Coronenetetraimide-Centered Cruciform Pentamers Containing Multiporphyrin Units: Synthesis and Sequential Photoinduced Energy- and Electron-Transfer Dynamics

Article abstract of DOI:10.1002/chem.201500766

A series of coronenetetraimide (CorTIm)-centered cruciform pentamers containing multiporphyrin units, in which four porphyrin units are covalently linked to a CorTIm core through benzyl linkages, were designed and synthesized to investigate their structur

Full text of DOI:10.1002/chem.201500766

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DOI: 10.1002/chem.201500766
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&
Energy Transfer
Coronenetetraimide-Centered Cruciform Pentamers Containing
Multiporphyrin Units: Synthesis and Sequential Photoinduced
Energy- and Electron-Transfer Dynamics
Taku Hasobe,*[a] Koichi Ida,[a] Hayato Sakai,[a] Kei Ohkubo,[b, c] and Shunichi Fukuzumi*[b, c, d]
Abstract: A series of coronenetetraimide (CorTIm)-centered
cruciform pentamers containing multiporphyrin units, in
which four porphyrin units are covalently linked to a CorTIm
core through benzyl linkages, were designed and synthe-
sized to investigate their structural, spectroscopic, and elec-
trochemical properties as well as photoinduced electron-
and energy-transfer dynamics. These systems afforded the
first synthetic case of coroneneimide derivatives covalently
linked with dye molecules. The steady-state absorption and
electrochemical results indicate that a CorTIm and four por-
phyrin units were successfully characterized by the corre-
sponding reference monomers. In contrast, the steady-state
fluorescence measurements demonstrated that strong fluo-
rescence quenching relative to the corresponding monomer
units was observed in these pentamers. Nanosecond laser
flash photolysis measurements revealed the occurrence of
intermolecular electron transfer from triplet excited state of
zinc porphyrins to CorTIm. Femtosecond laser-induced tran-
sient absorption measurements for excitation of the CorTIm
unit clearly demonstrate the sequential photoinduced
energy and electron transfer between CorTIm and porphyr-
ins, that is, occurrence of the initial energy transfer from
CorTIm (energy donor) to porphyrins (energy acceptor) and
subsequent electron transfer from porphyrins (electron
donor) to CorTIm (electron acceptor) in these pentamers,
whereas only the electron-transfer process from porphyrins
to CorTIm was observed when we mainly excite porphyrin
units. Finally, construction of high-order supramolecular pat-
terning of these pentamers was performed by utilizing self-
assembly and physical dewetting during the evaporation of
solvent.
Introduction
only as models for the study of energy and electron transfer in
organized systems,[2] but also as functional materials for device
applications such as light-emitting diodes (LED),[3] photovoltaic
cells,[2l,4] and field-effect transistors (FET).[5]
Disc-shaped molecules such as polycyclic aromatic hydrocar-
bons (PAHs) and their derivatives are finding increasing interest
among other self-assembled molecular systems because of
their tendency to form extended p-stacked assemblies.[1] Be-
cause of many potential properties such as a high electron
density, these molecules are of fundamental importance not
In contrast to the comparatively large collection of available
disk-shaped electron-donating molecules, disk-shaped elec-
tron-accepting molecular systems are still considerably under-
developed. Fullerenes possess three-dimensional and spherical
p-systems,[6] whereas the representative two-dimensional mol-
ecules are mainly perylenediimides (PDIs). PDIs exhibit reversi-
ble and successive first and second one-electron reduction.
Therefore, PDIs are widely utilized as good electron acceptors
in reaction center models for artificial photosynthesis[2b–f,h,7]
and electronic devices.[5f,8]
[a] Prof. T. Hasobe, K. Ida, Dr. H. Sakai
Department of Chemistry
Faculty of Science and Technology, Keio University
3-14-1 Hiyoshi, Yokohama, Kanagawa 223-8522 (Japan)
E-mail: hasobe@chem.keio.ac.jp
[b] Prof. K. Ohkubo, Prof. S. Fukuzumi
On the other hand, a systematic and rational synthesis of
coroneneimide derivatives has been recently reported.[9] Ac-
tually, the chemical structures of coronenes increased a great
diversity of synthetic strategies for coroneneimide derivatives.
For example, in the case of coronenetetraimide (CorTIm), four
maleimides can be fused to a coronene skeleton, whereas PDIs
have only two maleimides. Consequently, the photophysical
and redox properties can be systematically controlled by the
type and number of substituents.[9d] In particular, CorTIm
shows four reversible one-electron reduction waves because
the LUMO is energetically low-lying and doubly degenerate.[9d]
This is in sharp contrast with the above-mentioned trend of
Department of Material and Life Science
Graduate School of Engineering, Osaka University
ALCA and SENTAN, Japan Science and Technology Agency (JST)
Suita, Osaka, 565-0871 (Japan)
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[c] Prof. K. Ohkubo, Prof. S. Fukuzumi
Department of Bioinspired Chemistry
Ewha Womans University, Seoul 120-750 (Korea)
[d] Prof. S. Fukuzumi
Faculty of Science and Engineering, ALCA and SENTAN
Japan Science and Technology Agency (JST)
Meijo University, Nagoya, Aichi 468-0073 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500766.
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PDIs, which accept only two electrons. However, the synthesis
of covalently linked donor–acceptor systems composed of cor-
oneneimide derivatives have yet to be explored because of
synthetic difficulty.
Porphyrins possess an extensively two-dimensional p-conju-
gation and basically act as electron donors[2h,6a,c,e,g,10] or energy
acceptors,[11] but not electron acceptors because of the low
one-electron reduction potentials (i.e., high energy level of
LUMO). Therefore, synthetic covalent systems from molecular
dyads and triads[2e,6b,12] to macromolecules such as oligo-
mers[11d,13] and dendrimers[10,14] have been extensively reported
to investigate sequential energy and electron transfer. More-
over, in a combination of coronenetetraimide (CorTIm) and
porphyrin units, an overlap beween energy donor emission
(CorTIm) and acceptor absorption (porphyrin) is expected to
occur in the spectral range from about 500 to 600 nm.[9d] In
contrast, considering the reported first one-electron reduction
potential of coronenetetraimides (À0.65 V vs. SCE)[9d] and oxi-
dation potential of zinc porphyrins (ꢀ0.8 V vs. SCE),[15] the
energy level of the charge-separated state composed of
CorTIm radical anion and porphyrin radical cation is approxi-
mately estimated to be ꢀ1.5 eV, which is smaller than the
single and triplet excited energies of these molecules. Thus,
a combination of these two units seems ideal for enhancing
the light-harvesting efficiency and the resulting energy and
electron transfer throughout the solar spectrum.
On the basis of the above consideration, we newly synthe-
sized new coronenetetraimide (energy donor and electron ac-
ceptor)-centered cruciform pentamers containing four porphyr-
ins (energy acceptor and electron donor) as shown in Figure 1.
In this study, we report the details of the synthesis as well as
spectroscopic and photophysical properties of these molecules
including the sequential energy and electron transfer.
Figure 1. Chemical structures of donor–acceptor cruciform pentamers
((H2P)4ÀCorTIm and (ZnP)4ÀCorTIm) and reference compounds employed in
this work.
trifluoroacetic acid (TFA) to obtain the free base porphyrin 4.
Then, porphyrin 5, which is a precursor of (H2P)4ÀCorTIm, was
obtained by replacing the phthalimide of porphyrin 4 with an
amino group under basic conditions.
Results and Discussion
Synthesis
With regard to the synthesis of coroneneoctacarboxylic acid,
which is a precursor of (H2P)4-CorTIm, first, the isobutoxycar-
bonyl groups of CorDIm(iBu)4 were replaced with a carboxylate
anion under basic conditions.[9a] To synthesize coroneneocta-
carboxylic acid, protonation of carboxylate anion was carried
out by using acetic acid, which was followed by dehydration
under vacuum. Because of accomplished preparation of each
precursor for the synthesis of (H2P)4ÀCorTIm, (H2P)4ÀCorTIm
was synthesized by dehydration condensation of each precur-
sor. The synthesized pentamer was characterized by the
1H NMR and 13C NMR spectroscopy and MALDI-TOF MS (see
the Experimental Section and Figure S1 in the Supporting In-
formation). The synthesis of (ZnP)4ÀCorTIm was finally ach-
ieved by the insertion of zinc into the center of the free base
porphyrin macrocycle.
To achieve synthesis of cruciform pentamer, (H2P)4-CorTIm,
a synthetic scheme of maleimidation of coroneneimide was
proposed on the basis of the maleimidation of coroneneocta-
carboxilic acid derived from CorDIm(iBu)4 and porphyrin 5
(Scheme 1). Here it should be noted that direct introduction of
4-aminomethyl-phenyl group onto a porphyrin ring from mon-
obromo-substituted porphyrin by Suzuki–Miyaura coupling did
not proceed. Therefore, we employed a synthetic strategy to
protect the NH2 unit as a phthalimide as shown in Scheme 1.
In particular, the coupling reaction between monobromo-sub-
stituted freebase porphyrin and aryl boronic acid did not pro-
ceed well. Therefore, we employed zinc porphyrin derivatives
in the initial step and then removed the zinc from the porphy-
rin center to perform the subsequent reactions.
On the basis of on the above considerations, first, bromina-
tion of porphyrin 1 was carried out with NBS at 08C for synthe-
sis of porphyrin 2.[16] Next, porphyrin 3 was synthesized by
Suzuki–Miyaura coupling of porphyrin 2 and aryl boronic
acid.[17] To remove the zinc unit, porphyrin 3 was treated with
Steady-state absorption spectra
Figure 2 displays the absorption spectra of (H2P)4ÀCorTIm (Fig-
ure 2A) and (ZnP)4ÀCorTIm (Figure 2B) in benzonitrile (PhCN).
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Figure 2. Steady-state absorption spectra of (A) (a) (H2P)4ÀCorTIm (c),
(b) H2P-NHAc (a), and (c) CorTIm (c) in PhCN; (B) (a) (ZnP)4ÀCorTIm
(c), (b) ZnPÀNHAc (a), and (c) CorTIm (c) in PhCN. The concentra-
tions were 1.0 mm in all systems. The inset of (A) shows the enlarged spec-
trum of CorTIm in the long wavelength region for clarification.
Steady-state fluorescence spectra
Scheme 1. Synthetic schemes of porphyrin 5, (H2P)4ÀCorTIm, and (ZnP)4À
CorTIm.
Steady-state fluorescence spectra of (H2P)4ÀCorTIm and refer-
ence molecules (i.e., H2PÀNHAc and CorTIm) with the excita-
tion at 585 and 380 nm are shown in Figure 3A and 3B, re-
spectively. These excitation wavelengths mainly correspond to
absorption maxima of Q-band in porphyrin unit (585 nm) and
CorTIm unit (380 nm) (Figure 2A). Then, we fixed the same ab-
sorbance for excitation wavelength at 585 or 380 nm to com-
pare the quenching efficiencies between (H2P)4-CorTIm and
H2P-NHAc. The fluorescence spectrum of H2P-NHAc with excita-
tion at 585 nm (spectrum a in Figure 3A) shows only two
peaks at 645 and 709 nm, which are assigned to the emission
By comparing the spectra of (H2P)4ÀCorTIm and (ZnP)4ÀCorTIm
with those of the corresponding reference monomer com-
pounds, we can identify the respective peaks of porphyrin and
CorTIm units. For example, in (H2P)4ÀCorTIm (spectrum a in
Figure 2A), a CorTIm unit has an absorption maximum at
380 nm, whereas an intense Soret band at 419 nm and four Q-
bands at 511, 544, 586, and 648 nm were observed in porphy-
rin units, which are derived from the typical spectral features
in free base porphyrin monomer (spectra b and c in Fig-
ure 2A). More importantly, the absorption maxima of (H2P)4À from the porphyrin unit. In contrast, the emission of (H2P)4À
CorTIm are the same as those of the respective reference com-
pounds (H2PÀNHAc and CorTIm) as shown in spectra b and
c in Figure 2A. As a consequence, we can conclude that
a CorTIm and four porphyrin units in (H2P)4-CorTIm were basi-
cally characterized by the corresponding monomers, although
the broadened feature was slightly observed in the Soret and
CorTIm was strongly quenched (>99%; see spectrum b in Fig-
ure 3A). These results suggest additional deactivation path-
ways for the excited state of the porphyrin units arising from
the interaction between CorTIm and porphyrin units in (H2P)4À
CorTIm. This may result from photoinduced electron transfer
from the singlet excited state of porphyrin to CorTIm (see
Q-bands. These trends were also similarly observed for (ZnP)4À below). A similar trend was also observed in (ZnP)4ÀCorTIm
CorTIm in Figure 2B, which exhibits an absorption maximum
of CorTIm unit at 381 nm, an intense Soret band at 425 nm,
and three Q-bands at 513, 550, and 594 nm.
(see Figure S2 in the Supporting Information).
The spectrum a in Figure 3B exhibits strong fluorescence
emission bands of CorTIm at 513, 550, and 594 nm because
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similar trend was also observed in the case of ZnPÀNHAc and
(ZnP)4ÀCorTIm (Figure S2 in the Supporting Information).
Electrochemical measurements
We next performed electrochemical measurements by using
cyclic voltammetry to determine the first one-electron reduc-
tion and oxidation potentials of these pentamers. In these
measurements, we employed CH2Cl2 as a solvent because of
the limited solubility. Figure 4 shows the cyclic voltammograms
of (ZnP)4ÀCorTIm and the reference compounds in CH2Cl2. The
summarized electrochemical data are shown in Table 1. (ZnP)4À
Figure 3. Steady-state fluorescence spectra of (A) (a) H2P-NHAc and
(b) 0.25 mm (H2P)4-CorTIm in PhCN: lex =585 nm and (B) (a) CorTIm and
(b) 1.0 mm (H2P)4-CorTIm in PhCN: lex =380 nm.
Figure 4. Cyclic voltammograms of (A) (ZnP)4-CorTIm; (B) CorTIm; and
(C) ZnP-NHAc in CH2Cl2 with 0.3m n-Bu4NPF6 as the supporting electrolyte.
Scan rate: 200 mVsÀ1
.
the absorption band of CorTIm unit is excited. The fluores-
cence of (H2P)4ÀCorTIm is significantly quenched compared
with that of CorTIm (spectrum b in Figure 3B). The possible
reason is occurrence of energy transfer from CorTIm to por-
phyrins because of large overlap between emission bands of
CorTIm and absorption bands (Q-bands) of the porphyrins at
around about 500–600 nm region (see above). However, the
emission bands derived from the porphyrins were not clear be-
cause photoinduced electron transfer may occur through the
singlet excited states of porphyrins after energy transfer from
CorTIm to porphyrins. To carefully check the photodynamics,
the excitation spectrum of (H2P)4ÀCorTIm was measured (Fig-
ure S3 in the Supporting Information). The observed wave-
length was 650 nm, which corresponds to the emission band
of porphyrins. Although the signal-to-noise ratio (S/N) is not
low because of the occurrence of efficient photoinduced elec-
tron transfer, the excitation spectrum roughly agrees with the
corresponding absorption spectrum of (H2P)4ÀCorTIm with an
absorption peak at about 380 nm (Figure S3 in the Supporting
Information). On the basis of these results, we suggest that the
initial energy transfer from the singlet excited state of CorTIm
to porphyrins and subsequent electron transfer from the por-
phyrin to CorTIm occur when CorTIm is mainly excited. Addi-
tionally, femtosecond time-resolved transient absorption meas-
urements strongly support the above discussion (see below). A
Table 1. First one-electron reduction and oxidation potentials of (H2P)4À
CorTIm, (ZnP)4ÀCorTIm, and the reference compounds.
Compounds
Ered, V vs.
SCE[a] [V]
Eox, V vs.
SCE,[a] [V]
(ZnP)4ÀCorTIm
(H2P)4ÀCorTIm
CorTIm
ZnPÀNHAc
H2PÀNHAc
À0.72
À0.72
À0.65
À1.41
À1.26
+0.83
+1.00
–
+0.78
+0.95
[a] Measured in CH2Cl2 containing 0.3m n-Bu4NPF6 at 298 K.
CorTIm exhibits reversible redox waves, which are typical for
the first one-electron oxidation (ZnP +/ZnP) and reduction pro-
C
CÀ
cesses (CorTIm/CorTIm ). The first one-electron reduction (Ered
)
and oxidation (Eox) potentials were assigned to be À0.72 and
0.83 V versus SCE, respectively (Figure 4A). These values are
very similar to those of reference single components such as
Ered of CorTIm: À0.65 V and Eox of ZnP-NHAc: 0.78 V as shown
in Figure 4B and 4C. The electrochemical results indicate that
a CorTIm and four porphyrin units in (H2P)4-CorTIm were char-
acterized by the corresponding reference monomers in solu-
tion, which is similar to the trend of absorption spectra in
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