Surface-grafted multiporphyrin arrays as light-harvesting antennae to amplify photocurrent generation

Article abstract of DOI:10.1002/chem.200500040

Organized multiporphyrin arrays were developed on the conductive surface by a novel coordination-directed molecular architecture aiming at efficient photoelectric conversion. The basic strategy employs the mutual coordination of two imidazolylporphyrinato

Full text of DOI:10.1002/chem.200500040

FULL PAPER
DOI: 10.1002/chem.200500040
Surface-Grafted Multiporphyrin Arrays as Light-Harvesting Antennae to
Amplify Photocurrent Generation
Mitsuhiko Morisue, Shigeru Yamatsu, Noriko Haruta, and Yoshiaki Kobuke*[a]
Abstract: Organized multiporphyrin
arrays were developed on the conduc-
tive surface by a novel coordination-di-
rected molecular architecture aiming at
efficient photoelectric conversion. The
basic strategy employs the mutual coor-
dination of two imidazolylporphyrina-
assembled monolayer. The organized
Zn2(ImP)2 bearing allyl side chains was
covalently linked by ring-closing olefin
metathesis catalyzed with Grubbs cata-
lyst. Alternating coordination/metathe-
sis reactions allow the stepwise accu-
mulation of multiporphyrin arrays on
the gold electrode. A successive in-
crease in absorption over a wide wave-
length range occurred after each accu-
mulation step of Zn2(ImP)2 on the gold
electrode, and cathodic photocurrent
generation was enhanced in the aque-
ous electrolyte system, containing viol-
ogen as an electron carrier. The signifi-
cant increase of the photocurrent indi-
cates that the multiporphyrin array
works as a “light-harvesting antenna”
on the gold electrode.
tozinc(ii) units to form
a cofacial
dimer. Thus, meso,meso-linked bis(imi-
dazolylporphyrinatozinc) (Zn2(ImP)2)
was organized onto imidazolylporphyri-
natozinc on the gold substrate as a self-
Keywords: interfaces
devices porphyrinoids
self-assembly · zinc
· molecular
·
·
Introduction
blies.[2e,3f,3h] To mimick a natural “light-harvesting” function,
it is important to capture solar light over a wide wavelength
range as well as to mediate excitation-energy transport. The
small extinction of the monomolecular layers has been over-
come by employing porous nanocrystalline electrodes in
Grꢀtzelꢁs dye-sensitizing solar cell.[5]
The present paper proposes another type of molecular ar-
chitecture in fabricating multiporphyrin assemblies with in-
creased extinction. Metal–ligand interaction is a powerful
tool to build structurally regular multilayered assemblies on
conductive surfaces.[6] Besides, the fully conjugated metal
complexes may enhance electronic and optical communica-
tions among the individual layer constituents. The porphyrin
macrocycle is the fundamental chromophore framework in
natural photosynthetic systems, in which the special pair col-
lects solar energy from the peripheral light-harvesting anten-
nae with excellent yields.[7] Hence, considerable attention is
now focused on developing photoelectronic devices using
electrodes with porphyrins attached. Axial coordination of
metalloporphyrin has been exploited as a building tool to
immobilize porphyrins at conductive surfaces.[8]
Spontaneous molecular assembly on solid surfaces is a key
technique in engineering molecular-based devices. Particu-
larly, the field of photoenergy conversion systems is an area
of most intense research. In the development of such molec-
ular systems, well-arranged redox species are prerequisite in
controlling interfacial electron transport.[1] For this purpose,
various surface-fabrication methods have been applied for
alignment, for example, Langmuir–Blodgett (LB) films,[2]
self-assembled monolayers (SAMs),[3] or layer-by-layer
membranes.[4] These systems performed successfully as sim-
plified models for the primary process in photosynthesis.
Molecular systems for efficient artificial “light-harvesting”
photoenergy conversion have been constructed based on the
design by Fujihira and co-workers.[2e] These were accom-
plished by funneling the photoexcitation energy on the an-
tenna molecules into
a donor–sensitizer–acceptor triad
by lateral energy transfer within the monolayer assem-
[a] Dr. M. Morisue, S. Yamatsu, N. Haruta, Prof. Y. Kobuke
Graduate School of Materials Science
Nara Institute of Science and Technology
8916–5 Takayama, Ikoma, Nara 630–0101 (Japan)
Fax : (+81)743-72-6119
For tailoring extremely long multiporphyrin arrays, we
have focused on a self-organization methodology utilizing
imidazolylporphyrinatozinc(ii) units. Two such units mutual-
ly coordinate through imidazolyl groups attached to the cen-
tral zinc atom to form a cofacial dimeric structure, which
can also serve as a precise motif of the unit structure that
E-mail: kobuke@ms.naist.jp
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2005, 11, 5563 – 5574
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5563

constitutes the bacterial light-harvesting antenna system.[9,10]
Bis(imidazolylporphyrinatozinc(ii)) units spontaneously as-
semble into multiporphyrin arrays in isotropic media[11] and
even at the electrode surface modified with imidazolylpor-
phyrinatozinc as a SAM.[12] The cofacial dimer in a van der
Waals contact shows exciton coupling indicative of strong
ground-state electronic interactions and therefore delocaliz-
es photoexcited energy or a photogenerated hole in the co-
ordination dimer. Its continuous link is also an excellent
energy/electron/hole-transferral agent. The porphyrin wires
can be developed noncovalently by successive coordination-
dimer formation. The multiporphyrin array assembled on
the gold electrode was further modified to generate a large
photocurrent.[12a]
Covalent immobilization of the organized porphyrin will
produce advantages in the structural stability and in the mo-
lecular arrangement of the multiporphyrin array. The coor-
dination dimer unit of imidazolylporphyrin with allyl-sub-
stituents can be connected to each other by a ring-closing
olefin metathesis reaction catalyzed with Grubbs catalyst (a
ruthenium–carbene complex).[13,14] The metathesis reaction
quantitatively proceeds under mild conditions. The covalent-
ly linked cofacial dimer structures remain even in pyridine
due to the extremely large stability constant of the comple-
mentary coordination.[14a] We attempted stepwise immobili-
zation of the imidazolylporphyrinatozinc complexes to con-
struct heterogeneous multiporphyrin arrays on the imidazo-
lylporphyrinatozinc–SAM-modified gold electrode (see
Scheme 1). The photocurrent was therefore enhanced by ef-
ficient light excitation through large extinction—the so-
called “light-harvesting antenna” function.
was rinsed thoroughly with CH2Cl2 and then dried under a
stream of nitrogen. Zinc(ii) was introduced to the free-base
imidazolylporphyrin followed by gentle reflux for 2 h under
a nitrogen atmosphere in chloroform containing a small
amount of saturated zinc acetate in methanol (step ii) in
Scheme 1). The Soret band at 421 nm shifted to 428 nm, and
the Q-band at 516 nm, attributed to free-base porphyrin,
changed to 550 nm, indicating that zinc was introduced as
the central metal ion in the monolayer assemblies
(Figure 1). The characteristic oxidation peak of porphyrina-
tozinc also appeared around 680 mV in the cyclic voltammo-
gram for the SAM on the gold electrode (see Figure 11b
later in the text).
Surface concentration can be determined by means of
electrochemical desorption, a method well-established by
Porter and co-workers.[15] The one-electron reductive de-
sorption under alkaline conditions (pH>11) is a direct
ꢀ
method of observing the Au S covalent-bond cleavage. The
cyclic voltammogram of the porphyrinatozinc SAM in KOH
(0.5m) showed irreversible reduction peaks at ꢀ1069 mV
versus Ag/AgCl at 100 mVsꢀ1 scan rate (Figure 2). The re-
duction waves were assigned to the reductive cleavage of
ꢀ
the Au S covalent bond. In agreement with this assignment,
this peak gradually diminished in the course of potential
sweep cycles. The integrated current gives the surface con-
ꢀ
centration of Au S bonding, GT =(iV)/(ve), in which i, V, v,
and e denote current density, potential, sweep-rate, and ele-
mental charge, respectively. The surface coverage of the imi-
dazolylporphyrinatozinc complex (GT/2) is therefore calcu-
lated as 1.35ꢃ1013 molcmꢀ2, corresponding to the molecular
area of 7.4 nm2 moleculeꢀ1.
The SAM was also characterized by using [Fe(CN)6]3ꢀ as
a bulk redox probe.[16] The electrode surface covered with
the SAM was completely insulated from the bulk ferricya-
nide species, even after the zinc insertion (Figure 3b). The
imidazolylporphyrinatozinc complexes were thus assumed to
form a stable SAM on the gold surface.
Stepwise accumulation of surface-grafted multiporphyrin
arrays: The SAM-modified gold substrate was immersed for
1 h in Zn2(ImP)2 (1mm) in CH2Cl2 with methanol (20 equiv)
as coordinating solvent to partially dissociate the otherwise
extensively developed coordination. This was followed by
rinsing with CH2Cl2 to organize the imidazolyl-to-zinc coor-
dination by removal of methanol (step iii) in Scheme 1). The
organized Zn2(ImP)2 was covalently immobilized through
metathesis reactions of allyl groups by soaking for 10 min in
a dilute solution of Grubbs catalyst in CH2Cl2 (step iv) in
Scheme 1).[17] The gold substrate was thoroughly washed
successively with a large excess of CH2Cl2, methanol, and
water, and then dried under a stream of nitrogen.
Results and Discussion
Absorption spectra of species on the gold substrate dem-
onstrate the difference of coordination behavior of Zn2-
(ImP)2 depending on the surface chemical species. Only a
small amount of Zn2(ImP)2 can be adsorbed physically onto
a bare gold substrate or octanethiol-SAM-modified sub-
strate. For the Zn–porphyrin SAM or the H2–porphyrin
Formation of a SAM incorporating imidazolylporphyrin:
The first layer of imidazolylporphyrinatozinc was deposited
on the gold surface as a SAM. The gold substrate was
soaked in a solution of H2ImP(C10SH)2 in CH2Cl2 (0.5 mm)
for 20 h (step i) in Scheme 1). After immersion, the substrate
5564
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2005, 11, 5563 – 5574

Multiporphyrin Arrays
FULL PAPER
Scheme 1. Procedures for accumulation of the multiporphyrin arrays on the gold substrate. i) To form the SAM, the gold substrate was immersed in
H2ImP(C10SH)2 (0.1mm in CH2Cl2) for 20 h. ii) Zinc was introduced into the porphyrin in the SAM in CHCl3 containing a small amount of saturated Zn-
(OAc)2 in MeOH at 508C for 2 h. iii) The SAM-modified gold was soaked with Zn2(ImP)2 (1mm in CH2Cl2) containing MeOH (20mm) at room tempera-
ture for 1 h, and then rinsed with CH2Cl2 to organize Zn2(ImP)2 by removal of MeOH. iv) The substrate was soaked in a dilute solution of Grubbs cata-
lyst at room temperature for 10 min. The covalently linked organized porphyrin was obtained after thoroughly rinsing with MeOH and CH2Cl2. The rep-
etition steps iii) and iv) provided the multiporphyrin array. The most plausible structure for intradimer metathesis is illustrated.
Figure 1. Absorption spectra of H2– and Zn–porphyrin SAM on the gold
electrode.
Figure 2. Cyclic voltammograms for the SAM-incorporated imidazolyl-
porphyrinatozinc on the gold electrode. Conditions: KOH (0.5m) sup-
SAM, an apparently large increase of the absorbance was
found around 480 nm, attributable to Zn2(ImP)2 (Figure 4).
This comparison suggests that Zn2(ImP)2 is organized at the
central zinc through complementary coordination with Zn–
SAM or axial ligation to Zn2(ImP)2 on the H2 SAM surface.
porting electrolyte in aqueous media under continuous nitrogen stream
at a 100 mVsꢀ1 sweep rate. A Pt-wire electrode and a Ag/AgCl (satu-
rated KCl) electrode were used as the counter and the reference elec-
trode, respectively.
Chem. Eur. J. 2005, 11, 5563 – 5574
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5565

Y. Kobuke et al.
Figure 3. Cyclic voltammograms of K3Fe(CN)6 (1mm) in Na2SO4 (0.1m)
electrolyte solution. a) Redox behavior observed for the bare Au working
electrode at 10, 20, 50, 100, and 200 mVsꢀ1 and the proportional relation
between the peak-current ip versus v1/2 at 2.2ꢃ10ꢀ18 cm2 sꢀ2 of the diffu-
sion constant up to 100 mVsꢀ1 of the sweep rate (inset). b) The insulation
of the redox of K3[Fe(CN)6] (1mm) for the SAM-attached working elec-
trodes.
Figure 5. Effect of the metathesis reaction on the growth of Zn2(ImP)2 on
the imidazolylporphyrinatozinc SAM/Au substrate. a) The substrate
treated with alternate cycles of coordination organization/metathesis re-
actions. The spectrum was recorded after every metathesis reaction.
b) The substrate dipped repetitively in a solution of Zn2(ImP)2 without
metathesis.
ganization step. In contrast, the cumulative layering of Zn2-
(ImP)2 was difficult to control through noncovalent
deposition. Without treatment with Grubbs catalyst, the first
accumulation of Zn2(ImP)2 was observed, but no successive
increase was observed by the repetition (Figure 5b). In a so-
lution system, metathesis reaction of the allyl side chains
shows high specificity of the spacer chain length for the co-
valent linkage of the complementary porphyrin dimer.[14a]
Since the optimum spacer to give a 95% yield in solution
was utilized in the present system, the dimer connection
must occur efficiently on the gold substrate. In the present
case, however, interarray cross-linking may also occur to a
greater or lesser extent and contribute to the stabilization of
the multiporphyrin arrays. Both the complementary coordi-
nation through imidazolyl-to-zinc and covalent linking of
the allyl side chains are indispensable for the successive ac-
cumulation of multiporphyrin arrays.
The average thickness of the multiporphyrin layer on the
gold was evaluated by the surface plasmon resonance (SPR)
angles in water. The incident angle increased as a function
of the accumulation cycles. Based on the Fresnelꢁs fittings, a
layer with a thickness of approximately 0.9–1.2 nm grew in a
single accumulation cycle (Figure 6). The porphyrin should
interdigitate with the Zn–porphyrin terminal to be accumu-
lated at the surface. The distance from the terminal imida-
zolyl edge to the neighboring imidazolyl edge was estimated
Figure 4. Absorption spectra of the SAM-modified gold substrate after
soaking for 1 h in a solution of Zn2(ImP)2. The adsorbates forming the
SAM are indicated in the figure.
The substrate that had been immersed in a solution of
Zn2(ImP)2 over 2 h did not show further increase of the ab-
sorption in the subsequent cycles. Prolonged organization
may bridge the imidazolylporphyrinatozinc terminals to pre-
vent further coordination. These findings strongly suggest
that the outer terminal was a critical factor for sequential
deposition of Zn2(ImP)2 on the gold surface.
The organized Zn2(ImP)2 was then treated with Grubbs
catalyst for ring-closing metathesis. Alternating treatment of
the Zn–porphyrin SAM/Au substrate with a solution of Zn2-
(ImP)2 and Grubbs catalyst resulted in a gradual rise in the
absorption at 480 nm up to 0.18 after six cycles in Figure 5a.
The accumulation process requires dissociation and organi-
zation by addition and elimination, respectively, of the coor-
dinating solvent, methanol, in the above treatment. The co-
valently immobilized, organized porphyrin cannot be disso-
ciated by the addition of the coordinating solvent in each or-
5566
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2005, 11, 5563 – 5574

Multiporphyrin Arrays
FULL PAPER
Figure 6. SPR angles of the multiporphyrin array on the gold substrate in
water. The plot of the thickness as a function of the accumulation cycle
(inset).
to be around 1.5 nm by the molecular mechanics calculation
assuming complementary coordination (Figure 7). The sur-
face-grafted multiporphyrin arrays are therefore assumed to
be elongated by approximately a single Zn2(ImP)2 accumu-
lation cycle.
Figure 8. Photoresponse patterns during on/off cycles of white light at
various potentials. Conditions: Na2SO4 (0.1m) supporting electrolyte and
methylviologen (10mm) electron-carrier in the aqueous system under ni-
trogen streaming. The Zn2(ImP)2 was accumulated over four deposition
cycles.
range of 500 to ꢀ200 mV versus Ag/AgCl. A steep rise and
fall of the cathodic current was found as soon as the light
was turned on and off, respectively. The photocurrent densi-
ty gradually increased on applying a more negative poten-
tial. We observed only cathodic photocurrent for the violo-
gen–multiporphyrin–Au combination. The photorectifying
effect implies that most of the electrons flow unidirectional-
ly from the photoexcited porphyrin to the bulk viologen.
Furthermore, the cathodic photocurrent was amplified by
the increased accumulation of the porphyrin antennae,
keeping the dark current at the same level (Figure 9). The
multiporphyrin array could increase the magnitude of the
photocurrent depending on the degree of accumulation sub-
duing the effect of distance elongation between two interfa-
cial electron-transport sites.
Figure 7. Molecular model of Zn2(ImP)2 coordinating with imidazolylpor-
phyrinatozinc drawn by the molecular mechanics calculation software
(Cerius 2 supplied by Accelrys, Ver. 4.6/Force Field UNIVERSAL 1.02).
The view omits the allyl side chains for visual clarity.
The accumulated porphyrins were also observed by AFM.
The topographic images in the present system showed no
significant difference from those of the surface roughness of
the vacuum-deposited gold AFM images (see Figure S2 in
the Supporting Information). In the previous experiment,
the porphyrins were organized by evaporating pyridine from
nitrobenzene/pyridine (1:1 v/v), and were grown to a hetero-
geneous mixture of the multiporphyrin-like stalagmite.[12]
The stepwise coordination/metathesis can thus regulate the
gradual elongation of the porphyrin on the whole area of
the gold substrate.
Photocurrent generation by multiporphyrin antennae: We
measured the photocurrent response in the presence of viol-
ogen as an electron carrier by varying the applied potential
for the sample after several accumulation cycles. Prompt re-
sponse was maintained as stable without detectable change
of the photocurrent over several hours for the experimental
operation. Figure 8 depicts typical photocurrent response
patterns during on–off cycles of white light at a potential
Figure 9. Current-potential relationship for the dark and the photocurrent
(white light irradiation).
Chem. Eur. J. 2005, 11, 5563 – 5574
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5567

Y. Kobuke et al.
Hole migration :
PþC þ P ! P þ PþC
ð3Þ
ð4Þ
ð5Þ
ð6Þ
The photocurrent-action spectra obtained were normal-
ized to the illuminated light intensity at a constant value of
174.6 mWcmꢀ2. Close coincidence of these spectra with the
corresponding absorption spectra (Figure 5a) proves that the
photocurrent is generated by the photoexcitation of Zn–por-
Charge migration : PꢀC þ P ! P þ PꢀC
Redox reaction :
PþC þ eꢀ ! P
ꢀ
P þ e ! PꢀC
*
2þ
Viologen reduction : P þ MV ! PþC þ MVþC
*
ð7Þ
ð8Þ
PꢀC þ MV2þ ! P þ MVþC
*
*
Deactivation :
P þ Au ! P þ Au
ð9Þ
PþC þ PꢀC ! P þ P
ð10Þ
The primary photoinduced charge separation proceeds be-
tween the excited porphyrin and bulk viologen as the elec-
tron acceptor [Eq. (7)]. The hole generated migrates to-
wards the electrode surface [Eq. (3)] and receives an elec-
tron at negative potential to give the cathodic current
[Eq. (5)]. The redox potentials were determined by cyclic
voltammetry (Figure 11). Zn–porphyrin complexes on the
Au electrode possess a broad oxidation potential due to hy-
drophobicity and electroinactivity within the densely packed
porphyrin assemblies. The oxidation potential around
685 mV did not shift significantly with accumulation of Zn2-
(ImP)2. The redox potential of the Zn(ImP) unit in the
SAM may shift from the monomeric state, because the
Soret band showed a bathochromic shift suggesting J-type
Figure 10. Photocurrent-action spectra of the multiporphyrin on the gold
electrode at ꢀ200 mV versus Ag/AgCl. The spectra were normalized to a
constant light intensity at 174.6 mWcmꢀ2
.
phyrins (Figure 10). In addition, the accumulation of Zn2-
(ImP)2 induced a significant increase of the photocurrent
generation. The increase of the photocurrent was small for
the samples of the first two accumulation cycles, but became
significant for the samples of the last three. This agrees with
the observation that the excited singlet-state of Zn–porphy-
rin moieties is quenched strongly in the vicinity of the gold
surface.[3j,18] Photoexcitation at metal surfaces generally ac-
companies energy-transfer quenching by surface plasmons.
The imidazolylporphyrinatozinc SAM on the gold surface
hardly fluoresces. Hence, the photoexcitation of the inner
porphyrin contributed less to the photocurrent generation,
whereas the inner porphyrin played a role in spacing the
outer antennae from the gold surface in order to suppress
the deactivation by surface plasmon and to mediate hole mi-
gration along the porphyrin array. The photosensitization
was therefore enhanced by the accumulation of Zn2(ImP)2.
Photocurrent augmentation at 480 nm and the Q-bands was
larger than the increase at 430 nm, while the ratio of accu-
mulated absorbance did not change very much. This was in
line with a greater contribution of the Soret band around
480 nm and the Q-bands to fluorescence in CH2Cl2 (see Fig-
ure S1 in the Supporting Information). Successive accumula-
tion therefore provided improvement in photocurrent gener-
ation particularly sensitized by light excitation at longer
wavelengths.
The photocurrent generation can be composed of the fol-
lowing multi-elementary steps 1–10
Figure 11. Cyclic voltammograms. a) H2–porphyrin SAM on the gold
electrode. b) Zn–porphyrin SAM. c) A multiporphyrin array (one-cycle
deposition) on the gold electrode at 50 mVsꢀ1. Conditions: Na2SO4
(0.1m) supporting electrolyte in the aqueous system under a nitrogen
stream. Further accumulation did not increase the electrochemical re-
sponse under the conditions.
*
Photoexcitation :
P þ hn ! P
ð1Þ
ð2Þ
*
*
Energy migration : P þ P ! P þ P
5568
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2005, 11, 5563 – 5574

Multiporphyrin Arrays
FULL PAPER
aggregation in the SAM (Figure 1). We suppose that this is
the reason for no significant shift of the redox potentials on
the accumulation cycles of Zn2(ImP)2. Even though the oxi-
dation potentials have a wide distribution, the overall
charge and hole migration should proceed in a downhill di-
rection as shown in Scheme 2, which depicts the dominant
processes for the cathodic photocurrent generation.
An alternative mechanism may also account for the pres-
ent photocurrent conversion system. If electron injection
from the electrode to the excited porphyrin occurs at an
early stage [Eq. (6)], the multiporphyrin array must mediate
the excess charge [Eq. (4)] to reduce viologen in the bulk
solution [Eq. (8)]. Although it is difficult to choose the most
probable one from these two mechanisms, Equa-
tions (1),(7),(3),(5) and Equations (1),(6),(4),(8), the former
mechanism seems to be more likely for the present system,
because porphyrins generally act as p-type semiconduc-
tors.[19]
Regarding the photocurrent generation on the gold elec-
trode, the energy migration process along the porphyrin
arrays [Eq. (2)] should compete with quenching by surface
plasmon [Eq. (9)] as noted previously. This should reduce
the incident photon-to-current conversion efficiency (IPCE)
value. Nevertheless, the IPCE value shows a sharp rise up to
0.4% at 480 nm as the light-harvesting efficiency (LHE=
1ꢀ10ꢀA, derived from Beerꢁs law)[5a,20] increases (Figure 12).
This value, observed for the flat electrode, is relatively large
compared with the values (0.037% at 430 nm for the Zn–
porphyrin SAM in the present system) conventionally ob-
served for SAM systems.[3k]
*
*
Figure 12. Plots for the IPCE ( ) and quantum yield ( ) against LHE at
480 nm at ꢀ200 mV (versus Ag/AgCl). The numbers in the Figure
denote the deposition cycles.
The IPCE value is determined as Equation (11) fol-
lows:[5a,20]
IPCE ð%Þ ¼ 100 ði=eÞ=ðWl=hcÞ
ð11Þ
in which i, W, and l are photocurrent density (Acmꢀ2),
the incident photon flux (174.6ꢃ10ꢀ6 Wcmꢀ2), and wave-
length (nm), respectively. At the same time, the IPCE value
is related to the LHE and the quantum efficiency F after
light absorption by Equation (12).
F ð%Þ ¼ IPCE=LHE
ð12Þ
Scheme 2. Potential diagram for photoinduced charge-separation steps. The recombination processes, energy-transfer process, and distributed potentials
without contribution to photocurrent generation are omitted for simplification.
Chem. Eur. J. 2005, 11, 5563 – 5574
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5569

Y. Kobuke et al.
The plot of F against LHE at 480 nm also shows a sharp
increase as LHE increases (Figure 12, right-hand axis). This
means that the accumulation of Zn2(ImP)2 is effective not
only for light absorption, but also for generation of the elec-
tric current.
The above evaluation may be extended to all the absorp-
tion wavelengths. The results shown in Figure 10 demon-
strate that efficient light-to-current conversion is due to
light-capture over a wide range of wavelengths. The “light-
harvesting” task should then be evaluated by the integrated
current as a function of the total absorption (Figure 13). We
The C60-terminated antennae system: The stepwise immobi-
lization technique permits introduction of a wide variety of
substituent-appended imidazolylporphyrins to develop
redox- and energy-cascade systems for versatile applications.
Here, we show a preliminary trial for the C60-terminated
system on the gold, since C60 is an excellent electron accept-
or owing to its extremely small reorganization energy.[3d,h]
Zn(ImP)–C60 showed dramatically efficient quenching
(99.7%) of the photoexcited porphyrin by C60 compared
with Zn(ImP) (Figure 14), indicating the efficient electron-
transfer quenching from the excited singlet of the porphyrin
(1P*) to C60.
Figure 13. Plots of the photocurrent against the total absorbance at
ꢀ200 mV (versus Ag/AgCl). The total absorbance was estimated from
the integration of the absorption (arbitrary unit) from 350 to 750 nm in
*
Figure 14. Normalized absorption and emission spectra of Zn(ImP) (thin
lines) and Zn(ImP)–C60 (thick lines) in CH2Cl2. The fluorescence spec-
trum was obtained by excitation at the longer Soret band. The fluores-
cence of Zn(ImP)–C60 is magnified by 100 times.
wavenumbers (cmꢀ1). The total current density on the left-hand axis (
)
was calculated from the integration of the current density (Acmꢀ2) in
Figure 11 from 380 to 750 nm in wavenumbers (cmꢀ1). The current densi-
*
ty on the right-hand axis ( ) was obtained by white light irradiation as
shown in Figure 10. The numbers in the figure denote the deposition
cycles.
We soaked the gold surface modified with the multipor-
phyrin antennae in Zn(ImP)–C60 (0.1mm) and pyridine
(2mm) in CH2Cl2 for 2 h, followed by treatment with
Grubbs catalyst. No significant spectral change in the ab-
sorption was found after the treatment of Zn(ImP)–C60.[12b]
The photocurrent in the C60-terminated antennae, however,
increased the cathodic photocurrent by approximately three
times, in comparison to the case of the unmodified antennae
(Figure 15). The enhancement by termination with C60
obtained the total absorption and current values from the
integration of the cross-sectional absorption and current
density over the whole wavenumber range. The result from
the plot for the total photocurrent shows a good agreement
with the photocurrent observed by white light irradiation.
The sharp increase of both the total photocurrent and cur-
rent intensity increases the efficient “light-harvesting” func-
tion of the surface-grafted multiporphyrin arrays.
It is generally accepted that the disordered multichromo-
phoric assemblies dissipate photoexcitation energy at struc-
tural defects.[21] However, well-ordered arrangements of
chromophores can suppress deactivation pathways and con-
serve the photoexcited energy.[22] For instance, natural pho-
tosynthetic systems have developed sophisticated structures
for efficient photoenergy conversion. The multiporphyrin
array system composed of the dimer unit mimics these sys-
tems, since fluorescence is not quenched at all[11a] and effi-
cient long-range hole and/or energy transfer has been con-
firmed already.[23] The molecular axes of the multiporphyrin
array assist hole shift or energy transfer between the outer
terminal and the electrode surface, because the outer por-
phyrin terminal is connected to the electrode through the
surface-grafted chain structure.
Figure 15. Effect of termination with C60 at the multiporphyrin array on
the photocurrent response at ꢀ200 mV versus Ag/AgCl.
5570
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2005, 11, 5563 – 5574

Multiporphyrin Arrays
FULL PAPER
spectra were recorded on a HITACHI Fluorescence Spectrometer F-
4500.
should be attributed to the efficient photoinduced charge
separation at the array terminal.[3d,i,j] In the antenna system
without a C60 terminal, the electron-transfer reaction to viol-
5-(1-Methylimidazol-2-yl)-10,20-bis(3-allyloxypropyl)-15-di-3,5-(10-ace-
tylthiodecanoxy)phenylporphyrin (H2ImP(C10SAc)2): The linkage po-
phyrin was prepared by acid-catalyzed cyclization of a mixture of 2-
formyl-1-methylimidazole (45 mg, 1 equiv), di-3,5-(10-acetylthiodecanox-
y)benzaldehyde (see Supporting Information) (230 mg, 1 equiv), and 3-al-
lyloxypropyldipyrromethane (198 mg, 2 equiv) in chloroform (74 mL).
The mixture was stirred with trifluoroacetic acid (TFA) (0.20 mL,
2 equiv) for 5 h and then oxidized with p-chloranil (0.95 g, 3 equiv). The
porphyrin H2ImP(C10SAc)2 was eluted with chloroform/acetone (9:1 v/v)
3
ogen is assumed to undergo fast intersystem transfer to P*
followed by charge separation,[24] which must have been lim-
ited by the counter-complex formation with bulk viologen.
On the other hand, the C60 terminal directly oxidizes 1P*
and efficiently mediates the oxidation of the photoexcited
Zn2(ImP)2 and the electron transfer to bulk viologen. The
hole produced migrated to the electrode surface along the
multiporphyrin array. The heterodeposition of the imidazo-
lylporphyrins allows construction of the “light-harvesting”
system by incorporating a charge-separation unit.
1
from a silica-gel column (59.6 mg, 4.1%). H NMR (600 MHz) in CDCl3:
d=ꢀ2.65 (s, 2H; inner proton), 1.25–1.88 (br, 32H; alkyl), 2.28 (s, 6H;
ꢀ
ꢀ
SCOCH3), 2.79 (t, J=6.6 Hz, 4H; CH2CH2O-allyl), 2.83 (m, 4H;
ꢀ
ꢀ
ꢀ
CH2SCOMe), 3.40 (s, 3H; N CH3), 3.66 (t, J=4.8 Hz, 4H; CH2O-
ꢀ
ꢀ
allyl), 4.08 (m, 4H; OCH2CH=CH2), 4.12 (m, 4H; Ph-OCH2 ), 5.10 (t,
J=7.2 Hz, 4H; porphyrin-CH2(CH2)2O-allyl), 5.23 (dd, J=1.8, 9.6 Hz,
ꢀ
ꢀ
2H; CH=CHH), 5.42 (dd, J=1.8, 16.2 Hz, 2H; CH=CHH), 6.08 (ddt,
ꢀ
J=6.0, 9.6, 16.2 Hz, 2H; CH=CH2), 6.91 (s, 1H; Ph), 7.23 (s, 1H; Ph),
Conclusions
7.39 (s, 1H; Ph), 7.48 (s, 1H; 5-imidazolyl), 7.69 (s, 1H; 4-imidazolyl,
1H), 8.77 (d, J=5.4 Hz, 2H; b-pyrrole), 9.00 (d, J=5.4 Hz, 2H; b-pyr-
role), 9.47 (d, J=4.8 Hz, 2H; b-pyrrole), 9.53 ppm (d, J=4.8 Hz, 2H; b-
pyrrole); MALDI-TOF MS: m/z calcd: 1122.61; found: 1123.96 [M+H]+;
UV/Vis (CHCl3): lmax =414, 437, 566, 619 nm.
A surface-grafted multiporphyrin array has been successful-
ly constructed by repetitive coordination cycles of imidazo-
lylporphyrinatozinc and its covalent linkage by ring-closing
olefin metathesis of the allyl side chains. The resulting multi-
porphyrin array will provide a new class of molecular mate-
rials. Our results show that this array has dramatically im-
proved light absorption by splitting of absorption bands and
an accumulation effect, and thus brought about a significant
increase in the photocurrent generation by a so-called
“light-harvesting antenna” effect. The photocurrent density
was increased by efficient excitation energy/hole-transfer ca-
pability even without incorporation of any electron-acceptor
or -donor components. Moreover, an electron-acceptor unit
could be introduced at the terminal of the multiporphyrin
for construction of the redox-cascade system. The sequen-
tially arranged redox system led to a higher efficiency of the
electron-transfer reaction, sensitized by the multiporphyrin
array at the electrode surface. Further work is in progress to
improve the multiporphyrin systems by elaborate modifica-
tion. This approach will provide an interesting method to
improve the small extinction of the molecular assemblies
when a flat electrode surface is used.
5-(1-Methylimidazol-2-yl)-10,20-bis(3-allyloxypropyl)-15-di-3,5-(10-mer-
captodecanoxy)phenylporphyrin (H2ImP(C10SH)2): Acetyl-protected
thiol groups were deacylated by alkaline hydrolysis. To avoid decomposi-
tion under basic conditions, a central zinc atom was introduced into the
porphyrin ring of H2ImP(C10SAc)2 with zinc acetate (yield 90%). The so-
lution of thioacetyl-terminated porphyrin (27 mg) in chloroform/metha-
nol (10 mL, 4:1 v/v) was treated with potassium hydroxide (0.2 g) in
water/methanol (2:3 v/v) at room temperature for 0.5 h. After washing
with water, the organic layer was treated with hydrochloric acid to
remove the central zinc atom. The mixture was subjected to silica-gel
column chromatography to elute H2ImP(C10SH)2 with chloroform/metha-
nol (10:1 v/v). 1H NMR (600 MHz) in CDCl3: d=ꢀ2.77 (s, 2H; inner
ꢀ
proton), 0.77–1.81 (br, 32H; alkyl), 2.40 (m, 4H; CH3SH), 2.71 (m, 4H;
CH2CH2O-allyl), 3.31 (s, 3H; N CH3), 3.58 (t, J=6.0 Hz, 4H; CH2O-
allyl), 4.00 (m, 4H; OCH2CH=CH2), 4.06 (m, 4H; Ph-OCH2 ), 5.11 (t,
J=7.8 Hz, 4H; porphyrin-CH2(CH2)2O-allyl), 5.18 (dd, J=1.8, 10.2 Hz,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2H; CH=CHH), 5.34 (dd, J=1.8, 18.0 Hz, 2H; CH=CHH), 6.00 (ddt,
ꢀ
J=6.0, 10.2, 18.0 Hz, 2H; CH=CH2), 6.83 (s, 1H; Ph), 7.22 (s, 1H; Ph),
7.32 (s, 1H; Ph), 7.39 (s, 1H; 5-imidazolyl), 7.61 (s, 1H; 4-imidazolyl),
8.70 (d, J=4.2 Hz, 2H; b-pyrrole), 8.92 (d, J=4.2 Hz, 2H; b-pyrrole),
9.39 (d, J=4.2 Hz, 2H; b-pyrrole), 9.45 ppm (d, J=4.2 Hz, 2H; b-pyr-
role); no peaks were observed with MALDI-TOF MS because the thiol
groups were strongly bound to the gold plate. UV/Vis (CHCl3): lmax
=
419, 516, 554, 590, 648 nm.
5-(1-Methylimidazol-2-yl)-10,20-bis(2-methoxycarbonylethyl)porphyrina-
tozinc (Zn(ImPH)): A mixture of 2-formyl-1-methylimidazole (0.55 g,
1 equiv), paraformaldehyde (0.80 g, 4 equiv), and 2-methoxycarbonyl-
ethyldipyrromethane (2.3 g, 2 equiv) in chloroform (500 mL) were mixed
with trifluoroacetic acid (0.75 mL, 2 equiv). After oxidation with chlor-
anil (3.7 g, 3 equiv), and neutralization with saturated aq NaHCO3, the
desired porphyrin was roughly separated from the byproducts by means
of silica-gel column chromatography, followed by introduction of central
zinc by treatment with zinc acetate. Column chromatography with
chloroform/acetone (5:1 v/v) as eluent, and rinsing with diethyl ether
afforded the precursor porphyrin (yield 3%). 1H NMR (270 MHz) in
Experimental Section
Synthesis procedures: Benzylidenebis(tricyclohexylphospine)dichlororu-
thenium, the Grubbs catalyst, was purchased from Fluka and used with-
out further purification. 1-Octanethiol was dissolved in CH2Cl2 for ad-
sorption onto the gold surface as-received from TCI.
Imidazolylporphyrins were synthesized from the corresponding aldehydes
and dipyrromethane (Scheme 3). The preparation of 2-formyl-1-methyl-
imidazole, dipyrromethanes, and Zn(ImP) is described elsewhere.[14a] All
synthesized porphyrins gave satisfactory 1H NMR and MALDI-TOF
ꢀ
CDCl3: d=1.59 (s, 3H; N CH3, 3H), 1.85 (s, 1H; 5-imidazolyl), 3.87–
ꢀ
ꢀ
3.91 (m, 4H; CH2COOMe), 3.91 (s, 6H; COOCH3), 5.42 (s, 2H;
3,7-b-pyrrole), 5.44 (s, 1H; 4-imidazolyl), 5.52 (t, J=8.6 Hz, 4H;
1
mass spectra. H NMR spectra were recorded on a JEOL JNMEX 270 or
ꢀ
CH2CH2COOMe), 8.95 (d, J=4.6 Hz, 2H; 2, 8-b-pyrrole), 9.54, 9.74 (d,
JEOL ECP 600 by using TMS as an internal standard. For the MALDI-
TOF MS measurement on a PerSeptive Biosystems Voyager DE-STR,
the sample was mixed with dithranol (purchased from Aldrich) as a
matrix dye and was put on the gold target plate. UV-visible absorption
and emission spectra in isotropic media were measured by using a con-
ventional 1 cm quartz cell under aerobic conditions. The fluorescence
4H; 4.32 Hz, 12,13,17,18-b-pyrrole), 10.29 ppm (s, 1H; meso-porphyrin);
MALDI-TOF MS: m/zcalcd: 624.15; found: 625.35 [M+H]+; UV/Vis
(CHCl3): lmax =408, 430, 559, 608 nm.
15,15’-Bis{5-(1-methylimidazol-2-yl)-10,20-bis(2-methoxycarbonylethyl)-
porphyrin} (H4ImP2): A solution of the meso-free precursor porphyrin
Chem. Eur. J. 2005, 11, 5563 – 5574
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5571

Y. Kobuke et al.
Scheme 3. Synthesis of the imidazolylporphyrins.
Zn(ImPH) (174 mg, 1 equiv) in chloroform (90 mL) was deaerated by ni-
trogen bubbling. The addition of 0.5 equiv of AgPF6 (47 mg, 0.6 equiv)
and I2 (39 mg, 0.5 equiv) promoted a meso,meso-coupling reaction.[25] The
reaction was quenched within 20 min by adding aqueous sodium thiosul-
fate (ca. 100 mL). The mixture was then washed with aqueous sodium hy-
drogencarbonate and brine. The crude product was demetalated with
HCl/methanol (10:1 v/v). Free-base meso,meso-coupled bisporphyrin
(H4ImP2) was purified by silica-gel column chromatography with chloro-
15,15’-Bis{5-(1-methylimidazol-2-yl)-10,20-bis(2-allyloxycarbonylethyl)-
porphyrinatozinc} (Zn2(ImP)2): Allyl groups were introduced into the
side chains by transesterification catalyzed with 1,3-dichlorotetrabutyldis-
tannoxane.[26] The free-base bis-imidazolylporphyrin with four methyl
ester side chains was gently refluxed in toluene/allylalcohol (10:1 v/v) in
the presence of ten equivalents of the tin catalyst for 6 h. After removal
of the solvent, the tetraallyl-substituted bis-porphyrin was eluted from a
silica-gel column with chloroform/acetone (1:1 v/v) as eluent (yield
51%). 1H NMR (270 MHz) in CDCl3: d=ꢀ2.16 (s, 4H; NH), 3.53 (m,
1
form/acetone as eluent (5:1 ! 1:1 v/v) (yield 49%). H NMR (270 MHz)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
in CDCl3: d=ꢀ2.17 (s, 4H; NH), 3.53 (m, 14H; N CH3, CH2COOMe),
14H; N CH3, CH2COO-allyl), 4.63 (m, 8H; COOCH2CH=CH2), 5.14
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3.70 (d, J=3.24 Hz, 6H; COOCH3), 5.33 (m, 8H; CH2CH2COOMe),
7.56 (d, J=1.4 Hz, 2H; 5-imidazolyl), 7.76 (d, J=1.4 Hz, 2H; 4-imidazol-
yl), 7.99, 8.14 (d, J=4.7 Hz, 4H; 13,13’,17,17’-b-pyrrole), 8.94 (d, J=
4.1 Hz, 4H; 4,4’,7,7’-b-pyrrole), 9.14 (dd, J=10.8, 5.1 Hz, 4H;
12,12’,18,18’-b-pyrrole), 9.59 ppm (dd, J=2.3, 3.0 Hz, 4H; 3,3’,9,9’-b-pyr-
role); MALDI-TOF MS: m/z calcd: 1122.45; found: 1125.85 [M+H]+;
UV/Vis (CHCl3): lmax =423, 453, 526, 596 nm.
(m, 4H; CH=CHH), 5.23–5,30 (m, 12H; CH=CHH, porphyrin-CH2 ),
ꢀ
5.80–5.85 (m, 4H; CH=CH2), 7.57 (s, 2H; 5-imidazolyl), 7.76 (s, 2H; 4-
imidazolyl), 7.99, 8.14 (d, 4H; J=4.8 Hz, 13,13’,17,17’-b-pyrrole), 8.94 (d,
J=4.1 Hz, 4H; 4,4’,7,7’-b-pyrrole), 9.18 (dd, J=10.8, 5.1 Hz, 4H;
12,12’,18,18’-b-pyrrole), 9.59 ppm (dd, J=2.3, 3.0 Hz, 4H; 3,3’,9,9’-b-pyr-
role); MALDI-TOF MS: m/z calcd: 1226.51; found: 1227.58 [M+H]+;
UV/Vis (CHCl3): lmax =423, 453, 526, 596 nm.
5572
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2005, 11, 5563 – 5574

Multiporphyrin Arrays
FULL PAPER
The central zinc atom was introduced into the free-base bis-porphyrin
with zinc acetate. After washing with aqueous sodium hydrogencarbonate
and brine, Zn2(ImP)2 was eluted with chloroform/methanol (10:1 v/v)
from a silica-gel column. MALDI-TOF MS: m/z calcd: 1350.34; found:
1354.4 [M+H]+;UV/Vis (CH2Cl2): lmax =407, 490, 582, 633 nm (see also
Figure S1 in the Supporting Information).
total reflectance angles of the modified gold were estimated by Fresnelꢁs
fittings using the appropriate dielectric constants, ewater =1.793, eorganic
=
2.25. The gold substrate coupled with the prism (eprism =2.292) through
the matching oil was equipped with a goniometer in the Kretshmann con-
figuration (Nihon Laser Denshi Co.). The SPR angle of the gold sub-
strate in water was scanned by varying the incident angle of a 670 nm
laser and the photodetector. During the measurement, the SPR angle in-
creased, implying a swelling behavior. The stationary SPR angle after a
20 min immersion was used to estimate the thickness of the multipor-
phyrin array.
5-(1-Methylimidazol-2-yl)-10,20-bis(3-allyloxypropyl)-15-(p-formylphe-
nyl)porphyrin (H2ImP(CHO)): A catalytic amount of TFA (0.32 mL,
2 equiv) was added to a mixture of 2-formyl-1-methylimidazole (225 mg,
1 equiv), terephthalaldehyde (274 mg, 1 equiv), and 3-allyloxypropyldi-
pyrromethane (1.0 g, 2 equiv) in chloroform (400 mL), followed by stir-
ring for 3 h and further stirring after the addition of chloranil (1.5 g,
3 equiv). Elution over a silica-gel column with chloroform/acetone (1:1
v/v) afforded H2ImP(CHO) (yield 4%). 1H NMR (600 MHz) in CDCl3:
Electrochemical and photocurrent measurements: Electrochemical and
photocurrent measurements were performed in a typical three-electrode
configuration connected to a potentiostat (BAS, CV-50W). The gold elec-
trode, as the working-electrode, was mounted at the cell window (diame-
ter 6 mm) and exposed to the aqueous electrolyte solutions. A platinum-
wire electrode and a Ag/AgCl (saturated KCl) electrode were employed
as the counter and reference electrodes, respectively. Sodium sulfate solu-
tion (0.1m) was used as the supporting electrolyte, except for the alkali
desorption experiments, where potassium hydroxide (0.5m) was used. Po-
tassium ferricyanide (purchased from Nacalai Tesque) was used as a bulk
redox probe without further purification. All electrochemical measure-
ments were carried out under deaerated conditions.
ꢀ
d=ꢀ2.69 (s, 2H; inner proton), 2.78 (m, 4H; porphyrin-CH2CH2CH2O ),
ꢀ
ꢀ
3.31 (s, 3H; N CH3), 3.66 (t, J=5.4 Hz, 4H; CH2O-allyl), 4.08 (m, 4H;
ꢀ
ꢀ
OCH2CH=CH2), 5.01 (t, J=7.2 Hz, 4H; porphyrin-CH2 ), 5.26 (dd, J=
1.2, 10.2 Hz, 2H; OCH2CH=CHH), 5.43 (dd, J=1.2, 17.4 Hz, 2H;
OCH2CH=CHH), 6.08 (ddt, J=6.0, 9.6, 16.2 Hz, 2H; OCH2CH=
CH2), 7.49 (s, 1H; 5-imidazolyl), 7.69 (s, 1H; 4-imidazolyl), 8.28 (d, J=
7.2 Hz, 1H; Ph), 8.31 (t, 3H; Ph), 8.42 (d, J=7.2, 1H; Ph), 8.80 (d, J=
5.4 Hz, 2H; b-pyrrole), 8.81 (d, J=5.4 Hz, 2H; b-pyrrole), 9.51 (d, J=
ꢀ
ꢀ
ꢀ
ꢀ
5.4 Hz, 2H; b-pyrrole), 10.41 ppm (s, 1H; CHO); MALDI-TOF MS:
The experimental set-up for photocurrent measurements is presented in
Figure 16. The three-electrode equipment was placed in a dark box. The
electron-carrier methylviologen (10mm) (1,1’-dimethyl-4,4’-dipyridinium
m/z calcd: 690.33; found: 691.34 [M+H]+; UV/Vis (CHCl3): lmax =418,
516, 551, 591 nm.
5,15-Bis(allyloxypropyl)-10-[4-phenyl-2-(N-methyl)fulleropyrrolidinyl]-
20-(1-methylimidazol-2-yl)porphyrinatozinc (Zn(ImP)–C60): A mixture of
H2ImP(CHO) (37.5 mg, 1 equiv), fullerene (78.1 mg, 2 equiv), and N-
methylglycine (96.7 mg, 20 equiv) in toluene was gently refluxed under a
nitrogen atmosphere in the dark for 12 h.[27] The reaction mixture was
subjected to silica-gel column chromatographic separation. The desired
porphyrin–fullerene dyad (32 mg, 41%) was eluted with a gradient sol-
vent system from toluene to chloroform/acetone (9:1 v/v). 1H NMR
(600 MHz) in CDCl3: d=ꢀ2.66 (s, 2H; inner proton), 2.72 (m, 4H; por-
ꢀ
phyrin-CH2CH2CH2O ), 3.13 (s, 3H; pyrrolidine-NCH3), 3.68 (s, 3H;
ꢀ
ꢀ
imidazolyl-NCH3), 3.68 (t, J=5.4 Hz, 4H; CH2O-allyl), 4.05 (br, 4H;
ꢀ
OCH2CH=CH2), 4.43 (d, 1H; pyrrolidine-NCHH ), 5.01 (t, 4H; J=
ꢀ
ꢀ
7.2 Hz, porphyrin-CH2 ), 5.11, (d, 1H; pyrrolidine-NCHH ), 5.28 (s,
ꢀ
ꢀ
1H; pyrolidine-CHN ), 5.26 (dd, J=16.2 Hz, 2H; OCH2CH=CHH),
ꢀ
ꢀ
5.34 (dd, J=16.2 Hz, 2H; OCH2CH=CHH), 6.01 (br, 2H; OCH2CH=
CH2), 7.41 (s, 1H; 5-imidazolyl), 7.60 (s, 1H; 4-imidazolyl), 8.20 (s, 4H;
Ph), 8.68 (d, J=13.8 Hz, 4H; b-pyrrole), 9.31 (s, 2H; b-pyrrole),
9.45 ppm (s, 2H; b-pyrrole); MALDI-TOF MS: m/z calcd: 1437.38;
found: 1439.96 [M+H]+; UV/Vis (CHCl3): lmax =419, 516, 551, 592 nm.
We introduced the central zinc atom by treatment with zinc acetate in
chloroform. The organic layer was washed with aqueous sodium hydro-
gencarbonate, a large excess of water, and brine. Zn(ImP)–C60 (yield
90%) was purified over a silica-gel column with chloroform/acetone (9:1
v/v) as eluent. MALDI-TOF MS: m/z calcd: 1499.29; found: 1502.77
[M+H]+; UV/Vis (CHCl3): lmax =414, 439, 566, 620 nm. Very weak emis-
sions were observed around 621 and 675 nm (lex =438 nm); 99.7% of
these emissions were quenched relative to those observed for non-fuller-
ene-substituted imidazolylporphyrinatozinc Zn(ImP) (Figure 14).
Figure 16. Experimental set-up for the photocurrent measurements.
dichloride, purchased from TCI) was added to the electrolyte solution.
The photocurrent response was recorded under potentiostatic conditions
with exposure to the chopped light under a nitrogen stream. The front
side of the gold electrode was irradiated by a 150 W xenon arc lamp (Ha-
mamatsu Photonics, L2274 light source equipped with C7535 power
supply and C4251 starter) filtered with an IR cut-off filter (Sigma Koki,
HAF-50S-50H) or a monochromator (Shimadzu, SPG-120S). The elec-
trode was placed at a distance of 31 cm from the edge of the lamp hous-
ing. The profile of the white light exhibits properties similar to solar
light.[5a] The light intensity of the monochromatic light at the electrode
surface was monitored with a Si light detector (Advantest, Q8221 equip-
ped with Q82214).
Measurements
Absorption spectra: A gold substrate was prepared by a vacuum-deposi-
tion technique at 5ꢃ10ꢀ6 Torr; 50 ꢄ thickness of Cr and then 200 ꢄ of
Au onto a glass slide (ALVAC, VPC-260). The roughness factor, that is,
the mean area per apparent area, was found to be approximately 1.07 es-
timated from measurements made by non-contact mode atomic force mi-
croscopy (AFM, Seiko Instruments SPA400).
The substrates were placed perpendicular to the optical path for the gen-
eral absorption measurements. The absorption spectra, recorded on a
Shimadzu UV-3100PC spectrophotometer, were shown as the change in
the absorption with respect to the corresponding bare gold.
Acknowledgements
Surface plasmon resonance: Surface plasmon resonance (SPR) measure-
ments were employed to evaluate the thickness of the porphyrin layers
on the gold substrate.[28] The SPR measurements were performed in pure
water (18 MWcm purified by Milli-Q Labo reagent water system). The
Y.K. gratefully acknowledges the financial support of CREST (Core Re-
search for Evolutional Science and Technology) program from JST
Chem. Eur. J. 2005, 11, 5563 – 5574
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5573

Y. Kobuke et al.
(Japan Science and Technology Agency) and Grant-in-aid for Scientific
Research (No. 15205020) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan (Monbu Kagaku Sho).
Libman, G. Rubinstein, A. Shanzer, Angew. Chem. 1999, 111, 1333;
Angew. Chem. Int. Ed. 1999, 38, 1257; G. Kalyuzhny, A. Vaskevich,
G. Ashkenasy, A. Shanzer, I. Rubinstein, J. Phys. Chem. B 2000,
104, 8238; d) S. Zou, R. S. Clegg, F. C. Anson, Langmuir 2002, 18,
3241; e) T. A. Eberspacher, J. P. Collman, C. E. D. Chidsey, D. L.
Donohue, H. Van Ryswyk, Langmuir 2003, 19, 3814.
[1] a) A. Aviram, M. A, Ratner, Chem. Phys. Lett. 1974, 29, 277;
b) R. L. Carroll, C. B. Gorman, Angew. Chem. 2002, 114, 4556;
Angew. Chem. Int. Ed. 2002, 41, 4378; c) R. M. Metzger, Chem. Rev.
2003, 103, 3803.
[9] Y. Kobuke, H. Miyaji, J. Am. Chem. Soc. 1994, 116, 4111; Y.
Kobuke, H. Miyaji, Bull. Chem. Soc. Jpn. 1996, 69, 3563.
[10] R. Takahashi, Y. Kobuke, J. Am. Chem. Soc. 2003, 125, 2372.
[11] a) K. Ogawa, Y. Kobuke, Angew. Chem. 2000, 112, 4236; K. Ogawa,
Y. Kobuke, Angew. Chem. Int. Ed. 2000, 39, 4070; b) K. Ogawa, T.
Zhang, K. Yoshihara, Y. Kobuke, J. Am. Chem. Soc. 2002, 124, 22;
c) C. Ikeda, E. Fujiwara, A. Satake, Y. Kobuke, Chem. Commun.
2003, 616; d) K. Ogawa, A. Ohashi, Y. Kobuke, K. Kamada, K.
Ohta, J. Am. Chem. Soc. 2003, 125, 13356.
[12] a) A. Nomoto, Y. Kobuke, Chem. Commun. 2002, 1104; b) A.
Nomoto, H. Mitsuoka, H. Ozeki, Y. Kobuke, Chem. Commun. 2003,
1075.
[13] a) E. L. Dias, S. T. Nguyen, R. H. Grubbs, J. Am. Chem. Soc. 1997,
119, 3887; b) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34,
18.
[14] a) A. Ohashi, A. Satake, Y. Kobuke, Bull. Chem. Soc. Jpn. 2004, 77,
365; b) C. Ikeda, A. Satake, Y. Kobuke, Org. Lett. 2003, 5, 4935.
[15] a) C. A. Widrig, C. Chung, M. D. Porter, J. Electroanal. Chem. Inter-
facial Electochem. 1991, 310, 335; b) M. M. Walczak, D. D. Popenoe,
R. S. Deinhammer, B. D. Lamp, C. Chung, M. D. Porter, Langmuir
1991, 7, 2687.
[16] M. D. Porter, T. B. Bright, D. L. Allara, C. E. D. Chidsey, J. Am.
Chem. Soc. 1987, 109, 3559.
[17] M. Weck, J. J. Jackiw, R. R. Rossi, P. S. Weiss, R. H. Grubbs, J. Am.
Chem. Soc. 1999, 121, 4088.
[2] LB film approaches: a) H. Kuhn, D. Mçbius, Angew. Chem. 1971,
83, 672; Angew. Chem. Int. Ed. Engl. 1971, 10, 620; b) T. Miyasaka,
T. Watanabe, A. Fujishima, K. Honda, J. Am. Chem. Soc. 1978, 100,
6657; c) M. Fujihira, M. Sakomura, T. Kamei, Thin Solid Films 1985,
132, 77; d) I. Yamazaki, N. Tamai; Yamazaki, A. Murakami, M.
Mimuro, Y. Fujita, J. Phys. Chem. 1988, 92, 5053; e) M. Fujihira, M.
Sakomura, T. Kamei, Thin Solid Films 1989, 180, 43; f) T. Yamazaki,
I. Yamazaki, A. Osuka, J. Phys. Chem. B 1998, 102, 7858; g) B.
Choudhury, A. C. Weeden, J. R. Bolton, Langmuir 1998, 14, 6199;
h) A. Aoki, Y. Abe, T. Miyashita, Langmuir 1999, 15, 1463.
[3] SAM approaches: a) T. Kondo, T. Ito, S. Nomura, K. Uosaki, Thin
Solid Films 1996, 284–285, 652; b) K. Uosaki, T. Kondo, X.-Q.
Zhang, M. Yanagida, J. Am. Chem. Soc. 1997, 119, 8367; c) H. Ima-
hori, H. Norieda, S. Ozawa, K. Ushida, H. Yamada, T. Azuma, K.
Tamaki, Y. Sakata, Langmuir 1998, 14, 5335; d) H. Imahori, H. Nor-
ieda, Y. Nisimura, I. Yamazaki, K. Higuchi, N. Kato, T. Motohiro,
H. Yamada, K. Tamaki, M. Arimura, Y. Sakata, J. Phys. Chem. B
2000, 104, 1253; e) H. Imahori, H. Yamada, Y. Nisimura, I. Yamaza-
ki, Y. Sakata, J. Phys. Chem. B 2000, 104, 2099; f) T. Morita, S.
Kimura, S. Kobayashi, Y. Imanishi, Chem. Lett. 2000, 676; T. Morita,
S. Kimura, S. Kobayashi, Y. Imanishi, J. Am. Chem. Soc. 2000, 122,
2850; g) H. Imahori, T. Hasobe, H. Yamada, Y. Nisimura, I. Yama-
zaki, S. Fukuzumi, Langmuir 2001, 17, 4925; h) H. Imahori, H. Nor-
ieda, H. Yamada, Y. Nisimura, I. Yamazaki, Y. Sakata, S. Fukuzumi,
J. Am. Chem. Soc. 2001, 123, 100; i) D. Hirayama, K. Takiyama, Y.
Aso, T. Otsubo, T. Hasobe, H. Yamada, H. Imahori, S. Fukuzumi, Y.
Sakata, J. Am. Chem. Soc. 2002, 124, 532; j) H. Yamada, H. Imahori,
Y. Nishimura, I. Yamazaki, T.-K. Ahn, S.-K. Kim, D. Kim, S. Fuku-
zumi, J. Am. Chem. Soc. 2003, 125, 9129; k) T. Hasobe, H. Imahori,
H. Yamada, T. Sato, K. Ohkubo, S. Fukuzumi, Nano Lett. 2003, 3,
409.
[4] Layer-by-layer approaches: a) K. Araki, M. J. Wagner, M. S. Wright-
on, Langmuir 1996, 12, 5393; b) K. Ariga, Y. Lvov, T. Kunitake, J.
Am. Chem. Soc. 1997, 119, 2224; c) M. Lahav, T. Gabriel, A. N.
Shipway, I. Willner, J. Am. Chem. Soc. 1999, 121, 258; d) A. Ikeda,
T. Hatano, S. Shinkai, T. Akiyama, S. Yamada, J. Am. Chem. Soc.
2001, 123, 4855; e) F. B. Abdelrazzaq, R. C. Kwong, M. E. Thomp-
son, J. Am. Chem. Soc. 2002, 124, 4796; f) D.-J. Qian, C. Nakamura,
T. Ishida, S.-O. Wenk, T. Wakayama, S. Takeda, J. Miyake, Langmuir
2002, 18, 10237.
[5] a) M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E.
Mꢅller, P. Liska, N. Vlachopoulos, M. Grꢀtzel, J. Am. Chem. Soc.
1993, 115, 6382; b) A. Hagfeldt, M. Grꢀtzel, Chem. Rev. 1995, 95,
49.
[18] S. Ohshima, T. Kajiwara, M. Hiramoto, K. Hashimoto, T. Sakata, J.
Phys. Chem. 1986, 90, 4474.
[19] a) F. J. Kampas, M. Gouterman, J. Phys. Chem. 1977, 81, 690; b) F. J.
Kampas, K. Yamashita, J. Fajer, Nature 1980, 284, 40; c) Y. Harima,
K. Yamashita, J. Phys. Chem. 1985, 89, 5325; d) H. Yanagi, M.
Ashida, Y. Harima, K. Yamashita, Chem. Lett. 1990, 385.
[20] a) R. Dabestani, A. J. Bard, A. Campion, M. A. Fox, T. E. Mallouk,
S. E. Webber, J. M. White, J. Phys. Chem. 1988, 92, 1872; b) N. Vla-
chopoulos, P. Liska, J. Augustynski, M. Grꢀtzel, J. Am. Chem. Soc.
1988, 110, 1216; c) I. Bedja, P. V. Kamat, X. Hua, A. G. Lappin, S.
Hotchandani, Langmuir 1997, 13, 2398; d) F. Fungo, L. Otero, C. D.
Borsarelli, E. N. Durantini, J. J. Silber, L. Sereno, J. Phys. Chem. B
2002, 106, 4070.
[21] a) W. Klçpffer, J. Chem. Phys. 1969, 50, 2337; b) H. Nakamura, H.
Fujii, H. Sakaguchi, T. Matsuo, N. Nakashima, K. Yoshihara, T.
Ikeda, S. Tazuke, J. Phys. Chem. 1988, 92, 6151.
[22] a) A. Nakano, T. Yamazaki, Y. Nishimura, I. Yamazaki, A. Osuka,
Chem. Eur. J. 2000, 6, 3254; b) N. Aratani, H. S. Cho, T. K. Ahn, S.
Cho, D. Kim, H. Sumi, A. Osuka, J. Am. Chem. Soc. 2003, 125,
9668; c) D. H. Yoon, S. B. Lee, K.-H. Yoo, J. Kim, J. K. Lim, N. Ara-
tani, A. Tsuda, A. Osuka, D. Kim, J. Am. Chem. Soc. 2003, 125,
11062.
[23] D. Furutsu, A. Satake, Y. Kobuke, Inorg. Chem. 2005, 44, 4460.
[24] a) K. Kalyanasundaram, M. Neumann-Spallart, J. Phys. Chem. 1982,
86, 5163; b) T. Nagamura, N. Takeyama, K. Tanaka, T. Matsuo, J.
Phys. Chem. 1986, 90, 2247; c) A. Uehata, H. Nakamura, S. Usui, T.
Matsuo, J. Phys. Chem. 1989, 93, 8197.
[25] A. Osuka, H. Shimidzu, Angew. Chem. 1997, 109, 93; Angew. Chem.
Int. Ed. Engl. 1997, 35, 135.
[26] J. Otera, N. Dan-oh, H. Nozaki, J. Org. Chem. 1991, 56, 5307.
[27] a) M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 1993, 115,
9798; b) T. D. Ros, M. Prato, F. Novello, M. Maggini, E. Banfi, J.
Org. Chem. 1996, 61, 9070.
[6] Metal–ligand approaches: a) A. Hatzor, T. Moav, H. Cohen, S.
Matlis, J. Libman, A. Vaskevich, A. Shanzer, I. Rubinstein, J. Am.
Chem. Soc. 1998, 120, 13469; A. Hatzor, T. Boom-Moav, S. Yochelis,
A. Vaskevich, A. Shanzer, I. Rubinstein, Langmuir 2000, 16, 4420;
b) M. A. Ansell, A. C. Zeppenfeld, K. Yoshino, E. B. Cogan, C. J.
Page, Chem. Mater. 1996, 8, 591; c) M. Maskus, H. D. AbruÇa, Lang-
muir 1996, 12, 4455; d) D. L. Thimsen III, T. Phyly-Bobin, F. Papadi-
mitrakopoulos, J. Am. Chem. Soc. 1998, 120, 6177.
[7] a) G. McDermott, S. M. Prince, A. A. Freer, A. M. Haw-
thornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell, N. W. Isaacs,
Nature 1995, 374, 517; b) T. Pullerits, V. Sundstrçm, Acc. Chem. Res.
1996, 29, 381.
[28] K. Kobayashi, S. Imabayashi, K. Fujita, K. Nonaka, T. Kakiuchi, H.
Sasabe, W. Knoll, Bull. Chem. Soc. Jpn. 2000, 73, 1993.
[8] Metalloporphyrin–ligand approaches: a) D.-Q. Li, L. W. Moore, B. I.
Swanson, Langmuir 1994, 10, 1177; b) D. A. Offord, S. B. Sachs,
M. S. Ennis, T. A. Eberspacher, J. H. Griffin, C. E. D. Chidsey, J. P.
Collman, J. Am. Chem. Soc. 1998, 120, 4478; c) G. Ashkenasy, J.
Received: January 14, 2005
Published online: July 12, 2005
5574
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2005, 11, 5563 – 5574