] a) N. Tagmatarchis, A. Maigne, M. Yudasaka, S. Iijima, Small 2006,
2, 490–494; b) G. Pagona, J. Fan, N. Tagmatarchis, M. Yudasaka, S.
Iijima, Chem. Mater. 2006, 18, 3918–3920.
[4] For porphyrins noncovalently attached to carbon nanotubes, see:
a) Y.-P. Sun, K. Fu, Y. Lin, W. Huang, Acc. Chem. Res. 2002, 35,
1096–1104; b) H. Kataura, Y. Maniwa, M. Abe, A. Fujiwara, K. Ki-
kuchi, H. Imahori, Y. Misaki, S. Suzuki, Y. Achiba, Appl. Phys. A
2002, 74, 349–354; c) Y. Lin, B. Zhou, K. A. S. Fernando, P. Liu,
L. F. Allard, Y.-P. Sun, Macromolecules 2003, 36, 7199–7204; d) H.
Murakami, T. Nomura, N. Nakashima, Chem. Phys. Lett. 2003, 378,
481–485; e) M. C. Paiva, B. Zhou, K. A. S. Fernando, Y. Lin, J. M.
Kennedy, Y.-P. Sun, Carbon 2004, 42, 2849–2854; f) S. Barazzouk, S.
Hotchandani, K. Vinodgopal, P. V. Kamat, J. Phys. Chem. B 2004,
108, 17015–17018; g) Y. Lin, B. Zhou, Y. Lin, L. Gu, W. Wang,
K. A. S. Fernando, K. Kumar, L. F. Allard, Y.-P. Sun, J. Am. Chem.
Soc. 2004, 126, 1014–1015; h) I. Robel, B. A. Bunker, P. V. Kamat,
Adv. Mater. 2005, 17, 2458–2463; i) D. M. Guldi, H. Taieb, G. M. A.
Rahman, N. Tagmatarchis, M. Prato, Adv. Mater. 2005, 17, 871–875;
j) A. Satake, Y. Miyajima, Y. Kobuke, Chem. Mater. 2005, 17, 716–
724; k) J. Chen, C. P. Vollier, J. Phys. Chem. B 2005, 109, 7605–
7609; l) G. M. A. Rahman, D. M. Guldi, S. Campidelli, M. Prato, J.
Mater. Chem. 2006, 16, 62–65; m) M. Alvaro, P. Atienzar, P. de La
Cruz, J. L. Delgado, V. Troiani, H. Garcia, F. Langa, A. Palkar, L.
Echegoyen, J. Am. Chem. Soc. 2006, 128, 6626–6635; n) F. Cheng, S.
Zhang, A. Adronov, L. Echegoyen, F. Diederich, Chem. Eur. J.
2006, 12, 6062–6070; o) K. Schulte, J. C. Swarbric, N. A. Smith, F.
Bondino, E. Magnano, A. K. Khlobystov, Adv. Mater. 2007, 19,
3312–3316.
[14] P. M. S. Monk, The Viologens, Wiley, New York, 1998.
[15] G. Pagona, A. S. D. Sandanayaka, T. Hasobe, G. Charalambidis,
A. G. Coutsolelos, M. Yudasaka, S. Iijima, N. Tagmatarchis, J. Phys.
Chem. C 2008, 112, 15735–15741.
[16] a) R. Yuge, M. Yudasaka, J. Miyawaki, Y. Kubo, T. Ichihashi, H.
Imai, E. Nakamura, H. Isobe, H. Yorimitsu, S. Iijima, J. Phys. Chem.
B 2005, 109, 17861- 17867; b) J. Miyawaki, M. Yudasaka, H. Imai,
H. Yorimitsu, H. Isobe, E. Nakamura, S. Iijima, J. Phys. Chem. B
2006, 110, 5179–5181; c) H. Isobe, T. Tanaka, R. Maeda, E. Noiri,
N. Solin, M. Yudasaka, S. Iijima, E. Nakamura, Angew. Chem. 2006,
118, 6828–6832; Angew. Chem. Int. Ed. 2006, 45, 6676–6680;
d) A. S. D. Sandanayaka, O. Ito, T. Tanaka, H. Isobe, E. Nakamura,
M. Yudasaka, S. Iijima, New J. Chem. 2009, 33, 2261–2266.
[17] a) F. D’Souza, R. Chitta, S. Gadde, M. E. Zandler, A. S. D. Sanda-
nayaka, Y. Araki, O. Ito, Chem. Commun. 2005, 1279–1281; b) F.
D’Souza, R. Chitta, A. S. D. Sandanayaka, N. K. Subbaiyan, L.
D’Souza, Y. Araki, O. Ito, Chem. Eur. J. 2007, 13, 8277–8284; c) F.
D’Souza, O. Ito, Chem. Commun. 2009, 4913–4928.
[18] J. S. Lindsey, S. Prathapan, T. E. Johnson, R. W. Wagner, Tetrahe-
dron 1994, 50, 8941–8968.
[19] A. A. Yasseri, D. Syomin, R. S. Loewe, J. S. Lindsey, F. Zaera, D. F.
Bocian, J. Am. Chem. Soc. 2004, 126, 15603–15612.
[20] T. Yamaguchi, S. Bandow, S. Iijima, Chem. Phys. Lett. 2004, 389,
181–185.
[5] For porphyrins covalently attached to carbon nanotubes, see: a) H.
Li, R. B. Martin, B. A. Harruff, R. A. Carino, Y.-P. Sun, Adv. Mater.
2004, 16, 896–900; b) D. Baskaran, J. W. Mays, X. P. Zhang, M. S.
Bratcher, J. Am. Chem. Soc. 2005, 127, 6916–6917; c) Z. Guo, F.
Du, D. Ren, Y. Chen, J. Zheng, Z. Liu, J. Tian, J. Mater. Chem.
2006, 16, 3021–3030; d) B. Ballesteros, G. de La Torre, C. Ehli,
G. M. Aminur-Rahman, F. Agullꢀ-Rueda, D. M. Guldi, T. Torres, J.
Am. Chem. Soc. 2007, 129, 5061–5068.
10762
www.chemeurj.org
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 10752 – 10763

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Porphyrin Carbon Nanohorn Assembly
FULL PAPER
[21] S. Utsumi, H. Honda, Y. Hattori, H. Kanoh, K. Takahashi, H. Sakai,
M. Abe, M. Yudasaka, S. Iijima, K. Kaneko, J. Phys. Chem. C 2007,
111, 5572–5575.
of [MV2+]=0.5 mm can be evaluated as kfirst order =kDiff [MV2+]=5ꢄ
109 mꢀ1 sꢀ1 ꢄ5ꢄ10ꢀ4 mꢀ1 =2.5ꢄ106 sꢀ1, which is in good agreement
+
with the HV rise rate (5ꢄ106 sꢀ1). For the kDiff value, see: S. I.
C
[22] S. Giordani, J.-F. Colomer, F. Cattaruzza, J. Alfonsi, M. Meneghetti,
M. Prato, D. Bonifazi, Carbon 2009, 47, 578–588.
Murov, I. Carmichael, G. L. Hug, Handbook of Photochemistry,
Marcel-Dekker, New York, 1993.
[23] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Klein-
hammes, Y. Jia, Y. Wu, S. T. Nguyen, R. S. Ruoff, Carbon 2007, 45,
1558–1565.
[29] H. J. Lee, J. H. Yum, H. C. Leventis, S. M. Zakeeruddin, S. A.
Haque, P. Chen, S. I. Seok, M. Grꢆtzel, M. K. Nazeeruddin, J. Phys.
Chem. C 2008, 112, 11600–11608.
[24] E. Bekyarova, M. E. Itkis, P. Ramesh, C. Berger, M. Sprinkle, W. A.
de Heer, R. C. Haddon, J. Am. Chem. Soc. 2009, 131, 1336–1337.
[25] M. J. Gꢀmez-Escalonilla, P. Atienzar, J. L. G. Fierro, H. Garcꢁa, F.
Langa, J. Mater. Chem. 2008, 18, 1592–1600, and references therein.
[30] Y. Ueno, H. Minoura, T. Nishikawa, M. Tsuiki, J. Electrochem. Soc.
1983, 130, 43–47.
[31] T. Hasobe, Y. Kashiwagi, M. A. Absalom, J. Sly, K. Hosomizu, M. J.
Crossley, H. Imahori, P. V. Kamat, S. Fukuzumi, Adv. Mater. 2004,
16, 975–979.
[32] T. Hasobe, S. Fukuzumi, P. V. Kamat, J. Phys. Chem. B 2006, 110,
25477–25484.
[33] T. Umeyama, M. Fujita, N. Tezuka, N. Kadota, Y. Matano, K. Yoshi-
da, S. Isoda, H. Imahori, J. Phys. Chem. C 2007, 111, 11484–11493.
[26] kCS =(1/tf)sample
ꢀ
G
ꢀ
(tf)ref =2040 ns.
[27] a) S. Fukuzumi, S. Koumitsu, K. Hironaka, T. Tanaka, J. Am. Chem.
Soc. 1987, 109, 305–316; b) S. Fukuzumi, H. Imahori, K. Okamoto,
H. Yamada, M. Fujitsuka, O. Ito, D. M. Guldi, J. Phys. Chem. A
2002, 106, 1903–1908.
[28] The first-order rate constant for the intermolecular electron transfer
(kfirst order) as fast as a diffusion-controlled limit (kDiff) in the presence
Received: February 3, 2010
Published online: August 4, 2010
Chem. Eur. J. 2010, 16, 10752 – 10763
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10763