Ligand-assisted J-type aggregates of zinc porphyrin: Anticooperative molecular organization in self-assembled bolaamphiphile

Article abstract of DOI:10.1039/c004636a

Bolaamphiphilic zinc porphyrins bearing a 2-pyridylethynyl or 4-methoxyphenylethynyl group, Zn(PyPor) or Zn(ArPor), respectively, were newly synthesized in order to explore the molecular organizing behaviours within the interior hydrophobic layer of aqueous self-assemblies. Well-ordered J-type aggregates of Zn(PyPor) were formed in aqueous self-assembled bolaamphiphile, whereas J-type aggregates of Zn(ArPor) were disordered. The titration experiments suggest that axial coordination bond improves the structural uniformity of the J-type aggregates of Zn(PyPor). In a homogeneous isotropic media, on the other hand, Zn(PyPor) formed an antiparallel dimer through self-complementary coordination. Unprecedented molecular organization of ligand-assisted J-type aggregates is described in the term of anticooperative effect under the bulk conditions. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c004636a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Ligand-assisted J-type aggregates of zinc porphyrin: anticooperative
molecular organization in self-assembled bolaamphiphile†
Mitsuhiko Morisue,* Takefumi Morita and Yasuhisa Kuroda
Received 24th March 2010, Accepted 20th May 2010
First published as an Advance Article on the web 9th June 2010
DOI: 10.1039/c004636a
Bolaamphiphilic zinc porphyrins bearing a 2-pyridylethynyl or 4-methoxyphenylethynyl group,
Zn(PyPor) or Zn(ArPor), respectively, were newly synthesized in order to explore the molecular
organizing behaviours within the interior hydrophobic layer of aqueous self-assemblies. Well-ordered
J-type aggregates of Zn(PyPor) were formed in aqueous self-assembled bolaamphiphile, whereas J-type
aggregates of Zn(ArPor) were disordered. The titration experiments suggest that axial coordination
bond improves the structural uniformity of the J-type aggregates of Zn(PyPor). In a homogeneous
isotropic media, on the other hand, Zn(PyPor) formed an antiparallel dimer through
self-complementary coordination. Unprecedented molecular organization of ligand-assisted J-type
aggregates is described in the term of anticooperative effect under the bulk conditions.
Amphiphilic assemblies incorporating porphyrin have been
1. Introduction
reported as successful models for the primary photosynthesis.10
The self-assembled amphiphile partitions the interior hydrophobic
layer from the outer solvent and provides a condensed phase fluid
in aqueous solution.11 Such a two-dimensional layer may be seen as
analogous to a cuvette with nanometre-order optical path length.
Herein, we describe the synthesis of nonionic bolaamphiphilic por-
phyrins and subsequent investigation of the bulk self-coordinating
properties within the aqueous self-assembly.
The formation of organized porphyrin assemblies through axial
coordination has prompted significant efforts to construct sophis-
ticated molecular architectures aiming at artificial photosynthesis
and further novel photophysical applications.1 Fundamentally, an
association constant defines such dynamic structures as a function
of the concentration.2 The stationary dynamic structures are nor-
mally designed considering a homogeneous isotropic solution. The
organized geometry of multiporphyrin assemblies is controllable
by virtue of the rigid platform of porphyrin macrocycle.1 Of
particular importance, a cooperative effect stems from a preference
of intra-complex interactions over inter-molecular interactions in
isotropic solutions.2b In principle, if the contribution of solvent
molecules is ruled out, a coordination bond should be formed
regardless of the magnitude of binding constant, where a cooper-
ative effect no longer remains. Then, dynamic structures should
be altered into an unprecedented form under a bulk condition.
The ligand-appended porphyrinatozinc(II) is well known to
exclusively form an antiparallel dimer via self-complementary
coordination.1,3–6 This is accomplished by the positively allosteric
cooperativity possible by the intra-dimer coordination being much
more favorable than the initial coordination in isotropic solutions.
On the other hand, another possible staircase configuration
can emerge as a slipped-stacked array, the so-called J-type
aggregates, which has never been observed, otherwise than with
a specific aid such as porphyrin–graphite surface interaction.5
From the viewpoint of the structural and functional relevance to
photosynthetic light-harvesting complex, the staircase porphyrin
assemblies through electrostatic interaction have been the subject
of intense research.7–9 The present manuscript discloses an unusual
formation of J-type aggregates of zinc porphyrins in a bulk
condition employing the aqueous self-assemblies.
2. Results and discussion
2.1. Molecular organization in aqueous self-assemblies
The present molecular design of Zn(PyPor) introduces
nitrogenous-ligand aiming at pyridyl-to-zinc self-coordination,4,5
and dendritic oligoether groups as nonionic hydrophilic side-
chains to solubilize in the aqueous media (Scheme 1). The
homologous non-coordinating porphyrin, Zn(ArPor), has been
synthesized as a reference compound, as described in the Experi-
mental section (vide infra, Scheme 2).
Scheme 1 Chemical structures of Zn(PyPor) and Zn(ArPor).
Aqueous self-assemblies of Zn(PyPor) were prepared by injec-
tion of a methanol solution of the porphyrin into distilled water
followed by aging for a few days. A stationary Soret band emerged
at 464 nm indicating considerable expansion of the p-system above
that of the conventional antiparallel dimer (Fig. 1a). A remarkable
bathochromic shift should be the outcome of formation of
Department of Biomolecuar Engineering, Kyoto Institute of Technology,
Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan. E-mail: morisue@kit.ac.jp
† Electronic supplementary information (ESI) available: Syntheses, exper-
imental observations in isotropic solutions, and preparation of aqueous
self-assemblies. See DOI: 10.1039/c004636a
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Fig. 2 Emission spectra of the aqueous assemblies of Zn(PyPor) (thick
line) and Zn(ArPor) (thin line, magnified by twenty times) observed by
excitation at 580 nm.
Scheme 2 Synthetic routes of Zn(PyPor) and Zn(ArPor).
Scheme 3 Schematic energy diagrams of exciton coupling of J-type
aggregates with closer distance (a) and longer distance (b). Solid lines
denote optically allowed transitions and dotted lines are optically forbid-
den. Schematic illustration of fluorescent processes for ordered (c) and
disordered aggregates (d).
through sole p–p stacking. Highly ordered J-type aggregates are
expected to be driven by pyridyl-to-zinc axial coordination.
As long as the aqueous self-assemblies remain, spectral proper-
ties should be apparently relevant to the bulk organized structure.
To explore the bulk self-organization properties of Zn(PyPor),
the effect of a nitrogenous ligand was then investigated in aqueous
solution. Axial coordination of the external ligand should undergo
a permeation process limited by a hydration–dehydration process
as parameterized by the octanol/water partition coefficient, logP,
index.15–17 Therefore, introduction of an external ligand should
lead to diverse effects, i.e., dilution of the concentration of
porphyrin within the hydrophobic layer in aqueous self-assemblies
and competitive axial coordinaton. Titration experiments on
Zn(PyPor) showed a two-step discontinuous spectral change
indicating structural transition within the aqueous self-assemblies
followed by its collapse, coincident with changes in the morphol-
ogy as observed by epifluorescence microscopy. In this way, the
titrations with pyridine and 2,6-lutidine are performed up to the
point of collapse in the following section.
Fig. 1 Spectral titration of Zn(PyPor) (a) and Zn(ArPor) (c) with pyridine
in water at 25 ◦C, up to the collapse of the amphiphilic assemblies. Spectral
change was recorded after successive additions of ca. 5 ¥ 103 equiv of
pyridine. Plots of absorbance at the selected wavelengths (b and d).
J-type aggregates.7–9,12 Such bathochromism was also observed
for Zn(ArPor) in aqueous assemblies (Fig. 1c). The bulkiness of
dendritic oligoether side group may be effective to preclude from
H-type cofacial stacking and to form J-type aggregates.13
Epifluorescence microscopic observations identified fluorescent
spheres elucidating that the bolaamphiphilic porphyrin was
spontaneously assembled into spherical aggregates. The size of
spherical assemblies was in the order of micrometres, being at
least larger than 0.45 mm judging from complete disappearance of
the Soret band after passing the solution through membrane filter
with a pore diameter of 0.45 mm. Thus, J-type aggregates were
formed within the aqueous self-assemblies.
Notably, the pyridyl group plays a crucial role in organization
of the well-ordered J-type aggregates. The aqueous assemblies of
Zn(PyPor) exhibited emission seventy times stronger than that
of Zn(ArPor) (Fig. 2). In general, structural disorder dissipates
excited singlet through an energy sink leading to weak fluorescence
of the chromophore assemblies (Scheme 3d).14 Therefore, it is
clear that the pyridyl ligand significantly improves the structural
uniformity of the J-type assemblies as compared to that obtained
Upon addition of pyridine as the competitive ligand, the
bathochromic Soret band of Zn(PyPor) at 464 nm was duplicated
to give another bathochromic band at 474 nm (Fig. 1a). At the
same time, the shorter Soret band at 438 nm disappeared. Also
bathochromism of the Q band was enhanced from 637 nm to
643 nm. The enhanced bathochromism suggests larger dislocation
of interplane distance, based on Kasha’s molecular exciton
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theory (Scheme 3a and 3b).8b,12 The observation of an increased
bathochromic band was closely reminiscent of that observed for
aqueous assemblies of Zn(ArPor), and is considered to originate
from the intrinsic p-stacking properties of bolaamphiphilic por-
phyrin (Fig. 1c). On the other hand, the spectral titration of
Zn(ArPor) assemblies were faint. These spectral changes were
in sharp contrast to dissociation behavior, as emphasised in the
plots of the bathochromic and monomeric Soret bands for the
Zn(PyPor) assemblies showing biphasic change (Fig. 1b). On the
other hand, those for the Zn(ArPor) assemblies were dissociated
in a quasi-single phasic fashion (Fig. 1d). The susceptibility of
Zn(PyPor) to coordination by an axial ligand suggests that self-
coordination drives highly ordered J-type aggregates found in the
aqueous self-assemblies.
principles underlaying the bulk molecular organization, although
the observations should involve plural thermodynamic factors
in the strict sense. Further elucidation of the bulk molecular
organization in the heterogeneous system is difficult. The self-
organizing behaviours of Zn(PyPor) in isotropic organic media
may be predictive of some aspects of the stacking properties in
aqueous self-assemblies.
2.2. Self-complementary coordination in isotropic solutions
Zn(PyPor) showed self-organizing behaviour when using CDCl3
1
as a non-coordinating solvent in H NMR spectra. The protons
of b-pyrrole and pyridyl residue showed upfield shift, suggesting
ring-current shielding due to the close proximity to the mutual
porphyrin plane (Fig. 4). Assuming the equilibrium between
monomer and dimer, the concentration dependence of 1H NMR
spectra indicated an association constant (K1 ¥ K2, in Scheme 4)
of ~101 M-1 based on the nonlinear least square fitting.† On the
other hand, no organized behaviour was detected for Zn(PyPor)
when using CD3OD as a coordinating solvent. Further, Zn(ArPor)
showed no concentration dependence in either of the solvents.
Hence, the self-organization behaviour was a specific phenom-
ena for Zn(PyPor) in a non-coordinating solvent under highly
concentrated conditions. Self-organization is thus assignable to
formation of an antiparallel dimer through self-complementary
pyridyl–to–zinc coordination, according to a previous report.6 The
small binding constant for self-complementary coordination may
be attributed to low coordination ability of 2-ethynylpyridyl group
as follows.
Additionally, the titration experiments were conducted with
an aqueous solution of 2,6-lutidine. Steric hindrance of the two
methyl groups in this case significantly disturbs the coordination
capacity of nitrogen atom of 2,6-lutidine, while the degree of
permeation should be much higher than that of pyridine.15,16
A
dilution effect was observed for 2,6-lutidine in which the self-
coordination of Zn(PyPor) was dissociated into smaller aggregates
in the two-dimensional assemblies; here, monomeric species may
be predominant judging from the absorption maxima at 444 nm
(Fig. 3a). A dilution effect of 2,6-lutidine was also observed in
disstacking the p–p interaction of the Zn(ArPor) J-type aggregates
(Fig. 3b). These observations are pertinent to the small binding
constants for self-coordination of Zn(PyPor) in a homogeneous
isotropic solution (vide infra).
Fig. 4 Representative 1H NMR spectra (300 MHz) of Zn(PyPor) at
various concentrations in CDCl3; 9.2 ¥ 10-2 M (a), 1.0 ¥ 10-2 M (b),
7.5 ¥ 10-3 M (c), 5.0 ¥ 10-3 M (d), 2.5 ¥ 10-3 M (e), and 5.0 ¥ 10-4 M (f).
The initial binding constant K1 of the 2-ethynylpyridyl group
to the zinc atom was estimated to be less than ~10-5 M-1 from the
titration experiment of 2-phenylethynylpyridine with Zn(PyPor) in
chloroform.† This estimated value is too small to rationalize the
Fig. 3 Spectral titration of Zn(PyPor) (a) and Zn(ArPor) (c) with
2,6-lutidine in water at 25 ◦C, up to the collapse of the amphiphilic
assemblies. Spectral change was recorded after successive additions of
ca. 5 ¥ 103 equiv of pyridine. Plots of absorbance at selected wavelengths
(b and d).
1
self-organization behaviour observed in the H NMR. This fact
proves formation of self-complementary coordination through
a second binding constant K2 at least as large as 106 M0 to
account for the observed self-association constant of ~101 M-1.
Such a dimensionless constant would be much more favourable
Addition of external ligands was certainly effective in terms of
both competitive coordination and dilution of the amphiphilic
assemblies. The observations elucidates some parts of essential
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In aqueous media, on the other hand, the initial nucleation is
achieved by the formation of the aqueous self-assemblies. Likewise
the formation of the second coordination in an antiparallel
dimer, the nucleation compensates the small coordination capacity.
Moreover, the relative difference in the binding affinity should not
remain in the bulk condition—the so-called anticooperative effect,
wherein all the porphyrin constituents are equivalent. The possible
binding processes obey the sole dimensionless equilibria assuming
the single binding mode of the pyridyl-to-zinc coordination. To
this end, the formation of successive coordination bond should
be preferable to the self-complementary one in some probability
(Fig. 5). Along the bulk equilibria, small activation barrier for
dissociation of the antiparallel dimer may be proper for smooth
production of the J-type aggregates, avoiding kinetic entrapment
into the antiparallel dimer. Hence, Zn(PyPor) is likely to produce
successive coordination bonds in a unique fluidic medium within
the aqueous self-assemblies.
Scheme 4 Possible coordination equilibria of Zn(PyPor). Binding con-
stants; K1 for the single coordination to form a dimer, K2 for the second
intradimer coordination to form an antiparallel dimer via self-comple-
mentary coordination, and K3 for successive coordination to form J-type
aggregates.
3. Conclusions
than the initial binding process in fulfilling the prerequisite for
positively allosteric cooperativity to exclusively form an antipar-
allel Zn(PyPor) dimer.2b The initial nucleation induced the second
coordination bond through dimensionless binding equilibrium,
leading to self-complementary coordination in CDCl3. Eventually,
self-organizing behaviours of Zn(PyPor) are consistent with the
formation of a self-complementary dimer.
In summary, we have exploited a bulk molecular organization
within the interior hydrophobic layer of the self-assembled
bolaamphiphile. Comparable studies of molecular organizing
behaviour drew contrasting results in terms of the thermodynamic
equilibria between the assemblies in aqueous solution and those
in isotropic organic solvent. Consequently, highly ordered J-type
aggregates of Zn(PyPor) have been provided by the aid of self-
coordination under the conditions where solvent molecule is
ruled out. To the best of our knowledge, aqueous J-aggregates
of porphyrin normally assemble through electrostatic interactions
under acidic conditions.9,10 On the other hand, the structure
of the J-type aggregates of Zn(PyPor) was modulated by an
external ligand under neutral conditions. Such ligand-directed
structural tunability offers a great advantage in a functionalization
of porphyrin J-type aggregates. We are convinced that the antico-
operative molecular organization methodology opens the way to
design elaborate polymeric architectures.
2.3. Anticooperative effect in a bulk medium embedded in
self-assembled bolaamphiphile
Based on the above insights in homogeneous isotropic solutions,
let us consider the properties of molecular organization in the
aqueous self-assemblies. In isotropic solution, self-complementary
coordination is inherent in ligand-appended zinc porphyrins,
wherein a positively allosteric cooperative effect excludes the
formation of J-type aggregates because of disadvantage in the
entropy change. Thus, roles of the solvent must be key in the free
energy change. Consequently, the initial nucleation promotes the
second intra-complex association leading to self-complementary
coordination (Fig. 5).
4. Experimental section
4.1. General procedures
NMR spectra were recorded on AV-300 (BRUKER). MALDI-
TOF MS spectra were observed by using dithranol as the
matrix by Autoflex II (BRUKER). Recycling gel-permeation
chromatography (GPC) was carried out by LC-9101 (Japan
Analytical Industry Co., Ltd.) connecting column of JAIGEL-3H
and -2H with chloroform as the eluent. UV-visible and fluores-
cence spectra were recorded on the spectrophotometer (Multispec-
1500, Shimadzu) and the fluorescence photospectrometer (F-4500,
Hitachi), respectively.
4.2. Procedures of synthesis
All reagents were used as received otherwise described. De-
hydrated solvents were prepared by general procedures; N,N-
dimethylformamide (DMF) and triethylamine were distilled over
calcium hydride under vacuum, tetrahydrofuran (THF) and 1,4-
dioxane were distilled over sodium/benzophenone ketyl, toluene
Fig. 5 Schematic illustration of positively allosteric cooperative system
in homogeneous isotropic solution and anticooperative system in the
aqueous self-assemblies.
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was distilled over melting sodium. Pd2(dba)3 (dba = trans,trans-
dibenzylidenacetone) was prepared by the general procedure; to
the solution of palladium chloride (0.43 g, 1.8 mmol) and sodium
acetate (0.40 g, 4.9 mmol)◦in methanol (15 mL) was added dba
(0.17 g, 0.94 mmol) at 50 C under argon atmosphere and then
recrystallization of the precipitate obtained from chloroform.
3 was roughly purified by elution from a triethylamine-treated
silica-gel column chromatography with chloroform–ethyl acetate–
methanol (90/7/3, v/v/v) as a viscous green substance (103 mg,
0.095 mmol; 24%). This material was used for the next step without
further purification. MALDI-TOF MS (dithranol): m/z: calcd for
C81H106BrN5O24Zn: 1675.57; found: 1675.58.
4.2.1. 5,15-Bis[3,4,5-tris(9-methoxy-1,4,7-trioxanonyl)phe-
nyl]porphyrin (1). The titled compound 1 was prepared by
the modified literature method.18 The solution of 3,4,5-tris(9-
methoxy-1,4,7-trioxanonyl)benzaldehyde (0.86 g, 1.5 mmol) and
dipyrromethane (0.22 g, 1.5 mmol) dissolved in chloroform (300
mL) was deaerated by bubbling with nitrogen gas for 5 min.
The mixture was stirred with TFA (trifluoroacetic acid, 1.0 mL,
13 mmol) at room temperature in the dark for 14 h. To the
reaction mixture was added DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone, 0.33 g, 1.5 mmol) as the oxidant and stirred for
additional 3.5 h. The reaction mixture was washed successively
with aqueous saturated sodium hydrogencarbonate and brine. The
organic layer separated was dried over anhydrous sodium sulfate.
The crude material was subjected to the silica-gel chromatographic
separation with chloroform–methanol (95/5 and 98/2, v/v) as the
eluent. Porphyrin 1 was obtained as a viscous purple substance
4.2.4. 5-{N -(tert-butoxycarbonyl)-4-anilino}-15-(2-pyridyl-
ethynyl)-10,20-bis[3,4,5-tris(9-methoxy-1,4,7-trioxanonyl)phenyl]-
porphyrinatozinc(II), Zn(PyPor). The mixture of porphyrin
3
(0.12 g, 0.074 mmol) and pinacol {4-N-(tert-butyloxy-
carbonyl)aminophenyl}boronate (30 mg, 0.094 mmol) in DMF
(6 mL) and toluene (12 mL) was deoxygenated by freeze-pump-
thaw cycles and flushed with argon gas in a Schlenk flask. The
mixture was added caesium carbonate (26 mg, 81 mmol) and
Pd(PPh3)4 (PPh3 = triphenylphosphine) (12 mg, 9.8 mmol), and
then allowed for stirring at 80 ◦C in the dark for 5 h. The
reaction mixture was passed through Celite pad. After removal of
the solvent, the crude material dissolved in dichloromethane was
washed with water and brine. The titled material was purified by
silica-gel column chromatography with chloroform–ethyl acetate–
methanol (90/7/3, v/v/v). Further purification by recycling GPC
with chloroform as the eluent afforded Zn(PyPor) as a viscous
1
1
(0.59 g, 0.41 mmol; 57%). The compound was identified by H
green substance (83 mg, 46 mmol; 63%). H NMR (300 MHz,
NMR and MALDI-TOF MS, which were identical with those in
previous report.18
CD3OD): d = 1.56 (s, 9H; t-Bu), 2.9–4.4 (m, 90H; ether), 7.19 (t,
J = 6.5 Hz, 1H; 6-Py), 7.39 (s, 4H; Ph), 7.68 (t, J = 7.0 Hz, 1H;
4-Py), 7.77 (d, J = 8.3 Hz, 2H; Ph), 7.84 (d, J = 7.7 Hz, 1H; 5-Py),
8.00 (d, J = 8.3 Hz, 2H; Ph), 8.11 (d, J = 4.2 Hz; 3-Py), 8.79
(s, 4H; pyrrole-b), 8.90 (d, J = 4.5 Hz, 2H; pyrrole-b), 9.55 ppm
(d, J = 4.5 Hz, 2H; pyrrole-b). 13C NMR (75 MHz, CDCl3): d =
28.54, 58.39, 59.11, 69.36, 69.92, 70.08, 70.57, 70.66, 70.75, 70.81,
70.90, 71.14, 72.06, 72.85, 80.89, 93.88, 97.89, 115.68, 116.66,
121.75, 122.34, 122.99, 127.53, 131.13, 131.74, 132.20, 132.86,
135.02, 136.34, 137.56, 138.14, 138.21, 143.82, 149.61, 149.82,
150.09, 150.68, 152.79, 153.12 ppm (Figure S6). MALDI-TOF
MS (dithranol): m/z: calcd for C92H120N6O26Zn: 1788.75; found:
1788.83. lmax in CHCl3: 441, 509, 618 nm.
4.2.2. 5,15-Bisbromo-10,20-bis[3,4,5-tris(9-methoxy-1,4,7-tri-
oxanonyl)phenyl]porphyrinatozinc(II) (2). To the solution of por-
phyrin 1 (0.69 g, 0.48 mmol) in chloroform (50 mL) was added NBS
◦
(N-bromosuccinimide, 0.18 g, 0.99 mmol) at -40 C and stirred
for 60 min. Bromination was quenched by addition of acetone (ca.
5 mL) and the solvent was removed under reduced pressure. The
crude material redissolved in chloroform (50 mL) was treated with
saturated zinc acetate in methanol (3 mL) at room temperature
for 2 h. The reaction mixture was washed with aqueous saturated
sodium hydrogencarbonate and brine. The organic layer separated
was dried over anhydrous sodium sulfate. Purification by silica-gel
column chromatography with chloroform–methanol (98/2, v/v)
afforded dibrominated porphyrin 2 as a viscous brown substance
4.2.5. 5-Bromo-15-(4-methoxyphenylethynyl)-10,20-bis[3,4,5-
tris(9-methoxy-1,4,7-trioxanonyl)phenyl]-porphyrinatozinc(II) (4).
Porphyrin 4 was synthesized by the Mizoroki–Heck type cross-
coupling procedure, essentially same with that for the synthesis of
porphyrin 3. The deoxygenated mixture of porphyrin 2 (0.43 g,
0.26 mmol) and 4-ethynylanisole20 (36 mg, 0.27 mmol) in THF
(15 mL) and triethylamine (2.1 mL) was added triphenylarsine
(46 mg, 0.15 mmol) and Pd2(dba)3 (22 mg, 24 mmol) stirred at
50 ◦C in the dark for 5 h. The crude mixture was washed with
brine and the organic layer was dehydrated by anhydrous sodium
sulfate. The chromatographic separation with chloroform–ethyl
acetate–methanol (90/7/3, v/v/v) furnished the target por-
phyrin 4 and 5,15-bis(4-methoxyphenylethynyl)-10,20-bis[3,4,5-
tris(9-methoxy-1,4,7-trioxanonyl)phenyl]porphyrinatozinc(II) as
the byproduct. These porphyrins were difficult to be completely
separated by silica-gel column at this stage and used for the next
step without further purification. MALDI-TOF MS (dithranol):
m/z: calcd for C83H109BrN4O25Zn: 1704.59; found: 1704.24.
1
(0.78 g, 0.47 mmol; 98%). H NMR (300 MHz, CDCl3): d =
2.35–4.05 (m, 78H; ethylene), 4.35 (t, J = 5.0 Hz, 8H; Ph-3,5-
OCH2-), 4.50 (t, J = 4.9 Hz, 4H; Ph-4-OCH2-), 7.54 (s, 4H; Ph)
8.97 (d, J = 4.7 Hz, 4H; pyrrole-b), 9.68 (d, J = 4.7 Hz, 4H;
pyrrole-b) ppm. 13C NMR (75 MHz, CDCl3): d = 58.34, 58.64,
68.84, 69.47, 69.78, 70.12, 70.24, 70.38, 71.17, 71.56, 72.30, 77.79,
104.76, 115.18, 121.57, 132.88, 133.31, 137.65, 137.73, 150.00,
150.32, 150.62 ppm. MALDI-TOF MS (dithranol): m/z: calcd
for C74H102Br2N4O24Zn: 1652.45; found: 1652.36.
4.2.3. 5-Bromo-15-(2-pyridylethynyl)-10,20-bis[3,4,5-tris(9-
methoxy-1,4,7-trioxanonyl)phenyl]porphyrinatozinc(II) (3). The
mixture of porphyrin 2 (0.66 g, 0.40 mmol) and 2-ethynylpyridine19
(40 mg, 0.39 mmol) in THF (20 mL) with triethylamine (3.2
mL) was deoxygenated by freeze-pump-thaw cycles and purged
with argon gas in a Schlenk flask. The mixture was added
triphenylarsine (65 mg, 0.21 mmol) and Pd2(dba)3 (29 mg,
32 mmol), and then stirred at 50 ◦C in the dark for 24 h.
The crude mixture was washed with brine. The organic layer
separated was dehydrated by anhydrous sodium sulfate. Porphyrin
4.2.6. 5-{N-(tert-butoxycarbonyl)-4-anilino}-15-(4-methoxy-
phenylethynyl)-10,20-bis[3,4,5-tris(9-methoxy-1,4,7-trioxanonyl)-
phenyl]porphyrinatozinc(II),
Zn(ArPor). Zn(ArPor)
was
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prepared by the Suzuki–Miyaura cross-coupling procedure,
essentially same with that for Zn(PyPor). The mixture of
porphyrin 4 (0.21 g, 0.074 mmol) and pinacol 4-{N-(tert-
butyloxycarbonyl)aminophenyl}boronate (57 mg, 0.18 mmol)
in DMF (10 mL) and toluene (20 mL) was degassed by the
Schlenk technique. The mixture was added caesium carbonate
(50 mg, 0.15 mmol) and Pd(PPh3)4 (19 mg, 0.015 mmol), and
then allowed for stirring at 80 ◦C in the dark for 5 h. The
reaction mixture was passed through a Celite pad and the
solvent was removed under reduced pressure. The crude material
redissolved in dichloromethane was washed with water and brine.
The organic layer separated was dried over anhydrous sodium
sulfate. The titled material was roughly purified by silica-gel
column chromatography with chloroform–ethyl acetate–methanol
(90/7/3, v/v/v). Further purification was subjected to recycling
GPC with chloroform as the eluent. Zn(ArPor) was obtained as a
viscous green substance (29 mg, 0.016 mmol; 6% through 2 steps
from 420 nm to the stationary band at 463 nm, with remarkable
increment of fluorescent emission.
The properties of the porphyrin assemblies examined were
much affected by several experimental parameters. For example,
the content of methanol used for initial dissolution significantly
influenced the reproducibility of the spectral properties. When the
content of methanol was increased, the aqueous self-assemblies
were loosened to lose the susceptibility to the external ligand.
Also, spectral behaviour varied depending on ionic strength and
temperature. At higher temperature than 30 ◦C, bolaamphiphilic
porphyrin was precipitated, probably due to dehydration of oligo-
ether side-chains.23 On the other hand, the porphyrin assem-
blies were too robust to be coordinated by the external ligand
binding at lower temperature than 15 ◦C. When the aqueous
media was buffered with Na2HPO4–NaH2PO4, the solubility
of bolaamphiphilic porphyrin was seriously reduced. Then, the
present experiments were performed in distilled water (~pH 6.8)
at 25 ◦C.
In the main text, the dissociation behaviors were described
unless the amphiphilic assemblies collapsed. When the assemblies
were collapsed, the spectral change showed discontinuous in the
titration experiments. The collapse behaviours were coincidentally
observed epifluorescence microscopy. Addition of excess amount
of pyridyl ligand dissolved the amphiphilic assemblies into appar-
ently isotropic solution.
1
from 2). H NMR (300 MHz, CD3OD): d = 1.59 (s, 9H; t-Bu),
3.06–3.85 (m, 78H; ether), 3.73 (s, 3H; methoxy), 4.15 (m, 12H;
Ph-OCH2-), 6.90 (d, J = 8.7 Hz, 2H; Ph), 7.48 (s, 4H; Ph), 7.75
(d, J = 2.9 Hz, 2H; Ph), 7.78 (d, J = 3.0 Hz, 2H; Ph), 8.02 (d,
J = 8.5 Hz, 2H; Ph), 8.79 (d, J = 4.7 Hz, 2H; pyrrole-b), 8.83
(d, J = 4.7, 2H; pyrrole-b), 8.98 (d, J = 4.6 Hz, 2H; pyrrole-b),
9.71 ppm (d, J = 4.6 Hz, 2H; pyrrole-b) (Figure S9). 13C NMR
(75 MHz, CDCl3): d = 28.47, 55.48, 58.17, 59.07, 69.39, 69.71,
70.10, 70.51, 70.62, 70.70, 70.76, 70.86, 72.02, 72.76, 77.22,
77.44, 114.40, 115.70, 116.56, 133.07, 138.16, 150.64, 152.15 ppm.
MALDI-TOF MS (dithranol): m/z: calcd for C94H123N5O27Zn:
1817.77; found: 1817.37. lmax in CHCl3: 445, 573, 622 nm.
Acknowledgements
The authors are grateful to Prof. S. Asaoka and Dr S. Masuo
(Kyoto Institute of Technology) for GPC purification and epiflu-
orescence microscopic observation, respectively. This work was
partly aided by financial supports from JST (Japan Science and
Technology Agency) Innovation Plaza Kyoto, through a Grant-
in-Aid for Young Scientists (B) (No. 21750123) from JSPS (Japan
Society of the Promotion of Science), and the Kyoto-Advanced
Nanotechnology Network program with technical assistance by
Ms. Y. Nishikawa (Nara Institute of Science and Technology).
4.2.7. Pinacol 4-{N-(tert-butyloxycarbonyl)aminophenyl} bo-
ronate. Borylation of tert-butyl 4-iodophenylcarbamate21 was
carried out according to literature procedures.22 A solution of tert-
butyl 4-iodophenylcarbamate (0.36 mg, 1.1 mmol) in 1,4-dioxane
(10 mL) and triethylamine (0.5 mL) was degassed via freeze-
pump-thaw cycles and flushed with argon gas in a Shlenck flask.
To the solution was added pinacol borane (1.45 mL, 10 mmol)
and PdCl2(dppf) (dppf = 1,1¢-bis(diphenylphosphino)ferrocene)
(39 mg, 47 mmol). The mixture was gently refluxed at 100 ◦C for 2 h.
The reaction mixture diluted with dichloromethane was washed
with water and brine. The organic layer separated was dried over
anhydrous sodium sulfate. Recrystallization from methanol gave
the target material as colorless needle crystal (0.16 g, 0.45 mmol;
45%). 1H NMR (300 MHz, CD3OD): d = 1.33 (s, 12H, Me), 1.52
(s, 9H; tBu), 7.36 (d, J = 8.5 Hz, 2H; Ph), 7.73 ppm (d, J = 8.5 Hz,
2H; Ph).
Notes and references
1 (a) A. Satake and Y. Kobuke, Tetrahedron, 2005, 61, 13–41; (b) I.
Beletskaya, V. S. Tyurin, A. Y. Tsivadze, R. Guilard and C. Stern,
Chem. Rev., 2009, 109, 1659–1713, and cited therein.
2 (a) K. A. Connors, Binding Constant, The Measurement of Molecular
Complex Stability, John Wiley & Sons, 1987; (b) C. A. Hunter and
H. L. Anderson, Angew. Chem., Int. Ed., 2009, 48, 7488–7499.
3 (a) K. M. Smith, L. A. Kehres and J. Fajer, J. Am. Chem. Soc., 1983,
105, 1387–1389; (b) A. Jesorka, T. S. Balaban, A. R. Holzwarth and K.
Schaffner, Angew. Chem., Int. Ed. Engl., 1996, 35, 2861–2863.
4 (a) R. T. Stibrany, J. Vasudevan, S. Knapp, J. A. Potenza, T. Emge
and H. J. Schugar, J. Am. Chem. Soc., 1996, 118, 3980–3981; (b) N. N.
Gerasimchuk, A. A. Mokhir and K. R. Rodgers, Inorg. Chem., 1998,
37, 5641–56650.
5 M. Koepf, J. A. Wytko, J.-P. Bucher and J. Weiss, J. Am. Chem. Soc.,
2008, 130, 9994–10001.
6 (a) Y. Kobuke and H. Miyaji, J. Am. Chem. Soc., 1994, 116, 4111–4112;
(b) K. Kameyama, M. Morisue, A. Satake and Y. Kobuke, Angew.
Chem., Int. Ed., 2005, 44, 4763–4766; (c) M. Morisue and Y. Kobuke,
Chem.–Eur. J., 2008, 14, 4993–5000.
4.3. Preparation of aqueous self-assemblies
The porphyrins were dissolved in methanol, prior to dissolution
in water. Then, 5.0 mL of the methanol solution (ca. 1.2 mM of
Zn(PyPor) or Zn(ArPor)) was injected into 3.0 mL of distilled
water and adjusted to ca. 2 ¥ 10-6 M of concentration in water.
The solution was allowed to stand for a few days.
The Soret band emerged around 420 nm immediately after
dissolution of Zn(PyPor) in water. That gradually shifted to
around 470 nm over a few hours. In this period, fluorescence
intensity was very weak with remaining similar level. After aging
for a few days, spectral evolution of the Soret band was converged
7 (a) J. Ribo´, J. Crusats, F. Sague´s, J. Claret and R. Rubires, Science,
2001, 292, 2063–2066; (b) C. Escudero, J. Crusats, I. D´ıez-Pe´rez, Z.
El-Hachemi and J. M. Ribo´, Angew. Chem., Int. Ed., 2006, 45, 8032–
8035.
3462 | Org. Biomol. Chem., 2010, 8, 3457–3463
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
8 (a) M. Y. Choi, J. A. Pollard, M. A. Webb and J. L. McHale, J. Am.
Chem. Soc., 2003, 125, 810–820; (b) S. Okada and H. Segawa, J. Am.
Chem. Soc., 2003, 125, 2792–2796.
9 (a) G. McDermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-
Lawless, M. Z. Papiz, R. J. Cogdell and N. W. Lsaacs, Nature, 1995, 374,
517–521; (b) A. W. Roszak, T. D. Howard, J. Southall, A. T. Gardiner,
C. J. Law, N. W. Isaacs and R. J. Cogdell, Science, 2003, 302, 1969–
1972.
15 The binding constant of axial ligand in chloroform: pyridine; 2.9 ¥
103 M-1 for Zn(PyPor), and 1.8 ¥ 103 M-1 for Zn(ArPor). Axial
coordination was substantially not observed for 2,6-lutidine.
16 The logP value: 0.65 for pyridine and 1.68 for 2,6-lutidine.
17 (a) T. Mizutani, K. Wada and S. Kitagawa, J. Am. Chem. Soc., 1999,
121, 11425–11431; (b) R. Murakami, A. Minami and T. Mizutani, Org.
Biomol. Chem., 2009, 7, 1437–1444; (c) S. Tomas and L. Milanesi, J. Am.
Chem. Soc., 2009, 131, 6618–6623.
10 (a) G. Steinberg-Yfrach, J.-L. Rigaud, E. N. Durantini, A. L. Moore, D.
Gust and T. A. Moore, Nature, 1998, 392, 479–482; (b) T. Komatsu, M.
Moritake and E. Tsuchida, Chem.–Eur. J., 2003, 9, 4626–4633; (c) N.
Nagata, Y. Kuramochi and Y. Kobuke, J. Am. Chem. Soc., 2009, 131,
10–11.
11 (a) T. Kunitake, Angew. Chem., Int. Ed. Engl., 1992, 31, 709–726; (b)
J.-H. Fuhrhop and T. Wang, Chem. Rev., 2004, 104, 2901–2937; (c) J.-H.
Ryu, D.-J. Hong and M. Lee, Chem. Commun., 2008, 1043–1054.
12 M. Kasha, Radiat. Res., 1963, 20, 55–71.
13 T. Yamaguchi, T. Kimura, H. Matsuda and T. Aida, Angew. Chem.,
Int. Ed., 2004, 43, 6350–6355.
14 H. Nakamura, H. Fujii, H. Sakaguchi, T. Matsuo, N. Nakashima, K.
Yoshihara, T. Ikeda and S. Tazuke, J. Phys. Chem., 1988, 92, 6151–6156.
18 T. Sakurai, K. Shi, H. Sato, K. Tashiro, A. Osuka, A. Saeki, S. Seki, S.
Tagawa, S. Sasaki, H. Masunaga, K. Osaka, M. Tanaka and T. Aida,
J. Am. Chem. Soc., 2008, 130, 13812–13813.
19 S. Takahashi, Y. Kuroyama, K. Sonogashira and N. Hagihara, Synthe-
sis, 1980, 627–629.
20 L. F. Tietze, C. A. Vock, I. K. Krimmelbein, J. M. Wiegand, L. Nacke,
T. Ramachandar, K. M. D. Islam and C. Ga, Chem.–Eur. J., 2008, 14,
3670–3679.
21 A. Yokoyama, T. Maruyama, K. Tagami, H. Masu, K. Katagiri, I.
Azumaya and T. Yokozawa, Org. Lett., 2008, 10, 3207–3210.
22 O. Baudoin, D. Gue´nard and F. Gue´ritte, J. Org. Chem., 2000, 65,
9268–9271.
23 W. Li, A. Zhang and A. D. Schlu¨ter, Chem. Commun., 2008, 5523–5525.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 3457–3463 | 3463
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