Large porphyrin squares from the self-assembly of meso-Triazole-Appended L-Shaped meso-meso-Linked ZnII-Triporphyrins: synthesis and efficient energy transfer

Article abstract of DOI:10.1002/chem.200903195

meso-Triazolyl-appended Zn^IIporphyrins were readily prepared by Cu^I-catalyzed 1, 3-dipolar cycloaddi-tion of benzyl azide to meso-ethynylat-ed Zn^II-porphyrin (click chemistry). In noncoordinating CHCl₃ solvent, spon-taneous assembly occurred to form tet-rameric array (3)₂: from meso-meso-linked diporphyrins 3 ₁ and dodecameric porphyrin squares (4)₄ and (5) ₄ from the L-shaped meso-meso-linked tripor-phyrins 4 and 5. The structures of these assemblies were examined by ¹H NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the struc-tures of the dodecameric porphyrin squares (4) ₄ and (5)₄ were probed by small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in solu-tion using a synchrotron source. Excita-tion-energy migration processes in these assemblies were also investigated in detail by using both steady-state and time-resolved spectroscopic methods, which revealed efficient excited-energy transfer (EET) between the meso-meso-linked Zn^II-porphyrin units that occurred with time constants of 1.5 ps^-1 for(3)₂ and 8.8ps^-1 for (5)₄.

Full text of DOI:10.1002/chem.200903195

DOI: 10.1002/chem.200903195
Large Porphyrin Squares from the Self-Assembly of meso-Triazole-
Appended L-Shaped meso–meso-Linked ZnII–Triporphyrins:
Synthesis and Efficient Energy Transfer
Chihiro Maeda,[a] Pyosang Kim,[b] Sung Cho,[b] Jong Kang Park,[b] Jong Min Lim,[b]
Dongho Kim,*[b] Josh Vura-Weis,[c] Michael R. Wasielewski,*[c] Hiroshi Shinokubo,[d] and
Atsuhiro Osuka*[a]
1
Abstract:
meso-Triazolyl-appended
assemblies were examined by H NMR
spectra, absorption spectra, and their
gel permeation chromatography (GPC)
retention time. Furthermore, the struc-
tures of the dodecameric porphyrin
squares (4)4 and (5)4 were probed by
small- and wide-angle X-ray scattering
(SAXS/WAXS) measurements in solu-
tion using a synchrotron source. Excita-
tion-energy migration processes in
these assemblies were also investigated
in detail by using both steady-state and
time-resolved spectroscopic methods,
which revealed efficient excited-energy
transfer (EET) between the meso–
meso-linked ZnII–porphyrin units that
occurred with time constants of 1.5 psꢀ1
for (3)2 and 8.8 psꢀ1 for (5)4.
ZnII–porphyrins were readily prepared
by CuI-catalyzed 1,3-dipolar cycloaddi-
tion of benzyl azide to meso-ethynylat-
ed ZnII–porphyrin (click chemistry). In
noncoordinating CHCl3 solvent, spon-
taneous assembly occurred to form tet-
rameric array (3)2 from meso–meso-
linked diporphyrins 3, and dodecameric
porphyrin squares (4)4 and (5)4 from
the L-shaped meso–meso-linked tripor-
phyrins 4 and 5. The structures of these
Keywords: energy transfer · photo-
synthesis
assembly · triazoles
· porphyrinoids · self-
Introduction
In the photosynthetic antennae LH1 and LH2, efficient exci-
tation-energy transfers (EET) among chlorophylls are cru-
cially important for light-energy harvesting, which is realized
by regularly arranged cyclic wheel-like structures.[1] Continu-
ous efforts have been devoted to the synthesis and charac-
terization of artificial photosynthetic systems that can mimic
key features of EET in natural LHs.[2] Naturally, particular
attention has been focused on the construction of cyclic por-
phyrin arrays[3] by means of a covalent[4,5] or noncovalent
approach in view of their structural similarity to LH1 and
LH2.[6–9] Covalent synthesis has the advantages of robust
stability and precise control of the spatial arrangement. In
addition, a proper choice of spacer also allows well-defined
control of electronic interactions between constituent chro-
mophores. However, as the size becomes larger, covalent
synthesis becomes increasingly difficult and inefficient. As a
more convenient but promising strategy, noncovalent, supra-
molecular assembly has emerged as an effective means for
the construction of two- or three-dimensional architectures.
The importance of this noncovalent strategy comes from the
fact that the natural systems rely on noncovalent interac-
[a] C. Maeda, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto, 606-8502 (Japan)
Fax : (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
[b] P. Kim, S. Cho, J. K. Park, J. M. Lim, Prof. Dr. D. Kim
Center for Ultrafast Optical Characteristics Control and
Department of Chemistry
Yonsei University, Seoul, 120-749 (Korea)
Fax : (+2)2-2123-2434
E-mail: dongho@yonsei.ac.kr
[c] J. Vura-Weis, Prof. Dr. M. R. Wasielewski
Department of Chemistry and
Argonne-Northwestern Solar Energy Research
ACHTUNGTRENNUNG(ANSER) Center, Northwestern University
Evanston IL, 60208-3113 (USA)
Fax : (+1)847-467-1425
E-mail: m-wasielewski@northwestern.edu
[d] Prof. Dr. H. Shinokubo
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University, Chikusa-ku, Nagoya 464-8603 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903195.
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FULL PAPER
tions, as seen in LH1 and LH2
complexes. For such a noncova-
lent assembly, the coordination
interaction between zinc(II)–
porphyrin and pyridine[7,8] or
imidazole[9] moieties has been
particularly attractive because
of its large association constant
and favorable tendency not to
spoil the photoexcited-state dy-
namics of porphyrins.[10] Here it
is worth noting that the synthe-
Scheme 1. Syntheses of 1 and 2 and their self-assembling to (1)2 and (2)2. Ar=3,5-di-tert-butylphenyl.
ses of porphyrins that bear pyri-
dine or imidazole are not trivi-
al, since the usual porphyrin
synthesis on the basis of acid-catalyzed condensation is se-
verely hampered by such a coordinating basic moiety. In this
context, we recently demonstrated the utility of so-called
“click chemistry” that used a CuI-catalyzed 1,3-dipolar cy-
cloaddition reaction of benzyl azide to meso-ethynyl-substi-
tuted porphyrins for the preparation of meso-1,2,3-triazole-
substituted porphyrins (Scheme 1).[11,12] Actually, the synthe-
sis of triazolyl-appended ZnII–
assembly occurred to form porphyrin box (3)2 from 3, and
large porphyrin squares (4)4 and (5)4 from 4 and 5 by com-
plementary triazole–zinc(II) coordinative interactions. These
assemblies displayed larger absorption spectral coverage
due to exciton coupling and efficient intra-assembly EET
that has been confirmed by ultra-fast time-resolved meas-
urements.
porphyrin 1 was quite facile; it
formed
a face-to-face dimer,
(1)2, with an association con-
stant of 105 mꢀ1. The association
constant of the dimer was dra-
matically increased by imple-
menting additional hydrogen-
bonding interactions, as demon-
strated by the dimerization to
form (2)2 with an association
constant larger than 1011 mꢀ1.[11b]
In the latter case, the most
stable assembly (2)2 was chosen
from three possible atropiso-
meric face-to-face dimers with
respect to the position of the
ester group. Then it was sug-
gested that the relatively small
coordination ability of triazole,
as compared with the cor-
responding association con-
stants of the similar face-to-
face
diporphyrins
pyridine
(ꢁ108 mꢀ1)[7b] and imidazole
(ꢁ1011 mꢀ1)[6f] could be suitable
for more precise recognition
through multiple dynamic inter-
molecular interactions.
In this paper, 1,2,3-triazole
rings were introduced at the
meso positions of meso–meso-
Results
linked diporphyrin and triporphyrin[13] to provide 3, 4, and 5
with the aim of forming larger but well-ordered assemblies
by multiple coordination bonds. Actually, spontaneous self-
Synthesis: The syntheses and self-assembly behaviors of 1
and 2 were reported previously.[11] Synthetic routes to 3, 4,
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5053

D. Kim, M. R. Wasielewski, A. Osuka et al.
and 5 are illustrated in Scheme 2. Ethynyl groups were in-
troduced to meso–meso-linked ZnII–diporphyrin 6 by bromi-
nation with N-bromosuccinimide (NBS), subsequent Sono-
gashira coupling reaction with trimethylsilylacetylene, and
final deprotection of the trimethylsilyl group with tetrabutyl-
dibromo-substituted porphyrin 11 and 10 furnished ZnII
free-base hybrid triporphyrin 4 in 24% yield, which was
converted into ZnII–porphyrin trimer 5 by metalation with
ZnACHTNUTRGENNG(U OAc)2 in 85% yield.
1
A
Characterization: The H NMR spectrum of 3 in CDCl3 ex-
hibits a single set of signals with clear upfield shifts for the b
protons at d=6.3 and 6.2 ppm, thereby indicating the forma-
tion of a discrete face-to-face dimer, (3)2. The 1H NMR
spectrum of 4 in CDCl3 exhibits two sets of signals with
normal chemical shifts and upfield-shifted chemical shifts
for the b protons (d=5.8 and 5.5 ppm) in a ratio of 1:2,
thereby suggesting the formation of the expected tetrameric
porphyrin square. Actually, these two sets of peaks are con-
sidered to correspond to the corner four free-base porphyr-
ins and the four side-position-
ynyl-substituted meso–meso-linked ZnII–diporphyrin 7 with
benzyl azide gave 5,5’-bistriazolyl ZnII–diporphyrin 3 in
61% yield. The synthesis of 4 and 5 was performed as fol-
lows. Triazole-appended ZnII–porphyrin 9 was prepared by
the 1,3-dipolar cycloaddition of benzyl azide to meso-ethy-
nylated ZnII–porphyrin 8. 5-Borylated-15-triazolyl-substitut-
ed ZnII–porphyrin 10 was prepared from 9 by means of
meso-selective NBS bromination followed by Pd-catalyzed
borylation. A Suzuki–Miyaura coupling reaction[14] of 5,10-
ing face-to-face ZnII dimeric
pairs in a ratio of 1:2, since the
signals due to the latter parts
are observed in the range of a
high-field-shifted region. In
[D5]pyridine, these porphyrin
oligomers 3, 4, and 5 exhibit
1
simple H NMR spectra, there-
by indicating their monomeric
nature (see the Supporting In-
formation).
The molecular size of these
aggregates was examined by an-
alytical GPC-HPLC (Figure 1).
The GPC retention time was
distinctly shorter with CHCl3
than with coordinating THF as
eluent, thereby indicating that
assembly formation occurs in
CHCl3 but not in THF. It is
worthy of note that these por-
phyrins
formed
assemblies
under highly diluted conditions
in CHCl3. Importantly, the re-
tention times exhibited a good
correlation with increasing mo-
lecular weights of (2)2, (3)2,
(4)4, and (5)4 (Table 1 and
Figure 2), which were slightly
longer than those of the poly-
styrene standards that were
roughly linear in spite of their
propensity to take coiled con-
formations.
The structures of aggregates
(4)4 and (5)4 were also studied
by small- and wide-angle X-ray
Scheme 2. a) 1) NBS, CHCl3, pyridine, 08C; 2) trimethylsilylacetylene, CuI, [PdCl2ACHTUNTRGNEUNG(PPh3)2], THF, Et3N, RT;
scattering
(SAXS/WAXS)
3) TBAF, CHCl3, RT, 52%. b) BnN3, CuCl, toluene, 1008C, 61%. c) BnN3, CuCl, toluene, 1008C, 74%.
d) NBS, CHCl3, pyridine, 08C, then HBpin, [PdCl2A(PPh3)2], Et3N, dioxane, reflux, 91%. e) [Pd2A(dba)3] (dba=
(OAc)2, CHCl3,
measurements performed on
their 10ꢀ4 m solutions in toluene
using a high-flux synchrotron
C
CHTUNGTRENNUNG
dibenzylideneacetone), tri-2-furylphosphine, Cs2CO3, CsF, dioxane, DMF, reflux, 24%. f) ZnCAHTUNGTRNENNUG
MeOH, RT, 85%. Ar=3,5-di-tert-butylphenyl.
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Large Porphyrin Squares
FULL PAPER
the low-resolution scattering region (q<0.1 ꢁꢀ1), the scat-
tering intensity follows the Guinier relationship, I(q)=
2
I(0) expACTHUNGTRENNNUG
forward scattering amplitude, I(0), and the radius of gyra-
tion, Rg.[15] The linearity of the Guinier plot between 0.001<
q2 <1.0 ꢁꢀ2 (Figure 3b) indicated that the sample consisted
of a monodisperse structure.[16] The least-squares fit to the
linear data reveals a radius of gyration of 18.1 ꢁ. The mono-
disperse nature of (5)4 in toluene allowed direct compari-
sons between reciprocal-space scattering patterns and real-
space molecular models through atomic-pair distribution
function (PDF) analysis. The experimental PDF for (5)4 in
toluene was obtained using the X-ray scattering fitting pro-
gram GNOM (Figure 3c).[17] The peaks in the PDF at 23.5 ꢁ
were the result of atomic pair correlations between the side-
to-side pairs.[18] Optimized models of (5)4 revealed an aver-
ꢀ
age Zn Zn distance of 23.1 ꢁ for the analogous pair corre-
lations. For (4)4 the theoretical values of Rg calculated by
the PDF were 16.2 ꢁ for the experiment, which were a very
good match with the Rg value of 15.5 ꢁ for model (Fig-
ure S11 in the Supporting Information).[19] The PDF ob-
tained from monomeric model showed entirely different
peaks.
Figure 1. Analytical GPC-HPLC chromatograms of a) 2, b) 3, c) 4, and
d) 5 eluted with CHCl3 (solid line) and with THF (dashed line).
Table 1. Calculated molecular weights and GPC-HPLC retention times.
Compound
Molecular formula
Mcalcd
t [min]
2
(2)2
3
(3)2
4
(4)4
5
C59H61N7O2Zn
C118H122N14O4Zn2
C114H116N14Zn2
C228H232N28Zn4
C162H168N18Zn2
C648H672N72Zn8
C162H166N18Zn3
C648H664N72Zn12
965.6
25.0
23.4
23.5
22.3
23.2
20.3
23.1
20.1
Steady-state absorption, fluorescence, and fluorescence exci-
tation anisotropy: In the present study, we used CHCl3 and
pyridine as solvents, in which the triazole-appended por-
phyrins exist as aggregates and monomers, respectively.
Figure 4 shows the absorption spectra of 2, 3, and 5 in
CHCl3 and pyridine. As reported previously, the absorption
spectrum of 2 in pyridine exhibited a sharp Soret band at
433 nm, which was typical of a monomeric ZnII–porphyrin,
whereas that in CHCl3 displayed a blueshifted and split
Soret band that was characteristic of face-to-face dimers,
thus indicating the formation of (2)2.[9a,11b] The absorption
spectrum of 3 in pyridine featured a split Soret band at 430
and 469 nm characteristic of meso–meso-linked ZnII–dipor-
phyrin, and that in CHCl3 exhibited a narrowly split Soret
band that is characteristic of a porphyrin box that consists of
two complementary coordinating meso–meso-linked ZnII–di-
porphyrins,[8a,b] thus indicating the monomeric dissociation
in pyridine and the formation of dimeric assembly (3)2 in
CHCl3, respectively. The absorption spectrum of 5 in pyri-
dine was quite similar to that of L-shaped meso–meso-
linked ZnII–triporphyrin,[14b,c] whereas that in CHCl3 was
more broad and complicated in line with the formation of
(5)4. The absorption spectrum of 5 in CHCl3 will be dis-
cussed later.
1931.1
1813.1
3626.1
2498.0
9992.0
2561.4
10245.6
(5)4
The S1 fluorescence spectra of 2, 3, and 5 in pyridine were
observed with gradually redshifted peaks at 613, 641, and
646 nm. Compared with these spectra, the fluorescence
spectra of assemblies (2)2, (3)2, and (5)4 in CHCl3 become
slightly blueshifted and sharpened (Figure 4). Observed
changes in the fluorescence spectra of 3 and (3)2 are note-
worthy in that the vibronic structure is clearly restored in
the latter. This can be explained in terms of considerable ri-
gidification of the dihedral angle in the meso–meso-linked
*
Figure 2. Relationship of the retention time versus molecular weight (
:
*
^
aggregated structures; : dissociated structures; : polystyrene standard).
source (Advanced Photon Source at Argonne National Lab-
oratory). The solution-phase scattering pattern for (5)4 was
obtained for the q region (0.01<q<1.0 ꢁꢀ1; Figure 3a). In
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D. Kim, M. R. Wasielewski, A. Osuka et al.
(3)2, and (5)4, than monomeric
2, 3, and 5, respectively, thereby
suggesting that the self-assem-
bling leads to more rigid con-
formation.
The steady-state fluorescence
excitation anisotropy spectra
were measured for 2–5 and
(2)2–(5)4 and are compared in
Figure 5. The polarization an-
AHCTUNGTERGiNNUN sotropy measurement reveals
the relative orientation of emis-
sive transition dipole moment
with respect to that of excita-
tion, which changes through de-
polarization channels such as
molecular rotational motion
and excitation-energy transfer.
Whereas 2 and 3 displayed an-
AHCTUNGTRENGNiUN sotropy profiles that exhibited
negative anisotropy in the high-
energy Soret band and positive
anisotropies in the low-energy
Soret and Q bands, 5 showed a
distinct feature: relatively de-
creased anisotropy in the low-
energy Soret band and Q bands
in spite of a larger molecular
volume that caused slower rota-
tional diffusion time. A reason
for this feature may be a new
Figure 3. a) Experimental scattering intensity, I(q), versus scattering length vector, q, data for a solution of 5 in
toluene (4.7ꢂ10ꢀ4 m; 0.1<q<1.0 ꢁꢀ1 region). b) Scattering intensity versus q2 for the same sample (0.001<
q2 <0.01 ꢁꢀ2 region). Guinier fit to the data is also shown. c) Atomic pair distribution functions (PDFs) for 5
in toluene obtained using the program GNOM. d) Optimized structure of (5)4.
depolarization channel in 5, which is expected to be similar
to the energy-transfer process observed in the directly
meso–meso-linked porphyrin rings through the bidirectional
coupling with the nearest neighboring porphyrins.[5b] In (3)2,
an abrupt decrease in the anisotropy value was observed in
the entire spectral region as compared with 3, thereby sug-
gesting the occurrence of intracomplex fast energy-transfer
processes. In contrast to (3)2, (5)4 exhibited an increase in
the anisotropy value in the low-energy Soret and Q bands
compared with that of 5. Since (5)4 should have the same
depolarization channel as that of 5, the observed increase in
the anisotropy value for (5)4 was considered due to its much
slower rotational diffusion process.
Fluorescence decay measurements: The time-resolved fluo-
rescence decays of 2–5 and (2)2–(5)4 were measured and
their fitted fluorescence lifetimes are tabulated in Table 2.
Importantly, all the models retain relatively long fluores-
cence lifetimes (>1.45 ns), which is an important require-
ment for artificial photosynthetic antenna models. Their
fluorescence decay profiles exhibit monoexponential behav-
iors, and the overall fluorescence lifetimes are gradually re-
duced in going from 2–5 to (2)2–(5)4 and also in the order of
2>3>5 as well as (2)2 >(3)2 >(5)4. These decreased fluores-
cence lifetimes may be ascribed to increased conformational
heterogeneity associated with an increase in molecular size.
Figure 4. Steady-state absorption (left) and fluorescence (right) spectra of
a) 2, b) 3, and c) 5 in CHCl3 (solid line) and pyridine (dotted line).
diporphyrins due to the formation of the self-assembled
complex. In the case of (5)4, however, the fluorescence spec-
tral changes from that of 5 are only marginal because of the
greater conformational flexibility of (5)4. In addition, the
Stokes shifts are commonly smaller in the aggregates (2)2,
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Large Porphyrin Squares
FULL PAPER
sembling geometry, thus leading
to the reduced transition dipole
moment.
Intensity-dependent femtosec-
ond transient absorption (TA)
decay: To explore the excita-
tion-energy-transfer processes,
the pump-power dependencies
of the femtosecond transient
absorption measurements were
examined for (2)2–(5)4 using 2–
5 as reference molecules with
Q-band excitation at 570 nm
(Table 3, Figure 6). Monomeric
species 2–5 and the dimeric as-
sembly (2)2 revealed no pump-
power-dependent TA decays
that were in agreement with the
S1 state lifetimes measured by
time-correlated single-photon
counting (TCSPC) techniques.
On the other hand, (3)2 and
(5)4 showed fast decay compo-
nents t1 (1.5 ps) and t2 (7.5 ps),
respectively, the amplitudes of
which are sensitive to the pho-
toexcitation intensity as com-
pared to the S1 state lifetime.
The pump-power dependence
on the TA decay is a strong in-
dication of singlet–singlet anni-
hilation processes, because the
Figure 5. Steady-state fluorescence excitation anisotropy spectra of 2, 3, and 5 in pyridine (left) and CHCl3
(right). The spectra were obtained using parallel and perpendicular orientation between excitation and emis-
sion polarization at a fixed emission wavelength of 610 nm.
Table 2. Photophysical parameters for 2–5 and (2)2–(5)4.
[a]
Sample
Solvent
lA [nm]
lF [nm]
FF
t [ns]
kr [107 sꢀ1
]
knr [108 sꢀ1
]
F
[b] [ns]
2
(2)2
3
(3)2
5
(5)4
pyridine
CHCl3
pyridine
CHCl3
pyridine
CHCl3
433, 566, 607
613
607
641
623
646
636
0.035
0.020
0.058
0.023
0.068
0.040
2.04
1.88
1.90
1.58
1.86
1.45
1.72
1.06
3.05
1.46
3.66
2.76
4.85
5.19
4.95
6.20
5.01
6.62
0.44ꢂ0.05
0.64ꢂ0.14
1.07ꢂ0.34
–
414, 430, 561, 605
430, 469, 576, 618
428, 452, 575, 618
428, 468, 579, 625
408, 473, 576, 620
1.78ꢂ0.16
4.01ꢂ2.07
[a] Quantum yield was determined with reference to the value (0.033) of zinc(II) tetraphenylporphyrin in tolu-
ene. [b] The rotational diffusion time; see the Supporting Information.
Table 3. Pump-power-dependent transient absorption decay parameters
for (2)2, (3)2, and (5)4
To quantitatively analyze the characteristics of the S1
state, we calculated the radiative and nonradiative decay
rates using the fluorescence quantum yields and the S1 fluo-
rescence lifetimes. Consequently, the increase of fluores-
cence quantum yields in going from 2 to 5 and (2)2 to (5)4 is
mainly caused by the increase of radiative decay rates,
which give information about the difference in excitonic
coupling strength through the arrangement of transition
dipole moments that lead to a change in radiative decay
rates. Interestingly, the extent of increase in the radiative
decay rates has a different pattern between 2–5 and (2)2–
(5)4; the radiative rate is increased by 77% for 3 and 112%
for 5 as compared with 2 but the increase is 38 and 160%
for (3)2 and (5)4, respectively, as compared with (2)2. In spite
of the same bidirectional arrangements of transition dipole
moments, (5)4 has a larger increase (160%) than the case of
5 (112%), mainly because of additional linear arrangements
of transition dipole moments by the formation of a square-
type self-assembled complex. Additionally, the reason for
the small increase (38%) in (3)2 relative to 3 is probably
due to competition of H- and J-type coupling among the
transition dipole moments along the long axes by the self-as-
Pump power
Fitted decay times [ps]
[mW]
t1
t2
t3
(2)2
(3)2
(5)4
520
446
360
2000 (100%)
2000 (100%)
2000 (100%)
520
446
360
1.5 (18%)
1.5 (8%)
–
1640 (82%)
1640 (92%)
1640 (100%)
520
446
360
1.5 (30%)
1.8 (36%)
1.8 (24%)
7.5 (26%)
7.5 (15%)
7.5 (ꢁ2%)
1510 (44%)
1510 (49%)
1510 (74%)
intense excitation or high density of photons generates two
or more excitons within one self-assembled complex, and
then the recombination process between excitons leads to a
fast deactivation process. Because the TA decays of 3 and 5
showed no intensity dependence, the observed singlet–sin-
glet annihilation times in (3)2 and (5)4 suggested that the ex-
citon–exciton recombination processes occur in these assem-
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5057

D. Kim, M. R. Wasielewski, A. Osuka et al.
Discussion
The absorption spectral features of (2)2, (3)2, and (5)4 can be
explained in terms of the excitonic coupling theory
(Figure 7). The Soret band of porphyrin monomer 2 has two
perpendicular transition dipole moments Bx and By that are
degenerate. In cofacial diporphyrin (2)2, two transition
dipole moments Bx1 and Bx2 are coupled as an H-type inter-
action, thus giving rise to a blueshifted band (high-energy
Soret band), whereas By1 and By2 are considered to be cou-
pled as a J-type interaction. Fortunately, we have deter-
mined the crystal structure of (2)2,[11b] which is shown in
Figure 8; the arrangement of the transition dipole moments
is also indicated. This explains the absorption spectrum of
(2)2 nicely since the angle between By1 and By2 is 32.58,
which is characteristic for J-type interaction.[22] This geome-
try serves as an important structural motif in the exciton
coupling of (3)2 and (5)4. In the case of meso–meso-linked
diporphyrin 3, the Soret band is split to a redshifted band
and the original band because two dipole moments Bx along
with the meso–meso-linked axis are strongly coupled as J-
type interactions, but the coupling between By1 and Bz1 is
negligible due to the perpendicular orientation. In assembly
(3)2, By1, By2 and Bz1, Bz2 are coupled in a similar manner to
the low-energy Soret band of (2)2, but the remaining four
transition dipole moments Bx that are already J-type-cou-
pled undergo additional H-type interaction to give rise to a
blueshifted low-energy Soret band that is lower in energy
than that of 3 (Figure 7). As reported previously,[14b] L-
shaped triporphyrin 5 shows a split Soret band, the splitting
width of which is close to those of meso--meso-linked dipor-
phyrins such as 3. But in (5)4, the low-energy Soret band is
further redshifted because of the additional exciton coupling
with two transition dipole moments Bx2 and By2. For Bz com-
ponents, the H-type interaction and out-of-plane arrange-
ment of transition dipole moments create a high-energy
Soret band that is distinctly observed at 408 nm [Eq. (1),
Figure 7].
Figure 6. Power-dependent transient absorption decay profiles of (2)2
~
:
*
(top), (3)2 (middle), and (5)4 (bottom)
(
: 520 mW; &: 446 mW;
360 mW). The transient absorption decay profiles (520 mW) of 2, 3, and 5
is inserted. In the experiments, the pump and probe wavelengths were
570 and 520 nm, which are the Q-band excitation and induced absorption
probe, respectively. The solvent used was toluene.
th
ta ¼
ð1Þ
8 sin2ðp=2NÞ
blies. The faster t1 (1.5–1.8 ps) component in the TA decay
profile of (5)4 might be associated with high-order annihila-
tion processes, which could occur in multichromophoric sys-
tems, especially when the samples are excited by higher
pump-power pulses. In addition, according to descriptions of
the natural light-harvesting systems (LH1 and LH2), the sin-
glet–singlet annihilation process is directly indicative of Fçr-
ster-type incoherent energy transfer from the excited donor
to an adjacent excited acceptor, thereby resulting in the for-
mation of a doubly excited state that quickly relaxes to the
singly excited state. Thus, the singlet–singlet annihilation
process reflects the excitation-energy-transfer processes that
occur in multichromophoric molecular assemblies such as
(2)2–(5)4 in the present work.[20,21]
The observed exciton–exciton annihilation process is closely
associated with the excitation-energy-hopping process.
Equation (1) correlates the annihilation time (ta) and the
energy-hopping time (th) for a one-dimensional cyclic multi-
chromophoric system with N hopping units. Although (5)4
contains 12 porphyrin monomer units, the number of hop-
ping sites (N) is considered to be 4, that is, four of 5. Be-
cause it is conceived that the excitation-energy transfer
within approximately 200 fs occurs between meso–meso di-
rectly linked porphyrin monomers in the 5 moiety, we can
directly assign the slowest annihilation time component as
the energy-hopping time between four 5 units.[5,23] There-
fore, using the observed annihilation time t2 in Table 3, the
energy-hopping time th of (5)4 was evaluated to be 8.8 ps
(Figure 9). This method of determining the energy-hopping
5058
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Large Porphyrin Squares
FULL PAPER
Figure 7. Exciton coupling in the assemblies
quantum yield of the donor in the absence of acceptor, n is
the refractive index of the medium, N is Avogadroꢃs
number, r is the distance between donor and acceptor, tD is
the lifetime of the donor in the absence of acceptor, FD(l) is
the corrected fluorescence intensity of the donor normalized
to unity, and eA(l) is the extinction coefficient of the accept-
or at l, which is typically measured in mꢀ1 cmꢀ1. The distance
r between neighboring and next-nearest-neighboring mole-
cules in (3)2 and (5)4 are deter-
Figure 8. Crystal structure and schematic representation of dipole mo-
ments of (2)2.
mined from the Soret-band ex-
citonic dipole coupling and Q-
band shift. In other words, 5
and 3 have the similar peak po-
sitions of low-energy Soret
bands as well as Q bands, there-
by indicating that we can
assume the same transition
dipole moments for the hop-
ping units as that of 3. This as-
sumption results in the calculat-
Figure 9. Schematic representation of energy hopping in the assemblies.
ed distance of approximately
6.5 ꢁ for (3)2 and approximate-
ly 14 ꢁ for (5)4 (Figure 9).
rate is also applicable to (3)2, thereby resulting in an energy-
hopping time of 1.5 ps between ZnII–porphyrin units. In the
case of (3)2, the annihilation time represents directly the
energy-hopping time because it has only two energy-hop-
ping sites.
To confirm the energy-hopping model (Figure 9), the Fçr-
ster-type energy-transfer rate was calculated by using Equa-
tion (2).[23,24]
Using these values and k2 of 1 and 1.96 for (3)2 and (5)4, re-
spectively, the Fçrster-type energy-transfer rates were calcu-
lated to be (ꢁ0.71 ps)ꢀ1 and (ꢁ9.5 ps)ꢀ1, which are in good
agreement with the experimental hopping times. Therefore,
although (3)2 and (5)4 are very closely spaced, we demon-
strate that the experimental and calculated Fçrster-type
energy-transfer rates confirm the schematic dipole–dipole
interactions in (3)2 and (5)4 through the formation of self-as-
sembled complexes (Figure 9).
Assemblies (3)2 and (5)4 show excitation-energy-hopping
rates of (1.5 ps)ꢀ1 and (8.8 ps)ꢀ1 respectively. In particular, if
we consider that the Fçrster energy-transfer rate is propor-
tional to 1/r6, the (5)4 complex shows a faster excitation-
energy-hopping rate (ꢁ14 ꢁ, 8.8 psꢀ1) than (3)2 (ꢁ6.5 ꢁ,
1
ꢀ
ꢁ
Z
QDk2 90000ðln 10Þ
KTðrÞ ¼
FDðlÞeAðlÞl4dl
ð2Þ
tDr6
128p5Nn4
0
in which k2 is the orientation factor, QD is the fluorescence
Chem. Eur. J. 2010, 16, 5052 – 5061
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5059

D. Kim, M. R. Wasielewski, A. Osuka et al.
1.5 psꢀ1) or the previously reported porphyrin box (ꢁ10 ꢁ,
48 psꢀ1).[8b] In the case of (3)2, if we consider that the Fçrster
energy-transfer rate is proportional to 1/r6, the energy-trans-
fer rate of (5)4 (ꢁ14 ꢁ) is expected to be approximately
150 psꢀ1 as compared with that of (3)2, thereby resulting in a
rate around 17 times slower than that of (5)4. In the porphy-
rin box, when the distance between energy-hopping units is
simply considered, the porphyrin box with the center-to-
center distance of approximately 10 ꢁ between energy-hop-
ping units shows an excitation-energy-hopping rate around 5
times slower than that of (5)4 although the distance between
hopping units is larger by around 4 ꢁ. This result may come
from the competition between H- and J-type coupling of the
transition dipole moments along the meso–meso linkage in
(3)2 or the porphyrin box and the parallel arrangement of
transition dipole moments, thereby leading to much-de-
creased energy-transfer rates. On the contrary, the orienta-
tions between the transition dipole moments of 5 in (5)4 are
parallel to each other by the formation of square-type self-
assembled complexes.
cially supported by the Star Faculty and World Class University (no.
2008-8-1955) programs of the Ministry of Education, Science, and Tech-
nology of Korea and the AFSOR/AOARD grant (no. FA4869-08-1-
4097). Research at Northwestern University was supported by the Chem-
ical Sciences, Geosciences, and Biosciences Division, Office of Basic
Energy Sciences, DOE under grant no. DE-FG02-99ER14999. C.M.
thanks the JSPS fellowship for the young scientists. P.K., S.C., J.P., and
J.M.L. acknowledge the fellowship of the BK21 program of the Ministry
of Education, Science, and Technology of Korea.
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Conclusion
In this work, we have investigated the synthesis, self-assem-
bly, and excitation-energy-hopping processes that occurs in
self-assembled porphyrin array systems (3)2 and (5)4 con-
structed from 1,2,3-triazole-appended ZnII–porphyrins 3 and
5. The structures of these assemblies have been examined
by their 1H NMR, UV/Vis absorption spectroscopy; GPC
retention time; and SAXS/WAXS measurements. The exci-
ton–exciton annihilation process evidently represents the ex-
citation-energy hopping in (3)2 and (5)4. Assuming that the
number of energy-hopping sites (N) is 2 and 4, respectively,
the experimental observables give the excitation-energy-
hopping rates of 1.5 and 8.8 psꢀ1, which are in a good agree-
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In particular, in the case of (5)4, the excitation-energy-hop-
ping rate is faster than that of (3)2 or previously reported as-
semblies because of the strong exciton coupling that results
from the linear (J-type) orientation of transition dipole mo-
ments.
In conclusion, the strategy of this new square-type self-as-
sembly system is considered to be an alternative method for
increasing the energy-hopping efficiency in a self-assembled
artificial light-harvesting apparatus. A further increase in
the number of chromophores within this self-assembled
system may result in a more efficient energy-hopping pro-
cess with the ability to control its size. Our attempts to
design new porphyrin assembly systems to test this possibili-
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The work at Kyoto was supported by Grants-in-Aid (no. 19205006 (A)
and 20108001 “pi-Space“) from MEXT. The work at Yonsei was finan-
5060
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Chem. Eur. J. 2010, 16, 5052 – 5061

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Received: November 21, 2009
Published online: March 26, 2010
Chem. Eur. J. 2010, 16, 5052 – 5061
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5061