Construction of giant porphyrin macrorings self-assembled from thiophenylene-linked bisporphyrins for light-harvesting antennae

Article abstract of DOI:10.1002/chem.200801466

As a model of bacterial photosynthetic light-harvesting antenna, a large number of porphyrin units were organized into barrel-shaped macrorings. Two imidazolylporphyrinatozinc(II) molecules were linked through either unsubstituted thiophenes or 3,4-dioctylthiophenes la and lb, respectively. These structures were spontaneously organized by complementary coordination of the imidazolyl to zinc and produced a series of self-assembled fluorescent polygonal macrorings under high dilution conditions. The ring size increased compared with previous m-phenylene examples. The size distribution was also controlled by the presence of octyl substituents. A wide distribution of macrorings from 7- to > 15-mer was obtained from la, whereas macrorings ranging from 7- to 11-mer with a maximum population focused at the 8-mer were formed with lb. The size distribution was governed by competition between entropy-favored, smaller-ring formation and the enthalpy-favored, less-strained larger macroring. The UV/Vis spectra showed a gradual redshift for the larger rings reflecting an increase in the transition dipole interactions.

Full text of DOI:10.1002/chem.200801466

FULL PAPER
DOI: 10.1002/chem.200801466
Construction of Giant Porphyrin Macrorings Self-Assembled from
Thiophenylene-Linked Bisporphyrins for Light-Harvesting Antennae
Kaori Fujisawa,[a] Akiharu Satake,*[a] Shun Hirota,[a] and Yoshiaki Kobuke*[a, b]
Abstract: As a model of bacterial pho-
tosynthetic light-harvesting antenna, a
large number of porphyrin units were
organized into barrel-shaped macro-
rings. Two imidazolylporphyrinato-
zinc(II) molecules were linked through
either unsubstituted thiophenes or 3,4-
dioctylthiophenes 1a and 1b, respec-
tively. These structures were spontane-
ously organized by complementary co-
ordination of the imidazolyl to zinc
and produced a series of self-assembled
under high dilution conditions. The
ring size increased compared with pre-
vious m-phenylene examples. The size
distribution was also controlled by the
presence of octyl substituents. A wide
distribution of macrorings from 7- to
>15-mer was obtained from 1a, where-
as macrorings ranging from 7- to 11-
mer with a maximum population fo-
cused at the 8-mer were formed with
1b. The size distribution was governed
by competition between entropy-fa-
vored, smaller-ring formation and the
enthalpy-favored, less-strained larger
macroring. The UV/Vis spectra showed
a gradual redshift for the larger rings
reflecting an increase in the transition
dipole interactions.
Keywords: coordination modes
cyclization photosynthesis
porphyrinoids · self-assembly
·
·
·
fluorescent
polygonal
macrorings
Introduction
actions. This energy-cascade system allows efficient harvest-
ing of light energy to the reaction center. The ring structure
without any terminals is a key for intra- and inter-ring
energy migration by their degenerate energy levels.[7–9]
Construction of such antenna systems has long been a
challenging target in view of their importance in biological
energy transformation events. Porphyrin-based LH antennae
have occupied a central position in these efforts because of
their close similarity to the structural and photophysical
properties of chlorophylls. Various covalently[10,11] and non-
covalently[12–17] linked multiporphyrin systems have been
constructed, and extensively reviewed.[18–29] In the prepara-
tion of macrorings, the intramolecular cyclization of the
linear oligomeric species is important for suppressing poly-
mer formation. Template-assisted coupling reactions have
successfully afforded porphyrin macrorings,[30–37] whereas
other investigations have employed coupling reactions[38–42]
and self-assemblies by using coordination bonds[43,44] under
high dilution conditions.
Photosynthetic purple bacteria have developed the efficient
light-harvesting antenna systems[1–3] LH1[4,5] and LH2.[6] In
pigment array B850 of the LH2 complex, eighteen bacterio-
chlorophylls (Bchls) are arranged in a slipped cofacial barrel
structure supported by the protein matrix. Due to the close
packing of the neighboring Bchl units, the absorption maxi-
mum of the lowest excitation band of B850 is redshifted
from 800 nm, which is the lmax of the monomeric Bchl unit.
The absorption band of an even larger macroring of B870
(LH1) is shifted further to 870 nm by stronger exciton inter-
[a] K. Fujisawa, Dr. A. Satake, Prof. Dr. S. Hirota, Prof. Dr. Y. Kobuke
Graduate School of Materials Science
Nara Institute of Science and Technology (NAIST)
Takayama 8916-5, Ikoma, Nara 630-0192 (Japan)
Fax : (+81)743-72-6119
E-mail: satake@ms.naist.jp
We have developed a supramolecular methodology for
macroring formation by using complementary coordination
of two terminal imidazolylporphyrinatozinc(II) units con-
nected through an appropriate linker under dilute condi-
tions. Coordination equilibrium leads to the exclusive for-
mation of macrorings without production of linear byprod-
ucts. When a m-phenylene unit was used as the linker, pen-
[b] Prof. Dr. Y. Kobuke
Institute of Advanced Energy
Kyoto University
Gokasho, Uji 611-0011 (Japan)
Fax : (+81)774-38-4577
E-mail: kobuke@iae.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem200801466.
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tagonal and hexagonal macrorings were obtained in an
almost 1:1 mixture.[45,46] Interestingly, the hexagonal macror-
ings showed faster excitation energy hopping (EEH) rates
than the pentagonal structures.[47] The result suggested the
importance of the orientation factor of linker-separated
chromophore components in determining the EEH rates
and encouraged us to prepare larger macrorings.
Toward the noncovalent approach, only two examples of
the preparation of polygonal porphyrin rings larger than a
hexagon have been reported.[48,49] Unfortunately, these struc-
tures were nonfluorescent because cobalt–porphyrin[48] was
used or ferrocene[49] was directly connected to the porphy-
rin. Herein, we explored the use of a five-membered aro-
matic thiophene as the linker between bis(imidazolylpor-
phyrinatozinc(II)) molecules for making larger macrorings
with enlarged internal angles. Bisporphyrins 1a and 1b were
designed with the two porphyrin units connected through
thiophene as the 2,5-substituents. 3,4-Substituents were in-
troduced into the thiophene of 1b to control the ring size by
modulating the internal angle of the porphyrins.
Results
Synthesis of unit porphyrins: The synthetic schemes for 8a
and 8b are shown in Scheme 1. Condensation of dipyrrome-
thane 2 (2.0 equiv), monoacetal-protected thiophene-2,5-di-
carboxaldehyde 3 (1.5 equiv), and N-methylimidazolcarbox-
aldehyde 4 (1.0 equiv) in the presence of trifluoroacetic acid
gave acetal-protected thiophenylporphyrins 5a and 5b.
Acetal deprotection afforded 6a and 6b in yields of 5 and
4%, respectively, based on 4 and the second condensation
was then carried out by stepwise addition of dipyrromethane
2 and aldehyde 4. After neutralization with triethylamine
followed by oxidation with chloranil, compounds 8a and 8b
were obtained. Pure 8a was isolated by a combination of
SiO2 and gel permeation chromatographies (GPC) in a yield
of 10% based on 6a. Similarly, bisporphyrin 8b was ob-
tained in a yield of 6.5% based on 6b.
1H NMR spectra of 8a (Figure 1a) and 8b (Figure 1b) re-
vealed only one peak for 8a corresponding to a N-methyl
signal at d=3.44 ppm, and no atropisomer was observed. In
the case of dioctyl-substituted porphyrin 8b, however, three
N-methyl signals were observed
at
d=3.427,
3.422,
and
3.415 ppm, suggesting the pres-
ence of atropisomers (syn-anti
isomers; Figure 1d). These re-
sults indicate that the porphyrin
moieties for unsubstituted thio-
phenes rotate around the axis
of the thiophene–porphyrin
bond on the NMR spectroscopy
timescale, but no rotation
occurs with the dioctyl variant.
Similarly, the 1H NMR spec-
trum for 6b showed the pres-
ence of syn-anti isomers.
Scheme 1. Synthesis of thiophenylene-linked bisporphyrin.
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Construction of Giant Porphyrin Macrorings
FULL PAPER
Figure 1. 1H NMR (600 MHz) spectra of a) 8a and b) 8b in CDCl3 at RT. The inset shows an enlargement of N-methyl part. The proton assignments (c)
and atropisomers in 8b (d; A: anti-syn, B: syn-syn, C: anti-anti) are also shown.
Ring formation from 1a and 1b: Fundamental procedures
for ring formation based on strong complementary coordi-
nation of imidazolyl to zinc have been established in our
previous studies.[45,46,49–51] Thus, a dilute solution of bis(imi-
dazolylporphyrinatozinc(II)) units linked through an appro-
priate spacer was prepared in chloroform containing a small
amount of methanol or ethanol. After equilibrium was ach-
ieved, ring compounds were formed exclusively without
linear polymeric materials. The ring size should depend on
the internal angle between the two imidazolylporphyrinato-
zinc(II) units separated by the linker.
Zinc(II) ions were introduced into free-base bisporphyrins
8a and 8b to give 1a and 1b, respectively (Scheme 2). Com-
plementary coordination spontaneously started to give the
initial gel permeation chromatogram at t=0 (Figure 2).
Then, each sample of 1a or 1b was diluted to 10 mm by the
addition of chloroform containing 0.5% ethanol. In the case
of 1b, 1% methanol (v/v) was added to accelerate the reor-
ganization. The solutions were warmed to 478C. In a typical
GPC analysis of the progress of the reorganization for 1a
(Figure 2a), the initial peak maximum appeared at 8.4 min,
corresponding to 52000 Da, as estimated from polystyrene
standards. During the course of reorganization, the peak
maxima shifted progressively to longer retention times cor-
responding to increasingly smaller molecular weights. After
2.5 h, the shift of the peak maximum became significantly
slower, but the reorganization still proceeded, as determined
by the small shift and sharpening of the peak. By 18 h, the
progress had almost stopped at the retention time for the
peak maximum at 11.3 min, corresponding to an apparent
mass of 11000 Da; no further change was observed after
24 h, suggesting the final convergence (Figure 2a). For 1b,
the chromatogram after 18 h showed the peak at 11.7 min
(Figure 2b), which was identical to that at 24 h and suggest-
ed that final convergence had been achieved. The observed
behavior is common to the reorganization processes of
other m-phenylene-linked bis(imidazolylporphyrinatozin-
c(II)) structures[45] and the converged compounds suggest
macroring formation.[45,49,51,52] The whole reorganization pro-
cesses may be illustrated as shown in Scheme 2.
Since coordination-organized supramolecules give only
dissociated ionic species in the mass spectrum, the molecular
weight must be obtained after covalent linking of coordina-
tion pairs through a ring-closing metathesis reaction[46] with
the use of the Grubbs catalyst to produce the converged
sample of 1b (Scheme 3). In a MALDI-TOF mass spectrum
acquired after the metathesis reaction of the crude con-
verged samples, defined as C-(1b)mix to differentiate from
samples before metathesis, N-(1b)mix, 7- to 9-mers were ob-
served as the major products with lesser peaks of 10- and
11-mer also observed (Figure 3a). Fractions 1–4 of the meta-
thesis product, C-(1b)mix, were isolated by recycling GPC
using pyridine as the eluent (see Figure S1 in the Supporting
Information, fractions 1–4). The mass spectra of fractions 1–
4, which contained oligomers ranging from 7- to 10-mer,
gave a series of peak maxima that agreed well with the cal-
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A. Satake, Y. Kobuke, et al.
Scheme 2. Synthesis, self-assembly, and reorganization of 1a and 1b. Allyloxy propyl groups (R’) at the meso-position are omitted.
culated[53] molecular weights ([M+H]+) for C-(1b)n within
an error range of ꢀ2 Da (Figure 3b–e). The series of com-
pounds thus proved to be composed of macroring structures.
The size distribution of a mixture before metathesis (N-
(1b)mix) yields important information about the effect of the
internal angle on the ring size. The distribution of C-(1b)mix
may not be precise because although the metathesis reaction
proceeds very well it is not quantitative. Therefore, data
from the N-(1b)mix, showing two peaks and two shoulders in
twice-recycled GPC, were separated into fractions a–d. Frac-
tions a–d were assigned as 7- to 10-mers by comparing the
retention times with the calibration line prepared by C-
(1b)7–10 (Figure 4). Then, the recycling GPC chart of N-
(1b)mix was deconvoluted (Figure 5a). The peaks were ap-
proximated by a Gaussian function to give 7- to 11-mers as
major macrorings with a maximum at 8-mer: 7-mer=(27ꢀ
2), 8-mer=(36ꢀ2), 9-mer=(18ꢀ1), 10-mer=(11ꢀ2),
larger than 11-mer=(7ꢀ3)%. We also tried to estimate the
ring sizes and the distribution of the converged sample of
1a, N-(1a)mix (Figure 5b). At least ten peaks were partially
separated, with significant overlapping of the other peaks.
Therefore, the section of the chromatogram showing partial
separation (retention time ꢁ130 min) was deconvoluted,
and the higher oligomer section (40%) was ignored. Each
peak was assigned as 7- to >15-mer by comparing the reten-
UV/Vis absorption and fluorescence spectra: The UV/Vis
absorption spectra of a series of rings, C-(1b)7–10, were mea-
sured in pyridine (Figure 6). Even in strongly coordinating
solvents, such as pyridine, covalently linked macrorings
retain their cyclic structures.[46,47,51,53] The peak maxima of
the split Soret bands at longer wavelengths were gradually
redshifted in the order 7-<8-<9-<10-mer (insets of
Figure 6).
The fluorescence quantum yields (FF) of N-(1b)7–10 in
chloroform and C-(1b)7–9 in pyridine showed similar values
(0.015ꢀ0.008), which were about half of that of the unit
dimer 11.
Discussion
Macroring formation: If monomeric terminal units exist in
the converged N-(1b)mix, the size distribution of the mono-
mers in N-(1b)mix must depend on the concentration.[48,54]
We analyzed the samples of N-(1b)mix by GPC at two differ-
ent concentrations (80 and 10 mm), but obtained almost iden-
tical elution curves. More decisively, the mass spectra ob-
tained after the covalent linking gave the correct molecular
weights[53] calculated based on ring structures (Figure 3a–d).
We therefore concluded that converged N-(1b)n and its co-
valently linked analogue C-(1b)n are cyclic structures.[46,49,51]
tion times with the calibration line prepared by C-(1a)9–11
.
When evaluated in this manner, the size distribution of N-
(1a)7–15 was much wider: 7-mer=(12ꢀ1), 8-mer=(12ꢀ1),
9-mer=(13ꢀ1), 10-mer=(12ꢀ1), 11-mer=(11ꢀ1), 12-
mer=(10ꢀ1), 13-mer=(9ꢀ1), 14-mer=(8ꢀ1), larger than
15-mer=(10ꢀ2)%.
Rotation around the axis of the thiophene–porphyrin bond:
Observation of atropisomers in 6b and 8b indicated that
syn-anti isomerization was slowed by the introduction of
octyl groups. Based on variable-temperature NMR spectros-
copy studies performed in (CDCl2)2, the rate constant (kr)
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Construction of Giant Porphyrin Macrorings
FULL PAPER
Scheme 3. Covalent linking of N-(1b)n. Allyloxy propyl groups (R’) at
the meso-position.
Figure 2. GPC analyses of the reorganization process. a) 1a in chloro-
form: samples of 1a at various time intervals, from left to right, 0 (dotted
line), 0.5, 1, 1.5, 2.5, 5.5, 7.5, 18 (solid bold line), and 24 h (solid line).
b) For 1b in chloroform with 1% (v/v) methanol: 0 h (dotted line), after
18 h (solid bold line). Open triangles at the tops of graphs represent the
maximum peak position of polystyrene standards. Conditions: column:
JAIGEL 3H-A (polystyrene gel, diameter=8 mm, length=50 cm, exclu-
sion limit=70000 Da); eluent, CHCl3/0.05% Et3N.
and the activation free energy (DG°) of syn–anti isomeriza-
tion in 6b were estimated (Figure S2 in the Supporting In-
formation) to be kr(323 K) =12.0 sꢂ1 and kr(383 K) =28.0 sꢂ1, and
DG°323 K =72.5 kJmolꢂ1 and DG°383 K =83.8 kJmolꢂ1, respec-
tively. The DG°
value of 67.9 kJmolꢂ1 obtained from the
298 K
Eyring plot is slightly less than that of the meso-phenyl
group in the substituted imidazolyl porphyrin (DG°
=
298 K
71.1 kJmolꢂ1). Since the rotational energy of the meso-imi-
dazolyl group on the porphyrin is higher than that of the
meso-phenyl group,[55] the rotation around the porphyrin–di-
octylthiophenylene bond must account for the presence of
atropisomers rather than rotation about the imidazolyl
moiety. Consistent with this proposal, no atropisomer was
observed for 6a at 298 K, suggesting rapid rotation.
Figure 3. Mass spectra of a) C-(1b)mix and fractions 1 (b), 2 (c), 3 (d), and
4 (e); values indicate observed maximum peak number.
Although the conformational restriction was observed in
1
the H NMR spectrum of 8b based on the following obser-
confirming that isomerization among atropisomers occurs
after separation. Because reorganization of 1b usually re-
quires more than 10 h, the presence of atropisomerization
does not introduce any barriers for ring formation.
With respect to slow rotation around the axis connecting
the porphyrin and thiophene, structural isomers arise from
vations, rotation should occur under reorganization condi-
tions. Atropisomers of 8b could be detected as three spots
on a silica gel TLC plate. When any one of the spots were
isolated by column chromatography on SiO2, the collected
fraction showed three spots upon repeated TLC analysis,
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A. Satake, Y. Kobuke, et al.
Figure 6. UV/Vis spectra of C-(1b)7 (grey dashed line), C-(1b)8 (solid
line), C-(1b)9 (dotted line), and C-(1b)10 (grey line), in pyridine at RT.
The inset shows an enlargement of the Soret band at longer wavelengths.
as ring E contains one in-out (i-o) geometry and is repre-
sented by i-o, o-i, o-i, o-i, o-i, o-i, and o-i, from the right-
hand edge (Figure 7). These isomers could not be distin-
guished from each other by GPC analysis.
Figure 4. a) Logarithmic plots of the molecular weights against the reten-
tion time from Figure S1b in the Supporting Information. &: C-(1b)7–10
c: the calibration line for a series of C-(1b)n. b) Analytical GPC chro-
matograms of fractions a–d of separated N-(1b)mix. The conditions were
the same as those given in Figure 2. Fractions a–d were assigned as the 7-
to 10-mers by the calibration line given in (a).
,
Size population of reorganized macrorings for 1a and 1b:
Introduction of two octyl groups at thiophene b positions
may significantly affect the molecular structure. Molecular
orbital calculations (AM1)[56] showed that the torsional
angles between porphyrin and thiophene are 64 and 548 for
1a, but were 90 and 898 for 1b (Figure S3 in the Supporting
Information). The internal angles between two porphyrins
were 1548 for 1a and 1488 for 1b. Introduction of two octyl
groups induces not only an orthogonal conformation, but
also a smaller internal angle. The internal angle of 1a was
decreased to 1518 when two porphyrins were located orthog-
onally to the thiophene moiety. Thus, the internal angles
may vary between 151 and 1548 relative to the tilt motion
and widen the distribution for 1a. In addition, the internal
angle difference between the cyclic n-mer and the n+1-mer
decreases in larger polygons, and thus the stability differen-
ces become smaller. This factor would also contribute to the
wider size distribution of the macroring N-(1a)mix
.
The calculated internal angle (1488) of the unit bispor-
phyrin 1b suggests that the cyclic 11-mer is the macroring
with the least steric strain. Experimentally, the distribution
is shifted towards macrorings of smaller unit numbers. To
obtain a quantitative understanding, Gibbs energy differen-
ces were evaluated for equilibria between the 8-mer and the
cyclic n-mer [n=7, 9, and 11; Equilibria (a)–(c)].
8-mer Ð 7-mer
8-mer Ð 9-mer
8-mer Ð 11-mer
ðaÞ
ðbÞ
ðcÞ
Figure 5. Deconvolution analyses of recycling GPC charts for a) N-
(1b)mix, b) N-(1a)mix. Solid bold line: experimental data, solid line: decon-
voluted peaks, grey line: sum of the deconvoluted peaks. In this analysis,
40% of the higher molecular weight polymer (>15-mer) was ignored.
The enthalpy changes for the respective equilibrium of
the 8-mer and cyclic n-mer were calculated from the heats
of formation (Hf) by semi-empirical molecular orbital meth-
ods, AM1.[56,57] To facilitate the calculation, substituents on
out and in coordination.[55] Two of the possible eight geo-
metrical isomers are depicted schematically for the 7-mer in
Figure 7. Ring D only contains out-in (o-i) geometry, where-
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Construction of Giant Porphyrin Macrorings
FULL PAPER
UV/Vis spectra: The origin of
the split Soret bands in 1a, 1b,
and the macrorings may be ex-
plained on the basis of excitonic
coupling theory, in a manner
similar to earlier studies.[59,60]
The degenerated transition
dipole moments are split as a
result of slipped cofacial dimer
formation. In Figure 6, the peak
maxima of the longer Soret
bands were systematically red-
shifted in the order of increased
ring sizes. A similar tendency
Figure 7. Two of eight possible macroring structures of C-(1b)7. Ring D is composed of seven Aꢂs and ring E is
composed of five Aꢂs, one B, and one C.
the porphyrin at the meso-positions and on the thiopheny-
lene were replaced by protons and methyl groups, respec-
tively. The entropy changes were estimated by using a
model based on association for an aggregate of rigid parti-
cles.[58] The estimated thermodynamic parameters were
listed in Table 1 and plotted in Figure 8 as a function of the
internal angle of the n-mer. The entropy change per unit
bisporphyrin from 8-mer to cyclic n-mer decreases almost
linearly for larger macrorings and favors entropically smaller
macrorings. On the other hand, smaller macrorings are dis-
favored enthalpically. Calculated Gibbs energies (8-merꢃ7-
mer<9-mer<11-mer) of formation per unit bisporphyrin
successfully reflected the experimental population of the
macrorings (8-mer<7-mer<9-mer<10-mer<11-mer).
was observed for pentagonal and hexagonal macrocyclic
porphyrins.[47,51,60] In the present case, a more generalized
treatment may be required by including interactions among
not only the neighboring porphyrins, but also with other
component porphyrins in the macrorings.
Calculation of the interaction energies was undertaken as
follows: A macroring was placed in the center of x–y coordi-
nate. If the electric field of the input light comes from the x-
axis direction, only the x component of each transition
dipole interacts with the light. All of the transition dipoles,
mm, are divided into x and y components, mmx and mmy (see
Figure S4, left in the Supporting Information). The total ex-
citonic coupling energy for all of the porphyrins in each
macroring can be calculated by using Equation (1).[59,61]
X
2mmxmnxkmx,nx
E ¼
ð1Þ
R3mx,nx
n,mðm>nÞ
Table 1. Gibbs free energy change (DG) per unit porphyrin in Equili-
bria (a)–(c).
Equilibrium
DH [kcalmol]
TDS[a] [kcalmol]
DG[b] [kcalmolꢂ1
]
in which mmx and mnx are the x components of the transition
dipole moments of m and nth complementary dimer units,
Rmx,nx is the center-to-center distance between mmx and mnx,
and kmx,nx is the orientation factor of mmx and mnx. It is de-
fined by Equation (2) and Figure S4, right in the Supporting
Information.
a
b
c
6.3
ꢂ2.0
ꢂ3.2
5.8
ꢂ4.5
0.6
2.6
8.0
ꢂ11.2
[a] T=320 K. [b] DG=DHꢂTDS.
kmx,nx ¼ 1ꢂ3cos2q
ð2Þ
If the scalar of the transition dipoles, jmm j, is defined as
m0, the total coupling energies are expressed as shown in
Table 2. The coupling energy increases with increasing ring
size. Interestingly, the larger ring is associated with the
larger coupling interactions. Faster rates of EEH for the
hexagons rather than for the corresponding pentagons have
Table 2. The total excitonic coupling energy E in the 7- to 12-mer esti-
mated from Equation (1).
2
Ring size (n-mer)
Eꢁ106 (mo )
7
8
9
10
11
12
ꢂ1451
ꢂ1810
ꢂ1981
ꢂ2299
ꢂ2491
ꢂ2788
Figure 8. Approximate TDS (solid line, T=320 K), DH (dotted line), and
DG (bold line) changes from the 8-mer to the n-mer per coordination
dimer for 7- to 11-mers as a function of the internal angle of the n-mer.
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A. Satake, Y. Kobuke, et al.
already been observed[47,51,60] and measurements of the EEH
rates for a series of systematic ring variations of this study
may extend this tendency. Further studies are ongoing.
2,5-Bis(15-N-methylimidazolylporphyrinatozinc(II))-3,4-dioctyl-thiophene
(1b): A solution of zinc acetate dihydrate (4.0 mg, 18.2 mmol) in metha-
nol (0.5 mL) was added to a solution of 8b (2.1 mg, 1.4 mmol) in chloro-
form (1.0 mL). The mixture was stirred at RT for 3.5 h. Incorporation of
the zinc(II) ion was confirmed by UV/Vis and fluorescence spectroscop-
ies. A saturated aqueous solution of NaHCO3 (1.0 mL) was added to the
mixture. The organic layer was washed with water, dried over anhydrous
Na2SO4, and evaporated to dryness. The residue was purified by repreci-
pitation from chloroform and n-hexane to afford 1b (2.0 mg, 90.1%).
The crude porphyrinatozinc(II) compounds were analyzed by GPC (Fig-
ure 2b, dotted line). The peak maximum appeared at 9.1 min, corre-
sponding to 44000 Da estimated from polystyrene standard. MS
(MALDI-TOF, dithranol): m/z: 1604.9 [M+H]+ (Figure S18a).
Conclusion
Bis(imidazolylporphyrinatozinc(II)) compounds 1b and 1a
linked through either 3,4-dioctylthiophenylene or unsubsti-
tuted 2,5-thiophenylene units were synthesized. They were
linked by complementary coordination of imidazolyl to zinc
and produced a series of self-assembled fluorescent polygo-
nal macrorings larger than hexagons under the appropriate
reorganization conditions. The macroring size was controlled
by the internal angles between the two porphyrins linked
through thiophenylene and also by the introduction of the
octyl groups. The ring size distribution was rationalized by
the balance between favorable entropy and enthalpic insta-
bility due to angle strain for the smaller rings. A very wide
distribution of macrorings from 7- to >15-mer was obtained
from unsubstituted bisporphyrin 1a, whereas for 1b, the
macroring distribution was limited to 7- to 11-mer, with the
maximum population centering at the 8-mer. After covalent
linking of coordination pairs, cyclic 10-mer (C-(1b)10), 9-mer
(C-(1b)9), 8-mer (C-(1b)8), and 7-mer (C-(1b)7) were isolat-
ed in a pure form through recycling GPC. In the UV/Vis
spectra, the longer Soret bands in the polygonal macrorings
were gradually redshifted as the ring size increased. This ob-
servation was supported by a gradual increase of the cou-
pling interactions of porphyrin transition dipoles.[47,51]
2,5-Bis(15-N-methylimidazolylporphyrinatozinc(II))thiophene (1a): Com-
pound 1a was obtained according to a procedure similar to that for 1b
(3.6 mg, 81.5%). MS (MALDI-TOF, dithranol): m/z: 1380.8 [M+H]+
(Figure S18b).
Reorganization of 1b: A solution of 1b (0.8 mg, 0.49 mmol) in chloro-
form/1% MeOH (v/v; 50 mL) was kept at 478C in the dark. After 18 h,
the solution was cooled to ꢂ408C, and then evaporated at 08C under re-
duced pressure (ca. 10 hPa) to give N-(1b)mix (Figure 2b). The solution
was divided into several portions (less than 50 mL) and evaporated, since
the use of larger volumes significantly changed the size distribution. The
sample (N-(1b)mix) was separated by recycling GPC (JAIGEL 3HA,
eluent: chloroform/0.05% Et3N) to give fractions a–d. The separated sol-
utions were analyzed by analytical GPC (Figure 4b).
Reorganization of 1a: According to a procedure similar to that for 1b,
compound 1a was reorganized by using
a solution of 1a (0.5 mg,
0.36 mmol) in chloroform containing 0.5% ethanol (36 mL) (Figure 2a).
The sample (N-(1a)mix) was separated by recycling GPC (JAIGEL 3HA,
eluent: chloroform/0.05% Et3N). The seven separated fractions were an-
alyzed by analytical GPC.
Metathesis reaction of N-(1b)mix: The ring-closing metathesis reaction
was carried out for the reorganized sample (N-(1b)mix
) (1.4 mg,
0.88 mmol, in dichloromethane (10 mm)) by using the first generation
Grubbs catalyst (0.72 mg, 0.88 mmol). The reaction progress was moni-
tored by GPC analysis (Tosoh; eluent: pyridine). After 5 h the metathe-
sized sample (C-(1b)mix) was analyzed by MALDI-TOF mass spectrome-
try (Figure 3a). From the mixture of C-(1b)mix, 7-, 8-, 9-, and 10-mer mac-
rorings were isolated by recycling GPC with pyridine as the eluent using
two series columns (Tosoh) (Figure S1 in the Supporting Information).
The total amount of C-(1b)7, C-(1b)8, C-(1b)9, and C-(1b)10 was 0.5 mg.
These samples were reanalyzed by analytical GPC (Figure S1b in the
Supporting Information) to prepare the calibration line (Figure 4a). MS
(MALDI-TOF, trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-
malononitrile): m/z: 10832 (C-(1b)7), 12385 (C-(1b)8), 13940 (C-(1b)9),
15491 (C-(1b)10) [M+H]+.
Experimental Section
General: Analytical GPC was performed on a Hewlett-Packard HP1100
series instrument using a JAIGEL 3H-A column (Japan Analytical Indus-
try, polystyrene gel, diameter=8 mm, length=50 cm, exclusion limit=
70000 Da, eluent: CHCl3/0.05% Et3N) or a Shimadzu LC workstation
M10 equipped with an SPD-M10 AVP photodiode array detector using a
Tosoh TSK-GEL G3000HHR column (polystyrene gel, exclusion limit=
60000 Da). Recycling GPC was carried out on a recycling GPC–HPLC
system (Japan Analytical Industry, LC-908) connected with two series
columns (JAIGEL 3HA, diameter=20 mm, length=60 cmꢁ2, polystyr-
ene, exclusion limit=70000 Da, eluent: CHCl3/0.05% Et3N), or two
series columns of Tosoh TSK-GEL G3000HHR (polystyrene gel, exclusion
limit=60000 Da, eluent: pyridine). Column chromatography was per-
formed using silica gel (silica gel 60N (spherical, neutral) 63–210 mm,
KANTO) and aluminum oxide (aluminum oxide 90 active basic (0.063–
0.200 mm), Merck). Preparative GPC was performed on a glass column
(diameter=1 cm, length=100 cm) packed with Biobeads SX-3 (BioRad,
polystyrene, exclusion limit=2000 Da; flow rate: ca. 0.8 mLminꢂ1) using
toluene as the eluent. MALDI-TOF mass spectra were measured on
KRATOS AXIMA and Bruker autoflex II instruments with dithranol
(Aldrich) or trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]ma-
lononitrile (Fluka) as a matrix.
Estimation of macroring compositions from the recycling GPC chart of
N-(1b)n: The deconvolution analyses of GPC chromatograms were con-
ducted with the Origin Pro 7 software (OriginLab) with the peak fitting
module using the Gaussian function. The recycling GPC chart of N-(1b)n
(Figure 5a) was analyzed by using the initial parameters in Table S1 in
the Supporting Information. Five components (n=7–11) were prepared
as the initial set, their peak positions being adjusted manually to fit the
observed peaks. Half-band widths of 7- to 11-mers were set as (1.5ꢀ
0.5) min. This analysis was carried out five times to give the following
compositions: 7-mer=(27ꢀ2), 8-mer=(36ꢀ2), 9-mer=(18ꢀ1), 10-
mer=(11ꢀ2), and larger than 11-mer=(7ꢀ3)%. Similarly, deconvolu-
tion analysis of recycling GPC chart of N-(1a)n was carried out (Fig-
ure 5b). For this analysis higher oligomers (40% of the total) were ignor-
ed.
Estimated sum of interactions for all of the transition dipoles: To esti-
mate the sum of interactions among all of the transition dipoles, a mac-
roring was placed in the center of x–y coordinate. The numbers were
posted to the transition dipoles clockwise as m1, m2, and so forth. All of
the transition dipoles, mm, were divided into x and y components, mmx
and mmy. The center-to-center distance between the m and nth transition
dipoles was estimated mathematically by using the length of the bispor-
Synthesis of porphyrins: meso-(3-Allyloxypropyl)dipyrromethane 2[53]
and 1-methylimidazol-2-carboxaldehyde 4[62] were synthesized according
to reported procedures. Synthesis of 3,4-dioctylthiophene (S1), 3,4-di-
octylthiophene-1-carbaldehyde (S2), 5-(5,5-dimethyl-[1,3]dioxan-2-yl)-
3,4-dioctylthiophene (S3), compounds 3a, 3b, 5a, 6a, 6b, 8a, and 8b are
described in the Supporting Information.
10742
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Chem. Eur. J. 2008, 14, 10735 – 10744

Construction of Giant Porphyrin Macrorings
FULL PAPER
phyrin dimer (18.38 ꢃ) determined by AM1 method. The total excitonic
coupling energy for all of the porphyrins in each macroring was calculat-
ed by using Equations (1) and (2). The results are listed in Tables S3–S8
in the Supporting Information.
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Acknowledgements
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Received: July 19, 2008
Published online: October 16, 2008
10744
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Chem. Eur. J. 2008, 14, 10735 – 10744