Porphyrin boxes constructed by homochiral self-sorting assembly: Optical separation, exciton coupling, and efficient excitation energy migration

Article abstract of DOI:10.1021/ja046241e

meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-mesolinked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl ₃, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, form

Full text of DOI:10.1021/ja046241e

Published on Web 11/13/2004
Porphyrin Boxes Constructed by Homochiral Self-Sorting
Assembly: Optical Separation, Exciton Coupling, and
Efficient Excitation Energy Migration
In-Wook Hwang,† Taisuke Kamada,‡ Tae Kyu Ahn,† Dah Mee Ko,†
Takeshi Nakamura,‡ Akihiko Tsuda,‡ Atsuhiro Osuka,*,‡ and Dongho Kim*,†
Contribution from the Center for Ultrafast Optical Characteristics Control and Department of
Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, and Department of Chemistry, Graduate
School of Science, Kyoto UniVersity, and CREST (Core Research for EVolutional Science and
Technology) of the Japan Science and Technology Agency, Kyoto 606-8502, Japan
Received June 24, 2004; E-mail: osuka@kuchem.kyoto-u.ac.jp; dongho@yonsei.ac.kr
Abstract: meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble
spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and
porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-
sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers
of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm,
thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings.
Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved
spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power
dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles
are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-
exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Fo¨rster-
type incoherent energy hopping model by assuming a number of hopping sites of N ) 4 and an exciton
coherence length of L ) 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin
units have been estimated for B1 (48 ps)-1, B2 (98 ( 3 ps)-1, and B3 (361 ( 6 ps)-1. Overall, the self-
assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting
complex.
Introduction
However, as the size and complexity of these systems grow,
the covalent strategy becomes increasingly inefficient and
tedious.
Currently, much effort has been directed toward unraveling
the energy transport phenomena occurring in natural light-
harvesting complexes.1-3 At the same time, the mimicry of
natural light-harvesting complexes has been continuously at-
tempted by the covalent synthesis of various types of porphyrin
or related pigment arrays with the goal of applying these arrays
to artificial light-harvesting systems and molecular photonic
devices.4-11 The covalent approach has advantages of robust
stability and precise control in the spatial arrangement and
connecting spacer. The judicious choice of a spacer will allow
fine control of electronic interactions between chromophores.
Supramolecular chemistry, using a strategy of noncovalent
self-assembly of molecular units, has been developed as a highly
promising means for the construction of two- or three-
dimensional architectures that have specific structures, proper-
(9) (a) Kim, D.; Osuka, A. J. Phys. Chem. A 2003, 107, 8791. (b) Tsuda, A.;
Osuka, A. Science 2001, 293, 79. (c) Cho, H. S.; Jeong, D. H.; Cho, S.;
Kim, D.; Matsuzaki, Y.; Tanaka, K.; Tsuda, A.; Osuka, A. J. Am. Chem.
Soc. 2002, 124, 14642. (d) Aratani, N.; Cho, H. S.; Ahn, T. K.; Cho, S.;
Kim, D.; Sumi, H.; Osuka, A. J. Am. Chem. Soc. 2003, 125, 9668. (e)
Cho, H. S.; Rhee, H.; Song, J. K.; Min, C.-K.; Takase, M.; Aratani, N.;
Cho, S.; Osuka, A.; Joo, T.; Kim, D. J. Am. Chem. Soc. 2003, 125, 5849.
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Ed. 2004, 43, 150. (b) Choi, M.-S.; Aida, T.; Yamazaki, T.; Yamazaki, I.
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Sooambar, C. Chem. Commun. 2001, 2206. (d) Biemans, H. A. M.; Rowan,
A. E.; Verhoeven, A.; Vanoppen, P.; Latterini, L.; Foekema, J.; Schenning,
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† Yonsei University.
‡ Kyoto University and CREST of the Japan Science and Technology
Agency.
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J. AM. CHEM. SOC. 2004, 126, 16187-16198
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A R T I C L E S
Hwang et al.
Chart 1. Structures of D1-D3, D1H-D3H, T2H, and T2
ties, and functions.12 Inspired by the noncovalent nature of
natural photosynthetic systems, the self-assembly approach has
been increasingly attempted in the mimicry of light-harvesting
and charge separation systems.13 Among these, the coordination
interaction between zinc(II) porphyrin and pyridine is particu-
larly useful for its easy manipulation, relatively large association,
and a favorable tendency not to spoil the photo-excited-state
dynamics of porphyrins.14 Interesting examples so far reported
include oligomeric conjugated porphyrin ladders developed by
Anderson et al. that exhibit very large two-photon absorption
cross-section,15 energy-transfer and electron-transfer assemblies
reported by Hunter et al.,16 and giant porphyrin arrays and large
porphyrin wheels as a model of light-harvesting antenna reported
by Kobuke et al., where an imidazoyl substituent is used instead
of a pyridyl substituent.17 Spatial control of porphyrinic pigments
is crucial in supramolecular design of artificial photosynthetic
antennae, since it directly leads to control of the electronic
interactions between chromophores. In this respect, a precise
spatial control of porphyrin pigments with ample electronic
interactions still remains challenging. For use as light-harvesting
antennae, careful avoidance of energy sink that deactivates the
excited state is another important requirement.
Molecular self-assembly can translate the covalent connectiv-
ity and molecular shape of the components into tertiary structure.
In this context, the molecular component of a meso-meso-linked
diporphyrin18 is quite attractive because of its perpendicular
conformation, which may lead to unique architectures. In
addition, the two porphyrins in the component are strongly
coupled mainly with Coulombic interaction but not π-conjuga-
tion.18c This encourages the possibility that properly self-
assembled meso-meso-linked zinc(II) porphyrins serve as a
model for light-harvesting antennae.
Here we report self-assembly behaviors of meso-pyridine-
appended zinc(II) porphyrins M1-M3 and their meso-meso-
linked dimers D1-D3 (Chart 1). In noncoordinating solvents,
the monomeric porphyrins M1-M3 assemble into porphyrin
squares S1-S3, while the dimeric porphyrins D1-D3 self-
assemble into three-dimensional porphyrin boxes B1-B3 (Chart
2). Self-assembly formation of porphyrin squares has been
(12) (a) Lehn, J.-M. Science 2002, 295, 2400. (b) Hollingsworth, M. D. Science
2002, 295, 2410.
reported in other cases.19,20 Among these, we reported the first
X-ray crystal structure of S1.19 Formation of the porphyrin box
B1 is of interest in view of its large association constant. In
this study, we changed the size of the porphyrin box by inserting
one and two phenyl groups between the porphyrin and pyridyl
substituents. The three-dimensional zinc(II) porphyrin boxes
(13) (a) Hayashi, T.; Ogoshi, H. Chem. Soc. ReV. 1997, 26, 355. (b) Imamura,
T.; Fukushima, K. Coord. Chem. ReV. 2000, 198, 133. (c) Wojaczynski,
J.; Latos-Grazynski, L. Coord. Chem. ReV. 2000, 204, 113. (d) Chernook,
A. V.; Rempel, U.; van Borczyskowski, C.; Shulga, A. M.; Zenkevich, E.
I. Chem. Phys. Lett. 1996, 254, 229. (e) Flamigni, L.; Johnson, M. R. New
J. Chem. 2001, 25, 1368. (f) Hartnell, R. D.; Arnold, D. P. Organometallics
2004, 23, 391.
(14) (a) Hunter, C. A.; Sanders, J. K. M.; Beddard, G. S.; Evans, S. J. Chem.
Soc., Chem. Commun. 1989, 1765. (b) Anderson, S.; Anderson, H. L.;
Sanders, J. K. M. Acc. Chem. Res. 1993, 26, 469.
(15) (a) Wilson, G. S.; Anderson, H. L. Chem. Commun. 1999, 1539. (b) Taylor,
P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121, 11538. (c) Screen, T.
E.; Thorne, J. R. G.; Denning, R. G.; Bucknall, D. G.; Anderson, H. L. J.
Am. Chem. Soc. 2002, 124, 9712. (d) Screen, T. E.; Thorne, J. R. G.;
Denning, R. G.; Bucknall, D. G.; Anderson, H. L. J. Mater. Chem. 2003,
13, 2796.
(16) (a) Hunter, C. A.; Hyde, R. K. Angew. Chem., Int. Ed. Engl. 1996, 35,
1936. (b) Haycock, R. A.; Yartsev, A.; Michelsen, U.; Sundstro¨m, V.;
Hunter, C. A. Angew. Chem., Int. Ed. 2000, 39, 3616.
(17) (a) Ogawa, K.; Kobuke, Y. Angew. Chem., Int. Ed. 2000, 39, 4070. (b)
Ogawa, K.; Zhang, T.; Yoshihara, K.; Kobuke, Y. J. Am. Chem. Soc. 2002,
124, 22. (c) Takahashi, R.; Kobuke, Y. J. Am. Chem. Soc. 2003, 125, 2372.
(18) (a) Osuka, A.; Shimidzu, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 135.
(b) Aratani, N.; Osuka, A.; Kim, Y. H.; Jeong, D. H.; Kim, D. Angew.
Chem., Int. Ed. 2000, 39, 1458. (c) Kim, Y. H.; Jeong, D. H.; Kim, D.;
Jeoung, S. C.; Cho, H. S.; Kim, S. K.; Aratani, N.; Osuka, A. J. Am. Chem.
Soc. 2001, 123, 76.
(19) For a preliminary report of this work, see: Tsuda, A.; Nakamura, T.;
Sakamoto, S.; Yamaguchi, K.; Osuka, A. Angew. Chem., Int. Ed. 2002,
41, 2817.
(20) (a) Fleischer, E. B.; Shachter, A. M. Inorg. Chem. 1991, 30, 3763. (b)
Drain, C. M.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1994, 2313. (c)
Chi, X.; Guerin, A. J.; Haycock, R. A.; Hunter, C. A.; Sarson, L. D. J.
Chem. Soc., Chem. Commun. 1995, 2567. (d) Funatsu, K.; Kimura, A.;
Imamura, T.; Sasaki, Y. Chem. Lett. 1995, 765. (e) Fukushima, K.; Funatsu,
K.; Ichimura, A.; Sasaki, Y.; Suzuki, M.; Fujihara, T.; Tsuge, K.; Imamura,
T. Inorg. Chem. 2003, 42, 3187. (f) Stang, P. J.; Fan, J.; Olenyuk, B. Chem.
Commun. 1997, 1453. (g) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas,
J. Angew. Chem., Int. Ed. 1998, 37, 2344. (h) Merlau, M.; Mejia, M. de
P.; Nguyen, S. T.; Hupp, J. T. Angew. Chem., Int. Ed. 2001, 40, 4239. (i)
Mines, G. A.; Tzeng, B.-C.; Stevenson, K.; Li, J.; Hupp, J. T. Angew. Chem.,
Int. Ed. 2002, 41, 154. (j) Iengo, E.; Zangrando, E.; Minatel, R.; Alessio,
E. J. Am. Chem. Soc. 2002, 124, 1003. (k) Iengo, E.; Zangrando, E.; Alessio,
E. Eur. J. Inorg. Chem. 2003, 2371.
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Porphyrin Boxes Constructed by Self-Sorting Assembly
A R T I C L E S
Chart 2. Structures of S1-S3, B1-B3, and TB2a
a Only structures of (R)-isomers are shown for B2 and B3. Ar ) 3,5-dioctyloxyphenyl.
B1-B3 are well-defined discrete molecular entities that are
pertinent for the investigations of excitonic interactions and
excitation energy migration process. Thus, the excitation energy
migration processes within B1-B3 are comparatively investi-
gated, using steady-state absorption and fluorescence measure-
ments, the time-correlated single-photon counting technique
(TCSPC), time-resolved fluorescence anisotropy measurements,
and femtosecond transient absorption (TA) and transient absorp-
tion anisotropy (TAA) measurements. In the present study, Dn
molecules dissolved in strongly coordinating pyridine have been
taken as a reference of five-coordinated free meso-meso-linked
zinc(II) diporphyrin, and those dissolved in noncoordinative
CH2Cl2 have been used for the measurements of Bn. As detailed
later, the time-resolved fluorescence anisotropy decays have
shown relatively slow rotational diffusion times, while the
singlet-singlet exciton-exciton annihilation process in TA and
the anisotropy decay profile in TAA measurements have
identified and quantified the fast excitation energy hopping
processes within B1-B3.
respectively, using aldehydes 4 and 5 instead of 2. Suzuki-
Miyaura coupling of 6 and 7 with 4-pyridylboronic acid pinacol
ester (8)22 gave zinc(II) porphyrins M2 and M3 in 66% and
53% yields, respectively.
The meso-meso linkage in D1 was previously made by
Suzuki-Miyaura coupling of the appropriate partners, since Ag-
(I)-promoted dimerization of M1 failed to give D1 under the
standard coupling conditions.19 This time we found that Ag(I)-
promoted coupling reaction of M1 gave D1 by refluxing the
reaction mixture for a longer time. The Ag(I)-promoted coupling
reaction of M2 gave meso-meso-coupled diporphyrin D2 and
triporphyrin T2 along with higher oligomers. Since the separa-
tion of these oligomers was practically impossible due to serious
self-aggregation, these zinc(II) complexes were once demeta-
lated to the corresponding free-base porphyrin oligomers, which
were separated over preparative GPC to provide diporphyrin
D2H and triporphyrin T2H in 32% and 14% yields, respectively.
Similarly the Ag(I)-promoted coupling of M3 gave D3H in 20%
yield. Higher oligomers were formed but not isolated because
of their very poor solubility.
Results
Construction of Porphyrin Squares and Porphyrin Boxes.
Construction of porphyrin square S1 from M1 was indicated
by the characteristic 1H NMR spectrum of M1 in CDCl3 at [M1]
Synthesis of Porphyrin Blocks. Synthetic routes to M1-
M3 are summarized in Scheme 1. Porphyrin M1 was prepared
from the reaction of 3,5-dioctyloxybenzaldehyde (1) and
4-formylpyridine (2) with 2,2′-dipyrrylmethane (3).21 Similarly,
4-bromophenyl-appended and 4-bromobiphenyl-appended por-
phyrins 6 and 7 were prepared in 16% and 14% yields,
(21) Ka, J.-W.; Lee, C.-H. Tetrahedron Lett. 2000, 41, 4609.
(22) Li, W.; Nelson, D. P.; Jensen, M. S.; Hoerrner, R. S.; Cai, D.; Larsen, R.
D.; Reider, P. J. J. Org. Chem. 2002, 67, 5394.
9
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A R T I C L E S
Hwang et al.
Scheme 1. Synthesis of M1-M3
into T2H-a led to formation of insoluble and intractable
polymeric material, while T2H-b gave a soluble discrete zinc
complex that displayed a clear 1H NMR spectrum. The HR and
Hâ protons in the pyridyl substituents are observed at 2.82, 3.32,
6.09, and 6.28 ppm, hence suggesting the box formation. The
association of the meso-isomer leads to formation of a discrete
box assembly (TB2), while the association of the dl-isomer leads
to an infinite linear array. On the basis of this consideration,
T2H-a and T2H-b have been assigned, respectively, as dl-T3H
and meso-T3H isomers.
Interestingly, in the GPC analyses with CHCl3 as an eluent,
B1 exhibited a sharp elution band at a distinctly shorter retention
time than that of D1H, which indicated the preservation of a
discrete porphyrin box in the HPLC moving phase under highly
diluted conditions. Similarly, the porphyrin boxes B2 and B3
preserve the tertiary structures in the GPC moving phase. The
GPC eluting behaviors of M2, D2, and T2 and their corre-
sponding free-base counterparts M2H, D2H, and meso-T2H
are compared (Supporting Information). The retention times of
D2 and T2 are much shorter than those of D2H and meso-
T2H, indicating the box formation. On the other hand, the
retention time of M2 is almost the same as that of M2H,
indicating M2 exists as a monomer in the GPC moving phase.
The observed retention times reflect the molecular volumes of
these self-assembled entities.
Steady-State Absorption, Fluorescence, and Fluorescence
Excitation Anisotropy. Figure 1 shows the absorption and
fluorescence spectra of (a) D1-D3 in pyridine and (b) B1-B3
in CH2Cl2. The peak positions are summarized in Table 1.
The absorption spectra of Dn are characteristic of free meso-
meso-linked zinc(II) diporphyrin, exhibiting Soret bands (S0-
S2) that are split due to the exciton coupling between the
porphyrin units.18c,23 In the D1-D3 series, the Soret bands are
negligibly changed depending on the molecules, implying that
the meso-substituted peripheral pyridyl substituents do not affect
the exciton coupling between the porphyrin units. The fluores-
cence spectrum of D1 is broad with peaks at 629 and 667 nm,
which is also characteristic of five-coordinated free meso-meso-
linked zinc(II) diporphyrins. The diporphyrins D2 and D3
display similar broad fluorescence spectra, while the relative
intensity of the Q(0,0) fluorescence subband at short wavelength
increases in going from D1 to D2 and D3. As for other free
meso-meso-linked zinc(II) diporphyrins, the broad fluorescence
spectra of D1-D3 are interpreted in terms of a wide distribution
in the dihedral angle between the two porphyrins, since changes
in the dihedral angle influence the electronic interactions
between the two porphyrins.
On the other hand, the absorption spectrum of B1 (D1
dissolved in CH2Cl2) exhibits a split Soret band (λmax ) 426
and 448 nm), but its splitting width is much less than that of
free meso-meso-linked zinc(II) diporphyrins (e.g., the Soret
band of D1 in pyridine is split at 425 and 460 nm). This feature
is entirely concentration independent even at very low concen-
tration, ca. 10-7 M, which indicates a large association constant,
at least 1021 M-3. In contrast to those of the D1-D3 series, the
Soret bands of Bn exhibit a systematic spectral change. While
the high-energy Soret band remains at the same position, the
low-energy Soret band shifts to the blue in the order B3 < B2
> 3 mM, which revealed large upfield shifts for the pyridyl
protons. The X-ray crystal structure of S1 showed a mutually
coordinated square structure with nearly perpendicular dihedral
angles (89.1° and 90.9°) between the neighboring porphyrins.19
Almost quantitative formations of porphyrin squares S2 and S3
1
from M2 and M3 were both indicated by the characteristic H
NMR spectra in CDCl3 (δ(HR) ) 2.72 ppm and δ(Hâ) ) 5.97
ppm for S2 and δ(HR) ) 2.41 ppm and δ(Hâ) ) 5.70 ppm for
S3) (Supporting Information). On the basis of concentration-
dependent absorption changes, the association constants have
been estimated to be 1.4 × 1015 for S1, 1.8 × 1012 for S2, and
7 × 1011 M-3 for S3, indicating dependence of the association
constant on the length of the pyridyl side arm (Supporting
Information).
Quantitative self-assembly of D1 as its tetramer B1 was
1
confirmed by H NMR spectroscopy in CDCl3 and cold spray
ionization mass spectroscopy (CSI-MS).19 On the basis of the
practical concentration independence of its fluorescence spectral
shape (up to 1.0 × 10-8 M), the association constant of B1
was estimated to be at least >1025 M-3 in CHCl3.19
Formation of porphyrin boxes B2 and B3 from D2 and D3
1
was similarly indicated by H NMR spectra in CDCl3 (δ(HR)
) 2.78 ppm and δ(Hâ) ) 6.05 ppm for B2 and δ(HR) ) 2.79
ppm and δ(Hâ) ) 5.93 ppm for B3) (Supporting Information).
Typically, 1H NMR spectra of D3H and D3 (B3) in CDCl3 show
the characteristic large upfield shifts for the pyridyl side chain
of D3, supporting the formation of a discrete porphyrin box,
B3.
The free-base triporphyrin T2H was separated into two
diastereoisomers, T2H-a (fast eluting, 4%) and T2H-b (slow
eluting, 10%), through a silica gel column. Zinc(II) insertion
(23) Cho, H. S.; Song, N. W.; Kim, Y. H.; Jeoung, S. C.; Hahn, S.; Kim, D.;
Kim, S. K.; Yoshida, N.; Osuka, A. J. Phys. Chem. A 2000, 104, 3287.
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Porphyrin Boxes Constructed by Self-Sorting Assembly
A R T I C L E S
Table 1. Band Maxima of Absorption and Fluorescence Spectra of Dn and Bna
absorption (nm)
fluorescence (nm)
c
sample
solvent
Soret (high)b
Soret (low)b
∆ESoret
Q(1,0)
Q(0,0)
Q(0,0)
Q(0,1)
Φfd
D1
D2
D3
B1
B2
B3
pyridine
pyridine
pyridine
CH2Cl2
CH2Cl2
CH2Cl2
424.9 (23535)
426.2 (23463)
426.6 (23441)
426.3 (23457)
426.3 (23457)
426.6 (23441)
460.3 (21724)
460.4 (21720)
460.6 (21711)
447.9 (22328)
453.5 (22051)
456.6 (21901)
(1811)
(1743)
(1730)
(1129)
(1406)
(1540)
567
568
568
565
565
566
607
609
609
606
605
605
629
633
633
613
614
619
667
668
668
666
669
669
0.030
0.015
0.014
0.025
0.005
0.005
a The band position was determined by the Gaussian band-fitting process. b Values in parentheses are the absorption positions (cm-1). c ∆ESoret indicates
the Soret band splitting energy (cm-1). d Φf indicates the fluorescence quantum yield measured by excitation at 430 nm.
Figure 2. Steady-state fluorescence excitation anisotropy spectra of Dn
and Bn. The spectra were obtained using parallel and perpendicular
orientations between excitation and emission polarizations, at a fixed
emission wavelength of 610 nm.
Figure 1. Steady-state absorption (left) and fluorescence (right) spectra of
(a) Dn and (b) Bn. The fluorescence spectra were obtained using an
excitation wavelength of 430 nm, and the excitation wavelength dependence
on the fluorescence spectra was negligible.
spectra of the boxes B1-B3 are rather concentration indepen-
dent up to 1.0 × 10-8 M, from which the association constants
have been estimated to be at least 1024 M-3. These large
association constants have been interpreted in terms of effective
cooperativity, where eight pyridine-zinc ion coordinations work
simultaneously. Consistent with these results, the absorption
spectrum of TB2 exhibits Soret bands at 425.5 and 464.5 nm,
while T2 shows very split Soret bands at 424.5 and 480 nm
(Supporting Information).
The steady-state fluorescence excitation anisotropy spectra
were comparatively measured for D1-D3 and B1-B3 (Figure
2). This polarization anisotropy measurement is informative for
the excitation energy migration process, because the energy
migration between the same molecular units with different
orientations provides a depolarization channel.24 While D1-
D3 displayed common anisotropy profiles exhibiting negative
anisotropy in the high-energy Soret band and positive anisotro-
< B1, resulting in a progressive decrease in the energy
difference between the split Soret bands (∆Esoret) (Figure 1b
and Table 1). In contrast to the broad fluorescence spectra of
D1-D3, the fluorescence spectrum of B1 clearly shows a
vibrational structure (λmax ) 613 and 666 nm). We interpreted
the fluorescence spectrum of B1 in terms of a small electronic
interaction between the two porphyrins in the meso-meso-linked
zinc(II) diporphyrin component, which is brought about as a
rigidification of its perpendicular conformation upon box
formation. In going from B1 to B2 and B3, the Q(0,0)
fluorescence band is decreased in B2 and is red-shifted in B3,
while the Q(0,1) fluorescence subband at ∼670 nm is not
changed. In addition, the Stokes shifts are much less in B1-
B3 than those in D1-D3, probably reflecting more rigid
conformations in the former. These characteristic fluorescence
9
J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004 16191

A R T I C L E S
Hwang et al.
Table 2. Fitted Fluorescence Lifetimes and Anisotropy Decay
Parameters of Dn and Bna
anisotropy decay parametersc
fluorescence
lifetime
b (ns)
sample
solvent
τ
r0
Φ (ns)
D1
D2
D3
B1
B2
B3
pyridine
pyridine
pyridine
CH2Cl2
CH2Cl2
CH2Cl2
1.70 ( 0.02
1.79 ( 0.01
1.83 ( 0.01
1.61 ( 0.02
1.67 ( 0.01
1.75 ( 0.02
-0.046 ( 0.001
-0.061 ( 0.001
-0.053 ( 0.003
-0.029 ( 0.001
-0.030 ( 0.001
-0.034 ( 0.002
1.01 ( 0.24
1.39 ( 0.40
1.73 ( 0.50
4.69 ( 0.41
5.91 ( 0.61
9.30 ( 0.91
a An excitation wavelength of 420 nm was applied to all experiments.
The fluorescence lifetimes were obtained by averaging the fitted single
fluorescence lifetimes at several emission wavelengths. b Using the relation
I(t) ) A1 exp(-t/τ), where I(t) is the time-dependent fluorescence intensity,
A the amplitude, and τ the fitted fluorescence lifetime. The ø2 values of the
fittings were maintained as ∼1.0-1.5. c Using the relation r(t) ) r0 exp(-
t/Φ), where r(t) is the time-dependent fluorescence anisotropy [r(t) ) (I|(t)
- GI (t))/(I|(t) + 2GI (t)] and Φ the fitted rotational rise time.
as revealed by the absorption changes. The pyridine-induced
dissociation process was followed more clearly from the CD
spectral changes, where the Cotton effect was significantly
attenuated, less than one-tenth compared with those of B2.
Dissociated fraction I exhibited only the negative Cotton effect
at the high-energy Soret band (430 nm), and dissociated fraction
II exhibited the reversed Cotton effect (Figure 3b). The CD
spectra of optically separated B1 are similar to those of B2,
but the intensity at 450 nm is roughly doubled (Supporting
Information). The CD spectra of D1 are also similar to those of
D2.
Fluorescence Lifetime and Fluorescence Anisotropy De-
cay. The time-resolved fluorescence decays of Dn and Bn were
measured (Supporting Information), and their fitted fluorescence
lifetimes are tabulated in Table 2. The fluorescence decay
becomes slightly faster in going from Dn to Bn, and decreases
in the order D3 > D2 > D1, as well as B3 > B2 > B1 (see the
τ values in Table 2). The τ value, however, did not reflect the
energy migration process within Bn, because its time scale
(∼1.6-1.8 ns) was considerably slow in view of the close
proximity intermolecular coordination geometry of Bn. The
fluorescence lifetimes come from the different S1-state lifetimes
of the porphyrin units. The analogous S1-state lifetimes reflect
an avoidance of energy sink in the Bn boxes.
The fluorescence anisotropy decays of Dn and Bn were
measured, where the excitation of the high-energy Soret band
(λex ) 420 nm) was employed (Supporting Information). The
fitted decay parameters are also tabulated in Table 2. The
fluorescence anisotropy decay time constant (Φ) gradually
increased in the order D1 (1.0 ns) < D2 (1.4 ns) < D3 (1.7
ns), jumped to a level of 4.7 ns in B1, and further increased in
B2 (5.9 ns) and B3 (9.3 ns), reflecting an increasing molecular
volume of the box structure in this order. Therefore, the Φ
values have been interpreted to originate from the rotational
diffusion motion in solution. The fluorescence anisotropy decays
consistently revealed negative amplitudes in agreement with the
negative anisotropies at the high-energy Soret bands of the
steady-state excitation anisotropy spectra (Figure 2). The
excitation of the high-energy Soret band is known to induce a
coherent energy hopping within a meso-meso-linked zinc(II)
diporphyrin, which gives rise to a negative fluorescence
anisotropy.18c,23 Because this energy hopping proceeds on the
time scale of <200 fs, it cannot be probed by the TCSPC
system.23 Finally, the fluorescence excitation anisotropy decays
Figure 3. CD spectra of (a) B2 taken in CHCl3 and (b) D2 taken in CHCl3
and 20% pyridine. Absorption spectra taken under the same conditions are
shown at the bottom of each panel. The solid line is for the first fraction,
and the dotted line is for the second fraction.
pies in the low-energy Soret and Q bands,18c,23 B1-B3 showed
distinctly different characteristics. First, the absolute anisotropy
values over the entire absorption region are smaller in B1-B3.
Second, the anisotropy values decreased in the order B3 > B2
> B1, indicating the efficient excitation energy migration over
the Bn boxes, whose efficiency is enhanced as the size of the
porphyrin box decreases.
Optical Resolution of Chiral Porphyrin Boxes. Owing to
the different meso-aryl substituents, D1-D3 are all chiral, and
hence, two enantiomers are present in equal abundance in
solution, since a free rotation around the meso-meso linkage
is severely prohibited. Accordingly, the porphyrin boxes B1-
B3 are chiral and are formed by homochiral self-sorting
assembly of the respective (R)- and (S)-isomers of D1-D3. To
prove this, we attempted the optical resolution of the porphyrin
boxes against their noncovalent assembly nature. Actually the
optical resolution of B1 and B2 has been accomplished with a
chiral HPLC setup. Box B2 showed two bands (fractions I and
II) with comparable peak areas when chromatographed with
hexane/CH2Cl2 (62/38) as eluent (Supporting Information). The
two fractions were separated, both of which were found to be
identical to the original B2 in terms of the absorption and
fluorescence spectra, while the circular dichroism (CD) spectra
were perfect mirror images of each other (Figure 3a). Box B1
was separated similarly, and the CD spectra were also measured
(Supporting Information). In the case of B2, fraction I exhibited
Cotton effects, negative at 469 nm, positive at 454 nm, and
negative at 435 nm, and fraction II exhibited reverse Cotton
effects at the same positions (Figure 3a). Addition of pyridine
to a solution of chiral B2 led to dissociation into D2 fragments
(24) (a) Yatskou, M. M.; Koehorst, R. B. M.; van Hoek, A.; Donker, H.;
Schaafsma, T. J.; Gobets, B.; van Stokkum, I.; van Grondelle, R. J. Phys.
Chem. A 2001, 105, 11432. (b) Yatskou, M. M.; Donker, H.; Novikov, E.
G.; Koehorst, R. B. M.; van Hoek, A.; Apanasovich, V. V.; Schaafsma, T.
J. J. Phys. Chem. A 2001, 105, 9498.
9
16192 J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004

Porphyrin Boxes Constructed by Self-Sorting Assembly
A R T I C L E S
Figure 5. Transient absorption anisotropy decay profiles of Bn. The
transient absorption decays for parallel and perpendicular orientations
between pump and probe polarizations are included, and the fitted anisotropy
decay parameters are listed at the bottom of each panel. The transient
absorption anisotropy decay profiles of Dn are shown as insets in the upper
right corner of each panel. The pump and probe wavelengths were 570 and
490 nm, respectively.
Figure 4. Transient absorption decay profiles of Bn that include pump-
power dependence. The transient absorption decay profiles of Dn are shown
as insets in the upper right corner of each panel. In the experiments, the
pump and probe wavelengths are 570 and 490 nm, which are the Q band
excitation and induced absorption probe.
performed using three decay components (τ1, τ2, and τ3), where
the slowest decay components τ3 were fixed as the τ values in
the TCSPC measurements (Table 3). The TA decays of Bn were
interestingly dependent upon both the pump power and the
molecule. When the pump power was increased, the relative
contributions of the fast τ1 and τ2 components were enhanced,
as compared to that of τ3. In addition, both τ1 and τ2 values
systematically increased in the order B1 < B2 < B3 (Table 3).
The pump-power dependence on the decay is indicative of an
exciton-exciton annihilation process, because the intense
excitation or high density of photons generates two or more
excitons in one assembly unit, and then the recombination
between the excitons gives rise to a fast deactivation process.25,26
Figure 4 indicates that the exciton-exciton annihilation process
of Bn is due to the exciton-exciton recombination between
meso-meso-linked zinc(II) diporphyrins rather than zinc(II)
porphyrin monomers, because this process does not occur in
Dn and becomes slower as the distance between the meso-
revealed the excitation energy migration within Bn. The initial
r0 value of the fluorescence anisotropy decay became smaller
in going from Dn (∼-0.05) to Bn (∼-0.03), indicating the
presence of additional depolarization channels in Bn.
Femtosecond Transient Absorption and Transient Ab-
sorption Anisotropy. To investigate the fast excitation energy
migration processes within Bn, both TA including pump-power
dependence and TAA decays were measured (Figures 4 and
5), where the Q band excitation, i.e., λpump ) 570 nm, was
employed to avoid the involvement of S2-S1 relaxation.25 The
TA and TAA decays of Dn were also measured as references
(insets of Figures 4 and 5). Dn revealed slow TA decays that
are in agreement with the S1-state lifetimes (∼1.6-1.8 ns) found
in the TCSPC measurements, and concomitantly did not show
any anisotropy decay profiles in the time region of tens of
picoseconds (insets of Figures 4 and 5). On the other hand, Bn
showed relatively fast TA decays along with anisotropy rises
in the same time region, indicating fast depolarization processes.
The exponential fittings for the TA decays of Bn were
(26) (a) Trinkunas, G.; Herek, J. L.; Pol´ıvka, T.; Sundstro¨m, V.; Pullerits, T.
Phys. ReV. Lett. 2001, 86, 4167. (b) Trinkunas, G. J. Lumin. 2003, 102,
532. (c) Bru¨ggemann, B.; May, V. J. Chem. Phys. 2004, 120, 2325. (d)
Mu¨ller, M. G.; Hucke, M.; Reus, M.; Holzwarth, A. R. J. Phys. Chem.
1996, 100, 9537. (e) Bru¨ggemann, B.; Herek, J. L.; Sundstro¨m, V.; Pullerits,
T.; May, V. J. Phys. Chem. B 2001, 105, 11391.
(25) Bradforth, S. E.; Jimenez, R.; van Mourik, F.; van Grondelle, R.; Fleming,
G. R. J. Phys. Chem. 1995, 99, 16179.
9
J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004 16193

A R T I C L E S
Hwang et al.
Table 3. Transient Absorption Decay Parameters for Bn
excitation energy hopping between the covalently linked por-
phyrin monomer units proceeds on this time scale (<200 fs).18c,23
Depending on the Pump Powera
fitted decay timesb (ps)
pump power
Discussion
(mW)
τ1
τ2
τ3
Sample B1
One of the key requirements for molecular design of
photosynthetic models is a precise control of electronic interac-
tion between porphyrin-like chromophores, since the electronic
interaction is a key parameter in determining excitation energy
transfer and electron transfer. This control is often very difficult
in noncovalent assemblies, because of low chemical stability
and ambiguous spatial arrangement. This stands in sharp contrast
to strictly arranged chromophore arrays in natural antenna
systems.
1.0
0.5
0.2
3 (44%)
4 (46%)
4 (48%)
30 (40%)
30 (24%)
30 (14%)
1610 (16%)
1610 (30%)
1610 (38%)
Sample B2
1.0
0.5
0.2
9 (42%)
9 (42%)
10 (31%)
57 (39%)
60 (29%)
59 (22%)
1670 (19%)
1670 (29%)
1670 (47%)
Sample B3
1.0
0.5
0.2
24 (48%)
25 (37%)
26 (30%)
230 (47%)
228 (19%)
228 (16%)
1750 (5%)
1750 (44%)
1750 (54%)
Zinc(II) insertion into meso-pyridyl-appended porphyrins and
meso-pyridyl-appended meso-meso-linked diporphyrins drives
the formation of porphyrin squares and boxes, respectively.
Interestingly, box formation is accompanied by rigidification
of the perpendicular conformation of meso-meso-linked di-
porphyrin, leading to very precise fixation of the spatial
arrangement and hence the electronic coupling among porphy-
rins. While the association constant of porphyrin square Sn
depends on the length of the pyridyl arms, box formation
proceeds with an exceptionally large association constant
through simultaneous eight-point cooperative coordination,
which is practically not affected by the presence of one or two
1,4-phenylene spaces between the porphyrin meso-position and
4-pyridyl group. Owing to the large association constants, Bn
behave like a covalently linked single chemical entity in
noncoordinating solvents, which allows for the various steady-
state and ultrafast measurements as well as the optical separation
of B1 and B2.
a The pump and probe wavelengths are 570 and 490 nm, respectively.
b Using the relation ∆OD(t) ) A1 exp(-t/τ1) + A2 exp(-t/τ2) + A3 exp(-
t/τ3), where ∆OD(t) is the transient absorption intensity, A the amplitude
(noted in parentheses as the normalized percentage, i.e., [Ai/(A1 + A2
A3)] × 100), and τ the fitted decay time.
+
meso-linked zinc(II) diporphyrins increases in going from B1
to B2 and B3.
The TAA decays of Bn were fitted using one rising
component, i.e., 12 ps for B1, 25 ps for B2, and 89 ps for B3
(Figure 5, inset). Increasing TAA rising time in the order B1 <
B2 < B3 indicates that not energy hopping within a meso-
meso-linked zinc(II) diporphyrin but energy transfer between
neighboring meso-meso-linked zinc(II) diporphyrins is respon-
sible for this anisotropic rise. Here we note that the TAA of
Bn showed a single-exponential rising component, τr, even
though the TA of Bn revealed the two exciton-exciton
annihilation components τ1 and τ2. The discrepancy between
the exciton-exciton annihilation and the anisotropy depolar-
ization, however, has often been found in other multichro-
mophoric systems such as LH1 and LH2.26 In a multichro-
mophoric system, neither exciton-exciton annihilation nor
anisotropy depolarization time is directly ascribed to the
excitation energy hopping time, because they do not occur in a
donor-acceptor pair. The energy hopping time can only be
theoretically obtained by modeling the process.
The porphyrin boxes B1-B3 are formed via a homochiral
self-sorting assembly process of D1-D3, respectively. These
processes are aided by the dynamic nature of pyridine coordina-
tion toward zinc(II) porphyrin. Among many possible assembly
routes, the homochiral self-sorting process leads to a discrete
box, while the regularly alternate assembly of different enan-
tiomers of Dn leads to a linear infinite polymer network, and
nonspecific assembly gives rise to ill-defined oligomers or
polymers. The preferential formation of Bn may be understood
in terms of a favorable entropic gain toward other assembly
processes.
It is noteworthy that Bn displayed an anisotropy rise in the
TAA measurement, which converges to ∼0.1 at t ) ∞, whereas
Dn showed constant anisotropy of r0 ) r∞ ≈ 0, in the time
region of tens of picoseconds. Without rotational diffusion
Similar to other free meso-meso-linked zinc(II) diporphyrins,
Dn display split Soret bands due to exciton coupling.18c,23 The
Soret band of zinc(II) porphyrin monomer has two perpendicular
transition dipole moments, Bx and By, that are degenerate in a
simple monomer (Scheme 2, left). In a meso-meso-linked zinc-
(II) diporphyrin, however, parallel Bz dipole moments couple
effectively, whereas other dipole interactions should be virtually
zero because of an averaged perpendicular conformation
(Scheme 2, middle). Thus, the Soret bands of Dn are split into
a red-shifted Bz component and unperturbed Bx and By
components. It is known that the dihedral angle is not strictly
fixed at 90° but has a wide distribution of 90 ( 20°. This has
been taken as a cause of the broad Soret band of meso-meso-
linked zinc(II) diporphyrins. The electronic interaction should
be a minimum at exactly 90° and increase upon a decrease in
the dihedral angle. This was indeed confirmed by the examina-
tion of a series of permanently distorted 1,ω-dioxylmethylene-
strapped meso-meso-linked zinc(II) diporphyrins27 with dihedral
motion, the meso-meso-linked zinc(II) diporphryin has r∞
≈
0, because of its rodlike structure, and the zinc(II) porphyrin
monomer has r∞ ≈ 0.1, because of its disklike structure.25
Therefore, the TAA rise and r∞ ≈ 0.1 of Bn imply that the
excitation energy hopping process among the meso-meso-linked
zinc(II) diporphryins leads to an alteration of molecular
polarization from dimeric character to monomeric character.
To investigate the faster depolarization process within Dn
and Bn, the TAA measurements were performed in the time
region of hundreds of femtoseconds (Supporting Information).
The anisotropy decays consistently have decay times of <200
fs. In addition, the decay time becomes shorter in going from
Dn to Bn, as box formation occurs, where a more detailed
description of the decay time is not applicable, because of the
broad pump pulse (∼150 fs). According to our previous report
on the orthogonally linked zinc(II) porphyrin array, the coherent
9
16194 J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004

Porphyrin Boxes Constructed by Self-Sorting Assembly
A R T I C L E S
Scheme 2. Exciton Coupling in the Monomer, meso-meso-Linked Diporphyrin, and Porphyrin Box
angles of less than 90° and was used as a switching excitation-
energy-transfer component.28 In this context, it is interesting to
note that the boxes B1-B3 exhibit common features of a small
Soret band splitting and fluorescence spectrum with distinct
vibrational structure, which indicate the attenuation of the
electronic interaction within a meso-meso-linked zinc(II)
diporphyrin in comparison to those in normal meso-meso-linked
zinc(II) diporphyrins. We interpreted this attenuation in terms
of the rigidification of a strictly perpendicular conformation of
meso-meso-linked zinc(II) diporphyrin as a result of simulta-
neous multipoint coordinative interactions.
The CD spectra of optically active D1, B1, D2, and B2 are
quite informative on the orientations and mutual interactions
of the transition dipole moments. The CD spectra of optically
pure D1 and D2 exhibit weak bisignate exciton coupling at the
high-energy Soret band. This Cotton effect can be considered
to stem from the dipole coupling of Bx and By, which are
designated in Scheme 3, left, reflecting different meso-aryl
substituents (3,5-dioctyloxyphenyl versus 4-pyridyl or 4-py-
ridylphenyl). The relatively small CD intensity can be related
to a small different chiral environment caused by these different
substituents. According to the empirical exciton chirality
methods and related studies,29,30 the first eluting isomer with a
negative Cotton effect has been assigned as (R)-Dn, which has
a counterclockwise arrangement of appended pyridyl substitu-
ents. The Cotton effects are remarkably very enhanced in B1
and B2 at both the high- and low-energy Soret bands. In the
case of fraction I of B2, the first Cotton effect at 469 nm is
negative and the second one at 454 nm is positive, thus
indicating a counterclockwise arrangement of the transition
dipole moments involved. Among many complicated exciton
coupling interactions, these strong Cotton effects of B2 have
been assigned due to the coupling of coupled dipole moments
(Bz + Bz) with perpendicularly arranged Bx or By in the
neighboring diporphyrins in the porphyrin box formed from (R)-
D2. As shown in Scheme 3, the coupled dipole moments (Bz +
Bz) in the diporphyrin unit (III and IV) can interact strongly
with By(I) and Bx(VII) and weakly with Bx(V) and By(VI) but
cannot interact with Bz(I), Bx(II), Bz(II), Bz(V), Bz(VI), Bz(VII),
By(VIII), or Bz(VIII) due to the coplanar arrangements. Here it
is noted that the strongest interactions are apparently for the
coupling of (Bz + Bz)(III, IV) with By(I) and Bx(VII), and thus,
the spatial arrangements of these dipole moments are both
interpreted to be counterclockwise. This interpretation has been
supported by the CD spectra of B1, in which the corresponding
Cotton effects are increased by a factor of 2, since B1 has a
shorter distance between the dipole moments that are responsible
for the Cotton effects.
To explain the blue shift in the low-energy Soret band of Bn
(Figure 1b, left), the following excitonic dipole coupling scheme
(29) (a) Nakanishi, K.; Berova, N. In Circular Dichroism: Principles and
Applications; Nakanish, K., Berova, N., Woody, R. W., Eds.; VCH
Publishers Inc.: New York, 1994; p 361. (b) Mason, S. F. Molecular Optical
ActiVity and the Chiral Discriminations; Cambridge University Press:
Cambridge, 1982. (c) Harada, N.; Nakanishi, K. Acc. Chem. Res. 1972, 5,
257.
(27) (a) Yoshida, N. Osuka, A. Org. Lett. 2000, 2, 2963. (b) Yoshida, N.;
Ishizuka, T.; Osuka, A.; Jeong, D. H.; Cho, H. S.; Kim, D.; Matsuzaki, Y.;
Nogami, A.; Tanaka, K. Chem.sEur. J. 2003, 9, 58.
(28) Shinmori, H.; Ahn, T. K.; Cho, H. S. Kim, D.; Yoshida, N.; Osuka, A.
Angew. Chem., Int. Ed. 2003, 42, 2754.
(30) (a) Kurtan, T.; Nesnas, N.; Li, Y.-Q.; Huang, X.; Nakanishi, K. Berova,
N. J. Am. Chem. Soc. 2001, 123, 5962. (b) Pescitelli, G.; Gabriel, S.; Wang,
Y. K.; Fleischhauer, J.; Woody, R. W.; Berova, N. J. Am. Chem. Soc. 2003,
125, 7613. (c) Borovkov, V. V.; Lintuluoto, J. M.; Inoue, Y. J. Am. Chem.
Soc. 2001, 123, 2979.
9
J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004 16195

A R T I C L E S
Hwang et al.
Scheme 3. Structures of (R)-Dn and (R)-Bna
of LH1 and LH2, in which the exciton-exciton annihilation
was understood in terms of formation of a doubly excited state
as a consequence of a Fo¨rster-type incoherent energy-transfer
process upon the absorption of more than two photons in a single
molecular entity. The doubly excited state formed quickly
relaxes to the singly excited state.25,26 Consequently, the
observed exciton-exciton annihilation process of Bn is direct
evidence of the Fo¨rster-type incoherent energy hopping process
among many orthogonally associated zinc(II) diporphyrins. It
is noteworthy that the two fast annihilation components τ1 and
τ2 are observed in Bn, and the contribution of the latter is more
sensitive to the laser power (see the relative amplitudes in Table
3). It is well-known that the energy hopping process is a
migration-limited process rather than a trapping-limited process,
and the slowest exciton-exciton annihilation component de-
scribes well the Fo¨rster-type incoherent energy hopping process
over a multichromopore system.25,26
We have confirmed the excitation energy migration processes
within Bn, using the exciton-exciton annihilation and anisot-
ropy depolarization. Nevertheless, neither annihilation nor
depolarization time represents the straightforward energy hop-
ping times. To obtain the energy hopping times, a modeling of
energy migration and the simultaneous consideration of two
observables are necessary. The description of the exciton-
exciton annihilation and anisotropy depolarization has been well
developed for LH1 and LH2, which have cyclic multiporphyrin
structures.23,24,32 In LH1 and LH2, an explicit Fo¨rster-type
incoherent energy hopping model has been constructed assuming
a migration-limited character of exciton-exciton annihilation,
and a random walk formalism of anisotropy. In this context,
the analytical depolarization and exciton-exciton annihilation
times are connected with the excitation energy hopping time
by the following equations:25
a In (R)-Bn, coupled dipole moments (Bz + Bz) in diporphyrins III and
IV are coupled with By in porphyrin I, with Bx in porphyrin VII, and weakly
with Bx in porphyrin V and By in porphyrin VI.
is introduced. The Soret band splitting in the meso-meso-linked
zinc(II) diporphyrin is explained by simple excitonic dipole
coupling between zinc(II) porphyrin monomers.18c,23 In the case
of Bn, however, more complicated Soret band splittings are
expected, because various dipole-dipole excitonic interactions
are possible among eight mutually perpendicular porphyrin units
(Scheme 2, right). Among sixteen possible exciton coupling
states of Bn, only two states are transition-allowed (Supporting
Information).31 The low-energy Soret state indicates excitonic
dipole-dipole interaction among eight parallel transition dipole
moments along the z-axis (blue arrows in Scheme 2, right),
whereas the high-energy Soret state implies excitonic dipole-
dipole interaction among four parallel transition dipole moments
along the x- or y-axis (red or green arrows in Scheme 2, right).31
Using the reported transition dipole strength (9.5 ( 0.5 D)32 of
the Soret band of the pyridyl-substituted zinc(II) porphyrin
monomer (5-(4-pyridyl)triphenylporphyrin) and the porphyrin
center-to-center distances (8 × 8 × 10 Å3), the excitonic Soret
band splitting energy ∆ESoret of B1 was calculated to be ∼1112
cm-1, in good agreement with the experimental splitting value
of 1129 cm-1 (Supporting Information).31,33
The blue shift in the low-energy Soret band of B1 is due to
H-type dipole coupling between molecule I and molecules IV,
V, and VIII (Scheme 2, right). The H-type dipole coupling,
however, is expected to decrease in going from B1 to B2 and
B3, resulting in the small blue shift of the low-energy Soret
band. The excitonic Soret band splitting energies ∆ESoret of B2
and B3 were calculated by procedures similar to those used to
calculate ∆ESoret of B1, in which the different geometrical
parameters (8 × 14 × 14 Å3 for B2 and 8 × 18 × 18 Å3 for
B3) give rise to ∆ESoret values of ∼1635 and ∼1741 cm-1 for
B2 and B3, respectively, which are larger than ∼1112 cm-1 of
B1. The corresponding experimental values are 1406 and 1540
cm-1, obtained from Table 1. The differences between the
calculated values and the experimental ones may be rationalized
in terms of plausible conformational flexibilities in larger
porphyrin boxes.
τhopping
τhopping
τdepolarization
)
)
(1)
(2)
4(1 - cos2(2π/N)) 4(1 - cos2 R)
N2 - 1
24
τannihilation
)
τhopping
where N is the number of hopping sites and R the angle between
adjacent transition dipoles (Scheme 4, left). The number of
hopping sites N is defined by Nm/L, where Nm is the number of
molecules and L the exciton coherence length of the array.26
The τannihilation value in eq 2 is also defined as the slowest
exciton-exciton annihilation time assuming a migration-limited
character of the excitons.26
Now, we model the excitation energy migration process
within Bn, and simultaneously use the two different observables
to estimate the energy hopping times. The Bn boxes are regarded
as a cyclic multiporphyrin array that consists of four mutually
perpendicular meso-meso-linked zinc(II) diporphyrins. They
have eight mutually perpendicular porphyrin units; however,
the number of hopping sites is N ) 4, because the hopping
sites are four meso-meso-linked zinc(II) diporphyrins that have
an exciton coherence length of L ) 2 (Scheme 4, right).
Although the exciton coherence length of the orthogonally linked
zinc(II) porphyrin array is known to be L ≈ 4-5,9a,18c Bn has
an exciton coherence length of L ) 2, because of their
noncovalent linkages. To describe the random walk of the
anisotropy, the orientations of the molecular transition dipoles
The exciton-exciton annihilation, which depends on laser
power, has provided rich information about the excitation energy
migration processes within the natural light-harvesting antennae
(31) Hwang, I.-W.; Cho, H. S.; Jeong, D. H.; Kim, D.; Tsuda, A.; Nakamura,
T.; Osuka, A. J. Phys. Chem. B 2003, 107, 9977.
(32) Yatskou, M. M.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J. J. Phys.
Chem. A 2001, 105, 11425.
(33) (a) Kasha, M. Radiat. Res. 1963, 20, 55. (b) Kasha, M.; Rawls, H. R.;
El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (c) Scholes, G. D.;
Ghiggino, K. P. J. Phys. Chem. 1994, 98, 4580.
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16196 J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004

Porphyrin Boxes Constructed by Self-Sorting Assembly
A R T I C L E S
Scheme 4. Energy Hopping Model of Bn
are considered. As shown in Scheme 4, right, the transition
dipoles that are attributable to the change of anisotropy are
arranged in a rectangular cycle on the xy-plane; the energy
hopping process among the transition dipoles along the z-axis
does not change the anisotropy. Consequently, the anisotropy
decay profile of Bn reflects the energy hopping process in a
forward (black) or backward (gray) rectangular cycle that
consists of four mutually perpendicular transition dipole mo-
ments. This modeling indicates that the excitation energy
migration process within Bn can directly be described by eqs 1
and 2. Introducing R ) 90° and N ) 4, the relations τhopping
)
4τdepolarization and τhopping ) 1.6τannihilation are obtained for the Bn
boxes. As a consequence, the energy hopping times are
calculated to be 48, 100, and 356 ps in B1, B2, and B3 using
the anisotropy rise times, i.e., τr ) 12, 25, and 89 ps, given in
Figure 5. In a different approach, the energy hopping times are
estimated to be 48, 96, and 365 ps using the slowest exciton-
exciton annihilation components, i.e., τ2 ) 30, 60, and 228 ps,
listed in Table 3. It is noteworthy that the two different
observables, exciton-exciton annihilation and anisotropy de-
polarization, result in consistent excitation energy hopping times
within error ranges (48 ps in B1, 98 ( 3 ps in B2, and 361 (
6 ps in B3). This implies that the excitation energy migration
process within Bn is well described by the Fo¨rster-type
incoherent energy hopping model, because of their well-defined
and perpendicular orientations. The calculated energy hopping
times of Bn are also well expressed as a function of the distance
6
Figure 6. Energy hopping time (τEET) of Bn as a function of RDA
.
Conclusions
In noncoordinating solvent, Mn and Dn assemble to form
their tetrameric assemblies Sn and Bn, respectively. The
association constants of Bn are much larger than those of Sn
owing to double eight-point simultaneous coordinative interac-
tions. Bn are formed by a homochiral self-sorting assembly
process, which has been proved by the optical separation of B1
and B2. The features of exciton coupling within Bn have been
revealed by the examinations of the absorption spectra of Bn
and the CD spectra of the optically pure B1 and B2. The
porphyrin boxes Bn consist of four regularly arranged meso-
meso-linked zinc(II) diporphyrin subunits, which is favorable
for a model of light-harvesting antennae. In this regard, we have
investigated the excitation energy migration process within Bn.
The relatively small polarization anisotropy in the steady-state
fluorescence excitation spectra and the small initial anisotropy
value in the time-resolved fluorescence anisotropy decay reflect
the efficient excitation energy hopping processes. Both the
exciton-exciton annihilation and the anisotropy rise in femto-
second transient absorption and transient absorption anisotropy
describe well the different excitation energy hopping rates within
Bn. Modeling of the energy migration process within Bn with
N ) 4 and L ) 2 consistently gave excitation energy hopping
times of 48, 98 ( 3, and 361 ( 6 ps among the meso-meso-
linked zinc(II) diporphyrins for B1, B2, and B3, respectively.
These results indicate that the excitation energy migration
between adjacent meso-meso-linked zinc(II) diporphyrins
6
within Bn, i.e., τEET
RDA (Figure 6). Finally, using the
modeling, the anisotropy rise profiles of Bn that converge to
∼0.1 can be understood. Although the coherent energy hopping
process within the meso-meso-linked zinc(II) diporphyrins
results in r∞ ≈ 0, the energy migration process over the
rectangular cycle on the xy-plane can give rise to r∞ ≈ 0.1,
because this process proceeds among the transition dipoles of
zinc(II) porphyrin monomers. Similarly, the excitation energy
hopping process among the porphyrin monomers within LH1
and LH2 results in r∞ ≈ 0.1.25,34,35
(34) Jimenez, R.; Dikshit, S. N.; Bradforth, S. E.; Fleming, G. R. J. Phys. Chem.
1996, 100, 6825.
(35) Chachisvilis, M.; Ku¨hn, O.; Pullerits, T.; Sundstro¨m, V. J. Phys. Chem. B
1997, 101, 7275.
9
J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004 16197

A R T I C L E S
Hwang et al.
Time-Resolved Fluorescence Decay. A picosecond time-resolved
TCSPC system was used for the fluorescence decay and fluorescence
anisotropy decay measurements (Supporting Information).
Transient Absorption and Transient Absorption Anisotropy
Decay. A dual-beam femtosecond time-resolved transient absorption
spectrometer was employed for TA and TAA measurements (Supporting
Information).
processes within Bn are well described by the Fo¨rster-type
incoherent energy hopping model, because of their well-defined
arrangements and perpendicular orientations. Overall, the por-
phyrin boxes Bn are demonstrated to serve as a well-defined
light-harvesting model exhibiting excitation energy hopping rates
that depend on the vertical distance between the meso-meso-
linked zinc(II) diporphyrin subunits.
Acknowledgment. The work at Yonsei was financially
supported by the National Creative Research Initiatives Program
of the Ministry of Science and Technology of Korea (D.K.).
The work at Kyoto was partly supported by a Grant-in-Aid from
the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan, and the 21st Century COE Program Kyoto
University Alliance for Chemistry (A.O.). We thank Prof. H.
Tamiaki of Ritsumeikan University for his allowance for the
CD measurements.
Experimental Section
Steady-State Spectra. Solutions of the Dn diporphyrins were
prepared in approximately micromolar concentrations in pyridine and
CH2Cl2 solvents. All the solvents (∼99.9% purity) were purchased from
Merck Chemical Co. (HPLC grade). Absorption spectra were obtained
with a Shimadzu model 1601 UV spectrometer, and steady-state
fluorescence and excitation spectra were measured by a Hitachi model
F-4500 fluorescence spectrophotometer at room temperature. The
fluorescence quantum yield was obtained in comparison to the
fluorescence quantum yield of ∼1.0 of a Rhodamine 101 standard in
ethanol at room temperature.
Supporting Information Available: Experimental procedures
and characterization of the products. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA046241E
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16198 J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004