Hetero Face-to-Face Porphyrin Array with Cooperative Effects of Coordination and Host–Guest Complexation

Article abstract of DOI:10.1002/asia.201700738

We successfully synthesized a hetero face-to-face porphyrin array composed of ZnTPP and RuTPP(DABCO)₂ (TPP: 5, 10, 15, 20-tetraphenylporphyrin, DABCO: 1,4-diazabi-cyclo[2.2.2]octane) in 2:1 molar ratio. A cyclic Zn porphyrin dimer (ZnCP) was also used as the host molecule for the Ru porphyrin. In the latter, the Ru-DABCO bonding in RuTPP(DABCO)₂ was stabilized by the host-guest complexation. Reaction progress kinetic analysis of the ligand substitution reaction of RuTPP(DABCO)₂ and that in ZnCP revealed the stabilization mechanism of the Ru-DABCO bonding. Photoinduced electron transfer (PET) from the Zn porphyrin to the Ru porphyrin was observed in the porphyrin array. The host-guest stabilization of unstable complex for construction of a donor—acceptor–donor structure is expected to be a new method for an artificial photosynthesis.

Full text of DOI:10.1002/asia.201700738

CHEMISTRY
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Accepted Article
Title: Hetero Face-to-Face Porphyrin Array with the Cooperative
Effects of Coordination and Host-Guest Complexation
Authors: Yusuke Chiba, Maning Liu, Yasuhiro Tachibana, Tetsuaki
Fujihara, Yasushi Tsuji, and Jun Terao
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Hetero Face-to-Face Porphyrin Array with the Cooperative Effects
of Coordination and Host-Guest Complexation
Yusuke Chiba,[a] Maning Liu,[b] Yasuhiro Tachibana,*[b] Tetsuaki Fujihara,[a] Yasushi Tsuji,[a] Jun
Terao*[c]
Abstract: We successfully synthesized
porphyrin array composed of ZnTPP and RuTPP(DABCO)2 (TPP:
5,10,15,20-tetraphenylporphyrin, DABCO: 1,4-diazabi-
a
hetero face-to-face
metals (M and M’ in Figure 1). A stable hetero face-to-face
porphyrin array was also constructed using unsymmetrical
bidentate ligands, in order to create different binding enthalpies
with different metals.4 However, the pyridyl group in Ru- or Rh-
based porphyrins has low nucleophilicity, leading to weak metal-
ligand interaction. Zn porphyrin, being a promising unit for artificial
photosynthesis,5 has been used for the hetero porphyrin array in
only one case.4b However, the complex’s dissociation equilibrium
was observed at room temperature, causing ambiguous structure
determination. Therefore, it is important to develop a synthetic
strategy for preparing porphyrin arrays with versatile metals.
In this study, we report separate synthetic methods to prepare
two hetero face-to-face porphyrin arrays, and the products’ optical
properties. The complex of Ru porphyrin-bis-1,4-diazabi-
cyclo[2.2.2]octane (DABCO) was used as a unit in the two arrays.
RuTPP(DABCO)2 (TPP: 5,10,15,20-tetraphenyl) was synthesized
for the first time, and used for its ability to form six-coordinated
complex with DABCO, which has a higher affinity for Zn porphyrin
than pyridine.6 The two hetero face-to-face porphyrin arrays were
prepared from Zn and Ru porphyrins, with or without applied host-
guest chemistry (Figure 1b). A comparison of the two porphyrin
arrays revealed the stabilization of the weak Ru-DABCO bonding
in RuTPP(DABCO)2 by host-guest complexation.
cyclo[2.2.2]octane) in 2:1 molar ratio. A cyclic Zn porphyrin dimer
(ZnCP) was also used as the host molecule for the Ru porphyrin. In
the latter, the Ru-DABCO bonding in RuTPP(DABCO)2 was stabilized
by the host-guest complexation. Reaction progress kinetic analysis of
the ligand substitution reaction of RuTPP(DABCO)2 and that in ZnCP
revealed the stabilization mechanism of the Ru-DABCO bonding.
Photoinduced electron transfer (PET) from the Zn porphyrin to the Ru
porphyrin was observed in the porphyrin array. The host-guest
stabilization of unstable complex for construction of a donor-accepter-
donor structure is expected to be a new method for an artificial
photosynthesis.
The imitation of photosynthesis, which efficiently converts
sunlight into chemical energy, can provide important solutions to
the energy crisis.1 The detailed structures of photosynthetic
systems in nature revealed that their efficient use of light energy
is related to the sequential energy transfer among the light-
harvesting complex systems (LHCs).2 First, the LHC2 complex,
which consists of a 3D array of multi-porphyrins, absorbs
sunlight.3 The energy is quickly transferred to the LHC1 complex
and then to the reaction center. This directional energy flow is
based on the precise positional relationship among the LHCs and
the reaction center, which have different absorbance spectra.
One of the motifs in artificial photosynthesis is orientation-
controlled hetero porphyrin arrays composed of different
metalloporphyrins. Due to its directionality coordination bonding
is an easy and useful tool to position molecules regularly. Using
this tool, scientists have prepared a hetero face-to-face porphyrin
array with three porphyrins and two bidentate ligands (Figure 1a).4
The porphyrins in this array are parallel, as observed in natural
photosynthetic systems.2 In this case, the ligands should have two
abilities: (1) form a stable complex and (2) recognize different
.
[a]
Y. Chiba, Prof. Dr. T. Fujihara, Prof. Dr.Y. Tsuji
Department of Energy and Hydrocarbon Chemistry
Graduate School of Engineering, Kyoto University
Kyoto 615-8510 (Japan)
[b]
[c]
M Liu, Prof. Dr.Y. Tachibana
School of Engineering, RMIT University
Bundoora VIC 3083 (Australia)
Email: yasuhiro.tachibana@rmit.edu.au
Prof. Dr. J. Terao
Figure 1. Structures of hetero face-to-face porphyrin arrays.
Department of Basic Science, Graduate School of Arts and
Sciences, The University of Tokyo
Tokyo 153-8902 (Japan)
E-mail: cterao@mail.ecc.u-tokyo.ac.jp
Supporting information for this article is given via a link at the end of
the document
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Scheme 1. Preparation of a face-to-face porphyrin array from ZnTPP and
Scheme 2. Construction of a face-to-face porphyrin array from ZnCP and
RuTPP(DABCO)2.
RuTPP(DABCO)2.
C
H
Cl3
complexed
DABCO-Ha , Hb
b-H
(ZnCP)
PPM
-6
9
8
7
6
3
2
-4
-5
Figure 3. 1H NMR spectrum of ZnCP⊃RuTPP(DABCO)2 in CDCl3 at 298 K.
Figure 2. 1H NMR spectra of ZnTPP and RuTPP(DABCO)2 mixed in 2:1 molar
ratio after a) 20 min, b) 24 h, and c) 40 h in CDCl3 at 298 K.
RuTPP(DABCO)2 was synthesized as follows. RuTPP(CO)(py)
was prepared by a method modified from previous literature,7 and
stirred with excess DABCO in toluene under a high-pressure Hg
lamp at room temperature for 130 min. Evacuating the solvent and
excess DABCO in vacuum afforded RuTPP(DABCO)2. The
complex formation was supported by electrospray ionization
mass spectrometry (ESI-MS) and 1H NMR spectrum (included in
the SI). Two sets of NMR triplets are located at around -2.0 and -
5.4 ppm upfield from the methylene peak of free DABCO,
because of different ring-current effects of the porphyrin.
Figure 4. Conversion rates of ZnCP⊃RuTPP(DABCO)2 and RuTPP(DABCO)2
at 90 °C in toluene-d8 based on 1H NMR spectra.
The prepared RuTPP(DABCO)2 and ZnTPP were mixed in 1:2
molar ratio (Scheme 1). The formation of a hetero face-to-face
porphyrin array was indicated by the H NMR spectrum (Figure
In order to suppress the dissociation of DABCO from
RuTPP(DABCO)2, another face-to-face porphyrin was prepared
through host-guest complexation. Cyclic and cage-type porphyrin
dimers are well-known motifs in studying the mechanism of
aritificial photosynthetic devices, because the host molecules
encapsulate different fullerenes as promising accepter
molecules.9 We had previously reported cyclic porphyrin dimers
that can encapsulate molecules by various interactions, such as
coordination, bonding, - interaction, and hydrophilic-
hydrophobic interaction.10 However, there has been no report of
porphyrin dimers being used in a porphyrin array. In this study, a
cyclic Zn porphyrin dimer (ZnCP) was synthesized by a previously
reported method11 as the host molecule containing Zn porphyrin,
which can form a complex with amines (Scheme 2). The target
complex, ZnCP⊃RuTPP(DABCO)2, was selectively prepared by
mixing ZnCP and RuTPP(DABCO)2 in CHCl3 at room temperature,
1
2a). The two protons of DABCO in RuTPP(DABCO)2 were shifted
1
to a high field (from 0.85 and -2.52 ppm to -4.4 ppm). In the H
NMR spectrum, the protons of DABCO located between two
porphyrins are observed around -5 ppm.8 The broad singlet peak
observed at around -4.4 ppm corresponds to the chemical shift of
methylene protons of DABCO placed between two porphyrins.
However, this porphyrin array decomposed at room temperature,
since the NMR peaks gradually shifted to lower field due to the
smaller ring-current effect. The decomposition is attributed to the
instability of RuTPP(DABCO)2, whose 1H NMR spectra over time
are shown in Figure S1. The integral of free DABCO gradually
increased at room temperature, while that of RuTPP(DABCO)2
decreased. A comparison between the conversion rate of
RuTPP(DABCO)2 and the generation rate of free DABCO based
on the initial RuTPP(DABCO)2 concentration revealed that only
one DABCO dissociated from RuTPP(DABCO)2 with time
(Scheme S1 and Figure S2). Therefore, the Ru-DABCO bond is
thermodynamically unstable, thereby degrading the face-to-face
porphyrin array.
followed by purification by
a preparative gel permeation
chromatography (GPC). In the 1H NMR spectrum of the product,
the singlet peak at around -4.9 ppm again corresponds to the
chemical shift of methylene protons of DABCO placed between
two porphyrins (Figure 3).8 Two kinds of protons (Ha, Hb) of
DABCO in ZnCP⊃RuTPP(DABCO)2 underwent the ring current
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10.1002/asia.201700738
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effect to the same extent from Zn and Ru porphyrin, which was
the reason why only the singlet peak of the methlene protons was
observed. The 1H NMR and matrix-assisted laser
desorption/ionization time of flight (MALDI-TOF) MS spectra also
indicate that the ratio of ZnCP: RuTPP(DABCO)2 was 1:1,
suggesting the formation of ZnCP ⊃ RuTPP(DABCO)2. When
their 1H NMR spectra in CDCl3 at 55 °C are compared (Figure 4),
ZnCP ⊃ RuTPP(DABCO)2 was stable while RuTPP(DABCO)2
quickly decomposed. Therefore, RuTPP(DABCO)2 was stabilized
by host-guest complexation with ZnCP.
We further investigated the optical property of ZnCP ⊃
RuTPP(DABCO)2. According to the UV spectra (Figure 6a), in
RuTPP(DABCO)2 the Zn and Ru porphyrins did not interact with
each other in the ground and exciting states, because the
spectrum was a superposition of those of ZnCP-DABCO mix and
RuTPP(DABCO)2. ZnCP emits light at 610 nm based on a -*
transition (ex = 430 nm, Figure 6b). On the other hand,
RuTPP(DABCO)2 did not emit light, probably due to non-radiative
deactivation on the Ru metal (ex = 408 nm). When excited at 430
nm (mainly exciting Zn porphyrin), the intense fluorescence from
ZnCP⊃RuTPP(DABCO)2 at its maximum emission wavelength
was 85% lower than that from the ZnCP-DABCO mix. It was
expected that the quenched emission for ZnCP-DABCO mix was
caused by energy or electron transfer from the Zn porphyrins to
Ru porphyrin.
We further evaluated the stabilization mechanism of
RuTPP(DABCO)2 in ZnCP⊃RuTPP(DABCO)2 based on ligand
substitution reaction kinetics. Tert-butyl isocyanide (tBuNC) was
used as the substituting ligand, because isocyanide has a low
nucleophilicity and high ability to form complex with transition
metals. Hence, the DABCO in RuTPP(DABCO)2 can be replaced
with isocyanide to maintain Zn-DABCO bonds. After heating at
35 °C with excess tBuNC, RuTPP(DABCO)2 was gradually
1
converted to RuTPP(tBuNC)2 according to the H NMR spectra
over time (Figure S3). The mono-substituted complex was not
observed in this case, due to the strong trans effect of isocyanide
that promotes the dissociation of DABCO. The substitution
reaction was expected to occur through the seven-coordinate
Ru,11 because the conversion rate of RuTPP(DABCO)2 depended
on the equivalent of tBuNC (Figure S4). An excess amount of
tBuNC to the Ru porphyrin was used in the kinetic analyses. In
the case of ZnCP⊃RuTPP(DABCO)2, both RuTPP(tBuNC)2 and
RuTPP(DABCO)2 were produced at higher temperatures (Figure
S5). The measured rate constants at different temperatures lead
to the activation energy (Ea) and frequency factor (A), whose
values are shown in Table 1.
A comparison between
RuTPP(DABCO)2 and ZnCP⊃RuTPP(DABCO)2 revealed that
the dramatic reduction in A is key to the stabilization of Ru-
DABCO bonding in the latter, which can be rationalized as follows.
The sliding mode of DABCO in RuTPP(DABCO)2 (Figure 5a,
horizontal gray arrow) is the key step for the isocyanide to
associate with Ru in the substitution reaction. In the case of ZnCP
⊃RuTPP(DABCO)2, however, the cyclic porphyrin dimer inhibits
the formation of the transition state, by reducing the degrees of
freedom of DABCO (Figure 5b, horizontal gray arrow). Hence, the
Ru-DABCO bonding in ZnCP⊃RuTPP(DABCO)2 is stabilized by
the kinetic effect even at high temperature (Figure 4).
Figure 6. (a) Absorption and (b) emission spectra of ZnCP⊃RuTPP(DABCO)2
(orange lines), ZnCP mixed with 4 equivalents of DABCO (gray lines, excited at
430 nm), and RuTPP(DABCO)2 (gray dotted lines, excited at 408 nm), at
4.0×10-3 mM in CHCl3.
Table 1. Comparison of activation energy (Ea) and frequency factor (A) of
substitution reactions by isocyanide for RuTPP(DABCO)2 and ZnCP ⊃
RuTPP(DABCO)2.
Unit
Ea [kJ mol-1]
A×1016 [M-1 h-1]
Finally, we examined the detailed photoreaction dynamics of
ZnCP⊃RuTPP(DABCO)2. The transient absorption spectrum of
ZnCP at 1 ns after the excitation showed a negative value at 660
nm (Figure 7a), originated from the tail of the ZnCP singlet excited
state decay (in agreement with the emission shown in Figure
6b),12 while both spectra observed at 1 and 5 ns after the
excitation indicate two absorptions at 760 and 850 nm,
corresponding to the triplet excited state of Zn porphyrin (ex = 540
nm, Figure 7a).13 In the case of ZnCP⊃RuTPP(DABCO)2, a
RuTPP(DABCO)2
1.5
1.3
5.6×104
4.1
ZnCP⊃RuTPP (DABCO)2
single broad peak was observed at 720~850 nm from 1 ns (ex
=
550 nm to excite mainly the Zn porphyrin, Figure 7b). Photo
electrochemical measurement14 proved that the Zn porphyrin
formed a one-electron oxidation state at +1.37 V (Figures
Figure 5. Sliding modes of DABCO on Ru porphyrin to reach the transition state
(‡) of the ligand substitution reaction: a) without and b) with ZnCP. The arrows
mean the sliding movements of DABCO coordinated to porphyrin.
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S9~S12). Accordingly, the broad peak (600~850 nm) in the
transient absorption spectrum of ZnCP ⊃ RuTPP(DABCO)2
(Figure S13) is attributed to the one-electron oxidation state of
ZnCP. Therefore, a radical cation of Zn porphyrin is formed after
the photoexcitation of Zn porphyrin in ZnCP⊃RuTPP(DABCO)2.
The reason for the Zn porphyrin quenching in ZnCP ⊃
RuTPP(DABCO)2 can be explained as follows. At first, after being
excited at 550 nm, the electron is transferred from Zn porphyrin to
Ru porphyrin within 1 ns, followed by non-radiative deactivation.
The electron transfer reaction is not intermolecular but
intramolecular, because the measurement was conducted at
sufficiently diluted concentration (~40 M), so that intermolecular
reaction was negligible. ZnCP⊃RuTPP(DABCO)2 has a D-A-D
structure constructed through coordination bonding, in which Zn
porphyrin is the donor molecule (D) while Ru porphyrin is the
accepter molecule (A).
Acknowledgements
This research was supported by the Funding Program for
JSPSResearch Fellow and Grant–in–Aid for Scientific Research (B)
and
Scientific
Research
on
Innovative
Areas
(“MolecularArchitectonics” and “Soft Molecular Systems”) from
MEXT,Japan. This research was also supported by Tokuyama
ScienceFoundation and CREST, JST.
Keywords: Artificial photosynthesis • porphyrin • host-guest •
photoinduced electron transfer • coordination bonding
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Figure 7. Transient absorption spectra of a) ZnCP (excited at 540 nm) and b)
ZnCP⊃RuTPP(DABCO)2 (excited at 550 nm) with time delays of 1 and 5 ns in
CHCl3.
In conclusion, we successfully synthesized two face-to-face
type porphyrin arrays with Zn porphyrin, one of the promising units
for artificial photosynthesis, using RuTPP(DABCO)2. The
unstable Ru-DABCO bonding in RuTPP(DABCO)2 was stabilized
by a Zn cyclic porphyrin dimer through host-guest complexation.
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Entry for the Table of Contents
Layout 1:
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Text for Table of Contents
Yusuke Chiba, Maning Liu, Yasuhiro
Tachibana,* Tetsuaki Fujihara, Yasushi
Tsuji, Jun Terao*
Page No. – Page No.
Hetero Face-to-Face Porphyrin Array
with the Cooperative Effects of
Coordination and Host-Guest
Complexation
Kinetically stable
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