Assemblies of artificial photosynthetic reaction centres

Article abstract of DOI:10.1039/c2jm15585k

Nature harnesses solar energy for photosynthesis in which one reaction centre is associated with a number of light harvesting units. The reaction centre and light-harvesting units are assembled by non-covalent interactions such as hydrogen bonding and π-π interactions. This article presents various strategies to assemble artificial photosynthetic reaction centres composed of multiple light harvesting units and charge-separation units, which are connected by non-covalent bonding as well as covalent bonding. First light-harvesting units are assembled on alkanethiolate-monolayer-protected metal nanoparticles (MNPs), which are connected with electron acceptors by non-covalent bonding. Light-harvesting units can also be assembled using dendrimers and oligopeptides to combine with electron acceptors by π-π interactions. The cup-shaped nanocarbons generated by the electron-transfer reduction of cup-stacked carbon nanotubes have been functionalized with a number of porphyrins acting as light-harvesting units as well as electron donors. In each case, the photodynamics of assemblies of artificial photosynthetic reaction centres have revealed efficient energy transfer and electron transfer to afford long-lived charge-separated states.

Full text of DOI:10.1039/c2jm15585k

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FEATURE ARTICLE
Assemblies of artificial photosynthetic reaction centres
ab
and Kei Ohkuboa
*
Shunichi Fukuzumi
Received 1st November 2011, Accepted 13th December 2011
DOI: 10.1039/c2jm15585k
Nature harnesses solar energy for photosynthesis in which one reaction centre is associated with
a number of light harvesting units. The reaction centre and light-harvesting units are assembled by non-
covalent interactions such as hydrogen bonding and p–p interactions. This article presents various
strategies to assemble artificial photosynthetic reaction centres composed of multiple light harvesting
units and charge-separation units, which are connected by non-covalent bonding as well as covalent
bonding. First light-harvesting units are assembled on alkanethiolate-monolayer-protected metal
nanoparticles (MNPs), which are connected with electron acceptors by non-covalent bonding. Light-
harvesting units can also be assembled using dendrimers and oligopeptides to combine with electron
acceptors by p–p interactions. The cup-shaped nanocarbons generated by the electron-transfer
reduction of cup-stacked carbon nanotubes have been functionalized with a number of porphyrins
acting as light-harvesting units as well as electron donors. In each case, the photodynamics of
assemblies of artificial photosynthetic reaction centres have revealed efficient energy transfer and
electron transfer to afford long-lived charge-separated states.
demands in an environmentally benign manner, if efficient,
1. Introduction
low-cost systems other than natural systems can be developed for
making clean fuels such as hydrogen.1–3 In natural photosyn-
thesis, the photosynthetic reaction centre is associated with
a number of light harvesting units in which the harvested light
energy is efficiently transferred to the reaction centre where the
charge-separation occurs for the primary energy conversion
reactions of photosynthesis.4,5 Extensive efforts have so far been
devoted to develop artificial photosynthetic reaction centres,
which can mimic the energy and electron-transfer processes in
photosynthesis.6–13 However, covalent syntheses of large molec-
ular arrays are highly inefficient and costly. Thus, an appropriate
The development of artificial photosynthetic systems using solar
energy has been highy desired because of the rising concern of
environmental pollution caused by the use of fossil fuels.1–3 Solar
energy has the potential capacity to fulfill global human energy
aDepartment of Material and Life Science, Division of Advanced Science
and Biotechnology, Graduate School of Engineering, Osaka University,
ALCA, Japan Science and Technology Agency (JST), Suita, Osaka,
565-0871, Japan. E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: +81 6
6879 7370; Tel: +81 6 6879 7368
bDepartment of Bioinspired Science, Ewha Womans University, Seoul, 120-
750, Korea
Shunichi Fukuzumi earned his
Ph.D. degree in applied chem-
istry at Tokyo Institute of
Technology in 1978. He has
been a Full Professor of Osaka
University since 1994. He is the
director of an ALCA (Advanced
Kei Ohkubo earned his Ph.D.
degree in applied chemistry from
Osaka University in 2001. He
worked as a JSPS fellow and
JST research fellow at Osaka
University from 2001 to 2005.
He has been a designated asso-
ciate professor at Osaka
University since 2005.
Low
Carbon
Technology
Research and Development)
project and the leader of
a Global COE program, Global
Education and Research Centre
for Bio-Environmental Chem-
istry at Osaka University.
Shunichi Fukuzumi
Kei Ohkubo
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combination of covalent synthesis with the use of non-covalent
interactions is required in order to construct functionally inte-
grated ordered architectures from functional building blocks at
both the molecular and supramolecular level.
extremely slow charge recombination, has obvious advantages
with regard to synthetic feasibility.36
Mes–Acr+ was assembled on AuNPs by a condensation reac-
tion.36 A carboxylic acid group is introduced in the N-methyl
group in Mes–Acr+ as shown in Fig. 1.37 First, N-(2-methoxy-
ethoxymethyl)-9-acridone (1) was synthesized by the reaction of
9(10H)-acridone with 2-methoxy-ethoxymethyl chloride. Then,
9-mesitylacridine (2) was obtained by the reaction of N-protected
acridone with 2,4,6-trimethylphenylmagnesium bromide. 2 was
reacted with the triflate of benzylglycolate (3), which was
obtained by the reaction of benzyl glycolate with triflic anhydride
to yield 10-benzyloxycarbonylmethyl-9-mesitylacridinium tri-
flate (4). Finally the acridinium ester protected by a benzyl group
4 was reacted with hydrogen bromide/acetic acid to yield
In this article, we would like to focus on recent advancement in
construction of hierarchial assemblies of artificial photosynthetic
reaction centres by combination of covalent syntheses and the
use of a variety of non-covalent interactions. First, nanoparticles
are used to assemble artificial photosynthetic reaction centres.
Dendrimers, oligopeptides and carbon nanotubes are also
utilized to assemble electron donors and acceptors. We have also
used a new type of nanocarbon material with controlled size and
shape and also porphyrin nanotubes to construct artificial
photosynthetic reaction centres. With regard to the lifetime of
the charge-separated (CS) state of the artificial photosynthetic
reaction centres, a long CS lifetime has been achieved by
choosing a suitable chromophore and a redox component.
10-carboxylmethyl-9-mesitylacridinium
hexafluorophosphate
(Mes–Acr+–COOH, 43% yield). A reference compound, Mes-
Acr+–COOPh, was prepared by condensation of Mes–Acr+–
COOH and phenol under similar conditions as described above.
Then, the carboxyl-terminated Mes–Acr+–COOH was directly
coupled to 4-mercaptophenol-functionalized Au nanoparticles
(PhS-AuNPs) in the presence of N,N0-diisopropylcarbodiimide
(DIPC)38 and 4-(N,N-dimethylamino)pyridinium-4-toluenesul-
fonate (DPTS) as the standard coupling agents as shown in
Fig. 2a.36 The coverage of Mes–Acr+–COOH on the PhS–AuNP
was calculated to be 58 molecules per particle (75%).36 The
TEM image (Fig. 2b) revealed the mean diameter of the Au to be
1.7 ꢀ 0.3 nm.36
2. Assemblies on nanoparticles
2.1. Gold nanoparticles
Alkanethiolate-monolayer-protected gold nanoclusters (AuNPs)
have merited significant interest, because they provide three
dimensional (3D) architectures, which are stable in air and
soluble in both nonpolar and polar organic solvents, and also
capable of facile modification with other functional thiols
through exchange reactions or by couplings and nucleophilic
substitutions.14–19 Thus, AuNPs can be used as excellent
materials to assemble artificial photosynthetic reaction centres
(vide infra).
Nanosecond laser flash photolysis measurements of the
reference compound (Mes–Acr+–COOH) revealed the transient
absortion at 500 nm and a broad absorption band at 1000 nm in
the NIR region as shown in Fig. 3a,36 which are the same as
observed in the case of Mes–Acr+.39 The absorption band at
500 nm was previously assigned to the triplet excited state of the
Acr+ moiety.40,41 However, the absorption band at 500 nm is
accompanied by an NIR absorption, which was clearly assigned
to the p-dimer radical cation formed between the electron-
Extensive studies have so far been done to construct artificial
photosynthetic reaction centres starting from dyads, triads,
tetrads, and even pentads, which are composed of a chromo-
phore, electron donor(s), and/or acceptor(s), mimicking a multi-
step photoinduced electron-transfer processes in the
photosynthetic reaction centre.20–25 The multistep photoinduced
electron transfer from the spatially defined chromophore to the
terminal electron acceptor through a series of electron mediators
has well been established to attain fine-tuned and directed redox
gradients along donor–acceptor linked arrays for the formation
of the long-lived charge-separated state with high efficiency.20–25
However, such a long-lived CS state has only been attained in
compensation for a significant energy loss during the multi-step
electron-transfer processes, because each step loses a fraction of
the initial excitation energy with the increase in the distance
between the positive and negative charges. Instead of using
multistep photoinduced electron-transfer processes, one-step
photoexcitation of simple electron donor–acceptor dyads has
also been reported to afford long-lived CS states.26–35 Among
them, the 9-mesityl-10-methylacridinium ion (Mes–Acr+), which
has a small reorganization energy (l) of electron transfer because
the overall charge remains the same in the charge-shift electron
transfer, is capable of fast photoinduced electron transfer but
extremely slow back electron transfer to form a long-lived elec-
tron-transfer state and a p-dimer radical cation between the
ground state and electron-transfer state of the dyad.34 Such
a simple molecular dyad capable of fast charge separation, but
+
+ 39
,
transfer state (Mes –Acr ) and the ground state of Mes–Acr .
,
Fig. 1 Synthetic scheme for Mes–Acr+–COOH.
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Scheme 1 Photoinduced electron transfer of Mes–Acr+–COOPh and
subsequent formation of the p-dimer radical cation.
+
,
,
transfer state (Mes –Acr ) has both strong oxidizing ability and
reducing ability, which enables it to act as an excellent photo-
catalyst in various photoinducd redox reactions.50–56
Fig. 2 (a) Synthetic scheme for Mes–Acr+–PhS–AuNP. (b) TEM image
of Mes–Acr+–PhS–AuNPs.36
Femtosecond laser flash photolysis measurements of
Mes–Acr+–COOPh with excitation at 420 nm revealed formation
of the electron-transfer state as shown in Fig. 4a, where no NIR
absorption due to the p-dimer radical cation was observed at
500 ps, because no intermolecular reaction occurred at the time
scale in Fig. 4a. In sharp contast to this, femtosecond laser
excitation at 420 nm of Mes–Acr+–PhS–AuNPs revealed a tran-
sient absorption band at 490 nm together with a broad transient
The same NIR absorption band was observed in intermolecular
photoinduced electron transfer from mesitylene to the singlet
excited state of the 10-methylacridinium ion (1AcrH+*).39
A
similar near-IR absorption was also observed for the p-dimer
radical cation formed between the electron-transfer state of
2-phenyl-4-(1-naphthyl)quinolinium ion and the ground
absorption band in the NIR region due to the formation of the
p-dimer radical cation of the ET state of Mes–Acr+ (Mes
+
,
–
state.42,43 Thus, upon photoexcitation of Mes–Acr+–COOPh, the
+
Acr –PhS–AuNPs) with the neighboring Mes–Acr molecule.
,
+
,
electron transfer state (Mes –Acr –COOPh) is formed and then
,
ꢀ
From the distance between two Au atoms (4.06 A) of the crystal
+
+
,
,
the p-dimer radical cation [(Mes –Acr –COOPh)(Mes–Acr –
COOPh)] is formed with Mes–Acr+–COOPh (Scheme 1).36 Such
p-dimer radical cations have been well known to be formed
between p-conjugated compounds and the radical cations, which
exhibit NIR absorption.44–46 The decay of the p-dimer radical
cation obeyed second-order kinetics, because the intermolecular
back electron transfer predominates due to the slow intra-
molecular back electron transfer as observed in the case of
Mes–Acr+.36
lattice of Au, the distance between two neighboring Mes–Acr+
molecules on the Au surface is estimated to be 8.3 A.36 Such close
ꢀ
proximity of Mes–Acr+ molecules on the AuNPs makes it
possible to form the p-dimer radical cation via an intramolecular
p–p interaction upon photoinduced electron transfer. The
p-dimer radical cation decays with lifetime of 1.9 ps by rapid
electron transfer from the benzenethiol linker to the p-dimer
+
,
radical cation (Mes –Mes) moiety and back electron transfer to
the ground state with the lifetime of 380 ps as shown in the energy
diagram in Fig. 5.36
It should be pointed out that the energy of the acridinium
triplet was previously reported to be 1.96 eV, which is lower than
+
,
40
,
the the electron-transfer state (Mes –Acr ), but this value
corresponds to the strong phosphorescence of the corresponding
acridine, which remains as an impurity according to the reported
synthetic method.40 It is known that the triplet energy of the
acridinium ion is significantly higher than that of the corre-
sponding acridine.47–49 It should be emphasized that the electron-
Fig. 3 (a) Transient absorption spectra of Mes–Acr+–COOPh (0.1 mM)
in MeCN at room temperature taken 10 ms and 100 ms after nanosecond
Fig. 4 Transient absorption spectra observed in femtosecond laser flash
photolysis (lex ¼ 420 nm) of (a) Mes–Acr+–COOPh and (b) Mes–Acr+–
PhS–AuNPs in MeCN at 298 K.
laser excitation at 430 nm. (b) The time profiles for the decay of
+
,
Mes –Acr –COOPh at 500 nm and 980 nm.
,
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Thus, AuNPs provide an excellent scaffold to assemble
Mes–Acr+ molecules and the photoexcitation resulted in rapid
+
,
,
formation of the electron-transfer state (Mes –Acr ), accom-
panied by formation of the p-dimer radical cation with the
neighbouring Mes–Acr+ on AuNPs, whereas such p-dimer
radical cation formation occurs at a much slower time scale for
the reference compound (Mes–Acr+–COOPh) by the intermo-
+
+
,
,
lecular reaction between Mes –Acr –COOPh and Mes–Acr –
COOPh.
Fig. 6 TiO2 nanoparticles modified with 9-mesityl-10-carboxymethyl
acridinium ion and 10-carboxymethylacridinium ion.
2.2. TiO2 nanoparticles
Mes–Acr+–COOH and the reference compound (Acr+–COOH)
without the donor moiety (Fig. 6) can also be assembled on TiO2
nanoparticles (TiO2NPs) by immersing warmed TiO2NPs
(80–100 ꢁC) in an acetonitrile mixture containing Mes–Acr+–
COOH and Acr+–COOH for 12 h, respectively.57 After adsorb-
ing Mes–Acr+–COOH and Acr+–COOH, TiO2NPs were filtered,
and subsequent washing with acetonitrile and drying gave Mes–
Acr+–COO–TiO2NPs and Acr+–COO–TiO2NPs (Fig. 6).57 The
dye molecule was completely desorbed from the TiO2NPs into
solution by immersing TiO2NPs modified with the dye moieties
in methanol overnight. The amounts of Mes–Acr+–COOH and
Acr+–COOH adsorbed on TiO2NPs relative to the total weight
The scanning electron micrograph (SEM) of the OTE/SnO2/
(Mes–Acr+–COO–TiO2 + C60)n film in Fig. 7 exhibits closely
packed clusters of about 20–100 nm size with a networked
structure due to a supramolecular interaction between Mes–
Acr+–COO–TiO2 and C60 in the TiO2 nanoparticle matrix.57
Thus, TiO2 nanoparticles play an important role in the cluster
formation on the films.
Photocurrent measurements of the OTE/SnO2/(Mes–Acr+–
COO–TiO2 + C60)n electrode as a photoanode in acetonitrile
containing NaI (0.5 M) and I2 (0.01 M) under the bias of 0.2 V vs.
SCE revealed a significant increase in the photocurrent as
compared with those without TiO2.57 The incident photon-to-
photocurrent efficiency (IPCE) value of 37% was obtained for
were determined as 1.5 ꢂ 10ꢃ5 and 1.5 ꢂ 10ꢃ5 mol gꢃ1
,
respectively.57
the OTE/SnO2/(Mes–Acr+–COO–TiO2
+
C60)n electrode,
Mes–Acr+–COO–TiO2NPs were electrophoretically deposited
onto the OTE/SnO2 electrode in suspended solution to prepare
the OTE/SnO2/(Mes–Acr+–COO–TiO2)n electrode.57 A mixed
cluster suspension of Mes–Acr+–COO–TiO2 and C60 was also
prepared in the total concentration range from 0.025 to 0.13 mM
(molecular ratio of Mes–Acr+:C60 ¼ 1 : 5) in acetonitrile/toluene
(3/1, v/v).57 The clusters suspended in acetonitrile/toluene mixed
solvent were assembled electrophoretically as thin films on
a conducting glass electrode surface to obtain films of (Mes–
Acr+–COO–TiO2 + C60)n on nanostructured SnO2 films cast on
an optically conducting glass electrode, OTE/SnO2/(Mes–Acr+–
COO–TiO2 + C60)n.57 A broader wavelength absorption in the
visible region was observed in the OTE/SnO2/(Mes–Acr+–COO–
TiO2 + C60)n film relative to those in the OTE/SnO2/(Mes–Acr+–
COO–TiO2)n and OTE/SnO2/(C60)n films, suggesting the charge-
transfer (CT) interaction between Mes–Acr+ moiety and C60.57
whereas the maximum IPCE value of the OTE/SnO2/(Acr+–
COO–TiO2 + C60)n electrode without the electron donor moiety
(Mes) was significantly smaller (14%).57 This indicates that
photoinduced electron transfer from the donor moiety (Mes) to
the acceptor moiety (Acr+) occurs, followed by electron transfer
,
from the resulting acridinyl radical moiety (Acr ) to C60 in the
supramolecular complex, leading to enhancement of the photo-
current generation.
In the case of the OTE/SnO2/(Mes–Acr+–COO–TiO2 + C60)n
,
electrode, the long lifetime of the electron-transfer state (Acr –
+
,
,
Mes ) ensures efficient electron-transfer from Acr to C60 (C60/
ꢃ
C60 ¼ ꢃ0.2 V vs. NHE)58 to produce the C60 radical anion. The
,
formation of C60 ꢃ was confirmed by the nanosecond laser flash
,
photolysis measurements of a deoxygenated toluene–acetonitrile
(1 : 1, v/v) solution of 9-mesityl-10-methyl-acridinium ion
without carboxylic acid (Mes–Acr+)34 in the presence of C60,
Fig. 5 Energy diagram of photoinduced electron transfer in Mes–Acr+–
Fig. 7 SEM (scanning electron micrograph) images of OTE/SnO2/
(Mes–Acr+–COO–TiO2 + C60)n ([Mes–Acr+] ¼ 25 mM, [C60] ¼ 130 mM).
PhS–AuNPs.
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which clearly exhibits a broad absorption band at 1050 nm due to
ꢃ 59,60
,
C60
.
The reduced C60 clusters inject electrons into the
conduction band of SnO2, whereas the oxidized mesityl moiety
+
(Mes ) undergoes the electron-transfer reduction with iodide
,
ion in the electrolyte.57
2.3. Platinum nanoparticles
Among various metal nanoparticles, Pt nanoparticles (PtNPs)
have been known to be the most reactive as hydrogen evolution
catalysts.61 There have been extensive studies on hydrogen
evolution using metal complexes as photosensitizers combined
with electron mediators and hydrogen-evolution catalysts.62–67 In
order to improve the catalytic efficiency of hydrogen evolution,
an electron mediator (methyl viologen) was linked to PtNPs.68
The methyl viologen-modified platinum clusters (MVA2+–
PtNPs) were prepared by applying the same method as the
preparation of alkanethiolate-modified AuNPs,69 as shown in
Fig. 8a, where the reduction of H2PtCl4 with NaBH4 was per-
formed in water containing MVA2+ (MVA2+/H2PtCl4 ¼ 1/1). The
mean diameter of the platinum core (2RCORE) of MVA2+–PtNPs
determined by transmission electron microscopy (TEM in
Fig. 8b) is 1.9 nm (Fig. 8c), which is similar to the diameter
reported for the water-soluble PtNPs.70 The number of MVA2+
on the Pt surface is determined to be 67 from the elemental
analysis of MVA2+–PtNPs.68
Fig. 8 (a) Preparation of MVA2+–PtNPs. (b) Transmission electron
microscopy (TEM) image and (c) core size histograms of MVA2+–PtNPs.
+
,
,
state (Acr –Mes ). This is followed by the rapid electron-
transfer oxidation of NADH and/or the electron-transfer
reduction of MVA2+ in MVA2+–PtNPs. Both processes are
thermodynamically feasible, because the one-electron oxidation
potential of NADH (Eox ¼ 0.76 V vs. SCE)71,72 is less positive
than the one-electron reduction potential of the Mes + moiety of
,
MVA2+ and MVA2+–PtNPs were reduced by Na2S2O4 in
+
,
Mes –Acr and the one-electron oxidation potential of the Acr
,
aqueous solution at pH 7.0. The formation of a p-dimer radical
cation between MVA + and MVA on PtNPs is recognized in the
moiety in water (Eox ¼ ꢃ0.57 V vs. SCE) is more negative than
,
the one-electron reduction potential of the MVA2+ moiety in
+
,
UV-vis spectrum of MVA –PtNPs in comparision with that of
MVA2+–PtNPs (Er0ed ¼ ꢃ0.50 V). In contrast, electron transfer
+
+
,
MVA in Fig. 9. The absorption band of MVA –PtNPs is blue-
,
+
from Acr –Mes (Eox ¼ ꢃ0.57 V vs. SCE) to MV2+ (Ered
¼
,
,
+
,
shifted from 605 nm of MVA to 525 nm, accompanied by the
ꢃ0.67 V) is energetically unfavorable, because the Eox value of
appearance of the NIR absorption (Fig. 9b) as observed in the
0
Mes –Acr is more positive than the Ered value of MV2+. In this
+
,
,
case of the formation of the p-dimer radical cation between
+
case, MV2+ is reduced by NAD , which is produced via depro-
,
+
Mes –Acr and Mes–Acr in Fig. 3 and 4. This results from
,
,
+
,
,
tonation of NADH , because the electron transfer from NAD
closely packed MVA molecules on the Pt surface.68 The forma-
(Eox ¼ ꢃ1.1 V vs. SCE)71,72 to MV2+ (Ered ¼ ꢃ0.67 V) is highly
+
tion of the p-dimer radical cation between MVA and MVA2+
,
exergonic. Thus, MVA2+–PtNPs are more reactive in electron
at close proximity on PtNPs resulted in the positive shift in the
first one-electron reduction potential of MVA2+–PtNPs from
+
,
,
transfer from Mes –Acr and also electron transfer to PtNPs,
leading to the 10 times faster H2 evolution.
ꢃ0.67 V vs. SCE of MVA2+ to ꢃ0.50 V vs. SCE because of the
+
,
stabilization by the p-bond formed between MVA
+
and
,
MVA
.
2.4. Porphyrin assembled metal nanoparticles
The hydrogen-evolution efficiency was compared using the
photocatalytic systems composed of NADH, Mes–Acr+ and
MVA2+–PtNPs at pH 4.5 vs. NADH, Mes–Acr+ and a mixture of
the same amounts of MVA2+ and PtNPs under the same experi-
mental conditions.68 The amount of hydrogen evolution of the
NADH/Mes–Acr+/MVA2+–PtNPs system increases with the
photoirradiation time linearly and the rate was estimated to be
2.4 mmol hꢃ1, which is ten times faster than the rate of the NADH/
Mes–Acr+/MVA2+/PtNPs system.68 The more efficient hydrogen
evolution in the NADH/Mes–Acr+/MVA2+–PtNPs system
certainlyresults fromthe assembly effectof MV2+ onPtNPs, which
In order to expand the potential applications of metal nano-
particles (MNPs), systematic studies on chromophore-modified
facilitates electron transfer from MVA + to PtNPs, as compared
,
with intermolecular electron transfer from MVA + to PtNPs.
,
Scheme 2 summarizes the mechanism of hydrogen evolution in
the NADH/Mes–Acr+/MVA2+–PtNPs system.68 The photo-
catalytic hydrogen evolution starts by photoexcitation of
Mes–Acr+, which results in the formation of the electron-transfer
+
Fig. 9 UV-vis spectra of (a) MVA2+–PtNPs, (b) MVA –PtNPs, (c)
,
MVA2+ and (d) MVA + in aqueous solution at pH 7.0.
,
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Scheme 2 Catalytic cycle of hydrogen evolution from NADH and water
with Mes–Acr+ and MVA2+–PtNPs.
MNPs are required. Brust et al. developed a facile synthetic
method in which a quaternary ammonium salt is used to assist
the transfer of gold ion [(AuCl4)ꢃ] and reducing agent (BH4
ꢃ
)
from aqueous phase to organic phase to yield air-stable func-
tionalized MNPs with a mono dispersity.14 This method was
utilized to prepare porphyrin-modified metal nanoclusters with
different metal and different size of gold core (H2PC11AuNPs,
H2PC11AuNPs,
H2PC11AuNPs,
H2PC11PdNPs,
H2PC11PtNPs, H2PC11Au/AgNPs), as shown in Fig. 10.73
The different size of metal core was obtained by changing the
concentraion of porphyrin alkanethiol (Por-SH) as shown in the
TEM images of H2PC11AuNPs (Fig. 11).73 Based on the
elemental analysis, there are 25, 57 and 136 porphyrin alka-
nethiolate chains on the gold surface for H2PC11AuNPs with the
mean diameter 2RCORE ¼ 1.4 nm, 2.1 and 2.9 nm, respectively.19
The coverage ratios of porphyrin alkanethiolate chains of
H2PC11AuNPs with 2RCORE ¼ 1.4 nm, 2.1 and 2.9 nm to surface
Au atoms (g) were determined to be 46%, 40% and 46%,
respectively. These g values are remarkably large compared with
the coverage ratio (g ¼ 6.5%) of 2D porphyrin SAM H2PC11–
Au(111).74 Such enhanced packing of the large porphyrins is
made possible by the highly curved outermost surface of the
AuNPs, where the spacer is splayed outward from the gold core
to relieve steric crowding significantly. Similar results were
obtained for H2PC11Au/AgNPs, H2PC11PdNPs and
H2PC11PtNPs.73
Fig. 10 Schematic structures of porphyrin-modified gold nanoclusters
H2PCnAuNPs.
These porphyrin-modified MNPs form p-complexes with
fullerene molecules and they were clusterized in an acetonitrile/
toluene mixed solvent.79 Then, the highly colored composite
clusters were assembled as a three-dimensional array onto
nanostructured SnO2 films to afford the OTE/SnO2/
(H2PCnMNPs + C60)m electrode (n is the number of alkyl chains)
using an electrophoretic deposition method.79 The film of the
composite cluster with gold nanoparticle exhibits an incident
Although there are many porphyrins attached on MNPs, there
is no significant interaction between porphyrin molecules on
MNPs, because the lmax values of the Soret band of
H2PC11MNPs in benzene, THF and CHCl3 are nearly identical
to that of Por-ref in the three solvents. The surface plasmon
absorption due to the gold,75 palladium,76 platinum70b and alloy
gold–silver77 nanoparticles is much weaker than that of the
porphyrin moiety.
Fluorescence lifetimes of H2PC11AuNPs with 2RCORE
¼
1.4 nm, 2.1 and 2.9 nm were determined to be 0.17 ns (97%),
0.10 ns (96%), 0.12 ns (73%), respectively.73 The similar
quenching efficiency reveals that the difference in metal size
(1.4 nm–2.9 nm) has no appreciable impact on the quenching of
the porphyrin excited singlet state by the metal surfaces. It
should be noted that the fluorescence lifetime of H2PC11Au/
AgNPs [0.22 (81%) and 7.0 ns (19%) in CHCl3 (lobs ¼ 720 nm)] is
significantly longer than those of monometal H2PC11AuNPs.73
This suggests that interaction between the surface of the gold-
silver alloy and the porphyrin excited singlet state is attenuated
considerably relative to the mono-metal systems. Thus,
porphyrin-modified metal alloy nanoclusters are highly prom-
ising as new types of light-harvesting materials, photocatalysts,
and chemical and biochemical sensors.78
Fig. 11 Transmission electron microscopy (TEM) images of
H2PC11AuNPs with different sizes; (a) 2RCORE ¼ 1.4 nm with a standard
deviation (s ¼ 0.4 nm), (b) 2RCORE ¼ 2.1 nm with s ¼ 0.3 nm and (c)
2RCORE ¼ 2.9 nm with s ¼ 0.7 nm. The dark spots correspond to the
metal core.
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photon-to-photocurrent efficiency (IPCE) as high as 54% and
broad photocurrent action spectra (up to 1000 nm).79 The energy
conversion efficiency of this system reaches 1.5%, and this value
is 45 times higher than that of the reference system.79 Such
remarkable enhancement in the photoelectrochemical perfor-
mance as well as broader photoresponse in the visible and
infrared relative to the reference systems demonstrates that the
porphyrin-AuNPs and fullerene composites provide promising
perspective for the development of efficient organic solar cells.
3. Dendrimer supramolecular complexes
Because the morphology and the photochemical function of
porphyrin dendrimers are similar to those of the light-harvesting
units in photosynthesis, porphyrin dendrimers have attracted
considerable interest as a unique way to assemble porphyrins,
harvesting solar light in artificial photosynthesis.80–83 Porphyrin
dendrimers can also be used to combine light-harvesting units
with reaction centre units. For example, a zinc porphyrin den-
drimer [D(ZnP)16]84 is combined with an electron acceptor, N-
methyl-2-(40-pyridyl)-3,4-fulleropyrrolidine (PyC60), which has
a pyridine binding site,85 to form a supramolecular complex as
shown in Fig. 12.86 The porphyrin dendrimers were synthesized
by coupling of the porphyrin activated ester [5-amino-2-
{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)}]-5-oxopentanoic
acid 2,5-dioxopyrrolidin-1-yl ester with the appropriate first,
second, or third generation polypropylenimine dendrimer.83
Photoexcitation of the Soret band of D(ZnP)16 at 438 nm in
PhCN results in fluorescence at lmax ¼ 609 and 645 nm.11b The
fluorescence was efficiently quenched by addition of PyC60 to
a PhCN solution of D(ZnP)16.11b The fluorescence intensity
decreases to reach a constant value with increasing concentration
of PyC60. The formation constant (K) of 1 : 1 ZnP monomer
unit–PyC60 complex was determined from the fluorescence
quenching to be 5.0 ꢂ 104 Mꢃ1, which is significantly larger than
the K value (1.6 ꢂ 104 Mꢃ1) determined from the absorption
spectral change.11b The larger K value determined from the
fluorescence quenching results from the excited energy migration
between the porphyrin units, because the fluorescence of the
unbounded ZnP moiety is quenched by PyC60 bound to
a different ZnP moiety via the energy transfer. In contrast to the
case of D(ZnP)16, a ZnP monomer cannot form the supramo-
lecular complex with PyC60 in a coordinating PhCN solution,
when no fluorescence quenching occurred by PyC60.11b This
indicates clearly that the dendrimer affects the binding and
excited energy migration.
Fig. 12 A supramolecular complex between a zinc porphyrin dendrimer
[D(ZnP)16] and PyC60
.
laser power intensities (Fig. 13b).11b Because intermolecular back
electron transfer would obey second-order kinetics, the first-
order decay of the CS state results from back electron transfer in
the supramolecular complex. The lifetime of the CS state is
determined as 0.25 ms at 298 K.11b
Organic photovoltaic cells were constructed using clusters of
supramolecular complexes of porphyrin dendrimers with
fullerene exhibiting remarkable enhancement in the photo-
electrochemical performance as well as broader photoresponse in
the visible and near-infrared regions.84,86 This clearly indicates
that the p–p interaction between porphyrins and fullerenes in the
supramolecular clusters plays an important role in improving the
light energy conversion efficiency.
4. Porphyrin oligopeptide supramolecular complexes
In contrast to porphyrin dendrimers (Fig. 12) in which the
geometries of porphyrins are rather fixed, porphyrin
The occurrence of photoinduced charge separation (CS) in the
supramolecular complex was confirmed by nanosecond laser
flash photolysis measurements of the D(ZnP)16–PyC60 complex
in PhCN as shown in Fig. 13.11b The absorption band due to
PyC60 ꢃ is clearly observed at 1000 nm together with that due to
,
ZnP + at 630 nm after laser excitation at 561 nm where only the
,
ZnP moiety is excited (Fig. 13a).11b This indicates that the CS
state of the supramolecular complex is formed via photoinduced
electron transfer from the singlet excited state of the ZnP moiety
to the PyC60 moiety. The quantum yield (F) of the CS state of the
D(ZnP)16–PyC60 complex is determined to be 25%.11b The decay
Fig. 13 (a) Transient absorption spectra of D(ZnP)16 (2.3 ꢂ 10ꢃ5 M) in
the presence of PyC60 (5.2 ꢂ 10ꢃ5 M) in deaerated PhCN at 298 K taken
at 20 ms (solid line with black circles) and 88 ms (solid line with white
circles) after laser excitation at 561 nm, respectively. (b) Time profiles of
of the absorption band at 1000 nm due to PyC60 ꢃ obeyed clean
,
the absorption at 1000 nm of PyC60 ꢃ with different laser intensities (3.0,
,
first-order kinetics with the same slope irrespective of different
1.0, and 0.5 mJ/pulse) at 298 K. Inset: First-order plots.
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oligopeptides provide more flexible structures to assemble
porphyrins.87,88 Thus, zinc porphyrinic oligopeptides with
various numbers of porphyrin units [P(ZnP)n; n ¼ 2, 4, 8]87–89 are
used as light-harvesting multiporphyrin units, which are bound
to electron acceptors of fulleropyrrolidine bearing a pyridine
(PyC60)85,90 or imidazole coordinating ligand (ImC60)91 as shown
in Fig. 14.92
at 428 nm in the circular dichroism (CD) spectra of P(ZnP)16
by the formation of the supramolecular complex with
C60, which indicates the a-helix structure was changed to
accommodate C60 between the porphyrin rings by p–p
interaction.93
As in the case of the D(ZnP)16–PyC60 supramolecular complex
in Fig. 13a, the occurrence of photoinduced electron transfer in
the supramolecular complex in PhCN was confirmed by nano-
second laser flash photolysis measurements of the supramolec-
ular complex of P(ZnP)8 with PyC60, which revealed the
formation of the CS state, exhibiting the transient absorption
The fluorescence of P(ZnP)8 is strongly quenched by the
intrasupramolecular electron transfer from the singlet excited
state of the ZnP moiety (1ZnP*) in P(ZnP)8 to the bound
PyC60.92 The formation constant (K) of the supramolecular
complex of P(ZnP)8 with PyC60 was determined from the
fluorescence quenching to be 1.5 ꢂ 105 Mꢃ1, which is signifi-
cantly larger than the K value (2.4 ꢂ 104 Mꢃ1) determined
from the absorption spectral change.92 As in the case of the
supramolecular complex of zinc porphyrin dendrimers with
PyC60 (vide supra), the larger K value determined from the
fluorescence quenching results from the excited energy migra-
tion between the porphyrin units in P(ZnP)8. It should be
noted that the K value determined from the fluorescence
quenching of P(ZnP)8 by PyC60 (1.5 ꢂ 105 Mꢃ1) is significantly
larger than the corresponding value of D(ZnP)16 (5.0 ꢂ
104 Mꢃ1) but that the K values determined from the absorption
spectral change are similar between P(ZnP)8 and D(ZnP)16.92
This suggests that the flexible structure of the oligopeptide
chain enables more efficient energy migration as compared
with the rigid structure of the dendrimer. The flexibility of the
oligopeptide chain was clearly observed as the disappearance
of the Cotton effect originating from the porphyrin Soret band
ꢃ
,
+
,
band due to PyC60 at 1000 nm together with that due to ZnP
at 630 nm.92 The CS state decayed obeyed clean first-order
kinetics and the first-order plots at different initial CS concen-
trations afforded linear correlations with the same slope. If there
is any contribution of intermolecular back electron transfer from
ꢃ
,
+
,
unbound PyC60 to ZnP , the second-order kinetics would be
involved for the decay time profile. Thus, the decay of the CS
state results from back electron transfer in the supramolecular
complex rather than intermolecular back electron transfer from
ꢃ
+ 92
,
.
,
PyC60 to ZnP
The CS lifetimes of the supramolecular complexes of other
porphyrins [P(ZnP)n: n ¼ 2, 4] and fullerene derivative (ImC60)
become longer with increasing generation of porphyrinic oligo-
peptides.92 This may also result from efficient hole migration
between the porphyrin units following photoinduced electron
transfer in the supramolecular complex. The longest CS lifetime
was attained as 0.84 ms at 298 K for the P(ZnP)8–ImC60
supramolecular complex.92 Such a long-lived CS state was also
confirmed by the EPR measurements under photoirradiation of
the supramolecular complexes of P(ZnP)8 with PyC60 and ImC60
in frozen PhCN at 173 K. Under photoirradiation, the isotropic
EPR signals corresponding to the zinc porphyrin radical cation
and fullerene radical anion were observed at g ¼ 2.002 and 2.000,
respectively.92
The clusters of supramolecular complexes of porphyrin-
peptide oligomers with fullerenes can be assembled on nano-
structured SnO2 electrodes by an electrophoretic deposition
method to construct purely organic supramolecular solar
cells.93 There were significant effects of the number of
porphyrins in a poly-peptide unit [P(H2P)n and P(ZnP)n (n ¼ 1,
2, 4, 8, 16)] and of the types of porphyrins (H2P vs. ZnP) and
fullerenes (C60 vs. C60 derivatives) on the structures, spectro-
scopic, and photoelectrochemical properties of the porphyrin–
C60 composite electrodes.93 The best performance was obtained
for the (P(H2P)16 + C60)m system exhibiting a fill factor (FF) of
0.47, an open circuit voltage (Voc) of 320 mV, a short circuit
current density (Isc) of 0.36 mA cmꢃ2, and the overall power
conversion efficiency (h) of 1.6% at input power (Win) of 3.4
mW cmꢃ2, which is 40 times higher than the value (0.043%) of
the porphyrin monomer (P(H2P)1 + C60)m modified electro-
de.93b The broad photocurrent action spectra (with photo-
response extending up to 1000 nm) show the ability of these
composites to harvest photons in the visible and NIR. Thus,
a supramolecular approach between porphyrins and fullerenes
with flexible polypeptide structures seems to be promising,
making it possible to further improve the light energy conver-
sion effciency by using a much larger number of porphyrins in
a polypeptide unit.
Fig. 14 Supramolecular complex composed of the porphyrin–peptide
octamer [P(ZnP)8, Ar ¼ 3,5-(tert-Bu)2C6H3] and PyC60 or ImC60
.
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5. Porphyrin assembly on size-controlled cup-shaped
nanocarbons
In addition to fullerenes described above, single-walled carbon
nanotubes (SWNTs)94 also exhibit excellent chemical and phys-
ical properties that have been revealed by various potential
applications.95–99 Thus, extensive efforts have so far been devoted
to assemble electron donor and acceptor molecules on
SWNTs.7,100–106 However, the fine-control of size (i.e., length) of
SWNTs remains a formidable challenge, because SWNTs have
seamless cylinder structures made up of a hexagonal carbon
network, which leads to the difficulty of solubilisation/func-
tionalization without treatment of strong acid or vigorous soni-
cation.107,108 On the other hand, the cup-stacked carbon
nanotubes (CSCNTs) that consist of cup-shaped nanocarbon
units, which stack via van der Waals attractions, have merited
special attention from the viewpoint of conventional carbon
nanotube alternatives.109–112 The tube-tube van der Waals energy
between cup-shaped nanocarbons has been counterbalanced by
the thermal or photoinduced electron transfer multi-electron
reduction due to electrostatic repulsion, resulting in highly
dispersible cup-shaped nanocarbons with size homogeneity.113,114
The cup-shaped nanocarbons (CNC) with controlled size have
been functionalized with a large number of porphyrin mole-
cules.115 The general procedure for the synthesis of the
porphyrin-functionalized cup-shaped nanocarbons [CNC–
(H2P)n] is shown in Fig. 15a.115 The cup-shaped nanocarbons are
first functionalized with aniline as the precursor for the further
functionalization with porphyrins. The aniline-functionalized
nanocarbons react with the porphyrin derivatives to construct
the nanohybrids.
Fig. 15 (a) Synthetic procedure of CNC–(H2P)n. (b) TEM image of
CNC–(H2P)n.
CNC–(H2P)n and an efficient fluorescence quenching of
porphyrins in CNC–(H2P)n as compared to the ref-H2P may
result from the photoinduced electron transfer from the singlet
excited state of H2P (1H2P*) to CNC in CNC–(H2P)n. The
occurrence of the photoinduced electron transfer to afford the
charge-separated (CS) state of CNC–(H2P)n was confirmed by
The structure of the cup-shaped nanocarbons of the CNC–
(H2P)n nanohybrids is shown by the TEM in Fig. 15b, which
reveals cup-shaped nanocarbons with a hollow core along the
length of the nanocup with well-controlled diameter (ca. 50 nm)
and size (ca. 100 nm).114 The weight % of porphyrins attached to
the cup-shaped nanocarbons was determined by thermogravi-
metric analysis (TGA) and elemental analysis to be ca. 20%.114
This corresponds to one functional group per 640 carbon atoms
of the nanocup framework for the CNC–(H2P)n nanohybrid.
Thus, the p-framework of the CNC is not destroyed despite
attachment of a large number of porphyrin molecules on
the CNC.
Spectroscopic evidence for the covalent functionalization of
the CNC–(H2P)n nanohybrid was obtained by an intensity
increase of the Raman signal at 1353 cmꢃ1 (D band) in the
functionalized CNC as compared with the pristine CSCNTs,115
because the D band has been used for monitoring the process of
functionalization which transforms sp2 to sp3 sites.116 The UV-vis
absorption spectrum of the CNC–(H2P)n nanohybrid agreed
with that of the superposition of the reference porphyrin [tetra-
kis(N-octadecyl-4-aminocarboxyphenyl)porphyrin]
(ref-H2P)
and cup-shaped nanocarbons, indicating that there is no signif-
icant interaction between attached porphyrins and CSCNTs in
the ground states.115
Fig. 16 (a) Transient absorption spectra of (a) CNC–(H2P)n taken at
20 ms and 1.8 ms after laser excitation at 426 nm and (b) ref-H2P in
deaerated DMF at 298 K taken at 100 ms and 1.6 ms after laser excitation
at 426 nm. (c) Decay time profiles and (d) first-order plots at 470 nm with
different laser powers (5, 3, 2, and 1 mJ/pulse).
The fluorescence lifetime of CNC–(H2P)n was determined to
be 3.0 ꢀ 0.1 ns, which is much shorter than that of ref-H2P
(14.1 ꢀ 0.1 ns).115 The fluorescence emission at 650 nm was also
quenched in CNC–(H2P)n.115 The short fluorescence lifetime of
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nanosecond laser flash photolysis measurements in Fig. 16,
Porphyrin-functionalized cup-shaped nanocarbons CNC–
(H2P)n have also been assembled onto nanostructured SnO2 films
using an electrophoretic deposition method to examine the
photoelectrochemical properties.118 The resulting nanohybrid
film afforded the drastic enhancement in the photo-
electrochemical performance as well as the broader photo-
response in the visible region as compared with the reference
CNC system without porphyrins.119 The enhancement of
photocurrent generation is caused by the efficient electron
injection from the long-lived charge-separated state of CNC–
(H2P)n upon photoexcitation (vide supra).
where the absorption bands in the visible and NIR region are
+
,
attributed to H2P , which are clearly different from the triplet–
triplet absorption of ref-H2P.115 The formation of the CS state
was also confirmed by EPR measurements under photo-
irradiation of CNC–(H2P)n in frozen DMF at 153 K. The
observed isotropic EPR signal at g ¼ 2.0044 agrees with that of
+
,
ref-H2P
produced by the one-electron oxidation with
[Ru(bpy)3]3+ (bpy ¼ 2,20-bipyridine) in deaerated CHCl3.115 The
EPR signal corresponding to the reduced carbon-based nano-
materials was too broad to be detected probably due to delo-
calization of electrons in CNC.115
The CS state of CNC–(H2P)n detected in Fig. 16a decays
obeying clean first-order kinetics: the first-order plots at different
initial CS concentrations afford linear correlations with the same
slope (Fig. 16b).115 Thus, the decay of the CS state results from
back electron transfer in the nanohybrid rather than intermo-
6. Conclusions
In contrast to natural photosynthesis, where only a single reac-
tion centre is combined with light-harvesting units, multiple
reaction centres can be combined with light-harvesting units by
utilizing metal nanoparticles, dendrimers, oligopeptides and
nanocarbon materials, in which charge-separation molecules are
assembled in artificial systems as described above. The
construction of composites of multiple photosynthetic reaction
centres and light-harvesting units described in this article will
hopefully open a new strategy to develop efficient light-to-energy
conversion systems including organic solar cells and artificial
photosynthesis for production of solar fuels.
ꢃ
+
,
,
lecular back electron transfer from CNC to H2P . The CS
lifetime was determined from the first-order plots in Fig. 16b to
be 0.64 ꢀ 0.01 ms, which is the longest lifetime ever reported for
electron donor-attached nanocarbon materials.115 Such a long
CS lifetime may be ascribed to the efficient electron migration in
the cup-shaped nanocarbons following the charge separation.
The reduction potential (Ered) of the CNC moiety was esti-
mated by redox titration to be ꢃ0.15 V vs. SCE,115 which is about
the same as the reported value of the one-electron reduction
potential of CNT.117 In the presence of TCNQ (Ered ¼ 0.19 V vs.
SCE), which acts as an electron acceptor, photoinduced electron
transfer from H2P to CNC in CNC–(H2P)n was followed by
Acknowledgements
The authors gratefully acknowledge the contributions of their
collaborators and coworkers mentioned in the cited references,
and support by a Grant-in-Aid (Nos. 20108010 and 237500141),
a Global COE program, ‘‘the Global Education and Research
Center for Bio-Environmental Chemistry’’ from the Ministry of
Education, Culture, Sports, Science and Technology, Japan and
KOSEF/MEST through WCU project (R31-2008-000-10010-0),
Korea.
ꢃ
,
subsequent electron transfer from CNC of the CS state to
ꢃ
,
TCNQ to produce TCNQ (lmax ¼ 846 nm), whereas the
absorption bands due to H2P + at 480 nm and the near-infrared
,
region remain the same as shown in Fig. 17.115 This serves as
crucial testimony for the existence of the photolytically generated
ꢃ
ꢃ
,
+
,
,
CNC (i.e., CNC ꢃ(H2P)n ).
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